Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 1363
Electromagnetic Sensing of Partial Discharge in Air-insulated
Medium Voltage Switchgear
B. Zheng and A. Bojovschi
School of Electrical and Computer Engineering, RMIT University, Melbourne, VIC 3001, Australia
Abstract— The importance of detecting accurately the partial discharge in high voltage power
industry becomes obvious as the infrastructure ages. In this work electromagnetic sensing for
detecting the electromagnetic radiation, associated with partial discharge, in an air-insulated
medium voltage switchgear (Type D24-121114 of Driescher) is used. The study relies on Finite
Element Method as implemented in High Frequency Structure Simulator. The partial discharge
is approximated by Gaussian source. Coaxial patch antenna is employed for electromagnetic
sensing. This transducer is optimized to have highest efficiency in the frequency band of interest,
for partial discharge detection, of 800 MHz to 900 MHz. The antenna is placed in the switchgear
system and its ability to sense partial discharge in the air-insulated switchgear is addressed. The
optimized location in the switchgear system of the antenna for an efficient sensing is presented.
The current density induced in the electromagnetic sensors by the radiation emitted from the
partial discharges is used as an indicator of efficient radiative coupling. As repetitive partial
discharge leads to the failure of the air-insulated switchgear, this method provides a sensitive
method for pre-fault detection.
1. INTRODUCTION
With the increasing expectation of power system stakeholders on higher equipment reliability,
greater safety and lower cost, fault diagnosis of electrical equipment (e.g., switchgear) becomes a
vital task. As one of most important reasons for high voltage system failure, partial discharge (PD)
phenomenon can be detected accurately by electromagnetic (EM) radiation detection technique.
Among the techniques, ultra high frequency (UHF) method was initially applied for PD diagnostics
in Gas Insulated Substations twenty years ago [1]. In that work the efficiency, sensitivity and
applicability of UHF method are determined. Since then, the UHF method has been extensively
applied in gas insulated equipment worldwide with excellent results on-line or before commissioning.
This method overcame some of the well-known disadvantages of classical PD measurements [2, 3].
As a highly sensitive means of detection, UHF method is used to detect reliably PD signals in the
UHF band (300 MHz–3 GHz) because the noise level decreases at higher frequencies.
Recently PD in aged switchgear systems is a cause of concern. The EM radiation emitted by PD
is contained within the switchgear enclosure and can be detected by internal sensors purpose-fitted
to the enclosure or by external coupling device placed at appropriate apertures in the chamber.
PD identification and diagnosis in gas-insulted switchgear (GIS) using UHF sensors generated huge
interest and is actively studied worldwide [4–10], but UHF sensing of PD in air-insulated switchgear
(AIS) is rarely referred.
One of the sensors used for PD detection is patch antenna. Patch antenna features narrow operating bandwidths, satisfactory radiation properties, compact structures, light weight, inexpensive,
easiness of manufacturing. The patch antenna, also called as microstrip antenna, is used popularly
in the field of communication, such as mobile phones and personal computers. The investigation
of interaction between EM wave induced by PD and patch antenna can be analyzed based on a
simple transmission-line model [11]. It has been shown that PD on a twisted pair specimen of a
motor winding can be detected effectively by a patch antenna [12].
Stemming from above consideration, coaxial patch antenna is proposed and investigated based
on our pre-established AIS system to achieve EM sensing of PD. The coaxial patch antenna is
potentially capable of detecting accurately PD in the actual AIS system.
2. METHOD
The proposed coaxial patch antenna consists of a cupreous patch on a grounded substrate, which is
illustrated in Figure 1. The FR4 dielectric used as substrate has a thickness T = 48 mm, a relative
permittivity of 4.4 and loss tangent of 0.02. In the optimum design the radius of the circular patch
(Figure 1) is R = 6.8 mm. The size of the dielectric substrate and the ground is 40 mm × 40 mm
in the xy plane. The coaxial line feed has inner and outer radii of ri = 0.65 mm and ro = 1.1 mm
and is filled with teflon of relative permittivity 2.1.
