Low voltage capacitor banks, essential components in power systems, function like the heart in a body, supporting the smooth operation of the entire power grid. With precise design and compact structure, they operate efficiently in low voltage environments, significantly improving the power factor of the power system, reducing reactive power loss, and ensuring efficient utilization of electrical energy. Their applications span across industrial production, commercial offices, and residential areas, enhancing power supply quality, reducing line losses, lowering equipment failure rates, and extending the lifespan of devices, thereby ensuring stable power system operation. Here, we discuss several common issues in low voltage capacitor bank design.

1. Standards for Compensation Cabinets and Capacitors

Mechanical Standards:

JB7115-1993: Low Voltage Local Reactive Power Compensation Devices

JB7113-1993: Low Voltage Parallel Capacitor Devices

Power Industry Standards:

DL/T 597-1996: Technical Conditions for Low Voltage Reactive Power Compensation Controllers

National Standards:

GB 12747.2-2004: Self-healing Parallel Capacitors for AC Power Systems with a Nominal Voltage of 10kV and Below - Part 2: Aging Test, Self-healing Test, and Destruction Test

GBT 12747.1-2004: Self-healing Parallel Capacitors for AC Power Systems with a Nominal Voltage of 1kV and Below - Part 1: General

2. Why Install Power Capacitor Compensation Devices?

Inductive loads, such as AC asynchronous motors, electric welders, and induction cookers widely used in industrial production, cause the applied voltage to lead the current by a phase angle during energy conversion. The cosine of this angle, cosΦ, is known as the power factor. High reactive power (low power factor) can cause several issues:

Increased line current, resulting in greater line losses and wasted electrical energy.

Larger voltage drops over long transmission lines, potentially leading to inadequate voltage for equipment operation.

For transformers or generators, high reactive power means higher output currents, often reaching rated values, necessitating additional transformers or generator sets to handle increased loads, thereby wasting resources. With capacitor compensation, the transformer or generator output current significantly decreases under the same load, allowing for load increases without adding extra transformers or generators, thus saving resources.

If the monthly average power factor for industrial users is below 0.92 or for general users below 0.9, penalties are imposed by power management authorities.

Adding a parallel capacitor compensation cabinet is one method to improve the power factor (other methods include using overexcited synchronous motors, synchronous condensers, and synchronousization of asynchronous motors).

3. Current Standards for Power Capacitors for Compensation

The current standards are:

GB 12747.2-2004: Self-healing Parallel Capacitors for AC Power Systems with a Nominal Voltage of 10kV and Below - Part 2: Aging Test, Self-healing Test, and Destruction Test

GBT 12747.1-2004: Self-healing Parallel Capacitors for AC Power Systems with a Nominal Voltage of 1kV and Below - Part 1: General

4. Why Should Protection Control Devices and Wires for Capacitors be Chosen at 1.5 Times the Rated Current?

According to GB/T12747.1-2004/IEC60831-1:1996, switches, protective devices, and connectors should withstand continuous operation at 1.3 times the rated current under rated frequency and sinusoidal voltage. Given that capacitor capacitance may be up to 1.10 times the rated value, the maximum current can reach 1.3 * 1.1 times the rated current, approximately 1.43 times the rated current.

The selection of protection devices for different capacitors varies and is not simply 1.5 times the rated current. For example, fuses should be selected at 1.7-1.9 times the rated current for short circuit protection, while thermal relays or capacitor protectors are set at 1.15 times the rated current for overload protection. Since thermal relays do not operate within 2 hours at 1.2 times the set value, setting it at 1.5 times would render overload protection ineffective.

Capacitors generally operate normally at 1.1 times the rated voltage and 1.3 times the rated current. Considering capacitor capacity tolerance and harmonic voltage distortion rate (5%), the product of 1.1 and 1.3 equals 1.43 times, serving as the design basis for filter capacitors' field strength.

AC contactors must be specialized for capacitors. Ensure not to confuse the long-term reserved working current with the rated working current. For example, the CJ19-63 has a rated working current of 43A, while the CJ19-43 has a rated working current of 29A.

When selecting reactors (7%), the voltage at the capacitor terminals in a 400V system rises to 430V, so use 0.45kV or 0.48kV capacitors instead of 0.415kV ones. Comparatively, 0.48kV capacitors are more reliable in terms of voltage endurance but reduce compensation capacity, requiring comprehensive consideration.

5. Why Can't Fuses in Compensation Cabinets be Replaced with Miniature Circuit Breakers?

Fuses, used primarily for short circuit protection, should be fast-acting. Miniature circuit breakers (MCBs) have different characteristic curves and much lower breaking capacities (≤6000A). During faults, MCBs do not respond as quickly as fuses and cannot interrupt load currents in high harmonic conditions, causing switch damage and potentially expanding fault areas. Therefore, MCBs are unsuitable replacements for fuses in capacitor cabinets.

6. When Can Thermal Relays be Omitted in Compensation Cabinets?

In typical static compensation schemes: knife switch → fuse → contactor → thermal relay → (reactor) → capacitor.

Thermal relays provide overload protection. In modern grids, harmonics and overvoltage frequently cause capacitor overloads. Thermal relays protect capacitors by tripping during overloads. If the thermal relay has phase failure protection, it disconnects the capacitor during phase loss, ensuring protection.

Overload protection ranges for thermal relays are adjustable, whereas MCBs have fixed overload protection values. Therefore, MCBs can only replace thermal relays if their thermal trip current matches the protected capacitor. To save costs, some manufacturers simplify the scheme to: knife switch → MCB → contactor → (reactor) → capacitor.

This simplified scheme requires careful selection of protection devices (MCBs and contactors) for capacitors. MCBs must meet breaking capacity requirements and use D-type or higher breaking capacities (costlier and less reliable than fuses). The thermal trip setting must match the protected device. Note that after a thermal trip, manual reset is needed, which reduces the effectiveness of automatic compensation in unattended substations. Therefore, replacing thermal relays with MCBs is generally impractical.

Some capacitor banks, like ABB's CLMD series, have internal overload protection, eliminating the need for external thermal relays. However, older domestic products require thermal relays unless confirmed otherwise.

In dynamic compensation, where capacitors are switched by compound switches at zero crossing, no inrush current occurs. These switches often have built-in protection (phase failure, overcurrent, undervoltage), negating the need for thermal relays in such setups.