High Frequency Current Transformer Research Articles

On-Line Partial Discharge Monitoring System for Power Transformers Based on the Simultaneous Detection of High Frequency, Ultra-High Frequency, and Acoustic Emission Signals

The article presents a novel on-line partial discharge (PD) monitoring system for power transformers, whose functioning is based on the simultaneous use of three unconventional methods of PD detection: high-frequency (HF), ultra-high frequency (UHF), and acoustic emission (AE). It is the first monitoring system equipped in an active dielectric window (ADW), which is a combined ultrasonic and electromagnetic PD sensor. The article discusses in detail the process of designing and building individual modules of hardware and software layers of the system, wherein the most attention was paid to the PD sensors, i.e., meandered planar inverted-F antenna (MPIFA), high-frequency current transformer (HFCT), and active dielectric window with ultrasonic transducer, which were optimized for detection of PDs occurring in oil-paper insulation. The prototype of the hybrid monitoring system was first checked on a 330 MVA large power transformer during the induced voltage test with partial discharge measurement (IVPD). Next, it was installed on a 31.5 MVA substation power transformer and integrated according to the standard IEC 61850 with SCADA (Supervisory Control and Data Acquisition) system registering voltage, active power, and oil temperature of the monitored unit. The obtained results showed high sensitivity of the manufactured PD sensors as well as the advantages of the simultaneous use of three techniques of PD detection and the possibility of discharge parameter correlation with other power transformer parameters.

Practical aspects of partial discharge measurement for HVDC cables

High-voltage direct current (HVDC) cables are increasingly being installed to connect new offshore wind farms. Unplanned outages of these connectors can cause high economic impacts. Hence, there are requirements for condition-based maintenance that can improve operational reliability. Partial discharge (PD) is indicative of insulation defects. PD monitoring for AC cables is well established, but before applying the technique to HVDC cable connections, it is important to characterise PD behaviour under DC conditions and the attenuation in HVDC cables. This paper investigates PD activity under non-ideal DC stress, PD signal attenuation in HVDC cables, and electromagnetic noise in converter stations. Under the voltage of superimposed DC and harmonics, PD pulses tend to synchronise with the phase of harmonics. Therefore, synchronised recording of PD pulses can produce phase resolved patterns as an additional tool for insulation diagnostics. Modelling of attenuation in a HVDC transmission cable indicates that a detection bandwidth of tens of kHz to a few MHz may improve detection sensitivity when measuring PD current pulses over very long cable runs is carried out through sensors such as high frequency current transformers (HFCTs) installed at cable ends. Additionally, the RF spectrum measured in a converter station cable hall did not include any switching-related signals, demonstrating the viability of RF sensors based PD monitoring for the HV components associated with the cable connections.

Principles of Charge Estimation Methods Using High-Frequency Current Transformer Sensors in Partial Discharge Measurements.

This paper describes a simplified model and a generic model of high-frequency current transformer (HFCT) sensors. By analyzing the models, a universal charge estimation method based on the double time integral of the measured voltage is inferred. The method is demonstrated to be valid irrespective of HFCT sensor, assuming that its transfer function can be modelled as a combination of real zeros and poles. This paper describes the mathematical foundation of the method and its particularities when applied to measure nanosecond current pulses. In practice, the applicability of the method is subjected to the characteristics and frequency response of the sensor and the current pulse duration. Therefore, a proposal to use the double time integral or the simple time integral of the measured voltage is described depending upon the sensor response. The procedures used to obtain the respective calibration constants based on the frequency response of the HFCT sensors are explained. Two examples, one using a HFCT sensor with a broadband flat frequency response and another using a HFCT sensor with a non-flat frequency response, are presented.

