Proceedings of the World Congress on Engineering 2017 Vol I

WCE 2017, July 5-7, 2017, London, U.K.

A Study of Modelling and Inverter Controls for

AC Microgrid Simulation
Cheng-Yu Yu, Gary W. Chang, Member, IAENG, Yu-Jen Liu, Raymond Y. Chang, and Yee-Der Lee

 of the DER. When the microgrid is disconnected from the

Abstract—In this paper the simulation of an AC microgrid utility grid due to fault events, the MGCC will detect the
integrated with distributed energy resources (DERs) to increase islanding and command the specified DERs switch to
the renewable energy penetration and improve its operation Voltage/Frequency (i.e. V/F) control mode from P/Q control
reliability through different control strategies is presented.
Under normal operation, the microgrid is connected to the
mode to maintain the power balance between the supply and
utility grid through active/reactive power control for DERs and load demand. The MGCC performs functions such as power
energy storage units. It also includes Voltage/Var control for monitoring, operations management, and load shedding
the photovoltaic inverters to provide required reactive power. mechanism [2]-[5].
When the microgrid is disconnected from the utility grid due to In this paper, simulations of controlling the inverters of
external faults, the energy storage system will move to DERs and energy-storage units under different controls
voltage/frequency control to maintain the microgrid voltage
and frequency. Several scenarios including grid-connected and
models to enable the AC microgrid to robustly work for both
islanding modes with DER and energy storage unit controls are grid-connected and islanding modes are reported. An
simulated. Results show that proposed control strategies for the energy-storage battery in the microgrid adopts the P/Q
microgrid perform as expected to maintain a stable operation of control under the grid-connected mode and the P/Q control
the microgrid. will switch to V/F control to maintain power balance under
the islanding mode. The AC microgrid model under study is
Index Terms—Microgrid, photovoltaic, energy-storage built by Matlab/Simulink and the proposed inverter control
system, voltage/var control
strategies are verified through results of Matlab/Simulink
simulations. Results also show that the proposed control
I. INTRODUCTION methods are efficient while the microgrid is under stable
operation.
M icrogrids can manage and utilize the distributed energy
resources (DERs) effectively. The advantages of
distributed renewable generation include less emission
II. CONTROL METHODS OF DERS IN INER MICROGRID

of pollutants, high utilization of energy, less topographical A. Overview of INER Microgrid

restriction, and control flexibility. Microgrid concept assumes The one-line diagram of the microgrid under study is
a cluster of loads and microsources operating as a single depicted in Fig. 2, where microgrid of Institute of Nuclear
controllable system that provides both power and heat to its Energy Research (INER) is the first autonomous
local area. Users can design the microgrid to meet their needs hundred-kW 380-V microgrid in Taiwan with three-phase
of electric power [1]. The microgrid can be regarded as a four-wire configuration and information, communication and
controlled cell of the power system. Many distributed energy control systems of the microgrid have been established. The
resources are power electronic-based resources and static transfer switch (indicated by SS) is used to interconnect
controllable in the operation. These devices make energy between three zones, Zone 1, Zone 2 and Zone 3, of the INER
sources more flexible. Energy storage systems are also used in AC microgrid and the local electric utility. Zone 1 is in
the microgrid to enhance the reliability and supply the parallel with Zone 2 and Zone 2 is in series connection with
microgrid in islanding mode. Zone 3. The rms line voltage of distribution grid is 380 V and
The microgrid is normally connected to the utility grid the frequency is fS=60 Hz.
operated in grid-connected mode. In this mode, the microgrid Distributed energy resources in the microgrid are
control center (MGCC) commands every DER operated under photovoltaics, wind turbines, and microturbines. The energy
active and reactive power (i.e. P/Q) control to provide the storage system (ESS) can be used to regulate power flow and
most economical output in accordance with the characteristics power factor under grid-connected operation and stabilize the
system during the islanding operation or changing the
Manuscript received March 2, 2017. This work was supported in part by
Ministry of Science and Technology, Taiwan, under Grant MOST operation mode. The controllable load banks in Zone 1 and
106-3113-E-042A-001-CC2. Zone 2 consist of single-phase and three-phase loads. There
The authors are with the Department of Electrical Engineering, National is the only one single-phase load bank in Zone 3.
Chung Cheng University, Min-Hsiung, Chia-Yi 621, Taiwan. (phone:
+886-52729302; fax: +886-52720862; e-mail: garywkchang@gmail.com, B. Three-Phase Active and Reactive Control
azen0613@gmail.com, stu916466@gmail.com).
Raymond Y. Chang is with Institute of Nuclear Energy Research, The P/Q control is proposed to control each DER inverter
Tao-Yuan, 32546, Taiwan. (e-mail: raymond@iner.gov.tw, to output a preset or maximum power according to the DER
ydlee@iner.gov.tw). characteristics. In this mode the abc to dq0 reference frame

