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CIDEL Argentina 2010 Session 1 New Aspects for Neutral Grounding of Generators Considering Intermittent Faults Georg Koeppl, Dieter Braun Koeppl Power Experts ABB. Introduction:

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CIDEL Argentina 2010

Session 1

New Aspects for Neutral Grounding of Generators Considering Intermittent Faults

Georg Koeppl, Dieter Braun

Koeppl Power Experts ABB

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Introduction:

Most electrical faults are arcing faults (flash-over of an insulation), mostly they are treated as steady state, permanent faults however.

Justified, if there is practically no chance of fault arc extinction (high currents with high arc-channel ionisation).

Not justified for small currents (single-phase faults in systems with isolated or high impedance grounded systems).

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Single-phase fault tests in an 8kV-cable system with isolated neutral [1]

Voltages l-g

Load currents

10ms

Current at fault location

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Consequences of these tests:

  • Single-phase faults with arc channel in solid insulation systems behave like a re-striking switch:
  • An arc is initiated with a high transient (discharge- and recharge-) current far higher than the 50Hz steady state fault current.
  • This transient current may be extinguished at a current zero.
  • Then the recovery voltage (50Hz) rises to a certain magnitude where again a re-strike takes place.
  • This sequence may be repeated often and almost regularly.
  • The fault damage caused by the high transient currents is far higher than could be expected on the basis of the small 50Hz fault current.
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Transient fault currents

Discharge current:

Recharge current:

(50Hz fault current: 4.7A)

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Generator + Step-up Transformer

Typical Data: 20kV, 150MVA, 50Hz, Ctotal = 0.305mF/phase, Rf = 10W

Resonant grounding via Petersen coil: T = 2Lcoil / Rcoil = 2Q / w 0.12s >> 0.01s

steady state fault high resistance grounding of generator neutral
Steady state fault:High-resistance grounding of generator neutral

Energy in fault resistance (10W):

140J + 9.4J/cycle 610W

intermittent fault high resistance grounding of generator neutral
Intermittent fault:High-resistancegrounding of generator neutral

Energy in fault resistance (10W):

140J + 220J/cycle 11‘140W

intermittent fault resonant grounding of generator neutral
Intermittent fault:Resonant grounding of generator neutral

Energy in fault resistance (10W):

140J + 11J/cycle 690W

conclusions
Conclusions:
  • Single phase faults in stator windings are in most cases intermittent faults.
  • The corresponding transient fault currents are by a factor of 60 higher than the small steady state fault current and hence responsible for damages in stator iron and winding.
  • High resistance grounding (most usual) or resonant grounding of the generator neutral have practically no influence on height and shape of those transient currents.
  • With high resistance grounding the recovery voltage after fault arc extinction re-appears very quickly, leading to a high cadence of re-strikes and extinctions (2 per cycle) and a high amount of energy absorbed in the fault resistance.
conclusions continued
Conclusions (continued):
  • With resonant grounding the interval between extinction and re-strike is substantially prolonged due to a slowly rising recovery voltage. Energy absorption in the fault resistance is thus reduced by a factor of 20.
  • Earth fault protection relays are normally suited for high resistance grounding as well as for resonant grounding (different setting of course).
  • Resonant grounding of generator neutrals consequently is to be preferred to high resistance grounding.
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