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How to Protect Capacitor Banks? And Electrical Network Protection Guide by Schneider Electric

Several strategies are employed to protect capacitor banks, including not energizing them unless discharged, applying a time delay before re-energizing to prevent transient overvoltage, and utilizing rapid discharging reactors to minimize discharge time. Protective controls are also crucial, as they should disconnect the bank from service if any units exceed 110% of their voltage rating. Monitoring for imbalances is essential too, as it may reveal problems such as blown fuses or short circuits.

Capacitor Banks
How to Protect Capacitor Banks?

Capacitor banks compensate for the reactive energy consumed by electrical system loads and can also form filters to mitigate harmonic voltage. Their purpose is to enhance the electrical system's quality. They can be arranged in star, delta, or double-star configurations, depending on the voltage level and system demand.

A capacitor is typically encased, featuring insulating terminals at the top. It consists of individual capacitors with specified maximum voltages (for example, 2250 V) that are connected in series to achieve the necessary voltage tolerance and in parallel to reach the desired power rating.

Capacitor bank
Capacitor Bank

There are two types of capacitors:

  1. Those lacking internal protection,

  2. In capacitors with internal protection, each individual capacitance is paired with a fuse.

Capacitors come in various types, including:

  • Electrolytic capacitors, which are polarized and known for high capacitance.

  • Tantalum capacitors, recognized for their stability and reliability.

  • Ceramic capacitors, popular for their non-polarized nature and general-purpose use.

  • Film capacitors, valued for their precision and low loss.

  • Silver mica capacitors, appreciated for their accuracy and stability.

  • Variable capacitors, whose capacitance can be adjusted manually or electrically.

  • Niobium electrolytic capacitors, which are polarized and offer a low cost with a high temperature coefficient.

  • Super capacitors, known for their high energy density and rapid charge/discharge capabilities.

  • Paper capacitors, which are non-polarized, suitable for high voltage applications but prone to aging.

  • Glass capacitors, which are non-polarized, noise-free, and of high quality.

  • Trimmer capacitors, which are small, adjustable, and used for calibration purposes.


Table of Contents:


1| Types of Faults in Capacitor Banks

Capacitor banks are vital for enhancing power quality and efficiency in electrical systems. Now, let's explore the different types of faults that can impact these banks:

  1. Overload,

  2. Short Circuit,

  3. Frame Fault,

  4. Capacitor Component Short Circuit

1.1| Overload

Capacitor banks are essential for compensating reactive energy in electrical systems and improving overall system quality. Here are the key points related to overload protection:

  • An overload occurs due to temporary or continuous overcurrent affecting the capacitor bank.

  • Continuous overcurrent can result from:

  • Voltage fluctuations: When the power supply voltage increases.

  • Harmonic currents: Generated by non-linear loads (e.g., rectifiers, variable speed drives, arc furnaces).

  • Temporary overcurrent occurs during the energizing of a capacitor bank step.

  • Overloads lead to overheating, which adversely affects dielectric withstand and accelerates capacitor aging.

Temporary overcurrent linked to the energizing of a capacitor bank step. Overloads result in overheating which has an adverse effect on dielectric withstand and leads to premature capacitor aging.

Remember that proper protection ensures the longevity and reliable operation of capacitor banks. By addressing overloads and other faults, we maintain efficient power systems while safeguarding critical equipment.

1.2| Short Circuit

A short-circuit is an electrical fault that occurs internally or externally between live conductors, either phase-to-phase or phase-to-neutral, depending on the connection of the capacitors, whether delta or star. The presence of gas in the sealed chamber of the capacitor can create overpressure, potentially causing the casing to open and the dielectric to leak.

  • A short circuit can occur as an internal or external fault between live conductors. The nature of the fault depends on whether the capacitors are delta-connected or star-connected.

  • When a short circuit occurs, it may lead to overpressure within the gas-tight chamber of the capacitor. This overpressure can cause the opening of the case and potential leakage of the dielectric material.

  • To protect capacitor banks against short circuits and earth faults, consider the following measures:

    • Unbalance Relay: Detects asymmetry caused by blown internal fuses, short circuits across bushings, or faults between capacitor units and their mounting racks. Each capacitor unit consists of multiple elements protected by internal fuses.

    • Ordinary Short Circuit Protection: Utilize two- or three-phase short circuit protection combined with an earth overcurrent relay to safeguard against short circuits and earth faults.

In summary, proper protection ensures the longevity and reliable operation of capacitor banks. By addressing short circuits and other faults, we optimize the efficiency, reliability, and quality of the electrical power system

1.3| Frame Fault

A-frame fault occurs when there is an internal fault between a live component of a capacitor and the frame formed by its metal enclosure. This fault, akin to internal short circuits, can cause gas to accumulate in the capacitor's sealed chamber, resulting in overpressure. This overpressure might cause the casing to open and the dielectric to leak.

  • Frame Fault Description:

  • A frame fault occurs within the gas-tight chamber of the capacitor.

  • Similar to internal short circuits, the appearance of gas (due to a fault) creates overpressure.

  • This overpressure may lead to the opening of the case and leakage of the dielectric material.

  • Protection Considerations:

    • Detecting and addressing frame faults is crucial for maintaining the reliability of the capacitor bank.

    • Proper protection schemes must cover all faults, both internal and external to the capacitor bank.

