i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
Research on influence and resolution of the relay
protections with electric vehicle charging station
integrating into distribution network
Cheng Gong*, Longfei Ma, Baoqun Zhang, Yifeng Ding, Xianglong Li,
Shuo Yang, Ran Jiao, Huizhen Liu
State Grid Beijing Power Research Institute, Fengtai District, Beijing, 1000753, PR China
article info
abstract
Article history:
Electric vehicles have been widely used because of its significant environmental effect,
Received 22 February 2017
study the influence of the relay protection when electric vehicle charging station integrated
Received in revised form
into network is important. Three section current protections are configured in distribution
1 April 2017
network. In this paper, the equivalent model of the charging station is access to distribu-
Accepted 18 April 2017
tion network, different fault locations are set up, and the setting value of the corresponding
Available online xxx
protection are compared with the fault current, finally the impact of the three section
current protection is analyzed. A model is built in PSCAD to verify the correctness of the
Keywords:
Electric vehicles
analysis.
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Equivalent model
Three section current protection
Setting value
Back-up protection
Introduction
Electric vehicle (EV) has many advantages, such as high torque, zero pollution, low noise, etc. EV is developed in order to
relieve the double pressure of energy resource and environment preservation. Moreover, the capabilities of peak shaving
and voltage regulation and the reliability of power supply
network could be enhanced by EV charging station. When EV
charging station is switched into power distribution grid, it is
important that the influences on system protection and the
configuration issues are researched, for promoting EV and
keeping system security and stable [1,2].
At present, there are many researches on EV. Ref. [2e4]
focused on the load characteristics of EV charging station.
Power characteristics for charging station were studied in
Ref. [5], and this paper pointed out that there were two kinds
of charging modes, slow charging mode and rapid charging
mode. The former has the disadvantages of small charge
current and long time to charge. However the latter was
beneficial to the promotion of EV in the view of its merits, such
as larger charge current and shorter time to charge. But the
rapid charging mode tended to cause a short duration load
fluctuation and a massive impact on the power distribution
grid. And on account of harmonics injection, the power
quality would be reduced.
The influences of EV charging station on the power distribution grid have been studied now. In Refs. [6e9], this problem
was analyzed from two aspects separately, which were power
loss and voltage deviation. And the quantitative evaluation of
* Corresponding author.
E-mail address: 123.gc@163.com (C. Gong).
http://dx.doi.org/10.1016/j.ijhydene.2017.04.181
0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Gong C, et al., Research on influence and resolution of the relay protections with electric vehicle
charging station integrating into distribution network, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/
j.ijhydene.2017.04.181
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
the same issue, by means of quadratic programming model
and dynamic programming model, was proposed in Ref. [10].
Ref. [12e15] presented the harmonic influences on the power
distribution grid from the aspects of simulation model and
control strategy of charging station.
However, there are less studies about the influences of EV
charging station on the relay protection of power distribution
network. And the mechanism of the influences of EV charging
station on the relay protection of power distribution network
was merely mentioned in Ref. [11]. When EV charging stations
are integrated into power distribution network, three-section
current protection is applied [6]. Firstly an equivalent model
of charging station is presented, then the influences of EV
charging station on three-section current protection are
studied under the setting principle of three-section current
protection. Furthermore, when the EV charging station and
normal load are integrated into power distribution grid
respectively, the differences of current and voltage are
analyzed. Considering of the differences between electrical
elements for these two systems aforementioned, a method
based on the low-voltage opened over-current protection, is
presented to improve the sensitivity of back-up protection
device.
Finally a model is built in PSCAD to verify the correctness of
the analysis.
Load model of charging station
Before researching the protection of power distribution
network, the changes of electric parameters of the whole
charging station system need to be analyzed. Because the load
changes greatly, the load fluctuation models are studied in
the cases of both the slow charging mode and rapid charging
mode, respectively. As illustrated in Ref. [4], a holistic
approach to the system model is taken to simplify charging
station model, in consideration that high-frequency charger
is almost in constant power condition during charging
process.
At present, the block diagram of EV charger which is
mainly researched and used, is shown in Fig. 1. Three-phase
ac power source can be converted into direct current by
three-phase-bridge uncontrolled rectifier. And the filtered
direct current is used as input signal of high-frequency dc/dc
power converter. Meanwhile the power battery is charged by
the filtered output signal of dc/dc power converter.
The load fluctuation of EV charging station is comparatively large. The output power of each charger varies
depending on the types of power batteries, state of charge
(SOC), charging mode, and so on. The equivalent structure of
charging station is applied to simplify analysis. Because
high-frequency switches are adopted in charger and load
character is close to pure resistor element, the power factor
of charger is very high, even nears to unit 1, Thus the highfrequency input power converter can be instead by resistor
in the region of low frequency. The correspondence relation
between charging power and equivalent resistance is
expressed in (1).