Hybrid Sensing of Internal and Surface Partial Discharges in Air-Insulated Medium Voltage Switchgear

Partial discharge (PD) measurements have proved their reliability for health monitoring of insulation systems in power system components including synchronous generators, power transformers, switchgear and cables etc. Online condition monitoring and pro-active detection of PD faults have been highly demanded over the last two decades. This paper provides results from a research project to develop advanced non-intrusive sensing technologies that are cost effective, reliable and efficient for early detection of PD faults in medium voltage (MV) and high voltage (HV) air-insulated switchgear. Three sensors (high frequency E-field (D-dot) sensor, Rogowski coil and loop antenna) have been developed and tested under various PD faults and their performance were evaluated in comparison with high frequency current transformer (HFCT) which is being used commercially for PD testing and measurement. Among these three sensors, it is shown that D-dot sensor and Rogowski coil are more dependable when it comes to the PD measurements due to their high signal to noise ratio and hence high accuracy. These sensors can be customized according to a specific application and can be connected together with one data acquisition device while developing an online condition monitoring system.

Wideband UHF Antenna for Partial Discharge Detection

This paper presents a ultra-high frequency (UHF) antenna for partial discharge (PD) detection and the antenna sensor can be used near a conducting ground wire. The proposed UHF antenna has advantages of easy setup and higher-frequency detection over the high-frequency current transformer (HFCT) sensor. First, a variety of loop-shaped antennas are designed to compare each near field coupling capability. Then, a new UHF antenna is designed based on the loop-shaped antenna, which has the best near field coupling capability. Finally, the proposed UHF antenna is fabricated and measured. It provides a wide impedance bandwidth of 760 MHz (740–1500 MHz). Its simple setup configuration and wide bandwidth frequency response in the UHF band can provide a more efficient means for PD detection.

Research on Time-Domain Transfer Impedance Measurement Technology for High Frequency Current Transformers in Partial Discharge Detection of Cables

We focus on the high frequency current method which is widely applied in the partial discharge (PD) detection of cables. Aiming at guaranteeing the accuracy of this method, we study an innovative time-domain technology for effectively measuring the transfer impedance of the high frequency current transformers (HFCTs). The proposed technology called pulse injection method obtains the system response under the excitation of the wide-band instantaneous pulse signal. Firstly, by studying the working principle of HFCTs, we summarize that the bandwidth of the selected signal acquisition device should be at least 100MHz to ensure measurement accuracy. Secondly, Gauss pulse and square wave pulse are generated to determine the effects of different sources. The measurement results indicate that Gauss pulse is more suitable for pulse injection method, and the rise time should be under 10 ns to improve the starting frequency of oscillation distortion. Finally, the transfer impedance curves of five types of HFCTs are acquired by both pulse injection and traditional point-frequency methods. The measurement results show a remarkable consistency between two methods. However, pulse injection method requires the simpler operation and has a higher resolution, obviously improving the measurement efficiency and better displaying the details of the transfer impedance curves.

Novel Core Airgap Profiles Design Scheme for Winding and Thermal Loss Reduction in High-Frequency Current Transformer Sensors

Novel Core Airgap Profiles Design Scheme for Winding and Thermal Loss Reduction in High-Frequency Current Transformer Sensors

Separation of Partial Discharge Sources Measured in the High-Frequency Range with HFCT Sensors Using PRPD-teff Patterns.

During the last two decades, on-line partial discharge (PD) measurements have been proven as a very efficient test to evaluate the insulation condition of high-voltage (HV) installations in service. Among the different PD-measuring techniques, the non-conventional electromagnetic methods are the most used due to their effectiveness and versatility. However, there are two main difficulties to overcome in on-line PD measurements when these methods are applied: the ambient electric noise and the simultaneous presence of various types of PD or pulse-shaped signals in the HV facility to be evaluated. A practical and effective method is presented to separate and identify PD sources acting simultaneously in HV systems under test. This method enables testers to carry out a first accurate diagnosis of the installation while performing the measurements in situ with non-invasive high-frequency current transformers (HFCT) used as sensors. The data acquisition in real-time reduces the time of postprocessing by an expert. This method was implemented in a Matlab application named PRPD-time tool, which consists of the analysis of the Phase-Resolved Partial Discharge (PRPD) pattern in combination with two types of interactive graphic representations. These graphical depictions are obtained including a feature parameter, effective time (teff), related to the duration of single measured pulses as a third axis incorporated in a classical PRPD representation, named the PRPD-teff pattern. The resulting interactive diagrams are complementary and allow the pulse source separation of pulses and clustering. The effectiveness of the proposed method and the developed Matlab application for separating PD sources is demonstrated with a practical laboratory experiment where various PD sources and pulse-type noise interferences were simultaneously measured.

