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A Unified Power Controller for Photovoltaic

Generators in Microgrid

Du Yan Su Jianhui Shi Yong

School of Electrical Engineering and Automation, Hefei University of Technology, Hefei, China

Photovoltaic System Research Center of Education Ministry, Hefei, China

 Abstract  —the non-dispatchable energy output of PV generators

has an adverse effect on the stability of microgrid especially

capacity-limited microgrid with high PV penetration. To solve

this problem, a unified power control strategy for PV generators

is proposed to mimic the performance of synchronous generator

(SG) in power system. As a supplementary supply, fuel cell is

equipped in the PV generator to reduce the DC voltage ripple

and supply a virtual electrical inertia as well. This proposed PV

generator is regulated by the conventional control algorithm for

SG so that it can contribute to the voltage and frequency

regulation in an islanded microgrid while it can correct power

factor when the microgrid connects to the utility. This control

strategy for PV generators has been verified by the simulation

study. The simulation results show that it has excellent

performance in tracking the maximum solar power, participating

voltage and frequency control, sharing the load and correcting

the power factor.

 Keywords-microgrid; photovoltaic; VSI; inertia;

I.  I NTRODUCTION

Compared to a conventional centralized power system,distributed generation (DG) has become an attractive option forthe large-scale renewable energy resource. However, the high

 penetration of DGs always raises concern of power systemstability. To cope with this problem, a concept of microgrids[1][2] is proposed to improve the quality and reliability of thedelivered power by integrating different distributed generation(DG) and energy storage systems. Therefore microgrids can beregarded as the promising DG configurations.

As intelligent mini power systems, microgrids can workeither grid-connected operation mode or islanded operationmode. For an islanded microgrid, voltage and frequency aresupported by multiple inverter-based DGs since there is novoltage reference unlike the utility in the grid-connectedoperation mode. To improve the stability of microgrids, micro-sources are expected to participate in load sharing.

Solar energy is one of the most popular renewable energyin practice. Therefore, the photovoltaic (PV) generator isregarded as one of the important renewable energy resources ina microgrid. However, the disadvantage of PV generators isthat the PV output power fluctuates depending on the weathercondition. Therefore the conventional grid-connected PVgenerator is not able to follow the load change to support thefrequency and voltage in an islanded microgrid becauseFurthermore, this intermittent energy output may jeopardize thesystem’s stability in the case of a capacity-limited micro gridwith high PV penetration.

To reduce this adverse effect, various solutions have beendiscussed by several literatures. In [3], a ramp rate control of a

 photovoltaic (PV) generator is proposed to obtain a desirablePV generators output. In [4], a control strategy for a PV-dieselsystem without energy storage is discussed. In [5], a neuro-fuzzy MPPT controller for PV generators is presented tooptimize the energy output of PV in a microgrid.

In conventional power system, each synchronous generator(SG) could response to the frequency deviation because of the

kinetic energy stored in rotor [6]. This dynamic response isdetermined by the inertia of generator. Besides that, SG couldget involved in frequency regulation by an embedded primaryfrequency controller, thus the frequency deviation can bereduced during the load fluctuation.

However, PV generators are supplied by the inertia-lesssolar cells, the output power has no direct relation between

 power and frequency [7]. Moreover, a typical current-controlled PV generator is driven by the strategy of maximum

 power point tracking (MPPT). Therefore, this non-dispatchable power doesn’t have any contribution to the frequencyregulation.

In this paper, a unified power controller based on

synchronous generator model is proposed for PV generators toshare the load and to regulate the frequency and voltage as well.A virtual inertia can be obtained by the implementation of theswing equation. A virtual primary frequency controller (VPFC)is to regulate load frequency and a virtual excitation controller(VEC) is to control either feeder voltage or reactive power.Therefore, both frequency and voltage stability can beimproved in an autonomous microgrid since PV generators can

 participate in the frequency control.

The paper starts with a brief description of this designed PVsystem, and then the proposed power control strategy isintroduced. This is followed by the implementation of the

 proposed controller. Finally, the simulation studies are

 presented to verify this proposed unified power controller.

II.  SYSTEM DESCRIPTION

A microgrid includes PV generators is depicted in Fig.1.The microgrid is connected to the point of common coupling(PCC). When the switch is off, the local AC load is feed by thisislanded microgrid. In this islanded microgrid, the voltage andfrequency are supported by the distributed control of the VSIwith energy storage and PV generator as well. The designedPV generator, shown as Fig.1, is PV-Fuel Cell hybridgeneration system.

