Wednesday, October 5, 2011

Classification of Protective Relays

Protective Relaying (Part12)      
       All the relays consists of one or more elements which get energised and actuated by the electrical quantities of the circuit. Most of the relays used now a days are electro-mechanical type which work on the principles of electromagnetic attraction and electromagnetic induction.
1.1 Electromagnetic Attraction Type Relays
       The electromagnetic attraction type relays operate on the principle of attraction of an armature by the magnetic force produced by undesirable current or movement of plunger in a solenoid. These can be actuated by a.c. or d.c. quantities. The various types of these relays are,
1. Solenoid Type : In this relay, the plunger or iron core moves into a solenoid and the operation of the relays depends on the movement of the plunger.
2. Attached Armature Type : This relay operates on the current setting. When current in the circuit exceeds beyond the limit, the armature gets attracted by the magnetic force produced by the undesirable current. The current rating of the circuit in which relay is connected plays an important role in the operation of the relay.
3. Balanced Beam Type : In this relay, the armature is fastened to a balanced beam. For normal current, the beam remains horizontal but when current exceeds, the armature gets attracted and beam gets tilled causing the required operation.
1.2 Induction Type Relays
       These relays works on the principle of an electromagnetic induction. The use of these relays is limited to a.c. quantities. The various types of these relays are,
1. Induction Disc Type : In this relay, a metal disc is allowed to rotate between the two electromagnets. The electromagnets are energised by alternating currents. The two types of constructions used for this type are shaded pole type and watthour meter type.
2. Induction Cup Type : In this relay, electromagnets act as a stator and energised by relay coils. The rotor is metallic cylindrical cup type.
1.3 Directional Type Relays
       These relays work on the direction of current or power flow in the circuit. The various types of these relays are,
1. Reverse Current Type : The relay is actuated when the direction of the current is reversed or the phase of the current becomes more than the predetermined value.
2. Reverse Power Type : The relay is actuated when the phase displacement between applied voltage and current attains a specified value.
1.4 Relays Based On Timing
       In relays the time between instant of relay operation and instant at which tripping of contacts takes place, can be controlled. This time is called operation time. Based on this, the time relays are classified as,
1. Instantaneous Type : In this type no time is lost between operation of relay and tripping of contacts. No intentional time relay delay is provided.
2. Definite Time Lag Type : In this type intentionally a definite time lag is provided between operation of relay and tripping of contact.
3. Inverse Time Lag Type : In this type, the operating time is approximately inversely proportional to the magnitude of the actuating quantity.
1.5 Distance Type Relays
       These relays work on the principle of measurement of voltage to current ratio. In this type, there are two colis. One coil is energised by current while other by voltage. The torque produced is proportional to the ratio of the two quantities. When the ratio reduces below a set value, the operates. The various types of these relays are,
1. Impedance Type : In this type, the ratio of voltage to current is nothing but an impedance which is proportional to the distance of the relay from the fault point.
2. Reactance Type : The operating time is proportional to the reactance which is proportional to the distance of the relay from the fault point.
3. Admittance Type : This is also called mho type. In this type, the operating time is proportional to the admittance.
1.6 Differential Type Relays
       A differential relay operates when the vector difference of two or more electrical quantities in the circuit in which relay is connected, exceeds a set value. These are classified as,
1. Current Differential Type : In this type, the relay compares the current entering a section of the system and the current leaving the section. Under fault condition, these currents are different.
2. Voltage Differential Type : In this type, two transformer are used. The secondaries of the transformers are connected in series with the relay in such a way that the induced e.m.f.s are in opposition under normal conditions. Under fault condition, primaries carry different currents due to which induced e.m.f.s no longer remain in opposition and the relay operates.
1.7 Other Types of Relay
       Various other types of relays which are used in practice are,
1. Under voltage, current power relay : This relay operates when the voltage, current or power in a circuit falls below a set value.
2. Over voltage, current, power relay : This relay actuates when the voltage, current or power in a circuit rises above a set value.
3. Thermal Relay : This relay actuates due to the heat produced by the current in the relay coil.
4. Rectifier Relay : In this relay, the quantities to be sensed are rectified and then given to the moving coil unit of the relay.
5. Permanent Magnet Moving Coil Relay : In this relay, the coil carrying current is free to rotate in the magnetic field of a permanent magnet. This is used for d.c. only.
6. Static Relay : This relay uses some electronic method for sensing the actuating quantity. It uses a stationary circuit.
7. Gas Operated Relay : The gas pressure is adjusted according to the variations in the actuating quantity. This gas pressure is used to actuate the relay. Buchholz relay is an example of such type of relay.

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