Interpretation of Partial Discharge Patterns
I J Kemp
The electrical detection of individual partial discharges from high power electrical
plant insulation has p r o v e d to be a n e f f e c t i v e m e t h o d of monitoring a stress
condition which c a n cause severe degradation a n d possibly failure of plant
insulation.
In addition, such measurements can provide a basis for assessing the
overall integrity of t h e insulating system since degradation of t h e system,
irrespective of t h e p~articular causative stress, almost always results in partial
discharge activity. T h e n a t u r e of t h e discharge pattern can provide valuable
information on the form and extent of degradation and, in some instances, on its
location.
The discharge pattern produced by individual discharge events is generally recognised
as comprising the amplitude of individual discharge events, the number of discharge
pulses per power cycle and the distribution of these pulses within the power cycle ie
their phase relationship.
In addition to the discharge pattern produced by these
parameters at a given time and under a given applied electrical stress, one would
normally also be interested in any changes which occur in the pattern as a function
of the magnitude of applied electrical stress and time of application.
Armed with
this information, it may be possible to evaluate the nature of the degradation sites
and thereby provide an assessment
of insulation integrity.
However, although the
proceeding
paragraphs
might imply that this process
can be relatively
s t r a i g h t f o r w a r d , i n practice a variety of factors conspire to make interpretations
based on such patterns a highly complex affair.
Of these factors, the major culprits are external interference and the complexity of
the discharging insulating system.
External interference can generally be minimised
through the use of in-line filters and some form of discrimination circuit if the
problem is in the high voltage line.
I f the problem is airborne interference, e.g.
of noise
from rectifiers, it can be more difficult to eliminate but an assessment
activity and its characterisation can be made prior to the high voltage insulation
test.
In relation to the complexity of the discharging insulating system, this is
generally beyond one’s control.
The problem, in this case, lies in the vast number
of discharging sites and their variety.
This combination tends to swamp out the
characteristic p a t t e r n s associated w i t h specific discharge s i t e conditions. Having
indicated these caveats however, there is no doubt that the information contained in
a discharge pattern can be extremely useful in assessing insulation integrity.
Another aspect of pattern interpretation which should be noted at this stage since it
is not fully appreciated, is the importance of regular measurements on a given
insulating system. Ideally, patterns should be obtained a t regular intervals
throughout the life of the insulating system since there is no doubt that the trend
in the discharge pattern of a given insulating system with time provides far more
useful information on insulation integrity than any measurement at only one point in
time. Although it is always possible to cite examples where a single measurement can
be extremely beneficial, in general t h e r e a r e many situations w h e r e it provides
relatively little information.
As a diagnostician (be i t of plant or human beings)
once o n e has established that the absolute levels of t h e vital characteristic
parameters d o not indicate the imminent death of the patient,
one is primarily
interested in the g& at which these characteristic parameters are changing through
life in comparison with similar systems (or humans) of comparable design and stress
history.
I J Kemp, School of Engineering, Glasgow College
In interpreting the discharge pattern, one would normally start by considering the
location of the discharges on the power cycle waveform. Cavity-type discharge sites
generally yield a discharge pattern within which most pulses are in advance of the
voltage peaks i.e. quadrants one and three.
In contrast, discharge sites containing
a sharp metal surface generally produce a pattern with pulses symmetrically spaced on
both sides of the voltage peak(s).
C o n s i d e r i n g firstly t h e c a v i t y - t y p e discharge sites, one would next consider t h e
relative magnitudes of discharges on the positive and negative half cycles of applied
voltage. Broadly, similar magnitudes on both half cycles imply that both ends of the
discharge are in contact with similar physical surfaces ie insulating surfaces.
This
c o n t r a s t s w i t h t h e s i t u a t i o n which is prevalent if t h e p a t t e r n i n d i c a t e s d i f f e r e n t
magnitudes of pulses on the two half cycles.
In this instance, one would assume
that the discharge was active between a metallic and an insulating surface.
The
reasons for the variation in activity on each half cycle relate to the variation in
physical properties of a metal and an insulating surface - and specifically to the
ease with which a metal can supply initiatory electrons when the necessary cross-gap
stress is reached to produce a spark and the time which a n insulator takes to
dissipate s u r f a c e c h a r g e ( p o t e n t i a l ) b u i l d - u p c.f. a metal following a discharge
event.
H a v i n g d e c i d e d w h e t h e r the c a v i t y - t y p e discharge site(s) a r e i n s u l a t i n g - b o u n d o r
insulating-metallic, one would next consider the variation in d i s c h a r g e magnitude
with test voltage and then with the time of application of voltage.
If the discharge
magnitudes remain constant with increasing test voltage, this tends to imply that one
is observing discharge activity within fixed cavity dimensions.
This suggests either
a most unusual condition where all the cavities are similarly dimensioned or, far
more probably, one is observing a single cavity situation - possibly of quite large
s u r f a c e dimensions.
This contrasts with the condition in which t h e discharge
magnitudes of the pattern rise with test voltage.
In this instance, one is generally
observing cavities, or gaps between insulating surfaces, of differing size.
