On-line Partial Discharge Monitoring Research Articles

Adaptive templates for matched filter bank for continuous online partial discharge monitoring

Matched filtering can be applied to recorded signals for maximum detection sensitivity during online partial discharge (PD) measurements in medium-voltage cable circuits. PD waveform predictions are combined with a noise spectrum estimation to construct matched filters. The PD waveform is specific for each cable circuit under test and the predictions are based on online system identification measurements and a standard cable model. For some cable circuits, for instance with a ring-main-unit along the cable, the predicted waveforms only approximate the real waveforms due to signal distortion not accounted for in the model. This results in suboptimal detection sensitivity. This paper describes an automated procedure for updating the matched filters based on detected PD signals. The algorithm analyzes detected pulses to judge whether the current set of matched filters perform optimally. If not, the current set of matched filters is updated, increasing the PD detection sensitivity and potentially also the location accuracy. At the same time, recognition of recurring disturbances is improved and better rejection criteria can be defined. The method is tested on signals measured on live cable connections.

Partial discharge parameters to evaluate the insulation condition of on-line located defects in medium voltage cable networks

Partial discharge analysis has a long record as reliable diagnostic tool to assess the integrity and the quality of electrical insulation of power systems. On-line partial discharge monitoring systems for live medium voltage cable connections have recently been introduced in Dutch grids and also a few worldwide. To enhance the applicability of on-line cable diagnostics there is a need to develop a PD interpretation approach suitable for on-line monitoring. This paper discusses various PD parameters, including PD charge magnitude, PD occurrence rate and PD charge density and their related statistical values such as mean and maximum, used to interpret the PD activity. Trend watching of these parameters is employed to study the degradation state of cable connections and their components. The approach is applied to field data obtained over a year through monitoring several live circuits. In this paper, three examples are presented. Weak spots were detected and failures were prevented by early warning.

Influence of Ring Main Units and Substations on Online Partial-Discharge Detection and Location in Medium-Voltage Cable Networks

Online partial-discharge (PD) monitoring of medium-voltage power cables is traditionally performed on a cable between two consecutive ring main units (RMUs). PD location is achieved using a sensor at each cable end. Monitoring multiple consecutive cables, with RMUs in between, using a single monitoring system is more efficient. In addition, substations and RMUs without possibilities for sensor installation can be circumvented by installing the sensor at the next RMU. This paper studies how RMUs and substations along the cable under test affect online monitoring, including their influence on detection sensitivity, location accuracy, and charge estimate accuracy. Single-sided PD measurement, including PD location, with an RMU or substation at the far end is also considered. Models for RMUs and substations are proposed and verified by measurements. The performance of online PD monitoring is studied for a number of network configurations.

An Online PD Pulses Recognition Method to Extract PD Pulses Buried in External Interferences

Partial discharge (PD) pulses recognition, which is directly related to the credibility of PD online measurement results, is the key technology of the high-voltage (HV) equipment PD online monitoring. In this paper, a pulses recognition method during PD online measurement was put forward to extract PD pulses buried in external interferences, the method adopted three key steps to identify PD pulses: (a) Firstly, two pretreatments, including discrete spectrum interferences (DSI) suppression and single pulse boundaries determination, are used to convert several consecutive power cycles on-site monitoring signal to plenty of pulse - time sequences; (b) Then, an optimized adaptive clustering algorithm is adopted to classify all pulses into certain categories (c) Finally, the three-dimensional distribution of each category which indicated the pulses phase distribution with power cycles was calculated, and the PD pulses were identified by the statistics characteristics of their three-dimensional distribution. At present, the PD pulses recognition method has been used in industrial application, and on-site monitoring signal processing results has proved the effectiveness of the method.

