Partial Discharge Detection Sensitivity Research Articles

Novel dual‐resonance damped alternating current testing system for offline partial discharge measurement of power cables

AbstractTo accurately detect the early deterioration of electrical insulation in power cables, an energizing system is required for offline partial discharge (PD) measurements. The damped alternating current (DAC) testing system has emerged as an effective tool for inducing PD events. However, the existing systems often suffer from limitations such as signal interference, low sensitivity, and high costs. To address these issues, this study proposes a novel dual‐resonance DAC testing system that is simple in structure and cost‐effective. The prototype was built for testing 10‐kV distribution cables, and its effectiveness was validated through experimental tests on a cable sample. The results show that this approach significantly improves PD detection sensitivity and successfully completes PD localization. This research contributes to the development of more efficient and affordable DAC generation technology for power cable testing applications.

Hardware and Software Design of Programmable Medium and High-Speed Data Acquisition (DAQ) Board of Fiber Optic Signal for Partial Discharge Acquisition

The anti-electromagnetic interference capability of partial discharge (PD) acoustic signal conversion and collection circuits severely restrict the sensitivity of PD detection. The data acquisition (DAQ) systems available in the current market are costly and have limited functionality, making it difficult to satisfy the acquisition requirements for PD detection. This paper proposes a medium to high-speed fiber optic signal acquisition board with an adjustably controlled sampling rate and filter cutoff frequency. The circuit achieves a higher signal-to-noise (SNR) ratio by distributing the noise in each part of the signal acquisition chain reasonably. The temperature characteristics of the acquisition module are improved by utilizing the programmable T-type structure for transimpedance amplification of photocurrent. The DAQ card performs data acquisition and processing using STM32H743 internal ADC and caches data in bulk with an SRAM and SD card. A data uploading method based on time reference has been proposed, which enables full, effective information signal upload through a low-cost transmission interface. The research ultimately achieves a stable sampling of three channels at 1 MSps, SNR of 63 dB, and programmable gain amplification of the photocurrent with 0–60 dB. Finally, the system is used for PD acoustic signal acquisition in the frequency range of 20 Hz to 100 kHz.

Research on partial discharge detection based on distributed optical fiber acoustic sensor

Distributed acoustic sensors (DASs) have the advantage of long-distance and distributed monitoring of vibration signals. However, there are few studies on the application of DAS for partial discharge (PD) monitoring of power equipment. There is also a lack of related theoretical and methodological studies. In this paper, the DAS-measured phase signals are theoretically analyzed from the frequency and energy perspectives, and the method of PD broadband signal coverage using spectrum and energy accumulation is proposed. Fiber optic rings are employed for the detection of PD signals, and the precise localization of these signals is achieved by calculating the DAS energy spectrum. The fiber optic ring is wrapped around the cable joint to achieve the detection and locating of the PD signal. The results indicate that the use of a fiber optic ring can realize the sensitive detection and locating of PD. The signal strength is maximum when the length of the fiber ring is equal to the spatial resolution of DAS. The sensitivity of PD detection in cable joints can reach 40 pC, and the simultaneous detection and localization of PD at different locations can be realized. The research can provide support for theoretical and experimental for the monitoring of PD in power equipment.

A Thermal Excitation Based Partial Discharge Detection Method for Cable Accessory

Partial discharge (PD) test has been widely used in condition assessment for power cables. However, discharge extinguish phenomenon occurred in PD detection hinders the effective detection of potential defects in cable accessories. For this reason, a novel PD detection method based on thermal excitation is proposed in this paper to eliminate the extinguish phenomenon and to detect the potential defects in earlier stage. First, the physical processes in PD activity and the reasons for discharge extinguishing are introduced. Then, the principle and mechanism of the thermal excitation detection method are described and analyzed respectively, that is excitation by external heating to break the dynamic equilibrium of charge movement within the defect and further stimulate new discharges. And a finite element simulation model is constructed to determine the thermal excitation parameters. Finally, the effectiveness is validated by performing thermal excitation-based PD test on some defective cables. The experimental results show that this method can significantly increase the number of discharges in off-line test, while also stimulating PD of higher amplitudes in on-line monitoring. This method can improve the sensitivity of PD detection for cable accessories and detect potential defects in early stage.