ISBN: 978-988-14047-4-9 WCE 2017

ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
Proceedings of the World Congress on Engineering 2017 Vol I
WCE 2017, July 5-7, 2017, London, U.K.

transformation is adopted to calculate the reference real and  2 Pref

reactive power output of the inverter, where the idref 
 3 vd
transformation matrices are given in (1) [6].  (3)
To help maintain the microgrid operated at the nominal i 2 Qref

voltage and frequency, the DER inverters in the microgrid are  qref 3 vd
controlled by three different methods when connecting to the
As illustrated in Fig. 2, after determining the reference
utility grid, as shown in Fig. 2. In Fig. 2, the voltage source
current signals of (3), they are compared with the real
inverter (VSI) of a DER is controlled by the PI controller to inverter output currents and then obtain the error signals, εi
determine the inverter output voltage and current and thus and εv. The error signals are applied to the PI controllers, and
maintain the voltage and frequency at the point of common their outputs are computed with vpcc,d, vpcc,q, iinv,dwL and
coupling (PCC) [7]. iinv,qwL terms to generate the reference voltages, and
12×5 kW 150 kW 65 kW
HCPV WT
2×5 kW 25 kW 2×2 kW
HCPV WT
[8-10]. Then, the switching signals are produced by
µT WT

inverter inverter
PWM and are input to the VSI. Figure 3 depicts the control
block diagram to determine inverter reference output
380 V 380 V
voltages in dq0 reference frame. The abc reference voltages
are then obtained by (1).
400 kVA 150 kVA

380 V 380 V
25 m 25 m 25 m
v pcc ,d

INER power to Zone idref

69 kV TPC 11.4 kV
system microgr
380 V
to Zone 2 and Zone
Zone 3
2 L L
30 kW 30 kW
Zone
L
30 kW
Pref *
vinv ,d
id NO 2 3
GCB ACB NC NC NO
B0 B1 ss B2 380 V
Cable U
SCC 1714.2 MVA 2.5 km
X/R 8.02 10 MVA 500 kVA
7.26% 3.85%
6×50 kVAr
NFB
NC
4%
150 kVA
4%
150 kVA
iinv ,d
NC
L
B3 B4 B5 380 V
100 kVA 208V
L2
DMSC µT 480V L1
: Digital power meter, protective relay, and magnetic contactor
: No-fuse breaker (NFB)
65 kW 30 kW 30 kW inverter 12×3.6 kVA v pcc ,d
NO : Normal open
NC : Normal close
HCPV iinv ,q
Zone 1 21×1.5 kW

Fig. 1. Structure of the INER microgrid including three zones.

