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2011The International Conference on Advanced Power System Automation and Protection

APAP2011 www.apap2011.org

*Corresponding author (email: [email protected])

Comparative study of two stages and single stage topologies for Grid-TiePhotovoltaic Generation by PSCAD/EMTDC

ZHU YongLi1*, Yao JianGuo1, Wu Di1

1Research Center, State Grid Power Research Institute, Nanjing 210003, China

Abstract: This paper presents a comparison study of a typical grid-tie photovoltaic system between two types of topologies:

the two stages (boost circuit adds inverter) and the single stage (inverter alone adds a step-up transformer). Some essential

models built in this paper include PV array, Boost chopper and grid-tie inverter. The basic characteristics and descriptions of

these two topologies are described. Perturb and observe and incremental conductance methods are introduced for MPPT

(maximum power point tracking). As key-point, the out loop controller design details for the grid-tie inverter are investi-gated. The PV array, two system topologies and two MPPT strategies are implemented in PSCAD/EMTDC using its us-

er-defined facility. Simulation results show that the output power of PV array can achieve a stable and accurate MPPT function

under ambient conditions change. At the same time, the inverter can maintain a unit power factor by controlling the transferred

instantaneous reactive power to zero in the synchronous d-q frame. The simulating results are compared with that of the two

stages case in aspects including energy converting efficiency, power quality, and MPPT accuracy. Through this study work,

engineers and researchers can obtain an overall comprehension about advantages and disadvantages of these two topologies in

designing a PV system.

Keywords: photovoltaic, MPPT, single stage, two stages, perturb and observe, incremental conductance,

PSCAD/EMTDC

1 Introduction

Recently, photovoltaic (PV) generation has become a hot

issue in renewable energy development worldwide. To

achieve best operation efficiency and economic profit, most

PV system need to adopt MPPT (maximum power point

tracking strategy) strategy under different work condition,

including ambient temperature change, irradiation change,

and load condition change. Regard to MPPT algorithms, a

lot of versions have been advanced [1-3]. The most com-

monly used methods are CVT (constant voltage tracking),

P&O (perturb and observe) and Incremental conductance

method (in this paper it is denoted as IncCond). Eachmethod has its advantage and disadvantage, however, in

practice the P&O method is almost the first choice since its

idea is very concise and implemented easily in hardware.

Now there are mainly two types of power electronic to-

pologies being used in photovoltaic generation field, i.e.,

single stage topology and two stages topologies [4-9]. The

two stages topology is composed of former DC/DC part and

latter inverter part. One stage, as the term suggests, consists

of the inverter only. The merit of two stages topology lies in

the convenience of designing its control scheme, but has to

burden more power loss than that of single stage [8]. Singlestage, on the contrary, can achieve relatively higher power

efficiency, but the control scheme is more complex since the

inverter alone must achieve all of the control objectives:

grid current following, power factor constant and MPPT

function [4, 7]. About these two topologies, presently there

are not many literatures investigating the difference of their

design details and little conclusion refer to their most suita-

ble applying occasion respectively. The MPPT methods

comparison for these two topologies is rarely investigated,

so a rigorous comparative study between the two topologies

under different MPPT algorithms is of high necessity and

would be beneficial to the PV system research.

The main purpose of this paper is to evaluate the perfor-

mance of the two PV system topologies in aspects like

power efficiency, control scheme design, and MPPT accu-

racy. The PSCAD/EMTDC software is chosen as the simu-

lation tool. The article arrangement is as follows. The basic

structure and characteristics of a typical PV module are de-

scribed in second part. P&O and IncCond methods for

MPPT are briefly mentioned in part In part , four

cases are designed to validate the developed models and

corresponding MPPT algorithms. Simulation results and

___________________________________978-1-4244-9621-1/11/$26.00 2011 IEEE


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discussion are provided in next part. The last part ends this

article with some clear conclusions related to performance

comparison between these two topologies.

2 Two types of topologies

1) Two stages topology

Commonly, the output voltage of the PV array is not high

enough to connect to the grid. Moreover, the voltage source

inverter (VSI) usually has a voltage-down property, which

causes the PV array + Inverter topology to output a lower

voltage, thus two stages topology is suggested. This topol-

ogy adds a voltage-up link part, usually configured as Fig-

ure 1. The DC/DC part often adopts a Boost circuit or some

other derived versions, like Buck-boost, isolated Boost, etc.

[1, 3]. Besides voltage-up function, the Boost circuit can

also offer a more stable input voltage for the inverter. The

main advantage of the two stages topology is the flexibility

of designing its control scheme since it has a higher free-

dom degree, i.e. more controllable variables, which means

multiple control objectives (MPPT, grid connecting, var

compensating, active filter, etc.) can share by two stages

respectively simultaneously [6, 7].

Figure 1 Two stages topology

2) Single stage topology

Although two stages topology has advantages in control-

ler design, it also has some deficiencies [5, 8]. With the cir-

cuit stages increasing, the power loss rises as well that

makes the holistic energy transferring efficiency decrease;

more stages also adds system complexity, thereby reduce

the system reliability. To enhance system efficiency, system

only relying on the inverter, i.e. so-called single stage to-

pology has been suggested, as shown in Figure 2.

