Download Reactive Power Compensation in Transmission Lines Using Static

IJECT Vol. 5, Issue 3, July - Sept 2014
ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print)
Reactive Power Compensation in Transmission Lines Using
Static Var Compensator by Simulation in ETAP
Tamojit Chakraborty
Dept. of Electrical Engineering, Netaji Subhash Engineering College, Kolkata, West Bengal, India
Abstract
The study of shunt connected Flexible AC Transmission System
(FACTS) device is a connected field with the Reactive Power
Compensation and the better mitigation of transmission in Power
Systems. The FACTS technology based on Power Electronics
devices is used to enhance existing transmission capabilities
in order to make the Power Systems network more flexible
with independent operation thus increasing the power transfer
capability. [1] Static VAR Compensator (SVC) is one of the shunt
connected FACTS device, which can be utilized for the purpose
of reactive power compensation. This paper attempts to simulate
the distribution network substation by introducing SVC at the load
ends using the Electrical Transient Analyzer Program (ETAP)
environment. A comparative study is made before and after using
the FACTS device and subsequent results have been shown.
Keywords
FACTS, AC Transmission, Power Electronics, SVC, ETAP.
I. Introduction
During the past two decades, the increase in electrical energy
demand has presented higher requirements from the power
industry. More power plants, substations, and transmission lines
need to be constructed. However, the most commonly used devices
in present power grid are the mechanically-controlled circuit
breakers. The long switching periods and discrete operation make
them difficult to handle the frequently changed loads smoothly and
damp out the transient oscillations quickly. In order to compensate
these drawbacks, large operational margins and redundancies are
maintained to protect the system from dynamic variation and
recover from faults. This not only increases the cost and lowers
the efficiency, but also increases the complexity of the system
and augments the difficulty of operation and control. Severe
black-outs happened recently in power grids worldwide and
these have revealed that conventional transmission systems are
unable to manage the control requirements of the complicated
interconnections and variable power flow.
Therefore, investment is necessary for the studies into the security
and stability of the power grid, as well as the improved control
schemes of the transmission system. Different approaches such
as reactive power compensation and phase shifting have been
applied to increase the stability and the security of the power
systems. The demands of lower power losses, faster response to
system parameter change, and higher stability of system have
stimulated the development of the Flexible AC Transmission
systems (FACTS). FACTS has become the technology of choice
in voltage control, reactive/active power flow control, transient
and steady-state stabilization that improves the operation and
functionality of existing power transmission and distribution
system. The achievement of these studies enlarge the efficiency of
the existing generator units, reduce the overall generation capacity
and fuel consumption, and minimize the operation cost.
II. Static VAR Compensators (SVC):
Static Var Compensator is “a shunt-connected static Var generator
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or absorber whose output is adjusted to exchange capacitive or
inductive current so as to maintain or control specific parameters
of the electrical power system (typically bus voltage)”. [2]
SVC is based on thyristor without gate turn-off capability. The
operating principal and characteristics of thyristor realize SVC
variable reactive impedance. SVC includes two main components
and their combination:
1. Thyristor-controlled and Thyristor-switched Reactor (TCR
and TSR)
2. Thyristor-switched capacitor (TSC).
TCR and TSR are both composed of a shunt-connected reactor
controlled by two parallel, reverse-connected thyristor. TCR is
controlled with proper firing angle input to operate in a continuous
manner, while TSR is controlled without firing angle control
which results in a step change in reactance. TSC shares similar
composition and same operational mode as TSR, but the reactor
is replaced by a capacitor. The reactance can only be either fully
connected or fully disconnected zero due to the characteristic of
capacitor. With different combinations of TCR/TSR, TSC and
fixed capacitors, a SVC can meet various requirements to absorb/
supply reactive power from/to the transmission line.
Fig. 1: Static VAR Compensators (SVC): TCR/TSR, TSC, FC
and Mechanically Switched Resistor
III. Other FACTS Devices
Converter-based Compensator: Static Synchronous Compensator
(STATCOM) is one of the key Converter-based Compensators
which are usually based on the voltage source inverter (VSI) or
current source inverter (CSI). Unlike SVC, STATCOM controls the
output current independently of the AC system voltage, while the
DC side voltage is automatically maintained to serve as a voltage
source. Mostly, STATCOM is designed based on the VSI.
