This document discusses underexcitation protection, also called loss of field protection, for synchronous generators. It provides reasons for underexcitation including failures of the excitation device. Consequences of excitation failures can include rotor acceleration, overheating, and grid oscillations. The admittance protection criterion is proposed, which considers how the generator stability limit moves when voltage decreases. It involves transforming the generator capability diagram into an admittance diagram to set straight-line characteristics based on the stability limits.

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Siemens Power Academy TD
07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 1
Training: Generator Protection
Underexcitation Protection
(Loss of Field Protection)
Presenter: Dr. Hans-Joachim Herrmann
E D EA PRO LM1
Phone 0911-433-8266

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Siemens Power Academy TD
07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 2
Reasons for Underexcitation
 Failure of the excitation device
 short circuit in the excitation circuit
 interruption in the excitation circuit
 Maloperation of the automatic voltage
regulator
 Incorrect handling of the voltage regulator
(generator, transformer)
 Generator running with capacitive load
- Countermeasure:
Underexcitation Protection
excitation
device
GS
3~
ZLoad
Note: This protection is also called
Loss of Field Protection

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Siemens Power Academy TD
07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 3
Consequences of Excitation Failures
Influence Quantities:
 type of construction of the generator
 design of the excitation
 grid conditions
 magnitude of delivered active power
 type of the voltage and power regulator
Consequences:
 rotor acceleration
 local overheating in the rotor and stator
 over-voltages in the rotor
 mechanical shocks onto the foundation
 grid starts oscillating

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Siemens Power Academy TD
07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 4
machine equation: VP = V - I jX
cylindrical-rotor machine: X~Xd; VP = V- jXd I
(turbo generator)
salient-pole machine:
(hydro generator)
X:=Xq,Xd
exact: VP = V - j(XdId + XqIq)
reduced: VP ~ V - jXqI
ZL
V
VP
X I
Simplified equivalent circuit:
Vector diagram:
V
Iexc
I
Im
Re
J = rotor angle
j = load angle
j
Vp
I jX
J
V/jX
Relation of Current and Voltage in a Synchronous
Generator

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 5
Definition:
+P
(W)
+Q
(Var)
under excited
over excited
+P
(W)
+Q
(Var)
over
excited
under
excited
Operating
area
Operating
area
Steady-state
stability limit
Steady-state
stability
limit
Preferred design
Possible Design of the Generator Capability Diagram

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 6
Capability Curve of a Turbo Generator
type of generator: TLRI 108/46
nominal apparent power SN = 200 MVA
nominal voltage VN = 15.750 kV
nominal current IN = 7.331 kA
nominal frequency fN = 50.0 Hz
power factor cos jN = 0.8
cold-air temperature Tx = 40.00 °C
MVAr 140 120 MVAr
100 80 60 40 20 0 20 40 60 80 100 120 140 160 180
underexcited overexcited
Q
P
MW
220
200
180
160
140
120
100
80
60
40
0,2
0,4
0,6
0,7
0,8
0,85
0,9
0,95
0,975
0,975
0,95
0,9
0,2
0,4
0,6
0,7
0,8
0,85
cosphi
cosphi

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 7
Load Diagram of a Synchronous Machine (Cylindrical-
rotor Machine)
dynamic
stability
limit
steady
state
stability
limit
theoretical
limit
turbine limit
stator limit
rotor limit
P
overexcited
underexcited
Q
SN
VP If
jN
JN
N
d
2
N
;
'
S
X
V

 d
2
N
X
V
Xd: synchronous reactance
X‘d: transient reactance
The generator capability curve describes
the stability limits of the generator

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 8
In the case of an under-voltage the generator capability curve
moves to right and reduces the stability limits of the generator
1/xd
0.81/xd
0.85
1 U=1; I=1;
U=0.9; I= 1.11
Stability
limit
Q [p.u]
P [p.u]
Over-excited
Under-excited
Per Unit Capability Diagram of a Synchronous
Generator in the Case of Under-voltage (U = 0.9 UN)

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 9
A good under-excitation protection should consider both facts (1 and 2)
1. The generator capability curve describes in the
under-excitation region the stability limit of a generator
2. In the case of an under-voltage the stability becomes
much more critical (moves to active power axis)
The transformation of the generator diagram into the
admittance diagram is the solution, because:
 it is direct proportional to the per unit generator diagram
(only the reactive axis must be multiplied by -1)
 the settings can easily be read from the generator diagram
 it considers the under-voltage behaviour correctly
A
B
Conclusions for the Protection Design

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 10
Q
j
P
S
I
U
S 




B
j
G
Y
U
I
Y 


Transformation:
2
2
2
2
*
U
Q
j
-
U
P
U
Q
j
-
P
U
S
U
U
U
I
Y 




 

