Working Principle and Key Technologies of Thyristor Switched Capacitor (TSC)

Thyristor Switched Capacitor (TSC) is a dynamic reactive power compensation device based on the contactless switching characteristics of thyristors. Its core principle is to quickly and smoothly switch capacitor banks into or out of the power grid by utilizing the precise zero-crossing triggering capability of thyristors, realizing dynamic compensation for grid reactive power. Compared with traditional mechanically switched capacitors, TSC has significant advantages such as long operating life, contactless switching, strong resistance to mechanical stress, and fast dynamic response. In addition, by accurately controlling the switching instant, it can effectively suppress inrush current during the switching process, ensuring the stable operation of the power grid and equipment.

1. Classification of TSC

1.1 Classification by Voltage Level

According to the applied voltage level, TSC can be divided into low-voltage compensation and high-voltage compensation, in line with general specifications for reactive power compensation devices in the power industry:

Low-voltage Compensation: Mainly applicable to 0.4kV (400V) low-voltage distribution networks, covering reactive power compensation requirements for voltage levels of 1kV and below, mostly used at end-load sides such as industrial workshops and commercial buildings;

High-voltage Compensation: The compensation system is directly connected to the high-voltage power grid, mainly targeting voltage levels of 6kV, 10kV, and 35kV. It is suitable for centralized reactive power compensation scenarios such as substations and general step-down stations in industrial parks, to resolve the reactive power shortage in high-voltage power grids.

1.2 Classification by Application Scope

Based on the compensation scope and objects, TSC can be classified into load compensation and centralized compensation, which have clear divisions and complementary applications:

Load Compensation: Provides targeted dynamic compensation for single or a group of specific fluctuating loads (e.g., electric arc furnaces, frequency converters, electric welding machines) to offset reactive power impacts generated by loads in real time, preventing reactive power fluctuations from affecting grid voltage quality;

Centralized Compensation: Installed at power supply hubs of the power grid (e.g., substation busbar sides), it carries out systematic compensation for reactive power of the entire power supply area, solving the problem of overall reactive power fluctuation in the grid, improving grid power factor, and reducing line loss.

2. Operating States and Main Circuit Design of TSC

2.1 Operating States

TSC only has two operating states: switched-in state and switched-out state, with clear and controllable working mechanisms for both states:

● Switched-in State: The bidirectional thyristor (or anti-parallel thyristor group) conducts, and the capacitor bank is smoothly connected to the grid line. TSC outputs capacitive reactive power to the grid, offsetting inductive reactive power in the grid and improving the power factor;

● Switched-out State: The bidirectional thyristor (or anti-parallel thyristor group) is blocked, disconnecting the capacitor bank from the grid. At this time, the capacitor bank retains residual voltage close to the peak grid voltage, and the TSC branch no longer outputs reactive power to the grid. A special discharge device must be used to release the residual voltage to ensure equipment safety.

2.2 Core Requirements for Main Circuit Design

The TSC main circuit design must meet three core requirements: stepped fast compensation, inrush current suppression, and harmonic control. The key technology is to achieve zero-inrush current switching, avoiding damage to core components such as thyristors and capacitors caused by inrush current during switching.

In industrial applications, the common wiring mode of TSC is anti-parallel thyristor wiring (equivalent to a bidirectional thyristor), which enables bidirectional conduction and blocking in AC circuits, adapting to the operating characteristics of power-frequency AC power. In contrast, the thyristor-diode anti-parallel wiring mode can only control current in one direction, failing to meet normal switching requirements in AC scenarios. It is a non-mainstream structure, only used in special rectification and switching composite scenarios, and not recommended as a conventional TSC wiring scheme.

3. Core Wiring Modes and Performance Comparison of TSC

In conventional TSC systems, the anti-parallel thyristor wiring mode is the only mainstream scheme, with its performance characteristics and precautions as follows:

● Working Mechanism: Two anti-parallel thyristors are triggered alternately to connect and disconnect the compensation circuit, adapting to the positive and negative half-cycle operating requirements of power-frequency AC power;

● Reliability: It has high overall reliability. However, it should be noted that if one thyristor is damaged and short-circuited, it will cause half-wave conduction of the compensation branch, generating DC components and excessive inrush current, which will burn out the capacitor bank and other components. Therefore, complete fault detection and protection devices must be configured in practical applications;

● Reverse Voltage Withstand Capability: The peak reverse voltage borne by the thyristor valve is equal to the peak grid voltage after the residual voltage of the capacitor is released, complying with the rated voltage selection requirements of thyristor components.

The non-mainstream thyristor-diode anti-parallel structure features good economy and simple operation, but it cannot realize bidirectional current control, and its response speed cannot meet dynamic compensation demands. Moreover, the peak reverse voltage borne by the thyristor valve can reach twice the peak grid voltage, requiring higher component selection. It is only applicable to special scenarios with low requirements and small capacity, and is not included in the conventional TSC design category.

4. Selection and Function of Series Reactors

In the TSC main circuit, series reactors are indispensable core components. Their core functions are to limit switching inrush current, suppress high-order harmonics, and limit short-circuit current, ensuring the safe and stable operation of the system.

