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2010 China Inteational Conference on Electricity Distribution

Modeling and Simulation of Grid-connected Hybrid

Photovoltaic/Battery Distributed Generation System

Fe Dg\ eg L\ Bb Hg\ Fe G\ Ceg Dg, Cegs WgKey Laboratory of Power System Simulation and Control of Ministry of Education,Tianjin University, Tianjin 300072, China

E-MAIL: [email protected]

Abstract: Photovoltaic (PV) generation is the technique

which uses photovoltaic cell to convert solar energy to

electric energy. Nowadays, PV generation is developing

increasingly fast as a renewable energy source. However,

the disadvantage is that PV generation is intermittent fordepending on weather conditions. Thus, the battery

energy storage is necessary to help get a stable and

reliable output from PV generation system for loads and

improve both steady and dynamic behaviors of the whole

generation system. The paper presents detailed transient

models of the grid-connected PVIBattery hybrid

generation system, and all these models are simulated by

using MATLAB/Simulink. PV array is rstly connected to

the common dc bus by a boost converter, where the

battery is also connected by a bi-directional DC/DCconverter, and then integrated into the ac utility grid by a

common DC/AC inverter. Maximum power point tracking

helps PV array to generate the maximum power to the

grid, and the battery energy storage can be charged and

discharge to balance the power between PV generation

and utility grid. Finally, different cases are simulated, and

the results have veried the validity of models and control

schemes.

Keywords: Photovoltaic Generation; Battery Energy

Storage; Hybrid Simulation; Maximum Power Point

Tracking; Modeling

1 Introduction

Nowadays, renewable energy has been more and more

attractive due to the severe environmental protection

regulations and the shortage of conventional energy sources.

Photovoltaic PV) generation is the technique which uses

photovoltaic cell to convert solar energy to electric energy.

Photovoltaic energy is assuming increasingly important as a

renewable energy source because of its distinctive advantages,

such as simple conguration, easy allocation, free of pollution,

low maintenance cost, etc. However, the disadvantage is that

photovoltaic generation is intermittent, depending upon

weather conditions. Thus, energy storage element is necessary

to help get stable and reliable power from PV system for

loads or utility grid, and thus improve both steady anddynamic behaviors of the whole generation system.

Because of its mature technology, low cost and high

eciency, battery energy storage system (BESS is used

widely in distribution generation technology. BESS can be

integrated into PV generation system to form a hybrid

PV/Battery generation system, which can be more stable and

reliable. An integral grid-connected PVBattery generation

system is composed of PV array, battery, power electronic

converters, lters, controllers, local loads and utility grid.

The paper discusses the detailed transient models of agrid-connected PVBattery hybrid generation system. PV

array is connected to the utility grid by a boost converter and

DC/AC inverter. Meanwhile, the battery is connected to the

common dc bus via a bi-directional DC/DC converter. The

proposed models of PV system, battery energy storage system

and control system are all implemented in Matlab/Simulink.

Three different cases are simulated for the hybrid PV/Battery

system, and all simulation results have veried the validity of

models and effectiveness of control methods.

2. PVarray

Photovoltaic cell is the most basic generation part in PV

system. Single-diode mathematic model is applicable to

simulate silicon photovoltaic cells, which consists of a

photocurrent source h' a nonlinear diode, internal resistances

R" and Rh as shown in gure 1.

I.-D v

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2 200 China Inteational Conference on Electricity Distribution

Figure . Single-diode mathematic model of a PV cell

The mathematic relationship for the current and voltage

in the single-diode equivalent circuit can be described as

(,)V IRI = I -I (e - 1

_ +sph S Rsh ()

where, h is photocurrent; is diode saturation current; q iscoulomb constant (.602e19; k is Boltzman's constant(.38e-23 JK; T is cell temperature K; A is P junctionideality factor; and sh are intrinsic series resistances.

Photocurrent is the function of solar radiation and cell

temperature described as

(2)where, is the real solar radiation (W m); r' Trl phristhe solar radiation, cell absolute temperature, photo current in

standard test conditions respectively; Cr is the temperaturecoecient A

Diode saturation current varies with the cell temperature

(3)where. is the diode saturation current in standard test

conditions Eg is the band-gap energy of the cellsemiconductor e , depending on the cell material.When PV cells are arranged together in series and

parallel to form arrays these cells are usually considered to

have the same characteristics. The equivalent circuit of PV

array can be described as gure 2.

Np+

Ns /Phe NpNs V

Np h

Figure 2. Single-diode mathematic model of a PV array

The relationship of the voltage and current in PV array is

-(1) IRI = I - I eAkT Ns Np _ l)_- (+S )P P e R S P 4)where, and are cell numbers of the series and parallelcells respectively.

Use MATLAB/Simulink to implement the model of PV

array, shown in gure 3. All parameters of the model use thedata in table .

