Partial Discharge-Part X:
PD in Gas-Insulated Substations
-Measurement and Practical
Considerations
R. Baumgartner, B. Fruth, W. Lanz, and K. Pettersson, A B B Switzerland
his second article in a two-part series on PD
testing of gas-insulated substations (GIS)
provides an overview of engineeringpractice related to partial discharge (PD) measurements in GIs.
The previous article [l]discussed fundamental aspects
of I'D phenomena and measurements in solids and
gases as relevant to this topic. This article will address
the following questions:
Why measure PD in CIS?
In which phases of the life cycle should PD be
measured?
What PD measurements are presently available
and what results can be expected under various
measurement conditions?
The purpose of PD testing is not to improve the
quality of the GIs, since quality and reliability cannot
be "tested" into a product. A weak design will not be
improved by extensive testing or by the introduction of
new testing methods. Rather, the reason for testing is to
ensure that properly designed and potentially reliable
GIS is manufactured, packed, shipped, and assembled
to a standard which assures a reliable operation over the
design life of the equipment.
The application of high voltages for testing purposes
can either degrade or improve the insulation system
under test. When the only criterion for acceptance is the
absence of flashovers, such testing could be characterized as a "brute force" method which risks damaging
the equipment with little likelihood of revealing important information concerning the state of the insulation
system. Flashover can be highly probabilistic, so that
flashover may not occur even in the presence of a defect,
and if flashover does occur, it can damage the insulation
system and produce transient overvoltages which
cause secondary flashovers which may not be detected
but could impair dielectric integrity.
In contrast to simple high voltage testing, PD testing
is normally non-destructive, even in the case of intolerable PD activity. As such, PD testing is well suited
T
New partial discharge measurement procedures will make possible less risky, more
selective tests to assess the condition of high
voltage insulation in GIs.
to diagnostic testing of GIs. However, PD testing requires additional effort which comes at an expense.
Thus the cost effectiveness of PD testing must be
demonstrated, and this depends both on any additional
capital costs for built-in sensors which may be required
and the time required to conduct the tests.
Typical defects which result from errors in manufacturing, shipping, and assembly include loose or electri-
2 ns
Fig. 1. Typical partial discharge waveform from a positively
charged, 5-mm long particle fixed to one electrode of a
system which produces a "background field" (the field in
absence of the particle) of 3.5 kVlmm (peak) in sF6 at 400
kPa (45 psig).
0883-7554/92/$3.00 0 1992 IEEE
16
January/February 1992-Vol.8,No.1
IEEE Electrical Insulation Magazine
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
Table I
SUMMARY OF POSSIBLE PD-INDUCING DEFECTS IN GIS
1
Detectability
Kind of imperfection
Moving particles
The impact of a free conducting particle on the enclosure or on an insulating surface
causes an easily detectable acoustic signal. Essentially all types of dangerous particles
can be detected. Electrical signals are typically in the range of 2 to 10 pC.
Electrode Protrusions
A corona from a protrusion on an electrode generates a pressure wave in the sF6 which
propagates to the enclosure in which it generates an acoustic signal which can be
detected by an acoustic sensor. If the sensor is near the defect, sensitivity is generally
better than 2 pC. Electrical detection is also effective.
Fixed Particles on
Insulating Surfaces
Such defects can be detected if they produce corona. Sensitivity is generally in the range
of 2 p c .
"Floating Electrodes"
The large partial discharge between a floating electrode and an adjacent electrode
produces acoustic pressure waves of much greater energy than corona discharges and is
thus easily detected using acoustic sensors. Electricaldetection is also very simple as a
result of a PD magnitude which ranges up to 1,000,000 pC.
Loose, Non-floating
Electrodes
Such a defect, for example a loose corona shield, usually generates PD pulses which are
correlated to twice the frequency of the test voltage. Acoustic signals propagate from the
defect to the enclosure where they can be detected.
1I Voids
in Solid Insulation,
Delaminations
Filled epoxy absorbs high frequency acoustic energy strongly, so that acoustic partial
discharge detection is not very effective for detecting voids or delaminations in solid
dielectric components. The PD magnitude can range from fC to pC, depending on the
size and position of the void. Electical detection is generally effective for significant
defects.
cally floating corona shields, forgotten tools, scratches,
and poor electrical contacts (Table I).
Little PD activity can be tolerated in GIs, as PD
causes SF6 decomposition which results in corrosion
and eventual degradation of dielectric surfaces. The
precise level of tolerable PD activity remains a subject
of research; however, past experience indicates that the
small amount of PD activity from free conducting particles which survive high voltage testing process is
harmless, while strong PD activity from a floating component will result in failure in a matter of weeks to
months. Properly designed GIS exhibitsvery low levels
of PD, typically associated with a few remaining free
conducting particles which, if detectable,exhibit a very
characteristicPD signature. Such apparatus has proved
highly reliable, which indicates that such PD is not a
major problem in well-designed GIs.
PD Measurements in CIS
Partial discharges in sF6 generate extremely short
electrical pulses with a duration in the ns range (Fig. 1).
The frequency spectrum (amplitude density spectrum)
of such a pulse can be estimated by assuming that the
pulse has a Gaussian shape [2]. Assuming a pulse width
of 1 ns, the amplitude spectrum starts at DC and has a
-3 dB point of 320 MHz. At 960 MHz, the amplitude
IEEE Electrical Insulation Magazine
spectrum is down to 1%of the DC value. Quantitative
PD evaluation can be performed with measuring equipment which integrates the whole or part of the spectrum
to generate a voltage whrch is proportional to the apparent charge.
