Download Technical Constraints in Using Controlled Series Compensation for

Technical Constraints in Using Controlled Series Compensation for the
Optimization of Electrical Power Networks Operation
MOTI BIRJANDI A.A. , RYZHOV Yu.P .
Electrical power systems department
Moscow power engineering institute
(Technical university)
RUSSIA
Abstract : -The main technical constraints are considered which limit the range of controlled series
compensation of electric transmission line. The compensation limits are determined by the conditions of
line conductor heating, capacitor voltages and line efficiency.
Keywords: - FACTS, Controlled series compensation, Degree of compensation, Technical constraints,
Power systems.
1
Introduction
Transmission systems are becoming increasingly
loaded because of growing demand and restrictions
on new line construction.However most high voltage
transmission systems operate below their capacity.
Electric power systems (EPS) development is
caused by the increase in demand and power
generation, and it is accompanied by growing
complexity of existing network. Increasing power
flows in high voltage EPS and distribution networks
result in a number of new problems connected with
the
further development of power systems. These
problems are as follows:
• The necessity of efficient distribution of
powerflows between high voltage lines that
have higher transmission capacity, and lines
with lower voltage rating, which operate in
parallel with the high voltage lines.
• Increase in power transmission capacity of the
lines in transmission and distribution networks, since it can be more economical, than
construction of new lines, however it is
necessary to note that such increase is required
only when there is maximum load period.
• Existing rural and urban areas and unfavorable
land topography constitute problems facing
new transmission lines construction. Load
demand continues to increase despite these
difficulties. This growth has been
practically stabilized in developed nations,
but political and
environmental
constraints
make
the
construction of new lines difficult and
forces electrical utilities to a better use of the
existing network.
The problems mentioned above can be
solved by FACTS (Flexible AC Transmission
Systems). It may be noted, that FACTS changes
transmission line load flows and control EPS
reactive power balance. Hence FACTS can
stabilize nodal voltages [1].
However the use of FACTS in EPS, gives
rise to technical difficulties, which in turn limit
the control range of the devices. Technical
constraints
arising from the use of CSC, TCSC, STATCOM,
UPFC are considered below.
2
Configuration of controlled series
components
At present there exists a number of ways to
compensate line reactance.
The first of them changes the degree of compensation by shunting separate sections of series
connected capacitor banks by thyristor valves (CSC),
see Fig. 1, a, [2].
The second choice is a device consisting of
capacitor banks shunted by controlled reactors which
are switched in series to the line (TCSC), see Fig. 1,b.
Reactor control can be affected by a thyristor unit,
magnetic filed saturation, etc[3].
According to the third method of controllable
series compensation, the source of variable voltage is
introduced in the line which corresponds to the
capacitor voltage in phase and value and it is varied in
accordance with the system operational requirements.
This voltage can be introduced into the line with the
help of a transformer connected in series to the line.
Any controllable source of reactive power can be
connected to the secondary winding of the transformer,
see Fig.1 ,c (UPFC)[4].
Fig. 1: Controllable series components,
a) CSC, b) TCSC, c) UPFC
The majority of works on flexible lines are
devoted, basically to the analysis of the devices'
characteristics. At the same time, the networks and
systems aspect of using these devices have not
been given enough attention. In known literature,
as a rule, the active resistance, capacitive
susceptances, distributed parameters of the lines,
were not taken into consideration. An attempt to
take into account parameters of actual lines,
including long lines, is outlined below.
3
Constraints
Consider the constraints, which arise when
controlling the degree of compensation. When
degree of compensation increases then line
transmission capacity increases too and
consequently phase currents and temperature of
conductors increase. The maximum permissible
value of current can depend on cooling conditions
of conductors and of course on the ambient
temperature. Therefore permissible degree of
compensation should be selected with due account
for such constraints.
Another known constraint is that increase in
the degree of compensation gives rise to the
increase in voltage at the terminals of the
condenser, which under certain conditions can
reach the maximum permissible value. The same
phenomena will be observed when using
transformer device (Fig.1,c). Two factors
determine the increase in voltage. First of them, is
the operation conditions of a capacitor bank with a
variable capacity connected in series with the
inductance of a line. Second one is due to the fact
that with increasing degree of compensation,
compensated portion of a line inductance increases,
however line capacitance remains uncompensated,
which also results in voltage increase at the
condenser.