RC ¼
UB U2B hU2B hU2B
¼
¼
¼
I1
P1
P0
U0 I 0
(1)
where, h is charger efficiency, and I1 and I0 are the input and
output current, respectively.
The equivalent resistance can be calculated by the corresponding values of high-frequency power converter, and
changes with the load of charging station.
The influences on three-section current
protection of distribution power network
Petersen-coil grounded way and neutral non-grounded way
are widely used in distribution power system, which usually
adopts three-section current protection. Considering that
short-circuit current will sharply increase, an current increment protection method is proposed in this paper.
Under the condition that only one side of line current can
be achieved, a nice three-section current protection method is
designed for the purpose of perfect coordination of relay
protection. On the premise that both reliability and selectivity
are guaranteed, the purposes of the first section current protection is to ensure quickness. The goal of the second section
current protection is to ensure sensitivity. And the remaining
section current protection is acted as a backup protection. The
combination of these three sections is typically used for
satisfying four demands (selectivity, sensitivity, rapidity and
reliability). Engineering practices show that the merit of this
protection technology is that the relay operation has satisfactory reliability and high possibility of setting. The influences of EV charging station integrated into distribution
grid on three-section current protection are analyzed as
follows.
Fig. 1 e Block diagram of high frequency charger.
Please cite this article in press as: Gong C, et al., Research on influence and resolution of the relay protections with electric vehicle
charging station integrating into distribution network, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/
j.ijhydene.2017.04.181
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
The influences on current quick-break protection
Taking the current quick-break protection of protection-1 as
an example, when the model of EV charging station integrated
into power distribution network is replaced by equivalent
resistance, a hypothesis that the system keeps in the
maximum operation is applied in this paper.
The setting principle of current quick-break protection is
that the maximum short-circuit current occurred at end of the
line should be avoided. The specific setting formula is
expressed as follows.
IIset1 ¼ KIrel
E
Zs:min þ ZAB
(2)
where, KIrel is reliable coefficient of the current section-I protection, and its value is set between 1.2 and 1.3. E, whose value
is equal to average voltage, is phase electromotive force of
equivalent power source. ZAB is impedance of line-AB. Zs:min is
the minimum impedance between relay location and equivalent power source.
After EV charging station is integrated into power distribution network, the current values in different mode can be
calculated as follows.
The load current is calculated in the maximum operation
as follows.
IL:max ¼
E
Zs:min þ ZAB þ ZBC þ RC
< IIset:1
(3)
At this moment, the current quick-break protection is
reliable without false tripping.
When the fault occurs at point K1 in the range of current
quick-break protection, the fault current is expressed as
follows.
IK1 ¼
E
> II
Zs:min þ ZK1 set:1
(4)
At this moment, the current quick-break protection is also
reliable, as shown in Fig. 2.
Where, ZK1 is impedance between bus-A and short point.
In conclusion, current quick-break protection is not
affected by EV charging station integrated into power distribution network.
The influences on time-limit current quick-break protection
Taking the time-limit current quick-break protection of
protection-1 as an example, the setting principle of this protection (current section-II) is that the end of protection zone
should not exceed the range of current section-I protection of
adjacent line. Of course, that also means the end of adjacent
current section-I protection should be avoided. The setting
formula is expressed as follows.
IIIset:1 ¼ KIIrel Ik:C:max ¼ KIIrel IIset:2
(5)
KIIrel
is reliable coefficient of the current section-II, and
where,
its value is chosen between 1.2 and 1.3.
After EV charging station is switched into power distribution network:
The load current is calculated in the nominal operation as
follows.
IL:max ¼
E
< III
Zs:min þ ZAB þ ZBC þ RC set:1
(6)
At this moment, the time-limit current quick-break protection is reliable without false tripping.
When the fault occurs at point K2 in the range of current
section-II of protection-1, the fault current is expressed as
follows.
IK2 ¼
E
> III
Zs:min þ ZAB set:1
(7)
At this moment, the time-limit current quick-break protection is also reliable, as shown in Fig. 2.
The influences on definite time over-current protection
The setting principle of definite time over-current protection
(current section-III) is that the maximum load current should
be avoided. The setting formula is expressed as follows.