The pulse represented in Fig. 1 was measured in a
special experimental setup which allows the current
during the I'D to be measured directly by means of a
shunt resistor. In the case of a GIs, the outside observer
has no direct access to the location of the PD process.
He can only see a I'D-induced apparent charge transfer
between the test specimen and the high voltage power
supply. The apparent charge is substantially smaller
than the charge involved in the PD [2].
A PD event in GIS can be modelled by a current
source which injects a charge pulse into the GIS conductor to generate a travelling wave which propagates
away from the point of injection in each direction of the
GIS bus duct. The transmission line impedance (Z),
which gives the relationship between the travelling
current and voltage waves, is typically between 45 R
and 70 C
2 for a GIS bus duct. As the current injected is
split into two directions,the voltage wave to be detected
using a capacitive pickup near the enclosure is V = Z M
[3-71.
The busduct of the GIS branches many times
throughout the substation. When the travelling wave
January/February 1992-Vol.8,No.1
- ~-
17
____
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
generated by a PD reaches such a junction (for example
a "T"), it encounters an impedance mismatch which
results in the pulse being partly reflected and partly
transmitted (refracted)into the two branches of the "T."
This results in a reduction in the signal magnitude
detectable at the PD pickup. It can also cause the signal
to reach the pickup by many paths with arrival at
different times as measured on a ns scale. As a result of
all these effects, an observer at some distance from the
PD site will not observe a clean PD pulse. Nevertheless,
quantitative information can still be derived through
integration of the measured PD-induced voltage.
Electromagnetic interference (EMI) caused by surrounding high voltage equipment, radio transmitters,
etc. adversely affects the PD measurement sensitivity
and is one of the critical parameters which must be
evaluated.
GIS offers the unique feature of allowing a fully
encapsulated test configuration when a gas-insulated
test transformer and coupling capacitor are used in
conjunction with careful filtering of the voltage source.
This technique is primarily of value for factory testing
of shipping units and site testing of CIS with cable
connections. During commissioning tests of GIS which
includes SF6 to air bushings, the bushings act as antennas. As well, connection of the test transformerthrough
the bushings may be more convenient than costly
provision for a metal-enclosed test transformer. Fortunately, in the region of 100 MHz and higher, reasonably wide frequency bands can be found with little radio
interference at frequencies low enough that the ID
' still
has substantial energy. Using appropriate frequency
transformation and filtering techniques, quantitative
PD measurements can be carried out on site at little
additional cost to the high voltage commissioning program. Moreover, the phase relation of the PD signals can
be exploited for noise discrimination purposes.
The Use of Large Coupling Capacitors
For quality assurance on shipping units in the factory, tests are carried out using straight detection according to IEC 270 [7] with coupling capacitors around
1nF. l h s method uses the lower end of the PD-induced
frequency spectrum, up to some 100 kHz. As a result of
the large coupling capacitor, a high signal level is available for subsequent integration and readout. External
noise and interference are small because the tests can be
carried out with fully encapsulated apparatus. The
detection sensitivity in practice is better than 2 pC apparent charge.
Built-in PD Coupling Sensor
The use of a large gas-insulated coupling capacitor is
not convenient during on-site PD measurements because of the substantial weight, high reactive power
18
requirement, and difficult handling. Instead, permanently installed electric field sensors in the GIS are
preferred for quantitative electrical PD measurement.
Two sensors per phase is adequate, and the cost is low
as a result of the simple sensor design employed. The
design, function, and calibration procedure of such an
electric field sensor is well described in [5,6]. The sensor
consists of a metallic plate some 10 cm (4 inches) in
diameter which is insulated from the GIS enclosure.
Figure 2 shows a schematic for a sensor designed to be
fitted in a standard flange cover of a GIs. The high
voltage capacitance (typically Cz = 0.2 pF) is formed
between the GIS conductor and the sensor plate. The
low voltage capacitance is formed by the stray
capacitance of the plate to the enclosure (typically C2 =
20 pF). If the connection to the sensor is of very high
impedance, the sensor acts as a divider with a ratio of
about 1OO:l. If load on the sensor has a resistive impedance A, then the sensor acts as a high pass filter with
a cut off frequency of fc = I h n (cl
~ + cZ) = 8~109/fifor the
above parameters, so that when X = 50Q,fc = 160 MHz.
This high pass characteristic both attenuates the power
frequency component from the divider and minimizes
electromagnetic interference at lower frequencies. In
particular, corona in air generally generates longer,
lower risetime pulses than PD in SF6, so that the high
pass characteristic of the sensor reduces interference
caused by corona in air from the bushings and high
voltage test set.
A calibration in terms of apparent PD charge is useful, and this can be achieved by injecting a defined
charge with approximatelythe pulse waveform of a PD
pulse by means of one sensor and detecting the signal
generated using the other sensor on the same phase [8].
A voltage step of amplitude "UI" is applied to the
sensor from a mercury-wetted relay pulse generator
through a cable which is terminated with its characteristic impedance at the sensor input. A current impulse with a total charge q = UC1 is injected on the GIS
conductor through the C1= 0.2 pF capacitance between
the sensor and conductor.
This method provides the response function of the
total detection chain to a PD in SF6, as it involves all
stages from signal capture to the evaluation of the apparent charge by integration and readout. Correct
measurementsrequire amplitude linearity in the measurement channel. In case of doubt, calibration should
be carried out with the same order of magnitude signal
as expected during the measurement.
