SSSC (Static Synchronous Series Compensator)

A SSSC is a static synchronous generator operatedwithout an external electric energy source as a series compensator whose output voltage is in quadrature with, and controllable independently of the line current for the purpose of increasing ordecreasing the overall reactive voltage drop across the line and thereby controlling the transmittedelectric power. The SSSC may include transiently rated energy source or energy absorbing device to enhance the dynamic behaviour of the power system by additional temporary real power compensation, to increase or decrease momentarily, the overall real voltage drop across

the line.

An SSSC incorporates a solid state voltage sourceinverter that injects an almost sinusoidal voltage of variable magnitude in series with a transmission line. The SSSC has the same structure as that of a

STATCOM except that the coupling transformer of an SSSC is connected in series with the transmission line. The injected voltage is mainly in quadrature with the line current. A small part of injected voltage, which is in phase with the line current, provides the losses in the inverter. Most of injected voltage, which is in quadrature with the line current, emulates a series inductance or a series capacitance thereby altering the transmission line series reactance. This emulated reactance, which can be altered by varing the magnitude of injected voltage, favourably influences the electricpower flow in the transmission line. The structure

of SSSC shown in Fig.7.

SSSC is a solid-state synchronous voltage source employing an appropriate DC to AC inverter with

gate turn-off thyristor. It is similar to the STATCOM, as it is based on a DC capacitor fed VSI that generates a three-phase voltage, which is then injected in a transmission line through a transformer connected in series with the system. In SSSC, the resonance phenomena has been removed. So SSSC is having more superior performance as compare to TCSC. The main control objective of the SSSC is to directly control the current, and indirectly the power, flowing through the line by controlling the reactive power exchange between the SSSC and the AC system. The main advantage of this controller over a TCSC is that it does not significantly affect the impedance of the transmission system and, therefore, there is no danger of having resonance problem.

This device work the same way as the STATCOM. It has a voltage source converter serially connected to a transmission line through a transformer. It is necessary an energy source to provide a continuous voltage through a condenser and to compensate the losses of the VSC.

A SSSC is able to exchange active and reactive power with the transmission system. But if our only aim is to balance the reactive power , the energy source could be quite small. The injected voltage can be controlled in phase and magnitude if we have an energy source that is big enough for the purpose. With reactive power compensation only the voltage is controllable, because the voltage vector forms 90º degrees with the line intensity. In this case the serial injected voltage can delay or advanced the line current. This means that the SSSC can be uniformly controlled in any value, in the VSC working slot.

SSSC - A VSC interfaced in series to a transmission line

The Static Synchronous Series Compensator (SSSC) uses a VSC interfaced in series to a transmission line, as shown in the Figure.

Again, the active power exchanged with the line has to be maintained at zero hence, in steady state operation, SSSC is a functional equivalent of an infinitely variable series connected capacitor. The SSSC offers fast control and it is inherently neutral to sub-synchronous resonance. As mentioned, Static Synchronous Series Compensator (SSSC) is placed in the group of series connected FACTS devices, SSSC consists of a voltage source inverter connected in series through a coupling transformer to the transmission line. A source of energy is required for providing and maintaining the DC voltage across the DC capacitor and compensation of SSSC losses.

The line reactance is constant and by adding variable series (capacitive/inductive) reactance, the amount of compensation can be controlled. The degree of series compensation in this case is defined

where XL is the line inductive reactance and Xq is the emulated series reactance. In the inductive mode of operation, the line current decreases as the inductive reactance compensation level increases from 0% to 100%. In the capacitive mode of operation the line current increases with the capacitive reactance compensation level from 0% to 33%. It is worthy to note from the diagrams that SSSC not only can increase the transferable power but it can also decrease it, simply by reversing the polarity of the injected voltage. The reversed voltage is added directly to the line voltage drop as if the line impedance was increased. The effects of the compensating reactance on the normalized power flow in the transmission line are as follows: when the emulated reactance is inductive, the active and reactive power flow decrease and the effective reactance increases as the reactance compensation increases in the negative direction, and when the emulated reactance is capacitive the active and reactive power flow increases and the effective reactance decreases as the reactance compensation increases in the positive direction. We can also approve of the general use of a serial controllable condenser in the case of SSSC it controls the dynamic energy flow and improves the stability between the angle and the voltage. The fact that a SSSC can induce capacitive and inductive voltage increases the operation field of the device. During the stability slot the potential of damping electromechanical oscillations increases.