sustainability

Article
Soft-Switching Smart Transformer Design and Application for
Photovoltaic Integrated Smart City Power Distribution
Burak Esenboğa * and Tuğçe Demirdelen

Department of Electrical Electronics Engineering, Adana Alparslan Türkeş Science and Technology University,
Sarıçam 01330, Adana, Turkey
* Correspondence: besenboga@atu.edu.tr

Abstract: Smart city power distributions have become promising technologies to meet the demand
for energy in developed countries. However, increase in smart grids causes several power quality
problems on the smart grid, in particular, current and voltage harmonic distortions, sudden voltage
sag and swells, fault current, and isolation deterioration. Smart transformers are potential solutions
to improve the power quality on the electric grid. They present energy efficiency, ensure grid
reliability and power flow control, voltage regulation, bidirectional power flow, fault current limiting,
harmonic blocking, and galvanic isolation. Therefore, this paper offers an optimal selection of a
three-stage (AC-DC-DC-AC) smart transformer model and power control strategy for solar PV
power plant integrated smart grids. The topology of the rectifier, isolated bidirectional converter,
and inverter has soft-switching features. This enables low conduction loss, low electromagnetic
interference (EMI), high efficiency, achievable zero-voltage switching for converters, and zero-current
switching for electrical auxiliary systems. Operation strategies of the proposed ST, PWM control,
voltage, and current control between converters, including a medium-voltage (MV) high-frequency
transformer to realize a 10 kVA, 450 Vdc to 220 Vdc, or 220 Vac ST, are presented. Significantly, the ST
prototype achieves 96.7% conversion efficiency thanks to its control strategy, even under unstable
power generation conditions from the solar PV plant. Experimental results obtained on the 344 Vac
10.4 A load current validates the dv/dt rate 6.8 kV/us. The dynamic and experimental results of
the proposed bidirectional smart transformer demonstrate the success in preventing power quality
Citation: Esenboğa, B.; Demirdelen, problems for photovoltaic integrated smart city power distribution.
T. Soft-Switching Smart Transformer
Design and Application for
Keywords: smart transformer; power quality; smart city; power distribution; solar PV
Photovoltaic Integrated Smart City
Power Distribution. Sustainability
2023, 15, 32. https://doi.org/
10.3390/su15010032
1. Introduction
Academic Editors: Surender
In recent years, the increasing use of renewable energies and distributed generation
Reddy Salkuti and Brian Azzopardi
bring about energy quality problems in the smart power grid. Smart transformers (STs)
Received: 31 October 2022 are key and intelligent technologies that allow the incorporation of renewable and non-
Revised: 2 December 2022 renewable power plants into the same electric grid. STs have many advantages that
Accepted: 9 December 2022 are becoming an alternative to substitute the conventional power transformer such as
Published: 20 December 2022 reactive power compensation, voltage regulation, power flow control, bidirectional power
flow, fault current limiting, harmonic blocking, and galvanic isolation. One of the most
significant features of STs is that the weight and size are reduced considerably compared
with the conventional power transformers. This advantage is gained with the dc–dc isolated
Copyright: © 2022 by the authors.
converter structure of the ST. The dc–dc conversion stage with HF transformer is the main
Licensee MDPI, Basel, Switzerland.
part of the ST since it achieves some requirements such as high efficiency, high power
This article is an open access article
distributed under the terms and
density, intelligent power flow management, better isolation, reduced size, and weight. In
conditions of the Creative Commons
smart grids, it is the main part where power is controlled, and grid reliability is ensured.
Attribution (CC BY) license (https://
To meet these significant requirements in the different application areas of STs, researchers
creativecommons.org/licenses/by/ have performed many studies on STs integrated smart grids. The literature review presents
4.0/). the dc–dc converter-based ST studies so far. The literature review is summarized in Table S1.

Sustainability 2023, 15, 32. https://doi.org/10.3390/su15010032 https://www.mdpi.com/journal/sustainability

Sustainability 2023, 15, 32 2 of 27

A three-stage modular multilevel converter-based ST is presented [1–7]. This converter

provides bidirectional power flow due to its dual active structure. The control method
of the ISOP converter is the open-loop control (OLC) method. The study shows that the
proposed converter structure meets the requirements for isolation [1]. To increase the
power range, reduce the voltage harmonics, reduce the dv/dt stress and eliminate the back
power flow, the modular multilevel converter is proposed. The voltage regulation and
current balance operation are provided by the dual shift control method [2]. The modular
multilevel converter (MMC) has a high conversion ratio, zero current switching operation,
variable conversion ratio, and phase shift control. The faults are easily bypassed due to
their modular structure. Also, the resonant operation mode eases the ZCS mode. The
inductorless structure of the proposed converter provides low cost and high efficiency [3].
This converter topology is preferred to increase the converter power rating and gain high
efficiency under the ZVS operation of the power switches. PI controller-based phase
shift modulation is used for the control scheme. The control method enables the voltage
balance of the converter [4]. An isolated bi-directional dc–dc converter in an ISOP modular
topology is used in the design of a 100 kVA ST-based power substation to reduce the
freewheeling currents of DAB, which raise overall losses and lower the effective duty
cycle. To demonstrate the effectiveness of the suggested approach, simulation results in
the Matlab-Simulink environment are shown. For tracking a sinusoidal reference signal, a
proportional resonant (PR) controller performs better than a PI controller. However, due to
its limited bandwidth, a conventional PR controller is difficult to make stable. An updated
PR controller is incorporated into the design to address this issue. The developed controller
is stable and functions well thanks to the low steady-state error and reliable output [5].
The Quasi-2-Level (Q2L) modulation strategy is proposed for the control of the system
due to its effectiveness and superior current transfer capacity. Also, the proposed control
method provides to suppress the dv/dt on the HF transformer. The results show that the
Q2L modulation strategy achieves the capacitor voltage balance and lower electromagnetic
interference (EMI) [6].
A series resonant converter is proposed [8–18]. This converter topology is used to
meet the constant voltage-transfer requirement of the phase modular STs. The half-cycle
discontinuous conduction-mode operation principle is selected for the isolation stage of
the ST since this operation achieves zero current switching with high efficiency. However,
the operating strategy lacks power flow control. This operation mode provides to circulate
the MV side power fluctuations through the SRC to the common LV side [10]. A new
single-stage control method is used to simplify the system control. This converter has a
quasi-resonant frequency mode, because the constant voltage is obtained for the common
DC-side voltage thanks to this model [8]. The conventional series resonant converter is
reconfigured to provide voltage stability by using a voltage doubler rectifier. The proposed
converter structure achieves the post-fault operation and decreased additional power
devices [12]. A CLLC resonant converter is used for a next-generation DC data center. The
proposed converter provides ZVS operation of the primary power switches. To minimize
the power flow losses, synchronize rectifier control is used for output side power switches.
Also, the switching frequency and resonant frequency operate simultaneously to get better
conversion efficiency. The results show that the designed converter topology achieves 98%
peak efficiency and good power density [14]. The resonant soft-switching modulation is
applied for the control technique of the proposed dc–dc converter. The control method
increases the system’s efficiency because of its soft switch loss reduction capability [18].
A dual active bridge (DAB) converter is proposed for the MV dc–dc isolation stage
of the STs [19–47]. This converter has many superiorities such as zero-voltage switching
(ZVS), simple control, high power isolation, and bidirectional power flow. Yet, the parasitic
capacitances are especially high at high frequency and voltage levels. This case results in
the switching losses and slowing of dV/dt [19]. Contrary to conventional single-phase
shift control, the extended phase shift control method is applied to minimize power losses.
The proposed control strategy manages to adjust the direction and magnitude of power
Sustainability 2023, 15, 32 3 of 27

flow. The backflow power is reduced by extended phase shift control [30]. The phase angle
control method is only applied for the DAB voltage control due to its simple and easy
implementation capability. Also, the HF transformer is chosen as a star-delta connection to
reduce the turn ratio. The smaller and closer turns of the HF transformer enable the ZVS
operation [40]. To reduce the harmonics and inrush current effects, the soft-shift modulation
control is offered for the isolation stage of the ST. The proposed control method provides
the voltage balance and regulates the dc-link capacitor voltages of the dc–dc converter [44].
A power electronic-based intelligent universal transformer is proposed for the utility
grid. The isolation stage of the proposed smart transformer is an LLC based converter to
provide soft-switching operation. To manage the voltage and power flow, a DSP based
control card is used. The proposed converter allows the low magnetizing current thanks to
their leakage inductances. The converter provides 98% maximum efficiency under light
load conditions [46]. A medium frequency isolated transformer structure is presented for a
500 kVA industrial prototype electronic power transformer. The isolation stage consists of
cascaded voltage source converters and medium-frequency transformers. The pulse width
modulation control, current and voltage inner loop control structure are used in a DSP to
control the unbalanced output power. The fault protection and monitoring operation are
also achieved thanks to DSP [47].
A Quadruple-Active-Bridge (QAB) dc–dc converter is examined for a modular ST
configuration [48–50]. This converter topology is selected for soft-switching capability and
reduction in the number of high-frequency transformers. Triangular current modulation is
used to control the converter. This control strategy uses the duty cycle principle to control
the power switches. It provides to reduce the circulating reactive power in the isolation
stage of ST [48]. This converter type is used to reduce the number of HF transformers to
provide more bridges for the connection of HF transformers. The triangular current-mode
modulation control is applied to achieve the soft-switching operation of the power switch.
The control strategy enables it to reduce the power switching losses and ease the ZCS
operation for heavy loads [49].
The literature review presents the STs used in smart power grids. The studies reveal
that dual active bridge converters, resonant converters, modular multilevel converters,
and quadruple active bridge converters are widely used in ST applications. The four
main dc–dc conversion structures in different applications and powers are presented with
their control strategies. Related to this study, ST models for solar PV power systems are
presented in References [51–56]. To reduce the dual-side backflow power and increase the
DAB converter’s effectiveness for a variety of voltage conversion situations, a single-stage
dc smart transformer based on a DAB converter is developed. Enhanced triple-phase shift
control and single-phase shift control methods are applied to the DAB converter to show
the effectiveness of the proposed enhanced triple-phase shift control. The efficiency of
the DAB converter-based ST model is measured at 96% thanks to the proposed control
algorithm [51]. For MV-level grid-tied applications, such as power converters for renewable
energy systems and electric trains, single-stage loosely coupled resonant DAB converter-
based ST is used. Loosely coupled coils are used in place of the HF transformers used in
traditional STs. The efficiencies of the proposed DAB converter and ST model are 97.4%
and 95.2% at 2.4 kW [52]. A forward dual-active-bridge-based ST topology is presented.
This topology decreases the number of active switches, and it is aimed at further reduction
in the converter volume. The proposed topology is demonstrated to be superior to other
unidirectional topologies in three areas: higher power density with commercially available
power modules, lower part count, and ease of control [53]. The reactive power distribution
method is proposed for a cascaded multilevel converters-based ST topology. Due to the
increasing reactive power in solar cells, it is anticipated that the efficiency of such power
cells will decline as the imbalance rises. This method involves allowing some power cells
to carry a higher AC for a given active power than they would under balanced conditions.
It lessens unbalanced power on the solar cells [54]. The simulation model and management
of a three-stage ST with a solar PV power plant are presented. For the conversion of various
Sustainability 2023, 15, 32 4 of 27