Time-Resolved Measurements
In principle, a PD source can be located by evaluating
the relative time of arrival of the two PD-induced pulses
which propagate in opposite directions away from the
PD source. Although this is feasible in well-defined
geometries such as gas-insulated transmission line 191,
JanuaryFebruary 1992-Vol.8,No.1
IEEE Electrical Insulation Magazine
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
the method is very difficult to apply in real GIS unless
large numbers of sensors are built in [6,10-121. Other
methods of PD location are more easily employed.
Apparent Charge Evaluation in the
100 - 300 MHz Range
The short duration of a I'D pulse in GIS and the
resulting frequency domain bandwidth which extends
to 300 MHz (-3 dB) can be used to reduce the effects of
electromagnetic interference through the use of a
capacitive sensor with a high pass characteristic adjusted to suppress frequencies in which the strongest
interference occurs. The voltage signal at point A (Fig.
2) is further processed with a frequency transformation
circuit to bring the frequency to a region which can be
further processed by standard PD measurement apparatus (some 100 kHz). The next functional block is a
bandpass filter which integrates the signal to provide a
voltage which is proportional to the apparent charge.
The last functional block in the chain is the interface to
the observer. It must give a representation of the apparent charge of each I'D pulse.
Apeak-readingvolt meter provides an accurate reading if the time constant is adjusted to avoid effects of
superposition of successive impulses.A meter needle is
very easy to observe during a test of short duration. If
the phase angle at which the PD occurs must be observed, an oscilloscope with x-deflection synchronized
to the test voltage is often employed. The height of the
superimposed pulses on the y-axis is a measure of the
apparent charge magnitude. Any shift in the phase
angle between I'D onset and discharge at higher voltages can readily be observed. If more detailed statistics
are required, the apparent charge signal can be
processed with a Phase Resolved Partial Discharge
Analyzer (PRPDA) 1131. In such a system, a computer
stores the amplitude of the I'D signal along with the
time during the test and the electrical phase angle at
which it occurs. These data can be presented as the
number of events of a given amplitude vs the phase
angle. Figure 3 shows a partial discharge pattern of
x-ray induced I'D in a spherical cavity. The large number of data (pulses) produces the characteristic pattems
in the PRPDA representation which can help to characterize defects, as discussed in the last article of the series
Ill.
UHF Partial Discharge Detection
As noted above, near the source of a PD "event," the
spectral energy in a PD pulse starts to fall off at about
300 MHz but extends to beyond 1000 MHz. However,
at frequencies above 100 MHz, the signal attenuation is
fairly high as energy can be coupled into higher order
propagation modes (other than TEM,,) which suffer
from high attenuation. These characteristics have been
used to design a very sensitive frequency domain I'D
detection scheme for GIS which detects PD-stimulated
resonances within the GIS in the 1000 MHz frequency
region, where almost no interference occurs [lo-121.The
fact that detection takes place in the frequency domain
has the advantage that sources of interference, such as
radio transmitters, show up at well defined frequencies
which can be determined prior to connecting the instrumentation to the GIs. The primary disadvantage of
the technique is the attenuation of the PD signal at these
very high frequencies, which is in the range of 2 dB/m
I -
t-
0
I
I
50 R cable
50 R
4"
Fig. 2. PD detection and quantitative evaluation in GIS with electric field sensor. (1)Electric field sensor. (2) Calibrationsetup.
(3) Frequency transformation circuit from 200 MHz to some 100 kHz. (4) Bandpass filter for integration to get voltage
proportionalto the apparentcharge producedby the PD event in the GIS. (5) 'Scope for time domain measurement.(6)Optional
interfaces to the observer (a) peak-reading meter, (b) Lissajous display, (c)phase-resolved pulse height analysis.
IEEE Electrical Insulation Magazine
January/February 1992-Vo1.8,No.1
19
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
and requires sensors every 5 to 10 m so that 60 to 90
sensors are required for a typical CIS. The system has
been installed in several GIS and has been successful in
detecting PD sources both during commissioning and
during normal operation. No calibration method which
links the results of this PD detection technique to conventional PD detection technology has been reported.
Acoustic PD Detection
C
I
I
Phase Angle-
0
1
2 n
2
3
4
5
Number of pulses per half wave = NHW
c)
Fig. 3. Regular PD patterns for discharges with an abundant
supply of initiatory electrons. (a) Schematic description of
the discharge process, E&) is the field in the cavity without
discharges, E(t) is the field in the cavity with discharges
taking place; (b)experimental example from a cavity strongly irradiated with x-rays to produce abundant initiatory
electrons; (c) relationship between the dielectric over stress
and the frequency of PD (number of PD pulses per half
cycle).
20
Acoustic PD detection has been employed since the
earliest days of GIs. The first applications employed a
simplebut sensitive hand-held detector with metal contact probe. Such devices were suitable for detecting and
locating free conducting particles bouncing on the
enclosure and discharge from floating components.
More recently, much more sophisticated acoustic I'D
detection techniques have been developed [14-181, and
these more recent developments will be the subject of
the following discussion.
Acoustic methods detect the acoustic energy which
is generated by the PD process and which propagates
to the GIS enclosure through metallic and solid
dielectric components and/or through the sF6 gas. An
acoustic sensor is normally attached to the enclosure on
a temporary basis, where and when needed. Both accelerometers and acoustic emission sensors are used.
Acoustic emission sensors are resonant devices and
have a limited bandwidth around a well-defined center
frequency, while accelerometers are broadband devices
which generally require a high pass filter to limit the
low frequency acoustic noise which permeates a GIs.
