24/08/2007 STATCOM (Static Synchronous Compensator) It is a device connected in derivation, basically composed of a coupling transformer, that serves of link between the electrical power system (EPS) and the voltage synchronous controller (VSC), that generates the voltage wave comparing it to the one of the electric system to realize the exchange of reactive power. The control system of the STATCOM adjusts at each moment the inverse voltage so that the current injected In the network is in cuadrature to the network voltage, in these conditions P=0 and Q=0. In its most general way, the STATCOM can be modeled as a regulated voltage source Vi connected to a voltage bar Vs through a transformer. Donsin 24/08/2007 STATCOM (Static Synchronous Compensator) STATCOM - A VSC interfaced in shunt to a transmission line The STATic COMpensator (STATCOM) uses a VSC interfaced in shunt to a transmission line. In most cases the DC voltage support for the VSC will be provided by the DC capacitor of relatively small energy storage capability - hence, in steady state operation, active power exchanged with the line has to be maintained at zero, as shown symbolically in the Figure. Donsin 24/08/2007 STATCOM (Static Synchronous Compensator) STATCOM - A VSC interfaced in shunt to a transmission line With the active power constraint imposed, the control of the STATCOM is reduced to one degree of freedom, which is used to control the amount of reactive power exchanged with the line. Accordingly, a STATCOM is operated as a functional equivalent of a static VAR compensator; it provides faster control than an SVC and improved control range. Donsin 24/08/2007 STATCOM (Static Synchronous Compensator) Each GTO converter generates a voltage that is stepped up by a line- side-series-connected multi-stage converter transformer. The converter transformer enables the build-up of a sine-wave voltage in both magnitude and phase. Because STATCOMs with multi-stage converter transformers do not generate significant internal harmonics, they generally require minimal, or no, harmonic filtering. If the number of firing pulses for the GTOs is increased (i.e., pulse-width modulation (PWM) order), the harmonics are further decreased. High-side voltage is generally used as a controller input, as indicated in the figure. Donsin 24/08/2007 STATCOM (Static Synchronous Compensator) The figure shows the equivalent circuit of a STATCOM system. The GTO converter with a 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 leakage inductance. Equivalent circuit of a STATCOM Donsin 24/08/2007 STATCOM (Static Synchronous Compensator) Using the classical equations that describe the active and reactive power flow in a line in terms of Vi and Vs, the transformer impedance (which can be assumed as ideal) and the angle difference between both bars, we can defined P and Q. The angle between the Vs and Vi in the system is d. When the STATCOM operates with d=0 we can see how the active power send to the system device becomes zero while the reactive power will mainly depend on the voltage module. This operation condition means that the current that goes through the transformer must have a +/-90 phase difference to Vs. In other words, if Vi is bigger than Vs, the reactive will be send to the STATCOM of the system (capacitive operation), originating a current flow in this direction. In the contrary case, the reactive will be absorbed from the system through the STATCOM (inductive operation) and the current will flow in the opposite direction. Finally if the modules of Vs and Vi are equal, there wont 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 Vs and Vi voltages. The amount of the reactive power is proportional to the voltage difference between Vs and Vi. Donsin 24/08/2007 STATCOM (Static Synchronous Compensator) Principle of operation of a STATCOM There can be a little active power exchange between the STATCOM and the EPS. The exchange between the inverter and the AC system can be controlled adjusting the output voltage angle from the inverter to the voltage angle of the AC system. This means that the inverter can not provide active power to the AC system form the DC accumulated energy if the output voltage of the inverter goes before the voltage of the AC system. On the other hand, the inverter can absorb the active power of the AC system if its voltage is delayed in respect to the AC system voltage. Donsin 24/08/2007 STATCOM (Static Synchronous Compensator) V-I characteristic of a STATCOM The STATCOM smoothly and continuously controls voltage from V1 to V2. However, if the system voltage exceeds a low-voltage (V1) or high-voltage limit (V2), the STATCOMacts as a constant current source by controlling the converter voltage (Vi) appropriate