Types of Capacitor Banks

There are three main types of capacitor banks: internally fused, externally fused, and fuseless.

Internally Fused Capacitor Banks: In this type, the capacitors and fuse units are housed within the same casing. Each capacitor element is individually protected by a fuse unit. Even if one or more capacitor elements fail, the bank can continue to operate without interruption. This type of bank is easy to install and maintain. However, if multiple elements fail, the entire bank needs to be replaced as individual unit replacement is not possible.

Externally Fused Capacitor Banks: In externally fused banks, each capacitor unit has an external fuse unit. If a fault occurs in a capacitor unit, the fuse unit associated with it will be damaged, isolating the faulty unit. The bank can continue to function with the remaining units. However, the absence of a capacitor unit in a single phase affects the capacitance balance, resulting in voltage imbalances. The faulty unit can be visually identified by inspecting the blown fuse. This type of bank is susceptible to unbalance if any fuse unit fails. In Malaysia, we typically and commonly use Moulded Case Circuit Breaker (MCCB) for overcurrent protection instead of a fuse.

Fuseless Capacitor Banks: In fuseless banks, capacitor strings are connected in series, and multiple strings are connected in parallel to form a phase bank. There is no individual fuse protection for the capacitor strings. If a single string unit fails, the current flow is unaffected due to the presence of other capacitors in series. This allows for extended operation before replacing the faulty unit. Fuse units are not required for immediate replacement of the faulty unit in this type of bank. Fuseless capacitor banks are not common in Malaysia.

Each type of capacitor bank has its advantages and considerations. The choice depends on factors such as the desired protection level, maintenance requirements, and system design considerations.

Selection Considerations

Price should not be the sole consideration. When comparing prices, ensure you check on other details as well. A poor choice will lead to frequent capacitor damage. It’s not worth it. The TNB Power Factor Surcharge is significant. The surcharge or penalty can be as high as 50% of your TNB bill, depending on how low your power factor is.

  1. Ensure you select the right voltage for your systems. Capacitors can be rated as low as 380V to as high as 600V. Typically, any capacitors rated below 400V (commonly found in switchboards from China) are not suitable for use in Malaysia. Do not – all switchboards must be manufactured by a Suruhanjaya Tenaga approved Switchboard factory.
  2. Wet or dry type. Wet type of capacitors have an electrolyte, while the dry type does not. Selection will depend on how much harmonics (distortion of voltage and current waveform) is present in your electrical system.
  3. If harmonics are sufficiently high, you will want to consider a reactor-capacitor pair. A reactor will help mitigate damage to capacitors due to harmonics. The “size” of the reactor will be determine by your competent electrical engineer.
  4. What you need to note is, reactors are either made from copper or aluminum wire windings. Aluminum is cheaper but it is less energy efficient. It gives our more waste heat.
  5. Capacitor banks are typically connected to the bus-bar of MSBs. MSBs typically have high fault currents. As such, it is better to use moulded case circuit breakers (MCCBs) than miniature circuit breakers (MCBs) for over-current protection for individual capacitors. Discuss this with your consultants.

Maintenance

Generally, capacitors in capacitor banks do not require any maintenance. However, they do require periodic inspection. A few other items to periodically check includes:

  1. Check periodically to ensure capacitor bank and its power factor controller are working. We recommend weekly checks to avoid the monthly TNB Power Factor surcharge. To avoid manual checks, you can use our data logging system to send you automated alerts for low power factor.
  2. If the capacitor bank has ventilation fans, ensure that the ventilation fans are clean, its filters are clean and the fan works. Overheating or high temperatures will degrade your capacitors pre-maturely.
  3. Bulged capacitors must be isolated and replaced immediately.
  4. Leaking capacitors (for wet type) must also be isolated and replaced immediately. Be careful – some of the wet capacitors contain combustible liquid (electrolyte) and this requires careful handling.
  5. Ensure contactors in the capacitor banks are in good condition.
  6. During your scheduled maintenance, enquire all connections and screw terminals are properly re-torqued. Lose connection will cause arching and that will lead to unnecessary damage.
  7. You can also ask your qualified service engineer to measure the Equivalent Series Resistance (ESR) and Capacitance Measurement of the capacitors. This is to identify capacitors about to fail, but before it fails.

Disclaimer

The information provided in this website is for general knowledge only. Replacement of capacitors and capacitor banks must be done by qualified and competent persons. Capacitors can still store charge for long periods of time even after the power source has been isolated. The stored charge can cause serious injuries or even fatalities.

You must consult your competent person if you want to replace or install any capacitors. All maintenance work on electrical panels including but not limited to Main Switch Boards (MSBs) and Distribution Boards (DBs) must be done by qualified personnel and supervised by a Competent Person. Always remember: Safety First.