METHODS OF POWER FACTOR CORRECTION IN POWER SYSTEMS

Low system power factor is among the most important issues in power systems on grid and industrial establishments. Primary solution to this is the use of power factor correction capacitors. These could be either shunt capacitors or series capacitors. Low power factor affects the voltage drop over transmission line, draws significant lagging reactive current, and loads transformers, cables, wires and switchgear more heavily than necessary for active load KW demand.  Maximum demand of industry goes up, inviting avoidable demand charges.

Lagging reactive power is usually corrected by use of KVAR capacitors, which inject leading reactive power in the system, reducing system load and improving power quality thereby. Following methods and their combinations are in use.

Individual shunt capacitors

Capacitors are connected directly across individual load terminals (motors or lighting fittings). This reduces the current at the load point itself, as the capacitor nullifies the reactive current of load, thereby reducing current all the way in supply system. For example, a motor rated 10HP (7.5KW) at 0.8 power factor typically puts a load of 10 KVA on supply, with about 4 KVAR reactive component. A 4 KVAR capacitor in parallel will fully compensate and neutralize this 4 KVAR, and resultant load on supply becomes 7.5 KW at unity power factor. (In practice, power factor of load is kept at 0.98-0.99 for safety of motor.)

This method can be adopted at relatively large loads, while it may not be feasible in small loads or lighting loads. [Capacitors are often connected across tube lights, sodium or mercury lamps fittings to improve their power factor]

Group shunt capacitors

A group of loads are served by one capacitor at common point. This could be across a small group of motors or lighting branch, or a section in a factory. This combines a number of individual small loads to form a group, which can be provided with a single capacitor or bank.

Central point shunt capacitor banks

It is common in establishments to have a centralized group of capacitor banks, with a power factor meter. Capacitors are individually switched in and out as needed to maintain good power factor. Switching may be manual or automatic.

Manual control

Number of capacitors are connected to system at central location, which are monitored and switched individually by manual control. This is the simplest and cheapest proposition, but needs physical monitoring and frequent manual switching, done by a person assigned for the job.

Automatic Power Factor Correction (APFC) Panels

These are a standard equipment in most establishments. They do away the need for manual control, are more efficient as they sense and adjust the power factor by switching capacitors automatically.

Capacitors are switched in and out by sensing the power factor and judging the capacitor to be added or deducted from the banks. APFC panels are commonly used in industry and utility terminals, and consist of

1. Power factor sensing device
2. Switchgear
3. Number of capacitors /banks
4. Series reactors for capacitors
5. Algorithm to switch on / off capacitors as per system power factor status..

The algorithm usually takes care of life and health of capacitors by avoiding frequent switching of any particular capacitor unit. This allows switching of different capacitors in a group at different times, thereby extending the operational life of capacitors. In most cases, inductors are connected in series with individual capacitors (series reactors) to protect capacitors from surges.

Active Thyristor Switching Devices

These use electronic circuits with thyristor controls (SVC, FACT etc.) to fine-tune the power factor rapidly and ensure better control than APFC panels. Capacitors are switched in a manner to avoid switching surges, and closer control on power factor.

Flexible AC Transmission System (FACTS)

This is used for active control of power factor correction. APFC panels is a type of FACTS system. Static Var Compensation (SVC), Thyristor Controlled Capacitors (TSC) and Thyristor Controlled Reactors (TCR) are used since few decades, which offer fast switching and better performance.   General arrangement of VSC is given below, which includes TSC, TSR and harmonic filter.

Optimum Effective Power Factor Management setup

An effective and optimum power factor control regime is a combination of all the three methods of shunt compensation as shown in the figure.

Series Capacitors in Transmission line

Capacitors are introduced in the transmission line, forming a series connection with line to load.  These compensate for lagging reactive impedance of long transmission line on a continuous basis.

Series capacitors carry the load current passing through the line, and correction is automatic. They increase the transmission capacity and stability of line, where excessive line voltage drop is avoided. However, series capacitors are susceptible to fault conditions, and they have to be protected through automatic switches and arrestors.