Analysis and Solution of Overvoltage Problems Caused by Vacuum Circuit Breaker Switching Capacitor Banks

The overvoltage generated when vacuum circuit breakers switch capacitor banks is a common and significant issue in power systems that requires close attention. Such overvoltage may pose a threat to the insulation of capacitors, circuit breakers, and the entire system. The following is a systematic analysis of this problem and potential solutions:

The main reason can be attributed to the interaction between the breaking characteristics of vacuum circuit breakers and the energy storage characteristics of capacitor banks, which is specifically manifested as:

1. Switching inrush current and operation overvoltage

2. Mechanism: At the moment of closing, the voltage across the capacitor bank is zero, while the system voltage is at a certain instantaneous value. The large voltage difference between the two causes a high-frequency inrush current with a very large amplitude and high frequency (up to several to tens of times the rated current).

Impact: The high-frequency inrush current generates a high-frequency voltage drop across the system impedance, which may be superimposed on the power frequency voltage to form an operation overvoltage. In the case of multiple capacitor banks operating in parallel, when another capacitor bank is connected to a charged capacitor bank (or system), the voltage difference may be even greater, and the inrush current and overvoltage problems become more severe.

This is the most typical and challenging source of overvoltage when vacuum circuit breakers are used to switch capacitors, mainly related to the breaking characteristics of the vacuum medium:

Current chopping: The stability of the vacuum arc is poor. At low currents (such as below tens of amperes), the arc may suddenly extinguish before the current naturally crosses zero, which is known as "current chopping". The electric field energy (the charge on the capacitor) corresponding to the chopped current (mainly capacitive current) cannot be released immediately, resulting in a transient current chopping overvoltage on the capacitor that is higher than the system voltage.

Multiple restriking overvoltage (the most dangerous): This is the most severe form of overvoltage.

First restrike: After the circuit breaker opens, the contact gap gradually increases. When the residual voltage on the capacitor (DC or low-frequency) is in the opposite direction to the system supply voltage, the recovery voltage between the contacts may exceed the dielectric recovery strength of the vacuum gap at that time, causing the gap to be broken down and a restrike to occur. At the moment of restrike, the voltage on the capacitor will oscillate towards the system supply voltage through the circuit inductance.

Voltage "step" increase: The restrike generates a high-frequency oscillating current. Vacuum circuit breakers are particularly adept at extinguishing arcs at the zero-crossing of high-frequency currents. If the arc is successfully interrupted at the first or second zero-crossing of the high-frequency current, the capacitor will be "locked" at a new voltage value. Due to the restrike discharge process, this new voltage value may be much higher than the voltage before the restrike.

Repeating process: As the contact distance continues to increase, the recovery voltage rises again, and a second, third, or more restrikes may occur. Each restrike may cause a "step" increase in the voltage on the capacitor. Theoretically, after several restrikes, the peak overvoltage at both ends of the capacitor may reach three times or even higher than the system phase voltage.

1. For the capacitor itself: Overvoltage directly threatens the insulation of the capacitor elements, accelerates the aging of the dielectric, and long-term effects may lead to breakdown, causing the capacitor to explode.

2. For vacuum circuit breakers: Multiple reignitions can generate extremely high recovery voltages and reignition currents, intensifying the electrical wear of the contacts and potentially causing insulation damage to the circuit breaker itself.

3. For other equipment in the system: Overvoltage may be transmitted through the lines, endangering the insulation of connected transformers, instrument transformers, cables, and other equipment.

4. Triggering incorrect operation or failure to operate protection: The high-frequency transient process may interfere with the sampling and logical judgment of microcomputer-based protection devices.

The main solution approaches revolve around "limiting inrush current", "preventing reignition" and "absorbing/limiting overvoltage".

Optimize the selection and use of circuit breakers.