2
2
U
Q
-
B
U
P
G


Q
P
+
+
B
G
+
+
- -
Note: in the per unit calculation is: UN = 1
Complex Power: Admittance:
G: Conductance
B: Susceptance
In the per unit
representation
the diagrams
are the same,
only there is a
phase shift in
the reactive
part of 180°
Definitions for Converting the Generator Diagram into
the Admittance Diagram

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 11
a) Excitation Current (IEXC)
- stabile for over-excitation
- insecure for under-excitation (IEXC can be smaller than IEXC, N)
b) Direct Measuring of the Rotor Angle (J)
- steady stability limit depends on J or 2 J
- transversal reactance cannot be neglected with small excitation of
turbogenerators (Xq is also depending from 2 J)
c) Reactive power I-QI>, Impedance I-ZI<
- the reactive power protection gets more insensitive when voltage
decreases (at U<UN the stability limit curve moves to right)
- Impedance criterion is used by the competitors
Stability limit can not be clearly identified
Alternative Solutions for the Under-excitation
Protection

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 12
Underexcitation Protection with the Criterion
Admittance Y>
 Admittance calculation guarantees a right
behaviour, if the voltages decreases
 3 independent characteristics
and 3 timers
 characteristic 1,2 is adapted to the
steady-state limit curve;
 additional measurement of the field voltage
(release a short trip time)
 characteristic 3 is adapted to the
dynamic stability limit curve
 blocking of the protection at U<25% UN
a2
a3
a1
char.3 char.2 char.1
G[p.u.]
B[p.u.]
d1
x
1
d2
x
1
d3
x
1
Settings: Can directly be read out from the generator diagram
d
x
1
d1
x
1  a1 = 80°
d1
x
1
0.9
d2
x
1 
 = 90°
a2
d
x
2
or
1
d3
x
1 
 = 100° or 110°
a3

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 13
Combination of
 stator criterion (straight line characteristics)
 rotor criterion (under-voltage in the excitation circuit)
Case no. 1: only rotor criterion fulfilled:
no alarm, no trip
Case no. 2: only stator criterion fulfilled ( char. 1,2):
only alarm (10s); delayed trip (0,5s - 3s)
Case no. 3: rotor and stator criterion fulfilled (char. 1,2):
alarm and short-time delayed trip (0,5s - 1s)
Case no. 4: stator criterion fulfilled (char. 3):
alarm and short-time delayed trip (0 - 0,3s)
Reactions from the Protection

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 14
Example: Generator capability diagram (figure 9)
Generator: SN = 200 MVA CT, VT: knU =
UN = 15,75 kV
Stability limit: Q = 90 MVar knl = 8000A/1A
1. Calculation the longitudinal reactance :
2. Conversion into secondary values :
3. Setting value for Char. 1:












2,76
Q
3
Q
3 N
2
2
N
d
U
U
X 2,22
3
N
N
d
d 


U
X
x
I
2,38
k
k
W
pN,
G
N,
W
pN,
G
N,
d
nU
G
N,
Nsek
nl
Nsek
G
N,
d
dsek 








U
U
x
U
U
x
x
I
I
I
I

0,42
1
dsek
x 

 80
, 1
0,42
d1
x
1
a
3
100V
/
3
16kV
2,22
N
d 

Q
x S
or
Conversion of the Reactive Power into 1/xd

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Siemens Power Academy TD
07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 15
1) Filtering of the input values
2) Calculation of the positive sequence values
3) Calculation of the complex power
4) Calculation of the admittance values
Measuring Algorithm
GS
3~
iL1,2,3 uL1,2,3
7UM6
L
i
L
u
,
,
, L3
L2
L1 I
I
I
,
,
, L3
L2
L1 U
U
U
1
I
1
U
,
j iL
rL
L I
I
I 

,
j iL
rL
L U
U
U 

1
I
1
U
*
I
U
S  Q
j
P
S 

S 2
1
U
S
Y  B
j
G
1
j
1




X
R
Y
symmetr.
comp.
fourier filter
(50Hz)

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 16
Generator diagram is transferred to the impedance plane (e.g. X=U2/Q).
(Stability limit is represented as a circular characteristic)
 characteristic: Offset-MHO
 tripping zone inside the circle
 characteristic 1, tdelay  0...0,3 s (for high load
generator and field failure)
 characteristic 2, tdelay  0,5 - 3 s (for low load
generator, section field voltage failure)
R[p.u.]
X[p.u.]
xd
1
0.5 xd’
Char.2
Char.1
approximation
of stability
limit
Relay settings according
IEEE C37.102-1995
Summary:
• Measuring principle from electromechanical relays,
because impedance measuring elements were only
available
• circle characteristic is a compromise for adaptation
to the generator stability curve
Underexcitation Protection with Criterion Impedance I-ZI<

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 17
Transformation rule: A circle without zero crossing inverted becomes again a circle
R[p.u.]
X[p.u.]
xd
1
0.5 xd’
Char. 2
Char.1
G[p.u.]
B[p.u.]
d
,
d
d x
1
x
x
2
2



