The Function and Principles of Power System Protection

The objective of power system protection is to detect faults or abnormal operating conditions and initiate corrective action.

Principles of Power System Protection

There are different principles used in accomplishing power system protection, we have discussed them in the following sections:

Discrimination by Time

In simple radial circuits discrimination is achieved by giving the minimum tripping time setting to the relay furthest away from the power source. A small time delay is then added to each relay in turn, moving nearer to the source each time. This ensures that the relay closest to the fault trips first and as a result leaves the rest of the system between the source and the faulty section in service.

It is essential to allow a minimum grading interval or delay between successive relay settings in order to take account of:

• Circuit breaker tripping times – typically from 150 msec for an older circuit breaker to 50 msec for modern ones such as SF₆ and vacuum switchgear.
• Relay time delay errors – variation from the characteristic time delay curve for the relay as permitted by the appropriate standard e.g. 150 msec.
• Relay reset time – the relay must definitely fully reset when the current is 70% of pick-up value. Electromechanical relay reset at 90-95% of setting and a figure of 85% is considered for calculation purposes. Solid state relays have a better enhanced characteristic in this regard.
• Relay overshoot – an electromechanical relay must stop all forward movement or overshoot of the induction disc within 100 msec of the removal of current. Solid state relays and numerical relays have a better enhanced characteristic in this regard.

Therefore if all these items are additive then for discrimination to be achieved typical time grading intervals of 0.4-0.5 s are employed for electromechanical relays with oil circuit breakers and 0.25 s for modern solid state or numerical relays used to trip vacuum or SF₆ switchgear. The effect of current transformer errors on relay operating times is not expected to be additive and therefore such errors (about ±5%) are normally neglected when determining a discrimination margin.

Discrimination by Current Magnitude

The impedance of the power circuit between source and fault limits the fault current flowing at any point.  Thus by appropriately choosing the current setting at which a particular relay operates discrimination can be accomplished. Practically this is quite complex for transmission and distribution feeder circuits since different interconnection configurations significantly alter the fault level at any point in the network.

This technique works well for power transformer protection where instantaneous high set overcurrent relays can be used to protect the HV windings.

Discrimination by Time & Fault Direction

Directional sensing elements can be possibly be added to the relay protection system such that the relay responds to both the magnitude and one particular direction of the current flow. Common applications are for closed ring feeder systems, parallel feeders and parallel transformers.

Unit Protection

In these protection schemes, the current transformers (CTs) located at the end of a feeder, transformer or ‘unit’ of plant to be protected (the protected zone) are interconnected. A comparison of magnitude and phase angle of the current entering the protected zone with that leaving is established.

Two requirements are checked:

1. If the currents entering and leaving the protected zone are equal, operation of the protection must be prevented; this is referred to as the through fault stability requirement.
2. If the currents entering and leaving the protected zone are unequal the protection must operate; this is referred to as sensitivity to internal faults requirement.

The advantage of unit protection is that it provides a very fast (typically 200 msec or less) disconnection only of the plant being protected. Nonetheless, the disadvantage of this protection principle is that the interconnection between the relays requires communication systems which make the overall protection scheme costly than simple time graded schemes for long feeder lengths.

Signaling Channel Assistance

Rapid protection operation may be required for system stability. The speed of response of a protection system may be boosted by employing interconnecting signaling channels between relays.

Such signaling channels may be achieved by use of hard wire circuits (dedicated pilot wires, etc.) utilizing ON/OFF or low frequency. Alternatively, signal information superimposed upon carrier frequencies of several hundred kHz may be employed over the power circuits (power line carrier) to convey the information. Presently, fiber optic cables, which may form an integral part of an overhead line earth wire, are used.

Related articles:

• Protective Relays: Function, Features & Operation
• Overvoltage Protection Techniques & Equipment
• SF6 Circuit Breakers: Features, Operation & Applications
• Switchgear Equipment
• Common Busbar Protection Schemes