1. Functions of Shunt Capacitors

Shunt capacitors are primarily installed on the load side of the main transformer in substations or on the tertiary winding. They are used to supply capacitive reactive power to the system, improve the power factor, and enhance the power quality of the busbar. In the context of a capacitor bank in a substation, this function is essential for maintaining optimal operation.

For substations with voltage levels of 220kV and below, the capacitor bank in the substation mainly compensates for the reactive power losses of the transformer, partially compensates for the line losses, and also addresses the reactive power losses of the load. The capacity configuration should ensure that the power factor on the high-voltage side is not less than 0.95 at the maximum load of the main transformer. Typically, the capacity is selected to be 10% to 35% of the main transformer’s capacity.

Figure 1:

Shunt capacitors can be categorized into frame type and compact type based on their structure. The frame type, as shown in Figure 1, consists of a frame made of angle steel and channel steel, with capacitor units mounted on it. The capacitor units are open-structured, making on-site fault handling and maintenance convenient but with a high operational workload due to environmental influences. The compact type, as shown in Figure 2, encloses capacitor units within a sealed metal box, offering minimal environmental impact, simple external wiring, and lower operational workload, but on-site fault handling is less convenient. In either configuration, the capacitor bank in the substation plays a critical role in system stability and efficiency.

Figure 2:

2. Wiring Methods of Shunt Capacitors

The three-phase capacitor wiring methods for shunt capacitors mainly include star (Y) and delta (Δ) connections. The star connection can be further divided into single-star and double-star configurations, as illustrated below. The choice of wiring method significantly affects the performance of the capacitor bank in the substation.

From the diagram, under operating conditions, each phase capacitor in a delta connection bears the line voltage, whereas in a star connection, it bears the phase voltage. Consequently, a delta-connected capacitor of the same capacity delivers three times the reactive power compared to a star-connected capacitor.

Currently, for substations of 220kV and below, star connection dominates capacitor unit wiring. One reason is that in the event of a complete breakdown in a star-connected capacitor bank in the substation, the fault current is limited by the reactance of the healthy phases, significantly reducing the fault current from the system to no more than three times the rated current of the capacitor bank, thereby minimizing the risk of tank explosions.

3. Abnormal and Fault Conditions of Shunt Capacitors

Based on operational experience, the primary abnormal and fault conditions for a capacitor bank in a substation include:

(1) Internal faults, such as inter-electrode short circuits within the capacitor and faults among multiple capacitors within a group.

(2) Short-circuit/ground faults, including inter-phase short circuits and grounding faults on the connections between the capacitor and circuit breaker, as well as within the capacitor group.

(3) Overload of the capacitor.

(4) Overvoltage of the capacitor.

(5) Loss of voltage on the capacitor.

4. Protection of Shunt Capacitors

Shunt capacitors, especially in a capacitor bank in a substation, are typically equipped with two levels of protection: the first level consists mainly of external fuses or internal fuses, and the second level relies on relay protection based on electrical parameters.

External fuses and internal fuses protect individual units or components of the capacitor under internal and external influences. Since the number of breakdowns is usually small and the capacitor has some overload capacity, only the faulty units need to be isolated to prevent an accident, without triggering the entire group. The rated current of the fuse can be set to 1.5-2 times the rated current of the capacitor.

Other internal and external faults of the capacitor bank in the substation need to be handled by relay protection, which can trip the entire group. The protection configurations include unbalance protection, overvoltage protection, overcurrent protection, overload protection, and low-voltage protection.

4.1 Unbalance Protection of Capacitors

When a capacitor unit in a capacitor bank in a substation fails and is isolated by a fuse, the voltage and current of the parallel normal units increase, with the extent of increase proportional to the number of faulty units. If multiple capacitor units fail and are isolated by the dedicated fuses, the remaining units might experience unacceptable overload or overvoltage, necessitating the operation of unbalance protection to trip the entire capacitor group.

Unbalance protection methods vary with wiring configurations. Commonly used unbalance protection methods include single-star open-delta unbalance voltage protection, single-star voltage differential protection, double-star neutral point unbalance current protection, and single-star bridge differential unbalance current protection.

4.1.1 Single-Star Open-Delta Unbalance Voltage Protection

This protection is widely used for capacitor groups with a capacity below 5000kvar at 10kV. Each phase of the capacitor in the capacitor bank in the substation has a discharge coil connected in parallel, with the secondary side of the discharge coils forming an open-delta connection. A voltage relay is connected at the open point to detect unbalance voltage.

The principle is that a fault causes unbalance in the three-phase capacitors, shifting the neutral point and generating an unbalance voltage in the secondary side of the discharge coil, which triggers the protection.

4.1.2 Single-Star Voltage Differential Protection

Used for capacitor groups above 5000kvar at 10kV and below 20Mvar at 35kV, each phase capacitor consists of two series segments with a discharge coil having a common tap in parallel. The secondary winding of the discharge coil connects to a voltage relay to monitor differential voltage, which detects changes in the proportional distribution of voltage caused by faults in the capacitor bank in the substation.

4.1.3 Double-Star Neutral Point Unbalance Current Protection

Applied to capacitor groups above 5000kvar at 10kV and around 20Mvar at 35kV, this protection involves connecting a current transformer between the neutral points of two star configurations. The current transformer detects unbalance current caused by a fault in one branch, which shifts the neutral point voltage and creates an unbalance.

4.1.4 Single-Star Bridge Differential Unbalance Current Protection

For large-capacity capacitor groups at 35kV and above, each phase consists of four segments arranged in series-parallel with a current transformer bridging the midpoint. Any change in impedance due to a fault creates a current flow in the current transformer, triggering the protection.

4.1.5 Setting Unbalance Protection

The unbalance protection settings for a capacitor bank in a substation should be based on the overvoltage tolerance of the capacitor units. For capacitors protected by external fuses, the setting should reflect the allowable overvoltage of the capacitor units. For those with internal fuses or fuse-less designs, it should reflect the internal component's overvoltage tolerance. When multiple protection schemes coexist, the smallest value should be chosen.

4.2 Sectional Overcurrent Protection

Overcurrent protection addresses inter-phase short circuits and internal faults within the capacitor group, as well as overload conditions. Typically configured as two-stage or three-stage protection, the second stage also serves as overload protection with a definite time characteristic. The third stage can be set with either a definite or inverse time characteristic.

4.3 Overvoltage Protection

Overvoltage protection addresses excessive supply voltage (bus voltage) anomalies. Capacitors in a capacitor bank in a substation can only operate continuously at up to 1.1 times their rated voltage. If the supply bus voltage increases, overvoltage protection should activate, either signaling or tripping with a time delay. Though inverse time characteristics are recommended, definite time characteristics are commonly used for simplicity.

4.4 Low Voltage Protection

When the supply voltage disappears, capacitors in a capacitor bank in a substation discharge and their voltage gradually decreases. If the residual voltage has not dropped to 0.1 times the rated voltage before the supply is restored, the capacitors will face excessive inrush voltage, risking damage. Low voltage protection is required, activating based on the logic shown below.

The low voltage protection connects to the secondary side of the high-voltage bus voltage transformer. The protection activates only if all three-phase voltages drop to the set low voltage threshold, and current locking is used to prevent accidental trips due to minor breaker malfunctions. If current is detected, it indicates a voltage transformer trip, and the capacitor protection should not activate, thus preventing breaker tripping. If no current is detected, the low voltage protection activates, tripping the breaker. Additionally, protection is meaningful only when the breaker is closed, thus a breaker trip position lock function is added in microprocessor-based protections.