Partial Discharge Measurement System Research Articles

Partial Discharge Monitoring of Power Transformers by Calibrated UHF Measurements

Partial discharge (PD) measurement is an established technique to detect local defects in the oil/paper insulation. They can be measured by electrical PD measurements according to IEC 60270 or by electromagnetic measurements in the ultra-high frequency range (UHF: 300 MHz – 3 GHz). The electromagnetic emissions of PD are recorded using an UHF sensor which is inserted into the transformer tank. The importance of PD measurements is reflected in the standard for charge-based electrical measurements (IEC 60270). Because of the standard, apparent charge, QIEC, became an essential factor for determining insulation quality in transformers. To be an accepted factor for both, quality testing in factory acceptance tests (FAT) or site acceptance tests (SAT) and continuous PD monitoring, the UHF technology must be as reliable and reproducible as the electrical method. To achieve this, a standardized calibration procedure is needed: It makes UHF measurement results from different systems comparable. Because this calibration procedure was lacking in the past, the UHF technology does not provide comparability between measurement systems and different sensors, yet. However, it shows advantages for on-site acceptance tests like SAT, continuous monitoring, and diagnostic purposes. E.g., regarding signal to noise ratios, the possibility for localization of PD sources and also for practical reasons like preparation times and accessibility on-site. This contribution presents a PD monitoring system which can be calibrated according to the calibration process described recently in Cigré TB 861 and two different types of UHF PD sensors for power transformers. One sensor type is for new transformers using dielectric windows and the other is for retrofitting transformers using their DN50/80 drain valves. A recommendation or strategy on where to place window-type UHF sensors at the tank of new power transformers is also provided in accordance with TB 861. Additionally, this contribution provides a description of the calibration process for UHF PD measurement systems consisting of two steps: 1) Calibration of the measurement instrument analogous to the calibration of the electrical PD measurement circuit using a defined reference signal. 2) Calibration of a UHF sensor using its characteristic antenna factor (AF). Furthermore, it shows three case studies from different power transformers equipped with UHF sensors and UHF PD monitoring systems.

Nanosize Filler Effect on Propagation of Electron Avalanche in Terms of Waveform Characteristics of Partial Discharge

Nanocomposite materials have excellent insulating properties, such as improved treeing resistance. The propagation properties of electrical trees are often determined using optical observations and partial discharge (PD) characteristics regarding phase angle distribution patterns. There is a limit to confirming the effects of physical barriers on the PD by optical observation alone. Conversely, there is a high possibility that the PD waveform reflects the characteristics of the electron avalanche in the discharge space. In this study, we constructed a PD measurement system with the widest possible frequency range and acquired 1000 PD waveforms along the time series. Although the acquired waveforms have a single peak or multiple peaks, adding nano-fillers increased the observation ratio of the waveform with multiple peaks. Furthermore, the multiple peaks suggested that the electron avalanche progresses with difficulty considering that the nano-fillers easily captures electrons. We investigated the effect of adding nano-fillers from the viewpoint of the waveform characteristics. The time difference between the peaks and the peak ratio is also supported by the property changes in the electron avalanche because of the electron capture by nano-fillers.

Development of a Microcontroller-Based Phase-Resolved Partial Discharge Measurement System With Application to the Monitoring of Flash Sintering Discharge Patterns

Development of a Microcontroller-Based Phase-Resolved Partial Discharge Measurement System With Application to the Monitoring of Flash Sintering Discharge Patterns

Application of a Rogowski Coil Sensor for Separating Internal and External Partial Discharge Pulses in Power Transformers

Partial discharge (PD) measurement is an essential tool for evaluating the health condition of a transformer insulation system. The Conventional PD measurement systems have a major drawback since they cannot discriminate the internal PD pulses and external pulse-shaped interferences, especially in online and on-site conditions. Various de-noising techniques (e.g. wavelets) have been also used in the literature, which is effective for extracting the PD signal from the noisy signals or classifying the PD types. However, the PD pulses and some of the randomly external pulse-shaped interferences have similar time-frequency characteristics, which makes them difficult to separate. In this paper, a new cost-effective and simple logic method is proposed for the separation of these interferences. This uses the polarity comparison of the output voltages of the Rogowski Coil (RC) sensor, mounted on the bushing, and measurement impedance, connected to the test tap. The same polarity represents the internal pulse, while the opposite polarity indicates the external pulses. A distribution transformer ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$20/0.4\ {\bm{kV}},\ 500\ {\bm{kVA}})$</tex-math></inline-formula> and a condenser bushing ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$72.5\ {\bm{kV}})$</tex-math></inline-formula> were considered to evaluate the performance of the proposed system. The results confirm the effectiveness of the proposed PD measurement system for separating the pulses.

