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02a_Basis stator ground fault protection.pdf

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The year of Profitable Growth Global network of innovation Basis of Stator-Ground- Fault Protection
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Page 1: 02a_Basis stator ground fault protection.pdf

The year of Profitable Growth

Global network of innovation

Basis of Stator-Ground-Fault Protection

Page 2: 02a_Basis stator ground fault protection.pdf

Power Automation 2

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Presenter: Dr. Hans-Joachim Herrmann PTD PA13Phone +49 911 433 8266E-Mail: [email protected]

Generator ProtectionBasis of Stator-Ground-Fault

Protection

Page 3: 02a_Basis stator ground fault protection.pdf

Power Automation 3

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Test Results showing Effects of Arc Burning on Stator Core Laminations during Ground Faults

Page 4: 02a_Basis stator ground fault protection.pdf

Power Automation 4

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Methods of Neutral-Point Connection (1)

? Isolated or high ohmic groundedAdvantage: Small fault currentsDisadvantage: High transient overvoltage at

intermittent ground faults(2.5 – 3.5 ) Vph-E

? Compensated or reactive groundedAdvantage: Small fault currents on the fault

locationDisadvantage: Transient overvoltage

(<2.5Vph-E),Higher costs

Standard application

Very seldom; in older plants used

< 10 A

< 10 A

Page 5: 02a_Basis stator ground fault protection.pdf

Power Automation 5

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Methods of Neutral-Point Connection (2)

?Low ohmic groundedAdvantage: Lower transient overvoltage,

95 to 98% protective range ofSEF protection

Disadvantage: Great damages to generatorsat longer fault duration

? Solidly (effective) groundedAdvantage: 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

Page 6: 02a_Basis stator ground fault protection.pdf

Power Automation 6

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Voltages in Case of an Ground Fault

Voltage without ground fault

VL3,E VL3,E = VL31

VE = -VL1,E

VL1,E = 0

VL2,E = VL21

Voltages at the point of a ground fault at phase L1

? VLx,E ... phase-ground-voltage

? no displacement voltage (VE = 0)

? VL,E voltage decreases in the faulty phase (min ?? 0)

? VL,E voltage in the both “healthy” phasesare increase (max ?? ? phase-to-phase)

? VE displacement-voltage(can be measured at star point to ground)

MMVL1,E

VL2,E

L1L2L3

ABC

Page 7: 02a_Basis stator ground fault protection.pdf

Power Automation 7

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Currents in the Case of an Ground Fault

Model:

Equivalent voltage VE at the point of fault

IE =3 ?IE/3 = 3 ??VE/ZE

IE/3 ... ground fault current in one phase

ZE ... ground impedance at one phase

Vector diagram: ground fault in phase L1

VL3,E

VL2,E

IE =3V0

1

j? CE

= 3V0 ??j? CE

=3 VE

ZE

IC,2

IE

IC,3

L1L2L3

IC 33V0 =3VE

UE

IE/3

IE

ZE

L1L2L3

~ ~ ~

Page 8: 02a_Basis stator ground fault protection.pdf

Power Automation 8

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Correlation from Ground Fault Location

Displacement voltage VE (V0) and ground current IE (3I0) as a function of the fault location of an ground fault in the machine winding.

IE= 3VE

ZE

VE

VL2,EVL1,E

VL1,E VL2,E

VE ZE

VE

At faults close to the star-point the displacement voltage and the ground currents become small

Page 9: 02a_Basis stator ground fault protection.pdf

Power Automation 9

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Generators Connected via an Unit Transformer to the Grid

? generator is galvanic isolated

? under the assumption of an ideal transformer, the displacementvoltage caused by an ground fault, can only be measured at the generator

GGG

Page 10: 02a_Basis stator ground fault protection.pdf

Power Automation 10

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Unit Connection: Elimination of the Disturbance during a Ground Fault on the High Voltage

Groundingtransformer

üTR =

CLCG CTr

CK

VEO ?VN

3

(Limb transformation ratio)

VGen

31003

5003

VV

? ?R

RPrim

ü3

2TR

?VR

Problem: grid ground faults cause disturbancesdue to the coupling capacitancebetween the two transformer windings

Solution: attenuation by means ofa load resistor

Note:At solidly grounded transformer the VE0 is

appr. 80% of VN/?3

(Safety margin, if solidly grounding is open)

Page 11: 02a_Basis stator ground fault protection.pdf

Power Automation 11

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Unit Connection - Influence onto the Coupling Capacitance

VR,prim Rprim

CK

VC

VEO

VR,prim???Rprim VEO1Rprim + j? ?CK

Example: CK = 10 nF VEO = 220 kV3

VR,prim ??265V

?

??? ???

???? 665

3RR

2TR

Primü

VN,G = 10,5 kV R = 5 ?

üTr = 34,6

? VR,sek ??23V

? 23V500V

? 4,6% disturbanceinfluence

CE.

equivalent circuit disturbance voltage

CG+CL+CTr neglected

VEO displacement voltage on the high voltage sideCK three phase coupling capacitanceRprim primary load resistorüTr grounding transformer ratio

Page 12: 02a_Basis stator ground fault protection.pdf

Power Automation 12

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Unit Connection with Neutral Transformer

This design is very often used outside Germany, mostly at small generators

Generator Unit transformer

Rsek=Rprim

ü2TR

Design of Rprim so that the fault current is < 10A

R

VR

ÜTR =VGen

3VR

A high secondary nominal voltage VR (250V - 500V) is selected in order to avoid very small load resistors.

Page 13: 02a_Basis stator ground fault protection.pdf

Power Automation 13

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Generator Connected Directly to the Grid

? machines are galvanic connected

? displacement voltage caused by an ground faultcan be measured in all locations

G G G M

Page 14: 02a_Basis stator ground fault protection.pdf

Power Automation 14

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Direct Connection -Directional Stator Ground Fault Protection

L1 L2 L3

L1

L2

L3

CEIC + IR

Ohmic currentboost

IC

IR

ICL2

ICL1

3I0

3V0

VL1 VL2

Network

3i0 ?3V0

? DFT ?3V0>, 3I0>

? Direction(?3V0, 3I0)

Groundingtransformer

Page 15: 02a_Basis stator ground fault protection.pdf

Power Automation 15

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Fault Currents in Case of a Direct Connection

G1

G2

IMess

IMess

Page 16: 02a_Basis stator ground fault protection.pdf

Power Automation 16

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Fault Currents in Case of a Direct Connection with Grounding Transformer

G1

G2

IMeas

IMeas

Grounding transformer

Ohmic currentRB

Page 17: 02a_Basis stator ground fault protection.pdf

Power Automation 17

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Ground Current Detection via a Toroidal Current Transformer and Holmgreen Connection

Toroidal Current Transformer

IE3 per phase

I´E

L1 L3L2

IE

? I3 ~

Holmgreen connection(separate cores)

Holmgreenconnection(common neutral return connector

Sensitivity is limited

Problem:Large CT ratioleads to small currents on thesecondary side

IE3 per phase

L1

L3

L2

I´E

(IE = 3 I0)

? magnetic additionof ground currents,

? principle is sensitive

Page 18: 02a_Basis stator ground fault protection.pdf

Power Automation 18

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Directional Stator Ground Fault Protection Directional Characteristic

Page 19: 02a_Basis stator ground fault protection.pdf

Power Automation 19

Power Transmission and Distribution

Power AutomationProgress. It‘s that simple.

Directional Stator Ground Fault Protection - Logic


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