IIII
set:1 ¼
KIII
rel Kss
IL:max
Kre
(8)
where, KIII
rel is reliable coefficient of the current section-III, and
its value is set between 1.15 and 1.25. Kss , which is determined
by network connection and load characteristic, is selfestart
coefficient of motor, and its value is greater than 1. Kre is return
coefficient of current component, and its value range is from
0.85 to 0.95. According to operation mode, the maximum load
current at relay location IL:max is calculated by means of load
flow calculation.
(1) Local backup for protection-1
Fig. 2 e Schematic diagram of single power supply network
short circuit and setting values.
Taking short circuit fault of point K2 as example, if the
main protection of protection-1 is refused, current section-III
of protection-1 should be used as local backup protection to
remove breaker. And the fault current is expressed as
follows.
Please cite this article in press as: Gong C, et al., Research on influence and resolution of the relay protections with electric vehicle
charging station integrating into distribution network, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/
j.ijhydene.2017.04.181
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
IK2 ¼
E
> IIII
Zs:min þ ZAB set:1
(9)
At this moment, the section-III of protection-1 can
correctly operate.
(2) Remote backup for protection 2
Taking short circuit fault of point K3 as example, if the
main protection and the local backup protection of protection
2 are all refused, section-III of protection-1 should be used as
remote backup protection to remove breaker in the position of
protection-1. And the fault current is expressed as follows.
IK3
E
¼
Zs:min þ ZAB þ ZK3
(10)
When charging station is switched into power distribution
grid, it is very easy to cause a increase of load current. And
with the increase of charging station load, the corresponding
load impedance RC will change from very big to very small. If
the maximum load current is considered at the beginning of
relay system configuration, the setting value of current
section-III of protection-1 is very easy to become much
smaller, which will lead to reduction of remote backup
sensitivity. If the relay system configuration planning is went
beyond, it is difficult to meet the requirements of origin
configuration. Meanwhile the miss operation of remote
backup protection will occur. This situation ultimately results
in unreliable protection of the lines.
current. A way to distinguish between over-load and shortcircuit is finding differences of electric quantities of relay
location.
Taking no account of line resistance and system resistance, the voltage values of relay location in these two situations are expressed as follows:
When the short-circuit fault occurs at the short point
shown in Fig. 3, the voltage at the position of protection-1 can
be calculated as follows:
U_ 1:K ¼
E_
XL
XS þ XL
(11)
And the phase relationships between voltage vectors and
current vector in this case are described as shown in Fig. 4.1.
When the over-load fault occurs, the voltage at the position
of protection-1 can be calculated as follows:
U_ 1:L ¼
E_
ðjXL þ RC Þ
jXS þ jXL þ RC
(12)
And the phase relationships between voltage vectors and
current vector in this case are described as shown in Fig. 4.2.
_ C , U_ L ¼ jIX
_ L.
Where, I_ denotes system current, and U_ R ¼ IR
Fig. 4 shows that the short-current in short-circuit state
possesses inductive character. And there is a great voltage dip
of relay location. However, the over-current in over-load state
does not have inductive character, and the voltage dip of relay
location in this state is smaller than the former.
Resolution strategy analysis
Summary
Considering that both setting value and short-circuit current
of current section-I and section-II protection have less to do
with load, the effects of load on these two sections protection
are small.
As a remote backup of adjacent line protection and with
the circumstance in which breaker is miss operation, the
current section-III protects the short-circuit fault occurred at a
different place. Meanwhile it is regarded as local backup for
this line with miss operation of main protection. In general,
the protecting scope of current section-III is much bigger, and
the short-circuit fault is recognized according to the differences between normal circuit and short-circuit.
The fast increase of load will appear when large scale EV is
integrated into power grid. The origin load of residential areas,
that reaches peak at from 19:00 to 21:00, may lead to a sharp
increase of maximum load by adding up charging peak-load [7].
The above process reflected in (4) is the sharp increase of. This
situation induces setting value of current section-III into
increasing greatly. Meanwhile a significant decline of protective
range results in a decease of sensitivity of current section-III. And
the functions of remote backup of adjacent line may even be lost.
The current setting can be decreased by using above characterizes. The process is illustrated as follows: firstly, the
requirement of sensitivity should be satisfied, and then the
low voltage opened over-current protection is formed through
E
jXS
A1
jXL
C
RC
Fig. 3 e Over-load and short-circuit faults of single-powersupply system.
Resolution strategy
Electric quantities of relay location
The common characteristic of over-load fault and shortcircuit fault of charging station is that both states have over-
Fig. 4 e Phase relationships between voltage vectors and
current vector in two circumstances.
Please cite this article in press as: Gong C, et al., Research on influence and resolution of the relay protections with electric vehicle
charging station integrating into distribution network, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/
j.ijhydene.2017.04.181
5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
Table 1 e Short-circuit current of protection-1 under the
conditions of different short-circuit points.