1.Select "C2 grade" or "capacitive current breaking dedicated" vacuum circuit breakers: This is the most fundamental and effective measure. These circuit breakers have been verified through strict type tests and can ensure that no restriking occurs or the probability of restriking is extremely low when breaking the rated capacitive current. Their contact materials, magnetic field designs, and manufacturing processes are all optimized for capacitive loads.

Avoid using general-purpose or only "L75" tested circuit breakers: General-purpose circuit breakers may meet the breaking requirements of inductive loads, but they cannot guarantee the breaking performance for capacitive loads.

Ensure stable mechanical characteristics: Make sure the circuit breaker's opening speed is fast and stable enough to quickly establish a sufficient opening distance and enhance the dielectric recovery strength.

2. Installation of Overvoltage Protection Devices

Metal Oxide Arrester (MOA): Connected in parallel at the beginning of the capacitor bank or on the busbar side, it is a standard configuration for limiting the amplitude of overvoltage. It can clamp the overvoltage to a safe level. The appropriate model with suitable continuous operating voltage and residual voltage should be selected and installed as close as possible to the capacitor bank.

RC Damping Absorption Circuit: A parallel resistor-capacitor circuit is installed across the breaker contacts or between the capacitor bank and the breaker.

Function: To reduce the rate of rise of the recovery voltage (du/dt); to provide a low-impedance path for the high-frequency current that may occur after restriking and consume its energy; to suppress the current interruption overvoltage.

Design Key: The parameters (R and C values) need to be calculated based on system parameters to achieve the best damping effect.

3. Improve operation methods

Adopt synchronous switches (phase selection closing/tripping devices): By controlling the circuit breaker to close at the moment when the difference between the system voltage and the residual voltage of the capacitor is the smallest (such as at the zero crossing of the voltage), the inrush current and overvoltage at closing can be **greatly reduced**. Similarly, it can also be controlled to trip precisely at the zero crossing of the current, reducing the risk of current interruption. This is currently an advanced technology for suppressing operation overvoltage.

Optimize operation sequence: For parallel capacitor banks, it is recommended that the operation sequence be as follows: when power is off, disconnect the circuit breaker first, then the isolating switch; when power is on, close the isolating switch first, then the circuit breaker. Avoid operating charged capacitors with the isolating switch.

4. System-side Considerations

Series Reactors: Series reactors with a certain reactance rate (typically 0.5% to 1% to limit inrush current and 5% to 6% to suppress harmonic amplification) are connected in the capacitor bank circuit.

Functions: Limit the amplitude and frequency of inrush current; form a filter branch with capacitors; also can change the parameters of the transient process to some extent and affect the restriking conditions.

Reasonable Electrical Layout: Shorten the connection line length between the capacitor bank and the circuit breaker, reduce the loop inductan

The overvoltage problem caused by vacuum circuit breakers switching capacitor banks is fundamentally due to the conflict between the current interruption and restriking characteristics of vacuum arcs and the energy storage characteristics of capacitors.

The solution strategies should follow the following hierarchy:

1. Prevention first (addressing the root cause): During the design and procurement stages, only "C2 grade" or vacuum circuit breakers specifically designed for capacitor bank switching that have been certified by authoritative bodies should be selected.

2. Protection as a shield (addressing the symptoms): Standardize the configuration of metal oxide arresters (MOA) as the last line of defense against overvoltage.

3. Optimization as an auxiliary measure (enhancing efficiency): Depending on the project's importance and budget, consider installing RC damping circuits, synchronous switches, and rationally configuring series reactors.

4. Operation and maintenance as the foundation: Regularly inspect the mechanical characteristics of circuit breakers and the status of arresters, and strictly follow the correct operating procedures.

In practical engineering, a technical and economic comparison should be made by taking into account factors such as the system voltage level, capacitor bank capacity, operation mode and cost, to select one or more combined suppression measures to ensure the safe and reliable operation of the system.

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Shaanxi West Power Tongzhong Electrical Co., Ltd.
Contact: Ms.Grace Liu (Director of Sales Department)

Email：xdtz04@westpowerelectric.com

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