A Comprehensive Protection Scheme for Generator
Loss of Excitation
Nitesh Kumar D
R. Nagaraja
H. P. Khincha
Power Research & Development
Consultants Pvt. Ltd Bangalore
niteshostwal@gmail.com
Power Research & Development
Consultants Pvt. Ltd Bangalore
nagaraja@prdcinfotech.com
Advisor, Indian Institute of Science
Bangalore
hpk1598@gmail.com
Abstract— Power system is constantly evolving with
application of new technology to improve the reliability and
security of the system. At the same time technology upgradation
is being undertaken to reduce the cost of various power system
components, which can have an impact on protection techniques.
The paper evaluates the performance of loss of excitation
protection scheme for increase in generator direct axis
synchronous reactance. It is shown that for higher values of
direct axis synchronous reactance, the coordination between loss
of excitation protection, excitation control and capability limit is
not adequate. This may restrict the generator reactive power
absorption capability. The paper also reviews the conventional
philosophy for setting the excitation limiters and discusses its
impact on loss of excitation protection and system behavior. A
conventional protection scheme is modified to allow for full
utilization of machine capability and new power swing blocking
criteria is developed to improve the reliability during stable
power swings.
Keywords—Loss of excitation protection; Increase in Xd;
Under excitation limiter; Correlation function; Stable power swing;
Power swing blocking
I.
INTRODUCTION
The impact of loss of excitation (LOE) of a synchronous
generator on the machine and the system stability [1], has led
to considerable research in development of protection schemes
which can detect loss of excitation at the earliest and also make
the loss of excitation protection scheme immune to power
swings and other external disturbances.
The two variants of impedance measurement based
protection schemes can provide highly reliable protection for
loss of excitation occurring during any loading condition of the
generator [2]-[4]. Reference [5] suggests the need for
coordination of loss of excitation relay with generator
capability curve (GCC), steady state stability limits (SSSL) and
excitation controls in order to have an optimum protection
scheme. The impedance based schemes are known to mal
operate during certain external disturbances and either require
intentional time delay or special logic to prevent their operation
[6-7].
In recent times, certain generator parameters have
undergone significant change due to change in the design
aspects of machine. The loss of excitation protection settings
mainly depends on the generator direct axis synchronous
reactance (Xd) and generator direct axis transient reactance
(Xd’) value. A significant change in these parameters can have
978-1-4799-5141-3/14/$31.00 ©2014 IEEE
Fig. 1. Typical generator capability curve for different values of Xd
an impact on performance of the protection scheme. The paper
aims at studying the impact of change in machine parameter on
the performance of loss of excitation protection scheme and its
coordination with generator excitation control and capability
limits. The paper suggests modifications to the existing
protection scheme to allow full utilization of machine
capability. To overcome the limitation of existing scheme
during stable power swings, the paper discusses the use of
correlation technique which can classify the disturbance as
LOE or stable power swing and can accordingly block the relay
from tripping.
II.
CONSIDERATIONS IN PROTECTION SCHEME WITH
INCREASE IN XD
A. Coordination of LOE with SSSL and GCC
A survey of different generator data sheet of various ratings
is carried out, which shows that there has been a change in the
generator Xd and inertia values. It is observed that for the high
rated machines of capacity around 300 to 600 MW, the Xd
values is larger than the conventional value. This increase in
Xd can have an impact on coordination of the LOE relay with
SSSL and GCC. Fig 1 shows the generator capability curve
(with impedance in pu on its own rating) for machines with
different Xd rating. It can be observed that the capability curve
does not follow a specific trend with change in Xd and mainly
depends on the cooling techniques employed [8]. The
capability curves for machine of different Xd value generally
lie within a narrow band.
The calculation of SSSL [9] considering different values of
Xd shows the dependency between the SSSL and machine Xd
value. Stability margin decreases with increase in Xd and when
Fig. 2. Typical SSSL curve for different values of Xd and fixed system
reactance
Fig. 4. Coordination curve for Xd = 2.8pu
• For Xd = 2.0 pu, the three characteristic curves now lie
within a narrow band, with zone 2 of LOE and the GCC
almost overlapping each other. However this may not
have any significant impact.
• For Xd>2.0 pu, it is observed that the zone 2 of LOE
infringed into the GCC, thereby restricting the machine
reactive power absorption capacity.
