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LOSS OF EXCITATION PROTECTION FOR MODERN SYNCHRONOUS GENERATORS
LOSS OF EXCITATION PROTECTION FOR MODERN SYNCHRONOUS GENERATORS.pdf (Size: 165.09 KB / Downloads: 309)
ABSTRACT
This paper presents the results of a study into
the application and performance of the offset
mho distance relay for the loss of excitation protection
of synchronous generators. Included is
information on the loss of excitation characteristics
of modern generators, on relay performance
during transient swings and low frequency disturbances
and on generator protection.
INTRODUCTION
In 1949,1 a single phase offset mho relay was
introduced for the high speed detection of loss of
excitation in synchronous generators. This distance
relay approach was developed to provide improved
selectivity between loss of excitation and other
normal or abnormal operating conditions and to
provide the operating times necessary for optimum
protection of both the generator and the system.
Over the years, the offset mho relay has been
widely accepted for loss of excitation protection
and experience with the relay has been excellent.
The relay has demonstrated its capability of
detecting a variety of excitation system failures
and to discriminate between such failures and
other operating conditions. The relatively few
cases of incorrect operation that have occurred can
be attributed to incorrect relay connections (major
cause), and blown potential transformer fuses.
In spite of this excellent experience, there has
been some user apprehension about the performance
of distance type of relaying for loss of excitation
protection. In particular, there has been
concern over possible incorrect operation of the
relay when operating the generator in the underexcited
region, during stable transient swings and
during major system disturbances that cause underfrequency
conditions.
In view of this continuing concern over relay
performance and in view of the fact that machine
parameters have changed appreciably during the
past twenty years, a general study was initiated to
review the application and the performance of the
offset mho loss of excitation relay for a variety of
system conditions. This paper discusses the results
of this study and provides guidance on the application
of loss of excitation protection.
REVIEW OF RELAY CHARACTERISTICS AND SETTINGS
The offset mho loss of excitation relay is a single
phase, single element distance relay which isapplied
to the generator terminals and connected and set
to look into the machine. On the R-X diagram (see
Fig. 1) the relay characteristic is an offset circle
which has an angle of maximum torque that falls
on the (-X) ordinate. As viewed from the machine
terminals the relay will operate for any impedance
phasor that terminates inside the circular
characteristic.
When the relay was introduced in 1949, it was
recommended the offset be set equal to one-half
of the direct axis transient reactance (X’d/2) and
the diameter of the circle set equal to the direct
axis synchronous reactance (Xd). It was shown1
that with the machine reactances that existed at
that time, these settings would detect a loss of
excitation from any machine loading and that
there would be optimum selectivity against operation
during stable power swings. Machine direct
axis synchronous reactances were in the range of
1.1 to 1.2 per unit.
Fig. 1. Operating characteristic of loss-of-excitation relay.
3
In more recent machine designs, these synchronous
reactances have increased to a 1.5 to 2.0 per
unit range. With the advent of these higher impedance
machines there has been reluctance by some
utilities to use relay settings proportional to synchronous
reactance, mainly because of a fear that
the resulting large circle diameters might infringe
on the underexcited operating capability of the
machine. Therefore where this possibility was a
concern, it has been recommended that the relay
reach be limited to an assumed synchronous
reactance of 1.0 per unit. When this recommendation
was made it was recognized that this reduced
setting would detect a loss of excitation with high
machine loadings (the most severe condition for
both the machine and the system) but would not
provide coverage if the machine was lightly loaded.
While this limited coverage was acceptable to the
concerned user, there has been some question as to
the extent of protection being provided. Therefore,
one of the purposes of the study was to determine
quantitatively the protective limits of a reduced
setting.
probable mode of failure. For the less likely case of
an open field, the loss of excitation characteristics
will differ to some extent from those presented
here but the final impedances as viewed from the
generator terminal will be essentially the same as
for a short-circuited field.
While the discussion will be limited to steam
machines with specific parameters, the results and
phenomena described also apply to hydro-generators
and machines with other parameters.
As a point of interest, it should be noted that
the loss of excitation characteristics and phenomena
presented here do not differ appreciably
from those reported by Concordia,3 Temoshok
and Mason some twenty years ago.
Loss of Excitation - Tandem
Compound Generators
GENERATOR LOSS OF EXCITATION CHARACTERISTICS
This section presents and discusses in some
depth the loss of excitation characteristics of
modern tandem and cross compound generators.
As noted in reference 1, the loss of excitation
characteristic refers to the locus of the apparent
impedances as viewed from the generator terminals
during a loss of excitation condition. These
characteristics were determined for typical machine
designs in a digital computer study using a
comprehensive dynamic model2 of a turbine
generator.
Figure 2 shows the loss of excitation characteristics
for a typical large tandem compound generator
that is connected to a system through a stepup
transformer having a .15 per unit impedance on
the machine base. These characteristics are shown
as a function of both initial machine loading and
system impedance.
The following discussion will consider the effect
of initial generator loading and system impedance
on the impedance locus, on the generator terminal
voltage and on machine loading during a loss of
excitation condition. The discussion will also consider
the effect of voltage regulators on cross compound
generators. In all cases the loss of excitation
characteristics will be plotted with respect to two
relay settings: one setting will have a circle diameter
of 1.0 per unit, the other will have a circle
diameter equal to machine synchronous reactance.