Acoustic emission sensors, with a detection bandwidth
centered around 50 kHz, are optimum for detecting the
range of PD-inducing defects likely to be encountered
in GIS [ 151.
Acoustic PD detection in GIS has a number of characteristics which differ from electrical detection. Acoustic attenuation in the GIS is high, especially at metal
flanges or spacers. Thus for reasonable sensitivity, an
acoustic measurement must be made between every
pair of flanges or spacers. The magnitude of the acoustic
signal detected at the enclosure obviously depends on
the magnitude of the signal generated by the PD and
the attenuation of that signal between the source and
the point of detection. Some PD sources will involve
direct discharge to the enclosure and will generate large
acoustic signals on the enclosure at the point of generation which reduces in intensity fairly rapidly with distance from the point of generation. Other I'D sources,
which involve discharge to the GIS conductor, will
cause a relatively uniform PD magnitude over the
enclosure, as the signal propagates down the conductor
and across the SF6 gas to the enclosure. This wide range
of conditions complicates interpreting the signal from
acoustic PD measurement. However, the extensive
work carried out by a number of investigators [15-171
January/February 1992-Vol.8,No. 1
IEEE Electrical Insulation Magazine
~~
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
Characteristic
1
Time Resolved
Analysis
Modified PD
Technique
PRPDA
UHF Technique
Acoustic
Technique
3.2
3.3.1
3.3.2
3.3.2
3.3.3
3.4
in pC phase
voltage
in pC phase
apparent charge
in PC, phase rate
in 1min.
frequency
acoustic emission
Conventional
Technique
IEC 270
Reference
"c
PD sensor
PD sensor or
antenna
PD sensor
External
ultrasonic
transducer
accelerom.
I
no
Noise immunity
+
+
Detection range
in CIS
One phase
One phase
Localization of
no
Yes
handling,
evaluation
low
1
high
high
+
+
+
+
One phase
One phase
5-10 m
One
compartment
no
no
rough
Y"
low
medium
high
medium
medium
medium
i
has resulted in a field measurement sensitivity (typically 2 pC) and basis for interpretation of acoustic PD
measurements which provides a level of success and
specificity which is comparable to that for electrical
measurements. Table I1 summarizes the acoustic characteristics of various types of PD-producing defects in
CIS.
Acoustic I'D detection can be applied without builtin sensors, and the short detection range assures that
defects can be located easily. The technique is not sensitive to electromagnetic interference but is sensitive to
acoustic interference. However, this can be minimized
by restricting detection to above the audio band (e.g.,
by restricting detection from 20 kHz to 80 kHz 1151).
Finally, non-electrical vibrations can be detected, which
can be an advantage when a loose but non-discharging
part is detected or a disadvantagewhen a part vibrating
within a complex structure such as a circuit breaker
reduces the PD detection sensitivity.
The practical application of acoustic PD detection is
IEEE Electrical Insulation Magazine
i
quite easy, as no built-in sensors are required. However,
as a result of the large number of measurements which
must be made, conducting acoustic PD measurements
during GIS commissioning can extend the required test
time appreciably. Modem acoustic I'D detection has
been applied to over 350 GIS bays, which has resulted
in substantial accumulated experience with field measurements.
Chemical PD Detection
Partial discharge in SF6 causes decompositionof the
gas, primarily into SOF2 and S02F2 [19]. SOF2 can be
detected down to a few ppm using simple chemical
sensor tubes [20]. In some situations, PD in GIS is intermittent as a result of thermal expansion, current induced vibrations, etc. In such situations, I'D will not be
detected using electrical or acoustic techniquesif the PD
source is not active at the time of measurement.Chemical detection has the advantage that it detects a (usually)
January/February 1992-Vo1.8,No.1
21
~-
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
permanent and cumulative byproduct of I'D. Chemical
detection has the disadvantages that it cannot be applied to gas compartments which employ adsorbents or
desiccants (and a few GIS manufacturers use a desiccant
in every gas chamber), and sensitivity is limited for
chambers in which sparking is normal. For example,
each operation of a disconnector can generate as much
as 0.5 ppm of decomposition byproducts in disconnector chamber. Detection sensitivity of chemical techniques is compromised if the SF6 decomposition products
must diffuse a large distance from the I'D source to the
gas port at which their concentration can be measured.
Chemical PD detection is a valuable in-servicediagnostic for small gas chambers in which sparking does
not normally take place. This includes PT's, bushings,
short sectionsof bus duct, etc. Apparatus is commercially available which permits a chemical measurement of
I'D-induced byproducts to be carried out in the field in
about 10 minutes at a cost of about $5 in consumables
(the disposable chemical detection tube) [20]. The same
instrument can be used to sample the SF6 during fault
location.
mental conditions, a range of detection methods, equipment, and procedures have been employed. For example, the standard "coupling capacitor" method is not
easily applicable to on-site testing but is normally applied for factory and laboratory testing. On the other
hand, the acousticmethod and electricalmethods based
on small capacitive sensors provide good tools for onsite I'D testing.
Development PD Testing
I'D-measurement is an integral part of the normal
development testing procedure and is always applied
during development of new GIS components. Given
laboratory conditions with a low ambient noise level
and using the standard measuring techniques documented in IEC 270 [7],a detection sensitivitybetter than
2 pC is normally achieved. PD is typically measured
using a peak-reading meter and displayed in the standard Lissajous figure form. However statistical tools,
such as phase-resolved pulse height analysis, provide
more detailed information concerning the nature of any
I'D source detected and are becoming more common.
