Load flow analysis of transmission network with series compensation

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
_______________________________________________________________________________________
Volume: 03 Issue: 06 | Jun-2014, Available @ http://www.ijret.org 526
LOAD FLOW ANALYSIS OF TRANSMISSION NETWORK WITH
SERIES COMPENSATION
VinayaChavan1
, Sunil K. Patil2
1
Research Scholar, Electrical Engineering Department, GCE Karad, Maharashtra, India
2
Associate Professor, Electrical Engineering Department, GCE Karad, Maharashtra, India
Abstract
The increase in electrical energy demand has presented higher requirements from the power industry during past two decades. In
recent years, the highly increasing cost of building new transmission lines, compounded by the difficulty to obtain new
transmission corridors, has led to a search for increasing the transmission capacity of existing lines. Use of series capacitors for
compensating part of the inductive reactance of long transmission lines increases the power transmission capacity as well as
improves the system stability. Transmission line compensation implies a modification in the electric characteristic of the
transmission line with the objective of increase power transfer capability. The series compensator is primarily applied to solve
load flow problems which may be related to length of line or structure of transmission network. This paper presents an analysis of
the location of series capacitor, effect of series line compensation level on the line voltage profile, transferred power and
transmission losses. Also it gives information about problem formulation for load flow analysis. For this purpose simple three
bus model has been developed in MATLAB/SIMULINK. It gives basic mechanism of series compensation also highlight its
benefits for power system.
Keywords: Line Reactance, Series Compensation, Degree of Compensation, Capacitive Reactance, Transmission
Efficiency
--------------------------------------------------------------------***----------------------------------------------------------------------
1. INTRODUCTION
In AC power transmission system, compensation is the
management of reactive power to improve the reliability of
power system. To improve the system reliability, stability,
efficiency, cost effectiveness by means of compensation
there are mainly two methods,
1. Shunt compensation: The shunt compensator is
functionally a controlled reactive current source
which is connected in parallel with the transmission
line to control its voltage.
2. Series compensation: The series compensator is
functionally a controlled voltage source which is
connected in series with the transmission line to
control its current
As per Rolf Gruenbaum, Dr. G. Thomas Bellamhe, in
transmission system, due to the transmission line reactance,
there are limitations on power transmit ability of line so that
it leads towards building of new transmission line which is
costly affair. Series capacitive compensation is used to
increase power transmission capability by canceling the line
reactance [1], [2]. Ullasn Eminoglu reported that, voltage
regulation becomes an important and sometimes critical
issue in the presence of load, which varies the demands for
reactive power [3]. As per Rolf Gruenbaum, Belur S.
location of the compensator is also an important aspect [4],
[5]. Adebayo reported that series capacitor compensation
provides additional options for load flow control and voltage
stability of power system [6]. Compensation means the
modification of electrical characteristics of a transmission
line in order to increase its power transmission capacity, to
satisfy the fundamental requirements for transmission.
Compensation system ideally performs following functions:
1. It helps to improve voltage profile at all levels of
power transmission.
2. It improves stability by increasing the maximum
transmittable power.
3. It provides an economical means for meeting the
reactive power requirements of the transmission
system.
The paper is organized in different sections. First section
gives introduction about series capacitor compensation and
load flow power. The second section describes operating
principle of series compensation and its functional
capabilities. The third section deals with a selection of series
capacitor for series compensation and its location. Fourth
section gives the information about power system selection
and problem formulation for load flow analysis. Fifth
section deals with MATLAB/SIMULINK modeling and
results.
2. BASIC MECHANISM
Series capacitive compensation was introduced to cancel a
portion of the reactive line impedance and thereby increase
the transmittable power. The variable series compensation is
highly effective in both controlling power flow in the line
and improving stability. The usefulness of the concept can
be illustrated by means of example in Fig.1

