Capacitors in University Analog Electronics Labs vs. Substation Capacitors

Capacitors used in university analog electronics labs typically have capacitance values of several microfarads (μF). In contrast, capacitors used in substations, such as High Voltage Shunt Capacitors, usually have capacitance values in the range of farads (F).

Role of Capacitors in Substations

Capacitors in substations, including Static VAR Compensators (SVCs) and High Voltage Shunt Capacitors, primarily serve to provide reactive power and elevate bus voltage. The total capacitor capacity configuration is generally not less than 10% of the main transformer capacity. The installation capacity of shunt capacitors in substations typically ranges between 10% and 30% of the main transformer capacity. According to the newly established State Grid Corporation standards, the range is 10% to 30% for 220kV and 10% to 25% for 110kV. The power factor is usually required to be no less than 95% at maximum load.

Standard Reference

GB 50227-2008

Limiting Inrush Current

To limit inrush current, a certain amount of dry reactor is usually connected in series on the power supply side of the capacitor. The typical protection configurations for capacitors, such as High Voltage Shunt Capacitors, include: overcurrent protection, overvoltage protection, undervoltage protection, and differential pressure protection (unbalance protection).

For example, consider a capacitor with a capacity of 4.8Mvar connected at 10kV.

The protection configuration is as follows:

Overvoltage: 118.8V, 9s

Undervoltage: 50V, 0.5s

Undervoltage Lockout Current: 1.4A

Overcurrent Stage I: 11.5A, 0.1s

Overcurrent Stage II: 5.7A, 0.5s

Unbalance: 5.24V, 0.2s

Protection Mechanisms

Overvoltage and undervoltage protections monitor the line voltage of the bus. When the line voltage exceeds the set limit, the capacitor, whether it be a Static VAR Compensator or a High Voltage Shunt Capacitor, is tripped after a delay. Following a capacitor fault trip, the system usually opens in the AVC system. To prevent the undervoltage protection from operating if the PT circuit breaker trips and there is still current, undervoltage protection is locked. When the bus voltage fluctuates due to load variations (e.g., electric railway impacts), overvoltage or undervoltage protection may operate. After confirming no issues with the equipment, the capacitor can be re-closed. A critical but often overlooked condition is ensuring the switch is in the closed position. If the switch has opened and the bus is de-energized afterward, undervoltage protection will still operate. Some manufacturers do not incorporate switch position logic, leading to the transmission of fault information during power outages, causing unnecessary confusion.

Overcurrent Protection

Overcurrent protection monitors the CT current of the interval. When fault current exceeds the set limit, the protection trips. The overcurrent protection range includes all equipment in the interval, such as conductors, cables, disconnect switches, reactors, etc. If no issues are found in the external equipment like capacitors, a comprehensive inspection of cables and other equipment is necessary before attempting a re-close.

Unbalance Protection

Unbalance protection checks for voltage discrepancies between two branches of each phase. If the unbalance voltage exceeds the set limit, the protection trips. Unbalance protection reflects the differences between the good and faulty parts of the capacitor. Voltage for unbalance protection can be derived from the secondary coil of the discharge coil. It's important to note that the secondary cables for capacitor differential pressure are typically exposed outdoors, making their insulation prone to aging. Therefore, secondary cable protection should be enhanced, possibly using insulating tape and sheathing.

Surge Arresters and Surge Capacitors

Surge arresters for suppressing operational overvoltage are also installed on capacitor devices, typically positioned close to the power supply side of the capacitor. Additionally, Surge Capacitors are employed to protect the system from transient overvoltages, ensuring the stability and longevity of the High Voltage Shunt Capacitors and Static VAR Compensators. These surge protection devices play a crucial role in maintaining the reliability and efficiency of the power system by mitigating the impact of voltage spikes.