A fault that involves the ground is called an earth fault. Examples are—single line-to-ground (L-G) faults and double line-to-ground (2L-G) faults. Faults that do not involve ground are called phase faults. The protective scheme used for the protection of an element of a power system against such faults is known as earth fault protection. Similarly, the scheme that is used for the protection against phase faults is known as the phase fault protection scheme.
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In an electrical system, where unintended fault current reaches the earth, has the potential to create 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 mitigate this type of fault earth fault relays are used which are very fast in operations.
Earth fault protection relays 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
Relays that are used for the protection of a section or an element of the power system against earth faults are called earth fault relays. Similarly, relays used for the protection of a section of the power system against phase faults are called phase fault relays or overcurrent relays.
The operating principles and constructional features of earth fault protection relays and phase fault relays are the same. They differ only in the levels 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%. Earth fault relays are comparatively more sensitive to current than phase fault relays. The plug setting for phase fault relays varies from 50% to 200% of the CT secondary rating in steps of 25%.
The name phase fault relay or phase relays is not common. The common name for such relays is overcurrent relay. One should not confuse this term with the general meaning of overcurrent relay. In a general sense, a relay that operates when the current exceeds its pick-up value is called an overcurrent relay. But in the context under consideration, i.e. phase fault protection and earth fault protection, the relays which are used for the protection of the system against phase faults are called overcurrent relays.
Earth Fault Protection Schemes
An earth fault relay can be energized by a residual current. As shown in the diagram, ia, ib, and ic are currents in the secondary CTs of different phases. The sum (ia + ib + ic) is called residual current.
Under normal conditions, the residual current is zero. When an earth fault occurs, the residual current is non-zero. When it exceeds the pick-up value, the earth fault relay operates. In this scheme, the relay operates only for faults with the earth. During the balanced load condition, no current is carried to the earth fault relay. Theoretically, the current setting should therefore be any value that is greater than zero.
But in practice, it is not true as ideal conditions do not exist in the system. Usually, 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’s fault current depends on the fault impedance. In the case of an earth fault, the fault impedance depends on the system parameter, and also the type of earthing used for the neutral influences the impedance. The system’s neutral may be solidly earthed, or grounded through resistance or reactance.
The fault impedance for the earth fault is basically much greater than that of the phase fault. Therefore, the fault current in earth fault is low compared to the fault current in phase fault. An earth fault relay is set independent of load current. Its setting is below the normal load current. When an earth fault relay is set at lower values, its ohmic impedance is high, resulting in a high CT burden.
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.
COMBINED EARTH FAULT PROTECTION AND PHASE FAULT PROTECTIVE SCHEME
Here, two overcurrent relays for phase-to-phase fault relays and one earth fault relay are connected. When an earth fault occurs, the burden on the active CT is that of an overcurrent relay or phase fault relay and the earth fault relay in series. Thus, the burden of the current transformer becomes high and may lead to saturation of the current transformer.
PHASE FAULT PROTECTIVE SCHEME
Here, three overcurrent relays each connected at three phases for the protection of a three-phase system. This protection scheme is mainly utilized for the protection of the power system against phase faults. In cases, where no separate scheme for earth fault protection is available, the overcurrent relays, used in the scheme also sense the earth faults in the system but with a loss of sensitivity compared to that of earth fault relays.
DIRECTIONAL EARTH FAULT PROTECTION RELAY
For the protection against ground faults in one direction, only one directional overcurrent relay is required. Its operating principle and construction are like the directional overcurrent relays. These relays basically contain two elements, a directional element and an I.D.M.T. element. The directional element has two coils. One of the two coils is energized by current and the other by voltage. The current coil of the directional element is energized by residual current and the potential coil by residual voltage.
This connection is effective especially where the neutral point is unavailable. If the neutral of an alternator or transformer is grounded, connections are made as shown in diagram (b). If the neutral point is grounded through a VT, the voltage coil of the directional earth fault relay may be connected to the secondary of the VT. The I.D.M.T. element of the directional earth fault relay has a plug setting of 20% to 80%.
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
What is the ANSI code for earth fault relay?
The ANSI code for earth fault relay is 67Ns
What can cause an earth fault?
Any short circuit between a hot or live phase and the ground causes an earth fault. The current takes an unintended return path via the ground with minimum resistance.
What are the risks of earth fault?
A high amount of current flows from the phase to the ground, which can raise the ground potential, increasing the step and touch potential values beyond tolerable. It increases human and animal life risk.