Effect of Magnetic Field on Partial Discharge Initiated by Metallic Particle in Thermally Aged Natural Esters Under AC and Harmonic Voltages

This paper reports the experimental and theoretical investigations of particle levitation voltage on thermally aged ester fluid, under AC and harmonic AC voltages, in the presence of both electric and magnetic fields (130 mT and 160 mT). The results indicate a higher sensitivity to identify partial discharge (PD) initiated due to particle movement in aged ester fluids with an ultra-high frequency (UHF) sensor than the fluorescent fiber technique. The cause for the reduction in sensitivity of PD detection due to the fluorescent fiber technique with thermally aged fluid is analyzed using steady-state fluorescent measurement. The reduction in the levitation voltage noticed under high-frequency AC voltages is much more severe than its impact under the fundamental frequency of AC supply voltage. In addition, the presence of a magnetic field reduces the magnitude of levitation voltage substantially. The UHF signals generated due to particle movement-initiated discharges with aged ester fluids indicate a shift in its dominant frequency of 0.9 GHz under the absence of a magnetic field to around 0.6 GHz with the effect of a magnetic field.

A Partial Discharge Detection System for XLPE Cable Terminals and Acoustic Wave Sensing Characteristics Based on Fiber Optic Mach-Zehnder Interferometer

Monitoring the cable terminal’s partial discharge (PD) is of great significance to ensure the cable line’s safe and stable operation. To improve the anti-interference ability and real-time performance of partial discharge detection, this paper develops a partial discharge Mach-Zehnder interferometer (MZI) detection system for cross-linked polyethylene (XLPE) cable terminals. The optical fiber is implanted into the 10 kV XLPE cable terminal, and airgap defects of different volumes are set in the main insulation. The optical fiber inside the cable terminal serves as the sensing head of the MZI signal arm. PD test was carried out on the samples, and the test results of fiber optic MZI and high-frequency current transformer (HFCT) were compared and analyzed. The results showed that the 10 kV XLPE cable terminal does not produce PD due to optical fiber implantation. The partial discharge of 196 pC can be recognized by the fiber optic MZI for the cylindrical airgap defect sample with a base radius of 1.5 mm and a height of 3 mm. For the cylindrical airgap defect sample with a base radius of 1.5 mm and a height of 1 mm, the sensitivity of partial discharge detection by fiber optic MZI is higher than that by HFCT. The sensitivity of partial discharge inception voltage (PDIV) detection is increased by 27.5%.

De-noising of partial discharge signal using wavelet transform for GIS under HVDC

Abstract Detection and analysis of partial discharge (PD) have been regard as the most effective method for condition monitoring and asset management of gas-insulated structures (GIS) in the power system. However, PD detection sensitivity and accuracy are greatly influenced by on-site noise and interference, resulting in failures in PD severity assessment, defect identification or localization. Although de-noising of PD signal under AC was well studied, related investigations under DC have not been carried out. With the rapid development of HVDC technology, it is a new challenge to eliminate noise from PD signal under DC for diagnosis of related power facilities. Therefore, this paper dealt with the discrimination of PD signal based on wavelet transform (WT) techniques for HVDC GIS, aiming to improve the sensitivity and accuracy of insulation diagnosis. Experimental setup was configured to generate PD signal under DC and four types of artificial defects were fabricated to simulate typical insulation defects in GIS. The WT techniques were used to discriminate PD pulse sequences from background noise, amplitude modulation radio interference, non-sinusoidal noise, and switching impulse and the effectiveness was compared with a high-pass filter.

Impact of impulse voltage frequency on the partial discharge characteristic of electric vehicles motor insulation

The urgent need for electric vehicles is the leading research to safer operation of electric motors on high frequency impulsive voltages. For low voltage electric vehicles partial discharge (PD) inception voltage tests under impulse voltage waveforms are desired. However, measuring PD using repetitive impulse waveforms is critical in selecting generators with appropriate parameters to increase PD detection sensitivity. Here, we report on PD characteristics for such kind of voltage waveforms, demonstrated with the high frequency and thermal impact on different types of motor insulation models. The increasing frequency of impulsive voltages tends to result in PD activities with low magnitude and short delay times. In this case, some PD pulses are missed because the PD and disturbance pulses produced by power electronic devices will likely overlap. Therefore, these parameters are carefully considered to evaluate the insulation performance of EV motors.

