Static VAR Compensators: Improving Power System Stability and Quality

Static VAR Compensators
Background
Power systems are faced by challenges of voltage and stabilities. Requirement of the network and
load dictates on the quantity of reactive power to be supplied by the generator. Absence of reactive power
can lead to system breakdown, this makes it essential to control reactive power by consuming or
supplying the required quantity[1]. Loads connected to the grid consume reactive and real power, other
elements along the transmission line such as transformers consume reactive power[3]. Reactive power
consumption by loads and other elements will lower terminal voltage along the transmission line. Some
loads will supply reactive power increasing terminal voltages of the power system. Controlling the
terminal voltages along the transmission is necessary for safe operation of equipment connected to the
grid, as they manufactured to operate within a certain range [4].
Power systems compensation refers to improving alternating current (ac) performance by
regulating reactive power. Compensating quantity of reactive power in transmission is done to reduce
voltage fluctuations at a specified location of the transmission line. Compensation of the reactive power is
also done to improve power factor (pf) of the power system, this attains a balance in real power consumed
from power source thus eliminating harmonics produced by nonlinear loads in industrial set up[4].
Compensation in transmission increases transmitted active power on the transmission line thus helping in
improving the stability of power systems.
Power quality is another important factor that ought to be considered in power system analysis.
Harmonics are generated in the power system by specified loads such as inverters, rectifiers, switch-mode
power supply, speed drives etc. Harmonics generated by these loads adversely affect equipment
connected to the grid such as motor, transformers, capacitors and power cables[5]. These harmonics will
cause excess heating, creates harmonics currents in the rotor, in motors, cables are subjected to corona
and stress. Harmonics in transformers results to an increase in transformer losses and temperature. Above
all these effects causes adverse current and voltage distortion in the power system [6].
Static VAR compensator (SVC) is a device used in AC power transmission systems to help in
keeping line voltage in required level, regulating power factor for industrial loads, harmonics and general
stabilization of the system [2]. In high voltage transmission static VAR compensator is applied to assist in
regulating grid voltage. Schematic diagram of static VAR compensator is as shown in figure 1, its made
up of shunt capacitor paralleled with a reactor. Reactor has series of thyristor switches used for
controlling it[5]. The voltage across and current through the inductor is controlled by the firing angle of
the thyristor. It is in the same manner, the reactive power is drawn by inductor is controlled. Static VAR
compensator has no rotating parts and for that reason it is used for surge impedance compensation.
Advantages of static VAR compensator[6]
 Increases power factor of the load thus reducing power line losses
 Increases transmission capability of power lines
 It improve transient stability of the system.
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Figure 1. Static VAR compensator circuit.
Electricity generated in south Australia are sourced from non-renewable and renewable sources.
Coal fired was the earliest main source of energy generation, this has changed significantly as
concentration is now shifting to green energy. Its then transmitted from power stations by transmission
and distribution lines as shown in figure 2. National electricity Market are responsible for selling it to
retailers. Retailers then sell it to businesses and households. South Australia grid has interconnectors that
will enable electricity importation from eastern states when the demand is high [3].
Figure 2: power system
One of the major South Australia power outage is the 2016 blackout. This case was as a results
of extreme weather condition as well as circumstances that had not been considered [1]. Power
transmission and distribution was evaluated following the occurrence and the Australian Energy
regulation recommended that more weather monitoring processes be implemented, standardization of
notifications for participants in the market and improve training offered to operators. These
recommendations were implemented under the future power system security program to help avoid future
occurrence of power failure. Implementation of modern power system increase reliability on utility [7].
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Literature Review
Cases where static VAR compensators are mainly used are, when are used to regulate voltage to
be transmitted from one station to the other or to the load, and when they are connected near to the
industries, to improve the quality of power delivered to large loads. When used to regulate terminal
voltage in transmission lines, static VAR compensators uses thyristor controlled reactors to consume
reactive power for capacitive loads thus lowering system voltage. As for inductive loads capacitor loads
are switched in, applying reactive power to power system thus raising the voltage. In industrial set up,
compensators are placed near rapidly changing loads such furnaces to smoothen flickering terminal
voltages [6].
Elements making up the static VAR compensator are harmonic filters, capacitors and reactors
that are mechanically switched, thyristor switched capacitor and lastly thyristor controlled reactor.
Typical static VAR compensator has more than one bank of reactors and shunt capacitors each switched
using thyristors as shown in the figure 3. A bank of transformers is used to step down transmission
voltages say from 132kV to lower voltages say 66kV, stepping down voltages helps in reducing size and
number of components required in static VAR compensators, conductors however should be large as they
can withstand large levels of currents. In industrial applications, static VAR compensator can be
connected directly to voltage busbar, this saves the cost of using transformer [7].
Figure 3 single line diagram incorporating compensators
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