ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
PARTIAL DISCHARGE DIAGNOSTICS
ON GIS USING UHF AND ACOUSTIC METHOD
U. Schichler*, M. Reuter, J. Gorablenkow
Siemens AG
* E T HS MF B 13, Nonnendammallee 104, 13623 Berlin, Germany
Email: uwe.schichler@siemens.com
Abstract: PD measurement is an important measure for quality assurance during the life cycle of
GIS. In addition to the conventional method (IEC 60270), both UHF and acoustic methods are
suitable for PD diagnostics on GIS. The general principles of the UHF and acoustic method are
described. Different approaches for location of UHF couplers are discussed in detail for a typical
420 kV GIS. Acoustic signal characteristics of defects are presented and compared with UHF
pattern including the description of on-site measurements and return of experience.
- electrical methods
- conventional PD measurement (IEC 60270)
- HF/VHF method (3 - 30 MHz / 30 - 300 MHz)
- UHF method (300 - 3000 MHz)
- acoustic method
- optical method
- chemical method
1 INTRODUCTION
Nowadays, GIS have been in service for more than 40
years and they have shown a high level of reliability
with extremely small failure rates even for modern GIS
showing a high compactness. This is the result of
quality assurance during development, production,
installation and commissioning, as well as during
service. Quality assurance consists of different tests and
measurements during the life cycle of GIS which give
feedback to the manufacturer and user. All return of
experience should be taken into account for the
continuous improvement of the product quality.
However, the return of experience shows that some of
the in-service failures are related to defects in the GIS
insulation system. Many of these defects can be
detected by PD measurement.
PD measurement is an important part of the quality
assurance system and a mandatory measure at the type
test and during routine testing of GIS [1]. For these tests
conventional PD measurement according to IEC 60270
has to be performed. In addition to the conventional
method, both UHF and acoustic methods have been
used for PD diagnostics in GIS for more than 25 years.
These are less sensitive to noise than the conventional
method and they are beneficial for on-site PD measurement during commissioning and while in service.
Several publications describe the return of experience
and benefits of the UHF method whereas the application
of the acoustic method is mainly be limited to periodic
monitoring in service [2-7]. Standardization of both
methods is in progress [8].
Today’s dominant techniques for commissioning tests
and in-service monitoring on GIS are the UHF method
and with some limitations the acoustic method.
3 PD DETECTION BY UHF METHOD
3.1 General principle
The principle of the UHF method was developed more
than 25 years ago, and is now well-known worldwide
after being adopted by many GIS manufacturers and
utilities. It may be recalled that the current pulse which
forms the partial discharge has a very short risetime,
which can be less than 50 picoseconds. The rising edges
of these pulses excite the GIS modules into multiple
resonances at frequencies of up to at least some GHz.
Although the duration of the current pulse is less than a
few nanoseconds, the microwave resonances persist for
a relatively long time, typically a few microseconds.
2 METHODS FOR PD DIAGNOSTICS ON GIS
The insulation system of GIS (Figure 1) is made up of
three categories: SF6 gas, bulk material of insulators
(epoxy resin) and the insulator surfaces (interface
between gas and insulators). Electrical aging is not
expected at service voltage if critical defects are absent
from the insulation system and design was done
carefully according to electric field stress.
The detection of defects, like free or fixed particles,
floating components or insulator defects is possible by
different PD diagnostic techniques [2]:
Figure 1: Typical 420 kV GIS fitted with UHF couplers
The electromagnetic waves propagate within the GIS at
the speed of light as TEM-, TE- and TM-waves and are
Pg. 1
Paper D-9
ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
Figure 3 shows the UHF output of the five couplers at
location A - E for different defect positions within the
GIS configuration based on a simplified model of UHF
signal attenuation. Defects causing PD equivalent to at
least 5 pC are always detected by two adjacent couplers
which, in total, results in an increased detection sensitivity. The location of defects can be done easily using a
two channel wideband oscilloscope for time-of-flight
measurements because all detected defects are located
between two couplers. The fitting of couplers at the
interface to directly connected HV equipment like
cables, power transformer or shunt reactors may give
advantages for defect detection in such equipment.
only moderately influenced by open disconnectors or
closed earthing switches. However, an average loss of
signal strength of about 2 dB/m take place due to a combination of reflections, dispersion, division at T-junctions and attenuation. The UHF signals may readily be
picked up by couplers fitted either inside the GIS
modules, or over dielectric windows in the enclosure.
