The Effects of Low Power Factor on Electrical Equipment

The cosine of the angle between voltage and current in an AC circuit is referred to as the power factor. In a typical AC circuit, there is a phase difference Φ between the voltage and current. The term cos Φ is termed to as the power factor of the circuit; if the current is inductive, the current lags behind the voltage and the power factor is referred to as lagging. On the other hand, in a capacitive circuit, current leads the voltage and power factor is said to be leading.

Power factor may also be defined as the ratio of active power to the apparent power:

Power factor, cos Φ = active power/apparent power = kW/kVA

To help us understand the concept of power factor, consider the figure below:

We can resolve the circuit current I into two perpendicular components, namely:

• I cos Φ in phase with V, and
• I sin Φ, 90° out of phase with V

In this case, the component I cos Φ is referred to as the active or watt-ful component, on the other hand, component I sin Φ is called the reactive or watt-less component. The reactive component is a measure of the power factor, if the reactive component is small; the phase angle Φ is small and thus the power factor cos Φ will be high. Hence, a circuit having small reactive current (I sin Φ) will possess high power factor and vice-versa. It is important to remember that the value of power factor can never be more than unity.

The power consumed in an AC circuit depends upon the power factor; let us consider the following equations:

Power for single phase supply (P) = V_LI_L cos Φ

Thus, I_L = P/V_Lcos Φ

Power for three-phase supply (P) = √3 V_LI_L cos Φ

Hence, I_L = P/√3 V_LI_L cos Φ

We can clearly conclude from the above that for fixed power and voltage, the load current is inversely proportional to the power factor. The lower the power factor, the higher the load current and vice-versa.

A lower power factor has a number of detrimental effects on electrical equipment as discussed in the next section.

Low Power Factor Effects on Electrical Systems Equipment

A power factor that is lower than unity has the following disadvantages with regard to electrical equipment:

Large kVA rating of the Equipment large

We know that:

Power factor, cos Φ = active power/apparent power = kW/kVA

Therefore kVA = kW/cos Φ

The electrical machinery such as alternators, transformers and switchgears are usually rated in kVA (this due to the fact that the power factor of the load is not known when the equipment is made in the factory). From the above equation, the kVA rating of the equipment is inversely proportional to power factor. The smaller the power factor, the larger is the kVA rating. Consequently, at low power factor, the kVA rating of the equipment has to be made more, making the equipment larger and costly.

Larger Conductor Size

To transmit or distribute a fixed amount of power at constant voltage, the conductor will have to carry more current at low power factor. This requires large conductor size. We demonstrated the relationship between the load current and power factor in our introductory section above.

Greater Copper Losses

The large current at low power factor causes more I²R losses in all the elements of the supply system. This results in poor efficiency.

Reduced Handling Capacity of the System

The lagging power factor reduces the handling capacity of all the elements of the system. This is due to fact that the reactive component of current prevents the full utilization of installed capacity.

Poor Voltage Regulation

The large current at low lagging power factor causes bigger voltage drops in alternators, transformers, transmission lines, and distributors. As a result, it leads to the reduction of voltage available at the supply end, thus ruining the performance of utilization devices. In order to keep the receiving end voltage within permissible limits, additional equipment is needed such as (voltage regulators).

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