Insulation Deterioration Detection by Zero-Sequence Current Analysis for Induction Motors

Insulation tests for the stator coil, which decides the motor life, are carried out once several years in power and industrial plant. In the case of high voltage (HV) motors, insulation failure may adversely affect plant operation. Hence, maintenance activities, including insulation tests, research on insulation deterioration, and online Partial Discharge (PD) monitoring, are actively pursued for HV motors.On the other hand, the maintenance of low voltage (LV) motors is restricted to breakdown maintenance (BDM) because (1) a large number of LV motors are in operation in plants and (2) maintenance activities are relatively more expensive than those for HV motors. In this paper, the results of a study on insulation deterioration detection for LV motors are presented. For the deterioration detection, the deference in Zero-Sequence current waveforms between normal conditions and fault conditions is analyzed. This method helps in decreasing the maintenance cost and in online insulation diagnosis for LV motors.

Monitoring off-line and on-line PD under impulsive voltage on induction motors - Part 2: testing*

This article is the second of a series of three examining on-line partial discharge monitoring of Type I motors and highlighting critical aspects of IEC technical specification 60034-18-41. Motors passing the off-line type test can still experience premature failure when driven by variable-speed drives.

Application of data mining on partial discharge part I: predictive modelling classification

Innovations in computer technology have made possible continuous on-line monitoring of partial discharge (PD) activities. The power industry aims to assess the condition of power system equipment through on-line monitoring of PD activities. This involves long-term continuous data recording and it is very difficult to extract useful information from such a large amount of raw data, particularly if it is done manually. Instead, data mining can be applied in solving this problem. Data mining can be categorized into predictive modelling and descriptive modelling. In this paper, work was mainly focused on predictive data mining, which is classification of PD. The back propagation neural network (BPN), self-organizing map (SOM) and support vector machine (SVM) were used for classification and compared. Results indicate SVM is the best method in terms of classification accuracy and processing speed.

The Hardware Design of Partial Discharge Online Monitoring for Large Power Transformers System

The insulated state is one of the most important problems of large power transformer safely running in power system. The most efficient method in monitoring the running transformer’s insulated state is to extract partial discharge (PD) signals in real-time. In this paper, a hardware system of partial discharge online monitoring for large power transformers is presented. In hardware circuit implementation, notice to choose signal sensors, A/D convert unit and filter circuit are reasonably designed, at the same time, large capability storage circuit and the fault diagnosis system are designed.

A frequency-based RF partial discharge detector for low-power wireless sensing

Partial discharge (PD) monitoring has been the subject of significant research in recent years, which has given rise to a range of well-established PD detection and measurement techniques, such as acoustic and RF, on which condition monitoring systems for highvoltage equipment have been based. This paper presents a novel approach to partial discharge monitoring by using a low-cost, low-power RF detector. The detector employs a frequency-based technique that can distinguish between multiple partial discharge events and other impulsive noise sources within a substation, tracking defect severity over time and providing information pertaining to plant health. The detector is designed to operate as part of a wireless condition monitoring network, removing the need for additional wiring to be installed into substations whilst still gaining the benefits of the RF technique. This novel approach to PD detection not only provides a low-cost solution to on-line partial discharge monitoring, but also presents a means to deploy wide-scale RF monitoring without the associated costs of wide-band monitoring systems.

On-line partial discharge monitoring in medium voltage underground cables

For many years incipient partial discharge (PD) faults in power cables have been identified through off-line investigation techniques. More recently, in an effort to allow pro-active asset management of the medium-voltage (MV) cable network to be carried out, continuous on-line monitoring systems are being installed with the aim of reducing unexpected failures. This study presents work on the analysis and handling of acquired data, from the point of view of asset management and the PD activities observed in an on-line cable monitoring systems. Initially, a review of on-line against off-line cable PD monitoring is presented, in terms of their setups and their respective advantages and disadvantages. The study then presents the authors’ experience of applying wavelet-based denoising techniques [both the discrete wavelet transform (DWT) and the second generation wavelet transform (SGWT)] to PD data denoising. Results of a study on the on-line PD-based monitoring of MV underground cables that are presented in the following section demonstrate that PD activity which is observed in on-line monitoring also has an associated high level of electrical noise that must be removed to allow proper identification of the PD signals and the denoised PD activity is seen to vary with time. Finally, the necessity of developing a means by which knowledge rules can automatically be acquired from on-line condition monitoring data, to reduce reliance on human expertise, is discussed.