Shunt reactors are used for voltage reduction
at the terminals of the condensers. In series
controlled compensators, the reactors, obviously,
must be controllable. But the application of
controllable reactors will increase the price of the
device. Therefore, in all the cases compensation
limits without reactors are considered.
One more constraint on maximum compensation of reactive power is minimum
permissible efficiency of controlled transmission
line. As it was already noted, with the increase in
the degree of compensation, the line loses increase
and its efficiency is reduced. It is necessary to
consider it even in the cases when the current in the
line has not reached its thermal limits. This constraint
becomes important when the line with increased
transmission capacity is working for a long period.
Thus, the technical constraints to the degree of
compensation in EPS, which are considered below, are
as follows:
• thermal constraint,
• permissible voltage at the condenser terminals
constraint,
• line efficiency constraint.
4
Description of simulation study
Single line 220 and 500 kV without intermediate substation were studied, for the length range
characteristic to the corresponding voltage classes:
for 220 kV lines 100-500 km;
for 500 kV tines 100- 1000km.
The calculations were carried out with and without
account of line resistance for four types of conductors.
Corona power losses in all cases were not taken into
account.
Usually, in study of the effects of a controllable
serial capacitor, controlled transmission line is
considered as connecting two buses with constant
voltages. But, this assumption is true only in the cases
when these two buses infinite ones and also, the
internal reactance of the systems connected to these
two buses are equal to zero. In real conditions, the
network cannot be presented by infinite bus and the
buses, which the transmission line connect are only
network nodes. Since the reactance of the network can
affect the operation of the transmission line, it is
essential that they should be introduced by certain
equivalent values in the related calculations. The
quantity of this reactance can be obtained on the basis
of the three-phase short circuit capacity in the related
bus. For example, this capacity for a bus with 500 kV
voltage is about 20-28*103 MVA and for a bus with
voltage 220 kV about 10-12*103 MVA. Therefore, the
equivalent inductance of the system at these buses will
be about 9-12 Ohm and 4-6 Ohm, respectively. For
smaller systems, this inductance will be more. In the
conditions that the transmission line is connected to
these buses with an autotransformer, it is obvious that
this equivalent reactance could be obtained by high
voltage winding of the autotransformer. For example,
when two parallel autotransformers are used for
connecting to constant voltage buses two sides of the
transmission line, the reactance at high voltage side
will be about 20-30 Ohm for 500/220 kV
autotransformers and about 12-50 Ohm for 220/110
kV autotransformers. Consequently, in calculating
operating condition the transmission line, this
equivalent reactance will be considered in the 1050 Ohm range.
Since the lines, as a rule, are separated from
system buses by a certain reactance, it was taken
into account in calculations. Its value varied
between 0-50 Ohms. The lower limit corresponds
to constant voltage at terminal substation; the
upper corresponds to the case when the line is
connected to the buses via autotransformer. The
circuit diagram is shown in Figure 2.
Fig.2: Circuit diagram Of CSC in transmission
line, a) general circuit, b) single line diagram
The simulation studies are based on a simple
power system model in which two local networks
A and B are connected by a long transmission
consisting of a line with controllable bank
capacitor in the middle. Thus the left-hand and
right-hand boundaries correspond to a line
connected to system buses through an
autotransformer with constant voltage, UA and UB,
(see Fig.2), equivalent reactance of system being
equal to 50 Ohm (Xeq=50 Ohm). Degree of
compensation, KC %, varied in the range 0 - 60%,
(1)
X
c *100
K %
c
X
Line
where: XC - reactance of capacitor banks
Xline - line reactance
Degree of compensation here is understood
as the ratio of the reactance of capacitor banks to
the transmission line reactance expressed in per
cent. Higher degree of compensation was not
considered, as it tends to resonance and sharp
increased in voltage across capacitor banks.
