2011The International Conference on Advanced Power System Automation and Protection
Review of Impact of Distributed Generation on Distribution System
HOU Shuang, GAO QinXiang
School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China
Abstract˖Introduces distributed generation (DG), describes the types of DG and its benefits, analyses the prospect of distribution system access DG. As the particularity of the distributed power, DG will produce a series of important influence on
the distribution network. Mainly introduces three effect areas, DG effects distribution network voltage, distribution grid planning and relay protection, then puts forward the main causes and effect solutions.
Keywords˖distributed generation (DG), distribution system, voltage distribution, distribution grid planning, relay
protection
DG accesses systems in two ways, as shown in Figure 1 below.
(a) shows DG access the grid in parallel, (b) shows that by
switching to the grid. When DG connected to the grid in parallel,
under normal circumstances, the system bear the load, DG is in
hot spare mode, which can prevent power interruption. When DG
accesses the system by switching, if the main power outage happened, a brief power interruption occurs.
1 Introduction
The distributed generation technology is that the power in the
range of tens of kilowatt to ten megawatts, module type, distributed in the neighborhood of load, it's clean and environmental
power facilities. And can produce economic, efficient and reliable
power. DG is different from the traditional centralized power
generation, long-distance transmission, a large network of power
generation in the form. DG prime mover can be gas turbine, internal combustion engines, wind turbines, solar photovoltaic, fuel
cells, energy storage and other forms of energy conversion devices [1].
DG forms include wind power, photovoltaic battery, fuel cells,
miniature gas turbine, energy storage devices, etc. With the increasing maturity of the DG technology, the distributed power,
with its flexible, environment-friendly and other advantages are
access to distribution network system more and more, power
distribution network structure and operation have a significant
influence. It calculates the reliability of the distributed power
generation system, it is concluded that the distributed power as a
backup power supply can improve the reliability of the distribution network [2]. However, if DG and power grid run
grid-connected, it could reduce the parallel operation of the reliability of the system. This is because for a system that contains a
large number of DG, if the coordination between DG is poor, DG
will reduce the reliability of the system [3].DG penetration may
have numerous impacts on distribution networks, positive or negative. Concerned issues include energy losses, voltage control,
fault levels, etc.[4].Connection of distributed generators directly
to distribution systems has become a common practice worldwide. The connection of DGs brings a great change to configuration of the utility distribution network. As a result, this leads to a
big challenge for its control and protection system[5].This paper
mainly studies on DG effects distribution network voltage, power
grid planning and relay protection.
(a) DG access the grid in parallel
(b) DG access the grid by switching
Figure 1
The ways DG access the grid
2 Distribution network voltage
Voltage remains within the prescribed limits to ensure power
quality. The distribution system pressure regulating rules is based
on the trend of user-based flow from the substation, but when
access to the distributed power grid, the power distribution system changes from the radial structure to more power structure, the
APAP2011 www.apap2011.org
*Corresponding author (email: 10125620@bjtu.edu.cn)
___________________________________
978-1-4244-9621-1/11/$26.00 ©2011 IEEE
2011The International Conference on Advanced Power System Automation and Protection
size and direction of the trend likely to have enormous changes,
so that some parts of the power system voltage changes [6].
Qi
When a certain capacity of DG access the distribution network,
the voltage of feeders will be impacted. The specific impact has
important relationship with the total capacity, size and position of
DG. In general, the closer to the end node, the rate of change of
voltage is greater and the influence of the injection power of DG
is larger. In the distributed power generation with DG, where
contains a bigger rate of change is the place that the reactive
power compensation and voltage support should be taken into
account firstly. If the DG is larger than the network load, at this
time voltage of the system, in order to adapt to the higher voltage
that because of active, reactive power of DG injected should be
reduced. In order to maintain high power factor operation, under
normal circumstances, DG should produce more active power
and less reactive power. In the access locations of DG, install
some appropriate reactive power voltage support devices, it can
put into operation when DG run out. When the feeder has line
voltage regulator, if we want to reach the expected goal of voltage
regulation, we need to properly adjust the position of tap[7].In
short, when DG access to the distribution network, the voltage of
distribution network will be affected, we should take active
measures to minimize the harm to the system, and improve the
system power quality. We can also make full use of the ring operation of a distribution network. [8].