Short-circuit point
K1
K2
K3
Short-circuit current
1.51 kA
1.42 kA
1.28 kA
K3 shown in Fig. 2, the results of short-circuit current is listed
in Table 1.
When the short-circuit fault occurs at point K3, the shortcircuit voltage of protection-1 is calculated as shown in
Table 2. And the low voltage components is set according to
Eq. (14).
Fig. 5 e The logical diagram of low voltage opened overcurrent protection.
Table 2 e Voltage of relay location.
logic “And”, which is consisted of low voltage components and
current components.
The specific procedures are illustrated as follows:
Calculating over-current setting satisfied requirement of
sensitivity.
The expression of over-current setting is:
III
IIII
set:1 ¼ Ksen IK3:min
(13)
a) Calculating low voltage
Short circuit
voltage
3.19 kV
Low voltage
setting
Maximum load
voltage
4.0 kV
5.68 kV
Table 3 e Traditional method and modified method.
Protection
Traditional method
Improved method
Current setting
value
Remote backup
protection sensitivity
0.9 kA
1.4 kA
0.8
1.5
The expression of low voltage setting is:
VdIII ¼ KIII
rel:V VK3:min
(14)
where, KIII
rel:V is reliability coefficient of low voltage, and
>
1.
V
KIII
K3:min is the measured voltage when short-circuit
rel:V
fault occurs at the maximum point in the scope of protection.
b) Forming low voltage opened over-current protection
Base on the above calculations, the logical diagram of low
voltage over-current protection is shown in Fig. 5.
The sensitivity of current section-III protection can be
improved effectively, on the basis that the setting value is
decreased by a low voltage startup module.
Examples and simulation
Taking the model shown in Fig. 2 as example, the shortcircuit model is built in PSCAD. And a charging station is integrated into 10-kV single-power-supply system. The parameters are set as follows: Zs min ¼ j5 U, line-AB length is
5 km, line-BC length is 4 km, per unit impedance of line is
j0.4U. When the equivalent resistance of EV charging station
fluctuates from 100 U to 3.5 U, the maximum load current will
rise to 1.08 kA. In this over-load example, a hypothesis that
the equivalent resistance of charging station equals to 3.5U, is
taken.
The distance between point A and the point K1 is 0.8 times
the total length. And the other points K2 and K3 are the end of
line-AB and line-BC, respectively. Under the minimum operation, when the short-circuit faults occur at point K1, K2 and
Fig. 6 e The comparison results for current of relay
location.
Fig. 7 e The comparison results for voltage of relay
location.
Please cite this article in press as: Gong C, et al., Research on influence and resolution of the relay protections with electric vehicle
charging station integrating into distribution network, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/
j.ijhydene.2017.04.181
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
Table 4 e Comparison between current and setting value of protective installation.
Protection
Protection 1
Protection 1
Protection 1
Protection 2
Short-circuit point
(I segment)
(II segment)
(III segment)
(III segment/far backup)
K1
K2
K2
K3
Current section-III of protection-1 is set and verified
according to the values listed in Figs. 1 and 2. Meanwhile the
results of sensitivity are listed in Table 3.
The above results show that the sensitivity of current
section-III protection is influenced when the charging station
is integrated into distribution grid. And the sensitivity of
backup protection is improved through adopting low voltage
opened over-current protection.
The model shown in Fig. 3 is built in PSCAD. Current and
voltage of relay location of both states, which is short-circuit
and over-load, are simulated. And the comparison results
are shown in Figs. 6 and 7, respectively.
From Fig. 6, both short-circuit fault and over-load state
have very large over-current. From Fig. 7, there is a evident
voltage dip of relay location under short-circuit fault condition, but the voltage dip under over-load fault condition is
fewer.
The comparison between currents and setting values of
relay location are all listed in Table 4.
From Table 4, when EV charging station is switched into
power distribution network, the current section-I and sectionII protection will not be affected. But there may be a sharp
increase of load current, which will lead to an increase of
setting value of current section-III. Therefore if a short-circuit
fault occurs at the end of line-BC, the operating sensitivity of
current section-III of protection 1 will become very low, even
lose the protective function of remote backup of protection 2.
Meanwhile as shown in Figs. 3 and 4, when the setting value
listed in Fig. 3 is adopted, the protective sensitivity will increase greatly and satisfy the requirement of remote backup
protection.
The low voltage opened over-current protection, formed by
voltage of relay location, can improve effectively sensitivity.
