If the transformer were reenergized at the instant the voltage waveform corresponds to the residual magnetic density within the core, there would be a smooth continuation of the previous operation with no magnetic transient.  The flux, following the hysteresis loop, then falls to some residual value R. Initial inrush  When the excitation of a transformer bank is removed, the magnetizing current goes to O.  The factors controlling the duration and magnitude of the magnetizing inrush are:  Size and location of the transformer bank  Size of the power system  Resistance in the power system from the source to the transformer bank  Type of iron used in the transformer core and its saturation density  Prior history, or residual flux level, of the bank  How the bank is energized© ABB Power Technology1_114Q07- 6. This inrush current, which appears as an internal fault to the differentially connected relays, may reach instantaneous peaks of 8 to 30 times those for full load. Magnetizing inrush  When a transformer is first energized, a transient magnetizing or exciting inrush current may flow. Transformers protections  INTERNAL  ELECTRICAL  BUCHHOLZ (SPR)  SURGE ARRESTERS  THERMOMETER  OVERCURRENT RELAYS  THERMOSTAT  PHASE  NEUTRAL  THERMAL IMAGE  OIL LEVEL  DIFFERENTIAL RELAY  PRESSURE RELIEF  THERMAL RELAY VALVE  TANK RELAY  BUCHHOLZ-TAP  FUSES© ABB Power Technology CHANGER1_114Q07- 5. Possible faults in a transformer  EXTERNAL  INTERNAL  External Short  Short circuits Circuits  between turns  Overloads  between windings  Overvoltages  Ground faults  Overtemperature  Overpressure  Miss of oil© ABB Power Technology1_114Q07- 4.

© ABB Power Technology1_114Q07- 1 - Transformers Protection ProtectionsĪGENDA  PRINCIPLES  LINES PROTECTION  TRANSFORMERS PROTECTION  INTRODUCTION  SELECTING A PROTECTIVE SYSTEM  Differential protection  Sudden pressure relay  Overcurrent protection  Transformer tank protection  Typical protective scheme for power transformers© ABB Power Technology  STATION BUS PROTECTION1_114Q07- 2 -ĪGENDA  PRINCIPLES  LINES PROTECTION  TRANSFORMERS PROTECTION  INTRODUCTION  SELECTING A PROTECTIVE SYSTEM  Differential protection  Sudden pressure relay  Overcurrent protection  Transformer tank protection  Typical protective scheme for power transformers© ABB Power Technology  STATION BUS PROTECTION1_114Q07- 3. ABB - TRANSFORMERS-PROTECTION-COURSE (2001)

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