Georg Koeppl, Dieter Braun Koeppl Power Experts ABB 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).

EPR cable prepared for arcing fault

Current at fault location Single-phase fault tests in an 8kV-cable system with isolated neutral [1] Voltages l-g Load currents 10ms Current at fault location

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.

Transient fault currents Discharge current: Recharge current: (50Hz fault current: 4.7A)

Discharge- and recharge current:

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 Energy in fault resistance (10W): 140J + 9.4J/cycle 610W

Intermittent fault: High-resistance grounding of generator neutral Energy in fault resistance (10W): 140J + 220J/cycle 11‘140W

Intermittent fault: Resonant grounding of generator neutral Energy in fault resistance (10W): 140J + 11J/cycle 690W

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): 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.