The earth fault relay is basically a protection device used selectively for earth fault protection. These can be used for both primary and backup protection in an electrical system. Earth faulty relays generally have adjustable current settings with different current values to trip on.
As the time setting multiplier is 0.1 here, actually the moving parts of the relay have to travel only 0.1 × 100% or 10% of the total travel distance, to close the relay contacts. Hence, actual operating time of the relay is 3 × 0.1 = 0.3 sec.
Answer. The operating time is the time from when the rated voltage is applied to the coil, until the time when the contacts operate. With Relays that have multiple pairs of contacts, if there are no other conditions, then the operating time is the time required for the slowest pair of contacts to operate.
17.3 Relay Co-ordination for Earth-fault Relay (contd..)
Typical range available for setting is 1-4A. If standard inverse relay is also set to pick up at the same current in primary, which is 100A, then with 500:5 CT, pick up current of relay R1 referred to secondary is 1A.PSM = Fault Current / Actual Pick up = 500/167 = 2.99 Expected operating time for relay 2 = Operating time of relay 1+ CTI = 0.15 + 0.3 = 0.45sec. TMS = 0.07. Now for maximum fault current on section BC (1200A) PSM = Fault Current / Actual Pick up = 1200/167 = 7.185. with TMS = 0.07 operating time of relay 2.
Overcurrent relay are deployed extensively in the electric power system. They provide protection to important power system equipment including power transformers, generators, transmission lines, loads, motors, busbars etc. They are employed as primary protection as well as backup protection.
Non Directional relay: When there is fault in power system, power flows through fault. Non directional relays operate irrespective of direction of flow of current. So we use a non-directional relay. It has to operate in fault conditions irrespective of direction of power flow.
Directional overcurrent relays are normally used on incoming line circuit breakers on buses which have two or more sources. They are connected to trip an incoming line breaker for fault current flow back into the source, so that a fault on one source is not fed by the other sources.
It is inherently a directional relay as it detects the fault only in the forward direction. The relay is called Mho relay because its characteristic is a straight line, when plotted on an admittance diagram (G-B axes i.e., conductance – susceptance axes) as in the figure.
Distance relays are used for both phase fault and ground fault protection, and they provide higher speed for clearing the fault. It is also independent of changes in the magnitude of the short circuits, current and hence they are not much affected by the change in the generation capacity and the system configuration.
It is used to signal whether the power flow it is sensing is in the direction as set by the user. The difference between the two is that reverse power relay has a pre-assigned directional context, whereas the directional relay needs to have the direction set at the time of application.
Directional Overcurrent Relay Test Plan Summary
- Connect all currents and voltages.
- Choose the fault type to apply.
- Apply nominal balanced three-phase voltages.
- Cut the fault voltage in half.
- Raise the fault current more than 110% of the pickup setting.
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.
Directional overcurrent relaying (67) concepts. Abstract: Directional overcurrent relaying (67) refers to relaying that can use the phase relationship of voltage and current to determine direction to a fault. There are a variety of concepts by which this task is done.
Grading is initially carried out for the maximum short circuit current level at the relaying point under consideration, but verification is also made that the needed grading margin exists for all current levels between protection relay pick-up current and maximum anticipated short circuit current level.
The low-set instantaneous setting is typically made at a margin just above the maximum demand load current that a distribution or sub-transmission line would carry and is used as a sensitive fault detector or to trip on the first shot of reclosing to save a fuse from blowing for a temporary (weather/wild-life) fault on
