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: 705484965@qq.com, gq.pan@163.com
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
f
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
QU

I
(1)
where is magnitude of PCC voltage, and
PCC
Ud
I
, q
I
Copyright © 2013 SciRes. OJAppS
D. H. ZENG ET AL.
Copyright © 2013 SciRes. OJAppS
31
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
I
is the increment of reactive reference
current,
PCC
Uis 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
U
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
q0
I
equals
to 0. Therefore, when fault occurs,
qq_ref
II
=∆ .
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
I
would become as
ref
d_ref
PCCf
P
IU
= (3)
where,
ref
P
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
U
q_ref
I
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
22
dfrefPCCfmaxqf
max
min((), )
=min(/,)
=2..
IKUUI
IPUII
Ipu
⋅⋅
⋅⋅
=−
(4)
where
df
I
and
qf
I
represents the active and reactive
fault current of PV inverter respectively, and
max
I
represents the maximum short-circuit current.
3. Adaptive Voltage Support Control
Strategy
Figure 2 shows the diagram of distribution network with
PV system.
Where,
S
E
&
represents system equivalent voltage,
S
Z
and
L1
Z
,
L2
Z
represents system equivalent impedance
and line impedance of PCC upstream and downstream
lines.
PVf
I
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
U
&
as basis, which means
PCCfPCCf
0
UU
°
⋅⋅
=∠
&
.
PVf
I
can be written as
PVfdfqf
IIjI
⋅⋅
=+
&
(5)
With the node voltage method, the PCC voltage can be
expressed as
PCCfPVf
121
11
()
S
SLLSL
E
UI
ZZZZZ
⋅⋅
+=+
++
&
&&
(6)
According to the analysis of chapter 2, the maximum
magnitude of
PVf
I
is
max
I
, and the phase angle
changes from 0 to 90 degree. Equation (6) can be shown
with vector as Figure 3.
L1f
I
&
L2f
&
L1
Z
L2
Z
PCCf
U
&
Sf
I
&
f
PV
I
&
LD
f
S
Z
S
E
&
Figure 2. Diagram of the distribution network.
D. H. ZENG ET AL.
Copyright © 2013 SciRes. OJAppS
32
1
S
SL
E
ZZ
+
&
PCCf
12
11
()
SLL
U
ZZZ
+
+
&
PVf
I
&
max
I
Figure 3. Diagram of the vector analysis.
Figure 3 indicates that, when max
1
S
SL
E
I
ZZ
+
&
=,
PVf
I
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
I
can be expressed
as
ref
PVfPCCfPCC_ref
PCCf
()
P
IjKUU
U
⋅⋅
=+−
& (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
2
22
ref
PCCfPCC_refmax
PCCf
()()
P
jKUUI
U

+−=

(8)
Solving equation (8) and K should be
22
ref
max
PCC_refPCCfPCCf
1
()
P
KI
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
I
can be expressed as
2
2
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
I
. When the phase angle of
PVf
I
equals to
that of
1
S
SL
E
ZZ
+
&
, 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:
22
ref
max
PCCfPCC_refPCCf
1
max(2,())
P
KI
UUU
β
⋅⋅
=−
(11)
where,
β
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
S
E
&
= 10.5kV,
S
Z
= j2.3Ω,
1
L
Z
= 0.1+j0.2)Ω ,
2
L
Z
=
(0.2+j0.4)Ω,
LD
S
= 9.8+j0.5MVA, and the reli-
ability coefficient of K equals to 0.9.
dc_ref
U
dc
U
PCC_ref
U
PCCf
U
d_ref
I
q_ref
I
×
max
ref
P
+
+
Figure 4. The diagram of fault ride-through control strat-
egy.
D. H. ZENG ET AL.
Copyright © 2013 SciRes. OJAppS
33
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
R
()
Case 1 1 1 3
Case 2 3.5 3 3
Case 3 3.5 3 1
(c) Simulation result of inverter output current
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.
Copyright © 2013 SciRes. OJAppS
34
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 plants 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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