Over-current and Earth fault protection

 Over-current  / Earth fault relays are the basic protection relays. Used for protection of transformers and feeders from over current and earth faults. When excessive current flows in a circuit, it is necessary to trip the circuit breaker protecting that circuit.

The Example Over current/Earth fault relay is shown in the below figure. It is the most common Numerical relay. The following protection functions are used in this relay.

Protection Functions of Over current/Earth fault relays:

Protection Functions used in this relay are

1. 50/51 Phase Over Current Protection
   Definite time/Time over Current IEC
2. 50N/51N Earth over Current Protection
   Definite time/Time over Current IEC
3. Ground Fault Protection
   Definite time/Time over Current IEC
   Sensitive Earth Fault protection is provided by using CBCT
4. 74Trip circuit Supervision
   With 2 Binary inputs/With 1 Binary input
5. Breaker Failure

In the following figure the Over current/Earth fault relay is configured and LEDs are assigned as follows.

Over Current Relay

LED Configuration:

1. Device ok
   Relay is Operational and Protecting
2. General Trip
   This indicates when any protection tripping occur causes breaker trips.
3. Trip Circuit Healthy
   This indicates no problem in the trip circuit.
4. Over Current trip
   This indicates for any fault causes over current in the phase element of the relay that exceeds the setting value.
5. Earth Fault trip
   This indicates for any fault causes over current in the earth element of the relay that exceeds the setting value.
6. Breaker Failure
   If after a programmable time delay, the circuit breaker has not opened, breaker failure protection issues a trip signal to isolate the failure breaker by tripping other surrounding backup circuit breaker.
7. Interlock OK
   This indicates when certain conditions are satisfied. For example the conditions like LR Selector Switch in Remote Position, Breaker in Service, Bus Voltage Healthy, and Trip Circuit Healthy etc. If these conditions are satisfied we can close the breaker.

Explanation of over current protection:

Over current relay protection is usually provided by either instantaneous or time delay over current relays.

Instantaneous Over current Protection (50):

 This is typically applied on the final supply load or on any protection relay with sufficient circuit impedance between itself and the next downstream protection relay.

Relay Time-delay:

Time-delay built in the relays to provide coordination with other over current relays for selectivity.

 The selectivity is obtained by adjustment of current setting and time, using the most applicable of several time characteristics.

Over current/Earth fault Relays-Time Characteristics (51):

The relay time characteristics differ by the rate at which the time of operation of the relay decreases as the current increases. This can be represented graphically by the relays TCC curve (Time Current Characteristic).

Some common relay TCC curve families are identified as inverse, very inverse, extremely inverse, and definite time. There are many more and custom curves can be developed in microprocessor type relay.

Inverse time or definite time can be selected according to the following criteria: –

•  Definite time: Source impedance is large compared to the line impedance, that is, there is small current variation between near and far end faults.

• Inverse time: Longer lines, where the fault current is much less at the far end of the line than at the local end.

• Strong or extreme inverse-time: Lines where the line impedance is large compared to the source impedance (high difference for close-in and remote faults), or lines where coordination with fuses or reclosers is necessary. Steeper characteristics also provide higher stability on service restoration (cold load pickup and transformer inrush currents).

Over Current/ Earth fault relays Tripping and Dropout Times:

1. Calculation of Tripping Time:

For inverse time characteristic relays the tripping time can be calculated using the below formulae based on type of curve used.

The tripping time characteristic curve is drawn between I/Ip versus trip time t in secs.

Relay Pickup threshold is usually at 110 percent of pick-up current.

According to IEC

Trip-Time Formulae

2. Calculation of Drop-out Time:

The relay starts drop out usually at 95 percent of pick-up value without disk emulation and 90 percent of pick-up value with disk emulation.

Here Disk Emulation means the additional time taken into consideration to reset a Ferraris disk due to its inertia.