Power Capacitors - Power Factor Correction Equipment

Power Factor Correction Purpose, Definition & Benefits - Design Guidelines/Application

General

Under normal operating conditions certain electrical loads draw not only active power from the supply (kilowatts kW) but also reactive power (reactive kVA, kVAr). This reactive power has no useful function, but is necessary for the equipment to operate correctly. Loads such as induction motors, welding equipment, arc furnaces and fluorescent lighting would fall into this category.

Correction

Opposing reactive power resulting from the connection of a correctly sized capacitor can compensate for the reactive power required by the load. This ensures that only a small amount of reactive power is drawn from the supply.

Purpose

An apparent reduction in the total current drawn from the supply can be achieved as a result of connecting a capacitor to an inductive load.

Definition

The Power Factor of a load is defined as being the ratio of active power to total demand, that is to say kW + kVA. The uncorrected power factor of a load is cos Ø₁ (where Ø₁ is the phase angle between the uncorrected load and unity), and the corrected power factor is cos Ø₂ (where Ø₂ is the phase angle between the corrected load and unity). The nearer the ratio kW+kVA=cos Ø is to unity, the less reactive power is drawn from the supply.

Reactive Power Penalty Charges

The Reactive Power charge is the means by which consumers with a poor power factor pay more for their electricity than consumers with a good power factor. From April 2010, a common charging method is in force across all UK Electricity Supply Companies.

This applies to ALL half-hourly metered consumers. If your electricity bill does not yet include a charge for Reactive Power, this WILL be introduced over the coming months.

The way the Reactive Power charge is calculated is; the consumer will be ‘allowed’ to use Reactive Power up to a third of the active power consumed for free i.e. ‘allowed’ reactive power = 0.33 x total kWh

Any reactive power which is used over this ‘allowed’ figure is considered to be ‘excess’ usage and is chargeable.

By installing power factor correction equipment and ensuring that the average power factor is better than 0.95 lagging, no ‘excess’ reactive power is consumed. Smaller capacitor stages ensure that these charges are avoided under almost all load conditions.

This means that ALL excess Reactive Power charges are avoided.

Contact our team of sales engineers to discuss existing or imminent Reactive Power charges, and how to avoid them.

Technical Benefits

The connection of a capacitor capable of "correcting" half of the reactive power of a load leads to a reduction in the demand on the supply of approximately 15%. This results in the following:

a) The load on the cables and switches is reduced
b) The supply is now able to support additional load
c) The charges made by the electricity supply company are likely to be reduced

By reducing the load on cables and switches, power loss is reduced and life is extended. The facility to connect additional load is always useful to an expanding company.

An example: A fully loaded 1000kVA transformer supplying a load with a power factor of 0.80 can only supply 800kW of "useful" load. By correcting the power factor to 0.95, an additional 150kW of load may be connected, increasing the "useful" load capacity to 950kW.

Methods of Power Factor Improvement

1. Centralised Automatic Power Factor Correction This involves the connection of a number of capacitors, usually to the supply distribution point. The capacitors are controlled by a microprocessor based relay, which monitors the reactive power demand on the supply. The relay connects and/or disconnects the capacitors to automatically compensate for the reactive power on the system. Small stage ratings, detuning reactors and real-time switching should all be considered.

2. Group Power Factor Correction Group power factor correction is achieved by connecting one capacitor to a number of different loads usually sharing the same duty cycle. A shared duty cycle prevents the use of too much capacitance and avoids over-correction of the power factor.

3. Individual Power Factor Correction To individually correct the power factor of a multiple load, a suitably sized capacitor is connected to each element of the total load.

Harmonic Distortion & Filtering

In recent years the developments in semiconductor technology have led to constant increases in the number of converter-fed loads. These converters have undesirable effects on the incoming AC supply system since they draw non-sine-wave current. The supply system should be kept free from distortion in order to prevent equipment malfunction. When harmonic currents are superimposed on the AC supply, corresponding voltage distortion occurs which could lead to system disturbances and failures within the network. When the harmonic component is low, supply systems should be equipped with control units with reactor-connected capacitors in order to prevent resonance phenomena. Where the harmonic component is high, it may be necessary to systematically suppress the harmonics with a tuned filter circuit presenting a very low impedance to the individual harmonic currents.

Capacitor Sizing

To obtain capacitor sizes (in kVA) for a given power factor correction multiply the kW load by the number shown at the axis between existing power factor and required power factor.