Besides certain constraints may be imposed by
8500
by a line at bus A may be calculated by the following
expression:
* cos 11 
U a *U b
K z *Z12
6500
5500
(2)
Pmax [MW]
Pmax 12 
U a2
K z *Z11
Pmax- 500 кВ - Ас 3*400\51- L=100,500,1000 [км]
100-Xeq=0
7500
.. Where:
Ua and Ub - constant voltages at buses A and B
Z11,Z12 - desiring point and mutual impedances
of the lines
4500
500-Xeq=0
3500
+25 c
-5 c
2500
500-Xeq=50
1000-Xeq=0
100-Xeq=50
1500
1000-Xeq=50
500
 11 - Phase angle of Z11
-500
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Degree of compencation
5 Results of calculations
a)
Uc-500 кv - Ас 3*400\51 - L=100,500,1000 [км]
700
100-Xeq=0
650
1000-Xeq=50
600
Uc1 [kv]
Kz =1.2 - power stability margin
Under normal operating conditions the typical
case UA = UB was considered. Compensation modifies
values of Z11 and Z12 acts and correspondingly
changes maximum active power.
Efficiency of 90% was taken as minimum level.
Lower values are not acceptable for continuous
operation. However in extreme situations in an
electric power system the lower values of efficiency
are probably admissible.
525 kv
1000-Xeq=0
550
500
100-Xeq=50
450
500-Xeq=50
Some results of calculations for 220 kV line are
presented in a Table 1, Fig.3 and for 500 kV line in a
Table 2, Fig.4.
500-Xeq=0
400
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Degree of compencation
b)
Efficiency [%]- 500 кВ , Ас 3*400\51, L=100,500,1000 [км]
100
Table 1: Maximum Degree of 220kv line
compensation a) Xeq=0 b)Xeq=50
500-Xeq=50
95
Max. degree of Compencation [%] , Conductor-1*Ac
400/51 , Xeq=0 , 220 [kv]
90
90 %
1000-Xeq=0
85
Efficiency [%]
length Length L. Length L.
L. [km] [km]
[km]
100
300
500
Constraint
100-Xeq=50
1000-Xeq=50
500-Xeq=0
80
100-Xeq=0
75
Thermal
Cond.
Voltage
Comp.
0
0
5
44
0
43
0
42
0
70
0
Efficiency
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Degree of compencation
c)
a)
Max. degree of Compencation [%] , Conductor-1*Ac
400/51 , Xeq=50 , 220 [kv]
length
L.
Length L. Length L.
[km] [km]
[km]
100
300
500
Constraint
Thermal
Cond.
Voltage
Comp.
Efficiency
b)
0
30
52
60
60
60
0
60
0
Fig. 3: Variation max. power transmission,
voltage at the condenser terminals and
efficiency in the 220 kV line with
compensation degree for
length 100,300,500km.
As can be seen from these tables and curves,
for 220 kV lines in the studied length range (100500 km) for zero equivalent reactance at the ends
of the line the greatest possible transmitted
capacity lies beyond the limit determined by the
heating of line at any degree of compensation. The
results are shown in Table 1. Only for 300 km
lines and with equivalent reactance of 50 Ohm, it
8500
Pmax- 500 кВ - Ас 3*400\51- L=100,500,1000 [км]
100-Xeq=0
7500
6500
Pmax [MW]
5500
4500
500-Xeq=0
3500
+25 c
500-Xeq=50
1000-Xeq=0
100-Xeq=50
1500
1000-Xeq=50
500
-500
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Degree of compencation
a)
Uc-500 кv - Ас 3*400\51 - L=100,500,1000 [км]
700
100-Xeq=0
650
1000-Xeq=50
600
525 kv
1000-Xeq=0
550
Table 2 : Maximum Degree of 500kv line
compencation a) Xeq=0 b)Xeq=50
-5 c
2500
Uc1 [kv]
is possible to work below the level of the degree of
compensation up to 30 %.
Voltage limit at the condenser terminals for
220kV lines occurs only for lines with constant
voltages at the ends for the degree of compensation
equaling 42-44 %. Thus the allowable degree of
compensation practically it does not depend on the
length of the line. The transmission lines that are far
from constant voltage buses of the system can operate
with any degree of compensation.
Minimum allowable efficiency for lines
without the account of equivalent reactance
corresponds to the degree of compensation of about
5% and in this case it does not depend on the length
of the line. It reduces when the degree of
compensation is increased. When equivalent reactance
is taken into account the lines with 100 km length
can operate with acceptable efficiency for any degree
of compensation. Thus the increase in the degree of
compensation practically does not effect line
efficiency.