Design and Application of a Metamaterial Superstrate on a Bio-Inspired Antenna for Partial Discharge Detection through Dielectric Windows.

The adaptation of dielectric windows as metamaterial superstrate over a bio-inspired Printed Monopole Antenna (PMA) was evaluated in order to improve the detection sensitivity of Ultra High Frequency (UHF) sensors designed for Partial Discharge (PD) measurement. For this purpose, rectangular and circular Split Ring Resonators (SRR) structures were designed and evaluated aiming to achieve a metamaterial superstrate that improves the characteristics of the bio-inspired PMA as the gain, bandwidth, and radiation pattern. Measurements of the PMA with metamaterial superstrate were carried out in an anechoic chamber and compared to the simulations performed. The results show that the metamaterial superstrate insertion did not impact the original operating bandwidth, covering most of the characteristic frequency range of PD activity. Moreover, this insertion resulted in a mean gain enhancement of 0.7 dBi regarding the reference PMA, resulting in an antenna with better sensitivity for PD detection (mean gain of 3.61 dBi). The PMA-metamaterial set PD detection sensitivity was evaluated through laboratory tests with a point-to-plane PD generator setup and in field with measurements from a 230 kV current transformer. The developed PMA-metamaterial set was able to detect, successfully, the activity of PD for both tests, being classified as an optimized sensor for PD detection through dielectric windows.

A Novel PD Detection Technique for Use in GIS Based on a Combination of UHF and Optical Sensors

Detecting partial discharge (PD) in gas-insulated switchgear (GIS) is an effective way to diagnose the insulation state of GIS. When PD occurs in GIS, optical and electromagnetic signals will be emitted from the discharge source. PD can be detected by analyzing these signals. At present, the ultrahigh-frequency (UHF) method is one of the most popular methods for detecting the PD in GIS. However, the optical method has not been in the field due to the problem of mounting optical sensor into GIS, even though it has higher sensitivity and better anti-interference performance. To settle this problem, a novel GIS PD detection sensor integrated with the optical fiber and UHF method is developed in this paper, which can fully utilize the advantages both of UHF and optical methods. The simulation results indicate that the integrated sensor can be mounted into GIS on the premise of not affecting the electric field distribution in GIS. In addition, the integrated sensor has a good UHF performance. The PD signals of typical defects at several locations were measured in a physical GIS using the integrated sensor. The results show that the integrated sensor can detect the optical and UHF signals induced by PD at the same time. The optical method has higher sensitivity than the UHF method for the protrusion defect. The amplitude and the number of the optical signals induced by PD clearly decrease with the increase in the detection distance. The studies in this paper are helpful to improve the accuracy and sensitivity of PD detection in GIS on site.

Development of Acoustic Emission Sensor Optimized for Partial Discharge Monitoring in Power Transformers.

The acoustic emission (AE) technique is one of the unconventional methods of partial discharges (PD) detection. It plays a particularly important role in oil-filled power transformers diagnostics because it enables the detection and online monitoring of PDs as well as localization of their sources. The performance of this technique highly depends on measurement system configuration but mostly on the type of applied AE sensor. The paper presents, in detail, the design and manufacturing stages of an ultrasensitive AE sensor optimized for partial discharge detection in power transformers. The design assumptions were formulated based on extensive laboratory research, which allowed for the identification of dominant acoustic frequencies emitted by partial discharges in oil–paper insulation. The Krimholtz–Leedom–Matthaei (KLM) model was used to iteratively find optimal material and geometric properties of the main structures of the prototype AE sensor. It has two sensing elements with opposite polarization direction and different heights. The fully differential design allowed to obtain the desired properties of the transducer, i.e., a two-resonant (68 kHz and 90 kHz) and wide (30–100 kHz) frequency response curve, high peak sensitivity (−61.1 dB ref. V/µbar), and low noise. The laboratory tests confirmed that the prototype transducer is characterized by ultrahigh sensitivity of partial discharge detection. Compared to commonly used commercial AE sensors, the average amplitude of PD pulses registered with the prototype sensor was a minimum of 5.2 dB higher, and a maximum of 19.8 dB higher.