Comparative Study on Partial Discharge Characteristics of Seven Accessory Defects Under Four On-Site Testing Voltages of a 10-kV Cable Line

Due to the increase in voltage levels and line lengths of power cables, the commonly used alternating current resonance (ACR) voltage is challenging to meet the growth rate of the capacitance of ultra-high-voltage cables for the on-site partial discharge (PD) tests. PD research on power-frequency-like voltages such as damped alternating current (DAC) voltage, very low-frequency sine wave (VLF-Sine) voltage, and very low-frequency cosine rectangular wave (VLF-CR) voltage, has important engineering value for evaluating the cable insulation conditions. In this paper, a PD test and measurement system of a 10 kV XLPE cable line with a 2 km length is established. Seven typical accessory defects are made artificially, including insulation hole, insulation screen tip, ring-cut tool mark, and stress cone installation dislocation defects at the terminals, and connector tip, conductive impurity, and stress cone installation dislocation defects at the joints. PD tests under four kinds of testing voltages of ACR, DAC, VLF-Sine, and VLF-CR are carried out to compare the PD inception voltages (PDIV), phase-resolved PD (PRPD) patterns, and PD location results of various defects. PD equivalence between power-frequency-like voltages and ACR is characterized by PDIV equivalent coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PDIV</sub> , PRPD pattern characteristic quantities and discharge amplitudes. The results show that DAC and VLF-Sine have an outstanding ability to detect and characterize the severity of PD. DAC and VLF-CR have a good ability to recognize the type of PD. And DAC, VLF-Sine and VLF-CR can accurately locate PD. This study provides an important reference for on-site cable defect diagnosis and PD assessment.

Time–Frequency Maps for Multiple Partial Discharge Sources Separation in Cable Terminations

This letter proposes improved time-frequency maps for separating multiple partial discharge (PD) sources in cable terminations. PD signals are acquired through a digital measurement system and a high frequency current transformer. The waveform of each pulse is analyzed by synchrosqueezed wavelet transform, and six representative feature parameters are proposed to form time–frequency maps. The results show that the proposed algorithm is effective for discriminating mixed PD sources in cable terminations and pulses from power electronic devices detected simultaneously by the PD measurement system.

Partial Discharge Localization in Oil through Acoustic Emission Technique Utilizing Fuzzy Logic

This study presents an examination of partial discharge (PD) localization in oil based on acoustic emission technique utilizing fuzzy logic. First, the PD measurement and localization systems are developed. In total, 2 coordinates of needle-plane electrodes that represent the PD location and 336 acoustic emission sensor (AES) coordinates are examined. The electrical PD is measured through an impedance matching circuit (IMC). The acoustic PD is acquired through AES and its signal is boosted by a preamplifier gain unit. The acoustic and electrical PDs are collected once the voltage reaches 30 kV. The data are first preprocessed by discrete wavelet transform (DWT) whereby it will be further analyzed by time of arrival (TOA), fuzzy logic Mamdani (FLM), and fuzzy logic Takagi&#x2013;Sugeno (FLTS) methods. The arrival of time and distance between PD and AES are calculated based on TOA to find the PD location. Next, TOA and distance to PD location are used to define the input and output membership functions for FLM and FLTS to determine the PD location. It is found that FLTS has a great potential to be used for localization of the PD in oil whereby it has the lowest errors between actual PD location and AES. The performance of FLM and TOA is inferior to FLTS whereby the errors between actual PD location and AES are quite high for both PD locations.

Partial Discharge Detection Based on Optimization of Optical Probe and Sagnac Interference

Partial discharge (PD) measurement is necessary for insulation diagnosis and usually a required item of the maintenance procedure for most high voltage equipment assets. optical fiber sensor (OFS) provides an innovative and nondestructive approach to convert PD-induced acoustic signal into optical parameter, but the optical probe is challengeable since it is the key component to pick up the extremely weak PD-induced acoustic signal. A universal three-dimensional skeleton-type fiber probe is designed. To optimize the specific parameters of the optical probe, the multi-parameter sensitivity model of tiny skeleton-type optical fiber probe is proposed and established, involving in the material, physical height, radius, and the sensing fiber length. A skeleton-type probe with a wingding length of 60 m, the skeleton height of 12 mm, and peek material is designed for PD ultrasonic sensing. Theoretical modeling, mathematical simulation, sensitivity calibration, and comparative PD tests with regard to the sensing probe and the corresponding specific optical PD measurement system. PD results show that the average detection amplitude of the developed optical fiber sensor is 1.27 times that of commercial piezoelectric transducer (PZT). The flexible installation and the enhanced sensitivity make the Sagnac ultrasonic sensing system an excellent approach to detect and locate weak PD signals inside the power apparatus.