stages, a PWM rectifier, a DAB, and a three-phase voltage source control inverter are used.
Changes in solar irradiation, load demand, and irregularities in grid supply voltages have
all been found to affect ST. Under a variety of field disturbances, the performance of ST
has been deemed adequate [55]. A DAB converter-based ST model for solar PV systems
is created in MATLAB/Simpower, and a prototype is implemented in the laboratory. The
proposed ST can efficiently integrate solar PV modules into the grid/load, according to
simulation and experimental results, with the LCL filter demonstrating a good reduction of
undesirable harmonics. The ST with a dual active bridge increases efficiency and power
transfer based on the relative phase shift between each h-bridge switching [56]. In this
study, the cascade (AC–DC–DC–AC) soft-switching smart transformer model is used for
the first time for a real smart power grid. The proposed DAB converter-based ST model is
implemented in a real-time 12.5 kW solar PV power plant. Under partial shading conditions,
the output power obtained from solar PV panels is constantly changing. This causes large
power imbalances in the smart grid. Soft-switching features of the proposed ST will provide
low conduction loss, low electromagnetic interference (EMI) for the converters and high
efficiency, and balanced power for the end-users. With the proposed control method and
key driving strategies in converters, DAB converter and ST conversion efficiency is aimed
at over 98% and 96%, respectively. The proposed ST topology and easy-to-apply control
method are tested on a real solar PV power plant. The ability of the smart transformer to
control power against sudden power changes in solar PV plants has been verified with ZVS
gain, dv/dt ratio, and conversion efficiency. Also, the proposed system can be connected
to both ac and dc sources or loads thanks to bidirectional power flow. Additionally, the
proposed control algorithm of the ST is easy applicable and multifunctional to solve the
power sag and swell problems. Therefore, soft switching ST enables power control and
efficient use of electrical energy in a grid struggling with unwanted power flow from
interconnected grids such as grid-connected renewable power plants.
This paper is presented in 3 sections. All dc–dc converter structures used in different
ST applications are reviewed in the introduction section. In the literature review, the
converter topologies are evaluated in terms of switching characteristics, application areas,
frequency, isolation level, and ZVS and ZSC operations. Materials and methods consist
of soft switching medium voltage dc smart transformer topology, operation principle of
DAB converter, control schemes, and solar PV power plant, presented in Section 2. In
Section 3, a solar PV power plant-based ST dynamic model is created in MATLAB/Simulink
to show the performance analysis of the proposed photovoltaic integrated smart city power
distribution. The design and control application of the proposed ST model is examined in
detail. Finally, the inferences obtained from the simulation and experimental results, and
the proposed ST success are explained in the conclusion section.

2. Materials and Methods

2.1. Soft Switching Medium Voltage dc Smart Transformer Topology
The schematic of the proposed soft switching MVdc ST with a control strategy to
interface MVdc or low voltage ac (LVac) loads or sources with smart grids is shown in
Figure 1. The dc–dc converter module is based on the soft-switching smart transformer
topology with reduced conduction loss thanks to their voltage, current, and PWM control
strategy. The dc–dc converter module is formed by two reverse-blocking semiconductor
bridges, a high-frequency transformer to provide galvanic isolation, and auxiliary resonant
circuits to create ZVS conditions for all the major components.
The dc–dc conversion stage is the key part for three-level STs since the whole power
system efficiency generally depends on the dc–dc conversion efficiency. The high-frequency
isolated dc–dc converters are significant devices for future smart-grid applications due to
their acts as a DC transformers. However, the most challenging and remarkable stage is the
dc–dc conversion stage of the ST because it must meet some strict requirements to achieve
high conversion efficiency and galvanic isolation. These requirements are exemplified as a
fast-switching speed, ZVS, ZCS, zero load current, minimum hard turn-on loss, low di/dt
Sustainability 2023, 15, 32 5 of 27

feature, high-frequency isolation, high-rated power, soft switching, and power density.
Sustainability 2023, 15, 32 5 of 27
While providing some requirements, researchers used variable converter topologies in
different applications.

Figure 1. The proposed smart transformer topology for the smart city power grid.

The dc–dc conversion stage is the key part for three‐level STs since the whole power
system efficiency generally depends on the dc–dc conversion efficiency. The high‐
frequency isolated dc–dc converters are significant devices for future smart‐grid
applications due to their acts as a DC transformers. However, the most challenging and
remarkable stage is the dc–dc conversion stage of the ST because it must meet some strict
requirements to achieve high conversion efficiency and galvanic isolation. These
requirements are exemplified as a fast‐switching speed, ZVS, ZCS, zero load current
minimum hard turn‐on loss, low di/dt feature, high‐frequency isolation, high‐rated
power, soft switching, and power density. While providing some requirements
researchers used variable converter topologies in different applications.
FigureThe isolated
1. The proposed dual
smartactive bridge
transformer converters
topology for theare
smartwidely used
city power in many applications to
grid.
achieve bidirectional power flow and ZVS operation. Thus, the switching losses are
The dc–dc
The isolated dual active
conversion stagebridge
is the converters
key forare
partefficiencywidely used
three‐level in many
STs since applications
the density.
whole power
reduced thanks to ZVS operation for high and high power This feature
to achieve
system bidirectional
efficiency power
generally flow and
depends onZVSthe operation.
dc–dc Thus, theefficiency.
conversion switchingThe losses are
high‐
allows the
reduced use to
thanks ofZVS
DABoperation
converters for in DC
high microgrid
efficiency and applications
high power and battery
density. This management
feature
frequency isolated dc–dc converters are significant devices for future smart‐grid
systems.
allows theHowever,
use of to
DAB these converters
converters are exposed to the high andcirculating current at higher
applications due their acts as a inDC DC microgrid
transformers. applications
However, the battery
most management
challenging and
loads and
systems.
remarkable
ZVS
However, fails
stage is these
at converters
lightconversion
the dc–dc
loads. areThe switching
exposed
stage of tothethe
losses
SThigh
occur
circulating
because
under
it mustcurrent
theatlight
meet some higherload. The
strict
requirements to achieve high conversion efficiency and galvanic isolation. These in the
harmonics
loads and arise
ZVS from
fails at the
light high
loads. dv/dt,
The light
switching load, and
losses electromagnetic
occur under the lightinterference
load. The
harmonics
requirements arise
HF transformer. arefrom the highHF
Therefore,
exemplified dv/dt, light load,
as transformer
a fast‐switching and
design electromagnetic
speed,andZVS,
the selection interference
ZCS, zero ofload
the powerin theswitches
current,
HF transformer.
play a significant Therefore, HF
role inloss, transformer
the conversion design and
efficiency the selection of
of the converter. the power switches
The high‐rated
isolated DAB dc–
minimum hard turn‐on low di/dt feature, high‐frequency isolation,
play a significant role in the conversion efficiency of the converter. The isolated DAB dc–dc
dc converter
power, for the STs
soft switching, andis power
given in FigureWhile
density. 2. Also, Table 1some
providing presents the comparative
requirements,
converter for the STs is given in Figure 2. Also, Table 1 presents the comparative analysis of
researchers
analysis of used variable
the dc–dc converter used
converters topologies
in STs.in different applications.
the dc–dc converters used in STs.
The isolated dual active bridge converters are widely used in many applications to
achieve bidirectional power flow and ZVS operation. Thus, the switching losses are
reduced thanks to ZVS operation for high efficiency and high power density. This feature
allows the use of DAB converters in DC microgrid applications and battery management
systems. However, these converters are exposed to the high circulating current at higher
loads and ZVS fails at light loads. The switching losses occur under the light load. The
harmonics arise from the high dv/dt, light load, and electromagnetic interference in the
HF transformer. Therefore, HF transformer design and the selection of the power switches
play a significant role in the conversion efficiency of the converter. The isolated DAB dc–
dc converter for the STs is given in Figure 2. Also, Table 1 presents the comparative
analysis of the dc–dc converters used in STs.

Figure2.2.Isolated
Figure Isolateddual
dualactive bridge
active converter
bridge topology
converter for smart
topology transformer.
for smart transformer.

The proposed dual active bridge (DAB) dc–dc converter consists of the HF transformer
and two h bridge bridges of medium voltage and low voltage. The DAB converter with
10 kVA power has a voltage reduction feature from 800 Vdc to 400 Vdc. The HF transformer
is used at a power of 10 kVA according to the switching frequency of 20 kHz. Phase-shifted
duty square waves, voltage, and current control techniques are used to control the power
flow. The proposed DAB converter uses two inductors. These are the magnetizing in-
ductance and the primary leakage inductance. While reducing current harmonics and

Figure 2. Isolated dual active bridge converter topology for smart transformer.
Sustainability 2023, 15, 32 6 of 27

preventing unexpected voltage spikes, and leakage inductance, the high-frequency trans-
former’s circulating current causes electromagnetic interferences, which are lessened by
the magnetizing inductance. As a result, the HF transformer and power switches’ power
losses are reduced, and the converter’s efficiency is raised. The DAB converter can be used
in smart grids because it can provide bidirectional power flow, and interface the LVdc grid
or the LVac grid. Also, it can achieve higher efficiency by providing ZVS control with the
best control strategy. Thus, the proposed dc–dc converter module can feature a single-stage
high-frequency link power conversion architecture, reduced dv/dt, full-range ZVS, and
robustness with benign fault modes such as short circuit current limiting from the dc-link
inductor, desaturation protection in gate drivers, and limit parasitic-inductance-induced
voltage spikes.

Table 1. The comparative performance analysis of the dc–dc converters used for STs.

dv/dt at Bidirectional Multiple DC

Resonant Circuit Circulating Soft-Switching
Converter Turn-on of the Power Flow Cable
Connection Current Range
Main Switch Speed Connections
MC <10 kv/us Resonant Tank >1 us High >100 ns, <600 ns practicable
>10 kv/us, Series Resonant
SRC <1 us Medium >100 ns, <100 us impracticable
<100 kv/us Tank
DAB <10 kv/us LLC Resonant Tank <1 us Low >100 ns, <100 us practicable
QAB <10 kv/us No >1 us High >100 ns, <600 ns practicable

2.2. Operation Principle

Isolated bidirectional DAB converters consist of two full bridge circuits and an HF
transformer as shown in Figure 1. The leakage inductance of the high-frequency transformer
is presented. The voltage gain conversion ratio of DAB is:

aC MV
d= (1)
CLV

fs is given as the switching frequency, ϕ is a phase shift between the two bridge circuits,
Lsst is the leakage inductance of the MV side bridge circuit, and Ths is half of the switching
period. The transmission power given between two h-bridge circuits is:

aC MV CLV ϕ
PT = ϕ (1 − ) (2)
ws Lsst π

Also, leakage inductance currents of two bridge circuits are given as Isst1 and Isst2 .

Ths C MV
Isst1 = − (2dϕ + π (1 − d)) (3)
2πLsst

Ths C MV
Isst2 = (2ϕ + π (d − 1)) (4)
2πLsst
These currents determine the switch-on losses of the circuits. To achieve the zero-
voltage switching of two bridges, the states are Isst1 < 0 and Isst2 > 0, respectively. In this
situation, the body diode of the switch that needs to be turned on receives the current from
the leakage inductance. Additionally, the DAB converter ensures deadband to protect the
dependability of the high voltage and high power converters. In order to determine how
much of the leakage inductance’s current is necessary for the ZVS turn-on operation at the
start of the deadband, it is critical to study the converter at this point. Figure 3 shows the
key waveforms of the DAB during deadband ([t0 t2]). Figure 4 shows the equivalent circuits
of the DAB converter, which include the current path in various modes during deadband.
 determine how much of the leakage inductance’s current is necessary for t
operation at the start of the deadband, it is critical to study the convert
Figure 3 shows the key waveforms of the DAB during deadband ([t0 t2]).
the equivalent circuits of the DAB converter, which include the current
Sustainability 2023, 15, 32 7 of 27
modes during deadband.

