08_C_3_Basis Stator Earth Fault Protection.ppt

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Siemens Power Academy TD
08 Basis Stator Earth Fault Protection
Using numerical machine and motor protection
Version: C 3_Page 1
Training: Generator protection
Basics Stator Earth Fault Protection
Presenter: Dr. Hans-Joachim Herrmann
E D EA PRO LM1
Phone 0911-433-8266

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Version: C 3_Page 2
Test Results showing Effects of Arc Burning on
Stator Core Laminations during Earth Faults

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Version: C 3_Page 3
Methods of Neutral-Point Connection (1)
 Isolated or high ohmic earthed
Advantage: Small fault currents
Disadvantage: High transient overvoltage for
intermittent earth faults
(2.5 – 3.5 ) Uph-E
 Compensated or reactive earthed
Advantage: Small fault currents at the fault
location
Disadvantage: Transient overvoltage (<2.5Uph-E)
Higher costs
Standard application
Very seldom; used in older plants
< 10 A
< 10 A

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Version: C 3_Page 4
Methods of Neutral-Point Connection (2)
Low ohmic earthed
Advantage: Lower transient overvoltage,
95 to 98% protective range of
SEF protection
Disadvantage: Great damages to generators
at longer fault duration
 Solidly (effective) earthed
Advantage: Low transient voltage, better
measuring conditions for the
protection
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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Version: C 3_Page 5
Voltages in Case of an Earth Fault
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 increased (max  phase-to-phase)
 UE displacement-voltage
(can be measured at star point to earth)
M
M
UL1,E
UL2,E
L1
L2
L3

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Version: C 3_Page 6
Currents in the Case of an Earth Fault
Model:
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
jCE
= 3U0
 jCE
=
3 UE
ZE
IC,2
IE
IC,3
L1
L2
L3
IC 3
3U0 =3UE
UE
IE/3
IE
ZE
L1
L2
L3
~ ~ ~

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Version: C 3_Page 7
Basics of Stator Earth Fault Protection
Fault Location
Step-up
transformer
Auxiliary
transformer
Possible region of
earth faults
Region of earth faults Earth fault locations in the stator
U0
Low displacement voltage at
earth faults close to the star point

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Version: C 3_Page 8
 generator is galvanically isolated
 under the assumption of an ideal transformer, the displacement
voltage caused by an earth fault, can only be measured at the generator
G
G
G
Generators Connected via a Unit Transformer to the Grid

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Version: C 3_Page 9
Earthing
transformer
üTR =
CL
CG CTr
CK
UEO 
UN
3
(Limb transformation ratio)
UGen
3
100
3
500
3
V
V
 
R
RPrim
ü
3
2
TR

UR
Problem: grid earth faults cause disturbances
due to the coupling capacitance
between the two transformer windings
Solution: attenuation by means of
a load resistor
Note:
At solidly earthed
transformer the UE0 is
appr. 80% of UN/3
(Safety margin, if solid
earthing is open)
Unit Connection: Elimination of the Disturbance during
a High Voltage Earth Fault

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Version: C 3_Page 10
Unit Connection - Influence of the Coupling
Capacitance
UR,prim Rprim
CK
UC
UEO
UR,prim
Rprim
UEO
1
Rprim + jCK
Example: CK = 10 nF UEO =
220 kV
3
UR,prim 265V



 





 665
3
R
R
2
TR
Prim
ü
UN,G = 10,5 kV R = 5 
üTr = 34,6
 UR,sek 23V
 23V
500V
 4,6% disturbance
influence
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
üTr earthing transformer ratio

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Unit Connection with Neutral Transformer
This design is very often used outside Germany, mostly at small generators
Generator Unit transformer
Rsek=
Rprim
ü2
TR
Design of Rprim so that the fault current is < 10A
R
UR
ÜTR =
UGen
3
UR
A high secondary nominal voltage UR (250V - 500V) is selected in
order to avoid very small load resistors.

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 machines are galvanically connected
 displacement voltage caused by an earth fault
can be measured in all locations
G G G M
Generator Connected Directly to the Grid

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Direct Connection -
Directional Stator Earth Fault Protection
L1 L2 L3
L1
L2
L3
CE
IC + IR
Ohmic current
boost
IC
IR
ICL2
ICL1
3I0
3U0
UL1 UL2
grid
3i0 3u0
 DFT 3U0>, 3I0>
 Direction
(3U0, 3I0)
Earthing
transformer

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Toroidal Current Transformer
IE
3
per phase
I´E
L1 L3
L2
IE
I
3 ~
Holmgreen connection
(separate cores)
Holmgreen
connection
(common
neutral return
connector
Sensitivity is
limited
Problem:
Large CT ratio
leads to small
currents on the
secondary side
IE
3 per phase
L1
L3
L2
I´E
(IE = 3 I0)
 magnetic addition
of earth currents,
 principle is sensitive
Earth Current Detection via a Toroidal Current
Transformer and Holmgreen Connection

08_C_3_Basis Stator Earth Fault Protection.ppt

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08_C_3_Basis Stator Earth Fault Protection.ppt