STATCOM

STATCOM:

STATCOM is an acronym as a Static Synchronous Compensator. It is a shunt compensation device. This device is used to control power flow and improve the transient stability of the electrical power system. For switching of the capacitor, it uses GTO and IGBT.

STATCOM consists of,

* Coupling transformer

            * VSC or CSC (Voltage source converter or Current source converter)

            * DC capacitor

            * Harmonic filter

1) Coupling transformer:

                It provides inductive reactance and it connects the output of the inverter to the power system.

2) VSC or CSC:

                Voltage source converts the DC voltage (from capacitor bank) to AC voltage and this AC voltage is supplied to the power system. The VSC is divided into two types;

                * Square wave inverter using GTO (Gate turn OFF thyristor):

                * PWM inverter using IGBT (Insulated gate bipolar transistor):

                It controls the reactive power flow by using the capacitor. For this converter, the input is DC voltage and it is supplied from a capacitor or capacitor bank, the output is AC voltage and it is connected with power system network via a coupling transformer. The DC voltage can convert into AC by two methods. In the first method, the output is not sine wave but it is a square wave. For this method, GTO is used as a switching device. In this method, tertiary winding is used to reduce the harmonics. In the second method, the output is a sine wave and for this method, IGBT is used as a switching device. In this method, shunt filters are used to reduce harmonics.

3) DC capacitor:

                It is an energy storage device. This capacitor is used as input for the VSC and this DC power of a capacitor is converted into AC.

4) Harmonic filter:

                It is used to reduce harmonics. In the inverter, high-frequency harmonics are generated. This filter is used to prevent harmonics and maintain power quality.

By controlling the magnitude of the 3-phase output voltage of the converter, we can control the reactive power.

V-I characteristic of STATCOM:

Mode of operation of STATCOM:

1) Overexcited mode of operation:

                In this mode, the STATCOM generates reactive power for the ac system. In this mode of operation, the output voltage is increased above the system voltage. The current flows from STATCOM to the ac system.

2) Under the excited mode of operation:

                In this mode, the STATCOM absorbs the reactive power. In this mode of operation, the output voltage is decreased below the supply voltage. The current flows from the ac system to the STATCOM.

3) Normal floating excited mode:

                In this mode of operation, the output voltage is equal to the ac system voltage. The current flowing between ac system to STATCOM is zero. So, exchange of reactive power is not possible.

Application of STATCOM:

This device is used in long-distance transmission lines and power substation. For voltage stability, STATCOM is used in industries where a large amount of reactive load is connected. 

                * Reactive power compensation

                * Better voltage control

                * Maintain balance in each phase of the transmission line

                * Reduce harmonics

                *Power factor correction

                * Increase the power transfer capacity of the transmission line

                * Reduce temporary over-voltage condition in the transmission line

                * Improve transient stability