The document discusses rotor earth fault protection for generators. It describes two principles for detecting earth faults: using a 50/60 Hz injected voltage and using a low frequency (1-3 Hz) square wave voltage. The 50/60 Hz method directly measures the earth fault current, while the square wave method measures the voltage difference caused by charging of the rotor capacitance. Settings and logic for protection relays are provided for both methods. Considerations for parallel operation of the two types of protections are also covered.

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Rotor-Earth-Fault
Protection

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Presenter: Dr. Hans-Joachim Herrmann
PTD PA13
Phone +49 911 433 8266
E-Mail: Hans-Joachim.Herrmann@siemens.com
Generator Protection
Rotor-Earth-Fault Protection

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Requirement for Rotor Earth Fault Protection
⇒ in case of an earth fault, only small currents flow due to the galvanical isolation
Problem:
Double earth faults and interturn faults as a consequence of an earth fault cause:
• magnetical unbalance (unbalanced forces; violent vibration)
• high currents at the fault location
Task: Detection an earth fault already when it starts to build up
⇒ Destruction of the Rotor (Generator)
Earth fault in the rotor
RE
CE
Rotor
Excitation
system
+
-
Stator

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Protection Principle
Excitation
system
+
-
Voltage
Source
„Earthing brush “
Coupling
Unit
Measuring
- Incoupling of an AC voltage (50 Hz or 60 Hz)
- Measuring of the earth fault current
- Measuring of the earth fault resistance
- Incoupling of low frequency square wave voltage
Principles:
Higher
Sensitivity

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Earth Current Criterion
Principle (50 Hz/60Hz - Voltage Injection)
Coordinated
resonant circuit to fN
>40V
If disturbance influence from the excitation is to large
IE
Protection
Pick-up
limit:
IE,Fault > IE,Dist...
L1 L2 L3
IE,Distr.
IE,Fault
4µF105Ω
0,75H
Connection
on the earthing
brush

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Earth Current Measurement
Connection
Also IEE2
at 7UM62
is
possible
IEE1
J7
J8
1B1
1B3
1A1
1A3
+
-
4A1
4B1
3PP1336Err.
2B1
7UM6
Connection on the
phase to phase
voltage
7XR61
100 V - 125 V AC
105Ω
105Ω
AC Voltage
Source
appr. 42V or
65V
Documentation for Coupling Device in the Internet
www.siprotec.com
External resistors
at excitation voltages
> 150 V (circulating current >0,2A)

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Gain Characteristic of the R, C, L-Circuit
Z 50( ) 169.65= Z 60( ) 69.531=
0 50 100 150 200 250 300
0
500
1000
1500
2000
FilterverhaltenBandpaß
FrequenzinHz
ImpedanzinOhm
Z f( )
f
mA27
k1,5170
V45
I
RZ
U
I
fCoupling
≈
Ω+Ω
=
+
=
Imax approx. 300 mA

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Earth Current Criterion
Protection Settings
Protection with two stages:
Measuring circuit supervision
mA23
k1,5400
V45
I
RZ
U
I
fCoupling
≈
Ω+Ω
=
+
=
ZCouplingl(50Hz) = 400Ω
ZCouplingl(60Hz) = 335Ω
Imax ca. 100 mA
(voltage source decreases a little bit )
Note: Coupling impedance only with R and C
Finally setting during commissioning

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Earth Current Criterion
Logic

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Calculation of the Fault Resistance RE
(50Hz/60Hz- Voltage Injection)
100V 42V u
Digital
protection
(7UM62)
calculation
of RE
RE CE
RV CK
RV CK
L1 L2 L3
i
L1)
1) Recommended
at static excitation
with inject harmonics
(3rd harm.; 6th harm.)

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Calculation Formula of the Fault Resistance RE
(1) (2)
(3)
(4)
combining (3) and (4):
Note: RV* and XK* are measured during commissioning
Model:
Zers ZMess Z
X*K R*V
XE
RE
{ } VE
2
E
2
E
2
EE
*R-, ZR
XR
XR
R =
+
⋅
=
{ } { }ZZRZ j meMess I+=







+
+
+
+=
2
E
2
E
E
2
E
K
2
EE
2
EE
V --j
2ers
XR
XR
*X
XR
XR
*RZ
{ } Km2
E
2
E
E
2
E
-, *XZ
XR
XR
X I=
+
⋅
=
{ }( )
{ }
{ } V
Ve
2
Km
2
E -e-
--,
,
,
*RZR
*RZR
*XZ
R
R
X
R +=+=
I

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Earth Fault Resistance Calculation
Logic

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Earth Fault Resistance Calculation
Settings
Measured during commissioning
Measuring circuit supervision
Measured current can be influenced by disturbances
Correction during primary test,
(in most case the alarm stage is concerned)

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Injection of Square Wave Voltage with Low Frequency
Basic Diagram
Excitation
+
-
CE
RE
Digital
Protection
(7UM62)
UH
RV
RV
Ucontrol
Umeas.
RM
7XR6004
Controlling device
(7XT71)IE
Measuring
transducer
RE Fault resistance
RV Coupling resistor
UH Auxiliary supply ( ± 50V)
RM Measuring shunt resistor
CE Rotor capacitance
Typical frequency:
1 - 3 Hz