This work is supported by 973 Project of China (2009CB219705), National Natural Science Foundation of China (50777015), (51077033)

978-1-4577-0365-2/11/$26.00 ©2011 IEEE 1121

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Figure 1. The configuration of microgrid with PV generators

On DC side, a boost chopper for PV array and a bi-directional converter for fuel cell are controlled coordinately tomaintain a stable DC-link voltage. Boost chopper is used toconnect PV array which can boost the lower voltage to a more

stable higher voltage for the DC/AC inverter. To release orabsorb the extra energy during load or PV fluctuation, fuel cellis added to the system through bi-directional DC/DC converter.

The topology of DC/AC converter used here is three-phasefull-bridge VSI. Unlike typical current source, PV generator isregulated as a voltage-controlled voltage source in order toemulate synchronous generator. This proposed controller isexpected to regulate this DC/AC converter as a virtualsynchronous generator.

III.  PROPOSED CONTROL STRATEGY FOR PV GENERATORS

The control strategy for PV generator can be split into two parts: DC bus control and AC power control. On the DC side,

PV panel and fuel cell operate complementary to obtain acertain level of DC voltage for the grid-tied converter. Withrespect to AC power flow control, PV generator is either powerfactor control (PPF) mode or feeder voltage control (FVC)mode depending on the microgrid is islanded or not.

This paper is focused on the possibility that PV generatorscan be regulated as SG in power system and conventionalmethodology for SG can be introduced to control PVgenerators in microgrid.

 A.   DC Bus Control

The prime movers for proposed PV generator are PV array

and fuel cell. To improve the energy efficiency of renewable, boost chopper for PV array works as a maximum power pointtracking (MPPT) controller.

Fuel cell is functioned as a supplementary energy supply incase of the power fluctuation of load or PV panel. As aninevitable component for this proposed PV generator, fuel cellsupports the virtual primary frequency control as the virtualkinetic energy in rotor. Fuel cell can also filter the DC voltageripple in the irradiance variation by absorbing or releasing theextra energy on DC bus.

Since the emphasis is putted on the AC power control, theconventional charge and discharge controller for fuel cell andthe perturbation and observation (P&O) MPPT method areapplied to DC bus control. The control diagrams for fuel cellare shown as Fig.2.

*

dcU 

dc

U 

 

Figure 2. Control diagram of bi-directional DC/DC converter for fuel cell

 B.   AC Power Control

To mimic the control strategy for the SG’s performance, thegrid-connected inverter is regulated according to the swing

equation and the voltage equations of SG given by (1)[8].

2

0( )

af 

m em e

d d d q q a d  

q

q q q d d a q

af f 

q d f 

 f f 

 P P  d T T J D

dt di

u L X i R idt 

diu E L X i R i

dt 

 M  M E  E i E 

 L s L

ω ω 

ω 

ω 

−   ∆⎧− = = + ∆⎪

⎪⎪

= − + −⎪⎪⎨

= − − −⎪⎪⎪⎪   = + +⎪⎩

  (1)

Therefore, the active power is determined by power anglefrom the swing equation while the reactive power is controlled

 by the virtual excitation voltage. The scheme for simulating SG

is shown in Fig3.

δ 

 f  E 

m P 

q E 

*

qV 

*

d V    PWM 

 

Figure 3. Scheme of DC/AC converter to simulate SG

Furthermore, PV generator is controlled as a versatilecomponent by integrating the control methodology borrowed

from SG. PV generator participates in primary frequencythrough a virtual primary frequency regulator (VPR). When themicrogrid works in grid-connected mode, virtual excitationregulator (VER) control the power factor by regulating thereactive power. On the other side, VER works in feeder voltagecontrol (FVC) by regulating the voltage magnitude of PCC.

IV.  CONTROLLER IMPLEMENTATION 

The controller for DC/AC inverter to mimic SG contains athree-level control loop. The outer loop includes virtual a

 primary frequency regulator (VPR) and a virtual excitation

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regulator (VER) to get an inner virtual EMF. Then an adaptedSG model is employed to get a reference of terminal voltage.Finally, a voltage and current controller is designed to achievezero steady error of this reference voltage. Thus, theconventional SG for power system can be applied to PVgenerator.

 A.   Inertia Response

Inertia response is one of the most important characteristicof SG’s operation which is governed by the swing equation. It

can The swing equation of SG is given by (2)： 

0 0m e

d d  P P J D J D

dt dt  

ω ω ω ω ω ω ω ω  

∆ ∆− = + ∆ ≈ + ∆   (2)

Here,  P m  and  P e  are the mechanical power and

electromagnetic power separately； J is the moment of the

inertia；D is the damping coefficient; in the small excursion ofangular speed, ω  can be substituted by the reference angularvelocityω0 of the grid.

