DIAGNOSTIC TESTING OF GENERATOR INSULATION
WITHOUT SERVICE INTERRUPTION
M. KURTZ and G.C. STONE, Ontario Hydro
D. FREEMAN, V.R. MULHALL, and P. LONSETH, Canadian General Electric Co, Ltd (Canada)
SUMMARY
Coordinated efforts by the Canadian Electrical Association
(CEA) and industry in Canada have led to the development
of two new test methods which permit the evaluation of the
condition of a generator’s stator insulation system with no or
minimal service interruption. While both respond to partial
discharges in the winding, one relies on coupling capacitors
installed on the stator and the other, using an antenna
mounted on the rotor, senses the presence of discharges in
each slot as the antenna moves past. By employing these
sensitive tests routinely, those generators experiencing
insulation deterioration may be readily and inexpensively
identified at an early stage. Thus, revenue loss, and the cost
of maintenance inspection and repairs to the stator windings,
if necessary, are minimized.
The new approaches, which are insensitive to external
high noise levels, are based on the measurement of
electromagnetic disturbances related to individual partial
discharge deterioration mechanisms. The detected pulses are
analyzed by sophisticated electronic instruments, in order to
extract all pertinent information including the degree and,
sometimes, the location of insulation deterioration. Typical
test data are given. The Paper explores the differences
between the new diagnostic techniques and other methods
developed in the past.
Diagnostic Test, Generator, Partial Discharge, Slot
Discharge, Stator Insulation.
REPORT
1. INTRODUCTION
The rate of occurrence and the consequences of service
failures in high-voltage generator stator insulation systems
can be reduced by the use of sensitive diagnostic tests
designed to detect the early stages of insulation deterioration.
Degradation processes include insulation delamination,
shrinkage of wedges and/or side packing permitting
vibration and abrasion, and loss of function of gradient
control coatings. All these processes are almost invariably
accompanied by partial discharges which increase in severity
as the deterioration progresses, usually making an additional
contribution to the insulation damage rate. Especially
important with modem insulation systems are discharges
occurring in the slot between the electrical shield of the
stator bar and the core, usually referred to as slot discharge,
which can attain levels of energy ( > 5 000 pC) sufficient to
cause damage in times as short as several months [1,2].
Detection of discharges at the earliest practical stage,
and proper interpretation of test results can permit corrective
action to be taken before a winding deteriorates beyond
the point of economic salvage, and particularly before the
risk of a failure in service becomes unacceptably high.
Reliable early warning from a suitable diagnostic test may
permit relatively inexpensive repairs, such as re-establishing
ground connections, side-packing, re-wedging or touch-up of
stress grading paint, to be accomplished during a scheduled
outage.
In many power systems with mixed generation, hydraulic
machines are reluctantly removed from service for discharge
tests because of the relatively high cost of replacement fossil
fuel energy. Thus, a diagnostic test that, at least for screening
purposes, can be performed without service interruption,
presents a distinct advantage. For large thermal machines,
such a test also offers advantages in that the long time
restraints for shedding and picking up load may be avoided.
The new test methods described later in this Paper have
been designed to respond to partial discharges originating
in the stator insulation system, using signal- coupling
techniques inherently insensitive to system noise. The signalto-noise ratio is further improved by electronic processing of
the detected signals.
To put into perspective the diagnostic tests described in
this Paper, a review follows of the most widely employed
tests to ascertain stator insulation condition.
2. REVIEW OF DIAGNOSTIC TEST TECHNIQUES
2.1. Measurable Quantities which Correlate with
Damage
Mechanical vibration, gaseous products and partial discharges
are three quantities which can be monitored readily with
negligible service interruption, while providing information
with respect to the total stator insulation condition. The first
two quantities have only relatively recently been under study
and have yet to demonstrate their sensitivity and resolution,
since data correlating measurements with visual inspections
of stator condition are sparse.
2.2. Vibration
If a bar or coil side loosens within a stator slot, vibration can
cause ground wall erosion and wear, contributing to ultimate
insulation failure. Hence, the presence of a vibrating bar
indicates that the winding is loose and may eventually fail.
