Protection of Industrial Power Systems

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Protection of Industrial Power Systems
Mine Equipment Design Selection and Maintenance

• Lecture 3

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Faults

• Any abnormal state of the system
• Open circuit faults
• less common
• of less concern
• often followed by a short-circuit fault
• but may be a hazard for personnel
• Short-circuit faults
• due to short circuits
• more common and more severe consequences
• Overload currents
• Overvoltage ( voltage surges and transients) due
  to
• lightning strikes
• switching

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Dangers From Short Circuit (S/C) Faults

• If allowed to persist for an extended period S/C
  faults may cause
• Reduced stability of the power system
• Damage to the equipment in the vicinity of the
  fault
• Explosions in equipment containing insulating oil
  followed by fire, injuries to personnel and
  damage to other equipment
• Cascading effect - disruption in a large power
  system service area due to successive action of
  protective devices

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Role of System Protection

• to detect faults
• to selectively isolate faulted parts of the
  system
• to permit short clearance time to limit the fault
  power
• if a protection device fails, the upstream
  protection device must provide the
  protection
• selectivity(discrimination)
• a fault is detected by a number of protection
  devices, but only the protection device closest
  (and above) to the fault location is triggered

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Requirements of Protection of Wiring

• Must carry its rated current continuously without
  overheating or deterioration
• Small overloads of short duration should not
  activate them
• Small overloads of duration long enough to cause
  overheating should activate them
• protection must open the circuit before the
  damage is caused by fault currents
• Must be discriminative - only faulty circuit is
  isolated and other circuits remain unaffected and
  operative

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Protective Devices

• Most commonly used are
• Fuses
• Circuit Breakers
• Relays
• Reclosers
• Combination of those
• They have different switching(tripping or
  pre-arcing) characteristic
• is pre-arcing time vs. current - the elapsed
  time between the commencement of the current
  (large enough to melt the fuse or open circuit
  breaker contacts) and the instant of melting or
  opening.

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Reactors at Mining Sites (left) for Filtering
electrical noise (disturbances) produced by heavy
machinery. HV transformer with bushings (right).
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Grading of Series-Connected Fuses

• Absolute discrimination (selectivity) is achieved
  if the ratio of the current ratings for these two
  fuses is 1.6. (Fig.1.3/18)

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Grading of Circuit Breakers
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Grading of Circuit Breakers
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Typical Protection System

• Transducer (sensor)
• detects the increase of current and drop of
  voltage due to short circuit in the system
• CT (current transformer) or VT(voltage
  transformer)
• Relay
• read the signal from transducer
• energize the tripping circuit of the circuit
  breaker
• Circuit Breaker
• open their contacts to disconnect the section
  with the fault

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Circuit Breaker (left), Current Transformer
(middle), Voltage Transformer (right)
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Overcurrent Protection Current Transformers
Circuit breakers
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• Rated Switching Capacity
• Is the maximum value of the short-circuit current
  that the protection device can clear correctly.
• Back-up protection
• in case of fault larger than rated switching
  capacity the back-up must activate the upstream
  (above) protection device
• Zone protection
• protects sections of power system distribution
• zones overlap
• Motor Protection
• HRC fuses,
• fuzecontactorthermal overload relay
• switch gear assemblies with thermistor protection
  devices (for motors operating in tough
  conditions)
• motors with thermally critical rotor must have
  overload relay.