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Tuesday, 26 August 2014

STATCOM for Indian Grid: Advanced state-of-the-art Solution for Dynamic Reactive Compensation

Reactive Power balance in a Power System:

The reactive power balance in a power system varies with time. This variation in reactive power balance may bring in voltage variation or even voltage collapse. Dynamic i.e. the fast reactive power support, in the power system is very critical to maintain the system reliability. It helps to recover the voltage following a system disturbance.

To provide dynamic reactive capability, one of the following type of devices is needed.

  1. Static VAR compensator (SVC), or
  2. Static Synchronous Compensator (STATCOM), or
  3. Synchronous Condenser.

STATCOM belongs to the family of FACTS devices and is capable of providing both capacitive and inductive reactive power. It continuously provides the reactive power according to the voltage variations and thus helps to maintain the grid stability.

Principal Components and Working of STATCOM:

The principal components of  STATCOM are:
  1. the PWM Converter,
  2. the Coupling Reactor, and
  3. the DC Capacitor.
The converter topology depends on the rated voltage and power. STATCOM is usually a Voltage Source Converter (VSC) based compensator, but can be implemented using both the voltage or current source approach. The reactive power at the output depends on its terminal voltage. The device behaves as an inductor in case the AC system voltage is higher. The coupling reactor absorbs the instantaneous voltage difference between the grid voltage and the converter voltage. If the output voltage of the STATCOM is higher than the AC voltage at the point of connection, the STATCOM behaves as a capacitor. In either mode the reactance of the STATCOM can be varied continuously.

The coupling reactor also reduces the current harmonic distortion and helps to stabilize the switching frequency. These reactors are designed considering the maximum allowable current harmonics or the control scheme to be implemented. The DC capacitor maintains a constant DC voltage equal to the reference value. The selection of capacitor is based on the maximum allowable voltage harmonic distortion and the maximum tolerance value of DC voltage fluctuations. 

Advantages of STATCOM over SVC:

STATCOMs were preferred over Static VAR compensator (SVC) because of their faster response, reduced space requirement, and advanced technology.  The STATCOM has a much faster response than the SVC mainly because of the fast switching provided by the Insulated Gate Bipolar Transistor (IGBTs) of the VSC.  

The STATCOM also has better reactive power support at low AC voltages than the SVC, as the reactive power produced by a STATCOM decreases linearly with the AC voltage whereas in case of SVC the reactive power produced is proportional to the square of the AC voltage.  

STATCOM may also be combined with mechanically switched reactors and capacitors for the reactive power compensation under steady state. Installing STATCOMs  at one or more points in a power system helps in maintaining a smooth and stable voltage profile under varying operating conditions, thus enhancing the power transfer ability of the transmission system. 

Dynamic Reactive Compensation of Indian grid:

Power Grid Corporation of India Limited (POWERGRID), the Central Transmission Utility (CTU) of India, is responsible for planning, implementation and operation and maintenance of Inter-state Transmission System. It is also engaged in the operation of National and Regional Power Grids. 

POWERGRID currently owns and operates about 1,38,850 circuit kilometres of Extra High Voltage (E.H.V.) AC and DC Transmission lines throughout the country. It has 217 EHV AC and HVDC substations with a transformation capacity exceeding 2,88, 500 MVA. These transmission lines and systems transfers nearly half of the electrical energy produced in the country.  

POWERGRID, has a very good and proven track record. Its consistency rate is about 99% achieved through adopting global standards and deployment of hi-tech operation and maintenance practises. The Corporation after having interaction with various utilities and manufacturers, has decided to install STATCOM for the dynamic reactive compensation of Indian grid. It has proposed to install STATCOM of ± 200 and ± 300 MVAr at 13 locations in the WR, NR, ER, and SR of the Indian Power System.  

To further assist on the Flexible AC Transmission Systems (FACTS) and High Voltage Direct Current (HVDC) technology, POWERGRID has appointed Dr. Narain G. Hingorani, a consultant of international repute.

Ref: Study Report on Dynamic Reactive Compensation, POWERGRID