Analysis of a Three-Phase Grid-

Connected PV Power System using a

Modified Dual-Stage Inverter
Ashokkumar S Mr. Karthik S
PG Student, PSG College of Technology Assistant Professor (Sr. Gr.), PSG College of Technology
ashokbharath1489@gmail.com svk.eee@psgtech.ac.in

Abstract- This paper proposes a control methodology for in- buying power from the system owner for a
terfacing medium and large photovoltaic (PV) arrays to the specified period of time. For instance, the
power system distribution grid. First, the structure of the PV government of Ontario, Canada, has started the
system in grid-connected mode is introduced. It consists of a PV Feed-in Tariff (FIT) program, enabled by the Green
array in addition to a power conditioning system for grid Energy and Green Economy Act passed in 2009
interfacing pur-poses. The power conditioning system is [3]. Ontario Power Authority (OPA) is the entity in
composed of a DC-DC boost converter, followed by a current
controlled Voltage Source Inverter (VSI). The system was
charge of implementing the FIT pro- gram. The
controlled to inject a clean sinu-soidal current into the grid. program provides investors with a steady income
Maximum power point tracking (MPPT) and PV cell modeling that justifies the high capital cost of PV systems.
will be discussed as well. After Under the FIT program, PV power is sold back to
that, simulation results of the system using MATLAB the power system grid following a predetermined
SIMULINK are presented. Finally, a case study was conducted price scheme. Roof mounted PV systems below 10
to examine the effect of the interfacing transformer topology on
kW receive a contract price of 80.2 ¢/kWh for a
the system during fault conditions.
contract period of 20 years. Ground mounted PV
Keywords: Control, Photovoltaic power systems, Pulse width systems with capacity between 10 kW and 10 MW
modulated inverters, Maximum Power Point Tracking. receive a contract price of 44.3 ¢/kWh for the same
I. INTRODUCTION period.
Dispersed locations is not new, the introduction of 16000

renewable energy sources and small capacity 14000

PV Power Installed (MW)

fossil fuel generators to the centralized power grid 12000

is a relatively new concept. Several reasons have

10000
caused the shift towards a more decentralized
power system. Those reasons include but are not 8000

limited to: electricity market privatization and the 6000

increased environmen- tal concerns regarding

greenhouse gases [1]. Photovoltaic ar- rays are 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

one of the DG sources that have been growing Year

steadi- ly in the last few years. This is primarily due
Fig 1
to its increasing power conversion efficiency and
the decreasing costs of instal- ling new PV
modules. A survey report published by the Inter- Fig. 1. Total installed PV power in IEA participating
national Energy Agency (IEA) has indicated that countries by year (in- cluding both grid connected and stand
approximate- ly 5.68 GW of PV power was alone PV arrays). Source: International Energy Agency.
installed in year 2008 [2]. About 75% of that
additional capacity was installed in Spain and With the increase in the number of grid connected
Germany alone. The growth trend of PV power PV sys- tems, comes the challenge of implementing
installations since 1992 is shown in figure 1 to get an effective power conversion system (PCS). The
an understanding of that increase. Grid connected DC output power from the ar- ray needs to be
PV systems form the majority of the installed PV converted into AC power for it to be injected into the
power capacity as compared to stand-alone sys- grid. IEEE Std. 929-2000 [4] requires current
tems. In order to encourage investments in PV injected by PV systems into the distribution network
systems, some governments provide contracts for to have a total harmonic current distortion below 5%
at rated inverter output. From that, the power
conversion system has to perform two important
1
functions: 1) extract maximum power output from the A simplified electrical circuit equivalent to
PV array and 2) inject an almost harmonic free photovoltaic cell is shown in Figure.
sinusoidal current into the grid.
This paper focuses on the control of the PCS and
presents a control methodology to address the Although simple, this region is equal enough to
previously mentioned is- sues. The PV cell represent different types of photovoltaic cells
equivalent circuit model is presented in sec- tion II to when considering the effects of temperature
be used in simulating the system. Section III de- [61]. From [62], it is confirmed that the cells of
scribes the structure of a grid connected PV system the polycrystalline substance considered.
and its associated control blocks. Control of the PCS
is divided into two parts: controlling the DC converter
to extract maximum power from the array while
boosting the terminal voltage, and controlling the VSI Items are separated because the data
to inject 3 phase sinusoidal currents into the grid. obtained from the polycrystalline module data
Simulation results obtained using SIMULINK are
presented in section IV. Section V discusses the are used in simulation lessons; however,
effect of the interfacing transformer topology on the relevant features controls, shown from the
propagation of zero sequence currents between the
model, are not the only ones used for this type of
PV system and the grid. Final- ly, a brief conclusion
follows in section VI. item. In its entirety, the equivalent circuit of
Figure 10 has two electrical resistors, Rs and
Rp [12]. According to [12], both resistors can be
PV CELL EQUIVALENT CIRCUIT MODEL
neglected. However, it has been shown that the
The equivalent circuit model of a PV cell is needed in order
to simulate its real behavior. One of the models proposed series resistor, Rs, has a major impact on the
in literature is the double exponential model [5] depicted tendency of the I-V element curve, be more
in fig- ure 2. Using the physics of p-n junctions, a PV cell can
be modeled as a DC current source in parallel with two precise in the middle of the size point of power
diodes that represent currents escaping due to diffusion operation and open circuit voltage.
and charge recombination mechanisms. Two resistances,
Rs and Rp, are included to model the contact resistances
and the internal PV cell resistance respectively.
I=𝐼𝑝ℎ-𝐼𝑟. [𝑒 𝑞. 𝑉+1𝑅𝑠
.𝐾.𝑇
𝑛 − 1] (1)
where V and I: electrical and current energy
throughout the cell; : Ph: photocurrent; 𝐼𝑟:
currently the cell cell opposite the cell phone;
Photovoltaic Array Modelling
charge: charging of electron; Resistance:
It was previously noted that the proposed MPPT is resistance to internal cell series; :pn: p-n
based on the behavior of the photovoltaic array combination thinking factor; constant: Boltzmann's
through heat and radiation variation. us, the permanent; Temperature: temperature.
mathematical model of PV cells is used in the form Photocurrent relies on the sun's rays as well
of a current-controlled voltage source, makes temperature, given by (2):

