IN-SERVICE PARTIAL DISCHARGE TESTING OF GENERATOR INSULATION
M. Kurtz and G. C. Stone, Ontario Hydro, Research Division
Toronto, Canada
IEEE Trans. Electr. Insul, Vol EI-14 No 2, April 1979
ABSTRACT
Progressive degradation of high-voltage generator insulation,
whether due directly to the erosive effects of internal or slot
discharge or to other debilitating factors such as heat and
vibration, may be detected and monitored as a function of
time by in-situ partial-discharge tests. This paper describes
new diagnostic test techniques for assessing the condition of
generator insulation with a minimum of service interruption.
An improved means of coupling or data acquisition is
described, as well as a versatile single-channel pulse height
analyzer with characteristics adequate for a field application
of this type. Some typical test data are given. These studies
were sponsored by the Canadian Electrical Association and
are presented with their permission.
INTRODUCTION
Generator insulation systems, however perfect their initial
state, inevitably degrade in service. Heat, thermal cycling,
bar forces, vibration, mechanical shock, shrinkage of support
structure such as wedges and spacers, and the presence of a
high electric stress, act and interact to impair the integrity of
the dielectric system. At some stage, partial discharges will
start or begin to increase, providing an additional erosive
aging factor. This usually occurs at abraded outer shielding,
in cavities or delaminations, or in the end-turn structure.
The number, magnitude and distribution of the partial
discharge pulses can conceivably identify predominant aging
mechanisms. Changes in these quantities, as a function of
machine age, become a measure of cumulative insulation
deterioration.
“CONVENTIONAL” DIAGNOSTIC TEST
A relatively inexpensive technique for the detection and
display of partial-discharge pulses in generators has been
developed and used by Ontario Hydro for over 20 years for
the assessment of stator insulation condition [1]. In its present
form, portable discharge-free coupling units, fabricated from
approximately 2.5 m looped lengths of single conductor
27.6 kV concentric-neutral power cable are used, one for
each phase of the generator as shown in Figure 1.
The core conductors of the cables are connected to
the generator’s respective terminals at any conveniently
accessible point on the machine side of the disconnecting
switch. The cable shields, the “low” sides of these high
voltage coupling capacitors, are respectively connected, via
lengths of shielded coaxial cable, to three five-stage RC high
pass filters with a cut-off at about 30 kHz. After the machine
is brought up to rated speed and voltage, the filter outputs
are observed, together with a power-frequency trigger signal
taken from one of the filter inputs, on an oscilloscope with
a four-channel display as shown in Figure 2. The discharge
patterns in Figure 3 are typical of a new and an old winding.
Experience shows that as the magnitude of the highest partial
discharge pulse increases, damage to the stator insulation
also increases.
Fig. 1: Cable-type portable partial-discharge coupling capacitors being connected
to the three phase-conductors of a. machine prior to test.
Fig. 2: Typical display showing the partial discharges from three phases of a
machine and one power-frequency reference signal.
Tests are usually done with the machine (a) isolated from
the system, (b) synchronized at no load and (c) a full load.
In many cases, a skilled operator can distinguish between
pulses originating in the machine winding and external noise
entering from the power (?). Large differences between the
data for the (?) test modes may be caused by movement
of bars in response to dynamic forces as the load changes.
These movements result in a change in the contact resistance
between the bar shielding paint and the iron in the (?).
causing discharges in the slot to change. Such (?) are subject
to abrasion, cumulative damage to shielding, increasing slot
discharge with further (?). more loosening, more movement
and the beginning of a runaway condition to failure.
The major limitations of “conventional” partial-discharge
tests are:
1. Noise originating from external sources, whether from
other equipment in the station, or from the power system
in the synchronized-mode tests, tends to mask the desired
information. Such external noise can be as much as
an order of magnitude higher than the pulses related to
discharges in the machine. The phase position and the
general shape of the “external noise” pulses may be the
same as the “internal noise” pulses. (The pulse shapes are
indeed largely a characteristic of the generator, and the
coupling and filtering equipment characteristics.)
2. The discharge pulses, with rise times in the nano-second
range, usually do not recur at the same phase position in
successive cycles of power frequency. Estimation of the
highest pulse magnitude is difficult and test results tend
to be highly subjective, i.e., different operators report
different data. Capturing of the highest pulses with an
oscilloscope camera, even with fast film, is uncertain and
undependable.
Fig. 3: Partial discharge displays, one phase only, of asphaltic-mica insulated
windings. These are from two identical machines. The upper trace is from a 3
year old winding, the lower from a 24 year old winding.
