What is Meant by Power Factor Correction in a System?

Most load in a grid / power system consists of motors, lighting, pumps, welding, electromagnetic appliances etc., which are all inductive (many are highly inductive). Overall system load being inductive (lagging load), the power factor of the system is highly inductive, far below the ideal power factor of 1. This causes heavy inductive KVAR presence, increasing overall KVA of the system, with much larger currents in the system. Load on cables and switchgear increases, and cable and line losses go up. Generator ratings are mentioned in KVA / MVA, and a major part of the generator load is absorbed by the inductive KVAR load, limiting its KW / MW load supply capacity. The inductive load also causes voltage drop along the line, affecting load end voltage badly. Stability, efficiency and regulation of system is adversely affected. The power factor keeps varying with load at different time of day, depending on load.

Therefore, it becomes essential to compensate the inductive KVAR in the system through some compensating method. Most common and prevalent method logically is to compensate for the inductive KVAR by adding capacitive KVAR in the circuit, which is opposite in phase to inductive KVAR. Being opposite in phase, capacitor and inductor currents in parallel, reduce the load on source to the extent of difference in currents drawn by them.

It has become universal practice to use capacitors in parallel / series with the supply system from generating stations to load point, as also directly across load, depending on necessity. The capacitors offer leading power factor (with low losses), which are out of phase with inductive load by half cycle, or 180°. For example, a 100 KVAR capacitor can fully neutralize a 100 KVAR inductive load to bring power factor to unity. The process of improving the power factor of a system from highly inductive condition to a lesser desirable power factor is “Power Factor Correction”.

However, overall power factor is always maintained slightly below unity to avoid it from going capacitive (leading). This is because a leading load current is harmful for the generator as also the load, as it may lead to over-excitation of generators and motors, and also cause problems for switchgear operations. In practice, most utilities aim to keep power factor of system varying between 0.9 to 0.99, and impose a penalty on consumers for low power factor below, say 0.85, and some may offer incentives for high power factor.

The capacitors may be connected in parallel with load (parallel compensation) or in series with load (series compensation). Most common and convenient method is that of series compensation. Series compensation has the advantage that the capacitors are constant in value and need not change with load (load current and capacitor current are same). However, this needs very high capacitor voltages, and they need to be connected on transmission line. So series compensation is used judiciously on transmission lines when needed. Parallel compensation is the universal method of power factor correction.

Large heavy motors are often provided capacitors directly across their terminals to reduce overall motor current. This reduces motor current, as also current through wires / cables, leading to reduced voltage drop in them, as also reducing cable losses. The reduced current means reduced switchgear ratings, as also better control of motors.

Establishments usually have capacitors housed near supply points, which keep power factor under control. They are connected directly across mains terminals and across factory load, and are often continuously monitored (switched in and out as needed) to adjust capacitance values depending on changes in load. The load power factor is thus constantly monitored, which may be done manually, or as is done commonly through automatic power factor control panels.

Since load varies with time of day, and different loads keep switching in and out, measures are often taken to adjust the power factor in response to load. This could be done by manually switching capacitors, or managing them automatically through controls. Several systems are available for automatic control through Automatic Power Factor Control (AFC) Panels, FACTS etc.