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.

👉 Related Reading:

🔑 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.

👉 Suggested Reads:

Comments

Auto Transformer Tap Changing

Auto Transformer Tap Changing: Working Principle, Switching Sequence and Applications Auto transformer tap changing is a practical method of adjusting transformer output voltage without unnecessarily interrupting the electrical supply. In power transmission and distribution networks, the load does not remain constant throughout the day. As load current changes, voltage drops across transformers, cables, feeders, and transmission lines also change. A tap-changing arrangement compensates for these variations by modifying the effective number of turns in the transformer winding. A reactor-type on-load tap changer uses a center-tapped reactor, selector switches, and a bypass or short-circuiting switch to transfer the load from one tap to the next. The reactor limits the circulating current during the transition, allowing the tap position to change while the transformer continues supplying the connected load. This article explains the auto transformer tap-changing working principle , switch...

PLUG REVERSING OF CAPACITOR START MOTORS

The capacitor start motors suffer from the disadvantage that they are not easily reversible due to the centrifugal switch connected in the auxiliary winding. The motor cannot be instantaneously reversed by simple control as the auxiliary winding remains disconnected till the motor comes near to zero speed. However, by proper design of the control circuit the motor can be made instantly reversible. This is accomplished by using an electromagnetic relay along with a special two contact centrifugal switch as shown in Figure. The circuit shown in Figure is for a small hoist using a capacitor start motor. The upper and lower limits of travel are controlled by two limit switches viz. LSU and LSD. When UP-push button is pressed, contactor U gets energized. Its contacts U 1 and U 2 energies the main winding. Closing of its contact U 3 causes relay R to get energized through the centrifugal switch contacts A—B. After the energization of relay R auxiliary winding gets ene...

COMPONENTS OF HIGH VOLTAGE DC TRANSMISSION SYSTEM

Figure: A schematic of a bipolar HVDC system identifying main components

PRIMARY SECONDARY AND TERTIARY FREQUENCY CONTROL IN POWER SYSTEMS

Primary, Secondary and Tertiary Frequency Control in Power Systems Author: Engr. Aneel Kumar Keywords: frequency control, primary frequency control, automatic generation control (AGC), tertiary control, load-frequency control, grid stability. Frequency control keeps the power grid stable by balancing generation and load. When generation and demand drift apart, system frequency moves away from its nominal value (50 or 60 Hz). Grids rely on three hierarchical control layers — Primary , Secondary (AGC), and Tertiary — to arrest frequency deviation, restore the set-point and optimize generation dispatch. Related: Power System Stability — causes & mitigation Overview of primary, secondary and tertiary frequency control in power systems. ⚡ Primary Frequency Control (Droop Control) Primary control is a fast, local response implemented by generator governors (dro...

Advantages of Per Unit System in Power System Analysis | Electrical Engineering

Advantages of Per Unit System in Power System Analysis In electrical power engineering, the per unit (p.u.) system is one of the most widely used techniques for analyzing and modeling power systems. It is a method of expressing electrical quantities — such as voltage, current, power, and impedance — as fractions of chosen base values rather than their actual numerical magnitudes. This normalization technique provides a universal language for system calculations, minimizing errors, simplifying transformer modeling, and enabling consistency across multiple voltage levels. Because of these benefits, the per unit system is essential in fault analysis, load flow studies, transformer testing, and short-circuit calculations . ⚡ What is the Per Unit System? The per unit system is defined as: Q u a n t i t y ( p u ) = A c t u a l V a l u e B a s e V a l u e Quantity_{(pu)} = \dfrac{Actual \ Value}{Base \ Value} Q u an t i t y ( p u ) = B a se ...

Reversing the Direction of a Universal Motor: Methods and Circuit Diagram

How to Reverse the Direction of Rotation of a Universal Motor Introduction A universal motor is a high-speed electric motor that can operate on either an alternating-current or direct-current supply when designed for the applicable voltage and frequency. It is widely used in portable electric drills, grinders, vacuum cleaners, mixers, sewing machines, small machine tools, and other equipment requiring high starting torque and compact construction. The direction of rotation of a universal motor can be changed by reversing the direction of current through either the armature winding or the field winding relative to the other. The current must be reversed in only one of these windings. If the connections of both windings are reversed simultaneously, the relative direction of the field flux and armature current remains unchanged, and the motor continues rotating in the same direction. Before studying the reversing methods, readers may review the Universal Series Motors Electric Motors and...

Top 10 Transformer Manufacturers in the World

Top 10 Transformer Manufacturers in the World Transformers are among the most important and expensive assets in electrical generation, transmission, distribution, renewable-energy, industrial, infrastructure, and data-center projects. A power transformer changes voltage and current levels through electromagnetic induction while maintaining the system frequency. Generator step-up transformers increase generating-station voltage for efficient transmission, while grid and distribution transformers reduce voltage progressively before electricity reaches industrial, commercial, and residential consumers. The global transformer market continues to expand because of grid modernization, renewable-energy integration, industrial electrification, data-center development, replacement of aging equipment, and construction of new transmission infrastructure. One 2026 market assessment estimates that the global transformer market could grow from approximately USD 72.5 billion in 2026 to about USD 137....

ENGINEERING ARTICLES

Search This Blog