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Partial discharges and noise separation in high frequency signals using inductive sensors

J.M. Martínez-Tarifa1, M. Rojas,

G. Robles

Department of Electrical

Engineering Universidad Carlos III de Madrid Leganés (Madrid), Spain

jmmtarif@ing.uc3m.es1. B. MacPherson, P. Moore.

Elimpus Ltd,

Bellshill, United Kingdom

I. Portugués.

Electric Power Research Institute,

United States of America

Abstract—Partial discharges in high voltage equipment are a cause and consequence of premature ageing. Thus, its accurate measurement is an important diagnosis tool trying to avoid premature failures. Classical phase resolved patterns are now completed with pulse waveform analysis in order to discriminate between PD sources and PD from noise. For this purpose, high frequency inductive sensors are being used in recent years. In this paper, a fixed post-processing technique for PD pulse waveforms will be used to discriminate between discharges from electrical noise for two inductive sensors: a commercial High Frequency Current Transformer and a newly developed inductive loop sensor. Results will prove that this novel inductive loop sensor is able to discriminate PD from electrical noise for several test objects.

Keywords-component; partial discharges, electrical insulation, inductive sensor, pulse shape analysis, noise rejection.

I.I NTRODUCTION

Electrical equipment is usually exposed to several stresses through its whole lifetime. These devices (e.g. electrical machines and power cables) do not withstand voltages above their defined limits (dielectric strength) but, in addition to this, some of them show unexpected failures within their insulation systems at their rated voltages. A known source of insulation degradation are Partial Discharges (PD), that are a result of impurities or inhomogeneous areas inside insulation systems subjected to rated high voltages [1]. This phenomenon is a microscopic ionization localized at these sites, so they do not immediately lead to insulation breakdown; on the contrary, they are responsible for accelerated ageing through ion and electronic bombardment and resulting chemical attack [2].

Partial discharges are not only a cause, but a symptom of insulation degradation, so their measurement is an important technique for power equipment status assessment [3]. Conventional PD measuring systems detect the discharge pulse amplitude and represent it in a phase resolved pattern which is used for PD source identification. These systems are typically bandwidth limited to 400 kHz.

In order to help in PD interpretation it must be taken into account that not all kinds of PD are harmful for a long term operation of the equipment. In addition to this, PD measurements are commonly made on-line, so high levels of noise are always present in PD measurements [4]. Thus, conventional Phase Resolved Partial Discharge (PRPD) patterns sometimes cannot help in insulation diagnosis; furthermore, rejecting noise by means of increasing the trigger level is not reliable for PD detection in industrial environments because this means withdrawing low-magnitude PD pulses, that are more probable [1].

For these reasons, modern techniques have been developed to process PD pulse waveform information from high frequency sensors [5], [6], [7], [8]. These devices provide complementary information to conventional PRPD patterns for PD detection and classification. The most commonly used sensor is the High Frequency Current Transformer (HFCT), but other recently developed inductive loop sensors have been proposed as well [9]. Despite these loop sensors show less sensitivity, they are a cheaper approach for high frequency pulse detection than HFCTs, and do not suffer from saturation when applied to power cables feeding electrical equipment diagnosed on-line.

In this paper, PD pulse waveforms will be processed in order to distinguish them from noise pulses. This will be done using a previous technique proposed for pulse classification [5]. These pulses will be measured by means of an HFCT and an Inductive Loop Sensor (ILS). In this paper, the ILS pulse separation capability will be compared to that from HFCT for different test objects.

II.E XPERIMENTAL SETUP

Partial discharges have been measured in a classical indirect detection circuit, where high frequency pulses from ionization flow through the capacitive mesh formed by test object, and capacitive divider. A commercial PD detection circuit has been used in order to check PD activity in the test object. The inductive loop and the HFCT are placed at the low voltage terminal connecting the measurement impedance and ground (see Fig. 1).

This research has been supported by the Spanish Science and Technology Ministry under Contract No. DPI 2009-14628-C03-02. Tests have been carried out in the High Voltage Research and Tests Laboratory at Universidad Carlos III de Madrid (LINEALT). The research stage of Technician Brian MacPherson was financed by Elimpus Ltd.

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