Overview of PD Detection Techniques
Factory PD Testing
The characteristicsof the various I'D detection techniques are summarized in Table 111. The chemical technique is omitted, as it differs so much from the others
that it does not fit the categories used in the Table.
Experience with PD Measurements in GIS
The following description of I'D measurements in
GIS is based on the experience of ABB in various parts
of the world but should be typical of the state-of-the-art
in industrial practice. As a result of differing environ-
As for the case of development testing, factory testing is usually carried out according to IEC 270 and is
normally performed on shipping units only. Interference during such measurements is generally quite
low, as the test apparatus is completely metal-enclosed
and energized by a metal-enclosed test transformer.
Normally a measured I'D level of 2 pC is specified.
Although the GIS components are assembled with great
care, the I'D measurement during the factory high voltage test constitutes a very important check for cleanli-
Table I11
SUMMARY OF PD DETECTION TECHNIQUES
Method
22
Remark
Designation
Relative Cost
Applied
Cleanliness check
A
Low
Yes
Visual inspection
B
LOW
Yes
Penetrating test
C
Low
Yes
X-raying
D
Medium/High
No
AC test
E
Medium
Yes
AC test+ electrical PD, Conv. IEC
F
High
Yes
AC test+ electrical PD, Sensors
G
Medium
Yes
AC test+ acoustical PD
H
Medium
Yes
Also vibrations
LI test
I
Medium
No
Exceptionally
~-
January/February 1992-Vo1.8,No.1
IEEE Electrical Insulation Magazine
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
ness and defects in material quality, such as cavities or
delaminations in solid dielectric components or
protrusions on conducting electrodes. Normally less
than 1% of shipping units tested fail the PD test.
[Editor’s note: Some manufactures also conduct PD
tests on individual spacers and other solid dielectric
components. This is especiallycommon when the components are operated at relatively high electrical stresses.]
PD Testing during Commissioning
High voltage field commissioning tests for GIS are
not standardized. The applicable sections of IEC 517
and IEEE C37.122 are recommendations rather than
standards, and opinions concerning the optimum procedure vary. Good technicalreasons can be put forward
for a combination of AC testing and impulse testing. On
the other hand, good economic and practical reasons
can be provided for a combination of AC testing and PD
measurements. In selecting a test program, a balance
must be achieved between economic and technical considerations and between the value of a test in detecting
defects versus the likelihood of the test causing new
defects [21]. The high voltage testing of GIS is the sub-
ject of numerous technical publications and is far too
broad to be discussed in detail here.
Of the 65 GIS ABB has tested, an average of one defect
has been found in every fourth GIs. All of the defects
were caused by transport or site assembly errors, e.g.,
free conducting particles or loose components. These
defects were detected without difficulty during an AC
test, although almost half of the installations were also
tested with impulse voltage. ABB therefore feels that a
high voltage AC commissioning test, combined with
sophisticated field PD measurements, is adequate.
Recently, ABB commissioned the Pradella GIS in
Switzerland which provided an opportunity to compare various techniques for field measurement of I’D in
GIs. The 420 kV, 1425 kV BIL Pradella station consists
of two bays in a single bus arrangement. The GIS consists of circuit breakers, bushings, and other switchgear
components as seen in figure 4. The GIS is installed
alongside a conventional substation associated with the
Pradella generating plant.
The test voltage was generated with a variable frequency series resonant test set. The test procedure was
as follows:
1. Proof of a I‘D level <5 pC at 290 kV (1.2 pu)
2. Stepwise increase of the test voltage with PD
Fig. 4. The Pradella 420 kV,1425 kV BIL GIS at Pradella, Switzerland.
IEEE Electrical Insulation Magazine
January/February 1992-Vol.8, No.1
23
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
The PD test method described above clearly indimeasurement at 365 and 545 kV
cated that the Pradella GIS was free of partial discharge
3. Proof of a PD level <5 pC at 290 kV at the end of
to about the same apparent PD sensitivity as typically
the high voltage test
achieved during factory testing of shipping units. The
A PD measuring system was connected to each of
successful PD test, along with the AC high voltage test,
two built-in ID
' sensors. One was developed by the
indicated that the GIS could safely be placed in service.
Swiss Federal Institute of Technology (ETH) while the
other was an ABB system which provided phaseIn-Service PD Testing of GIS
resolved pulse height analysis. The two sensors in each
phase provided for PD calibration as described above.
Corona in air off the conventional switchgear in the
Since the early 1980's, interest has been expressed in
immediate vicinity of the GIS provided a very noisy
in-service testing of GIs. The acoustic method has been
measurement environment.In order to minimize interdeveloped and frequently been applied to this end. The
ference during the testing of phases Rand T, the Pradeltwo main exponents of this PD test method are the
la power plant and the conventional substation were
Norwegian State Power Board and the Swedish State
Power Board which, together, have tested more than
taken out of service for some hours. The tests on the S
350 GIS bays [15,17]. The primary purpose of such
phase were conducted with the power station and contesting is to detect, locate, and assess the risk from any
ventional switchgear in service for comparison and in
sources of PD within the GIS while, if necessary, they
order to obtain experience with PD measurements in
can still be eliminated through scheduled outages.
the presence of severe interference.
