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.
It became clear in the early eighties that apart from the mundane job of pumping lagging/leading VArs into the power system at chosen points ,VAr generators can assist in enhancing stability of the power system during large signal and small signal disturbances if only they were faster in the time domain. Also ,they can provide reactive support against a fluctuating load to maintain the bus voltage regulation and to reduce flicker problems,provide reactive support to control bus voltages against sag and swell conditions and provide reactive support to correct the voltage unbalance in the source – if only they were fast enough. PWM STATCOMs covered in this lecture are capable of delivering lagging/leading VArs to a load or to a bus in the power system in a rapidly controlled manner.
High Power STATCOMs of this type essentially consist of a three phase PWM Inverter using GTOs,Thyristors or IGBTs, a D.C. side capacitor which provides the D.C. voltage required by the inverter,filter components to filter out the high frequency components of inverter output voltage,a link inductor which links the inverter output to the a.c supply side,interface magnetics (if required) and the related control blocks. The Inverter generates a three-phase voltage, which is synchronized with the a.c supply ,from the D.C. side capacitor and the link inductance links up this voltage to the a.c source. The current drawn by the Inverter from the a.c supply is controlled to be mainly reactive(leading or lagging as per requirement) with a small active component needed to supply the losses in the Inverter and Link Inductor (and in the magnetics,if any).The D.C. side capacitor voltage is maintained constant( or allowed to vary with a definite relationship maintained between its value and the reactive power to be delivered by the Inverter) by controlling this small active current component. The currents are controlled indirectly by controlling the phase angle of Inverter output Voltage with respect to the a.c side source voltage in the “Synchronous Link Based Control Scheme” whereas they are controlled directly by current feedback in the case of “Current Controlled Scheme”.In the latter case the Inverter will be a Current Regulated one ,i.e. its switches are controlled in such a way that the Inverter delivers a commanded current at its output rather than a commanded voltage (the voltage required to see that the commanded current flows out of Inverter will automatically be synthesized by the Inverter).Current Control Scheme results in a very fast STATCOM which can adjust its reactive output within tens of microseconds of a sudden change in the reactive demand.