Q ref iqref *
vinv ,q

vinv ,abc iinv ,abc v pcc ,abc v pcc ,q

Vdc Fig. 3. Control block diagram for determining inverter reference output
Rf Lf voltages.
Cf
C. Three-Phase Volt/Var Control
The Volt/Var control is implemented to supply the reactive
* v
vinv ,abc power output based on controlling the PV inverter voltage
magnitude [11]. In the Volt/Var control, the voltage control is
accomplished with a linear droop characteristic, which
Fig. 2. Control mechanism of the microgrid DER. determines the reactive power injection as a function of the
voltage magnitude at the PV inverter terminals [12]. Figure 4
 vd   va   id   ia  shows the functional blocks of the three-phase Volt/Var
       
 vq   [C ]   vb  ,  iq   [C ]   ib  (1) controller and the inverter Volt/Var droop characteristic.
  v    i  The Volt/Var control is to obtain the active power output
 v0   c  i0  c of PV to compute maximum reactive power of (4), which is
 2 2  to limit the output without exceeding the inverter apparent
cos(wt) cos(wt  ) cos(wt  ) power rating, S, and then determine the microgrid voltage
 3 3 
2 2 2  magnitude for the droop function to compute the reactive
where [C]  sin(wt) sin(wt  ) sin(wt  ) ,
3 3 3  power output. If the reactive power output is within the limit,
 1 1 1  the PV output is to maintain voltage magnitude that meets the
  regulation requirement.
 2 2 2 
and w=2πfS. Qmax  S 2  P 2 (54)
The P/Q control is mainly achieved by controlling the real The control is recomputed at every fundamental period
and reactive reference currents in d- and q-axis. Equation (2) based on the active power and the apparent power rating of
shows the power calculation in the dq0 reference frame. the inverter. In Fig. 4(a) Q(-) is zero within the deadband
Since vq is 0, the inverter output reference current is given by since the voltage is within the preset voltage range (e.g.
(3). between 0.97 pu and 1.03 pu). When the measured voltage
 Pref  2( vd id  v qiq ) / 3 exceeds Vb shown in Fig. 4(b), the inverter starts absorbing
 (2) reactive power and reduces the voltage gradually. At Vb and
Qref  2( v qid  vd iq ) / 3 above, the inverter is asked to absorb the maximum reactive

ISBN: 978-988-14047-4-9 WCE 2017

ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
Proceedings of the World Congress on Engineering 2017 Vol I
WCE 2017, July 5-7, 2017, London, U.K.

power within the limit. This is a commonly seen PV *

Vinv,mag Vmagitude
operating mode because of the voltage rise phenomenon
caused by PV system. Nevertheless, this controller can also
contribute to the mitigation of low-voltage situation such as P
the locations are remote from the substation under heavy load Vinv
conditions, when the voltage drops below Va shown in Fig. Q
*
Vangle
4(b). V phase
I phase
Droop Q(-) Q
Limiter PV
function Fig. 5. Single-phase inverter Volt/Var control.
V P Qmax
E. Voltage/Frequency Control
2 2
S P If a cluster of DERs is operated within a microgrid
Microgrid connected to the utility network, all the inverters can be
operated in P/Q mode because the microgrid has the voltage
and frequency references. However, to operate the microgrid
in islanding mode a voltage source inverter can be used to
provide a reference for frequency and to smoothly switch to
Utility
islanding operation without changing the control mode of
any other inverters.
(a) The inverter output frequency is a produced sine wave
signal inside the controller. That signal is used as a
directional reference vector, which is in the d-axis of the dq0
Qmax reference frame. In this reference frame, the value of d-axis
and q-axis output voltages are
0 Vdref = U0 , Vqref = 0 (6)
where both Vdref and Vqref are the reference of the d-axis and
Qmax q-axis rms voltages, respectively. When the microgrid
operates at nominal voltage, U0 = 1 pu.
Va Vref Vb To achieve the voltage control, it usually uses voltage
(b) and current double close-loop controls. Voltage loop adjusts
Fig. 4. (a) Block diagram of a PV inverter with Volt/Var controller, (b) the the magnitude of the output voltage. The output of voltage
droop function. loop serves as the current loop input reference. The output
current references of the voltage loop are given in (7).
D. Volt/Var Control of Single-Phase Inverter
The Volt/Var control is also used in the single-phase
  K idu 

i dref   K pdu  s  Vdref  Vd  (7)
inverter. For such inverters, a widely-used control scheme is   
  K iqu 
to model three-phase inverter through time delay, which is to i qref   K pdu 
  s 