Figure 3. The voltage of the ZVS capacitor of C2 under deadband, the leakage inductance current Isst ,
Figure 3. The
and the PWM voltage
gate signals ofof theswitches.
power ZVS capacitor of C2 under deadband, the leakage in
Isst, and the PWM gate signals of power switches.
2.3. Control Scheme
The control strategies of the converters have a direct effect on the performance of the
STs. The variable control systems are applied for converters in the literature survey. The
voltage and current control method is used to provide voltage balance and output current
sharing in the ST. The voltage and current balance are significant operations to protect the
power switches from burnout. The dc voltage balance prevents the early failure of some
IGBT devices. Similarly, the current sharing control is an effective control strategy to reduce
the switching losses and heat stress on the power switches. Thus, the voltage and current
control strategy enable the avoidance of risks of overcurrent and overvoltage in the power
electronic system.
Frequency control is generally used in the series resonant converter-based STs. Due
to keeping the switching frequency close to the resonant frequency, power switches can
succeed in zero-voltage switching and zero-current switching. This case results in little
switching losses on the power switches. Also, the trapezoidal modulation can minimize the
turn-off losses of the power switches because the switch-off operation is achieved under the
ZVS. Discontinuous conduction mode operation is also used in series resonant converter-
(a)
based STs. This mode operates in the manner that the switching frequency is close to the
resonance frequency in the converter. This control method achieves the ZVS operation at the
MV side and the ZCS operation at the LV side. It reduces the electromagnetic interference
on the HF transformer due to its low di/dt capability. The Quasi-2-Level control strategy
enables the active power transfer capability. The Q2L control mechanism is based on the
capacitor voltage balance strategy. It protects the HF transformer from electromagnetic
interferences, reducing the dv/dt stress on the MF transformer.
 Sustainability 2023, 15, 32 Figure 3. The voltage of the ZVS capacitor of C2 under deadband, the leakage inductance current
8 of 27
Isst, and the PWM gate signals of power switches.

(a)

ustainability 2023, 15, 32 8 of 27

(b)

(c)
Figure 4. DAB converter
Figure 4. DABincluding
convertercurrent
includingpathcurrent
in different
pathmodes during
in different deadband:
modes (a)deadband:
during mode 0 ([t (a) mode 0
< t0]), (b) mode 1 ([t0(b)
([t < t0]), t1]), (c) mode
mode 1 ([t0 2t1]),
([t1(c)
t2]).
mode 2 ([t1 t2]).

2.3. Control Scheme

Duty cycle control has a significant role in the power conversion efficiency of the ST.
Considering the MV
The control strategies andconverters
of the LV side harmonic conditions,
have a direct effect the duty
on the cycles are adjusted
performance of the to enable
STs. The variable control systems are applied for converters in the literature survey. loss
a wide ZVS range, minimum circulating current flow, and power switching The reduction.
Triangular current modulation is a great solution to obtain the wide soft-switching
voltage and current control method is used to provide voltage balance and output current range
sharing inunder
the ST.the unstable
The voltageoutput capacitor
and current voltage
balance terms. Thisoperations
are significant control strategy manages
to protect the the duty
cycle to control and prevent the circulating reactive power. However,
power switches from burnout. The dc voltage balance prevents the early failure of some compared to the
conventional phase-shift modulation, the currents’ high root-mean-square
IGBT devices. Similarly, the current sharing control is an effective control strategy to (RMS) value
on the power switches and HF transformer lead to higher conduction
reduce the switching losses and heat stress on the power switches. Thus, the voltage and losses due to its
triangular shape. The D-Q control can provide ZVS operation of the
current control strategy enable the avoidance of risks of overcurrent and overvoltage in dc–dc converter by
adjusting the duty ratio. The LV side voltage of the converter is controlled while the D-axis
the power electronic system.
current operates freelance with the Q-axis current by arranging the duty ratio. This case
Frequency control is generally used in the series resonant converter‐based STs. Due
gains the fast response of the power switches and overcurrent protection.
to keeping the switching frequency close to the resonant frequency, power switches can
Phase shift modulation can control the power flow and cope with overcurrent prob-
succeed in zero‐voltage switching and zero‐current switching. This case results in little
lems. It allows the ZVS operation to turn on by changing the phase shift between the
switching losses on the power switches. Also, the trapezoidal modulation can minimize
bridges. Nevertheless, to avoid the sudden phase shift in the dc–dc converter, an additional
the turn‐off losses of the power switches because the switch‐off operation is achieved
under the ZVS. Discontinuous conduction mode operation is also used in series resonant
converter‐based STs. This mode operates in the manner that the switching frequency is
close to the resonance frequency in the converter. This control method achieves the ZVS
operation at the MV side and the ZCS operation at the LV side. It reduces the
Sustainability 2023, 15, 32 9 of 27

Sustainability 2023,
Sustainability 15,15,
2023, 32 32 method is proposed in such a quick phase‐shifted case. The increasing phase 9 ofshift
9 27
of 27 angle

is controlled with proportionality to the time step, so the LV side voltage of the dc–dc
converter is regulated. The commonly used control for all dc–dc converters is the pulse
method
control
width ismechanism
proposed strategy.
modulation inissuch a PWM
necessaryquickfor phase‐shifted
minimizing
signals case. Thetoincreasing
the peak
are applied current.
control the phase
The OLC
powershift angle is The
method
switches.
is controlled
proposed in with
such proportionality
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The so the
increasing
pulse width modulated signals operate with a 50% duty ratio. The two square wave LV side
phase voltage
shift angleof the
is dc–dc
controlled
converter is regulated. The
with proportionality to the commonly
time step, used
so thecontrol for all
LV side dc–dcof
voltage converters
the dc–dcisconverter
the pulse is
phase‐shifted signals control the turn‐on and turn‐off operation of the power switches, so
regulated.
width modulationThe commonly
strategy. PWM used control
signals for areall dc–dcto
applied converters
control the is power
the pulse width modu-
switches. The
thelation
power flowPWM
strategy.
is regulated. MV‐side and LV‐side capacitor voltages of width
the dc–dc
pulse width modulatedsignals signalsare applied
operate withto control
a 50% the duty power
ratio.switches.
The twoThe pulsewave
square
converters
phase‐shifted should
modulated signals control be adjusted
operatethe with according
a 50%and
turn‐on duty to the charging
ratio. operation
turn‐off The two squareand discharging
wave switches,
of the power time
phase-shifted soby using
the PWM
thesignals
power technique.
control
flow the Controlling
turn-on
is regulated. andMV‐side the operation
turn-off turn‐on
and and
of
LV‐side off times
thecapacitor
power of thesopower
switches,
voltages thethe
of switches
powerdc–dc flow and
input/output
is regulated.
converters voltages
shouldMV-side withLV-side
and
be adjusted PWM
according cantoreduce
capacitor thevoltagesthe circulating
charging ofand dc–dccurrent
thedischarging converters
timeand minimize
usingbe the
should
by
theadjusted
lossesPWM in STs.according Controlling
technique. to the charging and discharging
the turn‐on and off timestime by of using
the powerthe PWMswitchestechnique.
and
Controlling the turn-on and off times of the power switches
To summarize all the implications, phase shift modulation techniques arethethe most
input/output voltages with PWM can reduce the circulating and
current input/output
and minimize voltages
with
losses in
widely PWM
STs. can
used reduceof
in terms theease
circulating current and
of application and minimize
controlthe losses in STs.
capability. However, the phase
To summarize all the implications, phase shift modulation techniques arethethe most
To summarize all the implications, phase
shift modulation strategy is applied to control the switches, but thisare
shift modulation techniques control mostmethod is
widely
widely used used in terms of
in terms of ease ease of application
of application and control
and control capability.
capability. However,
However, the phase
the phase shift
suitable for
modulation
steady‐state
strategy
conditions.
is applied to control
Therefore, additional
the switches, but this
control algorithm methods are
shift modulation strategy is applied to control the switches, butcontrol method
this control is suitable
method is
used for the overcurrent,
for steady-state conditions. overvoltage,
Therefore, and soft‐switching
additional controlcontrol
algorithm operation.
methods areThese
used are the
suitable for steady‐state conditions. Therefore, additional algorithm methods arefor
extended phase
the overcurrent, shift control
overvoltage, method,
and soft-switching PI controller‐based
operation. These single phase
are theThese
extended shift control
used for the overcurrent, overvoltage, and soft‐switching operation. are phase
the
method,
extended and
shift control
phaseoutputshiftvoltage
method, control or currentPI
PI controller-based
method, control
single methods.
phase shift
controller‐based The additional
control
single method,
phase control
shiftand algorithms
output
control
are used
voltage for
or a grid‐connected
current control solar
methods. photovoltaic
The additional (PV)
control
method, and output voltage or current control methods. The additional control algorithms system.
algorithms Theyare achieve
used for athe current
grid-
and connected
voltage
are used solar
for abalance photovoltaic
controlsolar
grid‐connected (PV) system.
of converters. They
photovoltaicIn(PV) achieve the
thissystem. current
study,They and
the achieve voltage
PWM signals balance
the current of inverter
andcontrol
module of controlled
are
voltage converters.
balance control In using
by this
of study,
a DSP thecontrol
converters. PWM
In this signals
card and
study, of the
inverter
the PWMmodule signalsare
Matlab/Simulink of controlled
program. The
inverter
by usingare acontrolled
DSP control card aand
output
module voltage is measured
by using byDSPa the
LV25‐PMatlab/Simulink
control sensor
card andand program. The output
current
the Matlab/Simulink is measuredprogram. voltage
by aThe is
LA25‐NP
measured by a LV25-P sensor and current is measured by a LA25-NP sensor. These signals
output voltage
sensor. is measured
These signals are read by aandLV25‐P scaledsensor
withand thecurrent
Simulink is measured
blocks and by PWM
a LA25‐NP controls are
are read and scaled with the Simulink blocks and PWM controls are provided to obtain
sensor.
provided These signals
to obtain are read
reliable and scaled
output with
signals the Simulink blocks and PWM controls are cycle
reliable output signals as shown in Figure 5. as shown
Also, the duty in Figure 5. Also,
cycle control of thetheDAB duty is
provided
control to
of the obtain reliable output signals as shown in Figure 5. Also, the duty cycle
presented in DAB
Figureis6.presented in Figure 6.
control of the DAB is presented in Figure 6.

Figure 5. DSP PWM control model for inverter output voltage.

Figure 5. DSP PWM control model for inverter output voltage.
Figure 5. DSP PWM control model for inverter output voltage.

Figure
Figure 6. Duty
6. Duty cycle
cycle control
control of of DAB.
DAB.

Figure 6. Duty cycle control of DAB.

