devices are presented.

Index Terms--inverters, load modeling, power electronics,
power system voltage stability, shunt capacitors, static VAR
compensators, synchronous condenser.
I. N
OMENCLATURE

AVC: Adaptive VAR Compensator.
D-SMES: Distributed Superconducting Magnetic Energy
Storage.
DSTATCOM: Distribution Static Compensator.
D-VAR: Dynamic VAR Compensator.
Voltage Stability: The ability of a power system to maintain
steady voltages at all buses in the system after being subjected
to a disturbance from a given initial operating condition.
SVC: Static VAR Compensator.
II. I
NTRODUCTION

HE potential effects of voltage instability resulting from
the slow recovery of the power system voltages following
a major disturbance, such as a transmission line fault, are well
documented in the literature [1-3]. Transmission utilities have
traditionally addressed voltage stability concerns by installing
large SVCs or synchronous condensers to provide the
necessary dynamic reactive power support to the system
following a major disturbance.
The emergence of new advanced VAR compensators
utilizing power electronics with binary switched capacitors
and inverter-based systems with or without energy storage
provide utility transmission planning engineers with

Ernst H. Camm is with S&C Electric Company, Power Systems Services
Division, Chicago, IL USA (e-mail: ecamm@sandc.com).
Thompson Adu is with S&C Electric Company, Power Systems Services
Division, Chicago, IL USA (e-mail: tadu@sandc.com).
alternative solutions to the voltage stability problem.

Superconducting magnetic energy storage systems (D-SMES)
utilizing magnetic energy storage in the form of a
superconducting coil and inverter technology have lead the
way in utility applications of these new advanced VAR
compensators [4]. Other commercially-available advanced
VAR compensators are now increasingly being applied on
utility systems for voltage stability support as well as for
voltage regulation purposes. Also, these devices are used to
improve the fault ride-through capability of wind turbines in
wind farm applications.
An evaluation of commercially-available advanced VAR
compensators for improving power system voltage stability is
presented to highlight the differences in design and
performance of these devices. For the purposes of evaluation,
commercially-available advanced compensators are grouped
into three categories, namely:
Power-electronically-switched capacitors.
Inverter-based systems without energy storage.
Inverter-based systems with energy storage.
The evaluation includes a discussion of the design and
basic concept of operation, performance of the compensator
through dynamic simulation in a simple utility system,
application, operation, and other considerations.
III. E
VALUATION OF
A
DVANCED
V
AR
C
OMPENSATORS

A. Compensator Design and Concept of Operation
1) Power-electronically-switched capacitors
Compensators utilizing power-electronically-switched
capacitors (e.g., AVC) typically consist of three or more
stages of low-voltage capacitors. Capacitor stages are
typically sized in binary increments, i.e., if the size of the first
stage of capacitors is Q kvar per phase, the size of the second
and third stages would be 2Q and 4Q, respectively. Reactors
are typically used in series with each stage of capacitors for
detuning to eliminate harmonic resonance and large inrush
currents. Capacitors are charged to peak system voltage and
switched through thyristors at peak voltage to eliminate any
switching transients.
The AVC can respond to voltage fluctuations in one cycle,
or as fast as ½ cycle in specially-designed units. Single units
with capacity of up to 24 Mvar at 690 V or 120 Mvar at 15 kV
can be applied for dynamic voltage support. A step-up
Evaluating Advanced VAR Compensators for
Improving Power System Voltage Stability
Ernst H. Camm, Member, IEEE, and Thompson Adu, Senior Member, IEEE


©IEEE. Presented at the IEEE Power Systems Conference and Exposition New York, New York October 11 13, 2004
T