_______________________________________________________________________________________
Fig-1: Series compensated power transmission corridor
Fig-2: Phaser diagram
2.1 Stability Improvement
Power transmitted in system shown in Fig. 1 is given by
𝑃 =
Vs ∗ Vr
𝑋𝐿 − 𝑋 𝐶
sinδ (1)
Where
P - active power transfer
Vs - Sending end or Bus B1 voltage
Vr - Receiving end or Bus B2 voltage
δ - Angle between Vs and Vr
XL - line reactance
XC - series capacitive reactance
The effective transmission impedance with the series
capacitive compensation is given by
𝑋𝐿 − 𝑋 𝐶 = 1 − 𝑘 𝑋𝐿 (2)
Where, k is known as degree of compensation i.e.
𝑘 =
𝑋 𝐶
𝑋𝐿
0 ≤ 𝑘 < 1 (3)
From Fig.2 and equations 1, 2, 3 it is clear that with
increase in compensation level there is increase in capacitive
reactance and ultimately angular stability and also power
transfer capability.
2.2 Power Transfer Capability Improvement
From equation (1) it is evident that by decreasing the
effective series reactance active power transfer can be
increased.
2.3 Voltage Stability Improvement
Although series capacitors are not usually implemented for
voltage control, they do contribute to improve the system
voltage and reactive power balance. As magnitude of the
total voltage across the series line inductance is increased by
the magnitude of the opposite voltage, VC is developed
across the series compensator, due to the increase in the line
current.
2.4 Degree of Compensation
The degree of compensation is defined by the ratio of
capacitive reactance to the total inductive reactance of the
transmission line given by equation (3).
At 100% compensation the effective line reactance would be
zero and the line current and power flow would be
extremely sensitive to changes in the relative angles of
terminal voltages. In addition the circuit would be series
resonant at the fundamental frequency. High compensation
level also increases the complexity of protection circuit and
probability of sub synchronous resonance. Therefore a
practical limit to the series compensation is up to 75%.
3. SERIES CAPACITOR
Series capacitors have been used in transmission and
distribution lines to compensate inductive reactance. The
inductive reactance is reduced by the amount of inserted
capacitive reactance. Series capacitor compensation is self
regulating, that is, its reactive power output increases with
the line loading. Series capacitors are ideally suited for
effectively shortening long lines. Series capacitor reduces
both the characteristic impedance and electrical length of
line. As a result, both voltage regulation and stability are
significantly improved.
3.1 Calculation of Series Capacitor
Series capacitor is used in line to reduce line reactance.
Therefore,
𝑋𝑐 = 𝑘𝑋𝐿 (4)
Where, k is the degree of compensation
𝑋𝑐 =
1
2𝜋𝑓𝑐
(5)
𝐶 =
1
2πf𝑋𝑐
(6)
Series Compensation has following advantages,
1. Self regulating device.
2. Low risk of problems from load generated
harmonics.
3. Control of loading of two parallel lines to minimise
active and reactive losses.
4. Time overloading capability.