Sustainability 2023, 15, 32 10 of 27

To obtain voltage information from the single‐phase power system, a LEM LV 25‐P
Sustainability 2023, 15, 32 10 of 27
voltage sensor with a single‐phase error conversion ratio of 220/5 volts and a supply
voltage of +15 and −15 is connected. Voltage information is read from the output of the
voltage sensor and directed to the zero‐crossing detector. The circuit diagram of the
To obtain voltage information from the single-phase power system, a LEM LV 25-P
voltage sensor connected in parallel to the circuit is shown in Figure S1.
voltage sensor with a single-phase error conversion ratio of 220/5 volts and a supply
Rm measures the resistance, while Ri represents the major input resistance. The
voltage of +15 and −15 is connected. Voltage information is read from the output of the
observed power supply voltage for this study was a single‐phase 220 V system operating
voltage sensor and directed to the zero-crossing detector. The circuit diagram of the voltage
at a 50
sensor Hz line
connected frequency.
in parallel to theThe analog‐to‐digital
circuit is shown in Figure (ADC)
S1. range used by the DSP
TMS320F28335 must match the output waveform
Rm measures the resistance, while Ri represents the major input specified by this converter.
resistance. The The
conversion
observed ratiosupply
power for this LV 25‐P
voltage voltage
for this studyconverter is 2500:1000,
was a single-phase 220 Vaccording to the voltage
system operating at
converter’s data sheet [57]. Reducing the input voltage level
a 50 Hz line frequency. The analog-to-digital (ADC) range used by the DSP TMS320F28335from 220 V to 5 V is the
primary
must match goal.
the By applying
output waveform Ohm’s Law and
specified using
by this the primary
converter. rated RMS
The conversion current
ratio for thisof 10
mA,
LV resistors
25-P voltage are determined.
converter The secondary
is 2500:1000, according toside of the voltage
the voltage converter’sconverter’s
data sheetdesign
[57]. is
Reducing
maintained. the input voltage level
The secondary from 220
output V to is
current 5 Vpulled
is the primary
at 25 mA goal.
whenBy applying
the inputOhm’s
current is
Law
10mA and using thetoprimary
according rated RMS
the conversion current
ratio in theofdatasheet,
10 mA, resistors
which are determined.
is 2500:1000. As The
a result,
secondary side of the voltage converter’s design is maintained.
using Ohm’s Law, Rm is the measurement resistance and Ri is equal to 22 k. As The secondary output
a result,
current is pulled
the system at 25level,
voltage mA when which theininput
the current
circuit is 10mA according
diagram is 220 V,toisthe conversion
changed to 5 ratio
V at AC
in the datasheet,
RMS value. which is 2500:1000. As a result, using Ohm’s Law, Rm is the measurement
resistance and Ri is equal to 22 k. As a result, the system voltage level, which in the circuit
According to the design requirements, the current sensor must transform an input
diagram is 220 V, is changed to 5 V at AC RMS value.
current into a proportionate voltage value. In this investigation, the current transducer LA
According to the design requirements, the current sensor must transform an input
25‐NP is used. Figure S2 illustrates the preferred LA 25‐NP connection in detail. Voltage
current into a proportionate voltage value. In this investigation, the current transducer LA
and current
25-NP is used.transducers make upthe
Figure S2 illustrates thepreferred
single‐phase
LA 25-NPpower measurement
connection board.
in detail. Using a
Voltage
conversion
and circuit, the measured
current transducers make up the voltage values for
single-phase bothmeasurement
power transducers must board.beUsing
rescaled
a to
match the ADC voltage level in the specified DSP.
conversion circuit, the measured voltage values for both transducers must be rescaled to
matchBecause
the ADC the DSP accepts
voltage level inthe the 0–3 V input
specified DSP.data range, low‐level bipolar voltage signals
in the ±5 V range
Because the DSP areaccepts
now converted to thedata
the 0–3 V input 0–3range,
V range. The circuit
low-level bipolardetail forsignals
voltage the above
in the ±conversion
signal 5 V range are requirement
now converted is shown
to thein0–3
Figure S3. The circuit detail for the above
V range.
signal conversion requirement is shown in Figure S3.
2.4. Solar PV Power Plant
2.4. Solar PV Power Plant
Grid‐connected (on‐grid) solar systems are established to operate based on the
Grid-connected (on-grid) solar systems are established to operate based on the elec-
electricity produced by solar panels, where it is instantly consumed from storage, where
tricity produced by solar panels, where it is instantly consumed from storage, where it
it is produced, and the surplus is given to the grid. With the increase in the installation of
is produced, and the surplus is given to the grid. With the increase in the installation of
such solar PV systems, energy management and efficiency in power grids have become
such solar PV systems, energy management and efficiency in power grids have become
important. In
important. In this
thisstudy,
study,a a12.5
12.5kWp
kWp on‐grid
on-grid solar
solar PVPV power
power plant
plant waswas
usedused for smart
for smart
transformer input power. This solar PV power system is shown in Figure
transformer input power. This solar PV power system is shown in Figure 7. The solar 7. The solar PV
power
PV powerplant
plantconsists
consistsofof30
30units
units of
of 415
415 WWsolar
solarPVPVpanels
panels and
and 6 units
6 units of 2of
kW2 kW on‐grid
on-grid
inverter modules. The solar PV power plant is mounted on the roof to meet
inverter modules. The solar PV power plant is mounted on the roof to meet the needs of a ofthe needs
a house.
house. There
There are instantaneous
are instantaneous voltage according
voltage changes changes toaccording to variable
variable weather weather
conditions.
conditions. Experimental
Experimental studies of the studies of the
proposed proposed
smart smart transformer
transformer have been
have been carried out oncarried
this out
power
on thisplant to ensure
power the
plant to stability
ensure theofstability
the smart
of grid.
the smart grid.

Figure7.7.12.5
Figure 12.5kWp
kWpon-grid
on‐gridsolar PVPV
solar power plant.
power plant.
Sustainability 2023, 15, 32 11 of 27
Sustainability 2023, 15, 32 11 of 27

3. Dynamic and Experimental Model of Smart Transformer for Smart Grids

3. Dynamic and Experimental Model of Smart Transformer for Smart Grids
3.1.
3.1.Dynamic
DynamicModel Model
In
In this paper,ititisisdecided
this paper, decidedto toanalyze
analyzethe theimpedance
impedanceand andefficiency
efficiencycharacteristics
characteristicsof of
the
the bidirectional dc–dc dual active bridge (DAB) converter-based ST. To increasethe
bidirectional dc–dc dual active bridge (DAB) converter‐based ST. To increase thegrid
grid
reliability
reliabilityandandpower
powerstability
stabilityof
ofaasolar‐powered
solar-poweredsmart smartgrid,
grid,the
theperformance
performanceanalysis
analysisforfor
aasmart
smartgridgridisisproposed
proposedusing
usingaaDAB‐based
DAB-basedST. ST.Figure
Figure88presents
presentson‐grid
on-gridsolar
solarPVPVpower
power
plant‐based
plant-basedST. ST.The
TheDAB
DABconverter
converterisisnot notonly
onlyusedusedin inthe
theST STtotoenable
enablethetheDCDCmicro‐grid
micro-grid
but also serves as a power electronics interface for battery modules,
but also serves as a power electronics interface for battery modules, other sources, other sources, or loadsor
due to its bidirectional power flow capability, zero voltage switching
loads due to its bidirectional power flow capability, zero voltage switching (ZVS) ability, (ZVS) ability, and
galvanic isolation.
and galvanic Zero Voltage
isolation. Switching
Zero Voltage is one method
Switching that enables
is one method a returnatoreturn
that enables greaterto
switching frequencies
greater switching with increased
frequencies input voltage
with increased inputand voltage
voltage anddrop (ZVS).
voltage drop This method
(ZVS). This
uses
methodpulseuseswidth
pulse modulation (PWM)‐based
width modulation (PWM)-based operation,
operation,like like
almost allall
almost other
othermodern
modern
switching
switching voltage regulators,
regulators,butbutwith
with anan additional
additional independent
independent phase phase
to thetoPWMthe PWM
timing
timing to support
to support ZVS operation.
ZVS operation. ZVS offers ZVS offers
soft soft switching,
switching, preventing preventing
the switchingthe switching
losses that
losses thatexperienced
are often are often experienced during traditional
during traditional PWM operation PWM operation
and timing. andWhile
timing.ZVSWhile
runs
ZVS
on aruns on a constant
constant off‐time zero
off-time control, control, zeroswitching
current current switching
(ZCS) runs (ZCS)on runs on a constant
a constant on-time
on‐time
control. control. Whenvoltage
When output output control
voltageor control
powerorflow power flowiscontrol
control required,is required,
the DAB the DAB
converter
is more advantageous
converter because it because
is more advantageous works with activewith
it works control of the
active transferred
control of the power. Also,
transferred
under normal
power. load conditions,
Also, under normal load thisconditions,
isolated converter operates
this isolated at the resonant
converter operates point
at theto
resonant point to achieve zero voltage (ZVS) turn‐on at the primary side and zero currentat
achieve zero voltage (ZVS) turn-on at the primary side and zero current (ZCS) turn-off
the secondary
(ZCS) turn‐off at side.
the secondary side.

Figure
Figure8.8.The
Themodel
modelofofDAB
DABconverter‐based
converter-basedST
STfor
foraasmart
smartgrid.
grid.

Figure99shows
Figure showsthetheproposed
proposedsolar
solarPVPV power
power plant
plant model.
model. 12.5
12.5kWkW grid‐connected
grid-connected
solarPV
solar PVpower
powerplant
plantisismodeled
modeledto toprovide
providerenewable
renewablepower powerfor
foraa smart
smart grid.
grid. Because
Because
the average electricity consumption of a house is approximately
the average electricity consumption of a house is approximately 10 kW of power per 10 kW of power per
month, the ‘Perturb and Observe’ technique is used in the MPPT charge
month, the ‘Perturb and Observe’ technique is used in the MPPT charge controller. The controller. The
solarPV
solar PVpanel
panelvoltage
voltageand
andcurrent
currentarearemeasured,
measured,and andinstantaneous
instantaneouspower
powerisiscalculated
calculated
using these two values. Then, the output power change is observed
using these two values. Then, the output power change is observed by changing the solarby changing the
solar
PV PV operating
panel panel operating
voltage.voltage. If the power
If the output outputispower is increased,
increased, it is understood
it is understood that the
that the direction of change of the voltage is correct and by continuing
direction of change of the voltage is correct and by continuing in this direction, the in this direction,
the maximum
maximum powerpower
pointpoint is obtained
is obtained thanks
thanks to the
to the proposed
proposed method.
method. ThisThis method
method is
is proposed to improve power tracking accuracy and dynamic
proposed to improve power tracking accuracy and dynamic performance in rapidly performance in rapidly
changingenvironmental
changing environmentalconditions.
conditions.
In Figure S4, the solar PV panel output power and the load side output power are
calculated and compared using current and voltage values. Then, the voltages are com-
pared. Hereafter, the reference value (voltage) is increased or decreased by the specified
amount. When it reaches the maximum power point, it will constantly perturb and observe
operation at that point.
Sustainability 2023, 15, 32 12
12 of
of 27
27

The model
Figure 9. The model of
of a grid‐connected
grid-connected solar PV power plant for a solar‐powered
solar-powered smart grid.

