On-line PD System and PD Locator for XLPE Cables

This paper introduces a computerized online PD (Partial Discharge) monitoring and diagnostic system for power transformers, which has been installed in a 500 kV substation in China. It consists of wide-band active transducers and DAU (Data Acquisition Unit) with modularized and exchangeable components. This system has been developed to monitor multiple parameters of multiple power transformers, which is based on COM (Component Object Model). In addition, the system can also be used to monitor other power equipment. Some results of on-line monitoring are discussed.

Partial discharge (PD) based condition monitoring has been widely applied to power cables. However, difficulties in interpretation of measurement results (location and criticality) remain to be tackled. This paper aims to develop further knowledge in PD signal propagation in power cables and attenuation by the PD monitoring system devices to address the localization and criticality issues. As on-line or in-service PD monitoring sensors commonly comprise of a high frequency current transformer (HFCT) and a high-pass filter, the characteristics of detected PD pulses depend on the attenuation of the cable, the HFCT used and the filter applied. Simulation of pulse propagation in a cable and PD monitoring system are performed, based on analyses in the frequency domain using the concept of transfer functions. Results have been verified by laboratory experiments and using on-site PD measurements. The knowledge gained from the research on the change in pulse characteristics propagating in a cable and through a PD detection system can be very useful to PD denoising and for development of a PD localization technique.

The assessment of the insulation condition of high-voltage power transmission networks using a non-invasive method of online partial discharge (PD) monitoring is currently a common practice in maintenance programs for elements of the distribution and transmission networks. The joint-stock company „Elektromreža Srbije“ Belgrade (EMS AD) manages a large number of power equipment (EES), on which various partial discharge monitoring strategies can be applied to control the insulation condition. One of the goals of EMS AD is to implement a real-time monitoring system aimed at optimizing the maintenance process and conducting diagnostics for the early detection of insulation defects and predicting failures in cable lines. The paper describes the activities related to the implementation of an online monitoring system for partial discharge detection, which involves the installation of permanent high-frequency sensors at strategic locations to diagnose the insulation condition of cable lines. Additionally, different levels of diagnostics are presented through the use of a software tool for automatically eliminating partial discharges that do not originate from insulation defects, as well as a tool for localizing partial discharges along the length of the cable line.

This paper has been prepared to illustrate the value of partial discharge (PD) testing in assessing the condition of stator windings on operating generators and motors. Because of the magnitude of the data base accumulated by one particular PD system vendor during the last 15-20 years, this data base was chosen for study. It was felt that sufficient data had been accumulated to allow useful statistical analysis of the capabilities of PD testing. This paper is based somewhat narrowly on data from only one company; however, there are several other companies involved in PD testing and evaluation. As described below in this paper, these companies have also been successful in identifying stator winding problems through the use of PD equipment. PD monitoring systems appear to be finding conditions of concern on about 6% of generators where the equipment is installed. Probably no other generator monitoring system exceeds this rate of problem detection. Because of the power of PD testing, it is hoped and expected that use of PD monitoring will continue to expand at a significant rate in both utility and industrial power plants.

Online partial discharge monitoring system (PDMS) was developed to assess the condition of stator winding in high voltage motors. 80 pF capacitive couplers were connected to a 6.6 kV motor stator coil terminal. The voltage applied to stator coils were at 3.81 kV, 4.76 kV and 6.6 kV, respectively. These stator coils have various types of artificial insulation defects such as large voids, semi-conductive coating damage and strand insulation fault. On-line PD monitoring system, digital PD detector (PDD) and turbine generator analyzer (TGA) were used to measure PD activity. Most of the defect mechanisms of a large motor can be associated with PD patterns such as internal, slot and conductor surface discharges. PD patterns coincide with on-line PD monitoring system, PDD and TGA. These instruments have an input bandwidth of 100 kHz-50 MHz, 40-400 kHz and 0.1350 MHz. Continuous on-line PD monitoring system was measuring the discharge activity of large motors that were installed with 80 pF capacitive couplers at the terminal box.

Radio frequency (RF) detection of partial discharges (PD) is a long-established principle, being described in the international standard for partial discharge measurement. The growing market for mobile communication devices in recent years has led to big reductions in the cost of electronic hardware operating in the VHF and UHF bands, making advanced measurements of PD in these bands increasingly affordable. Some 20 years ago, ultra-high frequency methods for PD detection were pioneered in gas-insulated substations (GIS) and have since become widely accepted. Locating PD in GIS is essentially a one-dimensional problem. More recent research into PD location in power transformers, also using the UHF method, has had to address the more challenging problem of PD location in three-dimensions. Both the GIS and transformer applications monitor enclosed volumes, where the PD is within a metalclad system that traps the signal and provides good screening of external RF interference. At the same time, other researchers have developed RF detection and location systems for open environments, such as complete air-insulated substations. When deployed in a fixed installation or a surveying vehicle equipped with GPS, this system is capable of monitoring insulation defects as they develop, and provides a location capability using sophisticated signal processing. This paper begins with an overview of some aspects of radiometric monitoring techniques and goes on to outline some research challenges which it is hoped will attract the attention of the many researchers who are continuing to advance the areas of radiometric PD detection, measurement and location technology.

