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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


  • 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).



Voltages l-g

Load currents

10ms

Current at fault location


Consequences of these tests: isolated neutral [1]

  • 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.


Transient fault currents isolated neutral [1]

Discharge current:

Recharge current:

(50Hz fault current: 4.7A)


Discharge- and recharge current: isolated neutral [1]


Generator + Step-up Transformer isolated neutral [1]

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: isolated neutral [1]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: isolated neutral [1]High-resistancegrounding of generator neutral

Energy in fault resistance (10W):

140J + 220J/cycle 11‘140W


Intermittent fault resonant grounding of generator neutral
Intermittent isolated neutral [1] fault:Resonant grounding of generator neutral

Energy in fault resistance (10W):

140J + 11J/cycle 690W


Conclusions
Conclusions: isolated neutral [1]

  • 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): isolated neutral [1]

  • 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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