This approach was recently applied to the Filisur GIS
For Phase R, both PD measuring systems indicated a
PD background level of less than 2 pC at 290 kV both at
in Switzerland,which consists of one transformer bay,
the beginning and end of the high voltage test sequence.
two breaker bays, in a single bus arrangement. At the
When measuring phase S with the adjacent highvoltage
time of the test, Filisur had been in service for five years
without any failures. After determining the acoustic
substation in service, a PD detection threshold of 2 pC
background level, the acoustic signal level was
was also possible by comparing one measurement
synchronized with the fre:
quency of the high voltage set
(84 Hz and higher) and a
Table IV
second
measurement
SUMMARY OF THE ABB APPROACH TO QUALITY ASSURANCE OF GIS
synchronized with the 50 Hz
Imperfection
Qssurancemethods
Fault type
Stage of
power frequency voltage.
probability
Table I11
production
The disturbances synchronous to the 50 Hz line
Occasionally
Component
Conducting particles
A, B
frequency could clearly be
manufacturing
Rare
B
Loose, floating parts
distinguished from internal
disturbances, had any been
B
Very rare
Voids, protrusions
present. The use of a resonant
test set with a frequency
Negligible
C (Occasionally)
Delaminations
higher than the line frequenVery rare
Treeing
F (sample test)
cy allows the voltage transformers to be left in place and
Occasionally
Conducting particles
Final testing
A, B, F
tested during the high voltin the factory
age test. Such variable freRare
Loose, floating parts
B, F
quency resonant test sets
Very rare
Voids, protrusions
B, F
have been used by ABB at
many sites throughout the
Negligible
Delaminations
B, F
world during the past few
Occasionally
Conducting particles
Field testing at
years.
A, B, G, H
commissioning
PD from free conducting
Very rare
Floating parts
B, G, H
particles was detected by
both PD measuring systems
Occasionally
Loose parts
H
during testing of the T phase.
Very rare
Conducting particles
In service G, H
For precise location, a simple
periodic checking
acoustic test was employed.
Very rare
Floating parts
G, H
After the defective enclosure
was located and cleaned,
Very rare
Loose parts
H
phase T passed.
24
January/February 1992-Vol.8,No.1
IEEE Electrical Insulation Magazine
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
measured from each compartment with all measurements completed in a single day. The only signals above
the background level came from two potential transformers which showed weak mechanical vibration. As
past experience indicated that such PT's often vibrate,
these signal were considered irrelevant.
The ultra high frequency I'D detection technique has
also been applied under in-service conditions and has
located several defects associated with a recently commissioned GIS [lo-121.
PD Testing in the Total Quality Program
On the basis of past experience, ABB has established
a policy for quality assurance of GIS as summarized in
Table IV. Since cavities and delaminations are very rare
in present ABB designs, sensitive factory PD testing of
shipping units is adequate. On the rare occasion that a
cavity or delaminationis detected, the shipping section
must be partly disassembled to change the faulty component, an operation which is fairly costly and time
consuming. Since the probability of such an event is
extremely low, this is more cost effective than testing of
individual spacers and other solid dielectric components (disconnector operating rods, etc.)
ABB believes that as much as possible of the quality
control should take place in the factory, where sensitive
measurements are possible and where the cost of corrective action is relatively low. The purpose of testing at
site is simply to verify that the condition of the high
voltage system has remain unchanged during transport
and assembly.
ABB considers high frequency electric PD detection
using built-in sensors and acoustic PD detection using
sensors temporarily attached to the enclosure to be
well-proved and dependable technologies. If built-in
sensors are available, the electric method is generally
faster, as I'D from anywhere in an entire phase can be
detected simultaneously. The acoustic method is recommended for GIS without built-in electrical sensors, to
provide more precise localization as a complement to
the electrical method, and for detection of mechanical
vibration (loose parts, etc.) Acoustic detection is also
useful for PD detection in service. This is normally not
required, but could become desirable after some "significant event" such as an earthquake or if some other
circumstance indicates the possibility of PD within the
GIs. Acoustic PD testing is relatively quick and inexpensive, with two days required for PD testing of an
average size GIs.
From the experience with on-site PD measurements
we conclude that electrical methods using inexpensive,
built-in PD sensors are quite precise tools for I'D detection and quantification. In combination with high voltage excitation above the normal line frequency, PD
within the GIS can clearly be distinguished from external disturbances. Acoustic I'D testing is preferable for
IEEE Electrical Insulation Magazine
-
in-service testing as a result of the low cost of application and the ability to test the GIS noninvasively. This
method is, therefore, well suited for conducting tests on
a periodic basis to provide informationon the long term
behaviour of the GIS insulating system if built-in PD
sensors are not available.
Trends in PD Measurement of GIS
In the field of CIS diagnostics in general and I'D
measurement in particular, intense development is
going on at various laboratories around the world. New
methods are being developed, and existing methods are
being improved. This continued development will
result in increased sensitivity to and better identification and localization of I'D sources. International organizations such as IEEE and CIGRE have established
working groups for supervision and evaluation of this
development and for preparing recommendationsas a
basis for further standards. This development will influence the testing of GIS in the future. Some trends
which can be distinguished are mentioned below.
Another tendency is the development towards more
compact GIS designs, not by means of increasing critical
stresses and reducing accessibility, but by means of
more selective dimensioning and elimination of unnecessary functions in the layout. This will increases the
number of cases in which complete bays can be assembled, tested, and shipped from the factory, which
minimizes the exposure of the high voltage insulation
system at site.
The above considerations are expected to promote
development in the following directions:
Factory Testing: Test voltages are not expected to
change significantly, but more emphasis will be placed
on quality control. Better methods of defect detection
and identification will result from more sophisticated
arid sensitive electrical I'D measurements, which, in
combination with the increased size of shipping sections, will increase the quality of shipping units.