A Partial Discharge (PD) monitoring system for Air Insulated Substations (AIS) has been developed for monitoring of partial discharge activities of various insulators and insulating media that are distributed over an electrical substation. The proposed system utilizes four UHF sensors, which are to be placed around the area of interest so that any impulsive UHF signal originated from PD activity can be captured and subsequently processed for detection and localization of multiple PD sources. To improve the localization accuracy and to improve the detection specificity, i.e. to eliminate false impulsive UHF signals originated from other sources, a novel nearest-neighbour based approach is proposed, which utilizes the physical layout of the AIS to identify most probable PD source locations. The proposed algorithm can accurately find the PD source locations even in the presence of high time-delay-of-arrival (TDOA) estimation error by making necessary corrections in the PD source location estimation. The proposed algorithm has been field trialed in many small and medium-size distribution substations. A detailed comparative study with the other algorithms in different extreme situations has been performed with the help of simulation and laboratory-based testing. The obtained results are worthwhile for supporting widespread implementation of the proposed method.

Partial discharge (PD) monitoring applications enable the performance of appropriate diagnosis regarding the insulation condition of high-voltage (HV) electrical grids. During the last decade on-line PD monitoring systems have been increasingly implemented by utilities and large electricity consumers in HV installations. Therefore, the evaluation of their performance is interesting in order to check whether they are generally effective with respect to their functionalities. For the characterisation of the PD monitoring instruments and of the requirements of acquisition time (number of cycles per second), the performance of laboratory tests with real insulation defects is required. As these are monitoring tests, they entail a long time (at least one day), and during this time the monitored insulation defects must behave in a stable manner. The main scope of the study presented is to design, manufacture and characterise the behaviour of three test cells, each integrating a characteristic insulation defect, with the aim of using them in laboratory monitoring tests lasting at least one day. The considered insulation defects are as follows: an internal cavity in a solid dielectric, corona effect and a surface defect. The measurements carried out with the designed test cells have shown that they are suitable for their implementation in PD monitoring tests of long duration.

Rapidly increasing growth and automation of power are given the opportunity to power engineers to incorporate new ideas in the field of online monitoring of power apparatus for reliable operation of electrical system. As a part of new innovation in power system, photonics play an important role for driving innovation toward the sensor development, optical data communication to imaging, lighting and displays. In any power apparatus, degradation of insulation occurs due to the accumulation of mechanical, thermal and electrical effects for a long period. As the insulation of high voltage (HV) power apparatus are deteriorated by partial discharges (PDs), detection and measurement of PDs are of utmost important for the quality assessment of HV power equipment. In this chapter, an attempt has been made to detect the PDs with the help of photonic technology which eliminates the electrical interference; a challenge is being faced by power engineers in recent decades. It is understood from the several studies, PDs of HV power apparatus can be detected by electrical, chemical and acoustic methods. Electrical detection focuses on capturing the electrical impulses created by the current streamer in the void and impurities. Electrical detection method has some limitations. The primary limitation is its susceptibility to noise. The high voltage transformer (HVT) environment contains high levels of electrical noise, both narrowband and broadband. Chemical detection has also some drawbacks. Firstly, this detection method does not give any information about the location of the PD and the insulation condition. Secondly, the main problem is that chemical tests cannot be performed continuously when the HV equipment are in working operation. To explore the detection of PDs through acoustic method, acoustic phenomenon of PDs has been studied, and it is found that it leads to the propagation of acoustic waves having frequencies typically between 30 and 1000 kHz inside the transformer. The propagation of acoustic wave modulates the refractive index of the medium, which modulates the intensity of the laser beam in sympathy with the acoustic waves. Acoustic methods have many advantages over other methods in that they are immune to electromagnetic interference (EMI), which can greatly reduce the sensitivity of electrical methods and they can be applied to find failure spot and location of discharges. Optical technique is also capable to couple PDs with the optical signal. To capture the acoustic waves, an optical method is demonstrated in this chapter for detection and localization of PD sources by using photo detector inside the HV power apparatus. In this work, a new concept such as direct optical partial discharge detection technique has been introduced, where the energy exchange mechanism of electrical to acoustic is used for online monitoring of PDs. The acoustic signals are collected directly through the photonic technology, which is free from electrical interference, and finally they are converted to electrical signals for detection of amplitude as well as location of PDs occurring inside the HV power apparatus. This is a cost-effective tool for online monitoring of PDs and can be used as a predictive maintenance tool for utilities.