4.1 Mechanism of Series Reactors

Abnormal conditions such as thyristor false triggering and grid faults may cause instantaneous inrush current when the capacitor bank is switched on. Series reactors can limit the amplitude of inrush current through inductive impedance. Meanwhile, reactors and capacitor banks form an LC filter circuit, which can effectively suppress high-order harmonics in the grid (especially the 3rd and 5th harmonics), avoiding component damage caused by harmonic amplification.

Note: After connecting series reactors, the voltage across the capacitor will increase due to fundamental voltage drop and harmonic amplification effects. Therefore, the rated voltage of the capacitor must be higher than the grid voltage. For example, capacitors with a rated voltage of 450V are usually selected for 0.4kV grids, and capacitors with a rated voltage of 11/√3 kV for 10kV grids.

4.2 Types and Selection Principles of Reactors

Two common types of reactors are used in TSC systems: air-core reactors and iron-core reactors. They have obvious performance differences, and the selection should be comprehensively determined based on economic cost and technical requirements:

● Air-core Reactors: They have excellent current limiting effect, high linearity, are not easy to saturate or generate heat under harmonic conditions, and have strong operational stability, but with high cost. They are suitable for high-voltage, large-capacity TSC systems and scenarios with high requirements for compensation accuracy and stability;

● Iron-core Reactors: They have low cost and meet conventional current limiting requirements, but poor linearity. They are prone to saturation and heating under harmonic effects, and their current limiting effect is greatly affected by working conditions. They are suitable for low-voltage, small-capacity TSC systems and scenarios with strict cost control.

5. Main Circuit Wiring Modes of TSC

According to the connection mode between thyristor valves and capacitor banks, the main circuit wiring modes of TSC mainly include three-phase controlled delta connection and star connection, each with applicable scenarios. There is no mainstream "delta-star combined connection" (this combined connection is only theoretical derivation and not applied in industrial practice):

● Delta Connection: Mainly used in low-voltage TSC systems (e.g., 0.4kV), adopting three-phase common compensation mode. It has high compensation efficiency and simple wiring, can effectively offset three-phase unbalanced reactive power, and is suitable for reactive power compensation at end loads;

● Star Connection: Mainly used in high-voltage TSC systems (e.g., 6kV, 10kV, 35kV), usually with ungrounded neutral point. It can prevent the spread of single-phase faults, has high operational safety, and is suitable for substation centralized compensation scenarios.

6. Inrush Current Control for TSC Switching

Based on the core characteristic of capacitors that "voltage cannot change abruptly", a large difference between grid voltage and capacitor residual voltage (including amplitude and phase) during TSC switching will generate instantaneous inrush current, threatening component safety. Thus, inrush current control is the core of TSC switching control.

● Inrush Current Judgment Standard: The general engineering standard is that when the ratio of inrush current to the normal steady-state operating current of the capacitor is less than 1.2 to 1.5 times, it is considered harmless to thyristors, capacitors and other components. If the ratio exceeds this range, the switching control strategy needs to be optimized or current limiting measures added;

● Zero-Inrush Switching Implementation: The ideal switching state is "zero-crossing triggering". After switching stops, the capacitor retains the peak grid voltage. The thyristor is triggered and conducted at the zero-crossing point where the grid voltage and capacitor residual voltage are equal in amplitude and phase, with near-zero inrush current. During switching out, the thyristor is blocked at the current zero-crossing point to avoid overvoltage.

7. TSC Detection and Control System

The core function of the TSC detection system is to collect relevant electrical parameters of the power grid and load system in real time, providing accurate basis for switching control. It mainly consists of a phase sampling module, a voltage and current RMS calculation module, and a reactive power demand and reactive power calculation module.

Advanced control technologies in current industrial applications adopt microcomputer-based synchronous phase control technology and adaptive thyristor triggering technology. The working mechanism is: the detection system captures the amplitude and phase information of the voltage across the capacitor and the grid voltage in real time. When the two are equal in amplitude and consistent in phase, the thyristor is triggered instantly to achieve zero-inrush switching of the capacitor. During switching out, the thyristor is automatically blocked at the current zero-crossing point, without pre-charging the capacitor.

Important Note: Series reactors and special discharge devices (discharge coils or discharge resistors) are essential components in TSC systems and cannot be omitted. Series reactors are used for current limiting and harmonic suppression, and discharge devices release capacitor residual voltage after switching to avoid potential safety hazards caused by residual voltage. Only small-capacity low-voltage TSC can simplify the discharge device under specific working conditions, while high-voltage and large-capacity TSC must be equipped with complete current limiting and discharge components.

8. Conclusion

As an efficient and fast dynamic reactive power compensation device, TSC has core advantages of contactless switching, fast response speed and reliable operation. It can effectively solve the problem of grid reactive power fluctuation, improve grid voltage quality and reduce line loss. Its key technical points include zero-crossing triggering control, inrush current suppression, reactor selection and wiring mode adaptation. In practical design and application, it is necessary to strictly abide by power industry standards, avoid misunderstandings in component selection and control strategies, and ensure the safe, stable and efficient operation of the system.

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