According to the dq0 voltage equations of SG, the dq0

rotating frame can be derived from the swing equation of rotor.Therefore, the rotating angle θ of dq0 frame is defined by (3).

0

1 2

 P t 

k s k θ ω 

  ∆= +

+∫   (3)

Where, k 1= J ω0; k 2 = Dω0. 

 B.  Voltage Regulation

The topology of DC/AC converter used in this paper is athree-phase full-bridge VSI shown as Fig.4.The output voltageof LC filter can be regarded as the terminal voltage as long asthe dash part in Fig1 can be regulated as a SG according to the

model presented in Section III. Therefore the reference ofterminal voltage can be obtained from Eq.1.

caV 

cbV 

ccV 

 B

 A

C 

 

Figure 4. Topology of the DC/AC converter in the PV generator

However, the amplified high-frequency harmonics resultedfrom the differential term in the formula make the systemunstable during the large load step. Thanks to the constantsteady current in dq0 frame, the deviation term only worksduring the transient response. Therefore the differential term isomitted in terms of controller design since it has no impact onthe steady performance.

In order to achieve zero steady error of the referencevoltage, a dual-loop voltage and current controller is designed

here. In addition, the feed forwards of V od ，V oq are introduced

to improve the transient response of inner current loop. Thecontrol scheme for voltage regulation is depicted as Fig5.

Figure 5. Block diagram of the dual close-looped voltage regulator

C.   Primary Frequency Control

In power system, the speed governor on each generatingunit provides the primary speed control. Consequently, allgenerating units can contribute to the overall change regardlessof the location of load change. The primary frequency controlis based on a droop characteristic of the frequency deviation.The droop constant K  f  without  dimension is presented as (4):

f 

/

/

nom

nom

 P P  K 

ω ω 

∆=

∆ 

(4) 

Therefore, this power-frequency droop control for SG [9] isapplied to PV generators to share the load demand. Thediagram of virtual primary frequency regulator (VPFR) isshown as Fig.6.

Figure 6. Control diagram of the primary frequency controller(VPFR)

 D.  Virtual Excitation Voltage Control

The voltage drop is proportional to the reactive powerinjected into the network due to the inductance dominatedoutput impedance. Therefore, the regulation of excitationvoltage plays an important role in the control of the reactive

 power or terminal voltage. In the grid-connected mode, the PVgenerator can operate as a power conditioner by regulating the

 power factor at PCC. In the islanded mode, the primary voltagecontrol is achieved by a close loop control of terminal voltage.Fig.7 gives the control diagram of the virtual excitation

regulator (VER)

Figure 7. Control diagram of the virtual excitation voltage controller

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V.  SIMULTATION STUDY AND DISCUSSION 

To verify this proposed control strategy, the following

simulations have been accomplished.

 A.  Control of DC-link Voltage

As an auxiliary part, fuel cell in the PV generator maintainsthe DC-link voltage stable by absorbing or releasing the extraenergy flowing on the DC bus. Fig.8 gives DC voltage during

the fluctuation of irradiance. Fig.9 compares the DC voltage between fuel-cell-equipped and fuel-cell-less PV generators inthe condition of load fluctuation.

0.2 0.4 0.6 0.8 1 1.2 1.4700

750

800

850

900

950

1000

   U   d  c   (   V   )

 

With Fuel Cell

Without Fuel Cell

 

Figure 8. The response of Udc in irradiation fluctuation

0.5 1 1.5

800

850

900

950

1000

1050

1100

   U   d  c   (   V   )

 

With Fuel Cell

Without Fuel Cell

 

Figure 9. The response of Udc in load fluctuation

Those simulation results show the DC voltage has fewerripples than the conventional PV generator during thedisturbance. It also shows that fuel cell has the capability of

 balancing the extra energy as during the load fluctuation whichis the vital factor for the proposed ‘electrical inertia’.

 B.   Power Factor Control (PPF) Mode

In PPF  mode, the PV generator can supply reactive poweraccording to the reference of power factor. Therefore, it

contributes to the power factor correcting. Fig.10 and Fig.11depict the output for active power and reactive power in PPFmode separately. At 6s, the reference of power factor ischanged from 0.9 to 0.8 and the reference of active power isswitched from 2Kw to 3Kw at 10s. Those results illustrate thatthe proposed PV generator compensates the reactive powerrapidly in PPF  mode while delivers the maximum solar power. 

