Economic Limits of Power-Factor Correction

Author: E.E. Kimberly

If no cost were involved in power-factor correction, the power factors of all industrial loads would be regularly corrected to 1.0. In Schedule C of page 288 the power company imposes a penalty on power factors less than 0.75. In some contracts a penalty is imposed for power factors less than 0.85. The theoretical criterion that determines the choice of power factor below which a penalty will be imposed is the cost of correction relative to the value of released apparatus. Obviously, if it would cost $1000 in installed corrective apparatus to release $1000 worth of generating station and distribution apparatus, the venture would yield a mere trading of dollars and would be economically unsound where such release is the sole consideration. However, low power factor causes poor voltage regulation in many cases, and the correction of such regulation may justify a power-factor correction in excess of that dictated by other considerations. The PR losses in the generator and the transmission apparatus caused by the increase of current made necessary by low power factor must also be charged to this evil of low power factor.

Power-factor correction is strictly a problem of the power company, but may affect the consumer through a penalty, if one is imposed by the power company in its contract with the consumer. When the consumer must pay a penalty for low power factor, power-factor correction becomes one of his problems too.

In Fig. 25-1 is shown a kv-a diagram of a load, the power of which is OH and the kv-a of which is OA. Let it be assumed that seven equivalent static condensers, of no loss and of A B kv-a each, are available to be connected one by one in parallel with the load. By connecting one condenser the angle of lag is decreased from θ1 to θ2, and the kv-a drawn from the line is decreased by the amount AA₁. By the installation of AB kv-a in a static condenser, AA₁ kv-a in generating station and transmitting equipment is released for other service, As successive condensers are connected, the line kv-a is reduced successively, but not proportionately. When the last condenser, which costs as much as any other, is added to reduce the line reactive kv-a to zero, it releases only GGi kv-a in generating and distributing equipment. Thus, the effectiveness per unit of corrective kv-a becomes less and less as the power factor to be improved approaches 1.0. With the present cost of corrective apparatus, it is seldom sound economy to correct the power factor to any value greater than 0.85.

Fig. 25-1. Graphic Representation of Power-Factor Correction by Static Condenser Units