500
100-Xeq=50
450
500-Xeq=50
500-Xeq=0
400
0
Max. degree of Compencation,Conductor-3*Ac
400/51 , Xeq=0 , 500 [kv]
0,5
0,6
0,7
b)
Efficiency [%]- 500 кВ , Ас 3*400\51, L=100,500,1000 [км]
100
100-Xeq=50
1000-Xeq=50
0
22
500-Xeq=50
48
90
90 %
48
10
44
13
25
16
1000-Xeq=0
85
500-Xeq=0
80
a)
100-Xeq=0
75
length L. Length L. Length L.
[km]
[km]
[km]
100
500
1000
Constraint
Efficiency
0,4
95
Max. degree of Compencation,Conductor-3*Ac
400/51, Xeq=50 [ohm] , 500 [kv]
Thermal
Cond.
Voltage
Comp.
0,3
Efficiency [%]
Efficiency
0,2
Degree of compencation
length L. Length L. Length L.
[km]
[km]
[km]
100
500
1000
Constraint
Thermal
Cond.
Voltage
Comp.
0,1
0
58
60
60
60
60
60
0
48
b)
Fig. 4: Variation Max. Power transmission,
voltage at the condenser terminals and
efficiency in the 500 kV line with
compensation degree for length 100,500,1000
km.
For lines of 500 kV, with no equivalent
reactance, the limit of compensation due to heating
depends on the line length. For a line of 500 km it
is 22%, and for a line of 1000 km - 48%.
Transmission line with equivalent reactance may
operate for any degree of compensation in all
70
0
0,1
0,2
0,3
0,4
0,5
0,6
Degree of compencation
c)
lengths range.
Voltage at the condenser terminals for the
line of 1000 km without reactors exceeds permissible limit for any degree of compensation
whether equivalent reactance is introduced or not.
For lines of 100 to 500 km without equivalent
0,7
References
reactance the maximum permissible degree
compensation is 48-44%. But the line with equivalent
reactance can operate without exceeding voltage limit
for any degree of compensation.
As for efficiency of the line, it is possible to see
that (Figure 4), in the absence of equivalent reactance,
increase in compensation leads to the reduction in
efficiency to a value below accepted minimum. At the
same time, the presence of equivalent reactance,
allows transmission line of more than 100 km to
operate at allowable value of efficiency in all the
range of compensation. The exception are lines of
length close to 1000 km, for which maximum degree
of compensation is about 50%, which can be
considered sufficient.
Conclusion
1. The analysis of the effect of controlled series
compensation on the operating condition of EPS
shows that it is necessary to take into account
technical constraints on the degree of
compensation.
2. It is necessary to consider the equivalent reactance
at the ends of the lines with controlled series
compensation.
3. It is necessary to determine the maximum degree
of compensation for every given line with the
consideration of the type of conductors, line length
and its location in the network.
[1]
[2]
[3]
[4]
[5]
[6]
Ram Adara, "Facts system studies'',
Journal IEEE Power Engineering Review,
pp. 17 - 22, December 2002.
J. Deuse, M. Stubbe, B. Meyer, P.
Panciatici, "Modeling of facts for power
system analysis", CIGRE, Sym. Tokyo,
Session 320-04,1995
E. Lerch, D. Povh, R. Witzamann, B.
Helebcar, R. Mihalic "Simulation and
performance analysis of unified power
flow controller", CIGRE, Session 14-205,
1994.
E. V.Larson, K.Clark, S.A. Miske,Jr ,
J.Urbanek, "Characteristics
and rating
considerations of thyristor controlled series
compensation", IEEE Trans. Power
Delivery.Vol.9,No. 2,pp. 992-1000, April
1994.
Lashkar Ara A., Seyed Nabavi Niaki A.,
"Comparison of type facts equipment
operation in transmission and distribution",
Proc. 17th Inter. Con. On Electricity
Distribution, CIRED, Session 2,Paper No.
44, 12-15 May 2003.
Nukolavich E.V., Dumtrovich K.B.,"
Future Application Power Electronic
Converter in the Power System ",Tran.
Electrichectvo (Journal in Rus.), No. 9,
pp. 30-37, 2001.