_______________________________________________________________________________________
3.2 Location of Series Capacitor [3][4][5]
A series capacitor bank can theoretically be located
anywhere along the line. Factors influencing choice of
location include cost, accessibility, fault level,
maintainability of series capacitor, protective relaying
consideration, voltage profile and effectiveness in improving
power transfer capability. When a series capacitor is to be
installed in a transmission line mainly following locations
are considered.
1. Midpoint of the line.
2. Line terminals
3. 1/3 or 1/4 points of the line
Midpoint location:
Line terminals
1/3 or 1/4 points of the line
Fig-3: Typical series capacitors locations
The location of series capacitor banks is important for
several reasons:
• The compensation “effectiveness” of the series capacitor
varies as a function of the series capacitor location along the
line.
• The series capacitor location affects the voltage profile
along the line.
• The transmission line protection and the series capacitor
main circuit equipment are affected by the series capacitor
location.
• The series capacitor location affects the maintainability of
the series capacitor.
• For MOV protected series capacitors, the rating of the
MOV is very dependent upon the series capacitor location.
The midpoint location has advantage that the relaying
requirements are less complicated. If compensation is less
than 50% in addition short circuit current is lower, however
it is not very convenient in term of access for maintenance,
monitoring, securities.
4. PROBLEM FORMULATION
4.1 Bus Classification
Depending on the quantities that have been specified, the
buses are classified into 3 categories.
Load bus (Type 1) - In a load bus, the real power (P) and
the reactive power (Q) are known. The variables V and δ are
not specified. So this bus is also called as PQ bus.
Generator bus/Voltage Controlled bus (Type 2) - In a
generator bus, the voltage (V) is kept constant and the
output power (P) is fixed. These two items are controlled by
the excitation system and the governor. The unknown
variables are Q and δ. So this bus is also called as PV bus.
Swing bus/Slack bus (Type 3) - At the reference generator
or swing bus, the voltage (V) and the load angle (δ) are
known. The unknown variables are P and Q
Therefore for load flow analysis three bus power system has
been selected.
4.2 Power Flow Equation
A faster solution is obtained using the Newton Raphson
method and is suitable for large-scale problems [9]. In this
approach, the partial derivatives are used to construct the
Jacobian matrix. For the three-bus problem, the bus power
relations are given by:
𝑃1 = 𝑌11 𝑉1 + 𝑌12 𝑉2 + 𝑌31 𝑉3 𝑉1
𝑃2 = 𝑌21 𝑉1 + 𝑌22 𝑉2 + 𝑌23 𝑉3 𝑉2 7
𝑃3 = 𝑌31 𝑉1 + 𝑌32 𝑉2 + 𝑌33 𝑉3 𝑉3
The elements of the Jacobian matrix based on equations are
𝐼1
𝐼2
𝐼3
=
𝜕𝑃1
𝜕𝑉1
𝜕𝑃1
𝜕𝑉2
𝜕𝑃1
𝜕𝑉3
𝜕𝑃2
𝜕𝑉1
𝜕𝑃2
𝜕𝑉2
𝜕𝑃2
𝜕𝑉3
𝜕𝑃3
𝜕𝑉1
𝜕𝑃3
𝜕𝑉2
𝜕𝑃3
𝜕𝑉3
𝑉1
𝑉2
𝑉3
8
4.3 Power System
For the analysis of series compensation single phase system
shown in Fig.4 has been selected and it is scaled down
model of actual transmission line which is scaled down from
173 MVA to 8.66 KVA and 289 KV to 400 V for three
phase line. In the scaled down model a line has resistance
0.0048Ω/km, inductance of 0.1 mH/km and shunt
capacitance 0.1609µF/km. The line is made by connecting π
networks in series and one π network is designed as
equivalent circuit for 50 km line, having R=0.24Ω, L=5 mH
and C=4 µF. For analysis of series compensation in mesh
system three bus power system is modeled in Fig.4 and the
impedances for these lines are mentioned in table 1. Here
bus 1is slack bus having bus voltage 230V∠0, bus 2 taken as
generator bus since another generator is connected to
selected topology and bus 3 taken as load bus.

_______________________________________________________________________________________
Fig-4: Three bus mesh power system
5. MATLAB SIMULATION RESULTS
In MATLAB simulink software, three bus power system
modeled, simulated and load flow analysis has been done. If
line 2-3 (600 km line) is, compensated then total electrical
length of that line decreases. After compensation of line L2-
3, power of line L1-2, L1-3 changes accordingly. It is
verified by using simulation.
Fig-5: Simulation for 3 bus power system
Fig. 5 shows the simulation for 3 bus power system having
line lengths 100 Km, 500 km and 600 km having line
impedance, and required series capacitors for series
compensation are mentioned in table (1).
Table-1: Impedance and series capacitors for 3 bus power
system
Sr.
No.
Length
(km)
Impedance
(Ω)
k=25%
C (µF)
k=50%
C (µF)
k=75%
C (µF)
1. 100 0.48+ j3.14 4050 2027 1350
2. 500 2.4+ j15.7 810 405 270
3. 600 2.88 +j18.88 675 338 225
If we compensate 600 km line for 50%, means the electrical
length of 600 km line becomes approximately 300 km. So
the electrical lengths of line L1-3 and L2-3 becomes
approximately equal. Due to this both lines have to carry
approximately same power through them in compensation
mode.
Chart-1: Effect of compensation level on system voltage
As the compensation increases the load end voltage (i.e.
receiving end voltage) increases. This leads towards better
voltage regulation. This statement is verified by chart (1).
Chart (2), (3) and (4) shows the result of power system
when 600 km line is compensated at 50%. It shows that after
compensation of 600 km line active power transfer in that
line is increased; also power flow through the other lines is
changed. After compensation impedance on L2-3 and L1-3
are approximately same so power in those lines is
approximately equal which is verified by chart (2). Also due
to the reduction in transmission impedance losses are also
reduced.
Chart (3) and chart (4) gives the change in the current and
reactive power flow, before and after compensation
respectively.
Chart-2: Effect of compensation on power transfer