• It is also observed that the variation of Xd has no
impact on the coordination of LOE and SSSL.
Fig. 3. Coordination curve for Xd = 1.6pu
referred to impedance plane, the diameter of the SSSL circle
increases with increase in Xd, as shown in Fig 2.
Having shown that machine Xd has generally no direct
impact on the GCC but affects the SSSL, we move to analyse
the impact of this on the coordination of LOE characteristic,
GCC and the SSSL. The conventional impedance based
protection [3] is considered for analysing the change in loss of
excitation characteristic due to change in Xd and system
impedance (Xs). The value of Xd is varied from 1.6 pu to 2.8
pu and two values of Xs, i.e, 0.176 pu and 0 pu (infinite bus)
are considered. The GCC (in pu) is considered same for all the
cases and the under excitation limiter (UEL) is coordinated
with GCC.
For obtaining the loss of excitation and SSSL
characteristics it is considered that a generator of synchronous
reactance Xd is connected to a generator transformer having
impedance Xtg, which is then connected to the system of
impedance Xs, all impedance values referred to generator
MVA rating as the base. The characteristic curves are obtained
for different values of Xd and Xs by considering Xtg = 12.5%.
The characteristic curves of conventional loss of excitation
protection considering Xs equal to 0.176 pu for the case with
Xd equal to 1.6 pu and 2.8 pu are given in Fig 3 and 4
respectively.
The following observations can be drawn from the
coordination curves.
• For Xd less than 2.0 pu, it is observed that the LOE
relay characteristic is adequately coordinated with GCC
and SSSL.
We note that for certain values of Xd and GCC, the relay
characteristics can infringe into the normal operating region.
Reference [10] suggests that when such a situation is
encountered, the zone 2 reach be reduced by a margin such that
it is coordinated with GCC. As discussed in [2], a zone reach
equal to Xd is essential to detect LOE occurring at any initial
loading condition of machine, the reduced reach will reduce the
sensitivity of the scheme for LOE occurring during light
loading conditions of the machine.
B. Considerations for UEL setting
References [5], [11] suggest that UEL is to be coordinated
with the GCC, SSSL and loss of excitation relay whichever is
most constraining. This indicates that for scenarios where the
LOE characteristics infringe into the GCC, the UEL is to be
coordinated with SSSL. With this coordination, a significant
portion of the reactive power absorption capability of the
generator is compromised. In case of salient pole machine
having higher reactive power absorption capability than the
non-salient pole machines, the compromise can be even severe.
Two cases of UEL coordinated with GCC and UEL
coordinated with SSSL are shown in Fig 5 for which the loss of
reactive power absorption capability is clearly evident.
Since SSSL limit is valid only for generator operating in
manual excitation, it may not be idealistic to consider this in
the modern day power system. Reference [12] discusses in
detail the effect of coordination of UEL with SSSL, and
suggests that the role of UEL be only limited to prevent stator
end ring heating. It also shows that the stability margin of the
system increases with use of AVR. Further in several cases in
power system, redundancy is provided for AVR and in such
case manual excitation operation may never occur.
Fig. 5. Coordination of UEL with GCC and SSSL
The UEL output is fed as an input to AVR which in turn
controls the excitation voltage and hence limits the reactive
power absorption. However this action will not be available
during manual operation (for example AVR failure) and hence
coordination of UEL with SSSL considering manual excitation
has no significance and it unnecessarily limits the absorption
capacity of the machine which can be of at most importance
during severely disturbed system scenario [12]. Further, with
use of modern computational tools it is possible to compute the
dynamic stability limit with good accuracy and hence there is
no justification for use of SSSL for use in protection study
[11].
Another approach to this problem can be to coordinate the
UEL with LOE zone 2 characteristics. Though this along with
suitable time delay for zone 2 can be used as a possible relay
scheme, it still restricts some portion of the reactive power
absorption capacity of the machine. With UEL tuned to prevent
mal operation of LOE relay, the dynamic performance of UEL
becomes a very important consideration. The overshoot
occurring during UEL control action can cause the impedance
to lie in the operating region for considerable time which can
issue a false tripping [11].
Thus for machine with higher values of Xd and UEL
coordinated with SSSL or LOE relay, there exists a
compromise in operational capability and at the same time does
not provide full proof protection.