reactive power loss and active loss of branch i are as follows:
' Pi
'Qi
U1
R1 jX 1
U2
R 2 jX 2
Figure 2
Figure 2 shows that if no DG added, at the node i the active and
reactive power are:
(1 d i n )
Q i
U i2
2
Xi
(4)
P i
­
°
Pn
°
n
n
°
Pj ¦ ' Pj
®
¦
j i
j i 1
°
n
° n
P
° ¦ j ¦ ' P j PD G
j i 1
¯j i
Q i
­
°
Qn
°
n
n
°
Q
® ¦ j ¦ 'Q j
j i
j i 1
°
n
° n
° ¦ Q j ¦ ' Q j Q DG
j i 1
¯j i
i
n
(k i n )
1 d i d k
i
n
(k i n)
1 d i d k
The main task of distribution system planning during the network
planning is based on the results of load and the basic situation of
the existing network to determine the optimal system-building
programs. Under the premise that the construction of power distribution system and secure supply of electricity to minimum the
cost of distribution system construction and operation [10].At
present in the distribution network planning includes DG need to
be solved the following several problems. First, some kinds of
DG (mainly by using renewable energy to generate electricity,
such as solar energy, wind energy), its output is often have more
fluctuation compared to the traditional power plant. And the fluctuation due to climate and other natural conditions, can't be regulated effectively, therefore their output has a obvious random
characteristics. Necessary to study this type of distributed energy
with the variation of weather conditions and statistical properties,
establish the corresponding model. Second, in the electricity
market environment, the users may install a distributed power can
Un
n)
2
3 Distribution grid planning
Multi-node distribution system
(i
Pi
use the calculated power, at the node i the voltage that before and
after DG access system can be calculated by load flow calculation.
R n jX n
R k jX k
Ri
2
(6)
Pn jQn
Uk
Q i
U i2
(5)
network shown as figure 2. On the node k in the graph, there is a
DG with a power injection of PDG jQDG [9].
U0
2
after DG added in node k, at the node i the active and reactive
power are:
road the branch will be numbered, followed by l,2,Ă,n, line loss
of the corresponding branch is 'Pi j'Qi . The distribution
Pk jQk
P i
(3)
way, from the bus 0 of the system power for l,2,Ă,n , and the
load power of the corresponding node for Pi jQi . Along the
P2 jQ2
(i n )
(1 d i n )
(2)
Briefly introduce the changes of voltage that DG access distribution network. The line can be divided into n section, for a
short, the impedance is Ri jX i . Number the nodes along the
P1 jQ1
Qn
­
°n
n
® Q 'Q j
¦
¦
j
°̄ j i
j i 1
(1)
2011The International Conference on Advanced Power System Automation and Protection
radiation type, trunk type and ring network type. Currently, the
majority in our country distribution system is using radial wire;
this is mainly because of the simplicity of wiring operation and
the economy of over current protection. When DG access distribution network, the distribution network structure will change,
it will influence relay protection as follows.(1) Due to the access
of DG, the protection of upstream of the access point will produce
reverse current, causing such protection all action when upstream
and downstream fault reach the set values, thus losing the selectivity. At this time If downstream of DG access points have failure, short-circuit current flowing through the protection will be
reduced, which the distributed power has the role of split-flow,
when the shunt is large enough, the sensitivity of protection will
reduced, for time limit current protection, the scope of protection
may not cover their full range.(2)When the access points of DG is
in the upstream of protection and failures in the downstream of
protection, DG will have a positive fault current, fault current
distribution system will play an increasing role in flow, resulting
in that fault current flow will increase, so the range of protection
also will increase. When the capacity of DG is greater, the role in
increasing flow is greater, as increasing flow to a certain degree,
for the time limit current protection, the scope of protection will
be extended to the next level of the adjacent lines, which lead to
that the protection will lose selectivity [14].
