FIELD OF THE INVENTION
The present invention relates to planning maintenance schedules for
ensuring reliable operation of a Gas Insulated Switchgear (GIS). More
particularly, the proposed invention is directed to a method of
monitoring/evaluation of the partial discharge (PD)/partial discharge
source in a (GIS) without physically opening the GIS modules.
BACKGROUND OF THE INVENTION
Partial discharge (PD) is a discharge that occurs without causing a
complete flashover of the High voltage (HV) system. It is basically a mini-
spark or flashover within the GIS insulation. Measurement of partial
discharge provides an indication of the condition of the GIS insulation.
Partial discharge in insulation creates measurable effects like current
pulse, (Ultra high frequency) UHF emissions, sound, light and chemical
by products. The conventional method (or electrical method) involves
measurement of the PD generated current pulse using appropriate test
setups. Alternate unconventional methods are also being employed for
continuous online PD monitoring of electrical insulation. Of these the
UHF method has been the most acceptable and accurate method for
discharge measurements at GIS installations.
The generation of UHF signals inside GIS insulation is due to PD
generated current pulse which has a very short rise time of fraction of
nanoseconds. The PD current pulses excite the GIS modules into
multiple resonances in the GHz frequency range. Although the duration
of the current pulse is less than a few nanoseconds, the resonances
persist for a relatively long time. The generated electromagnetic waves
propagate within the GIS. The GIS enclosure acts as a waveguide and
help in propagation of the UHF signal. However, an average loss of signal

strength of about 1-2 dB/m take place due to a combination of
reflections, dispersion, division at T-junctions and attenuation.
The PD induced UHF signal propagates through the GIS and the
intensity of the UHF signal reduces as it moves away from the PD source.
The generated UHF signal can be captured by installing UHF couplers at
strategic locations inside the GIS. The captured UHF signals can be
processed by suitable hardware devices which amplify the actual PD
signal and reject the external noise signals captured by the couplers.
Dedicated software application can be used to display the magnitude of
the PD signal captured along with other relevant information as required.
In general, to monitor healthiness of GIS insulation, partial discharge
(PD) in the energized GIS is measured. The electrical method which is
used for routine PD measurement in GIS cannot be used for PD
measurement at site installations. This is because of significant noise
signals present at site installation which affect the measurement of PD
generated electrical pulses. The electrical PD measurement method
requires a “Faraday Cage” or a shielded room to block the external
electrical fields which affects the measurement of electrical PD pulses.
This type of arrangement is not possible at site installations. Also this
method cannot be used for continuous online PD measurement as it
employs a test setup consisting of a high voltage coupling capacitor and
impedance unit. This test arrangement can only be done for routine PD
testing at laboratories/shop floor. Further using electrical method the PD
fault location cannot be estimated as this method measures the total
apparent charge (pC) equivalent to the PD generated current pulse and is
basically a bulk PD measurement for the full insulation. To overcome
this problem UHF based PD measurement and monitoring is being
employed for GIS site installations.

In United States patent application number US08/030,277 is disclosed a
diagnostic measuring system for measurement and monitoring of the
UHF PD, suitable UHF couplers are fitted to the GIS pressure vessels.
Generated UHF signals are taken from the UHF couplers mounted on
inside of the hatch cover formed on the bus chamber. The couplers as
disclosed by said patent are designed to be mechanically robust and
reliable to withstand the pressure of SF6 (Sulfur hexafluoride) gas in the
GIS enclosure.
In the other conventional techniques for measuring partial discharges in
GIS two different sensors are mounted inside the GIS. For example, in a
US patent with application number US 09/463,763, the first sensor is
suitable for measurement in the HF range and a second sensor suitable
for detecting signal components in the UHF range, are arranged inside a
metal enclosure. Further, healthiness of the GIS insulation is
determined based on the output of these two sensors mounted inside the
GIS.
Moreover, the couplers/sensors which are traditionally used for
capturing of PD are mounted inside the GIS and are directly exposed to
PD induced UHF signals. High Voltage design of GIS modules also takes
into account these sensors and their positions in the GIS enclosure as
these sensors may affect the di-electric field strength and electrical field
distribution inside the GIS. Since these couplers are mounted inside the
GIS they are also required to withstand SF6 gas pressure continuously.
Further, once mounted, the positions of the sensors cannot be changed
online as they are embedded inside the GIS enclosure. Also in case the
sensors become defective, the replacement of the defective sensors would
require a complete shutdown of the concerned GIS bay.