III.
PROPOSED SCHEME
A. Modified Two Zone Scheme
Fig 6 shows the impedance trajectory for LOE occurring at
different initial loading conditions. It can be inferred that the
impedance trajectory during LOE occurring at light loading
conditions enters the relay operating zone from the bottom part
of the characteristic and does not penetrate too deep.
Fig. 7. Proposed modifications in LOE protection scheme
For the extreme case where the machine is operating as a
synchronous condenser, the impedance trajectory stops at the
co-ordinates - [0,-Xd]. This indicates that by reducing the zone
2 diameter and introducing an additional zone such that it
covers those impedance trajectories which will not enter the
reduced zone 2, we can achieve an optimal LOE protection
scheme which can detect LOE at all conditions and at the same
time allow better utilization of machine capability. The
proposed scheme is as shown in Fig 7.
The mod zone 1 is offset by an amount of –Xd’/2 and the
diameter is adjusted to coordinate with GCC. With a reduced
mod zone 1 reach, the impedance trajectory entry time for the
modified zone 1 is greater than entry time for original zone 2
reach. Therefore using a typical original zone 2 time delay of
1s may be inadequate to trip the machine before occurrence of
loss of synchronism. In order to improve the performance of
the scheme during stable power swings, a power swing
blocking criteria is used to supervise the tripping.
The proposed mod zone 2 is of quadrilateral characteristics
with its reactive reach starting from the point –1.1*(Xd’/2).
The resistive reach is defined as an angle with respect to Y-axis
such that mod zone 2 characteristic overlaps the LOE
impedance trajectory which just touches the modified zone 1
(just before losing synchronism) as shown in Fig 8. Typically
this angle is less than 10° and the overlapping ensures that the
scheme can detect LOE occurring at any loading conditions.
The mod zone 2 can be used to initiate an alarm as soon as the
impedance trajectory enters into it and can be configured to trip
the machine with some time delay.
Fig. 8. Overlapping of mod zone 1, mod zone 2 and LOE trajectory
Fig. 6. LOE trajectory for different initial loading
Fig. 9. Resistance variation during LOE
Fig. 10. Resistance variation during stable power swing
B. Power Swing Blocking Scheme
The power swing blocking criteria takes advantage of the
difference in resistance variation pattern during LOE and
power swing. Fig 9 and 10 shows the variation of resistance
during LOE and power swings respectively.
For use of correlation function as criteria for detecting
power swing conditions, one of the data set (x) is chosen as
generator terminal resistance. The start point of the data set is
at the time (t2) when mod zone 1 triggers and end point is at a
time t2+toss, where toss = 1/foss. Thus the data set consists of data
from t2 to t2+toss with a step time of Δt = 1/f, where f is the
nominal system frequency.
The initial variation of resistance during LOE is oscillatory
in nature, following which the variation becomes almost linear
with respect to time. This linear pattern can be observed till the
machine goes out of step, beyond which the variation in
resistance is erratic. By overlapping the impedance trajectory
during LOE on the relay characteristic as shown in Fig 6, it is
observed that for the time period when the trajectory is within
the characteristic zone, the variation in resistance is always
linear.
The variation of resistance during power swing conditions
is highly oscillatory and typically oscillates with a frequency
(foss) of 1Hz. The oscillatory variation in resistance can be
clearly observed even when the power swing locus infringes
into LOE relay characteristics.
The proposed scheme for power swing detection uses the
correlation function to classify the resistance variation as linear
or nonlinear, and tripping is permitted if the variation is linear.
Correlation function – r is computed using (1).
The second set of data can be any reference data set with
linearly increasing data. The ideal reference data set can be
time sequence which starts when mod zone 1 trigger, treating
this time t2 as 0s. The end time of the reference data set is toss,
with a step size of Δt. Thus the second set of data will consist
of data y = [0 Δt 2Δt 3Δt ………….toss].
Considering the inherent non linearity in measurement and
taking additional margin, the power swing blocking criteria can
be defined as:
Trip: -1 < r < -r1
Block: otherwise
r1 is a set threshold.
r = -1 indicates negative correlation. i.e. for linearly
increasing y, x decreases linearly.