 Vqref  Vq 
convert the 3-phase AC system to a 2-phase system using the 
Park transformation and then convert to a 2-phase dc system
[13]. Another control method is to use a different algorithm The output voltages of the current loop are given in (8).
to control the inverter. In the proposed control scheme, the
active power (P) and reactive power (Q) of the single-phase
 
Vid  Vd  wLi q   K pd 
K id 
s 
 i dref  i d  
inverter are shown in (5), respectively.  
 (8)
 K 
VV VV
P  c t sin   c t 
 Viq   wLi d   K pq  iq  iqref  i q
  s 
 
Xl Xl  
(5) Figures 6 and 7 depict the functional blocks of the double
Vt
Q Vc cos   Vt   Vt (Vc  Vt ) control loops.
Xl Xl
Vdref idref
where VC and Vt are the inverter rms output voltage VC and
the PCC voltage Vt, α is the phase angle difference of VC and
Vt, and Xl is the equivalent reactance of the transformer. Vd
Thus, the active power is controlled by regulating the Vq
phase angle difference α of VC and the reactive power is
controlled by the regulating of the VC and Vt. The simulation Vqref iqref
model is shown in Fig. 5, where the proposed decoupled
control algorithm can control the inverter’s active power and Fig. 6. Control block diagram of voltage loop.
reactive power independently and instantaneously. The fast
response of the control algorithm ensures that the PV system
can provide service under different system conditions, both
in steady and transient states and in grid-connecting and
islanding modes.

ISBN: 978-988-14047-4-9 WCE 2017

ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
Proceedings of the World Congress on Engineering 2017 Vol I
WCE 2017, July 5-7, 2017, London, U.K.

Vd

*
idref vinv ,d

f ref Vref
f0
iinv , d V0

iinv ,q
Pref P0 Qref Q0

Fig. 9. Droop control for frequency and voltage.

iqref *
vinv ,q

Fig. 7. Control block diagram of the current loop.

When the fault occurs at the utility grid side, the

microgrid will be disconnected from the grid. MGCC will
detect the current drop at PCC and commands the energy
storage system to operate at V/F control mode. When the
fault is clear, MGCC will decide when the microgrid is
reconnected to the grid based on the synchronization
conditions related to phase sequence, frequency, and voltage
Pa Pref Pb Qa
deviations. Qref Qb

Fig. 10. Frequency and voltage droop characteristics under V/F control.

III. REVIEW OF THREE INVERTER CONTROL STRATEGIES

Microgrid has many different control strategies for IV. CASE STUDY
different operation scenarios. In this paper, P/Q control, This paper applies Matlab/Simulink to develop the 380-V
droop control, and V/F control of inverters are used in and 60-Hz INER three-zone microgrid model, as shown in
grid-connecting and islanding modes. P/Q control is adopted Fig. 1. To simplify the modelling task and focus on assessing
in all DERs except the DER is under islanding control. The the effectiveness of the inverter controls of DERs and energy
P/Q control can generate the preset power (Pref and Qref) to storage systems, most DERs are modeled as constant
the grid and is independent of microgrid voltage and
dc-voltage sources. The Simulink model of Fig. 1 is depicted
frequency, as shown in Fig. 8.
in Fig. 11. The structure of the Zone 1 is illustrated in Fig. 12.
There are three DERs and two controllable loads in Zone 1.
Zone 2 includes a wind turbine and PV systems and Zone 3
includes a wind turbine and a microturbine. The microgrid is
connected to the utility grid by a static transfer system (SS).
Due to the space limit, only simulation parameters for Zone 1
are listed in Table 1.
The purpose of the study case is to simulate the INER
microgrid operated in grid-connected and islanding modes.
The initial total generation and load conditions at each zone
for the simulation are given in Table 2, where the DERs and
Pref Qref
loads are dispatchable and controllable. The simulation
Fig. 8. Frequency and voltage droop characteristics under P/Q control. scenarios are indicated in Table 3. At first, all three zones are
operated at the predetermined output power settings under
Droop control will change the output power which is
varied with the microgrid voltage and frequency [14]. If the the P/Q mode. At 0.6 s, the microgrid switches to islanding
microgrid voltage and frequency are dropped, the mode due to a fault occurred at the utility grid side. To meet
DERs/energy storage systems will increase the active and the load demand, the energy storage system in the Zone 1 is
reactive power output. Droop control can be used in two or switched to V/F control mode and regulates the power output
more DERs which are controllable to supply power to the to maintain the microgrid voltage and frequency. At 1.4 s, the
microgrid, as shown in Fig. 9. microturbine in Zone 1 increases its output power to 0.2 pu to
V/F control will change the power output of the energy take over the decreasing power output of the energy storage
storage system to maintain the microgrid voltage and system. At 1.6 s, the MGCC detects the fault is cleared, and
frequency at nominal values. Figure 10 illustrates the V/F
checks the differences of the voltage, frequency and phase
control characteristics of an inverter.
between the microgrid and the utility grid [15]. The
frequency and the rms voltage of the microgrid during the
simulation are shown in Fig. 13. The voltage and current
waveforms in three zones are also shown in Fig. 14. By
observing simulation results, the microgrid is well performed
and operated stably.

