Operating Principle of Different Relays | Electrical Engineering

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 quantities and there are various types of protective relays available, each of which is designed to recognize a particular difference and to operate in response to it. The difference may be in terms of the magnitude, frequency, phase angle, rate of change, harmonics or wave-shape, duration of the conditions etc.

The main principle employed in the operation of the relay is either electromagnetic attraction or electromagnetic induction. In an electromagnetic attraction relay, a plunger is drawn into a solenoid or an armature is attracted to the poles of an electromagnet. Such relays can be operated either by dc or ac. In the case of electromagnetic induction relays, the principle of induction motor is used and the torque is developed by electromagnetic induction. Such relays are operated by the ac quantities only.

Operating Principle of Electromagnetic Attraction Armature Relays:

These are the simplest type of relays and include plunger (or solenoid), hinged armature, rotating armature (or balanced beam) and moving iron polarized relays.

All of these relays operate on the same principle i.e., in such relays the operation is obtained by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. The electro­magnetic force exerted on the moving element is proportional to the square of flux in the air gap or the square of the current flowing through the coil. It is basically a single actuating quantity relay. Such relays respond to both ac and dc because operating torque is proportional to the square of current flowing through the coil. With dc the torque developed is constant and if it exceeds the restraining torque or force caused by the controlling spring, the relay operates reliably.

In case of ac quantity the electromagnetic force developed is given as:

F_e =K I² = K (I_max sin ωt)²

= ½ K [I² _max – I² _max cos 2ωt] … (2.1)

It shows that the electromagnetic force consists of two components, one constant independent of time (½ KI² _max) and another dependent upon time and pulsating at double the supply frequency (½ KI² _max cos 2 ωt). The total electromagnetic force, therefore, pulsates at double the supply frequency. Since the restraining force produced by the spring is constant and the electromagnetic force developed is pulsating, the relay armature vibrates at double the power supply frequency. This causes the relay to hum and produce noise and also is a source of damage to the relay contacts. This leads to sparking and unreliable operation of the relay operative circuit contacts due to make and break of the circuit.

To overcome this difficulty in ac electromag­netic attraction relays, the flux developing the electromagnetic force is splitted into two fluxes acting simultaneously but differing in time phase, so that the resultant deflecting force is always positive and constant. This can be easily achieved either by providing two windings on the electromagnet having a phase shifting network or by putting shading rings on the poles of the electromagnet. The latter method is simpler and is widely used.

Hinged armature relays are mainly employed as auxiliary relays, e.g., tripping relays, ac and dc voltage and current relays.

In the case of a balanced beam type relay, two quantities I A I² and I B I² are compared because electromagnetic forces developed vary as the square of the ampere-turns. Ratio of reset/operating current for such relays is low. If set for fast operation, it will have a tendency to over-reach on transient conditions.

The sensitivity of the hinged armature relays can be increased for dc operation by the addition of a permanent magnet. This is known as a polarized moving iron relay. It is more robust in construction; most of these employ leaf-spring supported armatures.

Modern attraction armature relays are compact, robust, reliable and fast. VA burden depends on construction, setting etc. For a typical relay it is of the order of 0.2 to 0.6 VA for current range of 0.1 to 0.4 A. Such relays are usually instantaneous type but can be made a definite time lag or inverse-time lag by using an oil dash pot, an air-escapement chamber, a clock-work mechanism or by placing a fuse in parallel with the relay. These relays do not have directional feature unless they are provided with additional polarized coil.

The attraction armature relays have fast operation and fast reset because of small length of travel and light moving parts. As these relays are fast and operate on dc as well as on ac they are affected by transients. The transients contain dc component in addition to ac wave. Therefore, though the steady-state value may be less than relays pick-up value, the relay may pick-up during transient state.

Attraction armature relays have many applications in protection of ac and dc equipment. They are, however, sensitive to starting currents, load fluctuations and current surges.

Attraction armature relays can be designed to respond to over- and under-current, over/under voltage, for both dc and ac operations. They are employed as measuring or auxiliary relays.

