Depending upon the time of operation, overcurrent relays may be categorized as: 1. Instantaneous Overcurrent Relay 2. Inverse-Time Overcurrent Relay 3. Definite Time Overcurrent Relay 4. Inverse Definite Minimum Time (IDMT) Relays 5. Very Inverse Relay 6. Extremely Inverse Relay. 1. Instantaneous Overcurrent Relay: An instantaneous overcurrent relay is one in which no intentional time delay is provided for operation. [...]
An induction type overcurrent relay giving inverse-time operation with a definite minimum time characteristic is shown in Fig. 3.25. It consists essentially of an ac energy meter mechanism with slight modification to give required characteristics. The relay has two electromagnets. The upper electromagnet has two windings, one of these is primary and is connected to the secondary of a CT [...]
Phase comparison technique is the most widely used technique for all practical directional, distance, differential and carrier relays. In a phase comparator, the operation of the relay takes place when the phase relation between two inputs S1 and S2 varies within certain specified limits. Both inputs must exist for an output to occur; ideally, operation is independent of their amplitudes, [...]
Whenever there is an unbalanced circuit, the unbalanced currents will have a negative phase sequence component. A negative phase sequence (or phase unbalance) relay is essentially provided for the protection of generators and motors against unbalanced loading that may arise due to phase-to-phase faults. Essentially such a relay has a filter circuit which is responsive only to the negative sequence [...]
In this article we will discuss about:- 1. Protection of Low-Voltage (Below 1,000 V) 3-Phase Induction Motors 2. Protection of Large Sized Motors 3. Protection for Exclusively Synchronous Motors. Protection of Low-Voltage (Below 1,000 V) 3-Phase Induction Motors: These are the most widely used industrial motors. Circuit diagram for a magnetic contactor starter for a low-voltage 3-phase induction motor is [...]
The different protection schemes for rotor protection of generators are described as follows: 1. Rotor Earth Fault Protection 2. Loss of Excitation (or Field Failure) Protection 3. Protection against Rotor Over-Heating Because of Over-Excitation 4. Rotor Temperature Alarm 5. Automatic Field Suppression and Use of Neutral Circuit Breaker. Scheme # 1. Rotor Earth Fault Protection: Field circuits are operated unearthed [...]
For power transformers, the protection is to be provided usually against dangerous overloads and excessive temperature rise. Dangerous overloads may be due to external faults or the internal ones. External faults, however, are cleared by the relay system outside the transformer within the shortest possible time in order to avoid any danger to the transformer due to these faults. Hence [...]
The differential protection used for power transformers is based on Merz-Price circulating current principle. Such protection provides protection against internal phase-to-phase and phase-to-earth faults and is generally used for transformers of rating exceeding 2 MVA. This protection system as applied to power transformers is fundamentally the same as that for generators but with certain complicating features not encountered in the [...]
There are several methods of classification of circuit breakers. On the basis of type of current they may be classified as ac circuit breakers and dc circuit breakers. AC circuit breakers may be classified on the basis of rated voltages. Circuit breakers below rated voltage of 1,000 V are known as the low-voltage circuit breakers and above 1,000 V are [...]
In this article we will discuss about operation and mounting of circuit breaker. Operation of Circuit Breaker: Each circuit breaker is provided with an operating mechanism. The functions of this mechanism are: (i) To close the breaker contacts rapidly without hesitation at all currents from zero to rated making current capacity (ii) To hold the breaker contacts together against the [...]