Partial Discharge localization on cross-bonded cable systems

When a Partial Discharge (PD) signal, which travels through a cross-bonded (CB) power cable system, reaches a CB joint, the signal not only continues its way on the defect phase but it also enters and propagates in the other faultless phases. For analyzing the effect of CB joints on PD signal propagation, an experimental model of CB cable systems is designed. The experimental setup uses 1km length of a PE insulated cable. The setup models one major section of real sectionalized CB high voltage three-phase cable systems. Transient signals are applied to the setup to model PD in different possible locations and the signals are detected in different locations. It is concluded how the faulty phase can be identified according to the results obtained from the CB and the non-CB joints. Single-phase PD measurements are also carried out, and the results are compared with those of PD in the CB cable system. It is suggested to use summation of the PD signals acquired from three high frequency current transformers installed on three screen conductors of three phases at each joint for PD localization and charge estimation. Finally, a monitoring algorithm is proposed for PD localization and defected phase identification.

Measuring method for partial discharges in a high voltage cable system subjected to impulse and superimposed voltage under laboratory conditions

A partial discharge (PD) measuring system has been deployed in order to identify and measure PD in a high voltage (HV) cable joint under impulse and superimposed voltages under laboratory conditions. The challenge is to enable the detection of PD during the impulse conditions. The method of measurement has been investigated by introducing an artificial defect in the cable joint in a controlled way to create conditions for partial discharges to occur. Next the HV cable system is subjected to AC, impulse and superimposed voltage. Two high frequency current transformers (HFCT) installed at both ends of the cable joint were used to identify PD from the cable joint and to separate PD from disturbance. Transient voltage suppressors and spark gaps are applied to protect the measuring equipment. Band pass filters with selected characteristics are applied to suppress transient disturbances and increase the chance to detect PD during the impulse. PD signals are separated from transient disturbances during data post processing and by means of pulse polarity analysis. The developed system enables the detection of so-called main and reverse discharges respectively occurring during the rise and tail time of the superimposed impulse. The measurement results obtained show the effectiveness of the presented PD measuring system for investigating the effects of voltage transients on a HV cable system in laboratory conditions.

A Novel Differential High-Frequency Current Transformer Sensor for Series Arc Fault Detection.

Fault arc detection is an important technology to ensure the safe operation of electrical equipment and prevent electrical fires. The high-frequency noise of the arc current is one of the typical arc characteristics of almost all loads. In order to accurately detect arc faults in a low-voltage alternating-current (AC) system, a novel differential high-frequency current transformer (D-HFCT) sensor for collecting high-frequency arc currents was proposed. The sensitivity and frequency band of the designed sensor were verified to ensure that the acquisition requirements of the high-frequency current were satisfied. A series arc fault simulation experiment system was built, and resistive, inductive, and non-linear load and high-power shielding load experiments were carried out. Experiments showed that the sensor output signal was close to zero in the non-arc state, and the sensor output response was a high-frequency glitch in the arc state. The results were consistent for different loads, and the discrimination between normal and fault states was obvious, which proved that the sensor is suitable for series arc fault detection.

Identifcation and Location of Partial Discharge Defects in Medium Voltage AC Cables

The presence of partial discharge (PD) in electricalequipment is the confirmation or indication of an ongoingdegradation of its dielectric insulation. PD study is a widely usedtool for condition monitoring of medium voltage (MV) cables.The location of the defects can be considered as one of the mostimportant tasks of diagnosis in underground installations of MVcables. A lot of references can be found on the location of singlePD sources along the cable. However, several PD defects can beactive simultaneously along a cable section. This paper presentsan experimental study on a MV cable having two PD sourcesand noise. Adopting one end measurement technique, a highfrequency current transformer (HFCT) is used to capture the PDcurrent pulses. A simple approach of identification of the PDsignals originated from different PD sources is presented basedon wave-shape study along with a time domain reflectometryanalysis. The presented measurement methodology and timedomain analysis technique provides a convenient way fordetection and location of more than one PD sources along a cablesection.

