Download Berechnung von Netztransienten

CIDEL Argentina 2010
Session 1
New Aspects for Neutral Grounding of Generators
Considering Intermittent Faults
Georg Koeppl,
Koeppl Power Experts
Dieter Braun
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
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
L"d
Discharge current:
i t1

U
 L G  e  t / R  C ,
Rf
i t1 
2  20000
3 10
If
e  t / 3.05 s  1633  e  t / 3.05 s A
Rneutral
or
Petersen Coil
Cg
Recharge current:
i t 2  ÛL G 
2C g
1.5L"d
 sin 2f o t
i t 2  221 .7 sin 2  3540 t ( A )
(ignoring R f ),
(50Hz fault current: 4.7A)
Rfault
Discharge- and recharge current:
Generator + Step-up Transformer
Typical Data: 20kV, 150MVA, 50Hz, Ctotal = 0.305F/phase, Rf = 10W
GKA
GKB
GN
GKC
LA
LB
LC
FAULT
GRD
Petersen
Coil
CGENERATOR
CSURGE + DUCTS
CSURGE + XFORMER
(DISTRIBUTED)
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.