Capacitor banks protection, cautions and maintenance tips

Power Quality and Cost Savings

The Role of Capacitor Banks

It would not be wrong to say that humanity has never consumed so much electricity, and to make the paradox bigger, there is still a lack of energy. The sharp increase in consumption in the last couple of decades has created a lot of problems in electrical networks around the world. The optimization of costs and reducing transmission and distribution losses are urgently needed. Here we come to the main topic of this article, how to handle all these problems using capacitor banks.

Requests for reactive power compensation, voltage stability, and harmonic filter mitigation have increased as a result of the integration of renewable energies many other technologies into the electrical system. Capacitor banks are abundantly utilized in substations for improving overall power quality.

Due to the neck-to-neck competition, every industry aims to reduce production expenses and better control and optimize electrical energy by employing power quality improvement.

Nowadays, in the light of the coming global crisis, this is important more than ever. Let’s talk about capacitor banks.

Table of Contents:

1. The Purpose of Capacitor Bank
2. Capacitor Bank Connections
3. Failure of Capacitor Banks
   1. Harmonics and Detuned Capacitors
   2. Resonance
   3. Relevant Load Changes
   4. Capacitors in Poor Condition
   5. Equipment Damage
4. Protection of Capacitor Banks
   1. Internal Resistors
   2. External Discharge Devices
   3. Internal Fault Protection for Capacitor Bank
   4. Element Protection
   5. Group of Element Protection
   6. Different Types of External Fault Protection
5. Maintenance of Capacitor Banks
6. Cautions to be Taken Care With Capacitor Banks
7. Case Study
8. Conclusion

1. Capacitor Bank Purpose

Let’s start with some basics. In a few words, capacitor banks provide stable voltage level, reactive power support, and increasing power transfer capability in the power system. They are also used to compensate for the losses in transmission systems. Capacitor banks reduce the phase difference between the voltage and current.

A capacitor bank is used for reactive power compensation and power factor correction in the power substations. Capacitor banks are mainly used to enhance the electrical supply quality and enhance the power systems efficiency.

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2. Capacitor Banks Connections

The capacitor bank is connected in two ways – star and delta, but most of the time, delta connection is used. Both of these two connections have their benefits and drawbacks. The main application is power factor correction because, in a 3-phase system, a 3-phase capacitor bank is used for the power factor correction which may be connected in star or delta.

Figure 1 – Delta-Connected Capacitor Bank

The star-connected capacitor bank is used for medium to high voltage applications. In star connection, the voltage across each capacitor is root 3 times lesser than the phase voltage, so the voltage stress across the capacitors is low even in high voltage applications.

There are two types of star connections in the capacitor bank:

1. Grounded star connection
2. Ungrounded star connection

Grounded star connection: The neutral point is grounded. In this type of connection, the unbiased point of the bank is stably earthed, which means the neutral should not be insulated toward the BIL level of the complete system.

An error on the 1-phase of the bank will not affect the rise of voltage within the remaining legs of the bank. So, a fault on one phase of the capacitor will not affect other phases.

Figure 2 – Grounded star connection of capacitor bank

Ungrounded Star Connection: The neutral point is isolated from the earth or ground. In this kind of connection, the capacitor bank’s neutral point is not connected toward earthing. So, this type of connection does not allow the supply of GND currents & zero series harmonic currents

Figure 3 – Ungrounded star connection of capacitor bank

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3. Failure of Capacitor Banks

Nowadays, modern capacitors use a “self-healing, safety disconnect” technology, in which the integrity of the capacitor dielectric is maintained very effectively. Under minor fault conditions, gases are released within the capacitor element to effectively weld and close any hole caused by the dielectric fault.

Typical failures of power factor correction capacitors can be attributed to several reasons as detailed below:

3.1 Harmonics and Detuned Capacitors

Harmonics are currents or voltages that are a multiple of the fundamental power frequency, harmonics are generated by some non-linear loads like variable speed drives, capacitors are particularly sensitive to harmonic currents since their impedance decreases proportionally to the order of the harmonics present.

This can result in a capacitor overload, shortening steadily its operating life.

Suggested Video – Using Harmonic Analysis Software

3.2 Resonance

Resonance is a situation where the capacitors and supply transformer creates a low impedance path for the circulating harmonic currents, when this happens, the electrical system could tune to the most dominant specific harmonic frequency thus increasing current flow through the electrical system.

There is no safe rule to avoid such resonant currents, but resonances above 1000 Hz will probably not cause problems except interference with telephone circuits.

Suggested Reading – Six network operating conditions under which ferroresonance can occur

Six network operating conditions under which ferroresonance can occur

3.3 Relevant Load Changes

Capacitor banks are designed for an original load. Over time, this changes. This can lead to the original capacitor bank being insufficient or unsuitable for the new situation. It can also be the case that the bank enters in resonance with the installation and becomes a harmonic generator, worsening the efficiency level of the installation.

3.4 Capacitors in Poor Condition

Poor maintenance of the capacitor bank may cause it to fail as expected. Capacitors lose their capacitance over time. If they are not replaced in time, the performance of the set is no longer effective: it does not perform its function correctly

3.5 Equipment damage

In the face of a power failure, the non-disconnection of the capacitor bank can cause a sudden surge of tension. This may damage sensitive equipment in the installation.

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4. Protection of Capacitor Banks

According to a large capacitor manufacturer, approximately half of all large industrial plants operate at a power factor of less than 0.85! At the same time it is commonly known that lower power factor results in higher utility cost.

It is worth mentioning that:

1. Raising power factor will also reduce peak charges.
2. System capacity will increase with a higher power factor.
3. Lighting and motor performance will benefit from improved power factor.
4. Since capacitor banks are prone to severe damages due to varied equipment connected to them, it is of utmost importance to ensure proper protection of same.

Suggested Reading – Unbalance protection of grounded & ungrounded wye shunt capacitor banks

Unbalance protection of grounded & ungrounded wye shunt capacitor banks

4.1 Internal Resistors

Internal resistors are specifically designed as individual capacitors to ensure the discharge of individual capacitor separately and hence the complete capacitor bank ultimately. With capacitor banks having multiple sections of capacitors in series, the residual voltage section in each section exactly equal to the residual voltage on the bank terminal.

4.2 External Discharge Devices

Every project site has different schemes and protection systems. Insulation level and strike distance shall be suitable to the structure and designing of each device. If the capacitors have no internal discharge resistors, there should be no isolating device between the capacitor bank and the discharge device.