Table of Contents
How does earth fault current flow?
Earth Fault is an inadvertent fault between the live conductor and the earth. When earth fault occurs, the electrical system gets short-circuited and the short-circuited current flows through the system. The fault current returns through the earth or any electrical equipment, which damages the equipment.
How do earth fault relays work?
Earth Fault relays In case of phase to ground faults, high currents flow to the grounded neutral of the transformer. Earth fault relay senses these faults. Usually, the contacts of the earth fault relay are connected to the trip circuit of circuit breakers at the output side of the transformer.
Which distance relay is used for earth fault?
Reactance relay
Reactance relay is normally preferred for protection against earth faults. Because ground resistance can be so variable, a ground distance relay must be practically unaffected by large variations in fault resistance.
What is over current earth fault relay?
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. It is the most common Numerical relay.
How does fault current occur?
The fault current is the electrical current which flows through a circuit during an electrical fault condition. A fault condition occurs when one or more electrical conductors short to each other or to ground. Fault interruption devices include fuses, circuit breakers and relays.
How does a distance relay work?
The distance relay is a distance protection element designed to measure the faulty point. The operation of this relay depends on the value of the impedance. It trips the circuit breaker and closes the contacts when the impedance of the faulty point is less than the impedance of the relay.
Why distance relay is used?
Distance relays are generally used for phase-fault primary and back-up protection on subtransmission lines, and on transmission lines where high-speed automatic reclosing is not necessary to maintain stability and where the short time delay for end-zone faults can be tolerated.
What is difference between over current relay and earth fault relay?
Over Current and Earth Fault Protection of Transformer The over current relays can not distinguish between external short circuit, over load and internal faults of the transformer. The normal range of current settings available on IDMT over current relays are 50% to 200% and on earth fault relay 20 to 80%.
What is distance relay?
A distance relay is a type of protection relay most often used for transmission line protection. Distance relays measure the impedance from the installation side to the fault location and operates in response to changes in the ratio of measured current and voltage.
What is the difference between earth fault and earth leakage current?
Earth fault current: Current flowing to earth due to an insulation fault. Earth leakage current: Current flowing from the live parts of the installation to earth in the absence of an insulation fault.
When to trip an over current earth fault relay?
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.
How does earth fault protection ( lvref ) work?
Under fault free or external fault conditions the CT currents will sum to zero and the relay will not operate. Under internal fault conditions within the transformer LV winding or connections the CT currents will become unbalanced and the LVREF protection will operate.
What happens to the E / L relay during a fault?
Earthing of both will short circuit the E/L relay and make it inoperative for faults. The two over current relays in R and B phases will respond to phase faults. At least one relay will operate for fault involving two phase. For fault involving ground reliance is placed on earth fault relay.
How are earth leakage relays used in a circuit?
Earth leakage relays are used to sense small magnitude of currents. Earth leakage relays are normally used in low voltage circuits only. These relays are designed in such a way that whenever they sense a preset value of current (usually settable between 10mA and 3A) it trips the circuit breaker and interrupts the flow of current.