Energy Automation

1. Energy Automation

Underexcitation Protection
© Siemens AG 2011
Infrastructure & Cities Sector

2. Content

Theory
Introduction
Stability limits of generators
Admittance measuring principle
Admittance measurement
Characteristic, settings
Comparison with the impedance measuring principle
Examples
Dynamic tests
Practical measurements
Page 2
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

3. Folie 3

Reasons for Underexcitation
Failure of the excitation device
short circuit in the excitation circuit
ZLoad
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
G
Page 3
excitation
device
Jun-23
Note: This protection is also called
Loss of Field Protection
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

4. Folie 4

Possible Design of the Generator
Capability Diagram
Definition:
Preferred design
+Q
(Var)
+P
(W)
Operating
area
over excited
+P
(W)
Static
stability
limit
under excited
Operating
area
under over
excited excited
+Q
(Var)
Static stability
limit
Page 4
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

5. Folie 5

0,85
0,8
0,85
0,9
MW
0,95
P
0,975
0,975
0,95
0,9
Capability Curve of a Turbo Generator
type of generator:
TLRI 108/46
220
nominal apparent power SN = 200 MVA
nominal voltage
VN = 15.750 kV
nominal current
IN = 7.331 kA
nominal frequency
fN = 50.0 Hz
0,7 power factor
cos N = 0.8
cold-air temperature
Tx = 40.00 °C
200
180
0,8
160
140
0,7
0,6
120
0,6
100
80
0,4
60
0,4
40
cosphi
0,2
cosphi
0,2
MVAr 140 120 100 80
60
40 20
underexcited
Page 5
Jun-23
0
20 40
Q
60 80 100 120 140 160 180 MVAr
overexcited
Energy Automation
© Siemens AG 2011
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6. Folie 6

Per Unit Capability Diagram of a Synchronous
Generator in the Case of Undervoltage (U = 0.9 UN)
P [p.u]
1
0.85
U=1; I=1;
U=0.9; I= 1.11
Stability
limit
overexcited
underexcited
Q [p.u]
0.81/xd
1/xd
In the case of an under-voltage the generator capability curve
moves to right and reduces the stability limits of the generator
Page 6
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

7. Folie 7

Summary of the Statements Regarding
the Stability Limits
practical
stability
limit
Stability
limit at
U < UN + P
(W)
theorectical
steady-state
stability
limit
maximum of
theorectical
dynamic
stability
limit
under
excited
over
excited
+Q
(Var)
U2
xd
U2
x'd
Page 7
Jun-23
Energy Automation
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Infrastructure & Cities Sector

8. Folie 8

Definitions for Converting the Generator Diagram
into the Admittance Diagram
Complex Power:
S U I
Admittance:
S P jQ
Y
I
U
Y G jB
B: Susceptance
Transformation:
I U
S*
P - jQ
Y
2
2
U
U
U U
+
P
P
G 2
U
Q
B
Q
U2
+
-
G: Conductance
P
Q
j
U2 U2
+
B
G
+
In the per unit
representation
the diagrams are the
same. There is only
- a phase shift in the
reactive part of 180°
Note: In the per unit calculation is UN = 1
Page 8
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

9. Folie 9

Characteristics of the Underexcitation Protection
with Admittance Measurement Y>
char.3
char.2
3 independent characteristics and 3 timer
char.1
G[p.u.]
characteristic 1,2 is adaptated on the
steady state curve
(delay: alarm only 10s; trip 0,5 – 3 s)
additional inquiry of the field voltage
(trip delay time: 0,5 – 1,0 s)
a3
B[p.u.] x
1
d3
a2
1
1
x d1 x d2
a1
characteristic 3 is adaptated on the
dynamic stability limit curve
(short trip delay time: 0 – 0,3 s)
blocking of the protection at U<25% UN
Settings: Can direct read out from the generator diagram
1
1
x d1 x
d
Page 9
a1= 80°
Jun-23
1
1
xd2 0.9 xd1
a2 = 90°
Energy Automation
1
2
xd3 1 or xd
a3 = 110°
© Siemens AG 2011
Infrastructure & Cities Sector

