2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
Analysis on Asymmetric Fault Current of Inverter Interfaced Distributed
Generator
1
Zhongxue Chang , Guobing Song1, Shanghua Guo2, Wei Zhang2
1. Xi’an Jiaotong University 2. Zhuhai XJ Electric Co., Ltd.
Abstract:
Inverter interfaced distributed generator (IIGD) is widely employed in power system. Fault characteristics of
IIGD are determined by control strategy of its grid-side converter. To deduce the fault current of IIDG is to analyze
its response under the action of turbine terminal voltage sags. To enhance the low voltage ride through (LVRT) ability
of IIDG when asymmetric fault occurs, the dual current control strategy is widely used. Based on this control
stratagy,the positive and negative sequence current expressions were deduced and the influence factors were analyzed
based on the expressions. It is obtained that the d-axis components of positive and negative sequence current meet
second-order response and the q-axis components are zero. The steady state values of the positive and negative
sequence current are determined by the unbalance degree of grid and the transient process of fault current is
determined by the control parameters of the grid-side converter. In the situation that the magnitude of the fault
current is higher than the maximum value of the inverter can transfer, the measure that the negative sequence
current should be limited is suggested.
1.
Introduction
Distributed generator, which is integrated to grid by voltage source converter(VSC), is called inverter
interfaced distributed generator (IIGD). When a fault occurs in AC side of the IIDG, the fault characteristics of
IIDG are determined by control strategy of its grid-side converter, which leads to the difference from traditional
synchronous generator, thus it is a big challenge for the operation and protection of power system and it is
necessary and urgent to analyze the fault characteristics of IIDG.
In order to further analyze the fault current characteristics of IIDG and theirs mechanism, the simplification
thought that grid-side converter of IIDG is actually an energy balance system under the action of control and its
external characteristics depend on the control strategies is employed in this paper. Based on this, the complex
topology of IIDG was simplified. Furthermore, the expressions of positive and negative sequence current of
IIDG whose grid-side converter control strategy is dual current control were theoretically deduced and the
factors which influence the fault current were analyzed.
2. Introduction of the Dual Current Control
When power system is asymmetric, the active and reactive power which are output by converter contain
second harmonic component, and there is ripple in DC bus voltage, which will influence the normal operation of
the converter. The dual current control strategy which can suppress DC voltage ripple very well and enhance the
low voltage ride-through capability of IIDG was proposed. The control diagram is shown in Fig.1.
udp
uqP udP
udc*
+
-
udc
PI
i d* c
q0*
i
P*
d
i
p0*
ps*2  0
M E1 idN *
pc*2  0
iqN *
-
p
q
i
iqP*
+
idN
+
iqN
uqN udN
+
p
d
-
ωL
+ vdp*
+
+
ωL
+
PI
-
PI
vqp*
+
-
-
ωL
ωL
PI
dP
PWM
Modu
+
udN
+
+
PI
lation
+
ia
P
d
i
ib
B
i
udP
udN
+ v
A
imax
ic
N*
+ vd
e  j 2 t
N*
q
i'

l
O
dN
idN ' idN
Fig.2. Relationship between d-axis components of the
positive and negative sequence current
-
Fig.1. Dual current control strategy diagram
3. Analysis on Fault Current
3.1 Theoretical Derivation
Fault current is the response to grid voltage sag. That is: i  f (u , K ) ,Where u 、 i represent phase
K  [ K1 , K 2 , …, K n ]T represents the control
parameters. Because the dual current control strategy is designed based on dq axis, it needs to calculate the
expressions of d-axis and q-axis components of positive and negative sequences currents firstly and then
three-phase current can be obtained by Parker inverse transformation. What should be noted here is that q-axis
current is constant zero according to grid codes. So it only needs to deduce the expressions of d-axis current.
According to the control strategy, it can be obtained that the d-axis component of positive sequence current
satisfies the equation (1) after a series of deduction.
voltage and current in AC side of the converter respectively;
d 2idP kip didP kii P kii P*

