Mohan Muniappan - Consulting - Static VAR Compensators (SVC)

Static VAR Compensators (SVC)

A Static VAR Compensator (SVC) is a shunt-connected static VAR generator or absorber whose output is adjusted to exchange capacitive or inductive reactive current to maintain or control specific parameters of the electrical power system (typically bus voltage).
The Static VAR Compensators (SVC) are the variable impedance type shunt connected device. The shunt compensator is primarily used for voltage control, reactive power (VAR) control, dynamic & transient stability, and power oscillation damping.
The SVC can be divided into the following topologies:

1. Thyristor-Controlled Reactor (TCR)

A thyristor-controlled reactor (TCR) is a shunt-connected reactor in series with a bidirectional thyristor valve. The schematic diagram of TCR and its firing delay angle control are shown in Fig. 1. The TCR is used to control the VAR absorption by controlling the inductive reactive current using firing delay angle control.

Fig. 1. The schematic diagram of TCR and firing delay angle control.

In TCR, the current flow through the reactor (iL) can be controlled from maximum (α=0) to zero (α=90°) by controlling the firing delay angle (α). The current flow through the reactor (iL), the fundamental reactive current (iLF) and the variable reactive admittance (BL) as a function of firing delay angle (α) can be expressed by,

Where, V is the amplitude of the applied AC voltage, L is the inductance of the TCR, α is the firing delay angle, σ is the conduction angle, and ω is the angular frequency of the applied voltage.
The V-I characteristics of TCR is shown in Fig. 2. Where, VLmax and ILmax are the voltage and current limits. BLmax is the maximum reactive admittance of the TCR.

2. Thyristor-Switched Reactor (TSR)

A thyristor-switched reactor (TSR) is a shunt-connected reactor in series with a bidirectional thyristor valve that is used to switch the reactor ON or OFF. The TSR provides fixed inductive admittance controllable in a step manner.
If the TCR switching is restricted to a fixed firing delay angle (α=0), then it becomes a thyristor-switched reactor (TSR). If the TSR is operated at α=0, the steady state current will be sinusoidal. Fig. 3 shows the V-I characteristics of the TSR. Where, BL is the admittance of the reactor.

3. Thyristor-Switched Capacitor (TSC)

A thyristor-switched capacitor (TSC) is a shunt-connected capacitor in series with a bidirectional thyristor valve which is used to switch the capacitor ON or OFF. Fig. 4 shows the schematic diagram of TSC and the conditions for transient free switching of TSC. The TSC can be used to control the VAR generation by controlling the capacitive reactive current.
A reactor is connected in series with the capacitor and the thyristor valve. This reactor is connected to limit the surge current in the thyristor valve under abnormal operating conditions (for example, control malfunctions cause capacitor switching at the wrong time) and also it is used to avoid resonance with the AC system impedance at the selected frequencies.

Fig. 4. The schematic diagram of TSC and the conditions for transient free switching.

The capacitor should be switched at the specific instants in each cycle to avoid the huge switching transients. The firing delay angle control is not suitable for capacitors. Therefore, the TSC can provide step change capacitive reactive current.
The conditions for transient free switching are given below as shown in Fig. 4:

If the residual capacitor voltage is lower than the peak AC voltage (VC<V), then the correct instant of switching is when the AC system voltage is equal to the capacitor voltage (V=VC).
If the residual capacitor voltage is less than the peak AC voltage (VC>V), then the correct instant of switching is at the peak of the AC voltage (Vmax) and when thyristor valve voltage (VSW) is minimum.

The V-I Characteristics of TSC is shown in Fig. 5. Where, VCmax and ICmax are the voltage and current limits, and BC is the capacitive admittance.

Fig. 5. V-I Characteristics of TSC.

4. Fixed Capacitor - Thyristor-Controlled Reactor (TCR)

The FC-TCR is the combination of fixed capacitors (FC) and thyristor-controlled reactor (TCR) and that is shown in Fig. 6. The fixed capacitors provide fixed capacitive VAR generation by injecting capacitive reactive current. The TCR provides variable VAR absorption by injecting variable inductive reactive current using firing delay angle control. The capacitive reactive output can be controlled by increasing the inductive reactive output which can be done by decreasing the firing angle α.

Fig. 6. The schematic diagram of FC-TCR.

Fig. 7. V-I characteristics of FC-TCR.

5. Thyristor-Switched Capacitor (TSC) - Thyristor-Controlled Reactor (TCR)

The TSC-TCR is primarily used for dynamic compensation of power transmission systems with the intention of minimizing standby losses and providing increased operating flexibility. The TSC provides VAR generation (capacitive reactive current) in a transient free switching manner. The TCR provides VAR absorption (inductive reactive current) by firing delay angle control.
Fig. 8 shows the schematic diagram of TSC-TCR and Fig. 9 shows the V-I characteristics of the TSC-TCR. The response of TSC-TCR is slower than FC-TCR because the maximum delay of switching in a single TSC is one full cycle and the maximum delay of TCR is only half cycle. The required capacitive output current is decided by the number of TSC branches.

Fig. 8. The schematic diagram of TSC-TCR.

Fig. 9. V-I characteristics of TSC-TCR.

References

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

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