Figure 3. The model of PV array in MATLAB/Simulink

LE I

PAAMETERS FOR PV MODELParameters Values

Referenced solar irradiance Sr 1000WlmReferenced cell temperature Tr 28KCell numbers of a PV module 36Parallel numbers of the PV modules Series numbers of the PV modules 20Photocurrent at standard condition phrj 3.35ABand-gap energy Eg e .237 eCell internal resistance 0.32P junction ideality factorA 54Temperature coecient Cj 0.065%

With different temperatures and solar radiations, output

characteristics of PV array are simulated as gure 4 andgure 5.

Figure 4. Characteristic curves of the PV array with dierentsolar irradiations

Figure 5. Characteristic curves of the PV array with dierent

cell temperatures

As shown in gure 4 and 5, PV array has nonlinearvoltage-current characteristics, and there is only one unique

operating point for a PV generation system with a maximum

output power under a particular environmental condition.

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21 China Inteational Conference on Electricity Distribution 3

3 Battry Mdl

Battery model can usually be divided into experimental

model, electrochemical model and equivalent circuit model.

The equivalent circuit model is most suitable for dynamic

simulation. Based on Shephred battery model, reference [9]

presents a generic battery model for dynamic simulation,

which assumes that the battery is composed of a

controlled-voltage source and a series resistance, shown as

gure This generic battery model considers the state of

charge (SOC) as the only state variable. v.Figure A generic battery model

The expression of the controlled voltage source is

5

where, Eh is noload voltage (V); Eo is battery constant

voltage (V); K is polarization voltage (V); Q is battery

capacity A; A is exponential zone amplitude (V); B is

exponential zone time constant inverse (Ah)0

This model assumes the internal resistance of the battery

keeps constant during both charge and discharge cycles. All

parameters are deduced from the discharge and assumed to be

same for charge. Figure 7 is discharge characteristics of the

battery at rated discharge current, and all parameters can be

calculated by three points marked in the figure, namely fully

charged voltage E(I), the end of exponential zone (Eexp ,Qexp)the end of nominal zone Elom Qnom

24 .

.

:

.

.

. .. ..L:i. -DischargeCUI'

u : : c Nial re..l E .:).. cExponeliarO nom n( : : :

J:(;,qt16 .", [ "? '''' .

400'.-O .5 20-.50- 00.50Aperhou )

Figure 7 Discharge characteristics curve of the battery at the

rated discharge current (VQ)

Formulas for calculating the parameters are

TABLE I

PAAMETERS FOR BATTERY MODEL

Parameters Values

Fully charged voltage Eju 2VExponential voltage Eexp 21V

Exponential capacity Qexp ANomianl voltage Enom 2VNominal capacity Qnom 2AInitial SOC % 60Battery internal resistance Rb 17Rated capacity A

Use MTLAB/Simulink to model the battery, as shown

in gure 8. Based on table 2 the battery discharge curves at

dirent discharge currents are obtained, shown in gure 9.

Figure 8. The battery model in MLTAB/Simulink

240

E220

- 20080

5 0 5 20Time (hus)

Figure 9. Discharge characteristics curves of the battery at

different discharge current (VQ)

4 Grid-cnnctd P/Battry Gnratin

Systm

Figure is the conguration of the gridconnected

PV/Battery generation system. PV array and battery are

connected to the common dc bus via a DCDC converter

respectively, and then interconnected to the ac grid via a

common DC/AC inverter. Battery energy storage can charge

and discharge to help balance the power between PV

generation and loads demand. When the generation exceeds

the demand, PV array will charge the battery to store the extra

power, meanwhile, when the generation is less than the

demand, the battery will discharge the stored power to supply

loads. Each of PV system, battery energy storage system and

the inverter has its independent control objective, and bycontrolling each part, the entire system is operating safely.

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cost will be reduced. A capacitor IS generally connected

between PV array and the boost circuit, which is used to

reduce high frequency harmonics. Figure 12 is the

conguration of the boost circuit and its control system.

ILc

Figure 12. Boost circuit and its control

Let D (O

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51 y pWhen the system is in steady state, solar irradiance S

1000 W/m, and temperature is 298K. From gure 4, it is

known that at this condition, the maximum output power of

PV system is 9800 Set the power demands to be 9800W,which is just equal to the generation, so now the BESS does

not work. Simulation time is 2s, and simulation step is 5 J s.

(a)DC reference voltage (b)Output voltage of PV array. :

(c) Output current of PV array

S

, . , a s '"

.

.+-..

-

t

(d)Output power of PV array

(e)Tnstaneous active power and reactive power at the ac bus

f

( Output power of battery

-;'meo-

(h)Outut voltage of battery

" 0) oc a It/

(g) State of charge

- Ch

(i) voltage at dc bus

"1 s t I $)hree-phase current of the ac line

(k)hree-phase voltage at the ac bus

Figure 17. Simulation results of the system in steady state

From gures (a)-(d), at this operation situation, the

reference voltage calculated by MPP controller is 352Y Tn

steady state, the operation point of PV array is just its

maximum power point and PV array generate the power of

9800 Since now the power generated from PV array is

equal to load demands, battery is inactivated, and its output

power is zero, its voltage and SOC are unchanged, as shown

in gures (-(h). Moreover, the voltage at dc bus is regulated

to be 400Y

52 gps ACS Line , L-\I -q ( 1.

y ,: . ,r GridSho-circuitPCC faultOO

Fig.18 Location of the fault in the system

At t=.5s, a single-phase fault happens in the line, and

the fault point is marked in gure 18. Aer 0.5s, the fault is

cleared. Simulation time is 3s.

