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 [...]
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 [...]
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 distance relays and its classification. Introduction to Distance Relays: Distance protection is the name given to the protection, whose action depends upon the distance of the feeding point to the fault. The time of operation of such a protection is a function of the ratio of voltage and current i.e., impedance. This impedance [...]
In this article we will discuss about induction type directional relays used in an electric circuit. Induction Type Directional Power Relay: The non-directional relay can operate for fault flow in either direction. In order to achieve operation for the fault flowing in a specific direction, it is necessary to add a directional element to the non-directional element. The directional (or [...]
Differential protective relaying is the most positive in selectivity and in action. It operates on the principle of comparison between the phase angle and magnitudes of two or more-similar electrical quantities. Comparing two electrical quantities in a circuit by means of differential relays is simple in application and positive in action. For example, in comparison of the current entering a [...]
It is noteworthy here that in electrical engineering, the term 'impedance' can be applied to resistance alone or reactance alone, or a combination of the two. In protective relaying terminology, however, an impedance relay has a characteristic that is different from that of a relay responding to any component of impedance and therefore, the "impedance relay" is very specific. An [...]