Comparison of SVC and STATCOM

1.  Principle of Operation

• SVC is the shunt-connected thyristor-controlled reactor (TCR) and thyristor-switched capacitor (TSC) to control the VAR generation and absorption by controlling the reactive admittance.

• STATCOM is a shunt-connected voltage source converter-based VAR generator to control the VAR generation and absorption.

2.  Operating Characteristics

• Fig 1 shows the V-I characteristics of STATCOM and SVC. The STTACOM can be operated for a full output current range even for low system voltage conditions (for example, 0.2 per unit). Therefore, the maximum capacitive and inductive current of the STATCOM can be achieved for the full AC system voltage.

• In the SVC, the maximum capacitive compensation current is decreased when the AC system voltage is reduced. Therefore, the STATCOM provides better voltage support than the SVC under reduced system voltage conditions when a large system disturbance happens.

3. Transient Stability

• The STATCOM provides a better response in terms of transient stability when compared to the SVC because the STATCOM can maintain the full capacitive compensation current under low voltage conditions. Due to this better voltage support, the transmittable power can be increased when the shunt compensation is provided by STATCOM.

• For the same stability margin, the rating of the STATCOM can be decreased below the SVC.

• The response time and the bandwidth of the closed-loop voltage regulation of the STATCOM are significantly better when compared to the SVC. Also, the STATCOM provides a much wider variation of the transmission network impedance than the SVC.

4. Unbalanced AC System Operation

• In the balanced operation, all the 3-phase voltages are in the balanced condition and therefore the compensator can control all the 3-phases of the output current in a combined manner. In the unbalanced condition, all the 3-phase currents may be different.

• The individual phase control of SVC in transmission line compensation is rarely employed because the triple-n harmonic current is different in each phase which would require third harmonic filters.

• The STATCOM can provide a better response in fault clearing and system stability because it gives strong voltage support during system faults. Also, the real and reactive power of the STATCOM (with energy storage device) can be controlled independently since the VSC has strong control systems.

5. Losses

• The total compensator losses for the STATCOM are higher than the SVC. The VSC in STATCOM uses an insulated gate bipolar transistor (IGBT) and the SVC uses conventional thyristors. The IGBT gives higher losses than conventional thyristors.

• The VSC is the latest technology in power converter developments and therefore, the overall losses of the STATCOM may be reduced in the future with new developments.

References

• Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS concepts and technology of flexible AC transmission systems”, 1999.