STATIC COMPENSATOR (STATCOM) DEFINITION BASIC AND TUTORIALS

This is a shunt device that does not require passive elements like inductors and capacitors. The schematic diagram of a SMIB power system that is compensated by a shunt compensator is shown in Figure 1.10. The STATCOM is built around a voltage source inverter, which is supplied by a dc capacitor. The inverter consists of GTO switches which are turned on and off through a gate drive circuit.

The output of the voltage source inverter is connected to that ac system through a coupling transformer. The inverter produces a quasi sinewave voltage Vo at the fundamental frequency. Let us assume that the losses in the inverter and the coupling transformer are negligible.

The inverter is then gated such that the output voltage of the inverter Vo is in phase with the local bus voltage v. In this situation two ac voltages that are in phase are connected together through a reactor, which is the leakage reactance of the coupling transformer.

Therefore the current ['I is a purely reactive. If the magnitude of the voltage Vm is more than that of the voltage Vo, the reactive current Iq flows from the bus to the inverter. Then the inverter will consume reactive power.

If, on the other hand, the magnitude of Vo is greater than that of Vm, then the inverter feeds reactive power to the system. Therefore through this arrangement the STATCOM can generate or absorb reactive power.

In practice how ever the losses are not negligible and must be drawn from the ac system. This is accomplished by slightly shifting the phase angle of the voltage Vo through a feedback mechanism such that the de capacitor voltage is held constant.

The structure of the GTO-based VSI must be so chosen that the lower order harmonics are eliminated from the output voltage. The VSI will then resemble a synchronous voltage source. Because the switching frequency of each GTOs must be kept low, overall switch ripple needs to be kept low without use of PWM.

This is accomplished by connecting a large number of basic inverter modules. The construction of a 48-step voltage source inverter is discussed in [19].

In this inverter, eight identical elementary 6-step inverters are operated from a common dc bus. Each of these 6-step inverters produces a compatible set of three-phase, quasi-square wave output voltage waveforms.

The outputs of these 6-step inverters are added through a magnetic circuit that contains eighteen single phase three winding transformers and six single-phase two winding transformers. This connection eliminates all low-order harmonics.

The lowest order harmonic on the ac side is 47th while that on the dc side is 48th . The line-to-line output\ voltage of the 48-step inverter is shown in Figure 1.11 along with the fundamental voltage. It can be seen that the output is a stepped approximation of the fundamental sinewave. The construction of a multilevel synchronous voltage source is given in.