Thus, in view of the forgoing, it would be clear to the person skilled in the
art that the existing techniques in the field of UHF PD monitoring of GIS
involves sensors to be integrated inside the GIS enclosure using different
technique. Thus there remains a need for a non-invasive and highly
sensitive approach for monitoring of GIS modules using UHF couplers or
sensors without physically opening the GIS modules.
OBJECTS OF THE INVENTION
An object of the invention is to overcome the aforementioned and other
drawbacks existing in prior art systems and methods.
More particularly, it is an object of the invention is to develop a Non-
invasive method for monitoring of partial discharge in a Gas Insulated
Switchgear.
Yet another object of the invention to develop a Non-invasive method for
estimating a partial discharge source in a Gas Insulated Switchgear.
Still another object of the invention is to implement techniques for
optimizing the sensor arrangement to maximize the PD signal to noise
ratio (SNR).
Yet another object of the invention is to facilitate the GIS modules to
capture PD generated UHF signal.
Further object of the invention is to develop multiple ways to measure PD
generated signals from GIS modules.

Still another object of the invention is to design different insulated
apertures with different profiles on GIS modules adapted to measure PD
generated signals. Yet another object of the invention is to design a
profile of insulated openings or apertures on enclosures of GIS modules
to measure PD generated signals.
These and other objects and advantages of the present invention will be
apparent to those skilled in the art after a consideration of the following
detailed description taken in conjunction with the accompanying
drawings in which a preferred form of the present invention is illustrated.
SUMMARY OF THE INVENTION:
The present application discloses a non-invasive method for monitoring
partial discharge (PD) in a Gas Insulated Switchgear (GIS) and estimation
of a PD source. The method further includes mounting a sensor on an
opening among a plurality of openings configured external to the GIS for
sensing a partial discharge (PD) induced high frequency signal. In an
aspect, the opening is configured as an insulated aperture on a support
insulator without metallic cover (01). In another aspect, the opening is
configured as an as an insulted aperture on a support insulator
integrated with metallic cover (04). In yet another aspect, the opening is
configured as an insulated earthing switch terminal (06). Further, in
another aspect, the opening is configured as an insulated viewing port
(07). Furthermore, in an aspect the opening is configured as an insulated
gas-to-cable terminal port (08).
Furthermore, the method includes identifying a partial discharge (PD)
source by capturing the partial discharge (PD) induced high frequency
signal sensed by two sensors mounted on two openings among the

multiple openings distributed across the surface of GIS. In an
embodiment, the PD source is located internal to the GIS. Further, the
method for identifying a partial discharge (PD) source by capturing the
partial discharge (PD) induced high frequency signal sensed by two
sensors involves determination of the distance between the two sensors
and calculating time of flight difference for the partial discharge (PD)
induced high frequency signal generated from the PD source to reach the
two sensors.
The above and additional advantages of the present invention will
become apparent to those skilled in the art from a reading of the
following detailed description when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The drawings refer to embodiments of the invention in which:
Figure.1 illustrates a typical PD generated UHF signals in GIS.
Figure.2 illustrates PD signal measurement through Support insulator
without metal cover.
Figure.3 illustrates PD signal measurement through Aperture of Support
insulator.
Figure.4 illustrates PD signal measurement through insulated Earthing
switch.
Figure.5 illustrates PD signal measurement through insulated viewing
port.