Active Dielectric Window: A New Concept of Combined Acoustic Emission and Electromagnetic Partial Discharge Detector for Power Transformers

The detection and location of partial discharge (PD) is of great significance in evaluating the insulation condition of power transformers. This paper presents an active dielectric window (ADW), which is a new concept of combined acoustic emission (AE) and electromagnetic PD detector intended for assembly in a transformer’s inspection hatch. The novelty of this design lies in the fact that all structural components of an ultrasonic transducer, i.e., the matching and backing layer, an active piezoelectric element with electrodes, and electrical leads, were built into a dielectric window. Due to the fact that its construction was optimized for work in mineral oil, it is characterized by much higher sensitivity of PD detection than a general-purpose AE sensor mounted outside a transformer tank. Laboratory tests showed that the amplitude of the AE pulses generated by creeping discharges, which were registered by the ADW, was around five times higher on average than the pulses registered by a commonly used contact transducer. A possibility of simultaneous detection of acoustic and electromagnetic pulses (with an integrated ultra-high frequency (UHF) antenna) is an important advantage of the ADW. It allows for an increase in the reliability of PD detection, the accuracy of defect location, and the effectiveness of disturbance identification. This paper describes in detail the applied methods of designing and modeling the ADW components, the manufacturing process of the prototype construction, and the results of preliminary laboratory tests, in which the detector’s sensitivity as well as the efficiency of the PD source location were evaluated.

Partial discharge detection by extracting UHF signal from inner grading electrode of insulating spacer in GIS

In this study, a novel ultra-high frequency (UHF) sensor using an inner grading electrode inside an insulting spacer is employed to detect gas-insulated switchgear (GIS) partial discharge (PD). The simulation of the waveguide modes of the 252 kV GIS and the frequency response characteristics of this UHF sensor are performed by using the finite difference time domain (FDTD) method. Three kinds of typical defects are employed to investigate the PD detection characteristics of this UHF sensor. It is shown that the resonance frequencies of this sensor are extremely abundant below 3 GHz, and the major resonance frequencies around 0.5–1.2 and 2.2–3 GHz. The phase-resolved PD (PRPD) images accumulated by using this UHF sensor present the typical PRPD images of defects. The main frequency spectrum of the metal protrusion on a high-voltage conductor and floating potential defects are distributed below 1.5 GHz, which is consistent with waveguide mode simulation. A resonance frequency of a grading electrode UHF sensor appears at about 115 MHz, which is close to the simulation value of a resonance frequency at 140 MHz. The PD detection sensitivity of this sensor is much higher than that of an external sensor; however, it is a little lower than that of an internal sensor.

Performance Comparison of Typical Built-In UHF Sensors Used for GIS PD Detection

The UHF method is one of the most important partial discharge (PD) detection means for GIS, while sensor technique is the key factor of the UHF method. In comparison with external sensors, built-in sensors can achieve higher sensitivity. In this paper, two parameters were quoted to evaluate the performance of the sensor, which are He and S11. He refers to the PD detection sensitivity of sensor; S11 refers to the impedance matching property of the sensor. The paper firstly designed a simulation model of sensor performance evaluation, and then studied the performance differences of four typically used or researched built-in UHF sensors (i.e., disc-type, dipole-type, spiral-type and logarithmic-type) by simulation tools of CST. The conclusions of the paper can provide a reference for performance evaluation and type selection of UHF built-in sensor. Keywords: Gas Insulated Switchgear (GIS), Partial Discharge (PD), UHF, Sensor, Sensitivity

Characteristics of External Loop Sensor Located Near Bushing on Partial Discharge Induced Electromagnetic Wave Measurement

For transmitting a huge electrical energy, high voltage transmission systems are commonly operated. Due to high reliability and less space required, gas insulated substation (GIS) are widely used. In a defect experienced GIS, partial discharge (PD) usually takes place. Partial discharge (PD) monitoring technique using UHF method becomes more popular because of its higher signal to noise (S/N) ratio. Amongst novel sensors, loop sensor could be a candidate for PD detector to measure UHF signal emitted from PD source. This paper presents the experiment results on characteristics of loop sensors. The characteristics of three loop sensors with varying diameter were examined using a spectrum analyzer and a network analyzer to obtain frequency characteristic and optimum operating frequency (S11) parameter, respectively. Then, PD experiment was conducted on 66kV GIS model having floating electrode and free metallic particles as its PD source. The loop sensors were placed far external to GIS bushing to measure leaked PD signal from it. The influences of the sensor diameter and the sensor distance from GIS bushing on PD detection sensitivity were also examined. Based on IEC-60270 standard measurement, the highest sensitivity of 30 pC was obtained from this investigation. The result suggests that higher sensitivity could be achieved with proper shielding and suitable filter design against external noise.

