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).

EPR cable prepared for arcing fault

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)

Discharge- and recharge current:

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.