Voltage Selection Relay MVAP 22

2 Features • Detects voltage circuit failures• Can be used to switch to analternative supply if the mainsupply fails• Compact design• Mechanically stable Application The MVAP 22 is essentially a fusefailure relay with changeover outputcontacts enabling its use for eithervoltage selection or fuse failureprotection. A typical application asa voltage selection relay is theautomatic connection of meteringequipment to an alternative supply ifthe normal or preferred supply fails.As a fuse failure relay it will monitorthe output of a voltage transformerand give an alarm or disconnectprotection circuits for VT fuse failure.The relay monitors the three phasevoltage supply and operates if thesupply is interrupted or becomesunbalanced due to failure of thevoltage transformers primary orsecondary fuses. Voltage failure detection ongenerator VTs The Type MVAP 22 relay can beapplied to provide an equivalentfunction to the voltage balance relayby using one MVAP 22 per voltagetransformer and connecting theoutput contacts of the two MVAP 22relays as shown in Figure 3.The relay combination can be usedto block relays or other devices thatwill operate incorrectly when avoltage transformer primary and/ orsecondary fuse( s) blow or to givean alarm for the faulty voltagetransformer.Each relay is supplied from its ownvoltage transformer and will detectblown fuses in either the primary orsecondary windings of the voltagetransformer or if the voltage supplybecomes interrupted.In the event of a generator shortcircuit causing the voltage suppliedto the relays to become appreciablyunbalanced the relays will operate, but the trip circuit via the generatorprotective relays ( ie., devices 21, 32 and 40) will be maintainedthrough the normally closed contactson the two changeover contactsA– 1 and B– 1, and an alarm will beprevented via the normally closedcontacts A– 3 and B– 2.Failure of any voltage transformersupply will cause operation of theappropriate MVAP 22 relay andblock tripping of the associatedgenerator relays which may beadversely affected by the loss ofvoltage supply via contacts A– 1 orB– 1 and give an alarm via A– 2 orB– 3. Description Two attracted armature units areemployed as shown in Figure 3.Unit A is fed from a resistance/ capacitance network which underhealthy conditions with negligibleharmonics has zero output.An operating voltage appearsacross the coils of unit A when thesecondary voltages becomeunbalanced due to loss of any oneprimary or secondary phase. Unit Bhas six changeover contacts forvoltage selection and is normallyenergised via the normally closedcontact on unit A from two phasesof the supply. Unit B drops off onoperation of unit A, on completeloss of supply, or on loss of twoprimary or secondary phases.If three limb core type starconnected voltage transformers areused, the relay will not operatecorrectly if the star points of theprimary windings are connected toneutral or earth