The influence of DG on the relay protection is divided into
several areas, the impact of three-current protection, the impact of
inverse time overcurrent protection and the impact of automatic
reclosing. Three-current protection is simple in principle and has
high reliability, so is widely used in the protection of distribution
network. When DG put into the distribution network, the trend of
system will be redistributed. The time short-circuit fault occurs,
the size and the flow of fault current will change dramatically.
Obviously, DG access in different positions, the size and flow of
fault current will be different, thus the protection action behavior
is different. Inverse time over current protection is a protection
that action limit time relates to the size of a fault current in the
protected line. Compared with the three-current protection, it can
be realized with only a relay, and when the short-circuit fault occurs in system, the actual time of remove the failure of every
point is short, so it is also commonly used in power distribution
system line protection[15].Also, different DG access points positions have different influence on inverse time overcurrent protection. Automatic reclosing is widely used in the distribution network. Before DG access distributed network, automatic reclosing
recloses breakers which occur transient fault, at this time it will
not cause too much impact on the system, the fault line generally
can resume normal power supply, can ensure grid reliability well.
But after DG access into the distribution network, when the line
occurred transient fault, DG is likely faille out of line, and continued to transfer current to the point of failure, so will lead to the
point of failure has sustained arc, which eventually led to the automatic reclosing failure [16]. Because of the incorporation of DG,
automatic reclosing will occurs to risks such as Continuous arc
fault and asynchronous closing etc, so it should be installed the
offset the power load directly, so as to impact the entire power
system load growth patterns, the result is impact the expansion
scale and progress of the power. Therefore, we must study the
influence that DG of the user side on the load of the grid side. The
third, the reasonable scale, the distributed power grid planning
and expand the stationing is worth studied. Study on the Cooperation between traditional and non-traditional compatibilizing
method and its technical and economic evaluation method. Research the capacity of DG that the electric power system will
accept. Also, an environment with distributed power supply under
low-voltage distribution system optimization planning is worth
researched, and research in the reliability that distributed power
supply for low-voltage distribution system [11].
DG affects power grid planning economy not only related to
their own technical parameters, generating cost, but also the network structure, natural environment closely related. Overall
speaking, make DG into the network planning and optimize its
reasonable choice, in this way, it can bring considerable economic
benefits, and lower the cost of DG power generation. The better
the natural conditions that DG installed, the higher the planning
economy considered of DG[12]. At this stage, the distributed
generator types and the diversification of energy sources that DG
used, making the reasonable power structure in distribution network and how to coordinate and effectively use various types of
power supply an urgent problem to be solved.
When DG is wind power generation, it is a process that wind
energy into mechanical energy and then converted into electrical
energy. Wind generator rotor is the most important part, it is made
up of the blade and hub components [13]. As shown in figure 3.
Figure 3
Wind turbine energy flow
Wind power grid affects the trend of the distribution obviously. Under
normal circumstances, wind speed in wind farms according to Weibull
distribution:
f v
k §v·
˜¨ ¸
c ©c¹
k 1
ª § v ·k º
˜ exp « ¨ ¸ »
«¬ © c ¹ »¼
(7)
After know the wind speed distribution, we can find the corresponding approximation relationship between wind speed and
output power of wind power generators, so conclude the random
distribution function of the output power, and conclude the influence on trend that it merged into the grid.
4 Relay protection
In The distribution network topology structure mainly includes
2011The International Conference on Advanced Power System Automation and Protection
low-cycle, low auto-splitting device at the side of DG. Before
the movement of reclosing, excise DG from the fault line, and to
avoid the continuing impact of the arc at the point of failure, the
action limit time of reclosing should be appropriately extended
[17]. DG is one of the main direction that power system develop,
but a large number of DG access into the network will profoundly
affect the distribution network structure, and the short-circuit
current size and direction of distribution network, so bring a lot of
negative effects to relay protection [18].Experts and scholars have
carried out researches for this problem, the aim is to solve the
problems that when DG access distribution network caused.
