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what is the effect of power factor to (a) voltage (b)
transformers (c) generators (d)transmission lines
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Answer Posted / deepak thapa magar
On an AC power system, there are two kinds of power - real power (watts) that actually does work, and reactive power (vars) that enables transformers to transform, generators to generate, and motors to rotate. Reactive power doesn't actually do any work, but it must be produced, transformed, transmitted and delivered for the power system to perform its function.
So, power factor is merely a way of describing the relationship that exists between real power (P) and reactive power (Q), and the complex combination that is sometimes referred to as apparent power (S), measured in volt-amperes. Mathematically,
pf = cos (arc tan (Q/P)
Therefore, power factor has a maximum value of 1 when the power is all real, and a value of zero when the power is all reactive.
Now, you asked about the effect of power factor. That's a very broad subject. Here are a few very general statements:
- the flow of reactive power results in voltage drop. That is, the magnitude of voltage will be higher near the source of reactive power and lower near the reactive power 'load'
- it is harder to regulate voltage when power factor is lower than it is when power factor is higher.
- reactive power doesn't travel well, and it is generally preferable to try to produce reactive power as close to the point of consumption as possible.
- transmission lines are a source of reactive power. Transmission lines contain both series inductive effects and shunt capacitive effects. Therefore, a lightly loaded transmission line will produce more reactive power than it 'consumes' in transmitting power. There is a phenomenon called "Ferrante rise" in which the open circuit voltage at the end of a transmission line is higher than the voltage at the source end of the as a result of this capacitive effect.
- Generators have a power factor rating. Ultimately, there are three limits governing the capabilities of a generator. For actual lagging load power factors (that means vars out of the generator) above the rating up to the maximum value of 1, and for leading power factors (which means vars INTO the generator) from 1 down to some lower value, the generator doesn't really care about the power factor. In this region, generator capability is determined by thermal heating in the stator winding which is purely a matter of current. For lagging power factors below the power factor rating, power factor is important. In that region, generator capability is governed by heating of the field winding, and power factor is important it determining how much field winding current is required for a given stator output. Finally, for leading power factor operation, the capability of the generator is governed by heating in the end rings, and again power factor is important.
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