Fault Ride-Through Capability Enhancement of PV System with Voltage Support Control Strategy

doi:10.4236/ojapps.2013.32B006

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Open Journal of Applied Sciences, 2013, 3, 30-34

doi:10.4236/ojapps.2013.32B006 Published Online June 2013 (http://www.scirp.org/journal/ojapps)

Fault Ride-Through Capability Enhancement of PV

System with Voltage Support Control Strategy*

Dehui Zeng, Gang Wang, Guoqing Pan, Haifeng Li

School of Electric Power, South China University of Technology, Guangzhou, China

Email: [email protected], [email protected]

Received 2013

ABSTRACT

With continuously increasing of photovoltaic (PV) plant’s penetration, it has become a critical issue to improve the fault

ride-through capability of PV plant. This paper refers to the German grid code, and the PV system is controlled to keep

grid connected, as well as inject reactive current to grid when fault occurs. The mathematical model of PV system is

established and the fault characteristic is studied with respect to the control strategy. By analyzing the effect of reactive

power supplied by the PV system to the point of common coupling (PCC) voltage, this paper proposes an adaptive

voltage support control strategy to enhance the fault ride-through capability of PV system. The control strategy fully

utilizes the PV system’s capability of voltage support and takes the safety of equipment into account as well. At last, the

proposed control strategy is verified by simulation.

Keywords: PV System; Fault Ride-Through; Voltage Support; Control Strategy

1. Introduction

Under the pressures of environment pollution and energy

shortages, power generation from renewable energy

sources has been increasing significantly. Photovoltaic

(PV) power generation can be used conveniently and

gives no pollution, and has become one of the most

widely used distributed generation technologies in recent

years[1].

With the increasing of PV penetration, the grid codes

from German[2,3], Japan[4] and China[5] have required

the PV system to have the low voltage ride-through ca-

pability, which defines as the PV inverters’ capability of

remaining grid-connected in the event of grid failures.

What’s more, in the German grid code, the PV inverter is

required to supply reactive power when voltage drops, so

as to support the voltage. This requirement can make full

use of the auxiliary functions of PV plant, which would

take benefit to the participation of the PV plant[6]. On

the other hand, the reactive power supplied by the PV

plant can support the voltage effectively with high pene-

tration of PV plant[7]. Therefore, when voltage drops,

making full use of the voltage support capability of the

PV plant will lead the trend of grid code in the future.

But at the moment, there is no theoretical research on the

effect of the reactive power from PV plant to voltage

support, which would limit the enhancement of fault

ride-through capability of PV system.

This paper refers to the German grid code, and the PV

inverter is controlled to supply reactive power when

voltage drops. The fault characteristic of the PV system

is studied with respect to the control strategy. Based on

this, the effect of the reactive power from PV inverter to

the point of common coupling (PCC) voltage is analyzed

and an adaptive voltage support control strategy is pro-

posed to enhance the fault ride-through capability of PV

system. At last, the proposed control strategy is verified

by simulation.

2. PV Inverter Mathematical Model and

Fault Characteristic

PV array produces dc current by photovoltaic effect. The

maximum power point tracking (MPPT) control strategy

is generally used to control the dc voltage to stay at the

maximum power point as the volt-ampere characteristic

shows serious nonlinearity[8]. With the synchronous

reference frame control, the output power of the PV in-

verter can be written as:

This work is supported by the National Basic Research Program o

China (973 Program) (2009CB219704), the Crucial Field and Key

Breakthrough Project in “Guangdong-Hongkong” (No. 2009A0913

00011), Guangdong Special Fund Project of Industry, University and

Research Institute Collaboration (2011A090200127,2011A090200074),

Guangdong Nature Science Foundation(S2012010008355).

outP CCd

outP CCq

PUI











I

(1)

where is magnitude of PCC voltage, and

PCC

, q

D. H. ZENG ET AL.

is the output current of PV inverter in d-axis and q-axis.

When fault occurs, according to the German grid code,

PV inverter should supply reactive power to the grid in

addition to remaining grid-connected. The relationship

between the increment of reactive current and the PCC

voltage deviation is shown as Figure 1 .

Where

q_ref

∆is the increment of reactive reference

current,

PCC

U∆is voltage deviation. The shaded area

means the control dead-band and voltage deviation be-

tween which would not lead to injection of additional

reactive current. The size of the area is generally influ-

enced by the allowed deviation of the system voltage.

When the voltage drops exceed the dead-band, the rela-

tionship between the increment of the reactive current

and the voltage deviation can be written as

q_refPCCPCCfPCC_ref

=()

IKUKUU

⋅

∆=∆− (2)

where

PCC_ref

U and

PCCf

⋅

represents the magnitude of

rated voltage and fault voltage at PCC respectively. K is

the coefficient of reactive power support. In general, to

make full use of the renewable energy in normal condi-

tion, no reactive power would be supplied and

equals

to 0. Therefore, when fault occurs,

qq_ref

=∆ .

