8690739 basis stator earth fault protection
TRANSCRIPT
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Basis Stator Earth Fault Protection
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Effects of Arc Burning on Stator Core Laminations
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Methods of Neutral-Point Connection (1)
Isolated or high ohmic earthedAdvantage: Small fault currents
Disadvantage: High transient overvoltage forintermittent earth faults
(2.5 3.5 ) Uph-E
Compensated or reactive earthedAdvantage: Small fault currents at the fault
locationDisadvantage: Transient overvoltage (
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Methods of Neutral-Point Connection (2)
Low ohmic earthedAdvantage: Lower transient overvoltage,
95 to 98% protective range ofSEF protection
Disadvantage: Great damages to generators
at longer fault duration
Solidly (effective) earthedAdvantage: Low transient voltage, better
measuring conditions for theprotection
Disadvantage: Great damages to generators,Leakage zero sequence currents
Application in industrial plants
Application at low voltage generators
G
< 200 - 400 A
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Voltages in Case of an Earth Fault
M
Voltage without earth fault
UL3,E UL3,E = UL31
UE = -UL1,E
UL1,E = 0
UL2,E = UL21
Voltages at the point of an earth fault at phase L1
ULx,E ... phase-earth-voltage
no displacement voltage (UE = 0)
UL,E voltage decreases in the faulty
phase (min 0)
UL,E voltage in the both healthy phases
are increase (max phase-to-phase)
UE
displacement-voltage(can be measured at star point to earth)
MUL1,E
UL2,E
L1
L2
L3
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Currents in the Case of an Earth FaultModel:
Equivalent voltage UE at the point of fault
IE =3.IE/3 = 3
.UE/ZE
IE/3 ... earth fault current in one phase
ZE ... earth impedance at one phase
Vector diagram: earth fault in phase L1
UL3,E
UL2,E
IE =3U0
1
jCE
= 3U0.jCE
=3 UEZE
IC,2
IE
IC,3
L1
L2
L3
IC 3
3U0 =3UE
UE
IE/3
IE
ZE
L1
L2
L3
~ ~ ~
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Correlation from Earth fault Location
Displacement voltage UE (U0) and earth current IE (3I0) as a function of the faultlocation of an earth fault in the machine winding.
IE= 3UEZE
UE
UL2,EUL1,E
UL1,E
UL2,E
UE ZE
UE
At faults close to the star-point the displacement voltage and the earthcurrents become small
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Generators Connected via an Unit Transformer
generator is galvanic isolated
under the assumption of an ideal transformer, the displacementvoltage caused by an earth fault, can only be measured at the generator
GGG
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Elimination of the Disturbance High Voltage Earth Fault
Earthingtransformer
TR =
CLCG CTr
CK
UEO U
N3
(Limb transformation ratio)
UGen
3
100
3
500
3VV
( )R
RPrim
3
2TR
=UR
Problem:grid earth faults cause disturbancesdue to the coupling capacitancebetween the two transformer windings
Solution:attenuation by means ofa load resistor
Note:At solidly earthed
transformer the UE0 is
appr. 80% of UN/3
(Safety margin, if solidlyearthing is open)
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Unit Connection - Influence on the Coupling Capacitance
UR,prim Rprim
CK
UC
UEO
UR,prim R
prim UEO1Rprim +j CK
Example: CK = 10 nF UEO =220 kV
3
UR,prim
=293V
==
735
3RR
2
TRPrim
UN,G = 10.5 kV R = 5
Tr = 36.4
UR,sek 24V
24V500V
4.83% disturbanceinfluence
CE.
equivalent circuit disturbance voltage
CG+CL+CTr neglected
UEO displacement voltage on the high voltage side
CK three phase coupling capacitance
Rprim primary load resistor
Tr earthing transformer ratio
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Unit Connection with Neutral Transformer
This design is very often used outside Germany, mostly on small generators
GeneratorUnit transformer
Rsek=Rprim
2TRDesign of Rprim so that the fault current is < 10A
R
UR
TR =
UGen
3 UR
A high secondary nominal voltage UR (250V - 500V) is selected inorder to avoid very small load resistors.
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Generator Connected Directly to the Grid
machines are galvanic connected
displacement voltage caused by an earth faultcan be measured in all locations
G G G M
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Directional Stator Earth Fault Protection
CE
L1 L2 L3
L1
L2
L3
IC + IR
Ohmic currentboost
IC
IR
ICL2
ICL1
3I0
3U0
UL1 UL2
Network
3i0 3u0
DFT 3U0>, 3I0>
Direction
(3U
0, 3I0)
Earthingtransformer
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Fault Currents - Direct Connection
G1
G2
IMess
IMess
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Fault Currents - Direct Connection with Earthing Transformer
G1
G2
IMeas
IMeas
Earthing transformer
Ohmic current
RB
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Earth Current Detection - Toroidal and Holmgreen Connection
Toroidal Current Transformer
IE3
per phase
I E
L1 L3L2
IE
I3 ~
Holmgreen connection
(separate cores)
Holmgreenconnection
(commonneutral returnconnector
Sensitivity is
limitedProblem:
Large CT ratioleads to smallcurrents on thesecondary side
IE3
per phase
L1
L3L2
I E
(IE = 3
I0)
magnetic additionof earth currents,
principle is sensitive
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Directional Stator Earth Fault Protection - Characteristic
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Directional Stator Earth Fault Protection - Logic