When the fault current flows through earth return path, the fault is called Earth Fault. Other faults which do not involve earth are called phase faults. Since earth faults are relatively frequent, earth fault protection is necessary. However, phase relays protection lacks sensitivity as its phase overcurrent pickup setting is greater than phase overcurrent pickup setting. Hence separate earth fault protection is generally provided to be sufficient sense the earth fault current.

Earth fault is a situation where non-earthed, live part of the network is connected to the earth trough relatively low impedance. It can be a permanent fault, which requires the personnel to rectify the fault situation, or it can be a transient fault, which can be managed via automatically controlled protection system.

• Three basic types can classify earth fault types:

o   Single Earth Fault: The simplest type is single-phase to earth fault this is most commonly caused by wire drop.

o   2-phase Earth Short circuit: In this fault type, two different phases are short circuited together with the earth.

o   Double Earth Fault: Where two different phases are simultaneously connected to the earth at the different locations. 

Relays equipped by Earth Fault protection functions that could operate in two modes:

a.  Measured

EF current is measured directly from the system; either by means of a separate CT connection (Core Balance CT CBCT) or via a residual connection of the three-line CTs.

b. Derived or Calculated

EF function operates from the residual current quantity which is derived internally from the vector summation of the three phase currents.

Like Directional Overcurrent, Directional Earth Fault (DEF) is used to differentiate between different fault types to ensure that co-ordination between protection devices such as distribution circuit breakers and reclosers is maintained. This principle is referred to as “Selectivity” in protection engineering. Earth faults pose an additional degree of complexity, as they don’t immediately follow nameplate value ratings of cables like Overcurrent does, but instead rely on understanding the full current path for the fault. 

Directional Earth Fault protection function is used as a backup protection for Overhead Transmission Lines to detect the high impedance faults that is out of all distance zones. And, to increase the selectivity and sensitivity the DEF is operating with one of the Teleprotection Schemes as POTT or PUTT or any other scheme

A. Polarization Quantity

The Earth Fault protection can use different polarizing quantity for determining the direction of the fault as:

a. Zero Sequence Polarization

With earth fault protection, the polarizing signal requires to be representative of the earth fault condition. As residual voltage is generated during earth fault conditions, this quantity is commonly used to polarize DEF elements.

The relay internally derives this voltage from the 3-phase voltage input which must be supplied from either a 5-limb or three single phase VTs. These types of VT design allow the passage of residual flux and consequently permit the relay to derive the required residual voltage. In addition, the primary star point of the VT must be earthed. A three-limb VT has no path for residual flux and is therefore unsuitable to supply the relay.

It is possible that small levels of residual voltage will be present under normal system conditions due to system imbalances, VT inaccuracies, relay tolerances etc. Hence, the relay includes a user settable threshold value which must be exceeded in order for the DEF function to be operational. Note that residual voltage is nominally 180° out of phase with residual current.

Consequently, the DEF elements are polarized from the "-Vres" quantity. This 180° phase shift is automatically introduced within the relays.

Important:

Some relays can trip even if VNpol is less than the set threshold. Provided that the superimposed current phase selector has identified the faulted phase (suppose phase A), it will remove that phase from the residual calculation Va + Vb + Vc, leaving only Vb + Vc. The resultant polarizing voltage will have a large magnitude and will be in the same direction as –Vres. This allows the relay to be applied even where very solid earthing behind the relay prevents residual voltage from being developed.

b. Negative Sequence Polarization

In certain applications, the use of residual voltage polarization of DEF may either be not possible to achieve, or problematic as

• Unavailability of a suitable type of VT, for example if only a three-limb VT was fitted.

• HV/EHV parallel line application where problems with zero sequence mutual coupling may exist.

In either of these situations, the problem may be solved by the use of negative phase sequence quantities for polarization. This method determines the fault direction by comparison of negative sequence voltage with negative sequence current. The operate quantity, however, is still residual current.

Negative sequence polarizing is not recommended for impedance earthed systems regardless of the type of VT feeding the relay. This is due to the reduced earth fault current limiting the voltage drop across the negative sequence source impedance to negligible levels.

B. DEF Characteristics

In many cases it is difficult to achieve selective earth-fault protection based on the magnitude of residual current only. To obtain a selective earth-fault protection scheme, it is necessary to take the phase angle of Io into account. This is done by comparing the phase angle of the operating and polarizing quantity.

The Characteristic angle setting, also known as Relay Characteristic Angle (RCA), Relay Base Angle or Maximum Torque Angle (MTA), is used to turn the directional characteristic if the expected fault current angle does not coincide with the polarizing quantity to produce the maximum torque. That is, RCA is the angle between the maximum torque line and polarizing quantity. If the polarizing quantity is in phase with the maximum torque line, RCA is 0 degrees. The angle is positive if the operating current lags the polarizing quantity and negative if it leads the polarizing quantity.