Power utilities are struggling to reduce power failure incidents in substations and their components to operate more reliably and economically [1]. Many power failures are produced directly or indirectly because of the insulation system of utility components [2], [3]. The selection of the insulation should ensure power plant operational continuity along with completely resolving or significantly limiting the actual power system's failures [4]. Gas insulated substations (GIS) have the best insulation performance which ensures achieving minimum failure incidents, although at high installation cost. The most common insulating gas used in GIS is Sulfur hexafluoride (SF 6 ) gas, which is widely used as an effective electrical insulation as well as an arc-quenching medium [5]. Basic GIS and gas insulated transmission lines (GITL or GIL) consist of a conductor supported by solid insulators inside an enclosure filled with SF 6 gas or its mixture [6].

Partial discharge detection and diagnosis in gas insulated switchgear: State of the art

Among the different solutions which allow onsite partial discharge measurement in energized power transformers, the UHF technique is gaining general interest. In order to apply this method in existing transformers, it is considered advantageous to design a UHF sensor to be fitted inside the transformer tank. This paper describes the constraints affecting the development of such sensor and the process followed to optimize its design. During this process, different solutions for broadband UHF antennas were analysed. The selection was based on computer simulation and experimental results. Computer simulation was used to evaluate differences in radiation pattern, antenna impedance, gain and effective area. Measurements of the antenna response to different partial discharge sources in oil were performed using an oil dielectric breakdown test set in an electromagnetic shielded laboratory. A group of selected antennas were then tested in a power transformer simulating the broadband signals generated by partial discharges by injecting controlled voltage pulses in the transformer. Finally, a sensor based on a conical monopole antenna structure was built and attached to a specially designed housing to be fitted inside the transformer tank through the drain valve. This final prototype was benchmarked during a high voltage factory test of a power transformer which showed a significant partial discharge activity.

Optimisation of a sensor for onsite detection of partial discharges in power transformers by the UHF method

During online partial discharge (PD), ultra-high-frequency (UHF) detection of gas-insulated switchgear (GIS), obtaining acceptable levels of sensitivity, still continues to remain a very challenging task, primarily due to strong electromagnetic interference (including white-noise, random, discrete spectral, and stochastic pulses). As white-noise interference stands in the breach existing in GIS PD UHF detection inevitably, the available suppression methods are unsatisfactory. According to the feature of white noise, the Daubechie's complex wavelet is constructed. Through the simple information (SI) and combined information (CI) of the complex wavelet transform (CWT), the denoising method of simulation signals and field signals of PD UHF is studied by the authors. In view of the estimation parameters, the results manifest that white-noise in GIS PD UHF signals can be suppressed effectively by the SI and CI of the CWT.

Application of Complex Wavelet Transform to Suppress White Noise in GIS UHF PD Signals

In view of the structure characteristic and operation performance of the field gas insulation switchgear (GIS), an external ultra high frequency (UHF) sensor, a novel microstrip antenna, is designed to detect UHF partial discharge (PD) signals for the field GIS. The microstrip antenna, which receives the UHF electromagnetic wave leaked from the GIS spacer, is just mounted on the GIS spacer, and has some effect to suppress the surrounding space electromagnetic interference. The analysis and on-site measurements manifest that the microstrip antenna working frequency bandwidth is 340 MHz to 440 MHz, which voltage standing-wave ratio (VSWR) is less than 2, the center frequency is 390 MHz, the relative bandwidth is 25.6% up to the wide bandwidth range, and its gain of the maximum radiation and reception directivity is covered 5.38 dBi. Based on the laboratory GIS simulation device measurements, the investigation of the relationship between UHF PD amplitude and apparent discharge quantity based on 1EC60270 is presented as linear curve. Meanwhile, comparison to the field 220 kV level GIS, the quantitative charge evaluated to about 40 pC by the UHF method could give a preliminary diagnosis for GIS insulation conditions. And the UHF measurement system is suited for GIS field partial discharge UHF on line detection and diagnosis.

GIS partial discharge quantitative measurements using UHF microstrip antenna sensors

Online partial discharge (PD) detection still remains a very challenging task because of the strong electromagnetic interferences. In this paper, a new method of de-noising, using complex Daubechies wavelet (CDW) transform, has been proposed. It is a relatively recent enhancement to the real-valued wavelet transform because of two important properties, which are nearly shift invariant and availability of phase information. Those properties give CDW transform superiority over other real-valued wavelet transform. On the basis of CDW transform, complex threshold algorithm and combined information are devised. In order to take into account the difference of the real part and imaginary part of complex wavelet coefficients, complex threshold is devised to modify the real part and imaginary part respectively. Because the restored signal by inverse CDW transform is complex one, combined information is devised to combine the real part and imaginary part of complex signal to construct a real one, furthermore making full use of the simple information from complex signal. Copyright © 2009 John Wiley & Sons, Ltd.

Suppressing white‐noise in partial discharge measurements—part 1: construction of complex Daubechies wavelet and complex threshold

This paper describes the practical application of the ultra-high-frequency (UHF) partial discharge (PD) detection method which VA TECH Reyrolle has routinely applied to high-voltage onsite commissioning tests on gas-insulated substations (GISs) for more than 10 yr. The location of the PD sensors has a significant effect on the sensitivity of the UHF method, and research has been undertaken to quantify the parameters which affect this (i.e., attenuation, GIS geometry, etc.). Verifying the correct operation of the detection system and its sensitivity is a crucial step to prepare for a high-voltage (HV) test. In order to do this, special test equipment has been developed to ensure that all of the cabling, connections, etc., attached to the detection equipment are correct and that the detection level of the couplers is confirmed. Finally, the paper discusses IEC 60517 (general guidelines for GIS HV testing), its most usual interpretation, and how the application of the UHF technique can transform this interpretation.

C. Charlson

Papers

High-Voltage on-Site Commissioning Tests for Gas-Insulated Substations Using UHF Partial Discharge Detection