ISBN: 978-988-14047-4-9 WCE 2017

ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
Proceedings of the World Congress on Engineering 2017 Vol I
WCE 2017, July 5-7, 2017, London, U.K.

TABLE 3. SIMULATION SCENARIOS OF THE THREE-ZONE MICROGRID

Event Operation condition Time (s)

0 Three zones operated normally 0

A fault occurred at the utility grid side
2 0.6
Microgrid operated under islanding mode
3 Microturbine started output 0.2 pu active power 1.4
4 Microgrid back to grid-connected mode 1.6
Microgrid Voltage Magnitude
1.1

1.05

V ( pu )
1

0.95

0.9
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
time ( s )
Microgrid Frequcncy
60.4
Fig. 11. Simulink model of the INER microgrid.
60.2

f ( Hz )
60

59.8

59.6
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
time ( s )

Fig. 13. RMS voltage and frequency of the microgrid.

Fig. 12. Structure of Zone 1 of the microgrid.

Table 1. Simulation parameters for Zone 1

Parameters Values
Inverter switching frequency 10 kHz
Inverter filter inductance 2 mH (a)
Energy Inverter filter capacitance 68.5 μF
Storage
System DC-link voltage 700 V
Maximum active power 60 kW
Maximum reactive power 100 kVAr
Inverter switching frequency 10 kHz
Inverter filter inductance 2 mH
PV system Inverter filter capacitance 68.5 μF
DC-link voltage 700 V
Maximum active power 51.5 kW
Inverter switching frequency 10 kHz
Inverter filter inductance 2 mH
Microturbine Inverter filter capacitance 68.5 μF
(b)
DC-link Voltage 700 V
Maximum active power 31.5 kW
Kp_P/Q 5
Ki_P/Q 500
PI Controller
Kp_V/F 6
Ki_V/F 30

TABLE 2. THE INITIAL CONDITIONS IN ZONE 1 TO ZONE 3 SYSTEMS

Initial condition
Zone 1 40 kW (0.4 pu)
Load 1 30 kW (0.3 pu)
Zone 2 40 kW (0.4 pu)
Load 2 60 kW (0.6 pu) (c)
Zone 3 40 kW (0.4 pu) Fig. 14. Three-phase voltages, three-phase currents, active and reactive
Load 3 30 kW (0.3 pu) power output of (a) Zone 1, (b) Zone 2, and (c) Zone 3 for the study case.

ISBN: 978-988-14047-4-9 WCE 2017

ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)
Proceedings of the World Congress on Engineering 2017 Vol I
WCE 2017, July 5-7, 2017, London, U.K.

