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Phase Angle Comparison

This type of relay compares the relative phase angle between two alternating-current quantities. It is commonly used to determine the direction of a current with respect to a reference quantity. Normal power flow in a given direction will result in the phase angle between the voltage and the current varying around the power factor angle (e.g., 30°) while power in the reverse direction will differ by 180°. Under fault conditions, since the impedance is primarily the inductance of the line, the phase angle of the current with respect to the voltage will be close to 90°.

Distance Measurement

This type of relay compares the local current with the local voltage. This is, in effect, a measurement of the impedance as seen by the relay. An impedance relay depends on the fact that the length of the line (i.e., its distance) for a given conductor diameter and spacing determines its impedance. This is the most commonly used relay for the protection of high voltage transmission lines.

Harmonic Content

Currents and voltages in a power system usually have a sinusoidal waveform of the fundamental power system frequency plus other normal harmonics (e.g., the third harmonic produced by generators). Abnormal or fault conditions can be detected by sensing any abnormal harmonics that accompany such conditions.

Frequency Sensing

Normal power system operation is at 50 or 60 Hz depending upon the country. Any deviation from these values indicates that a problem exists or is imminent.



The following discussion covers a very small sample of the possible designs. Specific details must be obtained from the manufacturers.


The fuse is a level detector and is both the sensor and the interrupting device. It is installed in series with the equipment being protected, and it operates by melting a fusible element in response to the current flow.

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