A capacitor bank is the most effective method to reduce reactive power and improve the Power Factor (Cos Phi) of your electrical installation. By compensating reactive current locally, you relieve cables and transformers and avoid fines from the grid operator. However, in modern installations with many power electronics, a standard capacitor bank is not without risk. Without proper engineering, resonances can occur, resulting in dangerous situations. This page tells you how to apply capacitor banks safely in low- and medium-voltage installations.
Short on time? Here are the key points you need to know:
Purpose: A capacitor bank provides the reactive power(reactive power) needed by inductive loads such as motors and transformers. As a result, the grid operator does not have to supply it.
Result: You avoid blind current penalties, reduce the load on your internal infrastructure (more amps available for business processes) and reduce energy losses.
Low vs medium voltage: Applied at both 400V level (locally at machines or in the main distribution panel) and medium voltage level (10kV/20kV) for large industrial grids.
The big risk (Resonance): In grids with harmonic contamination (due to LED, variable speed drives, EV chargers), a capacitor bank forms a vibration circuit together with the transformer. This leads to explosion hazard.
The solution: Always apply tuned capacitor banks (with barrier filters) in polluted environments and measure Power Quality beforehand.
Optimising Cos Phi through capacitor banks is crucial for organisations with significant inductive power demand. This concerns in particular:
A capacitor bank consists of switchable capacitors (usually in steps) placed in parallel with the load. Technically, a capacitor acts as a temporary storage for electrical charge.
The principle of reactive power Many electrical equipment (motors, transformers, ballasts) operate on the basis of magnetism. Reactive power (kVAr) is needed to build up this magnetic field. This power shuttles back and forth between the source and the consumer without being converted into actual work (kW). We call this reactive power. Although it does not perform any labour, this current does load your cables, switches and transformers.
Bank operation A capacitor bank provides this required reactive power locally. Instead of the reactive power having to flow all the way from the power plant via the transformer through your cables to the motor, the capacitor bank delivers this power 'around the corner'.
Comparison: Think of it as a warehouse right next to the production line. Instead of a forklift (the power) having to drive up and down to a distant distribution centre (the grid operator) for each component (magnetic field), you get it directly from the local warehouse (the capacitor bank). The road (your cable) remains free for other truck traffic.
The basic operation is identical, but the implementation and application differ.
Low-voltage capacitor banks (LV) These are used in 400V or 690V installations. They are often of modular design and are placed in the main distribution system or decentralised at large consumers.
Medium-voltage capacitor banks (MV) These installations (typically 10kV to 30kV) are deployed at very high power levels or directly behind the purchase station transformer.
Installing a capacitor bank often pays for itself within 1 to 2 years. The impact is threefold:
In traditional, purely inductive grids, installing a capacitor bank was simple. However, modern installations are full of non-linear loads such as variable speed drives, LED lighting and rectifiers. These devices cause harmonic pollution.
What goes wrong? A transformer has an inductive property (L) and a capacitor bank has a capacitive property (C). Together, they form a parallel LC circuit. Each LC circuit has a natural resonant frequency. If this resonant frequency happens to coincide with a harmonic frequency present in your installation (e.g. the 5th harmonic at 250Hz or the 7th at 350Hz), resonance occurs. Currents and voltages are then amplified uncontrollably.
Effects of resonance:
The Solution: The tuned capacitor bank To prevent this, we apply 'tuned' (detuned) capacitor banks. Here, a specific coil (reactor) is placed in series with the capacitor. This lowers the resonant frequency of the circuit to a safe point where no harmonic currents are present (e.g. 189Hz). The bank then behaves inductively for the harmonic frequencies, making resonance physically impossible.
Nuance: In installations with extremely high contamination or rapidly changing loads, even a tuned capacitor bank is sometimes not sufficient. In that case, a Hybrid Solution or an Active Harmonic Filter (AHF) with reactive current compensation function is the only safe option.
Want to increase capacity or avoid penalties? Follow these steps for a foolproof solution.
Although any home installer can put up a cabinet, specialist knowledge is required once the installation becomes more complex. Contact our engineers as:
Delve further into the subject matter via these related pages:
Want to free up space on your transformer immediately or avoid penalties, without the risk of breakdowns? Speak to an engineer from HyTEPS. We will analyse your situation and simulate the impact of a capacitor bank before installing it. So you are assured of results and safety.
HyTEPS
Beemdstraat 3
5653 MA Eindhoven