By measuring the magnitude and rate of increase of these
vibrations by means of accelerometers attached to the
stator frame, the expected remaining useful life might be
estimated.
2.3. Gaseous By-products
Certain dielectrics, exposed to partial discharges or to heat,
evolve various gaseous products. For insulations commonly
used in stator windings of hydrogen-cooled units, some
of these products can be readily distinguished against the
background by gas chromatography or spectroscopy. The
quantity of evolved gas car. indicate the degree of degradation
[3,4].
2.4. Partial Discharges
2.4.1. Tip-Up Test
The “tip-up” test [5,6] provides a measure of the void content
and partial discharge activity in a dielectric by measuring
the change in dissipation factor between two discrete voltage
stress levels, usually 50 percent and 100 percent of operating
voltage. Unfortunately, this test tends to be insensitive to
localized partial discharges because the loss component is
averaged throughout the entire test sample, unless individual
coils are isolated for test — an expensive procedure. Also,
the end-grading material will distort results forin-situ
measurements. The tip-up test requires an external supply
to energize the winding, thus applying maximum voltage
stresses to the entire winding which is not representative of
operating conditions.
2.4.2. Dielectric Loss Analyzer
The dielectric Loss Analyzer [7,8] reacts to the power loss
in an insulation system as a function of voltage per cycle,
thus indirectly measuring the presence and effects of partial
discharge. This test method, through more sensitive than
the “tip-up” test, can identify a number of weaknesses [9],
but cannot detect the presence of a small number of intense
discharge events in a background of many more moderate
discharges. This test also requires an external supply, though
the duration of the test outage may be comparatively short.
2.4.3. Inductive Probe
Inductively coupled radio frequency probes have been
employed to detect local discharges [10]. This test requires
a lengthy service interruption and an external high-voltage
supply, though it does have the capability of pin-pointing
those bars or coil sides suffering the most intense internal or
slot discharges.
2.4.4. Ultrasonic Detector
Signals from an ultrasonic probe have been introduced into
a conventional partial discharge measuring circuit with some
success, especially for locating specific discharge sites [11].
This procedure does not provide any advantage over the
Inductive Probe technique and is probably less quantitative.
2.4.5. Pulse Detection
Detection of individual partial-discharge pulses by direct
capacitive or inductive
coupling to a machine winding, with the generator self-excited
and thus supplying its own high voltage with normal voltage
distribution, has been implemented in various measurement
systems [12,13]. In this class of tests, the pattern of individual
pulses can be displayed on an oscilloscope or quantified by
a pulse-height analyzer. In the early days of this type of
measurement, the high partial-discharge repetition rate from
the many sites in a generator could result in the superposition
of pulses since tests were often performed with “pulseshaping” circuitry to lengthen the duration of the individual
pulses for easier observation. However, with modem wideband storage oscilloscopes and flat-response filters, it is
observed that actual superposition of pulses rarely occurs.
The rise times of partial- discharge pulses measured with
such equipment are about 10 ns or less. Ringing frequencies,
which depend only on generator winding parameters and the
measuring system, vary from about 1 to over 50 MHz and
are “second order” effects initiated by the original partial
discharge event. Pulse durations, including ringing, are
typically less than 1 psec., and consecutive partial discharges
are rarely observed at intervals less than 10 psec.
An inexpensive version of this test [13] has been in
routine use within Ontario Hydro for more than 20 years,
employing HV capacitors temporarily connected to the
generator, a high-pass filter and an oscilloscope for display.
This test has demonstrated that the condition of the stator
groundwall insulation is correlated with the magnitude of
the highest discharge pulse observed on the oscilloscope
[14]. However, distinguishing between generator insulation
partial discharges and external noise is sometimes difficult,
requiring an experienced operator. Additional difficulties
arise because of the nature of the partial-discharge pulses.
Since these pulses are extremely rapid, the peak magnitude
is difficult to determine at the slow oscilloscope sweep
speeds required to recognize partial discharges by their
phase position in the power frequency cycle, making the test
highly subjective.