3. Interruption to service is required. Though a Machine
need not necessarily be at standstill, it is desirable, from
safety considerations, to isolate the machine and open the
field breaker before connecting or removing the portable
coupling capacitors.
4. The portable couplers must be maintained in a safe
condition. Periodic high-voltage testing is required to
ensure that the couplers are discharge-free at 150% of
Maximum operating voltage.
To overcome these limitations the Canadian Electrical
Association undertook the sponsorship of the development
of a more objective technique making use of recent advances
in digital methods.
PULSE HEIGHT ANALYZER
Examination of the oscilloscope displays of discharge pulses
from different machines, insulated with a variety of systems
subject to different kinds and degrees of damage, discloses
subtle differences not easily characterized by quantities such
as maximum pulse height and the like, and often not readily
captured on film. There is clearly more information is the
pulse pattern than even a trained eye can realize. A method
of quantizing the data in the discharge pattern is required for
future correlation with Insulation condition.
A versatile dual-polarity single-channel pulse height
analyzer has been built to study the characteristics of discharge
pulses, particularly the uniqueness and repeatability of the
pulse height distributions. This Generator Corona Analyzer
(GCA) is designed specifically to accommodate the short
rise time and high repetition rate (multiple sites) of these
pulses and to be insensitive to “ringing”.
The GCA receives data from the high-pass filter Mentioned
above and counts the number of pulses during a one-second
interval which fall in the “window” between lower and upper
thresholds, manual 1’y variable from O.I to 1.4 V in 0.1 V
steps, with a 0.1 V interval between the thresholds. (These
values were selected after some experience with the first
prototype circuit which permitted continuous independent
variation of these thresholds.) A manual scan with a threshold
selector switch permits a complete pulse height analysis in
about 2 minutes. A “phase window”, adjustable in width and
position relative to the power frequency cycle, allows pulse
counts to be made in any desired portion of the cycle.
Fig. 4: Simplified schematic of analyzer.
of the output pulse from B Bust be set greater than the sum
of A’s output width and the signal rise time. This concept has
been used elsewhere [2].
A cross-inhibit feature between the above positive-pulse
channel and a similar negative-pulse channel, prevents either
channel from functioning for 3 us after the first channel has
been triggered, to avoid false counts due to opposite polarity
voltage swings in the partial discharge pulse. (Individual
partial discharge pulses are oscillatory, generally with a
duration of less than 1 us. Observation of partial discharges
on many different machines indicates that two consecutive
pulses are rarely less than 10µs apart.) In practice, the circuit
requires an overdrive of 20 mV for 25 ns to count a pulse,
and pulses separated by 5 us or more can be resolved.
Fig. 6: Typical GCA Pulse Height Distribution, Mountain Chute GS, Unit 2,
Blue Phase.
Fig. 5: Sequence of analyzer’s operation. Only pulses greater than the tower
threshold produce trigger signals.
The circuit uses two types of high-speed TTL 1C logic
devices: fast comparators and “non-retriggerable, edgetriggered” monostables which can also be inhibited by an
appropriate logic signal.
The operation of the magnitude window for the positivepositive-pulse channel may be explained with reference
to Figure 4. Monostables A and B are only triggered by a
positive pulse. C is triggered only when both the inhibit
signal is removed and a negative wave front appears at the
trigger input.
The sequence of operations for a pulse with a peak
magnitude falling within the window is shown in Figure
5(a). The pulse triggers an output from comparator A only,
while the inverted output from mono-stable B remains high
since it was not triggered, thus failing to inhibit C. When A
returns to the low level, monostable C is triggered by the
negative front, its output activating a pulse counter. Until
non-retriggerable monostable A returns to the low level, it
cannot perceive input signals, thus ignoring ringing for a
preselected time, in this case, 3µs.
When the input pulse exceeds the upper threshold, as
shown in Figure 5(b), both A and B are triggered. Since B’s
output is now low, C is inhibited from triggering when A’s
output drops; hence the counter is not activated. The width
It is interesting to note that the magnitudes of pulses in the 10
to 15 counts per second region correspond to the maximum
pulse magnitudes observed in the “conventional” test.
NOISE CORRECTION
From basic considerations, partial discharges occurring
during the rising portion of the applied high voltage wave,
are detected as negative pulses, and vice versa. The GCA
phase window may be adjusted so that in a sequence of
pulse counts, one may record the distribution of positive
and negative pulses in the rising and falling portions of the
power frequency cycle, respectively. On the assumption that
unwanted noise is more or less uniformly distributed in both
half cycles, a net distribution may be derived which ore truly
represents the discharge situation in the insulation system.
