PSMA Consulting - Power System Studies - Thyristor Controlled Series Capacitor (TCSC)

Thyristor Controlled Series Capacitor (TCSC)

The objective of series capacitive compensation is to vary (or reduce) the overall effective series transmission reactance between the sending and receiving ends. Thyristor-controlled series capacitor (TCSC) provides variable series capacitive compensation using the thyristor firing (or delay) angle control. The TCSC can be applied for power flow control, dynamic and transient stability, voltage stability, and damping oscillations caused by sub-synchronous resonance (SSR).

Need for Variable Series Compensation

The series capacitor provides fixed series capacitive compensation and it may not be suitable if there are changes in the power network configurations (i.e., outage of lines/network). In such cases, the TCSC can be applied to vary the compensation level depending on the network requirement or configuration.
The fixed series capacitor compensation can produce SSR problems (i.e., oscillations at sub-synchronous frequencies and instability) when it interacts with the generator-shaft torsional systems. The TCSC gives a better response in damping such oscillations and providing stability.

Principle of Operation of TCSC

The principle of TCSC is to provide the variable capacitive reactance continuously by varying the inductive reactance using firing angle control. The TCSC consists of a series capacitor in parallel with a thyristor-controlled reactor (TCR) as shown in Fig. 1. It consists of the fixed series capacitive reactance XC and variable inductive reactance XL(α) and SW refers to the thyristor switches. The firing angle (α) is measured from the peak of capacitor voltage or the zero crossing of line current. In the TCSC arrangement, the reactance of the TCR (XL) is smaller than the capacitor reactance (XC). The expression for the TCSC reactance XTCSC(α) is given in equations (1) & (2).

Fig. 1. Schematic Diagram of TCSC.

Where, XL(α) is the inductive reactance controlled by thyristor firing angle (α). XC is the capacitive reactance. If the inductive reactance XL is varied by changing the thyristor firing angle (α), the TCSC reactance XTCSC(α) can be controlled.

Modes of Operation of TCSC

The TCSC Reactance vs Firing Angle Characteristic is shown in Fig. 2. The TCSC has the following operating modes based on the thyristor firing angle ranges as given below.

- XTCSC(α) is capacitive for the firing angle range, αC lim ≤ α ≤ π/2
- XTCSC(α) is inductive for the firing angle range, 0 ≤ α ≤ αL lim
- Resonance at XL(α) = XC for the firing angle,αL lim ≤ α ≤ αC lim

Fig. 2. TCSC reactance vs firing angle characteristics.

When the thyristor switches (i.e., SW in Fig. 1) are closed, the following two actions can happen.

The capacitor is getting charged/discharged (depending on the switching instant) by the line current which will act as a constant current source.
The charging of the capacitor is reversed during the resonant half-cycle of the LC circuit. Therefore, the resonant charge reversal produces the DC offset in capacitor voltage.

The time duration of capacitor voltage reversal mainly depends on the XL / XC ratio. If XL << XC, then the voltage reversal is almost instantaneous. If XL is increased related to XC , the TCR conduction period will increase and the zero crossing of capacitor voltage majorly depends on the line current. In practice, the XL / XC ratio of TCSC is in the range of 0.1 to 0.3. The DC offset in capacitor voltage (which is produced due to charge reversal) can be controlled by adjusting (advancing or delaying) the firing instant of thyristor switches.

V-I Characteristics of TCSC

The V-I characteristic (compensating voltage vs line current) of TCSC for the voltage control mode is shown in Fig. 3. In the capacitive region, αC lim limits the maximum capacitive voltage (VC max) at low line currents (Imin) and α=90° limits the maximum capacitive voltage (VC max) at the maximum line current (Imax). In the inductive region, αL lim limits the maximum inductive voltage (VL max) at low line currents and α=0 limits the maximum inductive voltage (VL max) at the maximum line current (Imax).

Fig. 3. V-I characteristics of TCSC.

References

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