PSMA Consulting - Power System Studies - Static Synchronous Series Compensator (SSSC)

Static Synchronous Series Compensator (SSSC)

A static synchronous series compensator (SSSC) is a series-connected reactive compensator and it is developed based on a voltage source converter (VSC). The SSSC can provide active and reactive power compensation by injecting the AC voltage (i.e., compensating voltage) in series with the line. The SSSC can be applied for power flow control, dynamic and transient stability, voltage stability, and damping oscillations caused by sub-synchronous resonance (SSR).

Principle of Operation of SSSC

The schematic diagram of SSSC is shown in Fig. 1. The SSSC consists of the VSC, coupling transformer, and external DC power supply such as an energy storage element (optional). In SSSC, the coupling transformer is connected in series with the line. The principle of SSSC is to inject the compensating voltage in series with the line. The phase angle difference between injected compensation voltage and line current determines the active and reactive power compensation level. The phase angle of injected compensating voltage can be varied by the control action of the VSC (whether it can lead or lag the line current). The active and reactive power flow of the transmission line is given in equations (1) & (2).

Fig. 1. Schematic of SSSC.

Fig. 2. Active and reactive power flow scenarios.

Where, V is the magnitude of the voltage at the sending and receiving end of the line (assume both ends have the same voltage, VS=VR), δ is the phase angle difference between two ends of the line, VC is the injected compensating voltage, X is the series reactance of the line.

The active and reactive power flow scenarios of SSSC are shown in Fig. 2. Where, VC is the compensating voltage, VCP & VCq is the active and reactive component of the compensating voltage and I is the line current.

If the compensating voltage (VC) is 90° phase relation with the line current (I), then the reactive power (Q) can be exchanged. If the compensating voltage (VC) is leading the line current (I) by 90°, then the reactive power will be absorbed (inductive mode). If the compensating voltage (VC) is lagging the line current (I) by 90°, then the reactive power will be supplied (capacitive mode).

If the injected compensation voltage (VC) has any other phase relation with the line current (I), the active power (P) can be exchanged. The SSSC can provide active power compensation using an external DC power supply (i.e., energy storage element).

V-I Characteristics of SSSC

The capacitive/inductive compensating voltage vs line current Characteristics of SSSC are shown in Fig. 3. Where, VL max is the maximum inductive voltage (i.e., maximum compensation voltage in inductive mode), Vc max is the maximum capacitive voltage (i.e., maximum compensation voltage in capacitive mode) and Imax is the maximum line current. The SSSC can control the reactive power from -1 per unit (capacitive) to +1 per unit (inductive). The SSSC can exchange both active and reactive power with the transmission system when the energy storage element is connected to the DC side of the VSC.

Fig. 3. V-I characteristics of SSSC.

References

Mathur, R. Mohan, and Rajiv K. Varma, Thyristor-based FACTS controllers for electrical transmission systems, 2002.
Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS concepts and technology of flexible AC transmission systems”, 1999.
K.R. Padiyar, FACTS Controllers in Power Transmission and Distribution, 2007.