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CIDEL Argentina 2010Session 1
New Aspects for Neutral Grounding of Generators Considering Intermittent Faults
Georg Koeppl, Dieter BraunKoeppl 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).
Single-phase fault tests in an 8kV-cable system with isolated neutral [1]
Voltages l-g
Load currents
Current at fault location
10ms
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
RfaultC
g
IfI
L"d
Rneutralor Petersen Coil
Ae1633e103
200002i
,eR
Ui
s05.3/ts05.3/t1t
CR/t
f
GL1t
Discharge current:
Recharge current:
)A(t35402sin7.221i
),Rignoring(tf2sinL5.1
C2Ui
2t
fo"d
gGL2t
(50Hz fault current: 4.7A)
FAULT
CGENERATOR CSURGE + DUCTS CSURGE + XFORMER
(DISTRIBUTED)
GKA
GKB
GKC
LA
GN
GRD
LB
LC
PetersenCoil
Generator + Step-up Transformer
Typical Data: 20kV, 150MVA, 50Hz, Ctotal = 0.305F/phase, Rf = 10
Resonant grounding via Petersen coil: T = 2Lcoil / Rcoil = 2Q / 0.12s >> 0.01s
Steady state fault: High-resistance grounding of generator neutral
Energy in fault resistance (10):
140J + 9.4J/cycle 610W
Intermittent fault: High-resistance grounding of generator neutral
Energy in fault resistance (10):
140J + 220J/cycle 11‘140W
Intermittent fault: Resonant grounding of generator neutral
Energy in fault resistance (10):
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.