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.

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.

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.

### Concept of Static Synchronous Series Compensator:

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.

In
comparison to series capacitor, the SSSC can control the magnitude of injected
voltage

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:

*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.