Operating Modes of Thyristor Controlled Series Compensator (TCSC) in Power Systems

Operating Modes of Thyristor Controlled Series Compensator (TCSC)

In modern power transmission systems, the Thyristor Controlled Series Compensator (TCSC) plays a vital role in controlling power flow, enhancing stability, and improving power quality. TCSC is a key component of FACTS (Flexible AC Transmission Systems), widely used to optimize transmission efficiency and system reliability.

This article explains the operating modes of TCSC in detail — including Blocked Mode, Bypassed Mode, and Vernier Mode — along with their applications and advantages.

⚡ What is TCSC?

A Thyristor Controlled Series Compensator (TCSC) is a series-connected FACTS device designed to increase or decrease the effective impedance of transmission lines. By adjusting the thyristor firing angle, TCSC provides dynamic control of power flow, voltage stability, and transient oscillations.

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🔑 Operating Modes of TCSC

TCSC can be operated in three primary modes depending on the thyristor firing angle:

Thyristor Blocked Mode
Thyristor Bypassed Mode
Vernier Mode

Infographic showing operating modes of Thyristor Controlled Series Compensator (TCSC): Blocked Mode, Bypassed Mode, Capacitive Boost Mode, and Inductive Boost Mode

1️⃣ Thyristor Blocked Operating Mode

In this mode, the thyristors are not triggered, and the TCSC behaves as a fixed series capacitor.
The line reactance is reduced, improving power transfer capability.
Commonly used in normal steady-state conditions.

2️⃣ Thyristor Bypassed Operating Mode

Here, the thyristors are continuously triggered, keeping them in conduction.
TCSC behaves like a parallel combination of series capacitor and thyristor branch inductance (Ls).
In steady state, the voltage across the capacitor is inductive.
This mode reduces capacitor stress during system faults.

3️⃣ Vernier Operating Mode

In Vernier Mode, the TCSC provides dynamic, continuous control by varying the firing angle between 0° to 90°. It has two sub-modes:

(a) Capacitive Boost Mode

The thyristor is triggered just before the capacitor voltage crosses zero.
This results in a capacitor discharge current pulse that adds to line current, boosting capacitor voltage.
It effectively inserts up to 3x the fixed capacitor value.
This is the normal and preferred operating mode of TCSC.

(b) Inductive Boost Mode

The thyristor branch current becomes larger than the line current.
Produces high thyristor currents and distorted capacitor voltage waveform.
Causes stress and harmonic distortion, making it less desirable for steady-state operation.

👉 Learn more: Harmonics in Power Systems

📊 Comparison of FACTS Devices in Power Systems

FACTS devices, including SVC, STATCOM, TCSC, SSSC, and UPFC, each have unique characteristics.

FACTS Device	Power Flow	Voltage Control	Transient Stability	Oscillation Damping
SVC	*	***	*	**
STATCOM	*	***	**	**
TCSC	**	*	***	**
SSSC	***	*	***	**
UPFC	***	***	***	***

👉 Related Reading: Flexible AC Transmission System

✅ Conclusion

The operating modes of TCSC — blocked, bypassed, and vernier — make it one of the most versatile FACTS controllers. While Blocked Mode ensures simple capacitive compensation, Bypass Mode protects the system during faults, and Vernier Mode provides precise and dynamic control.

As the demand for efficient and reliable transmission networks grows, TCSC continues to be a cornerstone in modern power system engineering.

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