Page 2
transformer would typically be used to step the output voltage
up to distribution or transmission voltage level.
Since the AVC uses binary-switched capacitors, the
reactive power output occurs in discrete steps. In a three-
stage unit the total output can be varied over 7 discrete steps,
and in 15 steps in a four-stage unit. Since shunt-connected
capacitors are utilized to provide reactive power output, the
reactive power output is proportional to the square of the bus
voltage.
2) Inverter-based systems without energy storage
These compensators (e.g., D-VAR and DSTATCOM)
utilize shunt-connected voltage-source inverters to control the
reactive power flow. Reactive power flow is controlled by
adjusting the magnitude of the voltage output from the
inverter relative to the bus voltage. Units typically have
output filters and a step-up transformer to connect to the
distribution bus. Typical D-VAR units are rated 480 V and
consists of multiple 250 kVA inverter modules arranged for
an output of up to ±8 Mvar continuous. Units have a one-
second overload capability ranging from 2.3 to 3 times the
continuous rating. After one second the output ramps down to
its continuous rating in another second.
The reactive power output of an inverter-based
compensator is proportional to the bus voltage.
3) Inverter-based systems with energy storage
The D-SMES is currently the only commercially-available
inverter-based system that has been applied with energy
storage for voltage stability applications. The system is
similar to the D-VAR, with an additional superconducting
magnetic energy storage module with peak output power
capability of 3 MW and an average output power capability of
2.5 MW over the first 0.5 seconds of discharge [4].
The reactive power output of this compensator is also
proportional to the bus voltage.
B. Compensator Performance Evaluation
1) Simulation model of power system
To evaluate the relative performance of the three categories
of advanced VAR compensators for voltage stability support,
a 138-kV utility system with three relatively weak ties was
selected. See Figure 1. The three-phase short-circuit MVA at
the three ties were:
Bus #41: 670 MVA.
Bus #3: 335 MVA.
Bus #42: 1340 MVA.
Shaw Power Technologies Inc.s PSS/E load flow and
dynamic simulation software was used to perform the
dynamic simulations.
The dynamic response of the power system following a
major disturbance (i.e., short-term, large-disturbance voltage
stability) is largely determined by the characteristics of the
system loads and the strength of the power system. Analysis
involving system dynamic response to identify potential short-
term voltage instability is critically dependent on the modeling
of the system loads. Load modeling guidelines for power
flow and dynamic simulations are presented in [5].

Guidelines include recommendations on the modeling of
discharge lighting, dynamic induction motor models, dynamic
synchronous machine models, transformer saturation, load
shedding, dynamic constant energy load models, load changes
due to tap changer operation, etc. The effects of certain types
of air-conditioner motor loads, which may stall at voltage
levels below 60% of nominal lasting for 5 cycles or longer is
particularly important in considering the behavior of motor
loads [1].
Fig. 1. Simplified one-line diagram of 138-kV utility system used in the
dynamic simulations to evaluate performance of advanced VAR
compensators.

When using PSS/E for voltage stability simulation utility
system loads are typically split according to the percentage of
large induction motors, small induction motors, discharge
lighting, transformer saturation, constant power loads (other
than motor loads), and remaining loads. The remaining loads
are assumed to have a real power variation based on voltage
raised to a specified power (K
p
), and a reactive power
variation based on the square of the voltage. For the system
shown in Figure 1 the following load distribution was
assumed for loads at each load bus:
Small motor load: 45%.
Large motor load: 15%.
Discharge lighting: 20%.
Constant power: 5%.
Other loads: 15% (with K
p
equal to 1.55).
2) Simulation models of advanced VAR compensators
PSS/E simulation models of the AVC, D-VAR, and
D-SMES were used to represent the dynamic performance of
the three categories of advanced VAR compensators. A base
rating of ±8 Mvar continuous and one-second overload
capability of 18 Mvar were selected for the D-VAR and
connected through a step-up transformer at bus #21. This
rating was determined to ensure that the voltage recovers to
80% of nominal in 20 cycles (333 milliseconds) or less at all
transmission buses in the system as dictated by the

Page 3
NERC/WECC reliability criteria for a category B disturbance
[6]. Although these criteria do not specifically address short-
term, large-disturbance voltage stability performance
requirements, it was used in this paper as a measure of the rate
of voltage recovery for comparing the relative performance of
the advanced var compensators. All parameters used for the
D-VAR model were identical to those described in [4], but
without the superconducting magnetic energy storage