0 2 4 6 8 10 12 14 16 180

500

1000

1500

2000

2500

3000

3500

   A  c  t   i  v  e   P  o  w  e  r   (   W   )

 

Active Power

reference of Active Power

 

Figure 10. The response of active power in PPF mode

0 2 4 6 8 10 12 14 16 180

10

20

30

40

50

60

   R  e  a  c  t   i  v  e   P  o  w  e  r   (   W   )

 

Reactive Power

reference of Reactive Power

 

Figure 11. The response of reactive power in PPF mode

C.   Feeder Voltage Control (FVC) Mode

The active power in  FVC   mode is delivered by a power-frequency droop controller with primary frequency control.The difference is that the terminal voltage is regulated instead

of the power factor in an islanded microgrid. At 0.1s, a 1ΩAC

load parallels to the original 10Ωload.

Fig.12 shows that the proposed PV generator in FVC  modecompensates the feeder voltage drop compared with theconventional PV generators. Depicted as Fig.13, during theload fluctuation, the primary frequency controller can reducethe frequency deviation dramatically compared to the PVgenerator without primary frequency control. So the PVgenerator is involved in voltage and frequency regulation withthese two extraordinary characteristics and the stability of PVmicrogrid is improved as a result.

0.08 0.1 0.12 0.14 0.16 0.18 0.2210

212

214

216

218

220

222

224

   M  a  g  n   i  t  u

   d  e   (   V   )

 

With Feeder Voltage Control

Without Feeder Voltage Control

 

Figure 12. Feeder voltage with different controller in an islanded microgrid

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0 0.05 0.1 0.15 0.2 0.25 0.349.75

49.8

49.85

49.9

49.95

50

50.05

   F  r  e  q  u  e  n  c  y   (   H  z   )

 

Without Primary Frequency Control

With Primary Frequency Control

 

Figure 13. The frequency of an islanded microgrid with different controller

VI.  CONCLUSIONS 

In this paper, a novel PV generator with SG characteristic is presented. According to the mathematical model of SG, this proposed PV generator can be regulated as SG with virtualelectrical inertia. In addition, the control methodology of SG isintroduced so that PV generators can contribute the voltage andfrequency supporting in an islanded microgrid while correcting

the power factor in a grid-connected microgrid.

The feasibility and validity of this proposed control strategyhas been verified by the Matlab/Simulink-based simulation

 platform. The simulation results show this PV generator plays amulti-function role in a microgrid with this proposed operatingalgorithm including primary frequency control, primary

voltage control, power factor correcting and the maximumsolar energy exploitation.

R EFERENCES 

[1]  Kroposki, Benjamin, Lasseter, Robert, Ise, Toshifumi, Morozumi,Satoshi, Papathanassiou, Stavros, Hatziargyriou, Nikos, ‘Makingmicrogrids work’ , IEEE Power & Energy Magazine, vol. 6, pp 40-53,2008

[2] 

Lasseter, R.H., MicroGrids, vol.1 pp.305 August 2002. PowerEngineering Society Winter Meeting, IEEE

[3]   Naoto Kakimoto, Hiroyuki Satoh, Satoshi Takayama, Kouichi Nakamura ‘Ramp-Rate Control of Photovoltaic Generator WithElectric Double-Layer Capacitor’ IEEE Trans on Energy Conversion,Vol.24,No.2,June,2009 

[4]  A. Elmitwally, Mohamed Rashed ‘Flexible Operation Strategy for anIsolated PV-Diesel Microgrid Without Energy Storage’  IEEE Transon Energy Conversion, unpubished

[5]  Aymen Chaouachi, Rashad M. Kamel, Ken Nagasaka ‘microgridefficiency enhancement based on neuro-fuzzy mppt control for photovoltaic generator’, pp 2889-2894, Photovoltaic SpecialistConference 2010

[6]  Hadi Saadat, ‘Power System Analysis’, McGraw-Hill,1999

[7]  Johan Morren, Sjoerd W.H. de Haan, J.A.Ferreira ‘Contribution of

DG Units to Primary Frequency Control’ Future Power Systems, 2005International Conference, pp.6, 2005

[8]  P.M.Anderson,A.A.Fouad, ‘Power System Control and Stability’, TheLowa State University Press,1977

[9]  Hassan Bevrani, “Robust Power System Frequency Control,” Phil.Trans. Roy. Soc. London, Springer,2009

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