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Equipment protection
- 1. Equipment Protection Protection ofTransformer, Generator and Motor
- 2. Types of Faults •Faults in auxiliary equipment which is a part of transformer such as bushings, tap changer etc – Transformer oil; low level exposes live parts such as the leads to bushing. Oil level indicator with alarm give indication to this condition – Oil temperature; rise in oil temperature may be an indication of overload or failure of cooling system. A thermometer with alarms will indicate rise of oil temperature. • Faults in transformer winding and connections – Winding Insulation failure; high impulse voltage , mechanical forces, overload – Core insulation failure; • Overloads and external short circuits
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- 4. Transformer : Earthfault protection Magnitude of fault current depends on •Source impedance •Neutral earthing impedance •Transformer leakage reactance •Fault voltage •Winding connection
- 5. Fault current-DY transformer- neutral solidlygrounded Fault on secondary Y side. Current magnitude depends on leakage reactance of transformer winding which is minimum near neutral end and maximum at the middle of winding Primary fault current varies because of change in transformation ratio
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- 7. • Protection ofturbo-generators is the most complex and elaborate. – A Generator is a large machine and is connected to bus-bars. It is accompanied by unit transformers, auxiliary transformer and bus system.
- 8. Generator connection Typical smallgenerators connection to system Large Generator connection to system There are two methods by which generators are connected to Bus: Generators connected directly to bus Generators connected via delta-star unit transformer to HV bus i.e., to transmission Line
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- 11. Usual practices ofGenerator Protection
- 12. overloading • Continued overloading may increase the winding temperature to such an extent that the insulation will be damaged and its useful life reduced. • Temperature rise can also be caused by failure of cooling system. In large machines thermal elements (thermo-couples or resistance thermometers) are embedded in the stator slots and cooling system. • Electrical overcurrent protection can not sense the failure of cooling system
- 13. Unbalanced loading Un-balanced loadingon generator can be due to: 1. Unsymmetrical faults in the system near the generating station. 2. Mal-operation of a circuit-breaker near generating station, the three phases not being cleared. Negative sequence protection senses unbalanced loading of generators. Continued unbalanced loads, equal to or more than 10% of the rated current cause dangerous heating of the cylindrical rotor in turbo- generators. Salient pole rotors in hydro-generators often include damper windings and are, therefore, much less affected by unbalance loading (negative phase sequence currents).
- 14. Stator faults The statorfaults include: 1. Phase-to-earth faults 2. Phase-to-phase faults. 3. Stator inter-turn faults. Phase to phase faults and phase inter-turn faults are less common. Inter turn faults are more difficult to detect.
- 15. stator winding faults •Stator winding faults involve armature winding and must therefore be cleared quickly by complete shutdown of the generator. Only opening the stator circuit does not help since the e.m.f. is induced in the stator winding itself. The field is, therefore, opened and de-energized by "Field Suppression". • With resistance earthing, protection to full (100%) winding is not possible. • Leakage reactance depends on square of the number of turns of stator winding. • Fault at some x% of the winding, will therefore reduce leakage reactance by square of x% turns, making leakage reactance negligible . Current in fault loop then solely depends on earthing resistor
- 16. Phase to earthfault • These faults normally occur in the armature slots. The damage at the point of fault is directly related to the selected neutral earthing resistor. With fault currents less than 20 A, negligible burning of the iron core will result if the machine is tripped within few seconds. The repair work then amounts to changing the damaged coil without re- stacking the core laminations.
- 17. Methods of connectingalternator neutral to ground
- 18. • Resistor earthingis used when generator is directly connected to station bus. In this case the IDMT relay used in neutral is coordinated with other over current relays in the system • When generator is connected via delta-Y transformer, no coordination is required with other relays since earth loop is restricted to generator and transformer primary(delta) • For combined generator-transformer protection system, neutral is grounded through a voltage transformer.
- 19. • With Resistanceearthing it is impossible to provide protection to the whole winding, the percentage of stator winding protected being dependent on the value of earthing resistance. • Another problem with resistance earthing- resonance may occur between generator stator winding distribution capacitance and Earth resistance. The limiting value is given by fC Rn 6 106
- 20. If, however, theearthing resistor is selected to pass a much larger earth-fault current (> 200 A) severe burning of the stator core, will take place, necessitating re-stacking of laminations. Even when a high speed earth-fault differential protection is used, severe damage may be caused owing to the large time constant of the field-circuit and the relatively long time required to completely suppress the field flux. In the case of high earth-fault currents it is therefore normal practice to install a circuit breaker in the neutral of the generator in order to break the path of earth fault current.
- 21. Field suppression • Asthe source of fault current is itself the stator winding in which voltage is induced due to field flux, the exciter must be disconnected and stored field energy of both alternator and exciter be dissipated in the resistance . For this purpose, – one trip coil opens the connection of generator field winding from exciter and simultaneously shunts it to a resistance, – second trip coil opens the neutral circuit breaker to break the earth fault current path – And third trip coil opens the exciter armature circuit and shunts the exciter field winding across a resistance for flux collapse
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- 25. Possible Faults andabnormal conditions-Induction motor • Supply side problems – Faults at motor’s Terminal – Unbalanced supply – Single phasing – Reduced voltage – Phase reversal • Motor’s inside faults – Phase fault – Ground fault – Inter turn fault • Load side Faults – Bearing Failure – Prolonged over loads – Jam Rotor
- 26. Coordination of Overcurrent relay with motor starting current • Induction motor – Istart.= 6 to 8 IFL • To coordinate, OC relay characteristics must lie above starting characteristics and below thermal capability curve of the motor
- 27. Phase Faults At motorterminals, fault current is much greater than Full load current. Hence, an instantaneous OC relay with high pick up can be used Faults inside motor: Small size motor- protected by HRC fuse Big size motor- OC relay is used. Relay characteristics must lie below motor’s thermal characteristics High impedance ground fault: Fault current less than IFL Use of current balance type protection( core balance CT) is used
- 28. Unbalance supply Negative sequencecurrent flow at almost double frequency Rotor offers very high resistance due to skin effect and becomes excessively heat up and so is the stator Motors have certain I2t thermal capability ;usually around a value of 40
- 29. Overload Protection Overload devicethermal characteristics must lie below motor thermal curve (I2t) Thermal overload relays provide good protection against short, medium and long duration overloads but insufficient for heavy overloads Induction type overcurrent relay provides good protection against heavy overloads.