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ike 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. Earth fault magnitude, and its complexity, can be briefly summarised as:
The key to understanding DEF is to visualise and understand the fault current path, so that protection selectivity can be set appropriately. That is, we trip only on the faults we want to, and set up other circuit breakers to cover the other fault scenarios. We want to avoid “spurious tripping” (tripping when we aren’t supposed to), but must avoid failing to trip on a genuine fault.
Earth faults occur when there is an undesired loss of energy out of the distribution system. A few examples include trees brushing up against powerlines, broken powerlines landing on the ground, or contact with other items or personnel at earth potential. These single line-to-ground faults or double line-to-ground faults can be very serious, as they may involve life coming into contact with powerlines.
When earth faults occur, a new circuit is completed as the foreign interference creates an earth path. This results in an imbalance in the currents and voltages, and current flows out through the earth fault site, and returns at other points in the network. This is not the same as an overcurrent fault, because overcurrent assumes that only the existing circuit lines (powerlines) are involved, with no energy leaving the system.
n Fig. 1, a simple resistance earthed network is shown, where a single line to earth fault occurs on a feeder. When drawing the fault circuit, the resistive current flows out through the fault site, then returns through the star point of the upstream transformer. In a resistance earthed or solidly earthed network, this will be the largest component of the fault current is this resistive component.
However, the resistive component isn’t the only fault current path available. Powerlines, a charge carrier, with a dielectric substance (air) between an earth plane (the ground) act as a giant capacitor. The longer the cables, the more capacitance is available. Earth faults are unique in that the fault current path also includes a capacitive component where the fault current leaves at the fault site, and returns via all the other connected feeders in the reverse direction.
This is a common reason for DEF: How do you distinguish between a genuine downstream earth fault and a reverse capacitive current effect. These reverse capacitive currents are sometimes called “sympathetic” currents, as they are caused by faults on other feeders but still show up as an earth fault on the un-faulted lines. The current path for this can be seen in Fig. 1, where all the other feeders return capacitive current in the reverse direction.
This brings about the first observation of DEF – depending on the ratio between capacitive and resistive current, we determine what our relay characteristic angle (RCA) or torque angle should be set to.
Like overcurrent, earth fault protection uses a voltage vector as a reference for angle calculation, except DEF uses the zero sequence voltage and angle instead of the positive sequence. Fig. 2 shows a simple calculation with a resistive earth on how this is reached.
It is interesting to note that the zero sequence voltage angle is 180° out of phase (is a negative value) with the normal condition positive sequence. It is important to understand if the relay in use requires this 180° shift.
With this zero sequence voltage vector calculated, all that remains is to calculate the resistive and reactive (usually capacitive) fault current. Therefore, the RCA and tripping zone is likely to be similar to Fig. 3, with minimal variation away from a resistive earth fault current being determined as “Forward”.
Directional earth fault relay responds to single line to ground faults or double line to ground faults.
Directional earth fault relay can sense the direction in which earth fault occurs with respect to the relay location. This relay operates for a particular direction of the fault current with respect to its location.
Directional ground relays require that a reference quantity be established in order for the relay to determine the direction of the current flow at the relay location. This reference quantity is referred to as the polarizing quantity and for directional earth fault relaying it may be either zero sequence current or voltage. It is against this reference that an operating quantity is compared. The operating quantity will in all cases be proportional to and derived from the fault current at the relay location. Current may be used as the polarizing quantity at those points in the systems where power transformers having suitably grounded neutrals are located. The polarizing current may be obtained from the neutral(s) of the power transformer or the tertiary winding of the power transformer may be used as the source of polarizing current. Potential polarization may be used where a suitable polarizing current is not available or where dual polarization of the relay is desired. Regardless of the type of polarizing that is used, the polarizing quantity must be of sufficient magnitude and remain fixed in direction if the relay is to operate properly.
Negative sequence approach for the directional earth fault relay provides superior performance and has been widely used in the multi-function relays (static analog and digital).
always voltage B/W Earth & Neutral ,it should be 2 to 3v..if shows the too much frequency that means there is a earthing problem....etc,,
In the directional over-current protection the current coil of relay is actuated from secondary current of line CT. whereas the current coil of directional earth fault relay is actuated by residual current.
In directional over-current relay, the voltage coil is actuated by secondary of line VT. In directional earth fault relay, the voltage coil is actuated by the residual voltage.Directional earth fault relays sense the direction in which earth fault occurs with respect to the relay location and it operates for fault in a particular direction. The directional earth fault relay (single phase unit) has two coils. The polarizing quantity is obtained either from residual current. Using open delta connection we can check the residual voltage.
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