Download 1. Fault analysis of DC distribution system

2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
Research on a Line Protection Method for DC Distribution System Based
on Parameter Identification
YANG Qing, DUAN Jiandong, ZHOU Yi, SUN Yuanbing
Department of Electrical Engineering, Xi’an University of Technology, Xi’an, 710048, Shaanxi Province, China
Abstract:
In this paper, a DC distribution line protection method based on parameter identification is proposed. The
parameter equation in the conditions of monopolar grounding fault and pole-to-pole fault is deduced by analyzing DC
distribution line fault, the protection scheme for DC distribution line is researched based on the principle of
parameter identification. The protection scheme utilizes the voltage and current information at installation protection,
fault distance and transition resistance are calculated by method of the least square. The distinction of fault section
and determination of fault position are achieved by comparing fault distance with corresponding line length. The
model of DC distribution system is built on the real time simulation platform RT-LAB. Numerous tests are carried out
on different transition resistances and fault positions, different line types of DC distribution line fault. Simulation
results show that the protection scheme based on parameter identification is feasible.
1.
Fault analysis of DC distribution system
The fault equivalent circuit of DC distribution line was analyzed in the conditions of monopolar grounding
fault and pole-to-pole fault. The fault loop equation was deduced with the voltage and current as the known
parameters, the fault distance and the transition resistance as unknown parameters.
Taking positive pole grounding fault as an example, the fault equivalent circuit is shown in Figure 1:
up
xp
irecp
ip
icp
xpL0
xpR0
if
capacitor
Rf
converter valve
Figure 1. Positive pole grounding fault equivalent circuit
Based on KVL, fault loop equation of positive pole grounding fault is shown in equation (1):
u p  x p  R0i p  L0 (di f (t ) / dt )   i p R f
2.
Protection
principle
for
DC distribution
bus A
line
based
(1)
on
parameter
bus B
xp1
bus C
xp3
xp2
DAB
AB
identification
BC
DBC
DCD
CD
f1
converter valve
L2
L1
Figure 2. Monopolar grounding fault protection principle
Protection principle of DC distribution line can be explained by taking line BC as the protected object.
Assuming positive pole grounding fault occurs at BC line in Figure 2, fault distance x p1 , x p 2 , x p 3 are
calculated respectively by each level line protection DAB、DBC、DCD over equation (1).
For the next line protection DCD, the current flows from measuring point to fault line, therefore the
current direction is in the opposite direction of steady state. Because the polarity of voltage does not change
when fault occurs, the obtained fault distance is negative.
For the front line protection DAB, the current direction is same with the direction of steady state. When
CICED2010 Session x
Paper No xxx
Page1/2
2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
fault occurs, the polarity of voltage does not change, so the obtained fault distance is positive, and
theoretical value is L1+xp2. Similiarly, line protection DBC can also measure positive distance and theoretical
value is xp2.
Therefore, the protection criterion is shown in equation (2):
 x pi  0
reverse direction external fault

0  x pi  L set positive direction internal fault

positive direction external fault
L set  x pi
(i  1, 2, 3...)
(2)
where x pi = calculated fault distance. L set = threshold set by the corresponding line length.
3.
Practical algorithm for protection of dc distribution line
There are two unknown parameters in the parameter equation, so the data of the two positive voltage
and current are needed to solve the equation. In order to improve the accuracy of calculation, several
groups of a period of time voltage and current are often taken to solve parameter equation. The system's
parameter equations are transformed in matrix form (3). The least square method is used to solve it.
i3  i1


i2 
 R0 i2  L0 2t


 R i  L i4  i2
i3 
0
 u2 
 03
2t
u 


 3 
 R i  L i5  i3

i4
0 4
0
 u4 


2t






 x  
(3)


  R  




un 1 
 R i  L in  in  2 i 
0
n 1
u 
 0 n 1

2t
 n 


i

i
n

1
n

1
un 1 
 R0 in  L0
in 


2t


i i
 R0 in 1  L0 n  2 n in 1 
2t


4.
Simulation test and result analysis
The model of DC distribution system was built on the real time simulation platform RT-LAB. Numerous
tests were carried out on different transition resistances and fault locations, different line types of DC distribution
line fault. Simulation results indicate that the protection scheme can distinguish fault section correctly and
relatively determine fault position accurately.
5.
Conclusion
For the radialized monopolar DC distribution system, the equivalent circuit was analyzed when the DC
distribution line occurred to monopolar grounding fault and pole-to-pole fault. The parameter equation including
two unknown parameters (fault distance and transition resistance) was deduced in this paper. Fault distance and
transition resistance of DC distribution system were calculated. By comparing fault distance with corresponding
line length to distinguish fault section and determine fault position, the line protection scheme and practical
algorithm of DC distribution system based on the principle of parameter identification were constituted. The
model of the DC distribution system was built on the simulation platform RT-LAB and the numerous fault tests
were carried out. The results show that the proposed protection scheme is proved to be effective.
Keywords:
DC Distribution System; Line Protection; Parameter Identification;
Author’s brief introduction and contact information:
Yang Qing(1992-); girl; Master Degree; Major Research Direction is the line protection of DC distribution
system; Tel:18700919806; E-mail: [email protected]
CICED2010 Session x
Paper No xxx
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