What are the causes of Low Power Factor in Grid and Problems associated with it?

Majority of load in a grid system consists of motors, transformers, electromagnets, lighting etc., which are highly inductive in nature. These loads take magnetizing current from systems, which are necessary for functioning of these devices or equipment. Inductive current offers a lagging power factor to system, and causes lagging reactive currents to flow in the system. The result is a lagging power factor, which could form a significant part of grid current.

Inductive current does not play any part in real power utilization of generators or transmission lines, but are unavoidable and necessary component for working of equipment or devices. They draw ‘reactive power’ (reactive KVA, or KVAR) from grid, in addition to active or real power (KW) needed for say, heater loads, work being done by machinery, or power used for lamp loads. Relation between KW, KVA and KVAR is given by

Greater the reactive load, lower is the available active power from supply. In other words, lower power factor means lower efficiency and higher current for same generation capacity, limiting the load which can be served. Low Power Factor means higher reactive power is needed for a given load.

For example, 1000 KW load at 0.7 P.F. results in overall load of (1000/0.7=) 1428 on generator / grid. Thus 428 KVA more is needed for 1000 KW load. If power factor is unity, these 428 KVA can be used to supply that much additional load. At still lower power factor, reactive KVAR becomes more predominant factor of load, and at 0.5 power factor, 2000 KVA and 1750 KVAR may be needed to serve 1000 KW load. Such things are possible with welding loads, unless these lagging KVAR are neutralized by compensating measures.

At 0.7 P.F., reactive load in system is 1000 KVAR (inductive). If 1000 KVAR capacitors are connected in parallel, these will fully neutralize the inductive load and grid load becomes 1000 KW with 1000KVA. In practice, power factor is corrected to slightly less than unity.

Thus, problems on grid due to low power factor are:

1. Higher current load on cables and transmission lines, which can get overloaded. On the other hand, with higher power factor, smaller capacity cable can be used, saving on material and investment.
2. High copper losses in transformers and cables. Iron losses in transformers are also higher.
3. Load on circuit breakers and switchgear goes up, which see much higher currents, and also switching problems. Switchgear costs and size goes up, and switching problems also go up.
4. Generation capacity is blocked by the inductive reactance, which can otherwise be used for additional load and reduce necessity of new addition for expanding network. Generation capacity is decided by current capability of generators and equipment.
5. Voltage drop along the line increases, causing poor voltage regulation at load. Special measures are needed to improve regulation, involving additional costs and complexity in system.
6. Low power factor of one industry affects other nearby industries. Therefore, it is imperative to keep good power factor at load centers.
7. Higher currents cause higher losses in generation, transmission lines or cables. Operational costs go up.
8. Maximum demand (and contract demand) is higher due to higher reactive load. This may also affect utility bill, since most utilities charge for contract or maximum demand. Contract demand commits certain part of grid capacity of utility to an installation, which will go down if power factor is good and contract demand low.
9. Most utilities impose penalties for power factor below 0.85 / 0.9. Improving the power factor also benefits the consumer by avoiding.

These problems are all the more important when renewable energy sources are part of the grid system. Solar panels or wind turbines are costly, and must be utilized to their full capacity. Solar panels (or wind turbines) also occupy much more space if more currents are needed from them. Integration of renewable energy sources with grid becomes more complex if they have to deal with low power factor.

It is therefore very important to keep power factor of load to as closer to utility as possible. It is usually intentionally kept slightly lower than unity to avoid load going capacitive, which can cause other issues and hence must be avoided.

Industries and utilities employ measures to improve the power factor to desired level. These are part of any AC power system at various levels- from individual loads, group of loads, industry power supply center, transmission and distribution system, as also generation plants. Motors are designed to have high power factor of over 0.95 wherever possible. Use of Automatic Power Factor Correction (APFC) systems extensively, including electronically controlled devices, help keep system power factor close to unity.