V. CONCLUSION
In this paper, the inverter controls that adopted for DERs
and energy-storage system in the microgrid have been
presented. The inverter control is implemented to deal with
the microgrid for normal operation under both
grid-connected and islanding modes. Under the
grid-connected mode, the DERs generate the predetermined
power according to the characteristics of the DER units. The
PV Volt/Var control is used to control the voltage magnitude
of the microgrid. The single-phase PV system is also
included in the system. When the islanding is detected, the
microgrid is disconnected from the utility grid, which is
commanded by the MGCC. At the same time, the energy
storage system switches to V/F mode from P/Q mode to
provide active power to critical loads and maintain the
voltage and frequency of the microgrid.
The performances of the described inverter controls are
verified through simulations using Matlab/Simulink. Results
show that the functions of the controllers adopted in the study
performs as expected. The microgrid can reliably operate
either under grid-connected or islanding mode.

REFERENCES
[1] N. Jayawarna, X. Wu, Y. Zhang, N. Jenkins, and M. Barnes, “Stability
of a MicroGrid,” Proc. 3rd IET International Conference on Power
Electronics, Machines and Drives, pp. 316-320, March 2006.
[2] J. A. P. Lopes, C. L. Moriera, and F. O. Resende, “Control strategies for
microgrids black start and islanded operation,” Int. J. Distrib. Energy
Res., vol. 1, no. 3, pp. 241-261, 2005.
[3] B. S. Hartono, Y. Budiyanto, R. Setiabudy, “Review of microgrid
technology,” Proc. International Conference on QiR (Quality in
Research), pp. 127-132, June 2013.
[4] M. Milosevic and G. Andersson, “Generation control in small isolated
power systems,” Proc. 37th North Amer. Power Symp., pp. 524-529,
2005.
[5] R. H. Lasseter et. al., “Integration of Distributed Energy Resources: The
CERTS MicroGrid Concept,” Lawrence Berkeley National Laboratory,
LBNL-50829, April 2002.
[6] M. G. Villalva, J. R. Gazoli, and E. R. Filho,” Comprehensive Approach
to Modeling and Simulation of Photovoltaic Arrays,” IEEE Trans. on
Power Electronics, Vol. 24, No. 5, May 2009.
[7] P. Piagi and R. H. Lasseter, “Autonomous control of microgrids,” Proc.
2009 IEEE PES General Meeting, Montreal, July 2006.
[8] C. Jin, P. C. Loh, P. Wang, Y. Mi, and F. Blaabjerg, “Autonomous
operation of hybrid ac-dc microgrids,” Proc. 2010 IEEE Int. Conf.
Sustainable Energy Technologies (ICSET), Sri Lanka, 2010.
[9] M. Saeedifard, M. Graovac, R. F. Dias, and R. Iravani, “DC power
systems: Challenges and opportunities,” Proc. 2010 IEEE PES General
Meeting, July 2010.
[10] S. R. Guda, C. Wang, and M. H. Nehrir,” A Simulink-based
microturbine model for distributed generation studies,” Proc. 37th Annu.
North Amer. Power Symp., pp. 269-274, 2005.
[11] P. Jahangiri and D. C. Aliprantis, “Distributed volt/var control by PV
inverters,” IEEE Trans. Power Syst., vol. 28, no. 3, pp. 3429-3439, Aug.
2013.
[12] Y. Xu, H. Li, D. T. Rizy, F. Li, and J. D. Kueck, “Instantaneous active
and nonactive power control of distributed energy resources with a
current limiter,” Proc. 2010 IEEE Energy Conversion Congr. Expo, pp.
3855-3861, 2010.
[13] V. Dixit, M. B. Patil, M. C. Chandorkar,” Real Time Simulation of
Power Electronic Systems on Multi-core Processors,” Proc. 2009
International Conference on Power Electronics and Drive Systems, pp.
1524-1529, Nov. 2009.
[14] G. W. Chang, G. F. Zeng, H. j. Su, and L. Y. Hsu. “Modelling and
simulation for INER AC microgrid control,” Proc. 2014 IEEE PES
General Meeting, July 2014.
[15] IEEE Standard for Interconnecting Distributed Resources to Electric
Power Systems, IEEE Standard 1547-2003.

ISBN: 978-988-14047-4-9 WCE 2017

ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)