The effect that DG access distribution system is shown in Figure 4, short-circuit fault occurred in the F5. Before the DG access,
the current that protection 4 feel is the fault current. When DG
access, the protection 4 will fell the infeed current that provided
by DG, in this way, outside the scope of protection when
short-circuit occurred, protection 4 still feels the bigger current,
lead to that this line protection range is too big.
many, although there are also disadvantages, but we should take
advantage of the characteristics of DG so that it can serve the
power system better.
1
2
3
4
5
6
7
8
9
10
11
12
Figure 4
Effect on the scope of protection that DG access distribution system
13
5 Conclusion
14
When DG and distribution networks parallel operation, the advantages of distributed generators can given full play, reduce
pollution emissions and improve the utilization ratio of energy,
improve the power supply reliability, reduce the power network
loss. But with the continuous development of DG, its influence on
the distribution network also gradually obvious. This paper discusses when DG access to distribution network the change of
distribution network voltage, distribution grid planning and relay
protection, and also proposed some solutions. In our country the
development of DG is still not perfect currently, but we believe
that in the future the advantage of DG will be fully expressed, and
make contributions to the stable, reliable, efficient operation of
the power system.
In the following work, it needs further study on the specific effects after DG access distribution system, such as voltage drop,
voltage fluctuation and flicker all have significant impaction on
the system. The benefits that DG access distribution system are so
15
16
17
18
Wang Z J, Zhu S Z, Zhou S X, etal. Impacts of distributed generation on distribution system voltage profile[J].Automation of Electric Power Systems,2004,28(16):56-60
James AM, Robert AS. Distribution system reliability in a deregulated environment a case study[C]. Proceedings of IEEE PES, Transmission and Distribution Conference and Exposition, Dallas, USA,2003
Sui X X, Wang Q, Yang Y Q. Study on impact of distributed generation on
distribution system reliability[J].Journal of Electric Power, 2010, 25(1): 56-60
V. H. Mendez, J. Rivier, T. Gomez. Assessment of energy distribution losses
for increasing penetration of distributed generation [J]. IEEE Transactions On
Power Systems, vol. 21,2006:533-540
J. Sadeh, M. Bashir. Effect of distributed generation capacity on the
coordination of protection system of distribution network.2010 IEEE/PES
Transmission and Distribution Conference and Exposition: Latin
America:110-115
Wang T, Lan S L. Impacts of distributed generation on distribution network[J].East China Electric Power,2010,38(7): 1038-1039
Wang Z Q, Zhu S Z, Zhou S X. Study on location and penetration of distributed generations [J]. Proceedings of the CSU-EPSA, 2005, 17(1): 53-58.
Sami R , H Laaksonen, P Jhentausta, etal. A case study of a voltage rise
problem due to a large amount of distributed generation on a weak distribution network[J].Paper accepted for presentation at 2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy
Liu L, Jiang H, etal. Impact of distributed generation on distribution system
power loss and voltage profile [J]. Computer Simulation, 2010,
27(4):279-283
Miao W S, Zhang J H, Zheng Y X. Influence of distributed generation on distribution system[J].YAN JIU YU FEN XI,2010, 12(9):33-36
Wang M, Ding M. Distribution network planning including distributed
generation [J]. Proceedings of the CSU-EPSA, 2004, 16(6):5-9
Gao J Y, Ma X F. Distribution network expansion planning including distributed generation[J]. Electric Power, 2010, 43(2):39-43
Yang XY, Duan JD, etal. Study on the distributed generation impact on
power flow[J].Power System Protection and Control, 2009, 36(8):25-29
GUO M, AI Q. Analysis on the influence of connecting distributed generation on the relaying of distribution system. GON YONG DIAN, 2010,
27(1):1-3
HE J L, SONG C J. Theory of power system relay protection, Revised
Edition [M]. Beijing: China Electric Power Press,2004ˊ
Wen Y D, Wang X. Effect of distributed generation on protection in distribution system[J]ˊRelay, 2008, 24 (7):12-14
Pang J Y, Xia Xi B, Fang M. Impact of distributed generation to protection
of distribution system[J]ˊRelay, 2007, 35(11):5-8
Zhou W, Zhang Y, Xia C J, etal. Effect of distributed generation on relay
protection of distributed system[J].Power System Protection and Control,
2010, 38(3):1-5