Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 1363

Electromagnetic Sensing of Partial Discharge in Air-insulatedMedium Voltage Switchgear

B. Zheng and A. BojovschiSchool of Electrical and Computer Engineering, RMIT University, Melbourne, VIC 3001, Australia

Abstract— The importance of detecting accurately the partial discharge in high voltage powerindustry becomes obvious as the infrastructure ages. In this work electromagnetic sensing fordetecting the electromagnetic radiation, associated with partial discharge, in an air-insulatedmedium voltage switchgear (Type D24-121114 of Driescher) is used. The study relies on FiniteElement Method as implemented in High Frequency Structure Simulator. The partial dischargeis approximated by Gaussian source. Coaxial patch antenna is employed for electromagneticsensing. This transducer is optimized to have highest efficiency in the frequency band of interest,for partial discharge detection, of 800 MHz to 900MHz. The antenna is placed in the switchgearsystem and its ability to sense partial discharge in the air-insulated switchgear is addressed. Theoptimized 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 thepartial discharges is used as an indicator of efficient radiative coupling. As repetitive partialdischarge leads to the failure of the air-insulated switchgear, this method provides a sensitivemethod 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 avital 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 diagnosticsin Gas Insulated Substations twenty years ago [1]. In that work the efficiency, sensitivity andapplicability of UHF method are determined. Since then, the UHF method has been extensivelyapplied 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 theUHF 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 PDis contained within the switchgear enclosure and can be detected by internal sensors purpose-fittedto 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 hugeinterest 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 oper-ating bandwidths, satisfactory radiation properties, compact structures, light weight, inexpensive,easiness of manufacturing. The patch antenna, also called as microstrip antenna, is used popularlyin the field of communication, such as mobile phones and personal computers. The investigationof interaction between EM wave induced by PD and patch antenna can be analyzed based on asimple transmission-line model [11]. It has been shown that PD on a twisted pair specimen of amotor winding can be detected effectively by a patch antenna [12].

Stemming from above consideration, coaxial patch antenna is proposed and investigated basedon our pre-established AIS system to achieve EM sensing of PD. The coaxial patch antenna ispotentially 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 isillustrated in Figure 1. The FR4 dielectric used as substrate has a thickness T = 48 mm, a relativepermittivity of 4.4 and loss tangent of 0.02. In the optimum design the radius of the circular patch(Figure 1) is R = 6.8mm. The size of the dielectric substrate and the ground is 40 mm × 40mmin the xy plane. The coaxial line feed has inner and outer radii of ri = 0.65mm and ro = 1.1mmand is filled with teflon of relative permittivity 2.1.

1364 PIERS Proceedings, Moscow, Russia, August 19–23, 2012

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 themedium voltage switchgear system (Type D 24 — 121114 of DRIESCHER — Compact Switchgears24 kV). The PD source is simulated numerically in the AIS by a Gaussian pulse [14] with the centrefrequency of 750 MHz and a width of 200 MHz. The radius (ω0) of the Gaussian beam waist is 10mmwhich corresponds with the size of the surface PD. The intensity of the Gaussian source is of 1 V/mand it is set to propagate in the x direction. This is related with discharge distribution. Theproposed coaxial patch antenna is designed to operate around the 800 MHz to 900 MHz frequencyband. This was chosen as all the PD activities such as cavity discharge, corona, dry-band arcingemit in this frequency band [15, 16].

3. RESULTS

In the optimizing process, there are mainly three important design parameters that affect the coaxialpatch antenna performance. They are the radius of the patch R, the thickness of the dielectricsubstrate T , the x and y dimensions of the patch (Figure 1). In Figure 2, three design variablesare 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 decreaseaccordingly. 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. Theresonance frequency is at 850MHz. The E-plane radiation pattern for the optimum structure ofthe 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 theproposed coaxial patch antenna.

Figure 4: Simulated radiation patterns at 850 MHzof 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

fc=

899 MHz− 806 MHz850MHz

× 100% ≈ 10.94% (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. Thepropagation of radiation from the PD creates spectral distributions of different intensity in theswitchgear represented in Figure 5 by magnetic field lines. The electric field induce in the patchantenna is shown in the same figure. Its maximum intensity is of 3.42 V/m. Considering that inthis 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 806MHz to 899 MHz frequencyband have been presented. It has been shown that the performance of the antenna in terms of itsfrequency domain characteristics is mostly dependent on the radius of the patch, the thickness ofthe substrate and the dimensions of the x and y dimensions. The optimized location of antennain the particular AIS system for an efficient sensing is presented. This indicates that the optimumlocation for fault detection can be predicted computationally for a given switchgear system.

1366 PIERS Proceedings, Moscow, Russia, August 19–23, 2012

ACKNOWLEDGMENT

The authors acknowledge Dr. Hubert Schlapp (SebaKMT, Germany) for providing the initial in-terest to this topic and for fairly representative documents. The authors would like to expressgratitude to Mr. Thomas Benke and Mr. Kyrie Hadjiloizou for providing technical support.

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