The
In issue S4,
Figure thatthe
conventional
solar PV panel MPPT techniques
output powerare andtrying to solve
the load sideisoutput
how topower
automati-
are
cally determine the voltage or current at which a PV array
calculated and compared using current and voltage values. Then, the voltages arewill produce the most electricity
at a certain temperature
compared. Hereafter, the andreference
irradiance. By sampling
value (voltage)the is solar PV power
increased plant current
or decreased by and
the
voltage, the
specified MPPTWhen
amount. algorithm used in
it reaches thethe P&O technique
maximum power point,determines
it willthe solar PVperturb
constantly output
power
and and the
observe power change.
operation The calculation of the solar PV power plant output power
at that point.
change over successive cycles
The issue that conventional is the basis
MPPT fortechniques
this approach. areThe output
trying to power
solve value
is how of the
to
solar PV power plant during the current cycle (obtained by measuring
automatically determine the voltage or current at which a PV array will produce the most both the PV current
and voltage
electricity at values)
a certainistemperature
contrasted with and that of the cycle
irradiance. before. The
By sampling the algorithm
solar PV powerdetermines
plant
whether to raise or lower the reference current based on the results
current and voltage, the MPPT algorithm used in the P&O technique determines the solar of this comparison. If
the solar PV power plant output power is rising, the current
PV output power and the power change. The calculation of the solar PV power plantdirection of change will remain
unchanged from the prior direction. The direction of change for the current is reversed
output power change over successive cycles is the basis for this approach. The output
when the power difference between two consecutive cycles is negative. The output of
power value of the solar PV power plant during the current cycle (obtained by measuring
power oscillates around the peak power at a steady state because the current perturbation
both the PV current and voltage values) is contrasted with that of the cycle before. The
is fixed in each cycle.
algorithm determines whether to raise or lower the reference current based on the results
The main problem of renewable power plants is the occurrence of large imbalances in
of this comparison. If the solar PV power plant output power is rising, the current
the produced power and supplied power to the grid under constantly changing weather
direction of change will remain unchanged from the prior direction. The direction of
conditions [58,59]. Overcoming these problems occurring in the grid with the proposed
change for the current is reversed when the power difference between two consecutive
system extends the life of the devices used in smart homes and increases the profits of the
cycles is negative. The output of power oscillates around the peak power at a steady state
smart hosts. In this study, variable solar radiation amounts and variable temperatures are
because the current perturbation is fixed in each cycle.
applied to the solar PV panels. Thus, it is aimed to show the changes in power quality
The main problem of renewable power plants is the occurrence of large imbalances
affecting the system and the efficiency of the proposed solid-state transformer. Table 2
in the
presents produced
the parameterpowerofand supplied
the solar power
PV panel usedto in
thethegrid under constantly
simulation. changing
Figure 10 shows the
weather
power changes in solar PV plants under changing weather conditions. With 5with
conditions [58,59]. Overcoming these problems occurring in the grid the
parallel
proposed system in
string structures extends
the solar the PV
lifearray,
of thethedevices used voltage
produced in smartcan homes and increases
be constant. Since the
the
profits of the smart hosts. In this study, variable solar radiation amounts
decrease in the amount of solar radiation causes current losses in the solar cells, the change and variable
temperatures are applied
in power is observed to the solar
in proportion to PV
thepanels.
current.Thus, it is aimed to show the changes in
power The dc–dc bidirectional DAB convertersthe
quality affecting the system and efficiency
exhibit superior ofperformance
the proposed solid‐state
on power con-
transformer. Table 2 presents the parameter of the solar PV panel
version due to their low number of components and great controllability. The proposed used in the simulation.
Figure 10 shows topology
DAB converter the powerischanges
presented in solar PV plants
in Figure under
11. The changing
proposed weather operates
converter conditions.in
With 5 parallelThese
two manners. stringarestructures
forward and in the solar PV
backward powerarray,
flow.theThe
produced
power flow voltage can be
direction is
constant.
changed by Since thephase-shift
using decrease inmodulation.
the amount The of solar
powerradiation causesiscurrent
flow control achievedlosses in the
by phase-
solar
shiftedcells,
dutythe change
square in power
waves. Two is observedare
inductors in used
proportion to the current.
in the proposed DAB converter. These
 Table 2. The electrical characteristic of the solar PV panel used in the simulation.

Electrical Characteristics Value Electrical Characteristics Value

Sustainability 2023, 15, 32 Power Rating 415 W Power Density 192.13 W/m
13 of 227
Short Circuit Current (Isc) 6.21 A Power Tolerances 0%–+5%
Open Circuit Voltage (Voc) 85.6 V Peak Efficiency 19.66%
are primary leakage inductance (Lp) and magnetizing
Nominalinductance
Operating(Lm).
Cell While Lp prevents
Maximum System Voltage 600 V 45.8 °C
sudden voltage spikes and reduces current harmonics,
TemperatureLm reduces the electromagnetic
interferences caused by the circulating current in the high-frequency transformer. Thus, the
power losses on the power switches and HF transformer are decreased, and the efficiency
Sustainability 2023, 15, 32 13 of 27
of the converter is increased. The proposed dc–dc converter parameters are given in Table 3.
Two H-bridges are integrated with an HF transformer.

2. The
Table 2.
Table The electrical
electrical characteristic
characteristic of
of the
the solar
solar PV
PV panel
panel used
used in
in the
the simulation.
simulation.

Electrical Characteristics Electrical

Value CharacteristicsElectricalValue Electrical Characteristics
Characteristics Value Value
Power Rating Power
415 WRating 415 W
Power Density Power Density 192.13 W/m2 192.13 W/m2
Short Circuit Current (Isc )
Short Circuit
6.21 A
Current (Isc) 6.21 A
Power Tolerances
Power Tolerances
0%–+5%
0%–+5%
Open Circuit Voltage (Voc) 85.6 V Peak Efficiency 19.66%
Open Circuit Voltage (Voc ) 85.6 V Peak Efficiency 19.66%
Nominal Operating Cell
Maximum System Voltage Maximum
600 V System VoltageNominal 600 V
Operating Cell Temperature 45.8 ◦ C 45.8 °C
Temperature

Figure 10. Solar PV power plant: (a) solar irradiance, (b) PV array voltage, (c) generated power.

The dc–dc bidirectional DAB converters exhibit superior performance on power

conversion due to their low number of components and great controllability. The
proposed DAB converter topology is presented in Figure 11. The proposed converter
operates in two manners. These are forward and backward power flow. The power flow
direction is changed by using phase‐shift modulation. The power flow control is achieved
by phase‐shifted duty square waves. Two inductors are used in the proposed DAB
converter. These are primary leakage inductance (Lp) and magnetizing inductance (Lm).
While Lp prevents sudden voltage spikes and reduces current harmonics, Lm reduces the
electromagnetic interferences caused by the circulating current in the high‐frequency
transformer. Thus, the power losses on the power switches and HF transformer are
decreased, and the efficiency of the converter is increased. The proposed dc–dc converter
parameters
Figure arePV
10. Solar given in plant:
power Table(a)
3. solar
Two irradiance,
H‐bridges(b)
arePV
integrated with(c)angenerated
array voltage, HF transformer.
power.

The dc–dc bidirectional DAB converters exhibit superior performance on power

conversion due to their low number of components and great controllability. The
proposed DAB converter topology is presented in Figure 11. The proposed converter
operates in two manners. These are forward and backward power flow. The power flow
direction is changed by using phase‐shift modulation. The power flow control is achieved
by phase‐shifted duty square waves. Two inductors are used in the proposed DAB
converter. These are primary leakage inductance (Lp) and magnetizing inductance (Lm).
While Lp prevents sudden voltage spikes and reduces current harmonics, Lm reduces the
electromagnetic interferences caused by the circulating current in the high‐frequency
transformer. Thus, the power losses on the power switches and HF transformer are
decreased, and the efficiency of the converter is increased. The proposed dc–dc converter
parameters are given in Table 3. Two H‐bridges are integrated with an HF transformer.
Figure 11.
Figure 11. Bidirectional
Bidirectional dc–dc
dc–dc DAB
DAB converter
converter used
used for
forsolar
solarPV
PVpower
powerplant
plantintegrated
integratedST.
ST.

To reduce the power switch turn-on and turn-off losses and enable the ZVS operation,
1.2 kV IGBTs are used in the MV side of the converter. 600 V MOSFETS are used in the LV
levels under low power level conditions.
Sustainability 2023,
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32 14 of
14 of 27
27

Sustainability 2023, 15, 32 14 of 27

Table 3.
Table 3. The
The proposed
proposed DAB
DAB converter
converter parameters.
parameters.

Parameters
Table of
3. Theof
Parameters DAB Module
proposed
DAB Module
DAB converter Value Parameters of
parameters. Parameters
Value of HF
HF Transformer
Transformer Value
Value
Power
Power 12.5 kW
12.5 kW Input Capacitor
Input Capacitor 0.88 mF
0.88 mF
Parameters of DAB Module InputInput Voltage
Voltage
Value 750 V
Parameters
750 V of Output
HF Capacitor
Transformer
Output Capacitor Value 0.67 mF
0.67 mF
Power Output Voltage
Output Voltage
12.5 kW&& Current
Current 400 V,
400
Input V, 30 A
30 A Switching
Capacitor Switching Frequency
Frequency 0.88 mF 20 20 kHz
kHz
Input Voltage Parameters of
750of
Parameters HF Transformer
V HF Transformer Output Value
ValueCapacitor Parameters of HF Transformer Value
0.67 mF Value
Parameters of HF Transformer
Output Voltage & Current 400 V, 30 A Magnetization
Switching Frequency Resistance
20(Ω)
Magnetization Resistance (Ω)kHz
Magnetizing Inductance
Magnetizing Inductance (L
(Lmm)) 140 mH
140 mH 1.8 kΩ–3
1.8 kΩ–3 H
H
Parameters of HF Transformer Value Parameters of HF and
and Inductance
Transformer (H)
Inductance (H) Value
Primary Leakage
Primary Leakage Inductance
Inductance (L (LPP)) 2.2
2.2 mH
mH Turn ratio
Turn ratio (N)
(N) 2:1
2:1
Magnetization Resistance (Ω) and
Magnetizing Inductance (Lm ) 140 mH 1.8 kΩ–3 H
To reduce
reduce thethe power
power switch Inductance
switch (H) and turn‐off losses and enable the ZVS
turn‐on
To turn‐on and turn‐off losses and enable the ZVS
Primary Leakage Inductance (LPoperation,
) 2.2 mH
1.2 kV IGBTs are used in the MV side of the converter.2:1
Turn ratio (N) 600 V MOSFETS are
operation, 1.2 kV IGBTs are used in the MV side of the converter. 600 V MOSFETS are
used in
used in the
the LV
LV levels
levels under
under low
low power
power level
level conditions.
conditions.
The controlling operation is the key
The controlling operation is the key part of part of the
the DAB
DAB converter
converter design
converter design to
to gain
gain system
system
stability and higher efficiency of the system. To obtain the desired output voltage
stability and higher efficiency of the system. To obtain the desired output voltage from the from the
input voltage,
input voltage, PWM
PWM signals
signals are
signals are used
used inin the
the power
power switches.
power switches. As
switches. As shown
shown inin Figure
Figure 12,
12, the
the
output voltage
output voltage control
control is
is enabled
enabled by by using
using the
the voltage
voltage
voltage loop
loop control.
control.
control. The
The output
output voltage
voltage is
is
compared with the reference
compared with the reference voltage
reference voltage
voltage toto obtain
to obtain the error
obtain the error value. Thus, the difference between
error value. Thus, the difference between
the actual
the actual dc-link
dc‐link voltage
dc‐link voltage and
and reference
reference voltage
voltage undergoes
undergoes aa PI
PI control
control and
and then
then the
the limiter
limiter
to produce the required
to produce the required phase‐shift
required phase-shift angle and PWM signals.
phase‐shift angle and PWM signals. To achieveTo achieve regulated
achieve regulated output
regulated output
voltage,
voltage, the variable phase
phase angle
angle is
is applied
applied to
tothe
the LV‐side
LV-side PWM
PWM and
and MV‐side
MV-side
voltage, the variable phase angle is applied to the LV‐side PWM and MV‐side bridges. bridges.
bridges.

Figure 12.
Figure 12. Output
Output voltage
voltage control
control of
of the
the proposed
proposed DAB
proposed DAB converter.
DAB converter.
converter.

To verify
To
To verifythe
verify theperformance
the performanceofof
performance ofthe
thevoltage
the voltage
voltage control,
control, Figure
Figure
control, Figure 13
13 13
showsshows
shows the
thethe MV
MVMV and
andand LV‐
LV-side
LV‐
side capacitor
capacitor voltages.
voltages. Although
Although the the changing
changing power
power condition
condition is is realized
realized
side capacitor voltages. Although the changing power condition is realized in the solar in in
the the solar
solar PV
power
PV power
PV powerplant due due
plant
plant to the
due toweather
to the condition,
the weather
weather voltage
condition,
condition, stability
voltage
voltage is provided.
stability
stability Also,Also,
is provided.
is provided. an LCan
Also, filter
an LC
LC
is applied
filter is out
appliedof the
out PWM
of the rectifier
PWM to regulate
rectifier to the MV-side
regulate the capacitor
MV‐side voltage.
capacitor
filter is applied out of the PWM rectifier to regulate the MV‐side capacitor voltage. voltage.