Development of HFCT for partial discharge sensors

This paper presents partial discharge (PD) measurement techniques using a commercial sensor, High-Frequency Current Transformer (HFCT). An Output signal from HFCT is read by a digital oscilloscope and compared with the reading of convensional that PD detector. When PD occurs, the signals detected using high impedance that has a long time of oscillations and cause the delay of next reading of PD signals. To avoid this issue, the time of oscillations must be reduced by installing a damper which is a resistor connected in parallel between the sensor and the oscilloscope. The result of the measurement shows that the damper has the same reading on peak values as read by the convensional PD detector.

Partial Discharge Localization on Power Cables Using On-Line Transfer Function

An on-line method is proposed for cable transfer function (TF) identification. For a cable's TF identification with this method, no knowledge about the cable geometry and material specification is needed. Moreover, it does not require that the cable system go out of service. An identification signal is injected through a high-frequency current transformer (HFCT) to the shield of the cable. By measuring the injected signal via HFCTs clamped on the shield at two distinct locations along the cable, the TF of the cable can be identified. A novel partial discharge (PD) localization method is also introduced, which is based on asynchronous PD measurements. In this method, travel distance of the signal is estimated by the reconstruction of a reference signal from the measured signals, using the obtained cable TF. Reference signal refers to the typical waveform of PD signal at its source location. Both the cable identification and PD localization methods are validated using two experimental setups to simulate a diverse range of PD source distance. The test cables are a 666-m long, single-core, low-voltage (2 kV) signal cable with PE insulation, and a 51-km long, single-core, medium-voltage (24 kV) power cable with XLPE insulation.

Modeling of High-Frequency Current Transformer Based Partial Discharge Detection in High-Voltage Cables

Partial discharge (PD) testing of high-voltage (HV) cables and cable accessories has been implemented predominantly using high-frequency current transformers (HFCTs) as PD sensors. PD currents initiating at PD sources are coupled onto cable conductors. They travel away from the PD sources and are detected by HFCTs installed at cable terminations. In this paper, based on combining finite-difference time-domain (FDTD) modeling and transfer function theory, a hybrid modeling approach is proposed to investigate the processes of PD coupling and detection involved in HFCT-based PD testing of HV cables. This approach allows exciting a PD event anywhere in FDTD models of the cables and predicting output from HFCTs some distance away. Implementation of the method is illustrated using an 11 kV XLPE cable. Moreover, a “direct measurement” method of obtaining original PD pulses as the excitation source waveform is presented. The modeling approach introduced here will facilitate studies on the relationship between measured PD signals and those excited at PD sources, which can potentially give useful insight into the basic mechanisms behind PD detection in cables.

Identification and Location of PD Defects in Medium voltage Underground Power Cables Using High Frequency Current Transformer

Fault location is an important diagnostic task in condition monitoring of underground medium voltage cables. Available solutions are well capable of determining the location of a single partial discharge (PD) defect on a cable section. In case several PD defects are active simultaneously along a cable section, the interpretation of the measured data becomes complex to identify the presence of more than one PD sources. In this paper, experimental investigation of two PD defects/sources at different locations on a medium voltage (MV) cable section is presented. A high frequency current transformer is used for single end PD measurements. Time domain reflectometry-based in-depth study of the reflected pulses provides the most valuable information to identify the presence of PD sources which further leads to the location of the individual PD sources. In this paper, the proposed solution is presented for two PD sources, however, the same methodology can be extended to locate multiple PD sources on the cable.

Development of a High-Speed Data Acquisition Card for Partial Discharge Measurement

This study Development a high-speed data acquisition (DAQ) device by using AD9226 analog-to-digital converters, a field programmable gate array, and an ARM Cortex-A8 microprocessor for a self-designed synchronous 6-channel high-speed DAQ card that was able to transmit data to a computer through its network interface. Its cost was approximately 10% that of a commercial model, the National Instruments PXI-5105, and thus overcame the prohibitively high cost of commercial DAQ cards. A high-frequency current transformer (HFCT) was used to measure three types of typical partial discharge (PD) in self-made models to compare the performance of the self-designed DAQ card and that of the National Instruments PXI-5105. The HFCT signals were converted into three-dimensional PD patterns, and mean discharge was chosen as the feature to be extracted for the application of extension theory in the recognition of discharge models. The results revealed that the self-designed DAQ card was comparable to the commercial model in the recognition of high-frequency PD signals. Given the high price of commercial high-speed DAQ devices, the self-designed DAQ card was deemed to have considerable advantages in cost and expandability.

