Stator Earth Fault Protection
100 % Protection Range
Dr. Hans-Joachim Herrmann
Power Transmission
and Distribution
Content
Basics of stator earth fault protection
Fault locations
Disturbances from the high voltage side
100%-stator-earth-fault protection with 20-Hz-injection method
Basic idea
Practical design
Numerical measuring technique
Frequency operating range
Design of the protection schemes
Examples of real practical failures
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Basics of Stator Earth Fault Protection
Fault Location
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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Basics of Stator Earth Fault Protection
Disturbances from the High Voltage Side
An additional load resistor limits the disturbance influence
The design of the load resistor determines the protective range
(80% or 90 % or 95%)
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100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Goal and Basic Idea
Goal
Basic idea
max.
200 V
20 Hz
RE
I
Protection principle should cover
100 % protection range
Different measuring principles
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With injection of a 20-Hzvoltage into the star point a
e.m.f. is available in case of
an earth fault
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100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Practical Design
Two connections
are possible
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The protection measures the
injected 20-Hz-voltage and
current
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100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Numerical Measuring Technique
Replica:
isSEF
RE = Fault resistance
RE
20Hz
RB
CE
CE = Ground capacitance in
stator circuit
iSEF
VSEF
1) Filtering
VSEF
2) Calculation of the real part
V20 = Vr + jVi
Fourierfilter
(20Hz)
iSEF
I20 = Ir + jIi
3) Impedance replica of fault
Yers
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CE
RE
Yers =
 I 20 
Re 
=
V
 20 
Vr . Ir + Vi . Ii
V2r + V2i
4) Calculation of fault resistor from 2) and 3)
1
+ j ωCE
RE
RE =
V2r + V2i
Vr . Ir + Vi . Ii
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100%-Stator-Earth-Fault Protection with 20-Hz-Injection
Method – Relay Settings (secondary values)
Two Stages
Backup
overcurrent
Supervision
Calibration
Primary fault resistances:
1 to 3 kΩ
Ω
Trip stage
5 to 8 kΩ
Ω
Alarm stage
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Siemens. Innovation for generations.
100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Frequency Operating Range
Stand still
0Hz
Rated frequency
10Hz
40Hz
50Hz
70Hz
100%-Stator earth fault protection with 20 Hz injection
90% Stator earth fault protection with displacement voltage
Protection uses a frequency tracking for all protection functions
20-Hz-generator is activated before starting the generator
Due to disturbances from zero sequence system during starting (e.q. static
frequency converter) or braking of the generator an internal blocking avoids an
overfunction of the injection method
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Design of the Protection Schemes
Application used in Three Gorges
100% stator earth fault protection
Option:
90% stator earth fault protection
with calculated U0
90% stator earth fault protection
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Design of the Protection Schemes
Redundancy of 100%-Stator Earth Fault Protection
Group1
Group2
90% U0
calculated
90% U0
calculated
UL
UL
7UM62
UE
7UM62
100% SEF-1
IEE1
UE
IEE1
100% SEF-2
Neutral
transformer
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Commissioning
Primary Tests on a Generator (stand still and running)
Phase to ground
voltage
Displacement
voltage
Ground current
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The
secondary
signals are
very small
The capacitive
current is visible.
This disturbance
current will be
eliminated by the
algorithm.
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Real Practical Failures
External insulation test equipment failed
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Real Practical Failures
Calculated Secondary Fault Resistance
200
200
150
R in Ohm
REm
k
SES100Aus
k
100
86
Drop off
GAnr
k
60
Pickup
50
0
0
100
200
300
400
k .ta
t in ms
100
500
600
700
687.5
Resistance
Trip
Pickup
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Real Practical Failures
Earth Fault in the Cable of Static Frequency Converter
SFC
Starting of the generator with
static frequency converter
G
Permanently pickup by 100%-stator earth fault protection
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Real Practical Failures
Earth Fault in the Cable of Static Frequency Converter
Fault Record
frequency appr. 1,7 Hz
Input
signals
of the
100%
SEF
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Siemens. Innovation for generations.
Real Practical Failures
Earth Fault in the Cable of Static Frequency Converter
Calculated Secondary Fault Resistance
100
100
R in Ohm
80
REmk − 10
57
60
Drop off
48
SES100Ausk
Pickup
GAnr k
40
20
0
0
0
100
99.92
200
300
400
k⋅ ta
t in ms
500
600
700
686.95
Resistance
Trip
Pickup
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Siemens. Innovation for generations.