PIERS Proceedings, Moscow, Russia, August 19–23, 2012
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Figure 1: Schematic configuration of the proposed circular coaxial patch antenna.
(a)
(b)
Figure 2: Experimental S11 results of various R, T dimensions of the patch antenna.
Finite Element Method (FEM) implemented in Ansoft High Frequency Structure Simulator
(HFSS) 13 [13] is utilized in this work to design, optimize the patch and to simulate it in the
medium voltage switchgear system (Type D 24 — 121114 of DRIESCHER — Compact Switchgears
24 kV). The PD source is simulated numerically in the AIS by a Gaussian pulse [14] with the centre
frequency of 750 MHz and a width of 200 MHz. The radius (ω0 ) of the Gaussian beam waist is 10 mm
which corresponds with the size of the surface PD. The intensity of the Gaussian source is of 1 V/m
and it is set to propagate in the x direction. This is related with discharge distribution. The
proposed coaxial patch antenna is designed to operate around the 800 MHz to 900 MHz frequency
band. This was chosen as all the PD activities such as cavity discharge, corona, dry-band arcing
emit in this frequency band [15, 16].
3. RESULTS
In the optimizing process, there are mainly three important design parameters that affect the coaxial
patch antenna performance. They are the radius of the patch R, the thickness of the dielectric
substrate T , the x and y dimensions of the patch (Figure 1). In Figure 2, three design variables
are parameterized. This led to the optimum structure with the 800 to 900 MHz frequency band.
From Figure 2(a) it can be noted that with the increase of R, the resonance frequency will decrease
accordingly. The effect on the return loss (S11 ) curves of the T is also indicated in Figure 2(b).
They not only affect the resonance frequency, but also vary significantly the dB level.
Figure 3 shows the simulated return loss for the optimized structure of the patch antenna. The
resonance frequency is at 850 MHz. The E-plane radiation pattern for the optimum structure of
the patch antenna is shown in Figure 4. The fractional bandwidth (FBW) of the coaxial patch
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 1365
Figure 3: Simulated return loss curves (S11 ) of the
proposed coaxial patch antenna.
Figure 4: Simulated radiation patterns at 850 MHz
of the proposed coaxial patch antenna (E plane).
Figure 5: Optimized location of coaxial patch antenna in the AIS system.
antenna at −10 dB return loss can be calculated using the following equation:
FBW =
f2 − f1
899 MHz − 806 MHz
=
× 100% ≈ 10.94%
fc
850 MHz
(1)
The location of the PD sensors has a significant effect on the sensitivity of the UHF method.
The optimized location of the designed sensor in the switchgear enclosure is showed below. The
propagation of radiation from the PD creates spectral distributions of different intensity in the
switchgear represented in Figure 5 by magnetic field lines. The electric field induce in the patch
antenna is shown in the same figure. Its maximum intensity is of 3.42 V/m. Considering that in
this study the PD source has an intensity of 1 V/m an accurate detection is possible.
4. CONCLUSIONS
The simulation results of a circular coaxial patch antenna covering 806 MHz to 899 MHz frequency
band have been presented. It has been shown that the performance of the antenna in terms of its
frequency domain characteristics is mostly dependent on the radius of the patch, the thickness of
the substrate and the dimensions of the x and y dimensions. The optimized location of antenna
in the particular AIS system for an efficient sensing is presented. This indicates that the optimum
location for fault detection can be predicted computationally for a given switchgear system.
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PIERS Proceedings, Moscow, Russia, August 19–23, 2012
ACKNOWLEDGMENT
The authors acknowledge Dr. Hubert Schlapp (SebaKMT, Germany) for providing the initial interest to this topic and for fairly representative documents. The authors would like to express
gratitude to Mr. Thomas Benke and Mr. Kyrie Hadjiloizou for providing technical support.
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