The reactive current increases with the drops of PCC

voltage as equation (2) indicates. At the same time, as the

PCC voltage drops, the active current of the inverter is

controlled to increase, so as to output the reference active

power and keep dc voltage constant[9]. The active refer-

ence current

d_ref

would become as

ref

d_ref

PCCf

⋅

= (3)

where,

ref

represents the reference active power.

Equation (2) and (3) indicate the increment of refer-

ence current as PCC voltage drops. And the output active

and reactive current would follow under the effect of

control strategy, which would lead to increment of the

PCC

∆

q_ref

∆

Figure 1. Relationship between deviation of Upcc and incre-

ment of reactive current.

inverter fault current as a result. However, with the limi-

tation of the short-circuit capacity of the PV inverter, the

maximum fault current is about twice of rated current

[10]. And the reference current of the inverter is limited

to 2p.u., so as to protect the device. To support the volt-

age, the fault ride-through control strategy of PV system

controls the reactive power prior to the active power,

which represents that the reactive current would follow

the reference value, while the active current would be

limited. So the fault current of PV system can be ex-

pressed as

qfPCC_refPCCfmax

dfrefPCCfmaxqf

max

min((), )

=min(/,)

=2..

IKUUI

IPUII

Ipu

⋅⋅

⋅⋅⋅

=−





−







(4)

where

⋅

and

⋅

represents the active and reactive

fault current of PV inverter respectively, and

max

represents the maximum short-circuit current.

3. Adaptive Voltage Support Control

Strategy

Figure 2 shows the diagram of distribution network with

PV system.

Where,

represents system equivalent voltage,

and

represents system equivalent impedance

and line impedance of PCC upstream and downstream

lines.

PVf

⋅

represents the fault current supplied by PV

system. A three phase fault is assumed to be occurred at

the end of line 2.

Takes the phase of

PCCf

⋅

as basis, which means

PCCfPCCf

⋅⋅

=∠

PVf

⋅

can be written as

PVfdfqf

IIjI

⋅⋅⋅

(5)

With the node voltage method, the PCC voltage can be

expressed as

PCCfPVf

121

()

SLLSL

ZZZZZ

⋅⋅

+=+

(6)

According to the analysis of chapter 2, the maximum

magnitude of

PVf

⋅

max

, and the phase angle

changes from 0 to 90 degree. Equation (6) can be shown

with vector as Figure 3.

L1f

⋅

L2f

⋅

PCCf

⋅

Figure 2. Diagram of the distribution network.

D. H. ZENG ET AL.

PCCf

()

SLL

ZZZ

⋅

PVf

⋅

max

Figure 3. Diagram of the vector analysis.

Figure 3 indicates that, when max

PVf

⋅

can rarely influence the magnitude of PCC voltage.

It is to say, when the short-circuit capacity of inverter is

far more less than the short-circuit capacity at PCC sup-

plied by system, the reactive power from PV plant can

rarely support the voltage. Otherwise, the effect of reac-

tive power support can be substantial.

If the output current of the inverter does not reach the

maximum short-circuit current,

PVf

⋅

can be expressed

ref

PVfPCCfPCC_ref

PCCf

()

IjKUU

⋅⋅

⋅

=+−

& (7)

Substitute equation (7) into equation (6) and it is clear

to find that the magnitude of PCC voltage would increase

with the increment of K. Therefore, increasing of coeffi-

cient K at such condition can make full use of the voltage

support capability of PV plant and reduce the voltage

drop. To maximize the voltage support capability of PV

plant, K should meet the following equation

ref

PCCfPCC_refmax

PCCf

()()

jKUUI

U⋅

⋅



+−=



(8)

Solving equation (8) and K should be

ref

max

PCC_refPCCfPCCf

()

UUU

⋅⋅

=−

− (9)

In engineering practice, K can be multiplied by reli-

ability coefficient to get a certain margin for the control

system.

When the short-circuit current of the inverter reaches

the maximum value,

PVf

⋅

can be expressed as

PVfmaxPCCfPCC_ref

PCCfPCC_ref

()

IIKUU

jKUU

⋅⋅

⋅



=−−



+−

& (10)

Substitute equation (10) into equation (6). With the in-

crement of K, the magnitude of the PV short-circuit cur-

rent keeps constant, while the phase angle becomes

smaller. From Figure 3, it is obvious to find that with the

increment of K, the magnitude of the PCC voltage may

increase or decrease as well. It all depends on the phase

angle of

PVf

⋅

. When the phase angle of

PVf

⋅

equals to

that of

, the magnitude of PCC voltage be-

comes maximum. However, when the output current of

the inverter reaches the maximum, as the control system

output reactive power prior to active power and the ac-

tive power generated by the PV array would be limited,

which would lead to the increment of dc voltage. And

more serious damage would be brought to the equipment

with the increment of K. So from the point of protecting

equipment, K should be as smaller as possible. In Ger-

man grid code, K is required to be more than 2. So taking

all of this evidence together, K had better equals to 2 at

such condition.

