Basics of Static Synchronous Series Compensation

Power transfer over long AC transmission line is mainly limited by the series reactance of the line. The series capacitive compensation decreases the overall effective series reactance of the transmission line i.e. the series capacitive compensation cancels a portion of the line's inductive reactance and hence increases the transmittable power. 

Thus, controllable series line compensation can be applied to control the flow of power in transmission lines. Along with the application of fast controls we can minimize the effect of system disturbances, thereby reducing the required stability margin.

Degree of Compensation:

The ratio of the capacitive reactance of the compensator to that of the inductive reactance of the line is called degree of compensation. The transmittable power over a transmission line rapidly increases with the degree of series compensation.

Another explanation of the series compensation which is helpful in understanding the concept of converter based power flow controller is as given below:

“In order to increase the power flow over a transmission line and hence the current through the line, the voltage across the series reactance must be increased. This can be done by an appropriate series connected compensator, which produces a voltage Vc opposite to the existing voltage across the series reactance, thereby causing the voltage across reactance to increase.”

Thus the series capacitive compensation works by increasing the voltage across the inductive reactance of the transmission line, which results in corresponding increase in the transmission line current and hence the transmitted power. 

Concept of Static Synchronous Series Compensator:

The Static Synchronous Series Compensator (SSSC) was proposed by Dr L. Gyugyi in 1989. Similar to STATCOM, the SSSC uses a Voltage Sourced Converter (VSC) and is connected in series with the transmission line through a coupling transformer. It is also called series STATCOM. Figure 1 shows a SSSC connected in a transmission line through a coupling transformer.

 Fig.1:  SSSC connected in a transmission line through a coupling transformer.

The VSC generates an AC voltage that is applied to the coupling transformer. With suitable internal control, the magnitude and phase angle of the three alternating voltages produced by the controller i.e. VSC can be controlled. 

The SSSC can generate or absorb reactive power and can also exchange active power with the Power System when the DC terminal of the VSC is connected to a suitable storage battery or other adequate energy source.

Comparison with series capacitor:

In comparison to series capacitor, the SSSC can control the magnitude of injected voltage Vq independent of the transmission line current. The SSSC’s output voltage can be reversed by simple controlling to make it lag or lead the line current by 90^o. SSSC can increase the transmitted power by a fixed fraction of the maximum power transmittable by the uncompensated line, independent of the angle power angle δ. It can decrease, as well as increase the power flow to the same degree by just reversing the polarity of the compensating voltage.

SSSC is more capable of controlling the transmittable power when compared to controlled shunt compensation. Hence, SSSC can be more effective in increasing the transient stability limit and in damping power oscillations.