STATCOM(Static Synchronous Compensator) By: Flamin chacko Electrical EngineerIntroduction This s a small note discussing about one of compensators that is mostly used inpower system for improving quality of power transmitted form sourse to load.here in this paperits concentrated on the very basic of this compensator telling answer for why? What? When?How? etc.. As an electrical engineer in our day to day life we are dealing with real and reactivepower which can determine the quality of power transmitted from source to load.STATCOM A static synchronous compensator (STATCOM), also known as a "static synchronouscondenser" ("STATCON"), is a regulating device used on alternating current electricitytransmission networks. It is based on a power electronics voltage-source converter and can act aseither a source or sink of reactive AC power to an electricity network. If connected to a source ofpower it can also provide active AC power. It is a member of the FACTS family of devices.
The figure shows the equivalent circuit of a STATCOM system. The GTO converter witha dc voltage source and the power system are illustrated as variable ac voltages in this figure.These two voltages are connected by a reactance representing the transformer leakageinductance.Using the classical equations that describe the active and reactive power flow in a line in termsof Vi and Vs, the transformer impedance (which can be assumed as ideal) and the angledifference between both bars, we can defined P and Q. The angle between the Vs and Vi in thesystem is d. When the STATCOM operates with d=0 we can see how the active power send tothe system device becomes zero while the reactive power will mainly depend on the voltagemodule. This operation condition means that the current that goes through the transformer musthave a +/-90º phase difference to Vs. In other words, if Vi is bigger than Vs, the reactive will besend to the STATCOM of the system (capacitive operation), originating a current flow in thisdirection. In the contrary case, the reactive will be absorbed from the system through theSTATCOM (inductive operation) and the current will flow in the opposite direction. Finally ifthe modules of Vs and Vi are equal, there won´t be nor current nor reactive flow in the system.
Thus, we can say that in a stationary state Q only depends on the module difference between Vsand Vi voltages. The amount of the reactive power is proportional to the voltage differencebetween Vs and Vi.There can be a little active power exchange between the STATCOM and the EPS. Theexchange between the inverter and the AC system can be controlled adjusting the outputvoltage angle from the inverter to the voltage angle of the AC system. This means that theinverter can not provide active power to the AC system form the DC accumulated energy if theoutput voltage of the inverter goes before the voltage of the AC system. On the other hand, theinverter can absorb the active power of the AC system if its voltage is delayed in respect tothe AC system voltage.The STATCOM smoothly and continuously controls voltage from V1 to V2. However, if thesystem voltage exceeds a low-voltage (V1) or high-voltage limit (V2), the STATCOM acts as aconstant current source by controlling the converter voltage (Vi) appropriately.
V-I characteristic of a STATCOMThus, when operating at its voltage limits, the amount of reactive power compensation providedby the STATCOM is more than the most-common competing FACTS controller, namely theStatic Var Compensator (SVC). This is because at a low voltage limit, the reactive power dropsoff as the square of the voltage for the SVC, where Mvar=f(BV2), but drops off linearly with theSTATCOM, where Mvar=f(VI). This makes the reactive power controllability of the STATCOMsuperior to that of the SVC, particularly during times of system distress.Application of the Three-Phase STATCOM in Voltage Stability Voltage stability is one of the biggest problems in power systems. Engineers andresearchers have met with the purpose of discussing and trying to consolidate a definitionregarding to voltage stability, besides proposing techniques and methodologies for their analysis.Most of these techniques are based on the search of the point in which the system’s Jacobianbecomes singular; this point is referred as the point of voltage collapse or maximum load abilitypoint. The series and shunt compensation are able to increase the maximum transfer capabilitiesof power network .Concerning to voltage stability, such compensation has the purpose ofinjecting reactive power to maintain the voltage magnitude in the nodes close to the nominalvalues, besides, to reduce line currents and therefore the total system losses. At the present time,thanks to the development in the power electronics devices, the voltage magnitude in some nodeof the system can be adjusted through sophisticated and versatile devices named FACTS. One ofthem is the static synchronous compensator (STATCOM).
USES Usually a STATCOM is installed to support electricity networks that have a poor powerfactor and often poor voltage regulation. There are however, other uses, the most common use isfor voltage stability. A STATCOM is a voltage source converter (VSC)-based device, with thevoltage source behind a reactor. The voltage source is created from a DC capacitor and thereforea STATCOM has very little active power capability. However, its active power capability can beincreased if a suitable energy storage device is connected across the DC capacitor. The reactivepower at the terminals of the STATCOM depends on the amplitude of the voltage source. Forexample, if the terminal voltage of the VSC is higher than the AC voltage at the point ofconnection, the STATCOM generates reactive current; on the other hand, when the amplitude ofthe voltage source is lower than the AC voltage, it absorbs reactive power. The response time ofa STATCOM is shorter than that of an SVC, mainly due to the fast switching times provided bythe IGBTs of the voltage source converter. The STATCOM also provides better reactive powersupport at low AC voltages than an SVC, since the reactive power from a STATCOM decreaseslinearly with the AC voltage (as the current can be maintained at the rated value even down tolow AC voltage).
Other Compensators A static VAR compensator (SVC) can also be used for voltage stability. However, aSTATCOM has better characteristics than a SVC. When the system voltage drops sufficiently toforce the STATCOM output current to its ceiling, its maximum reactive output current will notbe affected by the voltage magnitude. Therefore, it exhibits constant current characteristics whenthe voltage is low under the limit. In contrast the SVCs reactive output is proportional to thesquare of the voltage magnitude. This makes the provided reactive power decrease rapidly whenvoltage decreases, thus reducing its stability. In addition, the speed of response of a STATCOMis faster than that of an SVC and the harmonic emission is lower. On the other hand STATCOMstypically exhibit higher losses and may be more expensive than SVCs, so the (older) SVCtechnology is still widespread.