Assessing Test Data

Online partial discharge (PD) testing has been used for more than half a century to provide useful information for diagnosing and monitoring the integrity of stator winding insulation of large generators and motors. Interest in online PD detection and monitoring has as the technology, which was originally used on machines rated higher than 10,000 V, has expanded its application to the smaller, lower-voltage (e.g., 4,160 or 6,900 V) machines. However, questions remain on how to assess stator insulation problems using PD test data. This article documents the most recent developments in research and applicable guides and standards related to online PD monitoring on rotating machines.

Research on Noise Recognition and Suppression of Partial Discharge On-line Monitoring in Transformer

Research on Noise Recognition and Suppression of Partial Discharge On-line Monitoring in Transformer

Effect of mechanical vibration on the behavior of partial discharges in generator windings

As mechanical vibration usually exists in HV motors and generators, its influence on the partial discharge (PD) needs to be studied so as to improve the reliability of on-line PD monitoring method. In this report, the mechanical vibration frequency was synchronized to the power frequency and the effects of vibration phase and frequency were investigated. Because the length of the discharge gap changed with the vibration, obvious variation in the slot-PD patterns was observed at different vibration lead phase and frequency. Moreover, when the vibration amplitude was large, it was also found that the PD behavior at low voltage just above the PD inception voltage was different from that at higher voltage. This was explained in terms of the effects of the field variation in the gap due to the vibration and the charges left on the insulating surface due to PD.

An improved optically based PD detection system for continuous on-line monitoring of HV cables

An optically-based remote sensing technique using a laser source, polarization scrambler, standard single mode fibers, fiber polarizer, electro-optic (EO) modulator and optical receiver, for continuous on-line monitoring of partial discharges (PD) in high voltage cable joints is proposed. Both the fiber polarizer and EO modulator, which are placed near cable joints, are passive without any power requirements. An EO modulator with no electrical bias, which operates close to the half-intensity point, was used and proved to have much better light transmission performance and higher measurement sensitivity than the bias modulator approach investigated earlier. Various multiplexing techniques including optical switches and a fused fiber coupler have been investigated in order to implement multi-channel monitoring so that one laser source can be used to provide light to several fibers. The revised PD optical remote sensing technique has been applied to practical PD measurements on both 132 kV cable joint at the University of Southampton in the UK and 400 kV cable joints at Sudkabel GmbH in Germany. Obtained results have verified the principle and demonstrated the effectiveness of the developed technique.

On-Line Partial Discharge Monitoring and Diagnostic System for Power Transformer

This paper introduces a computerized on-line partial discharge (PD) monitoring and diagnostic system for transformers. The system, which is already in use in a power station, uses wide-band active transducers and a data acquisition unit with modularized and exchangeable components. The system software is a power equipment monitoring and diagnostic system, which is based on the component object model, and was developed for monitoring multiple parameters in multiple power supply systems. The statistical characteristics of PDs in power transformers were studied using 7 experimental models for simulating PDs in transformers and 3 models for simulating interfering discharges in air. The discharge features were analyzed using a 3-D pattern chart with a three-layer back-propagation artificial neural network used to recognize the patterns. The results show that PDs in air and oil can be distinguished. The model can be used for interference rejection on-line monitoring of partial discharge in transformers.