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.

Investigation of Partial Discharge Phenomena in Artificial Cavity in Epoxy Cast Resin Insulation System

It is crucial for proper insulation design of cast resin transformer to consider voids and delamination which might exist in cast molding because of several surface boundaries between resin and conductor. Such defects in the insulator lead to reduction of the life of the apparatus. In this report, we investigate the relation between the void size and apparent charge of partial discharge (PD) occurring in a model simulating the insulation system of cast resin transformer. It is also important to determine necessary PD detection sensitivity of PD test in a factory as well as in a field. In this paper, we also discuss the detection sensitivity by considering the relation between charge and void radius using formula derived by L. Niemyer, M. Pedersen and their colleagues in the late 1980's to apply to the case of the cast resin transformer. Experiments were also performed to obtain PD inception voltage (PDIV) of epoxy resin slab including an artificial void with a given size. Investigation was also made on the effect of X-ray irradiation to the epoxy resin on PDIV characteristics.

Rogowski air coil sensor technique for on-line partial discharge measurement of power cables

For monitoring the insulation status of medium- and high-voltage power cables, on-line partial discharge (PD) diagnosis tests such as detection, location and identification are used to find the defects or faults and to assess the aging degree of the cable insulation, especially for XLPE cable systems. However, owing to signal propagation attenuation and noise pollution, the PD signals are weak, which impose challenges for PD detection. Fine detection methods require a sensor with high sensitivity and wide bandwidth to detect the PD pulses. Rogowski air coil sensor (RACS) technique has been used for on-line PD signal detection in recent years. It requires reasonable methods to raise the upper frequency under the condition of high sensitivity. Traditional RACS has narrow bandwidth because of the limitation of the discrete resonance frequencies. A novel physical design for on-line RACS is proposed. It is aimed at applying transmission line theory and adjustable balance impedance to restrain or eliminate the first resonance notch frequency. Based on this design, the bandwidth of RACS increases effectively, whereas the sensitivity remains essentially unchanged. A real RACS on the basis of coaxial cable windings is designed and developed. In comparison with the traditional capacitance sensor and RACS, experimental results verified that the RACS based on the proposed novel design technique can not only increase bandwidth but also improve the sensitivity of PD detection.

Comparison of Known PD Signals With the Developed and Commercial HFCT Sensors

Increasing need of cost effective identical sensors for partial discharge (PD) location prompted us to take up this research study. Four high-frequency current transformers (HFCT) were assembled and its performances with a used commercial sensor were compared with the known type of PD signals. In this paper, the responses due to surface, oil corona and air corona are analyzed. PD occurrence in 20 ms period and the shape of single PD with more sampled data points are characterized using seven techniques. The study indicates that HFCT with biflar winding is more sensitive in identifying different types of known PD. The number of turns and gauge of the used wire in HFCT play a role in increasing the sensitivity of PD detection.

Impulse and ac partial discharge inception and breakdown characteristics of LN2/polypropylene laminated paper composite insulation system

Partial discharge (PD) inception and breakdown (BD) characteristics of liquid nitrogen (LN2)/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables are presented. In this paper, we measured PD inception characteristics under lightning impulse and AC voltage applications by electrical, optical and acoustic methods. The impulse PD inception characteristics were compared with those under AC voltage application, and the PD generation mechanisms are discussed. Experimental results reveal that PD detection sensitivity under lightning impulse voltage application is as small as 6 pC. PD inception at the lower stress was verified to be generated not only in the butt gaps but also in micro gaps between PP laminated paper layers under both lightning impulse and AC voltage applications. After PD inception, PD activity could be developed in the butt gaps at the higher stress, finally leading to BD