As the
test voltage is i n c r e a s e d , in a d d i t i o n t o those cavities of low i n c e p t i o n voltage
d i s c h a r g i n g more o f t e n p e r cycle, cavities of higher i n c e p t i o n voltage a r e also
starting to discharge.
In this case one would be thinking of internal discharges in
a number of insulation-bound cavities of different size, external discharges across a
varying length gas gap ( b e t w e e n , f o r example, two t o u c h i n g insulated radial
conductors) or perhaps even surface discharges at an area of high tangential stress.
Having decided that one is dealing with a cavity-type degradation condition, that the
c a v i t y s u r f a c e s a r e e i t h e r insulation- bound o r metallic-insulation b o u n d , and
determined something of the cavity size/number distribution, observing the pattern at
a fixed voltage over a period of time may yield still more information. For example,
if the activity tends to lessen with time then, depending on the insulation system
under investigation, one might be observing a pressure increase in cavities (caused
by gaseous bi-product formation and resulting in a higher breakdown voltage for
given cavity dimensions), a build-up of surface charge within the cavity structure
( t h u s i n h i b i t i n g t h e realisation of s u f f i c i e n t potential d r o p across t h e c a v i t y to
p r o d u c e f u r t h e r d i s c h a r g e s following discharge activity and potential equalisation)
o r p e r h a p s a b u i l d - u p of water or a c i d s w i t h i n the cavity s t r u c t u r e (increasing
surface conductivity and allowing charge to leak away).
However, if, for example,
the discharge activity is observed to increase rapidly with time, one would probably
be observing the initiation of many new discharge sites which, for constant applied
voltage, would most commonly be due to internal discharges in gas bubbles in an
insulating liquid (the bubbles increase in size and number rapidly with discharge
activity). This could be confirmed by removing the electrical stress for a period of
time, thus permitting the bubbles, if present, to disssolve in the liquid, which, in
turn, would return the discharge pattern to its original condition.
4/2
The preceeding discussion has largely been related to cavity- type degradation sites identified by the location of discharges primarily in advance of the voltage peaks however, the same approach holds true for gaps with a sharp metallic boundary. For
example, depending on the numbers of discharges, their spacing and magnitude on one
half cycle compared with the other, one should be able to say something about the
relative sharpness of any points.
(This relates to the much higher electric stress
realised around a sharp point for a given applied voltage than that at a plane
s u r f a c e a n d t h e related s p a r k activity when the point a n d then the plane a r e
alternatively the cathode within a cycle of applied voltage).
There are many more conditions which could be discussed within this paper relating
discharge m a n i t u d e , phase d i s t r i b u t i o n , test voltage a n d time of application to
specific conditions.
However, it is not the purpose of this paper to provide a
comprehensive guide of this type.
Other papers within the literature are available
f o r this purpose.
R a t h e r , i t is to indicate to those unfamiliar to the
partial discharge patterns an effective and efficient way forward
interpretation of
in their use.
Although it may be sufficient to either memorise or have available a
"look-up'' table of specific discharge pattern characteristics from which one can make
a n interpretation, one will have a much greater possibility of making an accurate
interpretation if one understands the physical and chemical processes
which can
produce a given discharge pattern or result in a change in discharge pattern and can
relate these to t h e specific f o r m a n d design of the insulation system u n d e r
investigation.
Again, the importance of establishing a trend in the discharge pattern for a given
insulating system over its lifetime cannot be stressed too strongly.
In general, the
particular d e g r a d a t i o n characteristics a r e o f t e n developed relatively early in t h e
life of an insulating system.
Therafter, the parameters of primary importance are
the absolute discharge magnitude and, more importantly, its rate of rise over given
time periods - both relative to the values obtained from other insulating systems of
similar design and history.
Finally, most measurements of discharge patterns have, to date, been made from an
oscilloscopic trace - usually with the discharge pulses superimposed on the applied
This approach has made the accurate and
power cycle displayed as an ellipse.
complete recording of a discharge pattern in a form which is easily accessed and
retreaved extremely difficult. However, digital acquisition, storage and processing
of discharge pulses is now reasonably well established and a number of laboratorybased instruments a n d commercial detectors utilising this approach have been
developed.
These provide a method of accurately and objectively acquiring all of the
data available from a given discharge pattern, of storing this data in a convenient
way and of processing the data to provide such relationships as required by the user.
Pulse h e i g h t / p h a s e distributions w i t h time a n d pulse h e i g h t / p h a s e / n u m b e r
distributions at a given time are becoming increasingly popular ways of representing
this data - all gathered over a statistically valid number of power cycles rather
than a t o n e "frozen" m o m e n t in time.
More importantly, pattern recognition
algorithms c a n b e used t o i d e n t i f y subtle changes in pattern with ageing, t h u s
opening new avenues to discharge interpretation.
For example, recent work has
suggested a possible relationship between the onset of treeing (compared with cavity
degradation) in insulation with a change in the skewness of the discharge pulse
height/phase distribution.
T h e a d v e n t o f these i n s t r u m e n t s heralds a new phase in the interpretation of
discharge patterns.
Their power may hold the key to that most difficult of locks the accurate, reliable and objective assessment
of insulation integrity and remnant
life.
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