With the focus on reducing energy consumption, sustainable growth and reduced operating cost I thought it would be a good idea to review power factor. In 900 words or less, here is the abbreviated explanation.
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power, and is a number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50% pf. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit.
A power factor of one or “unity power factor” is the goal of any electric utility company since if the power factor is less than one, they have to supply more current to the user for a given amount of power use. In so doing, they incur more line losses.
Industrial facilities tend to have a “lagging power factor”, where the current lags the voltage (like an inductor). This is primarily the result of having a lot of electric induction motors – the windings of motors act as inductors as seen by the power supply. Capacitors have the opposite effect and can compensate for the inductive motor windings.
The significance of power factor lies in the fact that utility companies supply customers with volt-amperes, but bill them for watts. The relationship is (watts = volts x amperes x power factor). It is clear that power factors below 1.0 require a utility to generate more than the minimum volt-amperes necessary to supply the power (watts). This increases generation and transmission costs. Utilities may impose penalties on customers who do not have good power factors on their overall buildings.
Watts, or real power, is what a customer pays for. VARS is the extra “power” transmitted to compensate for a power factor less than 1.0. The combination of the two is called “apparent” power (VA or volt-amperes).
A low power factor is expensive and inefficient and some utility companies may charge additional fees when the power factor is less than 0.95. A low power factor will reduce the electrical system’s distribution capacity by increasing the current flow and causing voltage drops.
Consider this popular analogy to clarify the relationship between real and apparent power. We all know a glass of draft beer generally has a “head” on it. Let’s say your favorite pub institutes a new policy – you only pay for the beer, not the foam. While the foam is just aerated beer, it is not really usable in that form. If the glass of beer is half foam, you pay half the price. This is the same principle as electricity generation – the consumer only pays for the beer (real power), not the foam.
The main advantages of the Power Factor Correction are:
• The electrical load on the Utility is reduced, thereby allowing the Utility to supply the surplus power to other consumers, without increasing its generation capacity.
• Most of the Utilities impose low power factor penalties. By correcting the power factor, this penalty can be avoided.
• High power factor reduces the load currents. Therefore, a considerable saving is made in the hardware cost, such as cables, switchgear, substation transformers, etc.
Impact on the Power Utility
In order to generate electricity, the generator field or rotor must be electrically energized or excited. This is done by increasing the generator field current and voltage. In most power plants this is done automatically by the voltage regulator when the generator is coming up to speed. After the generator has been synchronized and some load has been placed on the generator, the power factor controller is placed in service. The generator filed current and voltage can be increased further or decreased with this controller, thereby adjusting the power factor. In some plants this is done with the rheostat.
When the generator is excited with a power factor of 1.0 or unity, only true or active power in watts is being generated to provide electrical energy for any load that dissipates energy. This type of load is purely resistive, and both current and voltage are in phase.
As more inductive load is added to the electrical system, the current begins lagging the voltage. The system power factor is therefore termed to be lagging.
Although the resistive load in MW’s has not changed, the added reactive load in MVAR’s results in increased MVA’s. For a constant system voltage this means the current has increased resulting in increased system losses. This can be corrected by adding fixed capacitors to provide offsetting capacitance, or the generator can be operated in a lagging mode to provide inductive MVAR’s. This is done by overexciting the generator thereby producing MVAR’s or moving them out to the line by increasing the generator field current and voltage by increasing the power factor controller set point.
So when you receive your electric bill with a charge for power factor correction at least you have a better understanding as to why.
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