Whether internal or external couplers are used, the UHF
signals can be amplified and displayed in different ways
where their characteristic patterns reveal the nature of
any defect that might be present in the GIS.
Extensive investigations in laboratories have confirmed
that PD detection using UHF technology results in
higher or at least the same sensitivity as detection by PD
measurement as set out in IEC 60270. Nowadays the
UHF method is used world-wide for quality assurance
during routine testing, on-site testing and for PD
monitoring in service.
3.2 Sensitivity Verification
The UHF method does not provide any direct
correlation with “pC-values” according to IEC 60270.
CIGRE has therefore developed a procedure verifying
that it is possible to detect “bouncing particles with an
apparent charge of 5 pC” in complete GIS substations
[9]. First, a laboratory test is carried out for each GIS
type and for the PD measuring system used, in order to
establish the direct correlation between the 5 pC value
and the amplitude of a voltage step generator. On-site
the voltage signal is fed in at a coupler and must be
detected at the adjacent couplers. This second step of
the sensitivity verification procedure is used to check
the correct location of the couplers in a complete GIS.
Following the current CIGRE procedure the maximum
distance between two couplers is between 10 m and
more than 25 m depending on the GIS type, bay design
and the substation layout. In case of long GIB or GIL
the distance can be much greater than 100 m.
The described procedure is currently under review by
the newly launched CIGRE WG D1.25 [10].
Figure 3a: UHF signal of couplers at location A - E for
different defect positions with PD equivalent to 5 pC
Figure 3b: UHF signal profile for five sets of couplers
The result of another approach to fit couplers to this
particular GIS is shown in Figure 4. Only two three
phase sets of couplers have to be installed on the GIS at
location B and D to fulfill the basic requirement for
detection of defects causing PD equivalent to 5 pC.
3.3 Coupler location
A 420 kV GIS with two bays as shown in Figure 2
needs to be fitted with five three phase sets of couplers
to achieve the requirements according to the current
CIGRE recommendation [9].
Figure 4a: UHF signal of couplers at location B and D
for different defect positions with PD equivalent to 5 pC
Figure 4b: UHF signal profile for two sets of couplers
Figure 2: 420 kV GIS with five coupler locations A - E
Pg. 2
Paper D-9
ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
techniques and the efficiency of the PD identification
algorithms. Nowadays, the suppression of noise and
other background signals like radar or mobile phone
signals is realized by combined hardware and software
filters. Current PD identification algorithms are based
on phase resolved pulse sequence analysis. The applied
redundant diagnosis systems (RDS) with hierarchical or
hybrid structures consists of PD feature extraction and
defect classification in combination with a proper
reference data base to identify the type and nature of the
insulation defect. The results from such RDS can have
an accuracy of correct identification in the range of
about 95%. Only a very small number of captured PD
data sets are classified as unknown defect or identified
in a wrong way.
The definition of criteria to create an alarm signal to the
substation control system is one of the open tasks for
today’s PDM systems. Different approaches are
available which all refer to defined thresholds for UHF
signal amplitude or number of detected UHF pulses per
time, number of defined PD events, trend analysis, the
criticality of the identified defect or a combination of all
the previously mentioned information. False alarms are
still being generated by the PDM systems and extensive
intervention of human experts is necessary.