Figure.6 illustrates PD signal measurement through insulator of gas-to-
cable termination.
Figure.7 illustrates installation of PD sensors on said insulated apertures
of GIS as shown in Figs 2-6. Insulated aperture for PD monitoring as
shown here in Fig. 7 is on a support insulator among the other insulated
apertures as aforesaid mentioned.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Although the disclosure hereof is detailed and exact to enable those
skilled in the art to practice the invention, the physical embodiments
herein disclosed merely exemplify the invention which may be embodied
in other specific structure. While the preferred embodiment has been
described, the details may be changed without departing from the
invention, which is defined by the claims.
It will be apparent, however, to one of ordinary skill in the art that the
present invention may be practiced without specific details of the well
known components and techniques. In other instances, well known
components or methods have not been described in detail but rather in
Figures in order to avoid unnecessarily obscuring the present invention.
Further specific numeric references should not be interpreted as a literal
sequential order. Thus, the specific details set forth are merely
exemplary. The specific details may be varied from and still be
contemplated to be within the scope of the present invention. The
features discussed in an embodiment may be implemented in another
embodiment.

Moreover occasional references to the conventional PD
monitoring/estimation techniques are made in order to better distinguish
the present inventive disclosure discussed later in greater detail. Few of
the details pertaining to said techniques are well-known in the art, and
therefore, are described herein only in the detail required to fully disclose
the present invention.
Improving upon the conventional techniques discussed at length above
(background), in the present disclosure the inventive design of the
apertures/openings for holding the sensors as shown in Fig. 2-7 clearly
makes the Non-invasive method for monitoring of partial discharge and
estimation of a partial discharge source in a Gas Insulated Switchgear
(GIS) as disclosed in the present application advantageous over the
existing arts as would also become clearer to the knowledgeable in the
art with the particulars of the aforesaid method being described below in
greater detail.
The method as disclosed in the present invention is configured to achieve
comparable PD monitoring sensitivity of the internally mounted UHF
couplers/sensors using a novel non-invasive approach. Further, Figure 1
shows the partial discharge generated UHF signal.
In an embodiment, the UHF couplers/sensors implemented for said
method are non-invasive passive type which can be mounted externally
without the need to open the GIS chamber or to take a shut-down of the
GIS installations. Special provision is made on the GIS for fixing the
couplers to them from outside. The installed couplers from outside
capture the leaking UHF signal which is generated inside the GIS due to
a PD source. The typical output of the coupler is a voltage signal. The
coupler output depends on the UHF field strength inside the GIS

enclosure, coupler design and the dimensions of the aperture on the GIS
spacer.
Further, the partial discharge signal, in general can emanate from
insulated flange/cover of support insulator where the strength of signal
depends on area of aperture/opening of insulation medium. In an
embodiment, the insulated apertures can be circular, rectangular,
elliptical or any other shape as required. The aforesaid insulated
apertures are provided on the LT of the support insulators for mounting
of the PD sensors. The profile of apertures is designed so as to have the
highest SNR for the leaking UHF signal.
Figure 2 shows the support insulator [01] without metallic flange/cover.
High frequency discharge signal emanate from support insulator [01] will
be sensed by sensor/sensors fixed to it. Different novel locations that can
be used for sensing has been identified in this present invention. The
sensing location to provide optimum sensor output is estimated though
experimental positioning of the sensors on the bare flange and choosing
the location which gives the best voltage output with less noise content.
In a preferred embodiment, one sensor is mounted on one of the multiple
openings positioned on the external surface of the GIS. In another
aspect, for the monitoring of discharge more than one sensor may be
mounted external to the GIS on the multiple openings positioned on the
external surface of the GIS.
In insulating flange type support insulator, in an aspect, multiple
numbers of copper strips [02] are connected for efficient current transfer
to the ground. In a preferred embodiment, sensor [03] is located between
the copper strips [02]. Herein, it is important to note that, in all the
installations support insulators may not be purely insulated type.