13. MV
Figure 13.
Figure 13.
and LV
LV side
MV and LV side voltages
side voltages of
voltages of the
of the DAB
the DAB converter.
DAB converter.
converter.
Sustainability
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2023, 15, 32
15, 32 15
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27

The
The DAB
DABconverter
convertertopology
topologyis is favorable
favorable to reduce
to reducethe the
switching
switchinglosses by using
losses ZVS
by using
operation. Figure 14 presents the ZVS operation of the proposed
ZVS operation. Figure 14 presents the ZVS operation of the proposed converter. The converter. The selection
of the power
selection switches
of the powerand control
switches methods
and controlplays
methodsa keyplays
role to achieve
a key theachieve
role to ZVS operation.
the ZVS
The waveforms
operation. of the MV‐side
The waveforms h‐bridge
of the MV-side output voltage
h-bridge (Vs1)voltage
output and the(Vs1)
output andcurrent (Is1)
the output
demonstrate the success the
current (Is1) demonstrate of the ZVSofoperation.
success This isolated
the ZVS operation. converter
This isolated operates
converter at the
operates
resonant point topoint
at the resonant achieve zero voltage
to achieve (ZVS) turn‐on
zero voltage at the primary
(ZVS) turn-on side. When
at the primary side.theWhen
MV‐
side h‐bridge input voltage (VP1) and VS1 reverse polarity, the
the MV-side h-bridge input voltage (VP1) and VS1 reverse polarity, the resonant mode resonant mode between
the transformer
between leakage inductance
the transformer and the capacitors
leakage inductance (C1 and C2)
and the capacitors (C1in andparallel
C2) inwith the
parallel
switches occurs. For
with the switches example,
occurs. in Figurein11,
For example, switches
Figure S2 and S3
11, switches S2are
andfully turned
S3 are fully on and on
turned S1
and S4 are at
S1 and S4theareoff stateoffbefore
at the the period
state before equals equals
the period zero. At thisAt
zero. time,
thisS2
time,is turned off at
S2 is turned
off atvoltage
zero zero voltage level (ZVS)
level (ZVS) whilewhileS3 is S3
keptis kept
in theinonthestate.
on state. Resonance
Resonance occursoccurs between
between Lp,
Lp, and
C1, C1, C2.
andTheC2.switch
The switch capacitors
capacitors reset thereset the capacitive
capacitive energy fromenergy S2 from
to S1.S2 Thisto topology
S1. This
topology
is known is as known as a quasi-resonant
a quasi‐resonant ZVS because ZVSthe because
resonant theoperation
resonant only
operation
occursonly for occurs
a brief
for a briefofduration
duration of the switching
the switching period. The period. The load
load current current is being
is therefore therefore being conducted
conducted by diode
by diode
D1, D1, andvoltage
and switch switch voltage
S2 is Vin1. S2 isNow,
Vin1. the Now, the resonant
resonant inductor
inductor is charged
is charged by the
by the dc
dc voltage.
voltage. TheThe active
active switch
switch is guaranteed
is guaranteed to turn
to turn on ZVS
on ZVS sincesince the anti-parallel
the anti‐parallel diode diode is
is still
still conducting.
conducting. S1 isS1conducting
is conducting andandcancanbe be turned
turned offoff
bybythethecontrol
controllogic
logicto to begin
begin the
subsequent resonance
subsequent resonance operation
operation whenwhen the
the load
load current
current changes
changes to to positive.
positive.

ZVS operation
Figure 14. ZVS operation of the bidirectional dc–dc DAB converter.

The essential
The essentialelements
elementsofofa DC–AC
a DC–AC inverter are depicted
inverter in Figure
are depicted 15. The15.
in Figure DCThe
source’s
DC
ripples or frequency distortions are eliminated by the input filter. It gives
source’s ripples or frequency distortions are eliminated by the input filter. It gives the the inverter
circuit a circuit
inverter pure voltage. The primary
a pure voltage. power circuit
The primary powerofcircuit
the entire
of thesystem
entire issystem
the inverter.
is the
The desired multilayer PWM waveform is created by this circuit by converting
inverter. The desired multilayer PWM waveform is created by this circuit by converting the DC
voltage.
the The output
DC voltage. Thefilter helps
output to create
filter helps atosignal
createthat is almost
a signal thatsinusoidal
is almost by attenuating
sinusoidal by
the high-frequency
attenuating componentscomponents
the high‐frequency of the PWMof waveform.
the PWM waveform.
In this design, the inverter is controlled by the Sinusoidal Pulse Width Modulation
In this design, the inverter is controlled by the Sinusoidal Pulse Width Modulation
(SPWM) approach because it can directly regulate the output voltage and output frequency
(SPWM) approach because it can directly regulate the output voltage and output
in accordance with sine functions. Constant amplitude pulses with variable cycle periods
frequency in accordance with sine functions. Constant amplitude pulses with variable
within each period define the SPWM technology. In order to regulate the converter output
cycle periods within each period define the SPWM technology. In order to regulate the
voltage and lower harmonics, the width of these pulses is varied. Three sine waves and
converter output voltage and lower harmonics, the width of these pulses is varied. Three
high-frequency triangular carrier waves are used to create the PWM signal in the sinusoidal
sine waves and high‐frequency triangular carrier waves are used to create the PWM signal
pulse width modulation approach. The amplitude of the voltage at the converter’s output
in the sinusoidal pulse width modulation approach. The amplitude of the voltage at the
depends on the amplitude modulation rate in the sinusoidal pulse width modulation
converter’s output depends on the amplitude modulation rate in the sinusoidal pulse
approach. Overmodulation and distorted sinusoidal PWM output results from raising the
width modulation approach. Overmodulation and distorted sinusoidal PWM output
modulation ratio at the high output voltage closer to 1. The output voltage must have its
results from raising the modulation ratio at the high output voltage closer to 1. The output
frequency and amplitude adjusted by the inverter. Pulse width modulation provides this
voltage
adjustingmust have itsAn
mechanism. frequency
inverter’sand amplitude
output voltageadjusted
must have byitsthe inverter.and
amplitude Pulse width
frequency
modulation provides this adjusting mechanism. An inverter’s output voltage
modified. The output voltage should be as near to the sinus form as possible while these must have
are
its amplitude and frequency modified. The output voltage should be as near to the sinus
form as possible while these are set. The frequency of the control signal must match the
 cycle periods within each period define the SPWM technology. In order to regulate the
converter output voltage and lower harmonics, the width of these pulses is varied. Three
sine waves and high‐frequency triangular carrier waves are used to create the PWM signal
in the sinusoidal pulse width modulation approach. The amplitude of the voltage at the
converter’s output depends on the amplitude modulation rate in the sinusoidal pulse
Sustainability 2023, 15,
Sustainability 2023, 15, 32
32 16
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27
width modulation approach. Overmodulation and distorted sinusoidal PWM output
results from raising the modulation ratio at the high output voltage closer to 1. The output
voltage must have its frequency and amplitude adjusted by the inverter. Pulse width
set. The frequency
modulation
intended provides
frequencyof this
the
at control signal
adjusting
the output inmust
mechanism.match
order the intended
Anproduce
to inverter’s frequency
a output at the
voltage
sinusoidal output
must
voltage athave in
that
order
its to produce
amplitude
frequency. a sinusoidal
and frequency voltage at
modified. that
The frequency.
output voltage should be as near to the sinus
form as possible while these are set. The frequency of the control signal must match the
intended frequency at the output in order to produce a sinusoidal voltage at that
frequency.

Figure 15. PWM inverter for DC–AC stage of the ST.

15. PWM inverter for DC–AC stage of
Figure 15. of the
the ST.
ST.
The
The end‐user
end-user obtains
obtains pure
pure electricity for the home. The most important part of the
The
efficient useend‐user
of obtains
electricity is pure electricity
realized electricity
by the
for
for the
the home.
proposed home.
dc–dc
The
The most important
most structure.
converter
part
part of
importantIncreasing of the
the
efficient
efficient use
use of
of electricity
electricity is
is realized
realized by
by the
the proposed
proposed dc–dc
dc–dc converter
converter structure.
structure. Increasing
Increasing
the
the power quality and ensuring power control provides efficient electricity output. InIn
the power
ideal power quality
quality
conditions, it is
and
and
hard
ensuring
ensuring
to obtain
power
power
the
control
purecontrol provides
provides
electrical
efficient
efficient
waveforms in
electricity
aelectricity
renewable
output.
output.
energy‐ In
ideal
ideal conditions,
conditions, it
it is
is hard
hard to
to obtain
obtain the pure
the pure electrical
electrical waveforms
waveforms in in aa renewable
renewable energy‐
energy-
connected
connected electric grid,
grid,butbutthisthis case
can can be possible
thanksthanks to the proposed DAB
connected electric
converter‐based electric
ST ingrid,
a renewable
case
but this case
energy
be possible
can be possible
integrated grid
to the proposed
thanks
system as toshown DAB converter-
the proposed
in Figure DAB
16.
based ST in a renewable
converter‐based energy integrated
ST in a renewable grid systemgrid
energy integrated as shown
systeminasFigure
shown16. in Figure 16.

Figure16.
Figure 16. End-user
16.End‐user regulatedvoltage.
End‐userregulated voltage.
voltage.

3.2.
3.2. Experimental
Experimental Model
Model
The
The gate driver circuit
gate driver circuit is
is an
an integral
integral part
part of
of power
power electronics
electronics systems.
systems. Gate
Gate drivers
drivers
form an important interface between high-power electronics and control circuitry and are
form an important interface between high‐power electronics and control circuitry and are
used to drive power semiconductor devices. The output of DA–DA converters or SMPS
used to drive power semiconductor devices. The output of DA–DA converters or SMPS
mainly depends on the behavior of the gate driver circuits. Therefore, the design of the gate
mainly depends on the behavior of the gate driver circuits. Therefore, the design of the
driver circuit is critical in the design of power electronics converters. In this study, an HCPL
gate driver circuit is critical in the design of power electronics converters. In this study, an
3120 gate driver is used to drive the high-power IGBT. A GaAsP LED is included inside the
HCPL 3120 gate driver is used to drive the high‐power IGBT. A GaAsP LED is included
HCPL-3120 gate driver optocouplers. A power output stage-equipped integrated circuit is
inside the HCPL‐3120 gate driver optocouplers. A power output stage‐equipped
optically connected to the LED, ideal for providing driving power to MOSFETs and IGBTs
integrated circuit is optically connected to the LED, ideal for providing driving power to
for inverted motor control applications. The output stage’s high operating voltage range
MOSFETs and IGBTs for inverted motor control applications. The output stage’s high
supplies the drive voltages needed by gate-controlled devices.
operating voltage range supplies the drive voltages needed by gate‐controlled devices.
Sustainability 2023, 15, 32 17 of 27

Sustainability 2023, 15, 32 17 of 27

It is the best option for driving IGBTs rated up to 1200 V/100 A due to the voltage
and current offered. The HCPL‐3120 series can be utilized to drive a separate power stage
It is the best option for driving IGBTs rated up to 1200 V/100 A due to the voltage
driving the IGBT gate for higher‐rated IGBTs. It features a 630 V peak insulation voltage.
and current offered. The HCPL-3120 series can be utilized to drive a separate power stage
Figure S5 depicts the HCPL‐3120’s application circuit for driving the IGBT.
driving the IGBT gate for higher-rated IGBTs. It features a 630 V peak insulation voltage.
To
Figuredrive high‐power
S5 depicts Mosfets, aapplication
the HCPL-3120’s driver circuit was
circuit forcreated
drivingwith the IR2104 IC together
the IGBT.
with a 4N25 isolated
To drive optocoupler.
high-power Mosfets, aThe typical
driver connection
circuit was createdstructure of the IC
with the IR2104 Mosfet
togetherdriver
circuit
withisapresented in Figure
4N25 isolated S6.
optocoupler. The typical connection structure of the Mosfet driver
DAB isbidirectional
circuit DC/DC
presented in Figure S6.converter is a topology with the advantages of reduced
DAB bidirectional
device count, soft‐switched DC/DC converter islow
commutations, a topology
cost, andwith the efficiency.
high advantagesThe of reduced
use of this
device count, soft-switched commutations, low cost, and high
topology is recommended for applications where power density, cost, weight, efficiency. The use of this and
topology is recommended for applications where power density, cost, weight, and reliability
reliability are critical factors. The Phase Shift Modulation method, which is frequently
are critical factors. The Phase Shift Modulation method, which is frequently preferred in
preferred in the literature, has been applied to generate trigger signals in the DA–DA
the literature, has been applied to generate trigger signals in the DA–DA Converter control
Converter
algorithm.control algorithm.
The advantage of The
Phase advantage of Phase
Shift Modulation Shift
is that Modulation
it allows is that
high power it allows
transfer
highcapacity
power between
transfer converters
capacity between converters
and is suitable for useand
for is suitable
variable DCforlinkuse for variable
voltages. If the DC
linkDC
voltages. If the DC link voltages are constant and close to each other
link voltages are constant and close to each other in accordance with the transformer in accordance
withconversion
the transformer
ratio, it conversion ratio, itefficiency.
provides maximum provides The maximum
proposed efficiency. The proposed
dual-direction DAB
converter was
dual‐direction DABdesigned and produced
converter was designedas an H bridge.
and The designed
produced as an HDAB converter
bridge. card
The designed
design and the produced card are shown in Figure 17.
DAB converter card design and the produced card are shown in Figure 17.