A further drawback to the test is that, in practice, only
the magnitude of the highest pulses is recorded. Information
such as the number of pulses and the distribution of pulse
magnitudes, that is, the relative abundance of large pulses
compared to small pulses, can only be noted qualitatively.
Yet, significant information about the nature and extent of
insulation degradation must be present in the total pulse
pattern.
3. IMPROVED GENERATOR TESTS
Although the partial discharge test [14] is successful in
quickly predicting stator insulation condition, the above
limitations have restricted use of the test outside Ontario
Hydro. As a consequence, CEA and one manufacturer began
separately the development of more sophisticated procedures
for observing and quantifying partial discharge activity in
generator stator insulation systems.
The test improvements described below comprise better
methods of acquiring and treating partial discharge data
with permanently installed coupling devices. The coupler
or “antenna” is mounted on the rotor in one system, while
couplers are installed on the stator in the system developed
for CEA. Both coupling techniques respond to the highfrequency energy in an actual discharge. Means for reducing
the influence of electrical noise are incorporated into both
coupling techniques, thus permitting diagnostic testing while
the generator is operating normally. Methods for quantifying
the signals from either coupling system differ, although in
principle both are based on pulse magnitude analysis.
3.1. Stater-Mounted Coupling System
The partial discharge signals are acquired using rugged
high-voltage capacitors of 50 to 100 pF which are solidly
connected to the stator winding. The low-voltage sides
of these couplers are connected to a convenient location
external to the generator housing by terminated 50 ^l coaxial
signal cable. The couplers are sensitive only to the highfrequency components of a discharge pulse. The placing and
functioning of the couplers depends on whether the stator
winding is in a hydraulic or a turbine generator.
3.1.1. Hydraulic Generators
In hydraulic generators, the couplers are often placed at or
close to the connection point of the circuit ring bus to each
split or parallel of each phase in the winding. Since noise
pulses entering the generator from the power system are first
attenuated by surge capacitors and transformers and may
be further reduced by impedance mismatches as the pulse
travels along the circuit ring buses, a measure of external
two capacitors installed on a hydraulic generator with a
symmetrical winding.
Referring to Figure 1 (a), when a noise pulse enters the
winding, voltage pulses travel along the ring bus and reach
both couplers about 25 ns, say, after signal injection. Since
the response is the same at each coupler, if these two signals
are combined in a differential amplifier there will be no
output, at least not until pulse reflections within the generator
winding start to build up.
For partial discharges, which usually occur near the
high-voltage end of each parallel, a net response is obtained
since the signal reaches one coupler almost 50 ns before it
reaches the other coupler in the pair (Fig. 1 (b)). This system
works because the partial discharge pulse rise times are
typically only 10 ns, much less than the pulse travel times
along the transmission-line-like path of the circuit ring bus,
(see paragraph 2.4.5.)
Practical hydraulic-machine windings are rarely
symmetrical about the terminals. Howewer, by the careful
placement of the couplers and the use of delay lines, such
permanent “differential” couplers can be installed on the
majority of windings currently in use. More than twenty
installations have been made in a number of Canadian
utilities. These installations are usually on generators which
either are difficult to obtain for test purposes because of
outage restraints or are subject to a high degree of external
noise interference.
3.1.2. Turbo-Alternators
Because of the much smaller rotor radius involved, the type of
differential coupling system described above is not possible
for most turbo-alternators since the electrical length of circuit
Figure 1: (a) Schematic diagram showing the response to an external noise source of a differential amplifier with inputs from a pair of balanced permanent couplers
on two of the parallels. (b) This shows the response of the circuit to a partial discharge event within the winding.
noise immunity is inherently present.
Additional attenuation of noise, including power
frequency and solid-state d.c. exciter noise, is afforded by
connecting pairs of capacitive couplers to a differential
amplifier in such a way as to cancel common-mode signals,
taking into account the pulse travel time from the machine
terminal to each coupler. For example, Figure 1 shows
ring bus is often shorter than the discharge-pulse rise time.
