Engineering, 2013, 5, 19-23
http://dx.doi.org/10.4236/eng.2013.59B004 Published Online September 2013 (http://www.scirp.org/journal/eng)
Copyright © 2013 SciRes. ENG
Analysis of Switching Over voltage in Regional Grid with
Small Hydropower Plants
Juan Chen, Haifeng Li
Electric Power College, South China University of Technology, Guangzhou, China
Email: 362360029@qq.com
Received June 2013
ABSTRACT
Present-day small hydropower plants (SHPs) have a large development potential because of the increasing interest in
renewable resources and distributed energy generation, therefore, there are many SHPs in places of China where are
rich in water resources. However, it has caused overvoltages in the distribution network, and which is even worse for
the switching overvoltage such as isolated network operation, changing power supply path. The simple network model
is used to analyze the reasons of the switching overvoltage, and the simulation software DIgSILENT/PowerFactory is
used to check out the results of the theoretical analysis.
Keywords: Small Hydropower; Isolated Operation; Changing Power Supply Path; Switching Overvoltage
1. Introduction
Hydropower is an important part of global energy pro-
duction with a rapid growth in recent years. Therefore,
there is a strong demand for the development of SHP
(short for small hydropower) for making the most of
clean hydropower. Ended to 2005, there are 50,00 0 SHPs,
widely distributed in more than 30 provinces in China,
the capacity of which is up to 38,530 MW [1]. Th e SHPs
are constructed in remote mountainous regions, besides,
they are primarily runoff hydropower plants defined as
providing little or no water storage, of which the active
power is nonadjustable, and hard to control [2,3].
Because of the long power transmission distance, the
small diameter of the transmission line, and the suppres-
sion of the grid voltage, overvoltage and high power fac-
tor are common problems of the SHPs [4]. What we
should pay more atte ntion to is, when the primary power
supply line is cut off, the SHPs still work as an isolated
network with the local load. If the power of the SHPs is
more than that of the loads, the isolated network will
suffer overvoltage and high frequency with damages to
the electrical equipment. Additionally, when the primary
power supply line requires maintenance, the SHPs should
change to another power supply path to prevent the loads’
power from being cut off, in case that the power supply
path is changed to a longer one (only this kind of chang-
ing power supply path is discussed this paper), which
will lead to overvoltage as well.
Currently, researches of SHP mainly focus on two
parts, the one is the reasons for overvoltage and high
power factor, including large transmission losses, non-
adjustable step-up transformer, and fierce conflicts be-
tween the dry season and the wet season [5]; the other is
the assessment of SHP’s reactive power based on optimal
power flow (OPF) [6]. Although the overvoltage problem
of SHP has been involved, there’s no rigorous theoretical
derivation for the switching overvoltage especially in
isolated operation and changing power supply path.
Therefore, in this paper, theoretical analyses are made
for the two kinds of switching overvoltage by the simple
network model; and the results of theoretical analysis are
verified by the simulation software DIgSILENT/Power-
Factory, meanwhile, some effectual advices are proposed
to restrain the switching overvoltage.
2. Analysis of Overvoltage in Region Grid
with Small Hydropower
The simple network model given in Figure 1 consists a
synchronous generator, a step-up transformer, the trans-
mission line, a load and the external network. The syn-
chronous generator runs as an isolated network with the
local load when the breaker connected to the external
network and previously closed is open. Without auto-
matic excitation regulator, the SHP can be equivalent to
be a constant voltage source with resistance in th eoretical
analyses.
2.1. Overvoltage in Isolated Operation
The regional grid always runs as an isolated network
when the primary po we r s u pply line is c ut off becau se of
J. CHEN, H. F. LI
Copyright © 2013 SciRes. ENG
20
Figure 1. Model of a simple network.
faults or other reasons. The voltage of bus3 shown in (1)
can be deduced by the model of the simple network.
( )
( )
2
coscos 4
2
q qVV
S
EEK RPXQ
V
K
δδ
ΣΣ
+ −+
= (1)
where,
( )
( )
22
1
LDLDLD LD
KRRXX RX
ΣΣ
=++ +
,
q
E
the no-load voltage of the generator,
δ
is the angle be-
tween
q
E
and S
V
,
Z
Σ
is the total impedance of the
synchronous generator, transmission line and step-up
transformer.
