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STATIC SYNCHRONOUS COMPENSATOR STATCOM

STATCOM is a Static synchronous generator operated as a shunt-connected static VAR compensator whose capacitive or inductive output current can be controlled independent of the ac system voltage. STATCOM is one of the key FACTS Controllers. A STATCOM is a controlled reactive power source. It provides voltage support by generating or absorbing capacitors banks. It regulates the voltage at its terminals by compensating the amount of reactive power in or out from the power system.

When the system voltage is low the STATCOM injects the reactive power to and when the voltage is high it absorbs the reactive power. The reactive power is fed from the Voltage Source Converter (VSC) which is connecting on the secondary side of a coupling transformer as shown in the Figure 1. By varying the magnitude of the output voltage the reactive power exchange can be regulated between the convertor and AC system. STATCOM is such a device in which the modern power electronic converters have been employed. These converters are capable of generating reactive power with no/very little need for large reactive energy storage elements.
Figure 1: Block diagram of STATCOM
OPERATING PRINCIPLE OF STATCOM:
The STATCOM generates a balanced 3-phase voltage whose magnitude and phase can be adjusted rapidly by using semiconductor switches. The STATCOM is composed of a voltage-source inverter with a dc capacitor, coupling transformer, and signal generation and control circuit.

Let V1 be the voltage of power system and V2 be the voltage produced by the voltage source (VSC). During steady state working condition, the voltage V2 produced by VSC is in phase with V1 (i.e. =0) in this case only reactive power is flowing. If the magnitude of the voltage V2 produced by the VSC is less than the magnitude of V1, the reactive power is flowing from power system to VSC (the STATCOM is absorbing the reactive power). If V2 is greater than V1 the reactive power is flowing from VSC to power system (the STATCOM is producing reactive power) and if the V2 is equal to V1 the reactive power exchange is zero. The amount of reactive power can be given as:
Q=V1(V1−V2)X

V-I CHARACTERISTICS OF STATCOM:

STATCOM exhibits constant current characteristics when the voltage is low/high under/over the limit. This allows STATCOM to delivers constant reactive power at the limits compared to SVC. Since SVC is based on nominal passive components, its maximum reactive current is proportional to the network voltage. While for STATCOM, its reactive current is determined by the voltage difference between the network and the converter voltages and therefore, its maximum reactive current is only limited by the converter capability and is independent of network variation.
Figure 2: V-I Characteristics of STATCOM
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