In this article we are going to talk about some important rally. First of all, we will learn about functional relay, after that we will learn about induction type overcurrent relay, induction type directional power relay, and induction type directional overcurrent relay. And will also study its construction, operation. So let’s start. And stay with us till the end .
What’s the functional relay type ?
Nowadays most relays in service of power system work on the principle of electromagnetic attraction or electromagnetic induction. Regardless of the principle involved, relays will generally be classified according to the function. Which they will be call upon to perform in the protection of an electrical power circuit.
For example, a relay that detects overcurrent in a circuit (it means the current which exceeds the tolerable limit). And initiates corrective measures would be termed as an overcurrent relay, regardless of the relay design.
In the same way an overvoltage relay is one that detects overvoltage in a circuit and initiates corrective measures. Even though there are many types of special function relays. Here only the following important types will be discuss in this article :
- Induction type overcurrent relays
- Induction type reverse power relays
- Distance relays
- Differential relays
- Translay scheme
See also this : What is static relay and its advantages?- Static relay definition.
What is induction type overcurrent relay?
This type of relay works on the induction principle. And will initiate corrective measures when the current in the circuit exceeds a predetermined value.
The actuating source is a current in the circuit supplied to the relay from a current transformer. These types of relays are use only on a. c. circuits and can operate for fault current flow in either direction.
Construction of induction type overcurrent relay
In this type of relay, there is a metallic (aluminum) disc. Which is free to rotate between the poles of two electromagnets. There are two windings in the upper electromagnet, one primary and the other secondary winding.
The primary is connected to the secondary of a C. T. in the line to be protected and is tapped at intervals. The tapping are connect to a plug-setting bridge. By which the number of active turns on the relay operating coil can be varied, giving the desired current setting.
The secondary winding is energized by induction from the primary and is connected in series with the winding on the lower magnet. The controlled torque is provided by a spiral spring.
The spindle of the disc has a moving contact that bridges the two stationary contacts (connected to the trip circuit) when the disc rotates through a predetermined angle.
This angle can be adjust to any value, only between 0° and 360°. By adjusting this angle the travel of the moving contact can be adjusted and hence the desired time setting can be given to the relay.
Operation of induction type overcurrent relay
The driving torque on the aluminum disc is set up due to the induction principle. This torque will be opposed by the stopping torque provided by the spring. Under normal operating conditions, the stopping torque produced by the relay coil current is greater than the driving torque.
Hence the aluminum disc remains stationary. However if the current in the protected circuit will exceed a pre-determined value. So the driving torque will be greater than the stopping torque.
As a result, the disc rotates and the moving contact bridges the fixed contacts when the disc rotates through a pre-set angle. The trip circuit operates the circuit breaker that isolates the faulty section.
See also this : Protective relays – What is relay – What is a basic relay?
What is induction type directional power relay?
When power in a circuit flows in a specific direction. Then this type of relay operates. Unlike a non-directional overcurrent relay, a directional power relay is designed such that it derives its operating torque from the interaction of magnetic fields derived from both the voltage and current sources of the circuit.
So this type of relay is essentially a wattmeter. And the direction of torque set up in a relay depends on the direction of current relative to the voltage it is connect to.
Construction of induction type directional power relay
This relay consists of an aluminum disc and two electromagnets. And this aluminum disc is free to rotate between the poles of two electromagnets.
The upper electromagnet has a winding (which is call the potential coil) on a central limb which will be connect to the circuit voltage source through a potential transformer (P. T.). The lower electromagnet has a separate winding (called a current coil) which will be connect to the secondary of (C. T.) the line to be protect.
The current coil is provided with a number of tappings connected to a plug-setting bridge. It allows any desired current setting. And the stopping torque is provided by a spiral spring
The spindle of the disc has a moving contact that bridges the two fixed contacts when the disc rotates through a predetermined angle. After adjusting this angle, the travel of the moving disc can be adjusted and hence the desired timing can be given to the relay.
See also this : What is fault in power system-Symmetrical and unsymmetrical fault
Operation of induction type directional power relay
The flux Φ1 due to the current in the potential coil will lag the applied voltage V by about 90°. The flux Φ2 due to the current coil will be approximately in phase with the operating current I (see vector diagram).
The interaction of the fluxes Φ1 and Φ2 with the eddy currents induced in the disc generates a driving torque :
T ∝ Φ1 Φ2 sin α
Since, Φ1 ∝ V, Φ2 ∝ I and α = 90−θ
T ∝ V I sin(90−θ)
∝ V I cos θ
∝ power in the circuit
It is clear that the direction of torque on the disc depends on the direction of current in the circuit to which the relay is connect. When power flows in the normal direction in the circuit, the driving torque and the restraining torque (due to the spring) help each other to pull the moving contact away from the stationary contact. As a result, the relay remains inoperative.
However, the reversal of current in the circuit reverses the direction of the driving torque on the disc. When the reverse driving torque is large enough, the disc rotates in the reverse direction and then the moving contact closes the trip circuit. And then this causes the operation of the circuit breaker which disconnects the faulty section.
See also this : What is electrical short circuit-Definition, Effects.
What is induction type directional overcurrent relay?
The directional power relay discussed above would be unsuitable for use as a directional protective relay under a short-circuit condition. When short-circuit occurs, the system voltage drops to a low value and its operation may result in insufficient torque developed in the relay.
This difficulty is overcome in the directional overcurrent relay which is design to be virtually independent of system voltage and power factor.
Construction of induction type directional overcurrent relay
So it consists of two relay elements mounted on a common case. The first is the directional element and the second is the non-directional element.
What are directional elements?
It is essentially a directional power relay that will operate when power is flowing in a specific direction. The potential wire of this element will be connect to the system voltage through a potential transformer (P. T.).
The current coil of the element is energized by the circuit current through a (C. T.). This winding is taken to the upper magnet of the non-directional element. The trip contacts (1 and 2) of the directional element are connect in series with the secondary circuit of the overcurrent element.
Therefore, the letter element does not start working until its secondary circuit is complete. In other words, for the overcurrent element to operate, the directional element must first operate (that is contacts 1 and 2 must be closed).
See also this : Switchgear in electrical : Features, Equipment.
Non-directional element
Operation of induction type directional overcurrent relay
Under normal operating conditions, power flows in the normal direction in the circuit protected by these relays. Therefore, the directional power relay (upper element) does not operate leaving the overcurrent element (lower element) inactive.
However when a short circuit occurs there is a tendency for current or power to flow in the opposite direction. If this happens the disc of the upper element rotates to bridge the fixed contacts 1 and 2. This completes the circuit for the overcurrent element.
The disc of this element rotates and the moving contact connected to it closes the trip circuit. It isolates the faulty section which operates the circuit breaker. The two relay elements are arranged in such a way that the final tripping of the current handled by them is noted until the following conditions are met :
- Current flows in one direction to operate the directional element.
- The current in the opposite direction exceeds the predetermined value.
- The excessive current persists for a period corresponding to the time setting of the overcurrent element.
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