Application and verification of partial discharge measurement system for power cable based on oscillating wave voltage

Abstract Compared with conventional methods for insulation performance evaluation of power cables, the oscillating wave test system used in partial discharge measurement for power cables has advantages of high integrity, easy operation, low power consumption, and compact size in addition, partial discharge, defect localization, and dielectric loss can be measured simultaneously without any damage to cable insulation. Therefore, the oscillating wave test system has been widely applied for insulation performance evaluation of newly installed as well as fault power cables. However, there is no study so far on the verification method of oscillating wave test system. This paper dealt with the application and verification for cable partial discharge measurement devices based on oscillating wave voltage, which is aimed to verify performances of oscillating wave voltage generator, partial discharge measurement, and partial discharge localization. The proposed verification system is expected to be applied in the admittance testing and regular verification of oscillating wave test system, for the purpose of improving the its accuracy and standardization in partial discharge measurement of power cable.

Modelling and simulation of online partial discharge measurement for medium voltage power cable

This paper presents the modelling of the online partial discharge (PD) measurement of the medium voltage (MV) power cable. Recently, PD monitoring trends are rapidly increasing due to high demand on reliable systems. Degradation are mainly due to the presence of PD in the high voltage power equipment used. PD measurement is therefore a highly recommended task to early detection of the degradation insulation for high voltage (HV) equipment in order to avoid breakdowns. Real network modelling is necessary to improvise system design in order to find the efficiency in a real power system network. In this paper, modelling focuses on a real distribution network by applying Rogowski coil (RC) as a detection sensor to trigger PD activity. The simulation is performed to determine the functionality and reliability of the system with the RC application in the network. The analysis is performed in the ATP-EMTP and MATLAB Simulink software environments. In addition, this paper contributed to justify the approach of a simplified PD sensor and measurement system. This PD measurement system provides a complete solution in the context of condition-oriented monitoring for the ability to apply the RC to trigger PD activity in the power distribution network.

Surface charge accumulation and pre‐flashover characteristics induced by metal particles on the insulator surfaces of 1100 kV GILs under AC voltage

With the increase in the voltage and capacitance of gas-insulated transmission lines (GILs), the insulation failure of GIL has attracted more and more attention. Further understanding of the partial discharge (PD) and flashover characteristics of metal particles on the insulator surface, as well as the flashover mechanism, is necessary to reduce the failure rate and improve the reliability of the equipment. In this study, an ultra-high voltage (UHV) AC insulator PD test and measurement system based on the cross-reference pulse current (PC) and ultra-high frequency (UHF) methods were established. The PD development and flashover characteristics of 5 mm-long metal particles at different positions on the surface of a UHV AC insulator, as well as the surface charge accumulation, were studied. The results show that the discharge of millimetre-scale metal particles on the insulator surface under PD test conditions is relatively low (generally lower than 2PC), and it is difficult to be detected by the conventional PC and UHF methods due to their insufficient sensitivity. Moreover, it is found that the weak PDs of the millimetre-scale metal particles will result in a charge accumulation on the insulator surface under AC voltage, which eventually will lead to the insulator flashover. The PDs of the UHV AC insulator before flashover are generally small, which will make it difficult for online monitoring systems to give an effective alarm before flashover.

Influence of temperature on partial discharge of insulating PET backsheet in photovoltaic module

Solar energy has been widely used as a kind of renewable power generation in recent years. In a photovoltaic (PV) system, polyethylene terephthalate (PET) is the core component of backsheet and affects the reliability of the system. In this paper, the effect of temperature on partial discharge (PD) of PET is studied. As shown by the PD measurement system, the amplitude of PD of PET films decreases significantly with temperature increasing. The mechanism of PD degradation, which is closely related to the operational temperature and the chemical change of the insulation backsheet, has been investigated.