Field Testing: To date, the predominant method for
field testing of GIS has been to apply one (or more) high
voltage (relative to the normal service voltage) test
waveforms for a short period of time. Flashover is
usually the only diagnostic. If a flashover occurs, it must
be judged self restoring or not, although a single
flashover cannot normally be located. If a flashover is
not self restoring, then costly disassembly and repair are
necessary. The information concerning the state of the
insulation system which can be determined from such
as test program is limited, and since flashover under
many conditions is probabilistic, the result of such a test
program is often a matter of chance. In addition, the test
voltage can damage the installation through flashovers
and transient overvoltages generated by flashovers
which can cause secondary flashovers that may not be
sensed. The high voltage test levels require that a costly
January/February 1992-Vo1.8,No. 1
25
-~__
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
test set be brought to the site.
Development toward more selective "diagnostic"
testing and less "brute force" testing can be foreseen as
a result of the more sophisticated diagnostic techniques
and the higher quality of shipping units. AC will be the
most common test waveform as most of the potentially
dangerous defects are activated by a power frequency
voltage and since the diagnostic tools now being
developed require a continuous voltage. As a result of
the sensitivity of the diagnostic methods under
development, GIS may be tested at service voltage or
slightly above using a GIS magnetic PT as the voltage
source. Such solutions are already being considered by
some users for small GIS or for limited extensions of
existing GIS where the costs of "conventional" high
voltage testing constitute a substantial part of the installation cost. Thus the on-going development of diagnostic techniques will create the possibility of more
sensitive testing with lower risk to the GIS at lower
testing cost.
Supervision In Service: Published fault statistics indicate that GIS are highly reliable. Unlike rotating
machines, the dielectric system of a GIS has no obvious
wear mechanisms and has no certain end-of-life. Unlike
a transformer, the substantial value of a GIS is spread
over a large physical area. As a result of these considerations and the appreciable complexity and cost of a
continuous PD monitoring system for GIs, such systems are unlikely to be cost effective except in a few
cases of know problems (associated with some very
early GIS designs).
Periodic checks for PD are already applied in some
CIS and will become increasingly simple as improved
diagnostic apparatus is developed. Period checking is
likely to be increasingly applied, especially during the
first year of service during which failure rate is highest
and for GIS with a high outage cost.
Conclusions
The application of diagnostic methods is important
for assessing the condition of high voltage insulation in
GIs, and PD measurement is among the most useful
diagnostics. The intense on-going development of PD
measurement procedures as well as other diagnostic
tools will provide possibilities for less risky and more
selective testing procedures and thus for still higher
reliability at lower testing cost, whereas equipment
costs will probably remain unchanged.
Rudolf Baumgartner was graduated as an electrical
engineer by the Swiss Federal Institute of Technology
in Zurich (ETHZ) in 1973. In 1977, he received the
degree of Doctor of Technical Sciences for his work on
the validity of the similarity law in SF6 at the High
Voltage Laboratory of ETHZ. He then joined the ABB
Power Lab in Baden as a development engineer in
charge of control and measurements. In 1984, he was
appointed head of the ABB Mechanical Testing Lab
doing development and type tests on GIs, live tank
breakers, and generator breakers. Since 1988, he has
been head of GIS product development as well as the
testing laboratories of ABB High Voltage Technologies
Ltd. in Zurich-Oerlikon.
Benhard Fruth received his M.Eng. degree in 1981
and his Dr.Eng. degree in 1986in Electrical Engineering
from the Aachen University of Technology, Germany
where he also held a position as Chief Engineer for two
years at the Institute for High Voltage Technology. He
joined ABB Corporate Research in 1987, where he is
now heading the Dielectrics Research Group. His current interests cover the investigation of ageing and
breakdown phenomena in gases, solids, and liquids,
charge injection and polarization mechanisms as well
as the development of insulating systems and diagnostic measurement procedures.
Werner Lanz received his M.Sc.Eng. in Electrical Engineering from ETHZ in 1981after which he joined ABB
as a test engineer in the Mechanical Laboratory. Presently he is Manager of the development laboratories of
ABB High Voltage Technologies Ltd. in Zurich-Oerlikon, including the High Voltage Laboratory.
Kjell Pettersson received his B.S. degree in Mechanical Engineering from the Eskilstuna Institute of Technology in 1959 and an MSc. degree in Electrical
Engineering from the Royal Institute of Technology
(Stockholm) in 1966. In 1960, he joined ASEA's Central
Research and Development Department in Vasteras,
Sweden where he was engaged in various development
projects. When ASEA initiated development of GIS in
1973, Kjell was appointed manager for the project and
moved to Ludvika. In 1978, when ASEA established a
more firm GIS organization, he was appointed the first
Technical Manager of the GIS department. After the
merger of ASEA and BBC, he transferred to ABB Switzerland (Zurich) where he is working in the GIS Business Unit to merge the GIS technology from various
parts of the ABB Group, structuring future product
programs, and similar development-oriented activity.
Acknowledgments
The authors are pleased to acknowledge useful contributions from the following colleagues: Prof. W.S.
Zaengl (ETH Zurich, Switzerland), M. Albiez (ETH
Zurich, Switzerland), Dr. L. Niemeyer (ABB Corporate
Research, Baden, Switzerland), and Dr. M. Leijon (ABB
Corporate Research, Vasteras, Sweden).
26
References
[l] Baumgartner, et al. "Partial DischargePart IX." IEEE Electrical
Insulation Magazine 7, No.6, November/December 1991.