 id  id
dt 2
L dt
L
L
CICED2016
Session x
Paper No xxx
(1)
Page1/2
2 p0*
udp . Make T  L / kip and K  kii / kip . When a fault occurs, the response
p 2
N 2
3[(u d )  (u d ) ]
P*
P
of id is the sum of id times of step response and zero input response. According to second-order
Where id 
P*
P
response, the response expression of id is:
idP  idP*  (idP (0)  idP* )e

kip
2L
2 Lkii
t
4kii L  k
2
ip
4kii L  kip2
sin(
2L
1
2
t   ) ,Where   tan ( 1   /  ) ，   kip / 2 Lkii .
The response expression of idN can be obtained by using the same method:
idN  idN*[1  e

kip
2L
t
2 Lkii
4kii L  kip2
sin(
4kii L  kip2
2L
t   )] ,Where id  
N*
2 p0*
udN
3[(u dp )2  (u dN ) 2 ] .
It should be noted that the derivation above is based on the assumption that fault current doesn’t exceed the
maximum current of the converter can transfer. When fault current exceeds its maximum value. In order to limit
the degree of unbalance in AC side of the grid-side converter, the strategy that the d-axis component of negative
sequence current is limited is adopted here. The strategy is illustrated in Fig.2.
3.2 Influencing Factors
According to the analysis above, the magnitude of the steady fault current of the converter is determined by
IIDG’s output power and the degree of grid unbalance, while the transient process is determined by the control
parameters of the converter.
4 Simulation And Verification
In order to verify the correctness of the deduction above, the d-axis components of positive and negative
sequence current which are obtained by deduction and those obtained by simulation on PSCAD are compared.
The results are as Fig.3- Fig.5 shown.
0.4
-0.8
-0.9
-1
0.3
0.35
0.4
0.45
0.5
t/(s)
0.55
0.6
0.65
0.7
Idn/(kA)
0.6
simulation
deduction
0.4
0.2
A
B
C
1
0.3
negative sequence current/(kA)
simulation
deduction
positive sequence current/(kA)
Idp/(kA)
-0.7
0.5
0
-0.5
A
B
C
0.2
0.1
0
-0.1
-0.2
-0.3
0
-0.2
0.3
0.35
0.4
0.45
0.5
t/(s)
0.55
0.6
0.65
0.7
-1
0.35
0.4
0.45
t/(s)
0.5
-0.4
0.35
0.4
0.45
0.5
t/(s)
Fig.3.Comparison of deduction and simulation Fig.4.Three-phase positive sequence currents Fig.5. Three-phase negative sequence current
5
Conclusion
1) The d-axis components of positive and negative sequence currents meet second-order response. The
steady values are related to fault type, transition resistance, network structure, distance to the fault point and so
on, while the transient characteristics are closely related to control parameters of the converter.
2) Due to the overshoot of the d-axis components, the trend of the positive and negative sequence currents
of the IIDG is firstly increasing and then decreasing to a steady value.
3) To limit the unbalanced degree of the grid, when fault current exceeds the maximum of the grid-side
converter, the strategy that the d-axis component of negative sequence current should be limited.
Keywords:
relay protection; IIDG; grid-side converter; dual current control; asymmetric fault current
Author’s brief introduction and contact information
Zhongxue Chang, received the B.S.degree from Shanghai University of Electric Power in 2013, and is
currently pursuing the M.Sc. degree at Xi’an Jiaotong University. His research interests include relaying
protection. Email:[email protected]
Guobing Song received the PH.D degree at the Xi’an Jiaotong University in 2005. Currently he works in
the Xi’an Jiaotong University. His research interests include transmission line fault location and protections.
Shanghua,Guo received the M.Sc. degree at Hunan University in 2003. Currently he is a senior engineer in
Zhuhai XJ Electric Co., Ltd. His research interests include the distribution network automation.
Wei Zhang received the M.Sc. degree at Shandong University of Science and Technology in 2010.
Currently he is an engineer in Zhuhai XJ Electric Co., Ltd. His research interests include the distribution network
automation.