Compared with SVC, the topology of a STATCOM is more
complicated. The switching device of a VSI is usually a gate
turn-off device paralleled by a reverse diode; this function endows
the VSI advanced controllability. Various combinations of the
switching devices and appropriate topology make it possible for
a STATCOM to vary the AC output voltage in both magnitude
and phase. Also, the combination of STATCOM with a different
storage device or power source endows the STATCOM the ability to
control the real power output. STATCOM has much better dynamic
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performance than conventional reactive power compensators like
SVC. The gate turn-off ability shortens the dynamic response
time from several utility period cycles to a portion of a period
cycle. STATCOM is also much faster in improving the transient
response than a SVC. This advantage also brings higher reliability
and larger operating range [3].
IV. Series-Connected Controllers
As shunt-connected controllers, series-connected FACTS
controllers can also be divided into either impedance type or
converter type. The former includes Thyristor-Switched Series
Capacitor (TSSC), Thyristor-Controlled Series Capacitor (TCSC),
Thyristor-Switched Series Reactor, and Thyristor-Controlled
Series Reactor. The latter, based on VSI, is usually in the form of a
Static Synchronous Series Compensator (SSSC). The composition
and operation of different types are similar to the operation of the
shunt-connected peers [4].
V. Load Flow Analysis
Load-flow studies are very common in power system analysis. It
allows us to know the present state of a system, given previous
known parameters and values. The power that is flowing through
the transmission line, the power that is being generated by the
generators, the power that is being consumed by the loads, the
losses occurring during the transfer of power from source to load,
and so on, are iteratively decided by the load flow solution, or also
known as power flow solution. In any system, the most important
quantity which is known or which is to be determined is the voltage
at different points throughout the system. Knowing these, we can
easily find out the currents flowing through each point or branch.
This in turn gives us the quantities through which we can find
out the power that is being handled at all these points. In earlier
days, small working models were used to find out the power flow
solution for any network. Because computing these quantities was
a hard task, the working models were not very useful in simulating
the actual one. It’s difficult to analyze a system where we need
to find out the quantities at a point very far away from the point
at which these quantities are known. Thus we need to make use
of iterative mathematical solutions to do this task, due to the fact
that there are no finite solutions to load flow.
The values more often converge to a particular value, yet do not
have a definite one. Mathematical algorithms are used to compute
the unknown quantities from the known ones through a process
of successive trial and error methods and consequently produce
a result. The initial values of the system are assumed and with
this as input, the program computes the successive quantities.
Thus, we study the load flow to determine the overloading of
particular elements in the system. It is also used to make sure
that the generators run at the ideal operating point, which ensures
that the demand will be met without overloading the facilities and
maintain them without compromising the security of neither the
system nor the demand.
The objective of any load-flow analysis is to produce the following
information:
• Voltage magnitude and phase angle at each bus.
• Real and reactive power flowing in each element.
• Reactive power loading on each generator.
VI. Load Flow Equation Solution Methods
To start with by solving the load flow equations, we first assume
values for the unknown variables in the bus system. For instance,
let us suppose that the unknown variables are the magnitude of
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International Journal of Electronics & Communication Technology
ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print)
the voltages and their angles at every bus except the Slack bus,
which makes them the load bus or the PQ bus. In this case, we
assume the initial values of all voltage angels as zero and the
magnitude as 1p.u. Meaning, we choose a flat voltage profile.
We then put these assumed values in our power flow equations,
knowing that these values don’t represent the actual system, even
though it should have been describing its state. So, now we iterate
this process of putting in the values of voltage magnitudes and
angles and replacing them with a better set.
So, as the flat voltage profile keeps converging to the actual values
of the magnitudes and angles, the mismatch between the P and
Q will reduce. Depending on the number of iterations we use
and our requirements we can end the process with values close
to the actual value. This process is called as the iterative solution
method. The final equations derived in the previous section are
the load flow equations where bus voltages are the variables. It
can be seen that these equations are nonlinear and they can be
solved using iterative methods like:
1. Gauss-Seidel method
2. Newton-Raphson method
VII. Power Flow Analysis with SVC
Fig. 2 below shows a power plant which is connected to a larger
system via a double line transmission system (upper part). The
lower part of the figure shows simplified the voltages at the end
of the transmission system for various load cases:
1. Heavy load
2. Light load
3. Outage of one line during heavy load condition
4. Load rejection at the end of the line
According to the loading conditions voltage decreases and increases
will occur with larger deviations at contingency conditions. An
SVC will be typically designed in size to limit voltage deviations
during normal load conditions and a good voltage profile is kept
for this operation. At other contingency conditions larger voltage
deviation will occur due to the sizing for normal conditions.
