Understanding Leading Power Factor Systems

The power factor of an electrical system indicates how effectively electrical power is being used. It is the ratio of real power used to do work to the apparent power supplied to the circuit. A power factor can be lagging, leading, or unity.

• Lagging Power Factor: This occurs in circuits with inductive loads, where the current lags behind the voltage. Examples include motors, transformers, and reactors.
• Leading Power Factor: This occurs in circuits with capacitive loads, where the current leads the voltage. Examples include capacitors, synchronous condensers operating in over-excited mode, and long transmission lines or cables.
• Unity Power Factor: This occurs in purely resistive circuits, where the current is in phase with the voltage.

Why Underground Cables Exhibit Leading Power Factor

Underground cables, especially long runs of high-voltage cables, have a significant amount of capacitance. This capacitance exists between the conductors and between the conductors and the surrounding earth or sheath. This capacitive effect causes the current to lead the voltage. Therefore, underground cables are a common example of a system component that contributes to a leading power factor, particularly under light load conditions or when the capacitive reactance is dominant.

Analysis of Other Options

• Reactors: Reactors are essentially inductors. Inductive loads cause the current to lag the voltage, resulting in a lagging power factor.
• Mercury arc rectifiers: Rectifiers are used to convert AC to DC. They are non-linear loads and introduce harmonics. Typically, they operate with a lagging power factor due to the phase delay in the current waveform relative to the voltage.
• Transformers: Transformers are primarily inductive loads because of their windings. While they also have resistance and some capacitance, their inductive reactance usually dominates, leading to a lagging power factor during normal operation.

Based on the characteristics of these electrical components, underground cables are the system listed that typically exhibits a leading power factor due to their capacitive nature.