Further as shown in Figure 3, in extra high voltage and ultra high
voltage class GIS, the support insulators are preferably integrated with
metallic cover [04]. Furthermore, these insulators have circular holes for
processing of insulators but these holes may not be good enough to
measure PD signals. Hence, in a preferred embodiment, opening is
provided with higher surface area and preferably rectangular/elliptical
[05] shape. This is mainly due to limited width (thickness) of support
insulator. Figure 3 specifically shows the proposed profile of this
rectangular slot/ aperture and its cutting view on support insulator.
Figure 4 shows insulated opening of earthing switch in GIS. One more
source to measure PD signals is insulated type of earthing switch. The
UHF signals being emanated from delrin/PTFE
(Polytetrafluoroethylene)/insulated Earth Switch (ES) terminal [06]
insulator is sufficient enough to measure PD generated inside GIS. In an
embodiment, said approach is available for both maintenance earthing
switch and fast acting earthing switch.
Further, in recent years, for long GIS bus ducts which runs into few tens
of meters, it is understood that support insulators is only mountable
inside the GIS and hence the proposed approach for collecting signal
with non-invasive type of sensors is not possible. This is due to the fact
that the exposed epoxy surface of the insulator is not available outside to
facilitate the external sensor mounting. The present disclosure
implements techniques for implementing said non-invasive PD
monitoring in GIS is as shown in Figure 5. Proposed herein are novel
insulated viewing ports [07] in the shape of rectangular or elliptical or
circular depending on the shape/dimension/configuration of sensor
base. These viewing ports [07] are further useful on switching elements
enclosures like disconnector switch, maintenance earthing switch and
fast acting earthing switch to check the status of contacts. The viewing

ports would thus serve dual purpose i.e. it can be used for non-invasive
PD monitoring as well as checking the status of arcing contacts/contact
system/switchgear.
As shown in Figure 6, one more source to measure PD signals is gas- to-
cable termination. The UHF signals emanate from this type of
termination has sufficient resolution to measure PD generated inside
GIS. Figure 6 particularly shows the insulated gas-to-cable terminal port
[08] through which PD signals can be measured.
The UHF PD response of these insulated apertures varies depending on
orientation and medium through which PD signals emanate out of GIS.
Precisely, in viewing ports, through the insulated glass, PD signals will
be received by sensor which can be placed at the Insulated Port [08].
However, in case of insulator, through epoxy material, PD signals will be
emitted into environment.
Figure 7 shows installation of PD sensor on novel insulated opening of
GIS. Through the opening on the GIS support insulator, the PD signal is
captured and monitored online. Designed profile of the opening provides
optimum PD measurement sensitivity. This sensitivity is comparable to
the sensitivity which can be achieved through the invasive mounting of
sensors internally in the GIS.
Further, in a preferred aspect, for identification of discharge location/PD
source/PD discharge location, it is important to capture PD signals from
two similar types of sensors for accurate estimation in a preferred
embodiment. In another aspect, for the identification of discharge
location more than two sensors may be mounted external to the GIS on
the multiple openings positioned on the external surface of the GIS.

Response from other types of sensors is being used for capturing PD
signals to re-ascertain location of discharge. It is also quite important
that discharge attenuation and pattern will be similar for similar type of
identification ports such as viewing ports, insulated apertures etc.
Advantage associated with such type of dual response systems is that,
verification of discharge location is possible with highest probability.
Locating the PD source i.e. the discharge location in the GIS is also an
important aspect. If the PD source can be effectively located inside the
GIS bay it helps in considerable saving of time in diagnosis and
rectification. This also reduces the GIS outage time in case of insulation
failures inside the GIS. Further in an embodiment, the identification
ports disclosed in the present invention may be distributed across the
GIS in any combination.
A conventional PD discharge of about 1 to 2 pC, could be easily detected
by the non-invasive discharge detection system of the present invention.
The tests were repeated for lower as well as higher value of pico-coulomb
(pC) PD and it was observed that the invented non-invasive technique
was quite sensitive in detecting the generated PD signals. For PD location
purpose, minimum two numbers of UHF PD couplers were mounted on
the GIS. The PD location technique works on calculating the time of flight
difference for the UHF signals to reach the two couplers from the same
PD source. We can detect PD discharge location anywhere between the
two sensors. This is subject to the condition that good quality PD signals
reaches the two sensors from the same PD source. For evaluating the
UHF PD location in GIS, the pulse propagation speed of UHF signal in
SF6 is required which is slightly less than speed of light in vacuum. Also
the actual distance between the two sensors is considered. The
corresponding UHF PD signal was captured by the two UHF couplers. By

