Detail MVAP 22 Voltage Selection Relay
2 Features • Detects voltage circuit failures • Can be used to switch to an alternative supply if the main supply fails • Compact design • Mechanically stable Application The MVAP 22 is essentially a fuse failure relay with changeover output contacts enabling its use for either voltage selection or fuse failure protection. A typical application as a voltage selection relay is the automatic connection of metering equipment to an alternative supply if the normal or preferred supply fails. As a fuse failure relay it will monitor the output of a voltage transformer and give an alarm or disconnect protection circuits for VT fuse failure. The relay monitors the three phase voltage supply and operates if the supply is interrupted or becomes unbalanced due to failure of the voltage transformers primary or secondary fuses. Voltage failure detection on generator VTs The Type MVAP 22 relay can be applied to provide an equivalent function to the voltage balance relay by using one MVAP 22 per voltage transformer and connecting the output contacts of the two MVAP 22 relays as shown in Figure 3. The relay combination can be used to block relays or other devices that will operate incorrectly when a voltage transformer primary and/ or secondary fuse( s) blow or to give an alarm for the faulty voltage transformer. Each relay is supplied from its own voltage transformer and will detect blown fuses in either the primary or secondary windings of the voltage transformer or if the voltage supply becomes interrupted. In the event of a generator short circuit causing the voltage supplied to the relays to become appreciably unbalanced the relays will operate, but the trip circuit via the generator protective relays ( ie., devices 21, 32 and 40) will be maintained through the normally closed contacts on the two changeover contacts A– 1 and B– 1, and an alarm will be prevented via the normally closed contacts A– 3 and B– 2. Failure of any voltage transformer supply will cause operation of the appropriate MVAP 22 relay and block tripping of the associated generator relays which may be adversely affected by the loss of voltage supply via contacts A– 1 or B– 1 and give an alarm via A– 2 or B– 3. Description Two attracted armature units are employed as shown in Figure 3. Unit A is fed from a resistance/ capacitance network which under healthy conditions with negligible harmonics has zero output. An operating voltage appears across the coils of unit A when the secondary voltages become unbalanced due to loss of any one primary or secondary phase. Unit B has six changeover contacts for voltage selection and is normally energised via the normally closed contact on unit A from two phases of the supply. Unit B drops off on operation of unit A, on complete loss of supply, or on loss of two primary or secondary phases. If three limb core type star connected voltage transformers are used, the relay will not operate correctly if the star points of the primary windings are connected to neutral or earth.