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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