The existing on-line monitoring systems of cable are poor in mobility, cost performance and networking mode. Aiming at these deficiencies, a mobile on-line partial discharge monitoring system of XLPE cable is designed and developed in this paper. Besides the analysis of partial discharge pulse and diagnosis for cable insulation, the system features on flexible networking modes. The paper describes the hardware theory, development process details and significant parameters. And the architecture and interface of the system’s software are also demonstrated. A series of test and installation proves that, this is a flexible, stable, reliable system which can satisfy the requirement on on-line partial discharge monitoring of XLPE cable.

For specific circuits the accuracy of online partial discharge (PD) detection and location (PD-OL) systems is impaired by complications in the cable circuit configuration, such as ring-main-units (RMUs) along the cable under test. For maximum sensitivity matched filtering is applied for extracting PD pulses from the recorded signal. The matched filters are based on a series of prior online system identification measurements and a standard cable model. Especially, with an RMU along the cable, the matched filters only approximate the real waveforms due to signal distortion. This results in sub-optimal detection sensitivity. This paper describes automated procedure for updating the matched filter bank based on measured (large) PD signals. The algorithm analyzes detected pulses to judge whether the current templates perform optimally. If not, a new template is created increasing the PD detection sensitivity and the location accuracy, and also improving the recognition of recurring disturbances. The method is tested on signals measured on a live cable connection.

Two major tasks of partial discharges (PD) measurements may be distinguished, (i) providing general evidence and the type of PD (detection) and (ii) the location of the PD. Dependent on the type of device under test the two issues have changing priority. For the on-line/on-site PD location in power transformers unconventional PD measuring methods like acoustic ultra-sonic measurements or electromagnetic measurements up to the UHF (ultra high frequencies) range are performed, while the Time Domain Reflectometry (TDR) of electric PD signals is a standard technique for the location of PD in power cables.

The main problem in power transformers is the degradation of the isolation, and partial discharges are a major contribution to this shortcoming. Partial discharges inside medium-and high-power transformers generate ultrasonic pressure waves which are usually detected by using acoustic piezoelectric sensors placed outside the transformers. In this paper we describe an optical fibre interferometric sensor we are developing for the detection and location of partial discharges by measuring inside the power transformers. This non-intrusive optical fibre sensor allows a high level of sensitivity to be achieved for the typical small pressure variations created by partial discharges, specially if compared with the results obtained by acoustic piezoelectric external sensors. We present interferometrically detected levels of partial discharge. These results indicate that the developed optical fibre sensor is able to detect a partial discharge in the range of 103 pC that produces an optical phase shift of about 2 rad with a resolution of 30 pC in charge means. Finally, the wavelet analysis is used to identify the acoustic patterns instead of the more commonly used Fourier analysis. When transforming the measured signal to the frequency domain, the temporal information disappears. But the acoustic signals really contain numerous transitory pieces of information to be considered. By looking at a Fourier-transformed signal it is impossible to state when a particular event took place. The wavelet analysis helps to solve this problem, and some results are presented here to show this.

The acoustic emission (AE) method could be used to detect and locate partial discharges (PD) in cast-resin dry-type transformers. However, due to the high sound attenuation in the filled epoxy, the signal is prone to interference from external noises and thus, in practice, there is little possibility of detecting PD. In this study, two techniques were developed to alleviate the shortcomings of the AE method. First, a waveguide is installed on the high-voltage (HV) windings, so that the acoustic signals of PD will propagate to the AE sensors that are installed on both terminals of the waveguide. The location of the winding that has PD can then be detected from the difference in arrival time of the acoustic signals. Test results indicate that the waveguide technique is able to enhance the safety of a measurement system and offers the advantages of easy installation and higher flexibility. Second, a specially designed AE sensor pair is used to distinguish whether acoustic signals are generated by PD inside the HV winding or by the corona outside the transformers. Using these two techniques of waveguide and AE sensor pair not only greatly improves sensitivity but also increases the reliability of the measurement system. Practical test results show that the new techniques can be used to locate precisely the PD in HV windings.

The objective of this work is to assess the efficacy and reliability of a certain technique of acoustic emission measurement for detection and location of partial discharges in power transformers. The possibility of locating a simulated partial discharge was studied in the laboratory. The experimental setup involved an arrangement of increasing complexity, starting with a simple tank filled with insulating oil, up to simulation of an unenergized transformer. It was concluded that, under careful control of experimental parameters, such location is effective. The study was complemented with observations on transformers in the field. In this case, results were less reliable. While the acoustic emission test was not sensitive to intense discharges within the internal winding of a transformer, problems in two tap-switches and one mechanical problem were detected. Considering that the literature reports doubts on the effectiveness of the technique, it was found that measurement of acoustic emission is useful for defects external to the transformer winding. Taking into account its success in locating some kinds of defects, partial discharge measurements by both acoustic and electromagnetic means supplement other usual tests to increase the reliability of the final diagnosis.