For LOE occurring during very low initial loading
(typically less than 5% of machine rating), it is observed that
rate of change of resistance is very slow and hence the
resistance value appears as a constant within the time frame
considered for power swing blocking criteria. Therefore the
power swing blocking criteria fails to detect LOE occurring at
extremely low initial loadings. As discussed earlier, the
proposed mod zone 2 is used to detect LOE occurring at very
low initial loadings and hence the power swing blocking
scheme is not applied for those impedance trajectories which
first enter the mod zone 2 characteristics. However the
probability of power swing entering the relay characteristic
through this small region is very remote and any impedance
trajectory entering through this region can be considered as
LOE.
r = 0 indicates no correlation. No linearity relation exist
between x and y.
The trip logic for LOE detection and power swing blocking
criteria is shown in Fig 11.
r = 1 indicates positive correlation. i.e for linearly
increasing y, x increases linearly.
Other values of r define the degree of linearity between the
two data sets.
function (r). With this, the proposed scheme has an internal
delay of 0.5s to classify the event and issue a trip signal.
Fig. 13. System considered for study
Fig. 11. Trip logic for proposed relay scheme
C. Additional Application of Proposed Scheme
The proposed scheme can also be used for cases where the
conventional LOE relay is well coordinated with GCC and
SSSL. The application of power swing blocking criteria will
help to improve the speed of relay operation and also improve
relay performance during stable power swing.
The relay characteristic for this application is shown in Fig 12.
The mod zone 2 characteristic is marginally extended beyond
the –(Xd+Xd’/2) point. This is done so that during very low
loading conditions, the impedance trajectory first enters the
mod zone 2 which blocks the use of power swing blocking
criteria.
LOE is simulated by reducing the field voltage of generator
1 to zero at t =1s. Case studies are performed considering seven
different initial loading conditions such that all possible
operating scenarios are covered. The results are presented in
table 1, case 1 to case 7. The time values given in table are
after applying the disturbance. It is evident that the proposed
scheme can successfully detect and classify LOE during all
operating scenarios and is capable of issuing trip signal before
loss of synchronism occurs. For case 5 to case 7, the mod zone
2 triggers first and hence power swing blocking criteria is
disabled. The tripping signal provided after a preset delay (D2)
after the trajectory enters mod zone 2 characteristic.
Power system disturbance like three phase to ground fault
and generator outage are simulated such that the machine does
not lose synchronism after the disturbance. Three phase to
ground fault is simulated considering two initial operating
conditions. The fault is created at t =1s on HV side of the
generator transformer with duration equal to its critical clearing
time. For the two cases of fault simulation, generator 2 is
considered to be out of service. The results are presented in
table 1, case 8 to case 10.
The proposed power swing blocking criteria can effectively
classify stable power swings entering the characteristic zone.
The scheme blocks the relay from tripping during all the above
cases.
Fig. 12. Characteristic of proposed relay for scenario where conventional LOE
scheme is adequtely coordinated with GCC
IV.
CASE STUDY AND RESULTS
In order to demonstrate the effectiveness of proposed
scheme in allowing full utilization of machine capability, and
to differentiate between LOE and stable power swings, the
scheme is applied to a test system. The system considered for
study is shown in Fig 13 and the system data is provided in
appendix. For the considered system the conventional LOE
scheme will result in zone 2 infringing the GCC and hence the
proposed scheme is used. The test system is subjected to
disturbance like full LOE, three phase fault and generator
outage, simulation studies are conducted using MiPower
software. For the system considered, the fundamental
oscillation frequency (foss) at the generator terminal is around 2
Hz. Hence the time period for use in power swing blocking
criteria is 0.5s. With each phasor extracted at 0.02s, a data set
of 25 samples is to be used for calculating the correlation
The locus of power swing during disturbance depends on
the electrical center. The change in electrical center will alter
the way in which the power swing enters the LOE
characteristic region. The power swing disturbances given in
case 8 to case 10 are simulated with line length reduced to
25km. The results are presented in table 1, case 11 to case 13.
The results demonstrate the capability of the power swing
blocking criteria for different electrical centers.
V.
CONCLUSION
An improved LOE protection scheme is presented in this
paper which allows the full utilization of machine reactive
power absorption capability and at the same time can detect
LOE before machine loses synchronism. A power swing
blocking criteria is developed which uses correlation function
to classify LOE and stable power swings, which enhances the
detection of LOE. The effectiveness of the proposed scheme is
demonstrated through extensive case studies.
