Static VAR Compensators: Improving Power System Stability and Quality

This article provides an overview of static VAR compensators (SVCs) and their role in improving power system stability, voltage regulation, and power quality. It explains how SVCs control reactive power and maintain terminal voltages along transmission lines. The article also discusses the advantages of SVCs, their operating modes, and their applications in transmission lines and industrial setups. Additionally, it highlights the use of SVCs in South Australia's power system and the challenges faced in maintaining reliability and security.

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.
1

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].
2

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
3

Static VAR compensators also have control system. control system has measuring system,
distribution system, voltage regulator and synchronizing system. Measuring unit measures voltage
positive sequence that is intended to be controlled. Synchronizing unit is responsible for sending a pulse
that will aid in control to the thyristor[6]. Voltage regulator utilizes voltage error so as to determine static
VAR susceptance required to maintain voltage at constant level. Lastly distribution system is responsible
in determining which thyristor switched reactors or capacitors is to be switched in or out. Distribution
system is one that determine firing angle of the thyristor switched reactors.
Static VAR compensator has two modes of operating namely VAR control mode and voltage
regulation mode[6]. in voltage regulation mode, V-I characteristic below is implemented
Figure 4 voltage current characteristics of static VAR compensator
Voltage current characteristic of static VAR compensators is explained with an aid of the
equations below
V=Vref+I*XS the equation applies when compensator is in regulation mode (−Bcmax<B<Blmax)
V=I/BImax if compensator is operating in inductive mode that is (B=Blmax),
V=-I/Bcmax if compensator is operating in capacitive mode that is (B=Bcmax)
where I is the reactive current, V the pu voltage positive sequence, X S is the droop reactance,
Blmax represent maximum inductive suspectance when all thyristor switched reactors are operational and
when thyristor controlled reactors are conducting fully. Bcmax on the other hand represent maximum
capacitive susceptance when all thyristor switched capacitors are operating and when no thyristor
switched reactor and thyristor controlled reactor is in service[8].
South Australia utilizes non-renewable and renewable sources to generate electricity. Renewable
energy sources are wind and rooftop solar systems. Wind farms generate electricity capable of supplying
a significant number of customers. Hornsdale wind farm is battery paired and offers stability to the
network and back up in case of emergency. Rooftop PV systems are available in more than 200 000
households, they generate power for use in the homes while excess power is fed to the grid [1]. The
capacity for PV systems is estimated at 930 MW and 15 MW for battery systems as at 2018. The main
source of non-renewable energy in South Australia is natural gas, 60 % of natural gas in South Australia
is used in power generation.
4

Transmission is done at 132 or 275 KV for long distance high voltage transmission. The high
voltage is transmitted to substations where it is stepped down to 66 or 33 kV. The power is then
transmitted to distribution substation and stepped down to 11 kV [1]. Ground level transformers are used
to step down voltage further to 415/240 volts for household consumption.
A number of procedures have been put in place for reliability and security maintenance of South
Australia power generation and network. Emergency generators are maintained under current schedules to
avoid risk of shortfall, more reserves have been identified to reduce scarcity during summer[9]. The
system frequency and voltage are maintained within limits [1].
5

References
[1] Aemo.com.au. South Australia electricity report: November 2018. [online] Available at:
https://www.aemo.com.au/-media/Files/Electricity/NEM/Planning_and_Forecasting/SA_Advisory/
2018/2018-South-Australian-Electricity-Report.pdf [Accessed 19 May 2019].
[2] C. Schauder and H. Mehta, "Vector analysis and control of advanced static VAr compensators,"
in IEE Proceedings C - Generation, Transmission and Distribution, vol. 140, no. 4, pp. 299-306, July
1993.
[3] L. Gyugyi, "Power electronics in electric utilities: static VAR compensators," in Proceedings of the
IEEE, vol. 76, no. 4, pp. 483-494, April 1988.
[4] P. E. Marken, A. C. Depoian, J. Skliutas and M. Verrier, "Modern synchronous condenser
performance considerations," 2011 IEEE Power and Energy Society General Meeting, San Diego, CA,
2011, pp. 1-5
[5] S. Teleke, T. Abdulahovic, T. Thiringer and J. Svensson, "Dynamic Performance Comparison of
Synchronous Condenser and SVC," in IEEE Transactions on Power Delivery, vol. 23, no. 3, pp. 1606-
1612, July 2008.
[6] Jumaat, Siti Amely, Ismail Musirin, and Mazliya Mohd Baharun. "A voltage improvement of
transmission system using static var compensator via matlab/simulink." Indonesian Journal of Electrical
Engineering and Computer Science 6.2 (2017): 330-337.
[7] Salas, Mateo, and Oliva Vianey. "Load balancing, power factor correction, and voltage regulation
using a static var compensator." (2017).
[8] Cheng, Miao-miao, et al. "Characteristics of the magnetic energy recovery switch as a static Var
compensator technology." IET Power Electronics 8.8 (2015): 1329-1338.
[9] Saddler, Hugh. "South Australia makes a fresh power play in its bid to end the blackouts." Chain
Reaction 129 (2017): 14.
6