(a)

(b)
Figure 17. 17.
Figure Designed
Designed primaryand
primary andsecondary
secondary circuits
circuits of
ofDAB
DABconverter: (a)(a)
converter: circuit diagram,
circuit (b) 2D
diagram, (b) 2D
cardcard
view.
view.

To drive the IGBT key elements at high speed, the driving process is carried out with
the additional design of the HCPL‐3120 IC for the PWM inverter. When IGBTs operate at
high current and voltage values, an isolated driver model is used to prevent the control
card from burning against reverse current flow. Gate signals with a 50% duty cycle are
 Sustainability 2023, 15, 32 18 of 27

To drive the IGBT key elements at high speed, the driving process is carried out with
Sustainability
Sustainability 2023,
2023, 15,15,
3232 the additional design of the HCPL-3120 IC for the PWM inverter. When IGBTs operate 1818
ofof 27
27at
high current and voltage values, an isolated driver model is used to prevent the control
card from burning against reverse current flow. Gate signals with a 50% duty cycle are
used for
usedfor
used each
foreach IGBT.
eachIGBT.
IGBT.The The control
Thecontrol card
controlcard driver
carddriver test
drivertest circuit
circuitisisiscarried
testcircuit carried out
carriedout on
outon the
onthe PCB
thePCB
PCBonon the
onthe
the
experimental table.
experimentaltable.
experimental It is designed
table.It Itisisdesigned
designedwithwith a PCB
witha aPCB card
PCBcard by designing
cardbybydesigning an isolated
designingananisolated gate
isolatedgate driver.
driver.InIn
gatedriver. In
Figure 18,
Figure18,
Figure the
18,the designed
thedesigned
designedgate gate driver
gatedriver model
drivermodel for
modelfor one
forone module
moduleisisisgiven.
onemodule given.
given.

(a)(a) (b)(b)
Figure
Figure
Figure 18.
18.
18. Isolated
Isolated
Isolated gate
gate
gate driver
driver
driver circuit
circuit for
circuit for
for one
one
one IGBT
IGBT
IGBT module:
module:
module:(a)(a)
(a) front
front
front view,
view,
view, (b)
(b)
(b) rear
rear
rear view.
view.
view.

Thegate
The
The gatesignals
gate signalsrequired
signals requiredtoto
required todrive
drivethe
drive theIGBTs
the IGBTsafter
IGBTs afterthe
after theisolated
the isolateddriver
isolated drivercircuit
driver circuitisis
circuit isasas
asinin
in
Figure19.
Figure
Figure 19.With
19. Withthe
With thecircuit
the circuitproposed
circuit proposedinin
proposed inFigure
Figure19,
Figure 19,the
19, thesignals
the signalsoperate
signals operateatat
operate ataaaswitching
switchingspeed
switching speed
speed
below600
below
below 600ns.
600 ns.
ns.

Figure
Figure
Figure 19.
19.
19. Isolated
Isolated
Isolated gate
gate
gate PWM
PWM
PWM inverter
inverter
inverter structure
structure
structure gate
gate
gate signals,
signals,
signals, and
and
anddcdc
dc input
input
input and
and
and ac
acac output
output
output signals.
signals.
signals.

Mosfetand
Mosfet
Mosfet andIGBT
and IGBTdrivers
IGBT driversneed
drivers needdc
need dcfeeds
dc feedsinin
feeds inconverter,
converter,converter,
converter, converter,and
converter, andinverter
and invertercard
inverter card
card
designs.
designs.
designs. Card
Card
Card designs
designs
designs are
are
are produced
produced
produced piece
piece
piece by by
by piece
piece
piece in
inin isolation
isolation
isolation so
soso that
that
that they
they
they are
are
are not
not
not affected
affected
affected
byelectromagnetic
by
by electromagneticinterference.
electromagnetic interference.With
interference. Withaaasingle
With single5V
single 55V supply,different
Vdcdcsupply,
supply, differentvoltage
different voltagedc
voltage dcsupplies
dc supplies
supplies
ofthe
ofof thedrivers
the driversare
drivers areprovided
are providedinin
provided inisolation
isolationon
isolation onall
on allcards.
all cards.Being
cards. Beingisolated
Being isolatedisisisimportant
isolated importantfor
important forthe
for the
the
earthing problem,
earthingproblem,
earthing harmonic
problem,harmonic fluctuations,
harmonicfluctuations,
fluctuations,and and short
andshort circuit
shortcircuit protections
protectionsinin
circuitprotections the
inthe cards.
thecards. These
cards.These
These
problems
problemsare
problems are
areeliminated
eliminated
eliminated with
with the
with themulti-output
the multi‐output
multi‐output isolated DC supply
isolated
isolated DCDCsupply design.
supply With the
design.
design. voltage
With
With thethe
that
voltage wethat
voltage get 5we
thatwe Vget
supply from
get5 5VVsupply
supply the control
from
from card, the
thecontrol
the control feeds
card,
card, the are
thefeedsgiven
feeds are to
aregiventhetoconverter
given tothe cards
theconverter
converter
with
cards a with
single
cardswith voltage
a asingle without
singlevoltage
voltage the need
without
without the
the for external
need
need for dc voltages.
forexternal
external The 5 V
dcdcvoltages.
voltages. voltage
The
The coming
5 5VVvoltage
voltage
comingfrom
coming fromthe thecontrol
controlcardcardisisfirst
firstpassed
passedthrough
throughinsulated
insulateddc–dcdc–dc(5(5V–5 V–5V)V)converters
converters
totoensure
ensureground
groundisolation.
isolation.InInFigure
FigureS7, S7,ananisolated
isolateddc–dc dc–dcdesign
designcircuit
circuitisispresented.
presented.
Theisolated
The isolatedDC/DC DC/DCconverter
converterisistypically
typicallyused usedinincost‐sensitive
cost‐sensitivegeneral‐purpose
general‐purpose
powerisolation
power isolationand andvoltage‐matching
voltage‐matchingapplications.
applications.Despite Despiteitsitslowlowcost,
cost,a afull‐featured
full‐featured
 Sustainability 2023, 15, 32 19 of 27

Sustainability 2023, 15, 32

from the control card is first passed through insulated dc–dc (5 V–5 V) converters to19ensure of 27
ground isolation. In Figure S7, an isolated dc–dc design circuit is presented.
The isolated DC/DC converter is typically used in cost-sensitive general-purpose
power isolation and voltage-matching applications. Despite its low cost, a full-featured
converter
converterwith with1 1kVDC
kVDCisolation
isolationandandananindustrial
industrialoperating
operatingtemperature
temperaturerange rangeofof−40
−40°C◦ C
toto
+85 °C◦ is used.
+85 C is used.
Adjustable
Adjustable supply
supplyvoltage
voltagebetween
between5V–50
5 V–50VV is is
provided
provided bybyXL6009
XL6009 IC. The
IC. TheXL6009
XL6009
regulator
regulator is a wide input range, current mode, and DC/DC converter that cangenerate
is a wide input range, current mode, and DC/DC converter that can generate
positive
positiveorornegative
negativeoutput
outputvoltages.
voltages.It It
can bebeconfigured
can configuredasasBoost,
Boost,flyback,
flyback,SEPIC,
SEPIC,oror
inverting converter. The XL6009 built‐in N‐channel power MOSFET,
inverting converter. The XL6009 built-in N-channel power MOSFET, fixed frequency fixed frequency
oscillator,
oscillator,andandcurrent
currentmode
modearchitecture
architectureensure
ensurestable
stableoperation
operationoverovera awide
widerange
rangeofof
supply
supply and
andoutput
outputvoltages.
voltages.TheTheapplication
application circuit
circuitis is
given ininFigure
given FigureS8.S8.The
Theisolated
isolateddcdc
card
cardfeeds
feedsdesign
designisisgiven
givenininFigure
FigureS9.
S9.
The
The1010kVAkVA380–380
380–380VVsoftsoftswitching
switching smart
smart transformer model
model is ispresented
presentedin inFigure
Figure20.
20. The model consists of a rectifier circuit, DAB converter, and PWM
The model consists of a rectifier circuit, DAB converter, and PWM inverter. Variable AC inverter. Variable
AC voltage
voltage producedbybya asolar
produced solarPV
PVpower
power plant
plant is converted
converted and andcontrolled
controlledby bythe
thesmart
smart
transformer
transformertotoenable
enableuser
uservoltage.
voltage.

Figure
Figure 20.20. Designed
Designed and
and produced
produced smart
smart transformer
transformer model
model for
for smart
smart power
power grids.
grids.

Power measurements, signal conditioning unit with amplifiers, and supply voltage
Power measurements, signal conditioning unit with amplifiers, and supply voltage
for converters and inverters are given in Figure 21. The main voltage is obtained from the
for converters and inverters are given in Figure 21. The main voltage is obtained from the
output power of the solar PV power plant. This voltage is reduced to 12 Vac by a 10 W,
output power of the solar PV power plant. This voltage is reduced to 12 Vac by a 10 W,
220/12 Vac power transformer. The 12 Vac voltage is converted to 5 Vdc with the diode
220/12 Vac power transformer. The 12 Vac voltage is converted to 5 Vdc with the diode
rectifier circuit. The 5 Vdc voltage is also converted to multi-output supply voltages with
rectifier circuit. The 5 Vdc voltage is also converted to multi‐output supply voltages with
isolated dc–dc converters. The 5 Vdc supply voltages are increased up to 50 Vdc with the
isolated dc–dc converters. The 5 Vdc supply voltages are increased up to 50 Vdc with the
help of a boost converter, providing supply voltages for converters and inverters. In the
help of a boost converter, providing supply voltages for converters and inverters. In the
measurement and signal conditioning circuit, the reference voltage (220 Vac) is converted
measurement and signal conditioning circuit, the reference voltage (220 Vac) is converted
to 5 Vac with the current/voltage measurement sensor. The 5 Vac obtained from the sensor
to 5 Vac with the current/voltage measurement sensor. The 5 Vac obtained from the sensor
is converted to the 3.3 Vac unipolar signal allowed by the DSP control card with the signal
is converted to the 3.3 Vac unipolar signal allowed by the DSP control card with the signal
scaling circuit. The current/voltage and PWM control in the DSP control card is provided
scaling circuit.
by control The via
blocks current/voltage and PWM control in the DSP control card is provided
MATLAB/Simulink.
by control
Withblocks viasupply
220 Vac MATLAB/Simulink.
voltage, the ST converted the ac–dc converter side to 450 Vdc
With 220 Vac supply
voltage. Based on the 450 voltage, the ST the
Vdc voltage, converted the ac–dc voltage
final conversion converterin side to 450 Vdc
the inverter part
voltage. Based on the 450 Vdc voltage, the final conversion voltage in the inverter
is 220 V according to the transformer conversion ratio. When the conversion efficiencies part is
220
of V according
the to the transformer
DAB converter and ST areconversion
calculated,ratio.
they When the conversion
are recorded as 98.6% efficiencies
and 96.7%ofat
the DAB converter and ST are calculated, they are recorded as 98.6% and
10 kVA. The input voltage obtained with the solar PV power plant passes through 96.7% at 10
thekVA.
smart
The input voltage
transformer. Here,obtained with
controlled the is
power solar
sentPV power
to the plantsmart
end-user passesgrid
through the smart
by controlling the
transformer. Here, controlled power is sent to the end‐user smart grid by controlling
variable input voltage. Figure 22 presents that the waveforms of the input and output the
variable
voltagesinput voltage.
are given Figure 22 presents that the waveforms of the input and output
sequentially.
voltages are given sequentially.
Sustainability 2023, 15, 32
Sustainability 2023, 15, 32 20 of 27

