STATCOM or Static Synchronous Compensator is a shunt device, which uses force-commutated power electronics (i.e. GTO, IGBT) to control power flow and improve transient stability on electrical power networks. It is also a member of the so-called Flexible AC Transmission System (FACTS) devices. The STATCOM basically performs the same function as the static var compensators but with some advantages.
The term Static Synchronous Compensator is derived from its capabilities and operating principle, which are similar to those of rotating synchronous compensators (i.e. generators), but with relatively faster operation.
STATCOMs are typically applied in long distance transmission systems, power substations and heavy industries where voltage stability is the primary concern.
In addition, static synchronous compensators are installed in select points in the power system to perform the following:
- Voltage support and control
- Voltage fluctuation and flicker mitigation
- Unsymmetrical load balancing
- Power factor correction
- Active harmonics cancellation
- Improve transient stability of the power system
A STATCOM is composed of the following components:
A. Voltage-Source Converter (VSC)
The voltage-source converter transforms the DC input voltage to an AC output voltage. Two of the most common VSC types are described below.
1. Square-wave Inverters using Gate Turn-Off Thyristors
Generally, four three-level inverters are utilized to make a 48-step voltage waveform. Subsequently, it controls reactive power flow by changing the DC capacitor input voltage, simply because the fundamental component of the converter output voltage is proportional to the DC voltage.
addition, special interconnection transformers are employed to neutralize
harmonics contained in the square waves produced by individual inverters.
|GTO-based STATCOM Simple Diagram|
2. PWM Inverters using Insulated Gate Bipolar Transistors (IGBT)
It uses Pulse-Width Modulation (PWM) technique to create a sinusoidal waveform from a DC voltage source with a typical chopping frequency of a few kHz. In contrast to the GTO-based type, the IGBT-based VSC utilizes a fixed DC voltage and varies its output AC voltage by changing the modulation index of the PWM modulator.
Moreover, harmonic voltages are mitigated by installing shunt filters at the AC side of the VSC.
This component provides the DC voltage for the inverter.
C. Inductive Reactance (X)
It connects the inverter output to the power system. This is usually the leakage inductance of a coupling transformer.
Mitigate harmonics and other high frequency components due to the inverters.
Basic Principle of Operation
In the case of two AC sources, which have the same frequency and are connected through a series reactance, the power flows will be:
- Active or Real Power flows from the leading source to the lagging source.
- Reactive Power flows from the higher to the lower voltage magnitude source.
Consequently, the phase angle difference between the sources decides the active power flow, while the voltage magnitude difference between the sources determines the reactive power flow. Based on this principle, a STATCOM can be used to regulate the reactive power flow by changing the output voltage of the voltage-source converter with respect to the system voltage.
Modes of Operation
The STATCOM can be operated in two different modes:
A. Voltage Regulation
The static synchronous compensator regulates voltage at its connection point by controlling the amount of reactive power that is absorbed from or injected into the power system through a voltage-source converter.
In steady-state operation, the voltage V2 generated by the VSC through the DC capacitor is in phase with the system voltage V1 (δ=0), so that only reactive power (Q) is flowing (P=0).
1. When system voltage is high, the STATCOM will absorb reactive power (inductive behavior)
2. When system voltage is low, the STATCOM will generate and inject reactive power into the system (capacitive).
Subsequently, the amount of reactive power flow is given by the equation:
Q = [V1(V1-V2)] / X
B. Var Control
In this mode, the STATCOM reactive power output is kept constant independent of other system parameter
STATCOM versus SVC
The STATCOM has the ability to provide more capacitive reactive power during faults, or when the system voltage drops abnormally, compared to ordinary static var compensator. This is because the maximum capacitive reactive power generated by a STATCOM decreases linearly with system voltage, while that of the SVC is proportional to the square of the voltage. Also, the STATCOM has a faster response as it has no time delay associated with thyristor firing. Nevertheless, these advantages come at a higher price (about 20% more).
Hingorani, N. and Gyugyi, L. (2000). Understanding FACTS; Concepts and Technology of Flexible AC Transmission Systems.