The expected positive impact of continuous PDM
systems to the GIS failure rate and insulation coordination is in general not proven up to now.
In this case the currently recommended on-site test for
the sensitivity verification need to be modified, because
the artificial voltage signal injected at coupler B can not
be measured at the adjacent coupler D due to the increased distance between the two couplers and due to
missing couplers at location A and E. Therefore, the
required sensitivity has to be verified by temporarily
fitted couplers for injection of the voltage signal or any
other suitable measures like calculations or comparison
with GIS of similar layout and proven coupler locations.
The benefit of this optimized coupler layout is confirmed by the reduced number of couplers and lower
investment cost for application of continuous PDM
systems. Locating of defects seems to be more difficult
in comparison with the coupler layout regarding
Figure 3 but can be realized in most cases by
sectionalizing, acoustic PD measurement or temporary
fitting of additional couplers.
3.4 Coupler sensitivity and specification
Different methods have been used to evaluate and to
optimize the performance of UHF couplers for PD
detection in GIS [11, 12]. The application of a GTEM
cell and the measurement of a frequency dependent
effective height seems to be favourable for coupler
specification. However, up to now a calibration procedure for UHF coupler sensitivity has not yet been
defined by CIGRE or IEC. Therefore, the CIGRE sensitivity verification procedure (step 1: laboratory tests)
covers the complete measuring chain consisting of
coupler and measuring system and confirms the
required sensitivity for PD detection. This approach is
suitable for practical use and has been proven in
practice.
A general definition of coupler sensitivity seems to be
difficult due to GIS type related UHF signal transfer
characteristics and output signals.
4 PD DETECTION BY ACOUSTIC METHOD
4.1 Fundamental aspects
Acoustic PD measurement on GIS is based on the
detection of mechanical waves emitted from defects.
These signals are picked up by the acoustic sensor
located outside the enclosure when e.g. free-moving
particles impinge the inside of the enclosure or when
discharges occur within the insulating gas, creating
pressure waves which propagate towards the enclosure
(Figure 5).
3.5 Partial discharge monitoring (PDM) systems
The return of experience shows that some of the inservice failures are related to defects in the insulation
system. Today many of these defects can be detected in
service by continuous PDM systems based on the UHF
method [5].
Modern PDM systems consist of standardized electronic
boards and commercially available components with
high reliability and an expected lifetime of more than 15
years. Different hardware modules can be easily
arranged to build up a customized PDM system, which
enables a sensitive PD monitoring with a detection level
of about -75 dBm. The man-machine-interface for
manual operation of a PDM system is realized by userfriendly software with flexible PD data display
including trend diagrams and customized reporting of
the monitoring results. Worldwide remote control of the
systems is state-of-the-art.
A PDM system should operate as a “black box”, which
captures UHF signals and submits alarm signals to the
substation control system only in case of service relevant PD activity. Therefore, the most important PDM
system features are the applied noise suppression
Figure 5: Propagation path of acoustic signals
The sound propagation within a metal enclosure is
almost free of attenuation (aluminum: 1 - 3 dB/m) and
flange connections cause considerable attenuation of
approx. 10 dB. The level of sound attenuation in the SF6
gas must also be taken into account for the purpose of
PD diagnostics, as this is highly dependent on both the
measuring frequency and the gas pressure. A measuring
frequency of 50 kHz and a gas pressure of 400 kPa will
result in an attenuation of approx. 40 dB/m. The lower
the gas pressure and the higher the measuring
frequency, the higher the attenuation will be. It is not
possible to detect PD from voids in cast-resin insulators
Pg. 3
Paper D-9
ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
increased voltage level, the discharges in the positive
half cycle are more significant. The phase resolved
acoustic PD pattern (phase angle not synchronized) and
the diagrams showing the time between two consecutive
acoustic PD pulses confirm the well-known influence of
the voltage level to the PD activity of a protrusion. The
parallel captured UHF PD patterns show that acoustic
PD measurement can be sensitive in detecting protrusions.
using the acoustic method, because within cast resin the
attenuation is approx. 100 dB/m. For acoustic PD
measurement usually acoustic emission sensors with a
high-pass characteristic, several points of resonance and
a low cut-off frequency of approx. 20 kHz are used.