Figure21.21. Power
Figure Powermeasurement,
measurement,supply voltagevoltage
supply circuit, circuit,
and signal conditioning
and circuit for
signal conditioning circu
smart transformer.
transformer.
Soft switching means eliminating the on or off switching losses of one or more power
switchesSoft
in switching means Soft
a dc-to-dc converter. eliminating the on ormeans
switching capability off switching lossescircuit
that the proposed of one or m
has full range ZVS, which is better than conventional solutions with no ZVS
switches in a dc‐to‐dc converter. Soft switching capability means that the propo or only limited
range ZVS in DAB. Figure 23 shows the experimental results for the ZVS (zero switching)
has taken
gain full across
rangetheZVS, whichside
high voltage is better than
of the HFT, conventional
corresponding solutions
to a current of 10.4 with
A on no ZV
limited
the primary range
side. ZVS in DAB. Figure 23 shows the experimental results for the
The high-frequency
switching) gain taken oscillation
across theissuehigh
brought on byside
voltage the transformer’s
of the HFT,capacitances
corresponding t
and the dual active bridge (DAB) converter’s high dv/dt increases in severity with higher
of 10.4 A on the primary side.
switching speed. Theoretically, the zero crossing points of the DAB dv/dt excitation might
be positioned at the oscillation frequency to solve the high-frequency oscillation problem.
By connecting parallel capacitors to the power components, the DAB dv/dt is changed.
The transformer’s reduced oscillation loss and superior soft-switching conditions not only
solve the high-frequency oscillation issue, but also allow for an increase in efficiency. Short
rise times can cause high dv/dt, which can cause voltage spikes and ringing on the voltage
pulse’s leading edge. This ringing is a high-frequency EMI source, and if the peak voltage is
sufficiently high, the voltage spikes could harm the loads. The resonant capacitor controls
the dv/dt in the proposed converter, whereas the DAB only controls the dv/dt within its
ZVS range and the dv/dt of the inverter is uncontrolled. Devices made of MV SiC can
have uncontrolled dv/dt rates of up to 10–100 kV/s [60]. The dv/dt rate of 6.8 kV/us is
validated by experimental data using a 344 Vac 10.4 A load current. The dv/dt analysis is
shown in Figure 24.
The ST performance in the sudden sag and swell of the power produced from the
12.5 kW solar PV power plant is presented in Figure 25 with current voltage measurements
of the ST output. Considering the signal fluctuations, it is observed that the end-user output
(a)
voltage is kept constant, and no visible harmonics occur in the load current.
 Soft switching means eliminating the on or off switching losses of one or more power
switches in a dc‐to‐dc converter. Soft switching capability means that the proposed circuit
has full range ZVS, which is better than conventional solutions with no ZVS or only
limited range ZVS in DAB. Figure 23 shows the experimental results for the ZVS (zero
Sustainability 2023, 15, 32 21 of 27
switching) gain taken across the high voltage side of the HFT, corresponding to a current
of 10.4 A on the primary side.

(a)

Sustainability 2023, 15, 32 21 of 27

(b)

(c)

(d)
Figure 22.22.
Figure Sequence
Sequence display
displayof system power
of system poweroutputs: (a) (a)
outputs: AC–DC
AC–DC rectifier input
rectifier voltage
input andand
voltage
converted dc dc
converted voltage, (b)(b)
voltage, thethe
voltage converted
voltage to alternating
converted current
to alternating in the
current primary
in the primary part of the
part HFHF
of the
transformer, (c) (c)
transformer, ACACvoltage at the
voltage secondary
at the of of
secondary thethe
HFHFtransformer,
transformer,(d)(d)
inverter
inverteroutput voltage.
output voltage.
 (d)
Figure 22. Sequence display of system power outputs: (a) AC–DC rectifier input volta
Sustainability 2023, 15, 32 22 of 27
converted dc voltage, (b) the voltage converted to alternating current in the primary part of
transformer, (c) AC voltage at the secondary of the HF transformer, (d) inverter output volt

Sustainability 2023, 15, 32

The high‐frequency oscillation (a) issue brought on by the transformer’s c

and the dual active bridge (DAB) converter’s high dv/dt increases in severity
switching speed. Theoretically, the zero crossing points of the DAB dv/dt excit
be positioned at the oscillation frequency to solve the high‐frequency oscillatio
By connecting parallel capacitors to the power components, the DAB dv/dt
The transformer’s reduced oscillation loss and superior soft‐switching conditio
solve the high‐frequency oscillation issue, but also allow for an increase in
Short rise times can cause high dv/dt, which can cause voltage spikes and rin
voltage pulse’s leading edge. This ringing is a high‐frequency EMI source, and
voltage is sufficiently high, the voltage spikes could harm the loads. Th
capacitor controls the dv/dt in the proposed converter, whereas the DAB only
dv/dt within its ZVS range and the dv/dt of the inverter is uncontrolled. Devi
MV SiC can have uncontrolled(b) dv/dt rates of up to 10–100 kV/s [60]. The dv/d
kV/us isZVS
validated
gain: (a)
(a) HF
by experimental data using a 344 measurements
Vac 10.4 Atoload current
Figure
Figure 23. 23.
ZVS gain: HFtransformer primary
transformer side, (b)
primary Switch
side, (b)voltage verify
Switch voltage measurements to
analysis
the
the ZVS. ZVS. is shown in Figure 24.

Figure
Figure 24.24.
TheThe controlled
controlled dv/dtthe
dv/dt across across the reverse‐blocking
reverse-blocking switches. switches.

The ST performance in the sudden sag and swell of the power produce
12.5 kW solar PV power plant is presented in Figure 25 with curre
measurements of the ST output. Considering the signal fluctuations, it is ob
the end‐user output voltage is kept constant, and no visible harmonics occur
current.
 Figure 24. The controlled dv/dt across the reverse‐blocking switches.

The ST performance in the sudden sag and swell of the power produced from the
12.5 kW solar PV power plant is presented in Figure 25 with current voltage
measurements of the ST output. Considering the signal fluctuations, it is observed that
Sustainability 2023, 15, 32 23 of 27
the end‐user output voltage is kept constant, and no visible harmonics occur in the load
current.

Figure25.
Figure 25.Output
Outputpower
powerofofthe
theST
STunder
underthe
thesudden
suddensag
sagand
andswell
swellofofthe
theSolar
SolarPV
PVpower.
power.

4.4.Conclusions
Conclusions
The
Thedemand
demandfor forrenewable
renewable energy
energy isisincreasing
increasing day
day by
by day,
day,thus
thusthe
theincrease
increaseofof
renewable
renewableenergy
energyintegration
integrationneeds
needsimprovements
improvementson onthe
thecontrollability
controllabilityside.
side. STs
STsare
are
potential solutions to enable power flow control, reactive power compensation, harmonic
control, ac–dc–dc–ac conversion, and renewable integration. The isolation stage is the main
part of the ST due to the gain of higher efficiency and controllability to the system. With
a comprehensive literature survey, the dc–dc converter topologies used in different ST
applications are examined. However, the optimal dc–dc converter topology selection for
grid-connected solar PV power plants has not been discussed in the literature.
Under the rated load condition, the proposed DAB converter runs at the fixed resonant
frequency with a 50% duty cycle. To reduce the backflow power and switch on-off current,
the magnetizing inductance (Lm) can be increased. Yet, magnetizing inductance should be
small enough to achieve the ZVS operation. Also, extending the deadband period results in
a larger magnetizing inductance to meet the optimal ZVS requirement. This case can reduce
the backflow power, but it leads to a reduction in the effective time for energy transfer.
The balance between the magnetizing inductance and the dead time specifies the ZVS
operation performance of the DAB. The switching power losses are reduced by extending
the ZVS range and adopting a control strategy. The selection of the power switches and
HF transformer parameters is significant to reduce the circulating current. The 10 kVA
380/380 Vac ST experimentally tested in terms of reduced dv/dt, ZVS gain, and conversion
efficiency at variable voltages. The dv/dt rate of 6.8 kV/us is achieved by experimental
data using a 344 Vac 10.4 A load current. ZVS gain is observed at HF transformer primary
side and switch voltage measurements to verify the success. The conversion efficiency is
calculated at 96.7% at full load conditions. The switching loss is close to ideal when looking
at the ZVS gain in dynamic analysis. In the experimental analysis, switch losses at full
load are clearly visible. In the experimental study, the ST output power was observed as
9.66 kW at 750 Vac peak to peak voltage and 12.89 A current, and in dynamic analysis it
was 9.9 kW at 750 Vac voltage and 13.2 A current. In the experimental study and dynamic
analysis, the ST efficiency was 96.67% and 99%, respectively. In this experimental study,
it is seen that besides the switching losses, there are also transmission losses and HF
transformer losses. The proposed ST has potentially reduced EMI from controlled dv/dt
and full-range ZVS capability Also, the control system gives better performance on the dc
voltage stability and presents a safe operation. The proposed ST has potentially reduced
EMI from controlled dv/dt and full-range ZVS capability. With a useful control method,
voltage balancing between the converter modules under the steady-state and the dynamic
conditions is achieved.
Sustainability 2023, 15, 32 24 of 27

Supplementary Materials: The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/su15010032/s1.
Author Contributions: Conceptualization, B.E. and T.D.; methodology, B.E.; software, B.E.; valida-
tion, B.E. and T.D.; formal analysis, B.E. and T.D.; investigation, B.E. and T.D.; resources, T.D.; data
curation, B.E.; writing—original draft preparation, B.E. and T.D.; writing—review and editing, B.E.
and T.D.; visualization, B.E.; supervision, T.D.; project administration, T.D.; funding acquisition, T.D.
All authors have read and agreed to the published version of the manuscript.
Funding: This research was supported by the Scientific Project Unit of Adana Alparslan Türkeş
Science and Technology University (Project Number: 20803001) and the Scientific and Technological
Research Council of Turkey (TÜBİTAK Project Number: 2210319).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

EMI Electromagnetic interference SRC Series resonant converter

ST Smart transformer MV Medium voltage
OLC open-loop control LV Low voltage
ISOP Input series output parallel QAB Quadruple-Active-Bridge
ZVS Zero voltage switching PWM Pulse width modulation
ZCS Zero current switching RMS Root mean square
DAB Dual Active Bridge PV Photovoltaic
PR Proportional resonant controller ADC Analog to Digital
Q2L Quasi-2-Level MPPT Maximum power point
HF High frequency SPWM Sinusoidal pulse width modulation
HFT High frequency transformer P&O Perturb and observe

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