Partial discharge measurement and analysis in PPIs

Electrical insulation capability is one of the most critical challenges for press-pack insulated gate bipolar transistors (IGBTs), but the knowledge of its insulation failure mechanism is much less mature. To understand the insulation failure mechanism, partial discharge (PD) measurement and analysis in press-pack IGBT (PPI) submodule were performed in this study. A dedicated PD measurement system for PPIs was designed and fabricated, and fast oscilloscope, PD detector and ultraviolet (UV) image camera were employed to observe the PD. PD-induced insulation failure phenomenon in the submodule was observed under direct current voltage, and UV images indicated that the discharge occurred at the periphery of the die. Phase-resolved PD (PRPD) analysis method for PPIs was developed and three elementary structures were used to simulate the PD in the electric field reinforcement areas identified by electric field analysis. According to PRPD patterns, the observed PD in the submodule was considered to be mainly caused by a mismatch between the electric field and critical electric field on the surface of passivation layers of the die. The inference was verified at different gas pressures because a decrease of PD inception voltage in the submodule and the bare die was observed at lower gas pressure.

Influence of Gas Flow on Partial Discharge Behaviors in Air and Nitrogen

Gas flow is an unavoidable factor to influence partial discharge (PD) behavior of insulators, e.g., in the cases of strong wind and high-speed train. In this paper, the surface charge distribution of PD sequences was obtained by the PD measurement system based on Pockels effect in different gas flows. Both phase-resolved PD (PRPD) and pulse sequence analysis method (dV-dV pattern) were employed to study the PDs behavior. We found that the air flow could homogenize the distribution of surface charge spots and influence the inception voltage, discharge number, as well as average discharge current. The discharge number increased with the airflow velocity until reaching its maximum at the threshold of 4 L/min. Although in nitrogen, with increasing the nitrogen flow velocity, the surface charge distribution became more uniform and the discharge number decreased together with its corresponding PDs pattern varying at the same time. This difference of PD behaviors in air and nitrogen was attributed to the presence of oxygen molecules in gas.

Implementation and Analysis of a Reference Partial Discharge Measurement System

Partial discharge (PD) detection is based on the measurement of apparent charge originated from PD in test sample. Since the measurement system configuration affects the PD measuring system response, a PD reference calibrator is the most important part of measuring system for the determination of scale factor in measurement system. In this paper, the design and implementation of reference partial discharge measurement system for the calibration of reference calibrator is presented. The reference pulses to be checked in the implemented measuring system are generated by the reference partial discharge generator or calibrator that is constructed for the characterization of the reference measuring system according to the requirement of IEC 60270. Proper correction for known errors and estimation of uncertainty contributions proved that traceable measurement of partial discharge can be made with an uncertainty of less than 1% in the range of between 2 pC and 1000 pC.

UHF PD measurement system with scanning and comparing method

Based on the periodic and repetitive characteristics of partial discharge (PD) under power frequency voltage, an ultra-high-frequency (UHF) PD measurement system using scanning and comparing method is realized without analog to digital (A/D) acquisition. The UHF PD pulses are compared with adjustable reference levels by high-speed comparator, and the number of pulses higher than the reference levels is recorded by a field programmable gate array (FPGA) along with corresponding phase information. The phases, amplitudes and number of discharge pulses can be extracted from the acquired data, which is verified in needle-plate PD experiment result analysis. And PD spectrograms of proposed method proves to be highly correlated with that of pulse current method and conditional UHF method. Influence factors in PD signal acquisition process are also analyzed.

Measurement of Partial Discharge Induced Electromagnetic Wave using Loop Antenna

Partial discharge (PD) detection by detecting the released energy in the form of electromagnetic (EM) wave during discharge using Ultra-High Frequency (UHF) antenna is one of the methods to determine insulation system quality of power apparatus. The main advantage of UHF PD detection system by using antenna are the capability to detect PD signal without firstly shut down the power apparatus. Therefore, the PD measurement system based on EM wave induced by PD detected by the sensor is needed. First, the sensors which be able to detect PD induced EM wave in ultra wide band need to be designed and manufactured. Second, the characteristics of PD induced EM wave measured by the sensors need to be investigated. This paper deals with measurement of PD induced EM wave by using loop antenna. The results show that the further the distance of loop antenna to PD source is, the lower Vpp is. This is due to EM wave attenuation. At the distance 225 cm from PD source, Vpp of PD is equal to Vpp of the background noise (BGN). Therefore, the maximum distance of the loop sensor to PD source is 225 cm.