>';m> H; .T l

(a)DC reference voltage

i - __

;

. + . "

,

,

(b)Output voltage of PV array

.i- .

-t:- .j--. -f i'I 1"

l

(c) Output current of PV array (d)Output power of PV array

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.,10'

" ; _,. - ,i ! -B , " . i O >m"

t I

(e) Output power of battery

+(g) State of charge

mos( Output voltage of battery

o

;7 -

(h) voltage at dc bus

(i) nstaneous active power and reactive power at the ac bus

) Line current aheadof the fault point

current from PV Battery

l

(k)Line current aerthe fault point

current from the grid

(l)Three-phase voltages at the fault point

Figure 19. Simulation results of the integral system in

single-phase fault

From gure (h), at t=.2s, the bi-directional DC/DC

converter in BESS can maintain the voltage at dc bus

constantly to be 400 Thus, output characteristics of PV

array which is in dc link does not uctuate during fault period,

as shown in gure (a)-(d). However, during fault period,

voltages and currents of the ac line are all changing: phase-c

voltage reduces to be zero, current rises to be very large.

Since the battery always tracks the grid power, the power

generated from the battery also uctuates during fault period,

as shown in gures (e)-(g).

52 Changes of solar irradiance

Assumin solar irradiance chanes: durin 0 to s, solar

irradiance is 1000 W m; during 1 s to 2s, solar irradiance is

W/m; during 2s to 3s, solar irradiance becomes

W/m Because power demands of the grid side keep 9800W,

PV generation will become not equal to the power demands,

and the battery will charge or discharge to maintain the power

balance. Simulation time is 3s.Figure 20 shows the simulation results of the PV system,

namely dc reference voltage calculated by MPPT controller,

PV array output voltage, output current and output power.

From these results, it can be known that when the solar

irradiance changes, V-I characteristics of PV array changes

and the maximum power point also changes. With dirent

sola iadiances, MPPT contolle can tack the MPP quickly

and make the PV array always generate the maximum power.

I .1 ('Hm., ...L . :u

- _.

"

f !

I

Figure 20. Simulation results of the PV system when solar

irradiance changes

Since the power generated from PV array doesn't keep

equal to the power demands, battery will charge or discharge

to make up with this power unbalance. Figure 21 shows the

simulation results of BESS. When solar irradiance is

W m, PV generation is less than 9800W, so the battery will

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discharge to provide power to the grid, as well as its SOC and

terminal voltage decreases; when solar irradiance IS1500W/m, PV generation is more than 9800W, so now PV

array will charge the battery to store extra energy, as well as

SOC and terminal voltage of the battery increases as a result.

2 f_, _+ . 1

" I . .J"'-is

r; ' I.t/

Figure 21. Simulation results of the BESS when solar

irradiance changes

When there is no battery in the generation system, power

delivered into the utility grid changes as PV generation

changes, moreover, the voltage and current at the ac bus will

also change as a result. On the contrary, when there is a

battery bank in the system, the battery can charge or discharge

to help maintain the voltage, current and power constant, and

thus improve the stability at the point of common coupling.

Figure 22 is the comparison between the simulation results of

the system with BESS and the system without BESS. Figure

(a) shows that if there is no BESS, the voltage at the common

dc bus will uctuate at the moment when the solar irradiance

changes; on the contrary, if there is a BESS, the voltage at the

common dc bus will always be stable. From gure (b )-(c),

when PV generation uctuates, the connection of BESS can

help keep the ac link in the generation system stable.

. -. .., .. . . . ...... _ ..; .. ,m

- _.. ...._, ...;-(a) voltage at the common dc bus

1,

' G '

r 1$

jMI. -BEW8S

;+,-tl(b) Instaneous active power and reactive power at the ac bus

(c) Three-phase voltages and currents at the ac bus

Figure 22. Comparison of the simulation results between the

system with BESS and the system without BESS

6 Conclusions

n this paper, a grid-connected hybrid PV /Battery

generation system is studied. In order to convert the solar

energy eciently, the maximum power point of the PV array

should be tracked to ensure the PV array to generate most

power to utility grid. Modied variable-step P&O method can

change the perturbation as the real operation point changes,

consequently, both of the tracking speed and algorithm

accuracy are satised. When solar irradiance or temperature

uctuates, PV generation will change as a result. Battery can

be charged or discharge to maintain the power balance

between PV generation and the demands, and thus improve

the stablity of the entire system. When there is a short-circuit

fault in the line, voltage, current and power at the ac grid will

uctuate. Because the bi-directional DC/DC converter in

BESS can control the dc bus voltage to be constant, dc side of

the system will not be affected during the fault period, and

thus it will reduce the impact of fault on the PV system.

Acknowledgeents

This paper is supported by National Natural Science

Foundation (No.50595412, 50625722), National 973 Project

(No. 2009CB219700).

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