[Z] Boggs, S.A. "Partial Discharge: Detection Sensitivity". IEEE
Electrical Insulation Magazine, 6, No. 5, September/October 1990.
[3] Boggs, S.A., G.L. Ford, and R.C. Madge. "Coupling Devices for
the Detection of Partial Discharges in Gas-Insulated Switchgear".
IEEE Trans. PAS-100, August, 1981, p 3969.
[4]Boggs, S.A. and N. Fujimoto. "Techniques and Instrumentation
January/February 1992-Vol.8,No. 1
IEEE Electrical Insulation Magazine
~~
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.
measurements in particle contaminated SF6 insulation”. Proc. Int.
Con& on Gas Discharges and Their Applications. Knoxville, 1990.
[14]Lundgaard, L.E., M. Runde, and B. Skyberg. “Acoustic Diagnostics of Gas-Insulated Substations: A Theoretical and Experimental
Basis”. ZEEE Trans PD-5, No. 4, October 1990. p. 1751.
[15]Lundgaard, L.E., G. Tangen, B. Skyberg, and K. Faugstad. “Acoustic diagnoses of GIs; field experience and development of expert
system”. Paper 91WM030-7 PWRD, IEEE Winter Power Meeting,
New York, 1991.
[16]Lundgaard et al. ”Acoustic diagnostic techniques”. Working
document prepared by CIGRE Working Group 15-03.
[17]Lord, W.S.G., P.E. Hoff, M.A.S. Leijon, and L. Skold. ”Experience
from on site testing with the acoustic method of GIs. Nord-IS,
1990.
[18]”Partial Discharge testing of gas-insulated substations”. Paper
91WM031-5 PWRD, IEEE Winter Power Meeting, New York, 1991.
[19]Chu, F.Y. ”SF6 Decomposition in Gas-Insulated Equipment”. ZEEE
Trans EZ-21, No. 5, October 1986. p. 693.
[20]Chu, F.Y. ”Novel Low-Cost SF6 Arcing Byproduct Detectors for
Field Use in Gas-Insulated Switchgear”. ZEEE Trans PD-2, No. 2,
April 1986. p. 81.
[21]Bargigia,A., W. Koltunowicz, and A. Pigini. ”Detection of Partial
Discharges in Gas Insulated Substations”. Paper 91 SM 373-1
PWRD presented to the 1991 IEEE PES Summer Power Meeting.
for Measurement of Transients in Gas-Insulated Switchgear”.
ZEEE Trans. EZ-29, No. 2. April, 1984, p. 87.
[5] Albiez, M. W. Zaengl, K. Diederich, and J. Mepelink. ”Design and
calibration of a universal sensor for the measurement of very fast
transients and partial discharges in GIS”. Proc. of 6th ISH, New
Orleans, 1989. paper 42.28.
[6] Albiez, M. and M. Leijon. ”I‘D Measurements in GIS with electric
field sensor and acoustic sensor”. Contribution submitted to ISH
Dresden, 1991.
[7] IEC: Partial discharge measurement, IEC Publication 270,1981.
[8] Albiez, M. “Praventives Messen in Elektrischer Anlagen”. ETG
Tagung: ”TE-Messungen in GIS-Anlagen”, Zurich 14 Sept. 1989.
Schweizerisher Elektrotechnischer Verein, Reihe ETG: Band 8d, p.
131.
[9] Boggs, S.A. ”Electromagnetic Techniques for Fault and Partial
Discharge Location in Gas-Insulated Switchgear”. ZEEE Trans
PAS-101, No. 7, July 1982.
[10]Hampton, B.F. ”Update on UHF PD diagnostic methods”. Working papers of the IERE Workshop ”Gas-Insulated Substations”.
Toronto, 1990.
[11]Pryor, B.M. “UHF partial discharge detection in GIS”. ibid.
[12]Hampton, B.F. and R.J. Meats. ”Diagnostic Measurement at UHF
in Gas Insulated Substations”. Proceedings of the ZEE, 235, JPt. C,
No. 2, March 1988.
[13]Niemeyer, L. and B. Fruth. ”Phase resolved partial discharge
IF YOU WERE UNABLE TO ATTEND A RECENT CONFERENCE
YOU CAN STILL ORDER THE CONFERENCE PROCEEDINGS
TO ORDER OR TO REQUEST A FREE CATALOG
TOLL-FREE IN THE U.S. AND CANADA CALL
1-800-678 IEEE
IN OTHER COUNTRIES CALL
(908) 98 1 -0060
FAX (908) 98 1 -9667
OR WRITE
IEEE CUSTOMER SERVICE DEPARTMENT
445 HOES LANE, PO BOX 1331
PISCATAWAY, NJ 08855-1331 U.S.A.
IN EUROPE, THE MIDDLE EAST, AFRICA OR THE USSR
CONTACT THE NEW BRUSSELS OFFICE
CALL 32.2.770.22.42.
FAX 32.2.770.85.05
OR WRITE
IEEE TAB OFFICE
13, AVENUE DE L’AQUILON
8-1200 BRUSSELS BELGIUM
IE~E
A SERVICE OF IEEE TECHNICAL ACTIVITIES DEPARTMENT
IEEE Electrical Insulation Magazine
-
JanuaryFebruary 1992-Vo1.8, No.1
~-
27
~
Authorized licensed use limited to: ABB Power Grids. Downloaded on September 16,2020 at 07:50:29 UTC from IEEE Xplore. Restrictions apply.