Typical data obtained in this way are shown in Figures 6
and 7. A calibration procedure to be described later, permits
conversion from a mV to a pC base.
Correction for transient noise occurring at the time of
measurement is achieved by a simultaneous display, renewed
every second, of the count of a selected pulse polarity in both
the rising and falling portions of the power frequency cycle,
both counted in the same one second interval.
This noise correction technique may be only partially
effective because the external noise (discharges on the
incoming transmission lines, transformers, bus work) tends
to be phase-oriented in exactly the same way as the generator
insulation discharge pulses. Yet one invariably finds that
the count in the “expected” polarity far exceeds the other.
A more effective approach to noise elimination is described
below.
COUPLER CONSTRUCTION
Essentially high-voltage power cable splice materials and
techniques are employed. A sufficient thickness of selfamalgamating insulating tape to insulate for 4 kV (about 2
mm) is applied first over the jumper insulation for a length
limited only by the local geometry (typically 20 cm). Over
this insulation, a semi-conductive, self-amalgamating tape
and tinned copper braid sandwich provides a discharge-free
capacitor electrode. The capacitance of the coupler is adjusted
to match symmetrical pairs to within about 2%. Depending
on the jumper length, a typical coupler capacitance will be
in the 70 to 90 pF range. The centre conductor of a coaxial cable (RGS8C/U) is soldered to the braid and securely
fastened in place, as shown in Figure 9. The shield is
removed to a point beyond the braid and left open at this end.
Insulating tape is applied overall for mechanical security and
a final finish layer similar to that normally used over jumpers
may be used.
Fig. 7: GCA Pulse Height Distribution, Barrett Chute GS, Unit 4, Red Phase.
PERMANENT COUPLING CAPACITORS
Permanent capacitive coupling devices may be installed in the
winding itself, on the circuit ring bus, or on suitable jumpers
between coils or coil groups. Such couplers permit observation
of the discharge condition in a machine in the presence of
external noise, and with no interruptions to service, though
some load variation during tests my be advantageous to
search for loose bars. These may be fabricated quite simply,
at least on hydraulic units, by applying a conductive layer
over the bus or jumper insulation in a convenient location
and providing for connection to detection equipment.
If the permanent couplers are placed at corresponding
points in a pair of parallels or “splits”, equidistant from the
machine terminal, “differential” noise rejection is possible.
(In cases where the circuit ring bus lengths are unequal, the
difference may be compensated by suitable delay lines to the
differential amplifiers.) A differential amplifier as shown in
Figure S will eliminate not only external or common-mode
noise, but also the power frequency and its harmonics, so
that a filter with its associated signal distortion is no longer
required.
It may be noted that pulses from a third split, entering
via the circuit ring bus, may be rejected like common-mode
noise. Thus in order to survey a machine winding completely,
every split must be equipped with a permanent coupler.
Couplers were installed initially on the six splits in one
phase of a 13.8 kV hydraulic machine. These were placed on
coil group jumpers half way down the winding between line
and neutral where the jumper conductor operates at about 4
kV to ground at 60 Hz
Fig. 8: Schematic of permanent couplers in machine splits.
The co-axial cables from all installed couplers are trained and
secured to a common junction box, external to the machine,
where each cable is terminated with a 50-ohm resistor and
connected to a BNC connector. This resistor eliminates
reflections and keeps the 60 Hz potential on the coupler and
cable down to less than a volt. It is important to note that the
cables from symmetrical coupler pairs must be matched in
length to better than 1 m.
PERMANENT COUPLER EVALUATION
Tests on generators with permanent couplers demonstrated
that pulses applied at the machine terminal were rejected in
the differential mode and could not be detected. However,
pulses injected near one of the couplers in a pair could be
observed directly on an oscilloscope, chiefly because the
difference in arrival time (50 ns) at the two couplers, and
hence at the differential amplifier inputs, resulted in a net
signal. Note that the polarity of an observed pulse depends
on the source split as well as its phase relative to power
frequency.
Effective noise rejection and correlation with conventional
techniques were demonstrated with permanent couplers
installed on four machines.
Inherent in the Use of the permanent couplers on all splits
is the capability, by observing pulse polarities, of identifying
the location of the source of anomalously high pulses to
within possibly a few coils at the line end of a single split.
