Thyristor controlled series compensation

Series compensation is a well established technology that is primarily used to reduce transfer reactances, most notably in bulk transmission corridors. The result is a significant increase in the transient and voltage stability in transmission systems.

Series Compensation with Thyristor Control (TCSC) enables rapid dynamic modulation of the inserted reactance. At interconnection points between transmission grids, this modulation will provide strong damping torque on inter-area electromechanical oscillations. As a consequence, a TCSC rated at around 100 Mvar makes it possible to interconnect grids having generating capacity in the many thousands of megawatts. Often the TCSC is combined with fixed series compensation to increase transient stability in the most cost effective way.

The TCSC concept also enables inherent immunity against subsynchronous resonance (SSR), and thus allows for extended use of series capacitors in specific transmission grids comprising thermal generation. The immunity to SSR is a result of the Hitachi Energy patented SVR control strategy.

Thyristor controlled series compensation principle
There are two main principles supporting TCSC technology.

• First, the TCSC provides electromechanical damping between large electrical systems by modulating the reactance of one or more specific interconnecting power lines. In other words, the TCSC will provide a variable capacitive reactance.
  
• Second, the TCSC will change its apparent impedance (as seen by the line current) for subsynchronous frequencies in such a way that a potential subsynchronous resonance is avoided.

The TCSC achieves both objectives by using control algorithms that work concurrently. The controls will function on the thyristor circuit (this in parallel to the main capacitor bank) such that controlled charges are added to the main capacitor, making it a variable capacitor at fundamental frequency but a "virtual inductor" at subsynchronous frequencies.

Thyristor controlled series compensation technology
From a principal technology point of view, the TCSC resembles the conventional series capacitor. The power equipment is located on an isolated steel platform, including the thyristor valve used to control the inductor in parallel with the capacitor bank. The inductor is placed on support insulators outside the platform. Control and protection systems are located on ground potential, together with other auxiliary systems.