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Injection of Square Wave Voltage
Connection Diagram (7UM62)
Connection on the
phase to phase
voltage
Exc.
17
15
11
25
+
-
27
7XR6004
25
27
7UM62
7XT71
TD1
K14
K13 +
TD2
K16
K15 +
40 kΩ
40 kΩ
Control voltage
Measuring voltage
100 V
110 V
120 V
9
7

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Injection of Square Wave Voltage with Low Frequency
Basic Principle
RV
2
RECE
UH
UMRM
UH
UM
UM
50V
- 50V
1,88V
- 1,88V
0,75V
- 0,75V
t
t
t
iE
50V
375
20k
2
H
M
V
±=
Ω=
Ω=
U
R
R
EMM iRU ⋅=
∞=ER
Ω≈ 5kER
0M ≈∆U
E
V
2
C
R
⋅≈τ
E
M
1
~
R
U∆
Equivalent circuit:

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Sources of Error and Error Compensation
Influence of field voltage and earth fault location
a) Earth fault location
Shifting of measuring voltage
with
a positive or negative dc voltage
b) Jumps in the field voltage
a change in the field voltage takes
to jumps in the dc-voltage shifting
Udc = dc voltage shifting
Solution:
Calculation of the difference voltage
∆ U = |UM1 - UM2|
∆U1 = |UM1 - UM2| ∆ U3 = |UM3 - UM4|
∆ U2 = |UM2 - UM3|
Solution:
Block of measuring
at jumps (e.g. ∆U1 = ∆U2)
UM
Udc
Udc1
UM1
UM2
UM3
UM4
UM1 UM2 Udc2
UM

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Calculation Formulas
RECE
UH
UM
RM
RV
2
UM
U1
U2
Algorithm
Voltage divider:
Filtering:
Amplitude-log frequency curve: fA = 800 Hz; N = 64
2
-1-2 V
M
M
H
E
M
ME
V
M
H R
R
U
U
R
R
RR
R
U
U






=
++
=
∑=∑=
==
NN
u
N
Uu
N
U
1i
i2,2
1i
i1,1
1
;
1
2
-
::
21
M
UU
UU =∆=
1KK II +∆≈∆ UU
∑∆=∆
=
8
1k
kU
8
1
U
0 30 60 90 120 150 180 210240 270 300
0.001
0.01
0.1
1
f in Hz
G(f)
Continuity supervision:
Validity requirement
otherwise

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Logic Diagram Rotor Earth Fault Protection (1-3Hz)

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Rotor Earth Fault Protection (1-3Hz)
Setting Values
Measuring circuit supervision
If the integrated test function is used,
pick-up value of test resistor
Advanced parameter
only visible in DIGSI

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Connection of the Rotor Earth Fault Protection
G RW
RE
CE
EM
EX-T
L+
RWUG
RE
CE
L-
(50/60 Hz)
(1 - 3 Hz)
(50/60 Hz)
(1 - 3 Hz)
40kΩ
4µF
4µF
a) rotating diodes
b) separate Exciter
(static excitation)
40kΩ

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Generator with Rotating Excitation
Fault Free Condition (Square Wave Principle)
Chance of charge of
rotor earth capacitance
Disturbances by the
excitation generator

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Generator with Rotating Excitation
Test Condition with a Fault Resistor
Fault resistor is inverse proportional to the difference voltage

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Parallel Operation of Rotor Earth Fault Protections
100V 42V
CK;4µF
CK;4µF
RK;105Ω
RK;105ΩRV;40kΩ
RV;40kΩ
RE
7UM62 7UM62
uControl
uMeas.
iREF
uREF
7UM61
nur
iREF
or
1- 3 Hz principle 50 Hz principle

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Parallel Operation of Rotor Earth Fault Protections
Measurement with the 50/60 Hz Principle
( )Ω20k
2
RV
*KR *KC
ER
2
ll:* V
EE
R
RR =
Ω==
∞=
20k
2
* V
E
E
R
R
R
Ω=
Ω=
4k*
5k
E
E
R
R
Measurement 7UM61 or 7UM62
(RV is earthed for an AC voltage)
Equivalent circuit:
seen from the 7UM6, RV already
is interpreted as a rotor-to-earth
resistance
Measurement:
measured as a fault resistance
Case 1:
Case 2:
alarm stage becomes less sensitive
 open brushes can not be find out

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RV
2
RERM
2CK
(8µF)
Umeas
∆U
2
Measurement 7UM62 (1- 3 Hz)
(CK is earthed for a DC voltage)
Equivalent circuit:
 seen from the 7UM6:
high rotor capacitance
 capacitors will not be
completely loaded
 ∆ U ~ RE
-1
under no-earth-fault conditions
a fault resistance is already measured
 alarm stage becomes less sensitive
(approx. 50kΩ)
 longer measuring time
Parallel Operation of Rotor Earth Fault Protections
Measurement with the Square Wave Principle