Sensitivity to external noise can be reduced, however, by the
use of two permanently installed “directional” couplers per
phase on the output bus of the generator (Fig. 2). External
noise can be greatly attenuated by differential sensitivity to
the direction of pulse travel on the bus, that is, either from
the generator (assumed to be partial discharges) or from the
power system (assumed to be noise).
Retrofitting of the “directional coupler” can often be
readily implemented on Isolated Phase Bus, since the
inspection covers, which are regularly placed in the bus
sheath, can provide sufficient capacitive coupling when
grounded through a suitable impedance.
3.1.3. Electronic Analysis
Partial Discharge Analyzers (PDA) have been constructed
to process the voltage pulses from pairs of couplers into
information about the repetition rate and magnitude of
the discharges [15]. An analyzer consists of an 80-MHz
bandwidth differential amplifier (Fig. 1 and 2) driving a
single channel, dual polarity, pulse-height analyzer fabricated
with ECL integrated circuits. The pulse-height analyzer is
designed to handle generator partial discharges. Specifically,
it responds to pulse rise times of less than 10 ns, ignores
ringing, inhibits
Figure 2: Directional coupler configuration. Pulses from the external system
arrive simultaneously at the differential amplifier inputs, resulting in cancellation
; pulses from the winding arrive separated in time, resulting in a signal at V.
the counting of pulse overshoots such as the negative
overshoot of a positive pulse which can cause a false
indication in the negative channel, ignores reflections in
noise signals, and accepts consecutive discharge pulses more
than 3 psec. apart. This single-channel pulse-height analyzer
provides multichannel operation by sequential variation of
the threshold levels. Fifteen 100-mV-wide channels with
lower thresholds ranging from 100 mV to 1 500 mVhave
been found satisfactory for completely determining pulse
magnitude spectra.
The PDA is controlled by a microprocessor which
automatically steps the pulse height analyzer through the
15 voltage channels. The microprocessor also controls the
counters which total the number of positive and negative
pulses per second which occur in each channel and at the
same time supervises a digital printer which provides the
pulse magnitude spectra. Also produced is information on
the generator’s operating voltage at the time of test, which is
obtained from the 10-mV power frequency signal appearing
on the couplers’s output. The source coupler of the partial
discharges is identified automatically by comparing the
polarity of the discharges with the phase of the power
frequency voltage. Facilities are also included in the PDA
for analyzing data from temporary couplers. These include
requisite filters and a circuit which removes the very strong
interference caused by thyristor excitation systems.
Various versions of the PDA have been in use for more
than 4 years and improvements are constantly being made.
Several of the PDA’S described above have now been
commercially manufactured and are in routine use by a
number of Canadian utilities.
3.1.4. Test Results
Test data on many operating hydraulic generators have
shown that external noise and interference caused by
thyristor excitation systems are reduced by more than 20 dB
when the permanent couplers and the PDA are employed,
whereas sensitivity to generator insulation partial discharges
is maintained. Results are consistent with those obtained
by skilled personnel using the “conventional” test [14].
Particularly the magnitude of pulses corresponding to a
partial discharge repetition rate of about 10 Hz was found to
correlate well with the magnitude of the peak discharge pulse
observed from the oscilloscope trace in the conventional
test.
Figure 3 indicates pulse magnitude spectra observed
on two of the parallels of a modern 200-MVA hydraulic
generator. The stator winding in this machine has been
visually examined and the parallel corresponding to the line
on the right side of Figure 3 was found to be damaged by slot
discharge deterioration.
3.2. Rotor-Mounted Coupling System
A second system presently being developed has the ability
to determine the location of slot discharge activity in an
operating generator. Signals are processed on a slot by slot
basis, providing detailed indication of the amplitude and
location of partial discharge activity. The system also holds
the promise of differentiation between various types of
partial discharge activity because of the selectable frequency
discrimination incorporated in the signal processing
package.
3.2.1. System Description
The system, as shown in Figure 4, consists of a specially
designed “antenna” mounted on one or two field poles. This
antenna does not interfere with the operation of the generator.