A Novel Approach for Partial Discharge Measurements on GIS Using HFCT Sensors.

This paper presents a novel measuring system for partial discharge (PD) measurements in Gas Insulated Systems (GIS) using high frequency current transformers (HFCT). The system is based on the measurement of the induced PD currents in the GIS enclosure. In opposition to the existing antenna technologies that measure the radiated energy in the very high frequency/ultra-high frequency (VHF/UHF) range, the proposed system measures the PD conducted currents in the high frequency (HF) range and below. The foundation of the measurements together with a detailed explanation of the sensor installed conveniently at the bolts of the GIS spacer are presented. An experimental study on the current distribution in the GIS enclosure is described to evaluate the impact of the sensor on the measurements. Laboratory experiments have been performed that show the suitability of this method to properly measure particle discharges caused by corona, surface and free moving particle discharges in SF6. Discharges in the range of 1 to 4 pC have been properly measured. An analysis to evaluate the performance of the method is shown, in comparison to VHF/UHF antenna measurements. The potential benefits of this novel technique rely on the small attenuation of PD signals in the GIS components in the HF range and sample rate reductions. Finally, a discussion on the potential applicability of present cluster and charge calculation techniques to the proposed PD GIS measurement using HFCT is presented.

Application of extension neural network algorithm and chaos synchronization detection method to partial discharge diagnosis of power capacitor

The power capacitor is an important equipment of a power system, which must run in a high-temperature and high-voltage environment for a long time, in order to maintain the stability of system. Hence, the failure rate of the power capacitor increases over time. Conventional capacitor testing methods mostly require multiple sensors and signal parameters, which increase system cost and complexity. This study attempted to propose a novel method for capacitor fault recognition. It developed a fault diagnosis system for power capacitor by employing the extension neural network (ENN) algorithm and the chaos synchronization detection method. In terms of signal acquisition, partial discharge was measured by hardware circuits, such as high-frequency current transformer (HFCT), high-pass filter, noninverting amplifier circuit, and high frequency oscillography. The ENN and the chaos method were integrated with hardware circuits to develop a human-machine interface fault diagnosis system designed with LabVIEW. The proposed method was also compared with extension method and artificial neural network algorithm. According to the results, the ENN has the best recognition result, and the huge data could be reduced greatly by the data pre-processing of the chaos synchronization detection method. Any subtle changes in the power capacitor discharge signal could be detected effectively, thus achieving an accurate operating state of the power capacitor.

The Rogowski Coil Sensor in High Current Application: A Review

Rogowski coil is used for measuring the alternating current (AC) and high-speed current pulses. However, the technology makes the Rogowski coil (RC) come out with more improvement, modification and until today it’s still being studied for the new application. The Rogowski coil has a few advantages compared to the high frequency current transformer (HFCT). A brief review on the basic theory and the application of Rogowski coil as a current sensor measurement that been done by previous researchers are presented and discussed in this paper. Additionally, the review also focused on the capability of Rogowski coil for high current sensor measurement and their application for fault detection, over voltage current sensor, lightning current sensor and high impulse current detection. The experimental set up, techniques and measurement parameters in models also been discussed. Finally, a brief review on the performance analysis of current sensor measurement of Rogowski coil likes sensitivity, the maximum and current detection which could be used as a guideline to another researcher in order to develop an advanced RC as high current sensor in future is presented. This review reveal that the RC has a very good performance in high current sensor detection in term of sensitivity which is up to a few nanosecond, higher bandwidth, excellent in detection of high fault and also could measuring lightning current up to 400kA and has many advantages compare to conventional current transformer(CT).