During acoustic PD measurements, all GIS compartments or GIS enclosures have to be tested separately. It
is essential to ensure a sufficient sensitivity during the
measurement and a time period of at least 1 min should
be applied on each individual test point. An acoustic PD
instrument or an oscilloscope can be used for the
measurements. The following information from the
acoustic signals is relevant for further analysis and
interpretation: signal amplitude, waveform, signal
periodicity, phase angle and the time between two
consecutive acoustic PD pulses. Today’s commercially
available acoustic PD instruments are able to analyze
the captured signal, store the measured data and create
time-of-flight diagrams, as well as showing the phase
correlation for the acoustic PD pulses (Figure 6).
Figure 6: Insulation monitoring on a 40 year old GIS by
acoustic PD measurement
Figure 7: Acoustic PD signals and UHF PD pattern of a
protrusion on the inner conductor at U = 60 kV (PD
inception voltage, left column) and U = 80 kV (right
column), p = 0.5 MPa
4.2 Acoustic signal characteristics of defects
A) Protrusion on inner conductor
Protrusions on the inner conductor can result in a failure
during transient voltage stress (LI, VFT). Acoustic PD
measurement can be used to detect the sound waves
from the discharge and, on the basis of the bandwidth of
the measured acoustic signal, it is also possible to
distinguish between protrusions on the inner conductor
and protrusions on the enclosure [2, 6].
Comparison measurements were performed to evaluate
the acoustic and UHF method on a small test set-up with
regard to the same defect. The test set-up consists of a
gas-insulated 750 kV transformer and two 420 kV GIS
modules. An acoustic PD instrument in conjunction
with an oscilloscope and in parallel an UHF PD monitoring system was used for the measurements [5-7]. All
measurements were done with sensors which were
located in the vicinity of the defect. Figure 7 shows
some results from the measurements, which were
recorded at the PD inception voltage of U = 60 kV and a
voltage level of U = 80 kV. The PD inception in the
negative half cycle of the time signal could be clearly
detected by the acoustic method if the average function
for noise suppression was used on the oscilloscope. At
B) Particle on insulation
The ability to detect particles on an insulator depends on
the shape and position of the particle, the shape of the
insulator as well as on surface charge [6]. Sound waves
from a particle on the inside of a conical spacer are
partially shielded by the spacer itself. Only very low
acoustic signals will be picked up on the GIS enclosure.
Particles on the outside of conical spacers, on the other
hand, were easily detected in experiments. If surface
charges build up in the vicinity of the particle, PD behaviour can become unstable and it is no longer possible
to detect this type of defect by periodic short term
measurements.
C) Free moving particle
It is well known that detection of even noncritical freemoving particles in GIS can be done easily by the
acoustic method [6]. By analyzing the typical time-offlight diagrams (Figure 8), it is possible to obtain a
rough estimate of the particle’s mass and length as well
Pg. 4
Paper D-9
ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
as the jump height inside the GIS, assuming that the
measuring equipment is correctly calibrated. For a risk
assessment the information about the jump height is of
particular importance. A simplified equation for calculating the jump height x is based on the particle’s
maximum elevation time Tmax:
x = 0.125 . g . Tmax2
signatures are detected for inductive voltage transformers and the adjacent enclosures which are based on
magnetostrictive noise and not relevant for service. This
was proven by electrical PD measurement showing a
PD level smaller than 1 pC.
5 CONCLUSION
(1)
The probability of a disruptive discharge is high if the
jump height of the particle is similar to the isolating
distance between enclosure and inner conductor.