sense for two inputs parameters, i.e., temperature Iph =[Isc + 𝛼.(T-Tr)].Psun/1000
(∘ C) the sun's rays power (W / m2). where 𝐼𝐼sc: short-circuit current; temperature:
temperature Coefficient of current temporary power;
2
Temperature: reference temperature, this standard. the other type of hybrid system. In this scheme the
renewable energy source and the diesel generator
condition; Sun: level of irradiance. Standard standard supply a portion of the load demand directly, resulting
1000 W / m2 power Current reverse satellite saturation in higher overall system efficiency. The diesel
generator and the inverter can operate in stand alone
varies according to temperature. or parallel mode. This offers some combination of the
where 𝐼𝑟: completion of rewind reference cell; source for meeting the load. When the load is low,
either the diesel generator or the battery can supply
Semiconductor bandgap power applied to the cell . the load. However, during peak load both sources are
caese estimates can be found in [14]. Solution for (1) operated in parallel mode. Due to this parallel
operation the initial capacity of diesel generator and
takes a feature curve with only one photovoltaic cell. inverter can be reduced.
However, the model is such that, when connected to the
file PV array form, can be treated as a single cell with
multiple associations in series and similarities
. Thus, I a photovoltaic array, corresponding to two
interconnected connections strings, imitated. Each
thread contains ten modules, almost producing a
working power of 263 V. Earlier, it was discovered that a
4 kWp range was developed by KC200GT modules from
Kyocera. performance obtained with a PVarray model
attached to DC-DC converter.

Dual-Stage Inverter
It is desirable that the system is suitable for high power
consumption, more than 10 kW. Therefore, the two-
phase inverter is a three-phase configuration.

DC-DC stage
When a single-phase DC-DC converter is replaced by a
single-phase transformer, several advantages arise

(i) prompt response times;

(ii) low pressures on active devices;

(iii) filters the reduction;

(iv) reduction of high voltage

transformer.

MATLAB Design
DC -AC configuration has superior performance over
3
RESULT

In this scheme a controller is needed to supervise the

operation of the system, selecting the most
appropriate mode of operation to supply a certain load
without power interruption.

FLC boost up the load voltage to the desired value

of 220V in 0.58s, whereas the system without FLC
reaches the desired output voltage at 1.7s. From the
Fig. 11 it can be observed that the system with FLC
implemented MPPT control brings the load current to
the desired value of 14.1A in 0.58s, whereas the
system without FLC control reaches the desired
output current at 1.7s.
The current waveform of inverter shown with
suitable filter is found be sinusoidal in nature.

Conclusion

In this paper, a modified dual-stage inverter

applied to grid-connected photovoltaic systems
performed for high power applications has been
studied. The modified dual-stage inverter contains
DC-DC stage and DC-AC stage. Through the
Behavior Matching, the DC-DC stage operates with
constant frequency and duty cycle and the DC-AC
stage becomes responsible for the maximum power
point tracking and grid- current control. I–V
characteristic of the PV array was reproduced in the
output of the DC- DC stage, without any control, which
was defined as Behavior Matching. Some sensors
4
could be avoided because the grid-current control control techniques for three-phase voltage-
source PWM converters: a survey," Industrial
apparatus produces the variables needed for the
Elec- tronics, IEEE Transactions on, vol. 45, pp.
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