1
x
2
2
,
d


x
2
,
d
Z
1
Y 
Impedance plane Admittance plane
Transformation of Criterion Impedance I-ZI<
into the Admittance plane

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 18
0,5 X‘d = 0,13
Diameter: Xd = 1,81
Diameter= 1
All values are per unit
Impedance Plane
1 0.5 0 0.5 1
2
1.67
1.33
1
0.67
0.33
0
Settings: 501- Generator x’d= 0,27; xd = 1,81
1/Xd = 0,55
2/Xd = 1,1
2.5 1.75 1 0.25 0.5
2.5
1.25
0
1.25
2.5
Admittance Plane
Setting Example for Impedance and Admittance
Principle

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 19
Settings:
x’d = 0,27
xd = 1,81
Note:
B-axis is for mathematical
reasons multiplied by -1
Generator
diagram
Trajectory in the
case of underexcitation
with 100% excitation
loss
1/Xd = 0,55
Impedance principle Admittance principle
2/X’d = 7,4
Admittance Plane
8 7 6 5 4 3 2 1 0 1
4
2
0
2
4
Both Measuring Principle in the Admittance Plane

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 20
Test condition: P=160 MW Q=25 MVar; If = 1,87 If0; Voltage regulator failure: U= 1,05 0,8
Relay settings: Char 1 = 0.55 80°, 10s; Char 2 = 0.51 90°, 10s; Char 3 =1.1 110°, 0s
Dynamic Test on a Network Model with RTDS
Fault Record

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07 Underexcitation Protection
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0 500 1000 1500 2000 2500
0
2500
5000
7500
1 10
4
Zeit in ms
Spannung
in
V
9.512 10
3

0.354
U0
i
U1
i
U2
i
2.483 10
3

94.986 ta
i
0 500 1000 1500 2000 2500
0
5000
1 10
4
1.5 10
4
Zeit in ms
Strom
in
A
1.229 10
4

0.086
I0
i
I1
i
I2
i
2.483 10
3

94.986 ta
i
Time in ms
Prim.
Voltage
in
V
Prim.
Current
in
A
Dynamic Test on a Network Model with RTDS
Calculated Symmetrical Components

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 22
1.5 1 0.5 0 0.5 1 1.5 2
2.5
2
1.5
1
0.5
0
0.5
0.5
2.5
X
i
XZ p
( )
XZS p
( )
2
1.5 R
i
RZ p
( )
 RZS p
( )

Load
point
All
impedances
are primary
values
Resistance in Ohm
Reactance
in
Ohm
Dynamic Test on a Network Model with RTDS
Results in the Impedance Plane

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 23
250 200 150 100 50 0 50
0
50
100
150
150
0
G1
i
Ch1 l
( )
Ch2 l
( )
Ch3 m
( )
12.87
237.814 B1
i
l
 l
 m

Scaling in percent - related to primary values X calculation every 50ms
Load
point
Dynamic Test on a Network Model with RTDS
Results in the Admittance Plane

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 24
Test condition: P=40 MW Q=25 MVar; If = 1,4 If0; Voltage regulator failure: U= 1,05 0,7
Relay settings: Char 1 = 0.55 80°, 10s; Char 2 = 0.51 90°, 10s; Char 3 =0.9 100°, 0s
Dynamic Test on a Network Model with RTDS
RMS Fault Record - Low Load Condition

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 25
2 1 0 1 2 3 4 5 6 7
4
2
0
2
4
Primärwiderstan in Ohm
Primärwiderstand
in
Ohm 3.339
4
X1
i
Im Z p
( )
( )
Im ZS p
( )
( )
6.446
1.103 R1
i
Re Z p
( )
( )
 Re ZS p
( )
( )

Resistance in Ohm
Reactance
in
Ohm
Generator oscillates
near pickup characteristic
Load
point
Dynamic Test on a Network Model with RTDS
Result in the Impedance Plane

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 26
Scaling in percent - related to primary values X calculation every 20ms
100 80 60 40 20 0 20
0
10
20
30
40
50
Leitwerte in Prozent
Leitwerte
in
Prozent
50
5
G i
( )
Ch1 l
( )
Ch2 l
( )
Ch3 m
( )
12.066
100 B i
( ) l
 l
 m

Load
point
Underexcited
region for 3.2 s
Oscillating near
the characteristic
Dynamic Test on a Network Model with RTDS
Result in the Admittance Plane

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07 Underexcitation Protection
Using numerical machine and motor protection
Version: C 3_Page 27
Motor Operation of a Pump Storage Station
After Problems with the Generator Circuit Breaker the
Field Breaker was switched OFF
Trip log of F11
Due to problems of the GCB the HVCB was switched off via breaker
failure protection (see trip delay in the fault record)

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Using numerical machine and motor protection
Version: C 3_Page 28
SIGRA Record of Trip by the Underexcitation
Protection
P, Q
X, R
0,5 s by 50 BF