Balanced beam relay is difficult to be designed over a wide range current because the force is proportional to square of current. Such relays are fast and instantaneous but can be made a definite time lag or inverse-time lag by using an oil dash pot, an air-escapement chamber, a clock-work mechanism or by placing a fuse in parallel with the relay. Such relays can have operating time of the order of 0.02 second. High ratio of resetting quantity can be had. VA burden depends on application. In current balance type the VA burden is of the order of 0.2, 0.4, 0.6 VA for 0.1 to 0.6 A current range. This relay is being largely superseded by permanent magnet moving coil relay having better accuracy and lower VA burden.

Operating Principle of Thermal Relays:

These relays operate on the principle of thermal effect of electric current. It consists of bimetallic strips which are used in small sizes and are heated by heating coils or strips supplied through a current transformer. An insulated lever arm carrying a contact is pivoted and is held in contact with this trip with the help of spring. The spring tension can be varied by rotating the sector shaped plate. Under normal operating condition the strip remains straight but under the action of fault current the strip is heated and bent and the tension of the spring is released. Thus the relay contacts are closed which energises the trip circuit.

The construction of the bimetal element consists of two nickel alloyed steel strips, welded together. These strips have a high heat resistivity and are free from thermal secondary effects and ageing.

These relays are mostly used for protection of low-voltage squirrel cage induction motors or dc motors of lower output ratings. The limitation of such a relay is the short-time overload withstand. Its heating element is usually designed to withstand short-time overload say up to 6 or 7 times full-load current. The thermal relay is not suitable for operation on short circuit as it will burn the element sufficiently before the strip may deflect so as to close the contacts. This type of relay is used in conjunction with instantaneous short- circuit relays of high setting or suitably graded time limit fuses.

Operating Principle of Gas Actuated Relay (Buchholz Relay):

Buchholz relay is a gas actuated relay. It is practically universally used on all oil immersed transformers having rating more than 500 kVA. Such relay can only be fitted to the transformers equipped with conservator tanks as it is installed in between the conservator tank and the main tank i.e., in the pipe connecting the two. It is employed in conjunction with some form of electrically operated protective gear because it provides protection only against transformer internal faults and does not respond to external bushing or cable connection faults.

Working Principle:

Whenever a fault occurs inside the transformer, the oil of the tank gets overheated and gases are generated. The generation of the gases may be slow or violent depending upon whether the fault is a minor or incipient one or heavy short circuit. Most short circuits are developed either by impulse breakdown between adjacent turns at the end turns of the winding or as a very poor initial contact which will immediately heat to arcing temperature. The heat generated by the high local current causes the transformer oil to decompose and produce gas which can be used to detect the winding faults. Buchholz relay operates only on this principle.

Construction:

Buchholz relay consists of two hinged floats in a metallic chamber located in the pipe connection between the conservator and the transformer tank. One of the floats is near the top of the chamber and actuates the mercury switch connected to the external alarm circuit. The other float is opposite the orifice of the pipe to the transformer and actuates the mercury switch connected to the tripping circuit.

Operation:

When a minor fault occurs, heat is produced due to current leakage, some of the oil in the transformer tank evaporates and some vapours collect in the top of the chamber while passing to the conservator tank. When a predetermined amount of vapours accumulate in the top of the chamber, the oil level falls, the mercury type switch attached to the float is tilted and so closes the alarm circuit and rings the bell. Thereby the operator knows that there is some incipient fault in the transformer.

The transformer is disconnected at the earliest possible and the gas sample is tested. The testing of gas provides clue regarding the type of insulation failure. Buchholz relay gives an alarm so that the transformer can be disconnected before the incipient fault grows into a severe one. A release cock is provided at the top of the chamber so that after operation the pressure in the chamber can be released and the gas emitted to allow the chamber to refill with oil.

When a severe fault occurs, large volume of gas is evolved so that the lower float containing a mercury switch mounted on a hinged type flat is tilted and the trip coil is energized. Thereafter the transformer is removed from the service.

The main advantage of Buchholz relays is that they indicate incipient faults, for example, between turns faults or core heating and so may enable a transformer to be taken out of service before serious damage occurs.

However, Buchholz relays have some limitations also i.e., only faults below oil levels are detected. For faults above the oil level, this relay is ignorant. These relays do not protect the connecting cables which must therefore have a separate protection. Setting of the mercury switch cannot be very sensitive, otherwise there can be a false operation by vibrations, earthquakes, mechanical shocks to the pipe, sitting of birds etc. The relay is slow, minimum operating time is about 0.1 second, average time 0.2 second. This is not desirable.