Static relays are classified according to the type of the measuring unit or comparator as follows: 1. Electronic Relays 2. Transductor (Magnetic Amplifier Relays) 3. Rectifier Bridge Relays 4. Transistor Relays 5. Hall Effect Relays 6. Gauss Effect Relays. 1. Electronic Relays: These were the first to be developed in the series of static relays. Fitzgerald presented a carrier current [...]
Distance relays are characterised by having two input quantities proportional to the voltage and current at a particular point in the power system, referred to as the relaying point. Ideal static distance relays have characteristics independent of actual magnitudes of voltage and current but dependent only on their ratio and phase angle between them. The versatile family of distance relays [...]
In this article we will discuss about the classification and design of relays used in an electric circuit. Classification of Relays: The protective relay may be defined as an electrical device interposed between the main circuit and the circuit breaker in such a manner that any abnormality in the circuit acts on the relay, which in turn, if the abnormality [...]
All the relays employed for protection against short circuits operate by virtue of the current and/or voltage supplied to them by CTs or PTs. Failures in the system are indicated by the individual or relative changes in the currents or voltages supplied to the protective relaying equipment. For every type and location failure, there is some distinctive difference in these [...]
Electromagnetic induction relays are the most widely used relays for protective relaying purposes involving only ac quantities. These relays operate on the simple principle of split-phase induction motor. Actuating force is developed on a moving element, that may be a disc or other form of rotor of non-magnetic current-conducting material (such as aluminium), by the interaction of electromagnetic fluxes with [...]
The application of a particular relay is governed by its characteristics and other factors such as accuracy, operating time, reliability, burden, method of setting adjustment etc. The function of a protective relay is to sense any abnormal condition in the system and send a signal to the breaker which in turn isolates the faulty section of the feeder from the [...]
Amplitude comparator compares the amplitudes of two (or more) input quantities. The phase angle between the quantities under comparison (inputs) is not recognized or noticed by the amplitude comparator. If the two input signals are S1 and S2 (say S1 the operating and S2 restraining), the amplitude comparator gives positive output only if – S2/S1 < K. The function is [...]
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, [...]