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Friday, 26 November 2010

DSTATCOM - Distribution SATATic COMpensator

Shunt Connected Controllers at distribution and transmission levels usually fall under two catogories - Static Synchronous Generators (SSG) and Static VAr Compensators (SVC).

A Static Synchronous Generator (SSG) is defined by IEEE as a self-commutated switching power converter supplied from from an appropriate electric energy source and operated to produce a set of adjustable multiphase voltages , which may be coupled to an ac power system for the purpose of exchanging independently controllable real and reactive power. When the active energy source (usually battery bank, Superconducting Magnetic Energy Storage etc) is dispensed with and replaced by a DC Capacitor which can not absorb or deliver real power except for short durations the SVG becomes a Static Synchronous Compensator (STATCOM) . STATCOM has no long term energy support in the DC Side and can not exchange real power with the ac system ; however it can exchange reactive power. Also , in principle, it can exchange harmonic power too. But when a STATCOM is designed to handle reactive power and harmonic currents together it gets a new name – Shunt Active Power Filter. So a STATCOM handles only fundamental reactive power exchange with the ac system.

STATCOMs are employed at distribution and transmission levels – though for different purposes. When a STATCOM is employed at the distribution level or at the load end for power factor improvement and voltage regulation alone it is called DSTATCOM. When it is used to do harmonic filtering in addition or exclusively it is called Active Power Filter. In the transmission system STATCOMs handle only fundamental reactive power and provide voltage support to buses. In addition STATCOMs in transmission system are also used to modulate bus voltages duting transient and dynamic disturbances in order to improve transient stability margins and to damp dynamic oscillations.

IEEE defines the second kind of Shunt Connected Controller called Static VAr Compensator (SVC) as a shunt connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system (typically bus voltage).Thyristor-switched or thyristor-controlled capacitors/inductors and combinations of such equipment with fixed capacitors and inductors come under this.This has been covered in an earlier lecture and this lecture focusses on STACOMs at distribution and transmission levels.

PWM Voltage Source Inverter based Static VAr Compensators (referred to as SVC here onwards) began to be considered a viable alternative to the existing passive shunt compensators and Thyristor Controlled Reactor (TCR ) based compensators from mid-eighties onwards. The disadvantages of capacitor/inductor compensation are well known. TCRs could overcome many of the disadvantages of passive compensators. However they suffered from two major disadvantages ;namely slow response to a VAr command and injection of considerable amount of harmonic currents into the power system which had to be cancelled by special transformers and filtered by heavy passive filters.