From (1), it can be seen clearly that if
V
P
and
V
Q
reduce to 0, the voltage of bus3 will rise sharply. Actua l-
ly, the SHPs supply power to both the local loads and the
external network (means
,
0
V
Q>
) in wet season,
when line fault (leading to the isolated operation) occurs
frequently by the thunderstorm. Thevenins theorem can
be used in the complex electricity network, so that the
complex network can be equivalent to a constant voltage
source with resistance, which is the same as the simple
network, so does the analysis method and the conclusion.
2.2. Overvoltage in Changing Power Supply Path
To ensure power distribution reliability, the SHPs change
to another power supply path when the primary power
supply line requires maintenance. Figure 1 shows that
the voltage of bus connected to the external grid is S
V
,
the power at
q
E
is
P jQ+
:
( )
2
2
cos
q qS
REZ EV
PZ
ϕδ
ΣΣ Σ
Σ
−+
=
(2 )
( )
2
2
sin
q qS
XEZ EV
QZ
ϕδ
ΣΣ Σ
Σ
−+
=
(3)
where
ϕ
Σ
is the impedance angle of
Z
Σ
.
The voltage S
V keeps invariable for the suppression
of the external grid, total impedance
Z
Σ
increases.
From (2) we can know that the electromagnetic power of
SHP will reduce, while the mechanical power holds
changeless because the water inflow keeps the same, thus,
the electromagnetic power is less than the mechanical
power, so that the rotor speeds up, and the power angle
rises up.
The resistance is almost equal to the reactance in dis-
tribution network, so we can’t ignore the resistance while
making theoretical analysis. In addition, more than 90%
of
Z
Σ
is the transmission line impedance, so the power
angle is about 50˚. Both the increase of the power angle
δ
and the total impedance
Z
Σ
will decrease the reac-
tive power (see (3)).
At this stage, the power management department take
power factor as assessment for the SHPs, if the SHP ge-
nerates less reactive power than that required, each 3 Kv
rah is equal to the benefit of 1 kWh active power as a
punishment. Based on the analysis above, the power fac-
tor gets higher and can’t meet the assessment require-
ment in changing power supply path, as a consequence,
the excitation voltage of the SHPs are raised up.
The reactive power against terminal voltage of SHP
can be expressed as (4) according to (3):
( )
2
2
sin
Sq SqGS
S
XEZ EV
QZ
ϕδ
−+
=
(4)
meanwhile
( )
22
sin
Sq S
GSq S
XEZ Q
VZE
ϕδ
=
+
(5)
where
S
X
is the internal reactance of generator,
S
ϕ
is
the impedance angle of
S
Z
,
δ
is the angle between
q
E
and
G
V
.
The impedance angle of
S
Z
is close to
0
90
because
of the small internal impedance of the generator. From (5)
we know that both the increase of
δ
and Q lead to the
increase of
G
V
, similarly, all the bus voltage shown in
Figure 1 will increase, then the whole network suffers
from overvoltage while the SHPs’ excitation voltage is
raised up at the same time.
3. Case Study
The Shaoguan grid is rich in hydropower resources,
where the installed capacity of hydropower has reached
1700 MW at the end of 2005. We select Bibei town
Ruyuan district Shaoguan as the overvoltage verification
point, because the available water resources reserves
amount to 104,500 kW (theoretical potential), the actual
available water is up to 76,000 kW and currently there
has developed two medium-sized hydropower stations
(Hengxi plant) and more than 10 SHPs. Figure 2 shows
the distribution network of 10 kV F36 Bibei line of
Hengxi plan t and 10 kV D aqiao lin e of Daqia o s ubs tation ,
and it has been built for simulation experiments in DIg-
SILENT/PowerFactory. Overvoltages are defined in per-
unit value s (p.u.).
3.1. Overvoltage in Isolated Operation
We set a short circuit on the line between Daling and
Wangcha at 30 s, and the breakers at each end of the line
(without the function of automatic reclosing) opens, that
makes the areas including Wangcha, Nakeng, Qingshi,
J. CHEN, H. F. LI
Copyright © 2013 SciRes. ENG
21
Figure 2. Distribution network of 10 kV F36 Bibei line of Hengxi plant and 10 kV Daqiao line of Daqiao substation.