10. Test Underexcitation

machine data:
300 MVA, 20 kV
1= 0.67 p.u. , a1= 70°
2= 0.50 p.u. , a2= 90°
P[p.u.]
UN
CT: 10000/1A,
VT: 20/0.1 kV
ING
I
1 Test1
Char. 1
Char. 2
φTest1
ITest2
2
a1
φTest2
φN
a2
λ 2 1 x d2
-1
1 Q[p.u.]
λ1 1 x d1
I Test InO
λ sin α
λ sin α
InO
sin γ
sin 90 α Test
Example : U U N 1p.u. ; Test1 30
InO (sec)
SN
I
300000 kVA
1A
CTsec
0.866 A
3 U N I CTprim
3 20 kV 10000 A
I Test1 0.866 A
Page 10
Jun-23
Energy Automation
0.67 sin 70
0.712 A
sin 90 70 30
© Siemens AG 2011
Infrastructure & Cities Sector

11. Folie 11

Underexcitation Protection with Criterion
Impedance I-ZI<
Generator diagram is transformed into the impedance plane (e.g. X=U2/Q).
Stability limit is represented as a circular arc.
characteristic: Offset-MHO
X[p.u.]
R[p.u.]
characteristic 1, tdelay 0...0.3 s (for high
load generator and field failure)
0.5 xd’
1
tripping zone inside the circle
Char.1
xd
Char.2
Relay settings according
IEEE C37.102-1995
approximation
of stability
limit
characteristic 2, tdelay 0.5 - 3 s (for low
load generator, section field voltage
failure)
Summary:
Measuring principle is from the electromechanical relays, because impedance
measuring elements were only available
circle characteristic is a compromise for
adaptation to the generator stability curve
Page 11
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

12. Folie 12

Transformation of Criterion Impedance I-ZI<
into the Admittance Plane
Transformation rule:
A circle which doesn’t touch the zero point becomes inverted a circle again.
Impedance plane
X[p.u.]
Admittance plane
1
Y
Z
G[p.u.]
R[p.u.]
0.5 xd’
1
Char.1
B[p.u.]
xd
2
1
2 x d x ,d x d
Char. 2
2
x ,d
Page 12
Jun-23
Energy Automation
2
1
,
2 xd
© Siemens AG 2011
Infrastructure & Cities Sector

13. Folie 13

Both Measuring Principles in the
Admittance Plane
Impedance
principle
Admittance
principle
Admittance Plane
Trajectory in the
case of underexcitation
with 100% excitation
loss
4
2
Generator
diagram
0
Settings:
x’d = 0,27
xd = 1,81
2
4
8
7
6
2/x’d = 7,4
Page 13
Jun-23
5
4
3
2
1
1/xd = 0,55
Energy Automation
0
1
Note:
B-axis is for mathematical
reasons multiplied by -1
© Siemens AG 2011
Infrastructure & Cities Sector

14. Folie 14

Dynamic Test on a Network Model with RTDS
Fault Record
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
Page 14
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

15. Folie 15

Dynamic Test on a Network Model with RTDS
Results in the Admittance Plane
150
150
100
G1
i
Load
point
Ch1( l )
Ch2( l )
Ch3( m )
50
0
0
250
200
237.814
150
100
B1 l l m
i
50
0
50
12.87
Scaling in percent - related to primary values X calculation every 50ms
Page 15
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

16. Folie 16

Dynamic Test on a Network Model with RTDS
RMS Fault Record - Low Load Condition
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
Page 16
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

17. Folie 17

Dynamic Test on a Network Model with RTDS
Result in the Admittance Plane
50
50
Leitwerte in Prozent
40
Underexcited
region for 3.2
s
30
G( i )
Ch1( l )
Ch2( l )
20
Load
point
Ch3( m )
Oscillating near
the characteristic
10
0
5
100
100
80
60
40
B( i ) l l m
Leitwerte in Prozent
Scaling in percent - related to primary values
Page 17
Jun-23
Energy Automation
20
0
20
12.066
X calculation every 20ms
© Siemens AG 2011
Infrastructure & Cities Sector

18. Folie 18

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)
Page 18
Jun-23
Energy Automation
© Siemens AG 2011
Infrastructure & Cities Sector

19. Folie 19

SIGRA Record of Trip by the Underexcitation
Protection
X, R
P, Q
0,5 s by 50 BF
Page 19
Jun-23
Energy Automation
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Infrastructure & Cities Sector