Table I.
CASE STUDY RESULTS
Case No
1
2
3
4
5
6
7
8
9
10
11
12
13
Disturbance
Initial
Loading
(pu, pf)
Mod Zone 1
Entry (s)
LOE
LOE
LOE
LOE
LOE
LOE
LOE
3P
3P
Gen Out
3P
3P
Gen Out
0.85,
0.98
0.85,
-0.98
0.35,
0.95
0.35,
-0.86
0.06,
-0.4
0.03,
0.23
0.03,
-0.22
0.7
-0.83
0.5
-0.71
0.85
-0.95
0.7
-0.83
0.5
-0.71
0.85
-0.95
2.84
1.20
8.70
6.92
86.08
NE
NE
0.14
0.18
1.82
0.16
0.20
NE
Mod Zone 1
Exit (s)
NL
NL
NL
NL
NL
NL
NL
0.78
0.92
2.76
0.92
0.92
NL
r
-0.998
-1.000
-1.000
-1.000
NV
NV
NV
0.716
0.819
0.485
0.029
0.789
NV
Mod Zone 1
Trip (s)
3.34
1.70
9.20
7.42
87.08
No
No
PS
Block
PS
Block
PS
Block
PS
Block
PS
Block
No
Mod Zone 2
Entry (s)
NE
NE
NE
NE
82.64
16.08
14.14
0.74
0.70
0.74
0.96
0.90
NE
Mod Zone 2
Exit (s)
NL
NL
NL
NL
NL
NL
NL
0.76
0.72
0.76
0.98
0.92
NL
No
No
No
No
83.64
17.08
15.14
No
No
No
No
No
No
3.70
2.98
12.12
10.76
89.72
178.58
182.78
Stable
Stable
Stable
Stable
Stable
Stable
Mod Zone 2
Trip (s)
Loss of
Synchronism
(s)
Pf: Power factor, Negative pf indicates leading power factor, 3P: Three phase to ground fault, Gen out: Generator outage, NE: Not entered, NL: Not left, NV: Not
valid, PS Block: Blocking signal from power swing blocking criteria.
REFERENCES
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CRC press.
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Generators”, AIEE transactions vol 68, Issue 2, July 1949, pp 12401245.
[3] John Berdy, “LOE Protection for Modern Synchronous Generators’,
IEEE Transactions on Power Apparatus and Systems, vol. PAS-94, no.
5, September/October 1975 pp 1457-1463.
[4] R. L Tremaine, J. L. Blackburn, “Loss of Field Protection for
Synchronous Generators”, Electrical Engineering, vol: 73, issue: 11,
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Excitation Control and Generator Capability”, Working Group J-5 of
rotating machinery subcommittee, IEEE PES, 2007
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Relay Augmented by Rate of Change of Reactance”, IEEE Power
Engineering Society General Meeting, Vol 2, June 2005, pp 1831-1835
[7] Z. P. Shi, J. P. Wang, “The Comparison and Analysis for Loss of
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[9] G Benmouyal, “The impact of synchronous generator excitation supply
on protection and relays”, Schweitzer Engineering Laboratories, Inc.
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APPENDIX
AVR Model
Generator Data
Rated MVA: 705.88 MVA
Tr: 0.01 s
AVR Data
Terminal Voltage: 20 kV
Tc2: 0.8 s, Tb: 2.6 s
Inertia constant: 2.5 MJ/MVA
Tc1: 0.01 s, Tb1: 0.08 s
Xd: 2.4 pu, Xq: 2.33 pu
Kr: 300 pu, Ts: 0.04 s
X′d: 0.2828 pu, X′q: 0.408 pu
Up+: 7.3, Up-: -7.3
X″d: 0.21 pu, X″q: 0.21 pu
Utility Data
Tdo′: 8.724 s, Tqo′: 0.969 s
3ph MVA: 15000 MVA
Tdo″: 0.046 s, Tqo″: 0.068 s
SLG MVA: 15000 MVA
Transmission Line Data
Voltage Level: 400 kV
Transformer Data
MVA: 705.88 MVA
Z: 0.015664+j 0.26816 ohm/km
Voltage Rating: 20/400 kV
B/2: j2.18437e-006 mho/km
Z: j0.125 pu