In summary, when the short-circuit current of inverter

reaches the maximum value, K equals to 2. Otherwise, K

should be adaptive to the capacity and output active

power of the PV plant, and the drops of PCC voltage as

well, so as to maximize the capability of voltage support

of the PV plant and enhance the fault ride-through capa-

bility. The principle of K can be expressed as follows:

ref

max

PCCfPCC_refPCCf

max(2,())

UUU

⋅⋅

=−

−

(11)

where,

represents the reliability coefficient.

The diagram of the proposed fault ride-through control

strategy with adaptive voltage support capability is

shown as Figure 4. The block of ‘K Control’ realizes the

function of adaptive voltage support.

4. Simulation Verification

The simulation model of distribution network as Figure

2 shows has been built in DIgSILENT to verify the ef-

fectiveness of the proposed control strategy. Where

= 10.5kV,

= j2.3Ω,

= （0.1+j0.2）Ω ,

(0.2+j0.4)Ω,

= （9.8+j0.5）MVA, and the reli-

ability coefficient of K equals to 0.9.

dc_ref

PCC_ref

PCCf

⋅

d_ref

q_ref

max

ref

Figure 4. The diagram of fault ride-through control strat-

egy.

D. H. ZENG ET AL.

Three different simulation cases have been done and

the rated capability and output power of PV inverter, and

the fault resistance are shown as Table 1.

According to the adaptive voltage support control

strategy proposed in this paper, K equals to 4.38, 7.03

and 2 respectively in three different simulation cases.

Figure 5 shows the simulation results of case 2. And the

analysis of simulation results when K equals to different

values is indicated in Table 2.

In case 1, with the proposed voltage support control

strategy, the magnitude of PCC voltage drops 28.68%,

and 1.56% higher than the situation when no voltage

support control strategy is used. The proposed control

strategy has improved the voltage to some extend, but the

effect is not obvious, and that is because the short-circuit

capability of PV inverter is far more less than that of

system supplied at PCC.

(a) Simulation result of PCC voltage magnitude

In case 2, the capability of the PV inverter has in-

creased and the voltage drops 21.96% with the proposed

control strategy, which is 6.24% higher than the control

strategy without voltage support. As the proposed control

strategy has left some margin, so the drops of PCC

voltage may decrease if K increases, such as K = 8.5. But

when the short-circuit current of inverter reaches the

maximum value, the dc voltage increases and makes

damage to the equipment.

In case 3, the fault resistance decreases from 3Ω to 1

Ω, and the short-circuit current reaches the maximum

value even K equals to 2. The voltage increases by

1.82% compared with no voltage support control strategy.

And the dc voltage has exceeds the rated value for

11.50%. The magnitude of PCC voltage may increase

and decrease as well if K increases, but the dc voltage

would keep increasing and make more serious damage to

the equipment.

(b) Simulation result of DC voltage magnitude

Table 1. The simulation parameter.

S(MVA) P(MW) f

(Ω)

Case 1 1 1 3

Case 2 3.5 3 3

Case 3 3.5 3 1

Figure 5. The diagram of case 2 simulation result.

Table 2. Analysis of simulation results.

Case 1 Case 2 Case 3

K 0 2 4.38 8.5 0 2 7.03 8.5 0 2 3 8.5

PCC %U 30.24% 29.50% 28.68% 29.33%28.20% 26.03%21.96%21.54% 56.24% 54.42% 54.35% 56.98%

dc %U 0 0 0 31.07%0 0 0 14.17%0 11.50% 14.40% 38.25%

PV f(..)ipu

 1.443 1.549 1.712 2 1.1861.2601.862 2 1.946 2 2 2

D. H. ZENG ET AL.

The adaptability of the proposed control strategy has

been proved from the simulations, as K equals to

different values when the condition changes. With the

proposed control strategy, the PV plant’s capability of

voltage support can be made full used and the safety of

the equipment has been taken into consideration as well.

5. Conclusions

This paper refers to the German grid code and control the

PV inverter to inject reactive current when the voltage

drops. The influence of reactive power from PV system

to the PCC voltage is analyzed and an adaptive voltage

support control strategy is proposed to improve the fault

ride-through capability of PV inverter. The study indi-

cates that when the difference between PV inverter

short-circuit capacity and the short-circuit capacity at

PCC supplied by system is not so large, with the pro-

posed control strategy, the voltage can be improved ef-

fectively. The proposed control strategy can maximize

the voltage support capability of PV plant while takes the

safety of equipment into consideration as well.

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