Relay coordination

Relay Co

-ordinatio

n

To determine the ratings and settings of fuses, breakers, relay, etc.

To isolate the fault or overloads.

Objective

RELAY………

An electrical device, typically incorporating an electromagnet, which is activated by a current or signal in one circuit to open or close another circuit.

How to work a relay………

Relays are switches that open and close circuits electromechanically or electronically. Relays control one electrical circuit by opening and closing contacts in another circuit. As relay diagrams show, when a relay contact is normally open (NO), there is an open contact and the relay is not energized. When a relay contact is Normally Closed (NC), there is a closed contact and the relay is not energized.

Basic Parts of Relay………

Frame: Heavy-duty frame that contains and supports the parts of the relay.

Coil: Wire is wound around a metal core. The coil of wire causes an electromagnetic field.

Armature: A relays moving part. The armature opens and closes the contacts. An attached spring returns the armature to its original position.

Contacts: The conducting part of the switch that makes (closes) or breaks (opens) a circuit.

Basic design or operation

A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts. The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay is closed, and the other set is open.

Protection vs. Coordination Coordination is not an exact

science Comparison between

protection and coordination

ComparisonCoordination: Protection:

Limit the extend and duration of service interruption

Selective fault isolation

Provide alternate circuits

Prevent injury to employees

Minimize damage to components

Quickly isolate the affected portion of the system.

Minimize the magnitude of available short-circuit

Fundamental requirements Of a Relay.

Selectivity Speed Sensitivity Reliability Simplicity Economy

Selectivity: maximum continuity of service with minimum system disconnection.

Speed : minimum fault duration and consequent equipment damage.

Sensitivity: operate with low value of actuating quantity.

Reliability: assurance that the protection will perform correctly.

Simplicity: minimum protective equipment and associated circuit to achieve the protection objectives.

Economics: maximum protection at minimum total cost

RELAY PROTECTION COORDINATION……

Relay Protection Coordination means that downstream devices (breakers/fuses) should activate before upstream devices. This minimizes the portion of the system affected by a fault or other disturbance. At the substation level, feeder breakers should trip before the main breaker. Likewise, downstream panel breakers should trip before the substation feeder supplying the panel.

ISO Standardized Relays

Classification of RelaysClassification can be done by different ways, such

as by function, input, performance characteristics an operating be divided into five types:

Protective Relays: Protective relays and fuses operate on the

intolerable power system conditions. They are applied to all parts of the power system; i.e. generates, buses, distribution lines and feeds,etc

Regulating Relays: Regulating relays are associated with tap

changer of TFs, on governor of generating equipment to control the voltages level with varying load (used during normal conditions).

Continue…… Monitoring Relays: Relays of this type are used in energizing

or restoring lines to service after an outage and in interconnecting pre-energizing parts of the systems.

Auxiliary Relay: An auxiliary relay is a relay

that assists another relay or device in performing an action. There are two categories: contact multiplication and circuit isolation

Other relay classification Protective relays classified by input

are known as current, voltage, power, frequency and temperature relays. Those classified by operating principles are electromechanical, solid state …etc.

Applications

Detecting and isolating faults on transmission and distribution lines by opening and closing circuit breakers.

Isolating the controlling circuit from the controlled circuit when the two are at different potentials.

In logic functions. Time delay functions. Used to control high voltage circuits

with the help of low voltage signals.

Advantages of Relays

Used as a kind of safety switch to allow a circuit with a small current through to switch on a circuit that will have a larger current flow through it.

Minimize the amount of damage to the system during a fault.

Small control signal controls a larger load current or voltage.

Decreased electrical noise when switching. Totally silent operation. Output resistance remains constant regardless

of amount of use.

Disadvantages High burden level instrument

transformers are required (CTs and PTs of high burden is required for operating the electromagnetic relays compared to static relays)

Requires periodic maintenance and testing unlike static relays

Relay operation can be affected due to ageing of the components and dust, pollution resulting in spurious trips

Operation speed for an electromagnetic relays is limited by the mechanical inertia of the component