Figure 2 Single stage topology

3 MPPT algorithm

The flow charts of the P&O and IncCond methods are de-

scribed below.

1) Perturb and Observe (P&O)The P&O is the most commonly used MPPT algorithm

due to its simplicity. Figure 3 displays its classic flow chart.

After each perturbs operation, the current power is com-

puted and compared with previous value to determine the

power variation'Pand'V . If 'P and 'Vbear the samesign, i.e.' 'P V> 0, then the perturb direction keeps un-

changed (C = 0), otherwise perturbs voltage inversely.

Figure 3 Flow chart of P&O algorithm

Figuratively speaking, this method is to mimic a hill

climbing. It works well in slow changing environment but

has some limitations under rapidly changing atmospheric

conditions: it may lead to a slow MPPT speed or even in-

correct tracking. To overcome such problems, some mod-

ified versions have been put forward, though they are not in

the focus of this paper [2, 3].

2) IncCond

The Incremental Conductance method implements the

MPPT function through a rigorous mathematic way. To

achieve the MPPT, from the P-V characteristic curve, fol-

lowing equations must be satisfied:

0dP dI

I UdU dU

dI I

dU U

(1)


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(1) is the condition of acquiring MPPT, i.e. when the varia-

tion of output conductance equals to negative conductance.

Figure 4 Flow chart of IncCond Method

3 Parts modeling in PSCAD/EMTDC

3.1 PV array

The PV array model is built according to [5]. The currentoutput equations of a PV module of m cells are:

PH DI I I (2)

1 1, 1,

,

[1 ( ) / ( )] ( ) aPH c c SC c norm SC c norma norm

GI T T I T I T

GD u u u (3)

( )/

0 1S Sq V IR N

mnkTDI I e

-

(4)

3

1

1

1 1

110 ( )/

1

( )

1

gn

OC S

qV

nk Tc Tcsc cqV T N

mnkT

I T TI eT

e

u u

(5)

PHI

DI

SHR

Figure 5 Equivalent circuit of a PV cell

The meanings of symbols in equations above can be

found in Figure 5 and Table 1. For a array of Ns modules in

series and Np modules in parallel, equations are adjusted to:

( )/

1S S

p PH p

q V IR N

NsmnkTI N I N e-

(6)

Table 1 Panel data of PV array*

Symbols Item Unit

Tc Solar cell temperature Kelvin

Ta ambient temperature Kelvin

Ga ambient irradiation W/m2

Tc1solar cell temperature at Standard

Test Conditions (STC)298.15K

Isc(Tc1) short circuit current at STC 3.8A

Ga,nom ambient irradiation at STC 1000 W/m2

Dtemperature coefficient of short

circuit current 0.0063

VOC(Tc1) open circuit voltage at STC 21.1V

q electron charge 1.610-19C

k Boltzmann constant 1.3810-23J/K

n diode ideality factor 1.5

Vg band gap voltage 1.12V

Ns Number of modules in series 10

Np Number of modules in parallel 9

m Number of cells in a PV module 36

* Data partly from PV panel manufacturers product information.

Figure 6 shows the PV array model in PSCAD. The pan-

el data listed in Table 1 is from a panel manufacturer. Run-

ning this model in PSCAD/EMTDC obtains Figure 7. As

can be seen, with the temperature increasing the output of

PV array decreases and the MPP voltage moves toward left.

Figure 6 PV array model in PSCAD/EMTDC

0 25 50 75 100 125 150 175 200 2250.0

1.0k

2.0k

3.0k

4.0k

5.0k+y

-y-x +x

25 C$

75 C$

50 C$

Figure 7 P-V characteristic of modeled PV array


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3.3 MPPT control scheme

The basic control inner loop of grid-tie inverter for both

topologies is the same, i.e. the classic current decoupled

technique in d-q frame for three-phase VSI [5, 7]. Thus, the

main difference lies in the out loop, as shown in Figure 8 to

11. The single stage uses the Edcref generated by the MPPT

block as the DC voltage set value; while the two stages set

this value to a fixed value (800V in this paper) and the Edc-

ref is only used to control PWM signals for Boost circuit.

1) Single stage + P&O

Figure 8 MPPT controller for Single stage with P&O

2) Single stage + IncCond

Edc

Edc_ref

*1000

Edc1

Boost PWM

IncCondIout

Edc1

I

PD

-

F

-

Iout

D+

F

-

Edc1

idref

Figure 9 MPPT controller for Single stage with IncCond

3) Two stages + P&O

Edc

Iout

dutyBoost_PWMBoost PWM

P & OEdc_ref

*1000.0

Figure 10 MPPT controller for Two stages with P&O

4) Two stages + IncCond

dutyBoost_PWM

Edc_ref

Boost PWM

IncCond

Edc

Iout

*1000.0

Figure 11 MPPT controller for Two stages with IncCond

T1

T1 T3

T4

T4 T6

T3

T2

1000.0

*Pin Pin

Ga

V

Ta

Ga

Iout

6600[uF]