Fig. 2: Voltages at the End of a Transmission System Under Various
Operating Conditions
Fig. 3 below shows the case of load rejection. The voltage
rises rapidly in will be reduced by the voltage control means
of the system i.e. voltage controllers in power plants. If a SVC
is connected close the voltage will also rise rapidly but will be
reduced in only a few cycles by the fast reaction of a SVC. The
first peak cannot be influenced because the SVC control must first
observe the increase and can only react afterwards.
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ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print)
Fig. 3 Action of a SVC on Load Rejection
Fig. 4 below shows a transmission system with strong active power
oscillations after a severe line fault followed by fault clearing and
switch off the faulted line.
IJECT Vol. 5, Issue 3, July - Sept 2014
Without a SVC the oscillations continue at low damping for a
long time. Using a SVC with voltage control already helps in
damping of the oscillations and reducing the oscillation time.
Using an SVC with a specific POD (power oscillation damping)
control function will even damp out the oscillations quite faster
and increase thus the margin in system stability [5].
VIII. Case Study
A Single line diagram of 33/11 KV Distribution Substation is
taken with fifteen buses (from Bus 1 to Bus15) as shown in
Fig. 1. It consists of two power transformers (T1 and T2), each
having capacity of 3 MVA and four distribution transformers
(T3, T4, T5 and T6). There are four static loads (from Load 1
to Load 4). There are two outgoing feeders connected to each
of power transformers. Incoming voltage level is 33KV and the
distribution voltage level is 11KV. Load receives a voltage of
0.435 KV. Bus 1is swing Bus. Buses from 2 to 7 are PV Buses
and Buses from 8 to 15 are PQ Buses. Power source to this
system is provided by Utility, U1. In order to see effect of two
SVCs on voltage profile, losses and power flows at each bus in
given single line diagram.
Fig. 4: Damping of Power Oscillations by SVC
Fig. 5:
ETAP Load Flow analysis calculates Bus voltages, Branch Power Factors, Currents and Power Flows throughout the Electrical
system in single line diagram. ETAP allows for swings, voltage regulated, unregulated power sources with multiple power grids and
generator connections. It is capable of performing on both radial and loop systems. ETAP allows feeding of all these above values in
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single line diagram for load flow analysis. ETAP provides three load flow calculation methods: Newton-Raphson, Fast-Decoupled,
and Accelerated Gauss- Seidel. They possess different convergent characteristics, and sometimes one is more favourable in terms
of achieving the best performance. Any one of them is selected depending on system configuration, generation, loading condition,
and the initial bus voltage. The following two figures shows load flow analysis of the above shown single line diagram with and
without using SVC.
Fig. 6: Load Flow Without Using SVC
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IJECT Vol. 5, Issue 3, July - Sept 2014
Fig. 7: Load Flow Using SVC
By using two SVCs, average value of voltage is changed from 77.366 to 78.071 i.e. increased by 0.705 units. Thus using two SVCs
in the single line diagram at location bus 9-10 where static load is present, it has been found that, we have increased voltage profile
and increased active power. Reduction of losses, increase of power transfer capability and voltage profile can also be optimized by
number of other optimization methods such as simulated annealing, fuzzy logic.
References
[1] Guneet Kour, Dr. G.S.Brar, Dr. Jaswanti, International Journal of Engineering Science and Technology (IJEST), Optimal
Placement of Static VAR Compensator in Power System.
[2] H. K. Tyll, Senior Member, IEEE,"Application of SVCs to Satisfy Reactive Power Needs of Power Systems".
[3] Y.H. Song, A.T. Johns,“Flexible ac transmission systems (FACTS)”, IEEE 1999.
[4] Juan Dixon (SM), Luis Morán (F), José Rodríguez (SM), Ricardo Domke, Reactive Power Compensation Technologies, State
of-the-Art Review.
[5] A. K. Chakraborty, A. E. Emanuel,“A Current regulated Switched Capacitor Static Volt Ampere Reactive Compensator”, IEEE
Transactions
[6] Power Electronics by Dr. P.S Bhimbra
[7] Elements of Power System Analysis by Stevenson
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