Ma’ao operate as an isolated network with the 10 SHPs
in 5 hydropower plants, such as Wangcha plant, Hongyu
plant, Nankeng plant, Yongyuan plant and Agongtan
plant. The bus voltage of Wangcha and Nankeng are
shown in Figure 3:
We can see it clearly that the buses’ voltage are all in-
creased, the bus voltage of Wangcha rises to 2.342 p.u.
from 1.056 p.u., while the bus voltage of Nankeng is up
to 2.340 p.u. from 1.066 p.u. Thus, the isolated operation
results in serious overvoltage.
3.2. Overvoltage in Changing Power Supply Path
The contacted switch named Changshanzi changes the
state from cold standby to hot standby, as a result, the
power of Bibei Town is supplied by Daqiao substation
instead of Hengxi plant. We disconnect the primary
power supply line by opening the outlet cir cuit b reaker of
Hengxi plant at t = 30s, and close the Changshanzi
breaker at t = 32s to simulate the process of changing
power sup ply path.
It is obvious in Figure 4 and Figure 5 that the bus
voltage of Daling reaches the value of 1.21 p.u. from
1.037 p.u., and the bus voltage of Wangcha is up to 1.202
Figure 3. Bus voltage of Datangkeng and Guikeng in iso-
lated operation.
p.u. fro m 1.034 p.u. On the other hand, the reactive pow-
er of Zhaimian plant is reduced from 0.105 MVar to
0.069 MVar, the power factor is over the assessment
scope by rising from 0.925 to 0.966, which makes the
excitation voltage raised and the who le network overvol-
taged.
J. CHEN, H. F. LI
Copyright © 2013 SciRes. ENG
22
Figure 4. Bus voltage of Daling and Wangcha in changing
power supply path.
Figure 5. The reactive power of Zhaimian plant in changing
power supply path.
4. Some Effective Measures to Restrain
Switching Overvoltage in Region Grid
There are many measures to restrain overvoltage, the
following three must be more effective.
1) Put into intelligent automatic bus transfer equip-
ment. The quality of voltage and frequency in the iso-
lated network are too poor which can often damage the
electrical appliances. The conventional automatic bus
transfer equipment has the defect of needing more time
to detect bus non-voltage, at the cost of islanding all the
SHPs and taking a very long time to restore power supply.
In this case, not only the power quality is poor, but also
the reliability of power supply is low. The automatic bus
transfer has two modes, the one is that the SHPs are
quickly selected and tripped in low water period, while
the other one (in the mode of high frequency) is that the
SHPs are tripped round by round and the automatic syn-
chronous capture reclose in high flow period, which
enables regional power supply to restore more quickly
and has greatly improved the quality of frequency and
voltage [7].
2) Equip the small hydropower with automatic excita-
tion regulator. As we all know that the AVR (short for
automatic excitation regulator) can regulate the voltage
automatically, better distribution of the system’s reactive
power. It is clear from formulas (2) and (3) that
q
E
has a
significant effect on the active power and reactive power
of the generators. The unbalanced power which can
cause great disturbance to the system and is not condu-
cive to the stability of the system can be reduced by the
regulation of the AVR. Putting into the AVR for SHPs
has significant effect on restraining overvoltages.
Install the voltage control terminal of smart distribu-
tion grid in the load side. The voltage control terminal
makes use of the modern power electronic technology,
and its primary components include silicon controlled
rectifier and the PWM inverter, which can effectively
achieve the isolation and control of the voltage. The ter-
minal can convert the unqualified voltage into the one
that become available to be used by the user directly, so
as to keep the overvoltages from damaging the electrical
equipment effectively.
5. Conclusions
On the basis of the analysis of theory and simulation in
overvoltages, the following conclusions can be drawn:
1) There will be severe overvoltages in isolated opera-
tion, changing power supply path and stepping into the
wet season.
2) Some effective measures have been proposed to re-
strain overvoltage in region grid, like putting into intelli-
gent automatic bus transfer equipment in order to restore
power supply more quickly; equipping the SHPs with
AVR to regulate the too high or too low voltage auto-
matically; and installing the voltage control terminal in
the load side to prevent the overvoltages from damaging
the electric equipment.
6. Acknowledgements
The work described in this paper is supported by
Guangdong Special Fund Project of Industry, University
and Research Institute Collaboration (2011A090200127,
2011A090200074).
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