PD measurement is an important measure for quality
assurance on GIS. In addition to the conventional
method (IEC 60270), both UHF and acoustic methods
are have been used for PD diagnostics on GIS for more
than 25 years. Today’s dominant techniques for
commissioning tests and in-service monitoring are the
UHF method and with some limitations the acoustic
method.
• The current CIGRE sensitivity verification procedure for the UHF method and the location of UHF
couplers on a GIS can be optimized by a new
approach. One of the open tasks for today’s PDM
systems is the definition of criteria to create alarm
signals and to avoid false alarms.
• The acoustic method is mainly be limited to periodic
PD monitoring in service. Return of experience is
available from testing of more than 1,500 enclosures. Experiments in comparison with the UHF
method show that sensitive detection of protrusions
is possible. Suitable acoustic PD instruments are
available.
Figure 8: Time-of-flight diagram for moving particle
from acoustic PD measurement
4.3 On-site measurements and return of experience
During planning of an acoustic PD measurement, the
individual sensor locations on the GIS bays have to be
defined. Furthermore, access to all parts of the GIS bays
has to be ensured and any acoustic noise that may be
present must also be taken into account. It will take
approx. two hours to perform the PD measurements on a
typical GIS bay during service (GIS type according to
Figure 9). This means that an entire GIS can be tested
in just a few days. Should any PD signals be detected,
the easiest way to locate the defect is to alter the
position of the acoustic sensor until the signal amplitude
is at its highest.
6 REFERENCES
[1] IEC 62271-203, “High-voltage Switchgear and Controlgear - Part 203: Gas-insulated metal-enclosed switchgear
for rated voltages above 52 kV”, 2003
[2] CIGRE WG 15.03, “Diagnostic Methods for GIS Insulating Systems”, CIGRE, Paris, 1992, Report 15/23-01
[3] Diessner, Gorablenkow, Hashoff, Schreieder, “Experience with Diagnostic Techniques for Electrical Insulation
in GIS”, CIGRE Symposium, Berlin, 1993, Report 11023
[4] Schichler, Gorablenkow, “Experience with UHF PD
Detection in GIS Substations”, 6th ICPADM, Xian, 2000
[5] Achatz, Gorablenkow, Schichler, Hampton, Pearson,
“Features and Benefits of UHF Partial Discharge
Monitoring Systems for GIS”, ISEIM, Japan, 2005,
Report P2-50
[6] Lundgaard, Skyberg, Diessner, Schei, “Method and
Instrumentation for Acoustic Diagnoses in GIS”, CIGRE,
Paris, 2000, Report 15-309
[7] Schichler, Wurster, Glaubitz, Fritsch, “Anwendung der
akustischen TE-Messung für die Zustandsbewertung von
gasisolierten Schaltanlagen”, VDE/ETG conference,
Kassel, 2006
[8] IEC 62478, “Measurement of Partial Discharges by
Electromagnetic and Acoustic Methods”, in progress
[9] CIGRE TF 15/33.03.05, “Partial Discharge Detection
System for GIS: Sensitivity Verification for the UHF
Method and the Acoustic Method”, ÉLECTRA, No. 183,
1999
[10] CIGRE WG D1.25, “Application Guide for PD Detection
in GIS using UHF and Acoustic Method”, in progress
[11] Judd, Farish, Coventry, “UHF Couplers for GIS Sensitivity and Specification”, 10th ISH, Canada, 1997
[12] Wanninger, “Antennas as Coupling Devices for UHF
Diagnostics in GIS”, 9th ISH, Graz, 1995
Figure 9: Typical locations for acoustic sensors on a
145 kV GIS (8DN2 GIS of installed base, see Fig. 6)
Up to now the acoustic PD measurement has been used
for the periodic monitoring in service of a number of
GIS with more than 1,500 enclosures. No critical PD
defects were detected and so there was no need to check
GIS compartments. However, sometimes high acoustic
Pg. 5
Paper D-9