GCA. Some 60 machines have been tested with this technique
to date, though the opportunity has not yet arisen to make
measurements on a machine before and after maintenance,
such as re-wedging or the like. Good correspondence has
been noted between the new and the “conventional” test
data.
The “conventional” test, using portable couplers and the
oscilloscope display, has correctly identified windings in
the process of deterioration. Test data have ranged from 10
to 20 mV on machines considered to be in good running
condition, to 50 mV to 500 mV (2000 to 5000 pC, depending
on individual machine Vibrations) for machines in trouble.
One large hydraulic generator suffering from severe ground
wall erosion failed within a few months of the 50 mV
measurement.
Fig. 9: Unfinished permanent coupler installed on jumper in generator winding.
Kote the co-axial cable attachment.
CALIBRATION
The conversion of test data in terms of millivolts of
deflection on an oscilloscope screen, into apparent charge in
picocoulombs is of paramount importance for rendering the
data “independent” of the detection equipment parameters,
machine size and the loading effects of connected bus, etc.
For this purpose, it is necessary to inject a known charge
at the machine terminals and to note the response on the
detection equipment.
A simple means of injecting such a pulse is achieved by
isolating about 15 cm at one end of the shield on a portable
discharge coupler by separating the concentric neutral
conductors and cutting a I mm gap in the extruded shield
of the cable. This provides a 30-pF high-voltage coupling
capacitance through which a suitable square wave may be
supplied via a length of co-axial cable, for calibration [3].
A 50-Q terminating resistor, connected to the co-axial cable
at the coupling capacitor, limits the 60 Hz rise in potential
of the HV cable shield to the order of 4 mV for a 13.8 kV
generator winding, and eliminates reflections.
A typical input square wave and output response is shown
in Figure 10. A 200 ns rise time was selected since faster
fronts produced unacceptable, ringing in the response.
Typical calibrations obtained to date range from 10 to 35 pC
per mV, depending on the machine size. A pulse generator at
a repetition frequency of 5 kHz, capable of driving 50 Ω to
20 V, has been built into the GCA and may also be used to
check its operation.
TEST RESULTS
Pulse height distribution data is being collected using the
Fig. 10: Calibration of pulse voltage magnitudes into picocoulombs. The lower
trace shows the front of a negative square wave which is applied through a 50 pF
ideating capacitor. The response through the filter is shown in the upper trace.
The injected charge, about 200 pC, yields a calibration in this case, of 20 pC/mV.
For several machines, maximum pulse height has been
decreased by an order of magnitude following treatment such
as slot wedge tightening, re-finishing of semi-conductive
slot paint, injection of semi-conductive silicone rubber in
slot sides, etc. [4].
CONCLUSIONS AND FUTURE WORK
The goal of a more objective and reliable test method with
reduced system cost and independence from the interfering
effects of external noise has been achieved. Further
refinement and time reduction for the test is possible using
a multichannel analyzer. A commercial analyzer has been
modified to accept pulses at intervals down to about 10
us, yielding test curves identical to those obtained with the
single-channel Generator Corona Analyzer, but in a much
shorter time.
A prototype system has been demonstrated using a
programmable calculator and a fast digitally controllable
counter for the permanent installation and continuous
pre-programmed periodic monitoring of discharge pulse
distribution as a function of load and time.
Now that data may be obtained in a consistent and
repeatable manner, the enormous task remains of collecting
data from the field and establishing correlations with
observed insulation damage in service as a function
probably of the type of insulation system in question. When
such correlations are confirmed, early warning of incipient
problems will hopefully be obtainable so that suitable repair
and maintenance work may be scheduled.
REFERENCES
[1] M. Kurtz, “A Partial Discharge Test for Generator
Insulation,” Ontario Hydro Research Quarterly, Vol. 23, No
4, 1973, pp 1-4.
[2] H. A. Cole, “A Differential Pulse Height Discriminator,”
Nuclear Instrumentation and Methods, Nov. 1976, p 551.
[3] “Detection and Measurement of Discharge (Corona)
Pulses in Evaluation of Insulation Systems, ASTM 0 1868,
Part 39.
[4] M. Kurtz and J. F. Lyies “Generator Insulation Diagnostic
Testing”, IEEE Paper So. F7918S-0, PES “Winter Meeting,
New York, N. Y., Feb. 1979.
This paper was presented at the 1978 International
Symposium on Electrical Insulation, Philadelphia, June
1978.
Manuscript was received 11 August 1378, in revised form 30
November 1978.