Operating Principles of Protective Relays
The conventional types of electromagnetic relays (electro-mechanical relay) has one or more coils, movable elements, contact system etc and its operation depends on whether the operating torque or force is greater than the restraining torque or force. The relay operates when the operating force is greater than the restraining force. The operating torque is produced by electro-magnetic attraction or by electromagnetic induction. The restraining torque is given by the springs.
Types of Electromagnetic Relays
- Attracted armature type
- Balanced Beam type
- Induction Disc type
- Induction cup type
- Moving coil relay
Attracted Armature Type Relay
This has four types i.e., plunger, hinged armature, balanced beam and moving iron polarized. These are the simplest types which responds to AC and DC.
The force of attraction, F = Kl2 = K(Im sin ωt)2
1/2 K (I2m – I2m cos2ωt)
where k is a constant and I2mcos2ωt is pulsating
The coil is energized by Current or voltage, a proportionate operating quantity, which produces a magnetic flux, thereby creating an electromagnetic force. This force is proportional to the square of the flux in the air gap or square of the current. This force increases, as the armature approaches the pole of electromagnet.
This type of relay is used for protection of small machines and also used for auxiliary relays such as indicating relays (flags) alarm and annunciator relays etc.
Balanced Beam Type Relays
Principle
All these relays have the same principle. That is electromagnetic force produced by the magnetic flux which in turn is produced by the operating quantity. The force exerted on the moving element is proportional to the square of the flux in the air gap or square of the current. In DC electromagnetic relays this force is constant. If this force exceeds the restraining force, the relay operates.
F ∝ Φ2 (AC), F = K (DC)
This type of balanced beam relay has a fixed beam and an electromagnet (EM) as shown in the figure. The EM has two windings, one is energized by the voltage and the other energized by the current. Under normal conditions, the pull due to the voltage (restraint quantity) will be high and hence the contact remains open. When a fault occurs, the current increases, the pull due to current will be more than the pull due to voltage. This closes the contacts of the trip circuit.
This balanced beam type relay shown in the figure consists of a horizontal beam pivoted centrally, with one armature attached to either side. There are two coils, one on each side. The beam remains horizontal till the operating force becomes greater than the restraining force. Current in coil gives the operating force and in the other, the restraining force. When the operating force or torque increases, the beam tilts and then the contact closes. The relay then actuates the tripping mechanism of the circuit breaker controlling the line / equipment.
Induction Disc Types of Electromagnetic Relays
Watt Meter Type or Non Directional Relay
It has a metallic disc free to rotate between the poles of two electro magnets.
The spindle of this disc carries a moving contact which bridges two fixed contacts when the disc rotates through an angle, which is adjustable between 0o to 360o. By adjusting this angle the travel of moving contact can be adjusted so that the relay can be given any desired time setting which is indicated by a pointer. The dial is calibrated from 0-1. The relay time from name plate cure is to be multiplied by time multiplier setting.
The upper magnet has two windings. The primary coil is connected to the secondary of CT through tappings in it. These tappings are connected to plug Settings Bridge. The secondary is connected to the lower electro magnet. The torque exerted on the disc is due to the interaction of eddy current produced therein by the flux from the upper EM and the lower EM. The relay setting is 50% to 200% in steps of 25%.
Shaded Pole Type or Non Directional Relay
The rotating disc is made of aluminum. In the above type one half of each of electromagnet shown in fig, is surrounded by a copper band called as the shading ring. The shaded portion of the pole products a flux which is displaced in space and time w.r.t the flux produced by the unshaded portion of the pole. These two alternating fluxes cut the disc and produce eddy currents in it. Torques are produced by the interaction of each flux with the eddy current produced by the other flux. The resultant torque causes the disc to rotate.
A spring is used to supply the resetting torque. A permanent magnet is employed to produce eddy current braking to the disc. The braking torque is proportional to the speed of the disc. When the operating current exceeds pick-up value, driving torque is produced and the disc accelerates to a speed where the braking torque balances the driving torque. The disc rotates at a speed proportional to the driving torque.
At a current below the pick-up value, the disc remains stationary by the tension of the control spring acting against the normal direction of disc rotation. The disc rests against a backstop. The position of the back-stop is adjustable. So that the relay-contacts travel distance can be varied according to need. This is called time-setting of the relay.
In disc-type relays, there are a number of tappings provided on coil, to select the desired pick-up value of the current.
Introduction of Cup Relay or Directional Relay
- It is a double actuating quantity relay (current and voltage)
- Highly sensitive
- High speed
- Steady, non-vibrating torque
- Ration of re-set to pickup is high (above 95%)
- It has operating time less than 0.01sec
The operating is similar to induction motor. It consists of a stationary rotor iron core and moving rotor conductor (cup). The moving element is a hollow cylinder or cup which turns on its axis. The driving element is a four or eight pole structure placed radially around the outside of the cylinder and joined by an yoke. To shorten the air-gap, a stationary iron core is placed inside the rotate cylinder.
It is a high speed induction relay since inertia is less. the two pairs of coils are displaced by 90o. When these coils are excited, a rotating magnetic field is produced. The rotor cuts the magnetic field and an emf is induced in the rotor. which results in a current due to short circuit nature of the rotor. A torque is produced due to the interaction between the rotating flux and the induced current which causes the rotation of the cylinder. The movement of the rotor closes the trip circuit. The torque produced is proportional to |I1| |I2| sin α or Φ1 Φ2 sin α, where Φ1 Φ2 are the fluxes due to excitation currents I1 and I2 of coil pairs and α is the phase difference between the two fluxes.
Due to low weight of the rotor and efficient magnetic system, its torque is found to be three times that of an induction disc types. As this type of relay has high torque /weight ratio, high speed operation is possible.
Moving Coil Relays
It has two types:
- Rotary Moving coil
- Axial moving coil
Rotary Moving Coil Relay
It consists of a permanent magnet coil wound on a non-magnetic former, spring, spindle etc. as shown in the fig. The coil is energised by the fault current. Due to the interaction of the permanent magnetic field and the field due to the coil, moving torque is developed. Due to this the spindle rotates and closes the trip circuits. It has negative time-current characteristics as shown in the figure. It has high torque / weight ration.
F α NHIL
where F = Force, N= No. of turns, H=Magnetic Field, I=Current in the coil, L=Length of coil
Axial Moving Coil Relay
It has axially suspended coil wound on a former. The coil has only axial movement. When the coil is energized by the current, magnetic field is developed and this magnetic field is repelled by the already existing permanent magnet. Due to this, contacts get closed.
This relay is more sensitive and faster than the rotary moving coil relay. This relay has inverse operating time-current characteristics.