Conclusion
The charging station equivalent models of slow charging
mode and rapid charging mode are analyzed firstly in this
paper. Then the equivalent model of power distribution grid
with charging station is established. Considering that the
relay system configuration for distribution grid is usually very
simple, the influence of EV charging station on three-section
current protection is analyzed. The analysis indicates that
setting values and short-current values of current section-I
and section-II protection have little relationship with load,
however section-III is affected evidently by load. And the influence of local backup protection of this line is smaller than
remote backup protection of the next line.
Analyzing that electrical quantities of relay location under
short-circuit fault and the maximum load current situations,
Measured value
1.59
1.49
1.49
1.17
kA
kA
kA
kA
Setting value
1.9
1.7
0.9
0.9
kA
kA
kA
kA
Result
Startup
Startup
Startup
Low sensitivity
the voltage dip of short-circuit fault is greater than the other
situation. Taking use of above properties, a low voltage
opened over-current protection is adopted to improve the
sensitivity of backup protection. Meanwhile the examples and
simulation results verify the correctness of the theory proposed in this paper.
With the fast development of EV charging station, the
influences of charging station on power distribution grid
become bigger and bigger. Analyzing the influence of distribution grid with charging station and proposing corresponding strategy, have important implications for the
development of EV and the secure and stable operation of
power system.
Acknowledgment
In this paper, the research was sponsored by the National
High Technology Research and Development Program (“863”
Program) of China (No. 2011AA05A109).
references
[1] Liu Wen-feng, Zhu Cong, Li Bin. Status and development
proposals of standards about electric vehicle charging/
battery swap infrastructure in China. Transp Res
2015;1(6):1e7.
[2] Huang Run. Impacts of electric vehicles charging on the load
of power system. Shanghai: Shanghai Jiao Tong University;
2012. p. 14e8.
[3] Ouyang Jian, Li Di, Liu Jun-cheng. The overview of research
of power battery SOC estimation methods use for electric
vehicle. Mech Electr Eng Technol 2016;45(1):52e7.
[4] Qian Kejun, Zhou Chengke, Allan Malcolm, Yuan Yue. Load
model for prediction of electric vehicle charging demand.
In: International conference on power system technology;
2010.
[5] Gu Yue. Study on off-board electric vehicle charger and its
electrical performance test. Beijing: Beijing Jiaotong
University; 2012.
[6] Ma Lingling, Yang Jun, Fu Cong, Sun Yuanzhang. Review on
impact of electric car charging and discharging on power
grid. Power Syst Prot Control 2013;41(3):140e8.
[7] Sunxing, Lichunping, Jiangnan. Stablity analysis of power
system consider the large-scale electric vehicle access. Appl
Electron Tech 2015;42(11):149e53.
[8] He Jinhan, Xie Yuyu. Influence of electric vehicles charging
modes on active network distribution. Electr Power Constr
2015;36(1):97e103.
[9] Hu Zechun, Song Yonghua, Xu Zhiwei. Impacts and
utilization of electric vehicles integration into power
systems. Proc CSEE 2012;32(4):1e10.
[10] Chen Lin, Zhong Jin, Ni Yixin, Gan Deqiang. Optimal reactive
power planning of radial distribution systems with
Please cite this article in press as: Gong C, et al., Research on influence and resolution of the relay protections with electric vehicle
charging station integrating into distribution network, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/
j.ijhydene.2017.04.181
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
distributed generation. Automation Electr Power Syst
2006;30(14):20e4.
[11] Guo Yuhua, Fan Chunju. Research on relaying technologies
of distribution network including mass electric vehicles.
Power Syst Prot Control 2015;43(8):14e20.
[12] Liu Yuexian, Wang Tianyu, Yang Yayu. Modeling and
simulation of electric vehicles' charge and discharge system.
Power Syst Prot Control 2014;42(13):70e6.
[13] Li Ruisheng, Wang Xiaolei, Zhou Fengquan, Li Xianwei. The
system of electric vehicle intelligence charge station with
7
smart power flow control. Power Syst Prot Control
2010;38(21):87e90.
[14] Zhang Qian, Han Weijian, Yu Jihui, Li Chunyan, Shi Lefeng.
Simulation model of electric vehicle charging station and the
harmonic analysis on power grid. Trans China Electrotech
Soc 2012;27(2):159e64.
[15] Guo Wei, Wang Yue, Wang Zhaoan, Dong Xiao,
Wang Fengren, et al. Control strategy for PWM rectifier in
electrical vehicle charging station. Trans China Electrotech
Soc 2012;27(2):153e8.
Please cite this article in press as: Gong C, et al., Research on influence and resolution of the relay protections with electric vehicle
charging station integrating into distribution network, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/
j.ijhydene.2017.04.181