STATCOM | Static Synchronous Compensator

STATCOM

STATCOM is an acronym as a Static Synchronous Compensator. It is a shunt-connected compensation device. The STATCOM installed to support the Power System network that has poor Power Factor and poor voltage regulation.

This device used to control the power flow and improve the voltage stability of the electrical power system. The STATCOM is a power electronics device that uses power electronics switches  like GTO and IGBT.

How STATCOM works?

It is a voltage source converter-based device. The STATCOM has very little active power capability because of the voltage created from the DC Capacitors. But the active power capability can increase by connecting a suitable energy source with the DC capacitor.

The amplitude of the voltage source will decide the reactive power at the terminals of the STATCOM.

If the terminal voltage of VSC is higher than the AC voltage at the point of connection, the STATCOM generates a reactive current. And if the terminal voltage of VSC is lower than the AC voltage at the point of connection, the STATCOM absorbs the reactive power.

The response time of the STATCOM is shorter than the Static VAR Compensation (SVC). Due to fast switching using IGBTs of voltage source converter.

The STATCOM provides better reactive power support at low AC voltage than an SVC. Since the reactive power from a STATCOM decreases linearly with the AC voltage.

STATCOM
STATCOM

Which Equipment needs to install the STATCOM?

It 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

The voltage source converts the DC voltage (from capacitor bank) to AC voltage and this AC voltage supplied to the power system. The VSC 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 supplied from a capacitor or capacitor bank, the output is AC voltage and it connected with a 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 a sine wave but it is a square wave. For this method, GTO used as a switching device. In this method, tertiary winding used to reduce the harmonics.

In the second method, the output is a sine wave and for this method, IGBT used as a switching device. And in this method, shunt filters used to reduce harmonics.

3) DC Capacitor

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

4) Harmonic Filter

It used to reduce harmonics. In the inverter, high-frequency harmonics generated. This filter 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

V-I characteristic of STATCOM
V-I characteristic of STATCOM

Mode of Operation

1) Over-excited mode of operation

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

2) Under-excited mode of operation

In this mode, the STATCOM absorbs reactive power. In this mode of operation, the output voltage 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 the AC system to STATCOM is zero. So, the exchange of reactive power is not possible.

Application

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

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