_______________________________________________________________________________________
Chart-3: Effect of compensation on current flow in 3 bus
power system
Chart-4: Effect of compensation on Reactive power flow in
3 bus power system
6. CONCLUSIONS
Series compensation is one of the methods of reactive power
compensation. Location of series capacitor is an important
issue. Series compensation improves line power handling
capacity. In this paper three bus power system has been
modeled and effect of series compensation for three bus
mesh system is analyzed using MATLAB/SIMULINK. This
paper concludes that series compensation is the effective
means of improving electrical characteristics of transmission
line. Voltage profile, power transmission capability, stability
are enhanced by series compensation and mid of the
transmission line is the best location for series
compensation.
ACKNOWLEDGMENTS
The authors would like to acknowledge the management of
organization, head of the institution, teaching, non teaching
staff and friends for their continuous support to complete
this work.
REFERENCES
[1] Rolf Gruenbaum, Jon Rasmussen “Series Capacitors
for Increased Power Transmission Capability of a
500 kV Grid Intertie” IEEE conference on electrical
power and energy,2012
[2] Dr. G. Thomas Bellamhe, “ Optimum Series
Compensated High Voltage Transmission Lines”
1997 IEEE
[3] UllasnEminoglu, M. Hakan Hocaoglu, atankut
Yalkinoz, Transmission line shunt and series
compensation with voltage sensitive loads.
International Journal of Electrical Engineering
Education 46/45.
[4] Rolf Gruenbaum, J. Samuelesson “Series Capacitors
Facilitate long distance AC Power Transmission
Capability of a 500 kV Grid Intertie” IEEE
conference on electrical power and energy,2012
[5] Belur S. Ashok Kumar, K. ParthaArathy, F. S.
Prabhakara, and H. P. Khincha “Effectiveness of
Series Capacitors in Long Distance Transmission
Lines” IEEE transaction on power apparatus systems,
May/Jun1970.
[6] Adebayo, I.G., Adejumobi, I.A., Olajire, O.S., “
Power Flow Analysis and Voltage Stability
Enhancement Using Thyristor Controlled Series
Capacitor (TCSC) Facts Controller”, International
Journal of Engineering and Advanced Technology
(IJEAT) ISSN: 2249 – 8958, Volume-2, Issue-3,
February 2013
[7] N.G. Hingorani, L. Gyugyi, “Understanding FACTS,
Concepts and Technology of Flexible AC
Transmission systems”, IEEE Press 2000S (2002)
[8] T.J.E.Miller, “Reactive power control in electric
system” John Wiley and sons, Inc
[9] Hadi Sadat, “Power System Analysis”, TATA Mc-
Grew Hill publication
[10] Abhijit Chakrabharti, Sunita Halder, “Power System
Analysis Operation and Control” PHI learinig
BIOGRAPHIES
Vinaya Chavan received her B.E. degree in
Electrical Engineering from Shivaji
University, Kolhapur, Maharashtra. She is
currently pursuing M.E. degree in Electrical
Engineering (Power System). Her research
interests include power system FACTS, and
custom power devices.
Prof. Sunil K Patil, working as faculty of
Electrical Engineering since last 22 years in
Government Engineering College in
Maharashtra. His research area is FACTS.
He is Member of IE(India).

Load flow analysis of transmission network with series compensation

More Related Content

What's hot (19)

Viewers also liked (20)

Similar to Load flow analysis of transmission network with series compensation (20)

More from eSAT Publishing House (20)

Recently uploaded (20)

Load flow analysis of transmission network with series compensation