A novel calibration method for PD measurements in power cables and joints using capacitive couplers

Partial discharge (PD) measurements are universally accepted as a technique giving some indication of the state of the insulation in high-voltage apparatus. Cable end users are keen to adopt online PD monitoring during commissioning of systems. However, because of noisy environments and the problems of interference the conventional methods are difficult to implement. As a consequence, ultra-high frequency (UHF)/very high frequency (VHF) techniques of on-site detection of partial discharges have been developed. A technique based on capacitive coupling has received much attention. It has been demonstrated that it is possible to use the technique to monitor the partial discharge in cables, particularly in joints/terminations. However, in order to obtain quantitative information about PD, calibration is required for this technique. Existing calibration methods are difficult to implement on-site. In this paper, a novel method is proposed and compared with the conventional method on a short piece of cable. It has been shown that an individual capacitive coupler can also be accurately calibrated on-site and online using the new method, therefore it provides quantitative information about the amount of apparent discharge. In practice, this is important for electricity utilities as the quantitative information about PD can be used to determine the quality of the cable system and to decide whether the system needs to be repaired or replaced.

Advances in Online Monitoring and Localization of Partial Discharges in Large Rotating Machines

Some of the problems facing partial discharge (PD) measurements in rotating machines, especially during online measurements are: i) calibration of the measurements; ii) high noise levels interfering with both the measurement system and the winding under the test; as well as iii) attenuation and distortion of PD signals during transmission from the source point to the winding terminals. This paper presents a new technique and a measuring system for online PD monitoring of large rotating machines, developed by the authors. With this technique, the above mentioned problems are improved considerably. PD detection is implemented for every stator coil and the coil that PD comes from can also be identified. Inductive coils are used for decoupling of PD pulses and processing of the detected signals is in accordance with the developed technique, denoted Intech. Remote monitoring and navigation via a telephone link is one of the system features. The system was tested on both hydro and turbo generators with good results. The detection sensitivity of the system for the conventional machines is about 1 nC under operating condition.

A Development of Online Partial Discharge Monitoring System for High Voltage Motors, considering Insulation Deterioration Trend Analysis

High voltage motors have been applied to various industrial applications. Among the high voltage motors in service today, significant proportions were manufactured in the 1960s and 1970s. Machines of this generations will need to be refreshed sooner or later, due to the insulation deterioration.As to insulation deterioration diagnosis for high voltage motors, automatic and accurate Off-Line diagnosis system, where electric diagnosis, mechanical diagnosis and visual inspections are employed with Expert System for total evaluations has been already developed and is applied to various machine treatment and renewal cases.This time, authors have developed On-Line partial discharge monitoring system for those motors. Features of the system are, (1) Simple system configuration using single Split-Type High-Frequency CT, (2) Micro-computer aided Statistical Noise Reduction System based on Phase Synchronous Algorithm, (3) Micro computer aided automatic Likelihood valuation system, implementing expert evaluation for Partial Discharge Phenomena of the motors.This paper describes the system configurations and features. This paper also describes Insulation diagnosis method using the system, based on more than 30 experiences by actual machines of 300kW-10000kW

Development of an on-line partial discharge monitoring system for gas insulated substations

This paper describes the results of tests and discussions carried out to develop a system for partial discharge monitoring in GIS. They include the propagation characteristics of partial discharge pulses in each GIS component, as well as the test results obtained using an actual 300-kV GIS. We also report the results of verification test on the GIS partial discharge monitoring and location system developed by the authors. © 1999 Scripta Technica, Electr Eng Jpn, 129(2): 40–50, 1999

On-line partial discharge calibration and monitoring for power transformers

Partial discharge (PD) is one of the major factors causing insulation damage of power transformers. This paper describes the development of on-line partial discharge monitoring system for transformers. The developed set can monitor up to four power transformers continuously. It uses various methods to reject interference and enhances monitoring sensitivity of PD effectively. The optical cable is applied to communicate between measurement system near transformers and the upper computer in the main control building. On-line calibration of partial discharge quantity is requisite for on-line detection or monitoring. This paper describes an on-line calibration method using pulse injection through tap of high voltage transformer bushings. It also discusses the influence of limit inductance and bushing capacitance, from tap of bushing to ground, in on-line calibration circuits. Influences of multi-grounding of the transformer tank for monitoring and calibration are also discussed. Simulation in laboratory and site test shows that the method is practically valid for on-line PD calibration of power transformers within permissible engineering error range under certain defined conditions. This system has been in operation steadily in 500 kV substations for more than 2 years.