EARTH FAULT PROTECTION RELAY 50N/51N

Earth fault protection is a protective scheme used for the protection of an element of a power system against electrical faults involving the earth. An earth fault is an electrical fault that involves the ground. Examples are—single line-to-ground (L-G) faults and double line-to-ground (2L-G) faults. Faults that are caused in the absence of the ground are called phase faults. Similarly, the scheme that is used for the protection against phase faults is known as the phase fault protection scheme.

In any electrical system, where the fault current reaches the earth caused by the link with the phase, creates a high potential of a fire hazard. Causes are

• failure of electrical insulation,
• breakdown of metal enclosure of equipment,
• foreign objects or trees touching the naked live conductors, and many others.

To protect against this type of fault, earth fault relays are used which are very fast in operations.

Relays used for Earth fault protection and overcurrent relays are mostly similar in function with a little variation. A detailed elaboration is discussed in the following.

Earth Fault Relay and Overcurrent Relay

The operating principles and constructional features of earth fault protection relays and phase fault relays are the same. The difference is only in the level of current in the operation. The plug setting for earth fault relays varies from 20% to 80% of the current transformer’s secondary current rating in steps of 10% while for overcurrent relays it varies from 50% to 200% of the CT secondary rating in steps of 25%. The overcurrent relays are less sensitive to the current compared to the earth fault relay.

It may be noted that the earth fault is detected by the imbalance current flowing in the neutral but for phase protection monitoring of the phase current is necessary. However, in the modern digital relay or numeric relay, both the overcurrent and earth fault functions are integrated into a single relay.

Earth Fault Protection Schemes

An earth fault relay is energized only by a residual current. As shown in the diagram, ia, ib, and ic are the secondary CT currents of different phases. The sum (ia + ib + ic) is called residual current, which under normal conditions has to be zero.

When an earth fault occurs, this residual current becomes non-zero. When the residual current exceeds the pick-up value, kept between 20-80% of the CT secondary current, the earth fault relay operates. In this scheme, the relay operates based on residual current only, which means only for faults with the earth.  For normal operation of the power system, no current is carried to the earth fault relay as there remains no residual current. Theoretically, the current setting should therefore be any value that is greater than zero.

But practically, ideal conditions do not exist in the system and therefore a margin of 20% is necessary. This is why, the minimum plug setting is kept between 20% to 30%. The manufacturer provides a range of plug settings for earth fault relays from 20% to 80% of the CT secondary rating in steps of 10%.

The magnitude of the earth fault current depends on the fault impedance. The fault impedance depends on the system parameter and the type of earthing used for the neutral. The system’s neutral may be solidly earthed, or grounded through resistance or reactance.

 An earth fault relay used for the protection of a transformer and an alternator, respectively is shown in the figure. When an earth fault occurs, zero-sequence current flows through the neutral. It actuates earth fault relay.

The connection of an earth fault protection relay using a special type of CT known as a core-balance CT, which encircles the three-phase conductors is also shown in the figure.

EARTH FAULT PROTECTION VS PHASE FAULT PROTECTION

• The fault impedance during the earth fault is usually much greater than that of the phase fault.
• Therefore, the fault current during an earth fault is lesser compared to the fault current during a phase fault.
• The earth fault relay is not dependent on the load current in contrast to the phase fault relay.
• The current setting is always below the normal load current but the current settings of the phase fault relay are mostly above the full load load current.
• Zero sequence current is involved in the ground fault, whereas positive and negative sequence current are present in a phase fault.

Note: An earth fault relay is set at lower values, with high ohmic impedance, resulting in a high CT burden.

DIRECTIONAL EARTH FAULT PROTECTION RELAY

For earth fault protection in one direction, only one directional overcurrent relay is needed. The operating principle and construction are like the directional overcurrent relays which contain two elements, a directional element for fault direction identification and an I.D.M.T. element for time grading. The directional element has a voltage coil energized by the residual voltage and a current coil powered by the residual current.

The connection is effective in case of unavailability of a neutral point. In the case of a grounded neutral, connections are to be made as shown in diagram (b). If the neutral point is grounded through a VT, the voltage coil of the directional earth fault relay shall be connected to the secondary of the VT.

APPLICATIONS OF EARTH FAULT RELAYS

They are used in the following

• Transmission system as a secondary layer of protection in feeders
• Transformer protection, to detect any winding or internal faults
• Generator protection increases the reliability of power generation as it safeguards the generator.
• Industrial application: help detect ground faults and isolate them, saving human and plant machinery by quick isolation.

FAQ’s

This article is a part of the Protection System, where other articles related to the protection of electrical equipments are discussed in details.