Condition monitoring of high voltage transformers using Fiber Bragg Grating Sensor

High voltage (HV) power apparatus determines the stability of any electrical power system. Failure of such apparatus is mainly due to damage of insulation normally caused by the Partial Discharge (PD). Therefore PD measurements are globally considered as most essential and favored testing procedure for most of the HV power apparatus. Online condition monitoring may help to take proper corrective action at proper time, which reduces unscheduled downtime and can save money for electrical supply utilities.HV power transformers are being considered as heart of any electrical system and they have received much attention in recent years. In HV transformers, PD is one of the main reasons for degradation of insulation quality. Therefore its detection can provide early indication of the insulation failures of transformers. PD can be traced through direct measurement of current, electromagnetic radiation, chemical changes or acoustic emission. Electrical, acoustic, chemical and optical methods of PD measurement are the popularly accepted methods of PD detection in the high voltage (HV) power transformers. Over the past decades various PD sensors have been developed, most of these require electrical connections closed to measurement site and electrical wirings and amplifiers for transmission of measured signals. Placing electrical components and wirings closer to HV terminals is difficult for installation of such sensors; moreover, due to proximity of HV terminals these sensors suffer from electromagnetic interference (EMI).In this context, development of optical fiber based PD sensor can be promising as the optical fibers are made of dielectric material so can be easily placed inside the equipment without any additional insulation arrangements. On the other hand, it has very low transmission loss; therefore, the measured signal can be transmitted to the sub-station control room without use of any electrical amplification system and it is free from EMI. In this work, a Fiber Bragg Grating (FBG) based optical PD sensor has been developed for online detection of PD within HV power transformer. The shift in wavelength of FBG due to impulsive acoustic pressure generated during PD event has been used for the detection. The performance of the developed FBG based sensor has been validated experimentally with standard electrical (IEC60270) PD measurement system. Experimental results show that the developed sensor is a suitable substitute for conventional PD measuring system and it can be placed inside the apparatus.

New aspects of UHF PD diagnostics on gas-insulated systems

Gas insulated switchgears (GIS) have been increasingly introduced as main substation equipment since the late 1960s thanks to their high reliability, safety, and compactness. The UHF method to diagnose partial discharge (PD) is broadly employed in the field as an advanced insulation diagnostic technology. There are three primary factors related to the PD signal detection characteristics using this UHF method; namely the frequency spectrum of the PD signal itself inside GIS, the propagation characteristics of the PD signal inside the GIS tank, and the detection characteristics of the sensor measuring the PD signal. Accordingly, to further improvement of the diagnostic technique based on the UHF method, the PD phenomenon itself and its propagation characteristics should be clarified and the measurement system refined. From these perspectives, the present study reviewed the recent and latest findings concerning the PD phenomenon and its propagation characteristics inside GIS as well as the PD measurement system verification methods and diagnostic technologies, and summarized typical examples. Firstly, with regard to the PD phenomenon, measurement up to the high frequency band exceeding 10 GHz (even 30 GHz) clarified that the rise time of the PD current waveform is several tens of picoseconds, shorter than previously known. In the case of PD in micro-defects inside the epoxy insulator, however, the rise time is relatively long, in the order of nanoseconds, and a crack is the most critical defect. Subsequently, with regard to UHF PD diagnostics/monitoring technology, a novel technique using PD current waveform characteristics has been developed, and a more advanced PD diagnostic algorithm has been established by clarifying the influence of the shape and GIS internal structures on the electromagnetic wave propagation characteristics. Less uncertain and simplified calibration and verification technologies are also proposed both for a single UHF sensor and the entire UHF measurement system. These new technologies and further advanced studies in future are expected to make the UHF method more convenient and sophisticated.

Automatic Blind Equalization and Thresholding for Partial Discharge Measurement in Power Transformer

Partial discharge (PD) signals acquired from on-line measurements of power transformers are easily overwhelmed by various interference and noise. This paper proposes an automatic blind equalization (BE) and morphological thresholding method for PD signal de-noising. Firstly, BE automatically selects an equalized signal that reveals PD impulses from an acquired noise-corrupted signal. Then, automatic morphological thresholding (AMT) is adopted for determining thresholds on the equalized signal. After de-noising with BE and AMT, phase-resolved pulse sequence (PRPS) is constructed and used for analyzing the types of insulation defects that cause discharges. To verify the proposed method, PD measurements on experimental PD models and a distribution transformer have been conducted. The results show that PD impulses can be extracted from severely noise-corrupted signals by using the proposed method. Also, PRPS constructed from de-noised signals can achieve consistency in revealing the types of insulation defects even different types of PD sensors and measurement systems are used.