The offset, in both cases, will be equal to X’d/2.
0 . 5
-I3-
1
2
1 2 3 4
PER UNIT IMPEDANCE
CURVE INITIAL LOADING (per unit) SYSTEM IMPEDANCE
0.93 MVA 0.92 PF Lagging 0.4 PU
0.98 MVA 0.98 PF Lagging 0.2 PU
0.92 MVA 0.90 PF Lagging 0
0.31 MVA 0.95 PF Leading 0.4 PU
0.30 MVA 1.00 PF 0.2 PU
In all cases, it was assumed the loss of excitation Fig. 2. Loss-of-excitation characteristics for a tandemwas
caused by a short-circuited field, the most compound generator.
4
As noted in the diagram, curves (a), (b) and ©
show the impedance locii as a function of system
impedance with the machine operating initially at
or near full load. Curves (d) and (e) show the locii
at two values of system impedance with the machine
initially at about 30% load.
For the case of the machine operating at full
load, all of the impedance locii terminate in an area
to the right of the (-X) ordinate and will approach
impedance values, which at the final steady-state
slip, will be somewhat higher than the average of
the direct and quadrature axis subtransient impedances
of the generator. The final impedances will
always be greater than the offset setting (X’d/2)
and therefore will always fall inside the relay characteristics
as shown in Fig. 2.
For system impedances of zero and 0.2 per unit,
the impedance locii (b, c) go directly to this area
while the impedance locus for a .4 system spirals
into the area as indicated by curve (a). The traverse
time from the initial load point to the relay characteristic
of the impedance locii will be between 2
to 7 seconds. The .4 system locus travels the fastest
(2 seconds). It should be noted that when the impedances
reach the area to the right of (-X) ordinate,
the machine will be operating as an induction
generator at a speed of 2 to 5% above normal. It
will be supplying some reduced power to the system
and will be receiving its excitation (VARS)
from the system. The machine slip and the power
output will be a function of the machine slip-torque
characteristic (which in turn is a function of machine
and system impedances) and governor characteristic.
High system impedances produce a high
slip and a low power output.
For the case of the machine operating initially at
30% load, the impedance swing is more gradual and
only goes as far as point (A) just inside the 1.0 per
unit circle before it reverses. The swing will oscillate
in the region between points (A) and (B). The
traverse time from the initial point to point B is
around 7 to 9 seconds while the time to traverse
the distance B-A can be up around 10 to 15 seconds
or higher. For this initial loading, the machine
speed will only be 0.1 to .2% above normal and as
before it will be operating as an induction generator.
For initial machine loadings between .3 and 1.0
per unit, the impedance locii will terminate inside
the 1.0 per unit circle in the region above point A.
For loadings below .3 per unit the locii will terminate
below point A and will only appear in the
large circle (diameter = Xd). For a loss of excitation
from no load, the relay will see an impedance
which in the limit will vary between the direct and
quadrature axis synchronous impedances (Xd’ Xq).
Machine Loading and Terminal Voltage: Figure 3
shows the effect of loss of excitation on terminal
voltage, power output and reactive power for a 0.1
per unit system impedance and for a machine operating
initially at full load. The abscissa is given in
seconds while ordinates specify per unit volts,
power and VARS. It should be noted that negative
VARS signify VARS into the machine.
I I I I I I I
Fig. 3. Variation in terminal voltage, power, vars for loss
of excitation on tandem-compound generator.
As noted in this diagram, the voltage decreases
and oscillates around an average of 0.5 per unit,
the power output decreases and averages about 0.3
per unit and the VARS go negative and average
around-O.93 per unit.
For the case of a lightly loaded machine, the
variation in loading and terminal voltage will be
considerably less when excitation is lost. For example,
consider the case of a generator connected to a
.2 system and with an initial loading of P = .3, Q =
-. 156, VT = 1.0 per unit. Thirty (30) seconds after
losing excitation, the lowest voltage reached was
.78 per unit, the power dropped only to .275 per
unit and the VARS reached -.6 per unit.
There are several points to note from these
results. First, when a lightly loaded machine loses
5
excitation, the final MVA loading will probably
not be damaging to the machine but the VAR
drain may be detrimental to the system. In the case
discussed the final machine MVA loading is .66 per
unit and the stator current only reaches .85 per
unit. When the machine is initially operating at full
load, a loss of excitation can be damaging to both
the machine and the system. While the final loading
in terms of MVA is not excessive, the machine
in Fig. 3 will have stator currents in excess of 2.0
per unit. The high current is due to the fact that
the resulting machine loading is at a substantially
reduced terminal voltage. Of course, the VAR
drain from the system can depress system voltages
and thereby affect the performance of other generators
in the same station or elsewhere on a system.
In addition, the increased reactive flow across the
system can cause tripping of transmission lines and
thereby adversely affect system stability. For
example, in 1951 a utility reported4 that loss of
excitation on a 50 MW generator caused system
wide instability, the tripping of interconnections
and tie lines and over 100 breaker operations
before the disturbance subsided. In this case, it
was evident that other generators and interconnections
could not stand the additional reactive load
imposed on the system. The possible effects on
other generation will be discussed in a later section.