using this technique, it is possible to find the discharge location within
the range of 0.5 m.
The foregoing is considered as illustrative only of the principles of the
invention. Furthermore, since numerous modifications and changes will
readily occur to those skilled in the art, it is not desired to limit the
invention to the exact construction and operation shown and described.
While the preferred embodiment has been described, the details may be
changed without departing from the invention, which is defined by the
claims.

We Claim:
1. A non-invasive method for monitoring partial discharge (PD) in a
Gas Insulated Switchgear (GIS) including estimation of a PD
source, the method comprising:
mounting a sensor on an opening among a plurality of
openings configured external to the GIS for sensing a partial
discharge (PD) induced high frequency signal; and
identifying a partial discharge (PD) source by capturing the
partial discharge (PD) induced high frequency signal sensed
by two sensors mounted on two openings among the
plurality of openings, wherein the PD source is located
internal to the GIS.
2. The method as claimed in claim 1, wherein the opening configured
external to the GIS is configured as an insulated aperture on a
support insulator without metallic cover (01), and wherein the
partial discharge (PD) induced high frequency signal generated
from the PD source is sensed by the sensor mountably attached
between a plurality of copper strips (02) connected to the aperture.
3. The method as claimed in claim 1, wherein the opening configured
external to the GIS is configured as an insulted aperture on a
support insulator integrated with metallic cover (04), and wherein
the partial discharge (PD) induced high frequency signal generated
from the PD source is sensed by the sensor mountably attached
between a plurality of copper strips (02) connected to the aperture.
4. The method as claimed in claim 1, wherein the opening configured
external to the GIS is configured as an insulated earthing switch
terminal (06), and wherein the partial discharge (PD) induced high
frequency signal generated from the PD source is sensed by the

sensor mountably attached to the insulated earthing switch
terminal (06).
5. The method as claimed in claim 4, wherein the insulated earthing
switch terminal (06) is of type maintenance earthing switch and
fast acting earthing switch.
6. The method as claimed in claim 1, wherein the opening configured
external to the GIS is configured as an insulated viewing port (07),
and wherein the partial discharge (PD) induced high frequency
signal generated from the PD source is sensed by the sensor
mountably attached to the insulated viewing port (07).
7. The method as claimed in claim 1, wherein the opening configured
external to the GIS is configured as an insulated gas-to-cable
terminal port (08), and wherein the partial discharge (PD) induced
high frequency signal generated from the PD source is sensed by
the sensor mountably attached to the insulated gas-to-cable
terminal port (08).
8. The method as claimed in claim 1, wherein the opening is one of a
rectangular, an elliptical and a circular shape, and wherein the
shape is selected based on shape of base of the sensors.
9. The method as claimed in claim 1, wherein the identifying the
partial discharge (PD) source by capturing the partial discharge (PD)
induced high frequency signal sensed by two sensors, further
comprises the steps of:
determining distance between the two sensors; and
calculating time of flight difference for the partial discharge
(PD) induced high frequency signal generated from the PD
source to reach the two sensors.

10.The method as claimed in claim 1, wherein the sensors are ultra
high frequency (UHF) sensors, and wherein each of the two
sensors are identical.