Edc

T3

T4

T2

T5 Ia

Ea

6600[uF]

0.005 [H]

0.005[H]

0.005[H]

Ib

Ic

Ipv

Eb

Ec

P = -4.472

Q = 0.4708

V = 0.1029

V

A

0.01 [ohm]

0.01 [ohm]

0.01 [ohm]

Edc

*1000.0

*0.001

Iout

Eab

If_c

If_b

If_a

AC Filter

#1 #2

100.0 [kVA]100.0 [V] / 380.0 [V]

T

25.0

T1

T1 T3

T4

T4 T6

T3

T2

1000.0

*Pin Ppv

Ga

V

Ta

Ga

Iout

6600[uF]

Edc

T3

T4

T2

T5 Ia

Ea

6600[uF]

0.005 [H]

0.005[H]

0.005[H]

Ib

Ic

Ipv

Eb

Ec

P = -4.448

Q = 0.6257

V = 0.3913

V

AEdc

*1000.0

*0.001

Iout

Eab

If_c

If_b

If_a

T

0.1 [H]

1000[

uF]

Boost

Vboost L-L,RMS,380V,AC

Ta

AC_Filter 3

a b c

25 27 29

53 55

25.0Boost_PWM

Figure 12 Overall architectures of single stage and two stages topologies built in PSCAD

4 Case study and simulation results

Since the MPPT voltage transfers more sharply under tem-

perature change, to verify the model accuracy and compare

the two topologies more rigorously, simulations are carried

out during a temperature slope change: from 3s to 4s, T va-

ries from $25 C to $50 C. Considering two topologies and


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two MPPT methods mixed here, four cases are designed:

1) Single Stage + P & O

0.0 1.0 2.0 3.0 4.0 5.0 6.0

0.0

1.0k

2.0k

3.0k

4.0k

5.0k Pin

Figure 13 PV array output and MPP tracking curve

2) Single Stage + IncCond

Main : Graphs

0.0 1.0 2.0 3.0 4.0 5.0 6.0

0.0

1.0k

2.0k

3.0k

4.0k

5.0kPin


3) Two stages + P&O

0.0 1.0 2.0 3.0 4.0 5.0 6.0

0.0

1.0k

2.0k

3.0k

4.0k

5.0kPin


4) Two Stages + IncCond

P_PV

0.0 1.0 2.0 3.0 4.0 5.0 6.0

0.0

1.0k

2.0k

3.0k

4.0k

5.0kPin


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0.0 1.0 2.0 3.0 4.0 5.0 6.0

-0.050

-0.040

-0.030

-0.020

-0.010

0.000

0.0100.020

0.030

0.040

0.050if_a Usa

-0.080

-0.060

-0.040

-0.020

0.000

0.020

0.040

0.060

0.080

y

if_a Usa

Main ...

Q

-8.80641

Figure 17 Grid current and voltage of S + Inc

Table 2 Performance comparison between the two topologies

S +P&O S + Inc T+ P&O T + Inc

Power efficiency 98.48% 99.49% 98.11% 97.52%

THD (phase voltage) 4.5% 2.7% 0.27% 0.28%

MPPT accuracy

(ideal: 4000w)3977w 3856.6w 3910.6w 3917.3w

The validity of the MPPT controllers can be watched di-

rectly by comparing the P-V characteristic curve in Figure

13 to 16 with Figure 7. Figure 17 shows that the grid current

follow the grid voltage narrowly (since the current positive

direction defined in this model is flowing out of grid, so in

the picture the current is inverse phase against voltage) and

the reactive power is nearly zero (8.8w) compared with ac-

tive power (3856.6w).

Table 2 compares the performance from three indexes:

power efficiency, phase voltage THD and MPPT accuracy,

which represent economy, power quality and controller va-

lidity respectively. According to expectation, the power ef-

ficiency of one stage is higher than that of two stages. The

fact that the power efficiency of IncCond is less than that of

P&O can be explained by the oscillation caused by the step

selection in IncCond method. It can be drawn from the THD

item that, there are less non-characteristic and low order

harmonics in the AC side of two stages than that of single

stage due to a more stable DC voltage control effect. All thePV array output values in four cases are around the MPP

(losses are caused by filters dissipation and control oscilla-

tion). It indicates that there is not too much difference in

MPPT accuracy for the two topologies.

5 Conclusion

This paper compares the single stage topology and two

stages topology in power efficiency, power quality and

MPPT accuracy through modeling a complete grid-tie PVgeneration system in PSCAD/EMTDC. Two classic MPPT

algorithms are implemented and some design details of the

out loop controllers are presented. Acquired results validate

the effectiveness of developed models and controllers.

Comparisons between the two topologies show that

1) Power efficiency: single stage topology better

2) DC Voltage stability and AC side voltage THD: two

stages topology better

3) MPPT accuracy: in an acceptable error range of con-

troller oscillation and ac filter loss, both can satisfied

accuracy requirements.

In a word, the PV system models developed in this paper

can be used in simulation studies for a class of grid-tie PV

generation occasion. The simulation results can help engi-

neers to choose a proper topology in PV system design.

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