    • Identifying faulty capacitor units becomes more complicated with specific fusing types, such as internally fused or fuseless banks, which poses challenges for maintenance and fault investigation.

In summary, frame faults present a risk to the capacitor bank's integrity. The implementation of effective protective measures is crucial for ensuring safe operation and reducing the likelihood of damage.

1.4| Capacitor Component Short Circuit

A short circuit in a capacitor is often the result of a flashover within a single capacitance unit.

With no internal protection, the parallel-wired individual capacitances are shunted by the faulty unit:

  • The capacitor impedance is modified

  • The applied voltage is distributed to one less group in the series

  • Each group is submitted to greater stress, which may result in further, cascading flashovers, up to a full short-circuit.

With internal protection, the melting of the associated internal fuse removes the defective individual capacitance, ensuring the capacitor remains operational with an accordingly adjusted impedance.

A short circuit in a capacitor indicates the absence of resistance (impedance) between its two terminals. This can occur for several reasons:

  • Definition of a Short Circuit:

  • A component is considered short-circuited or shorted if there is a direct wire connection between its terminals.

  • In your circuit diagram, the vertical wire drawn next to the vertical capacitor effectively shorts the two terminals of capacitor C2.

  • This wire bypasses the capacitor, making it non-operational in the circuit.

  • Voltage Across the Shorted Capacitor:

  • The short circuit puts points A and B at the same voltage, effectively bypassing capacitor C2.

  • As a result, the output capacitance becomes C/2 instead of the expected C/3 due to the shorted capacitor.

Understanding short circuits is essential for circuit analysis and ensuring their correct functioning. Capacitors are critical in electrical systems, and their response to faults significantly affects the system's performance.

2| The Protection Devices of Capacitor Banks

Capacitors should not be energized unless they have been discharged. Re-energizing must be time-delayed in order to avoid transient overvoltage. A 10-minute time delay allows sufficient natural discharging.

Fast discharging reactors may be used to reduce discharging time.

2.1| Overloads

Long-duration overcurrent caused by an increase in power supply voltage can be prevented with overvoltage protection that monitors the system's voltage. While this protection can be specific to the capacitor, it is typically part of a broader electrical system protection strategy.

Considering that a capacitor can typically handle a voltage of 110% of its rated capacity for up to 12 hours daily, additional protection may not always be required.

Overcurrent of long duration due to the flow of harmonic current is detected by an overload protection of one the following types:

  • Thermal overload

  • Time-delayed overcurrent

provided it takes harmonic frequencies into account. The amplitude of overcurrent of short duration due to the energizing of capacitor bank steps is limited by series-mounting impulse reactors with each step.

2.2| Short Circuits

Short-circuits are detected by a time-delayed overcurrent protection device. Current and time delay settings make it possible to operate with the maximum permissible load current and to close and switch steps.

2.3| Frame Faults

Protection depends on the grounding system. If the neutral is grounded, a time-delayed earth fault protection device is used.

Capacitor component short-circuits: Detection is based on the change in impedance created by the short-circuiting of the component for capacitors with no internal protection by the elimination of the faulty individual capacitance for capacitors with internal fuses.

When the capacitor bank is double star-connected, the unbalance created by the change in impedance in one of the stars causes current to flow in the connection between the netural points. This unbalance is detected by a sensitive overcurrent protection device.

3| Examples of Capacitor Bank Protection

Capacitor banks are crucial for various purposes in electrical systems, such as power factor correction, voltage regulation, harmonic filtering, and transient suppression. Here are a few protection methods commonly used for capacitor banks:

  • Unbalance Relay:

    • The unbalanced relay detects asymmetry in the capacitor bank caused by:

    • Blown internal fuses.

    • Short circuits across bushings.

      • Faults between capacitor units and their mounting racks.

      • Each capacitor unit consists of multiple elements protected by internal fuses.

    • When faulty elements disconnect due to blown fuses, overvoltages occur across healthy capacitor units.

    • The unbalance relay monitors the current between the two neutrals and gives an alarm if unbalance exceeds a set level (usually 50% of the maximum permitted level).

    • If not addressed, the capacitor bank will be tripped when the allowed unbalance current level is exceeded.

  • Capacitor Bank Overload Relay:

    • Capacitors today have minimal losses due to overcurrent heating.

    • Overload is mainly caused by overvoltages (total peak voltage, fundamental, and harmonic voltages combined).

    • Capacitors can withstand 110% of their rated voltage continuously.

    • The capability curve follows an inverse time characteristic (approximately 1 second at 180%, 10 cycles at 210%).

    • Detecting overload by measuring busbar voltage is challenging due to the series connection with a reactor.

  • ABB SPAJ 160C Relay:

    • ABB Transmit Oy designed the SPAJ 160C relay, which:

    • Measures the current in the capacitor bank.

    • Transforms this current into a voltage corresponding to the voltage across the capacitor elements.

    • Includes unbalanced protection, overload protection, and undercurrent relay.

Remember that proper protection ensures the longevity and reliable operation of capacitor banks. These methods help optimize efficiency, reliability, and quality in electrical power systems.

Double star connected capacitor bank for reactive power compensation

Double star connected capacitor bank for reactive power compensation
Double star connected capacitor bank



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Setting Information


Electrical Network Protection Guide by Schneider Electric




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