As the antenna is carried by the rotor, a sample of discharge
activity is received from each slot. Every slot is sampled in
sequence during each revolution of the rotor. The antenna is
is the occurrence of energy exchange from the broad band
frequency spectrum in the actual discharge to specific
harmonically related frequency components. When a partial
discharge event occurs, the bars of the stator winding act as
tuned circuits, producing energy in the 20 to 100 MHz region.
Each bar acts as a structure for radiating electromagnetic
energy directly from the front of each slot in the winding.
These radiated signals are in the form of damped sine waves
lasting approximately 1 jusec. for each slot discharge event.
The higher harmonics tend to be restricted to the slot in
which the discharge occurs because of reflections from the
impedance discontinuity where the bar leaves the slot. Using
the higher frequency components allows the monitoring
system to resolve partial discharge activity on a slot by slot
basis.
Figure 3: Pulse magnitude spectra of the positive partial discharge pulses only,
from two parallels on one phase of a 200-MVA hydraulic generator.
located on a pole so as to maximize the probability of signal
reception by being correctly positioned in front of each slot
during the interval of the a.c. voltage cycle when discharges
are expected to take place. The antenna has a bandwidth of 10
to 100 MHz. The received signals are passed by a balanced,
matched transmission line to a specially designed matched
radio frequency strip-line slip-ring coupling system located
on the shaft below the rotor. These signals are then passed on
to signal-conditioning electronics for analysis. To identify a
specific source of slot discharge, a timing or synchronizing
track is applied to the generator shaft near the location of the
slip-ring coupler.
3.2.2. Theory of Operation
The fundamental principle upon which the system is based
3.2.3. Signal Processing and Analysis
The signal analysis electronics consists of a tuned radio
frequency amplifier, envelope detector, high-speed
logarithmic pulse amplifier and pulse stretcher. The timing
signals from the shaft report the exact position of the antenna
as a function of shaft rotation. Suitable counting circuits
allow pulses from each slot to be studied individually or,
alternatively, pulses from any or all slots may be displayed
together. The data from each slot is then processed by pulse
height analysis, giving statistical significance to the observed
results.
3.2.4. Test Results
Much of the work to date has been the solution of application
problems. Results of field tests so far indicate that the theory
upon which the system is based is generally correct. The
Figure 4: Schematic diagram of rotor-mounted partial discharge monitoring system.
full significance of many of the results is still not clear,
although it is certain that the data reveal much detail about
partial discharges which is unachievable by other detection
systems.
4. CONCLUSION
The two new systems described in this Paper illustrate the
possibility of low-cost on-line monitoring of partial discharge
activity in the stator windings of large generators. These tests
may be performed on machines in their normal operating
condition without an interruption to service, because they
incorporate noise rejection. Test results are based on the
direct measurement of partial discharge quantities, with
none of the “dilution” effects inherent in other techniques.
Both systems will identify the part of the winding, and one
system the exact slot where the most significant discharge
activity is located.
Future developments must strive for the establishment of
clear definitions of acceptable limits for damaging discharge
in different machines. In particular:
- It must be possible to positively identify high-intensity slot
discharge so that it can be eliminated without delay.
- The identification and measurement of surface or internal
insulation discharges will permit the monitoring of any
progressive increase in intensity throughout the life of
the winding, as an indicator of insulation aging. Probably
little can be done to reduce or eliminate internal discharge
activity so that the usefulness of monitoring this mechanism
lies mainly in making possible efficient scheduling of rewinding. Surface discharges may be reduced, when their
intensity warrants, by suitable maintenance procedures.
The greatest value from these diagnostic techniques will
be obtained only after data from many different generators
have been reviewed in relation to their design characteristics
and their operating and maintenance histories, and correlated
with the result of other more traditional diagnostic quality
assessments of the bar insulation itself.
To achieve this, continued close cooperation between
manufacturers and users is essential.
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Montreal, PQ, Canada).
Extrait de la Conference Internationale des Grands
Reseaux Electriques
Session de 1980