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The accurate measurement and the investigation of electromagnetic transients are becoming more important, especially with the increasing integration of renewable energy sources into the power grid. These sources introduce new transient phenomena due to the extensive use of power electronics. To achieve this, the measurement devices must have a broadband response capable of measuring fast transients. This paper presents a capacitive electric field sensor-based measurement system to measure transient overvoltages in high-voltage substations. The concept and design of the measurement system are first presented. Then, the design and concept are validated using tests performed in a high-voltage laboratory. Afterwards, two different calibration techniques are discussed: the simplified method (SM) and the coupling capacitance compensation (CCC) method. Finally, three recorded transients are evaluated using the calibration methods. The investigation revealed that the SM tends to overestimate the maximum overvoltage, highlighting the CCC method as a more suitable approach for calibrating transient overvoltage measurements. This measurement system has been validated using various measurements and can be an efficient and flexible solution for the long-term monitoring of transient overvoltages in high-voltage substations. [ABSTRACT FROM AUTHOR]

To increase the acceptance of electric vehicles (EVs), inductive charging technology can be an important tool because of the simplified charging process for the user. This paper presents the fundamentals of wireless power transfer (WPT) for EVs, while focusing on electromagnetic compatibility (EMC). This work deals with the investigation of the conducted and field-bound interference emissions using a WPT system with a max. input power of 3.6   k W . During the research, a new frequency-tracking algorithm is developed, to find the optimal operating frequency at any coil misalignment. The impedance behavior as well as the possible interference paths are investigated, showing the great geometric influence of the test bench setup. The conducted interference currents are analyzed and subsequently filtered. The filter shows good performance in attenuating common mode currents. The measured radiated magnetic field is directly rated against the proposed limits of various standards. Finally, the EMC influence of the direct current (DC) power supply line to the inverter is examined, which is not defined precisely in the standard. This underlines the significance of a standardized test setup, since the limit values can be met under different geometric circumstances of the DC cable. [ABSTRACT FROM AUTHOR]

Although partial discharge (PD) measurement is a well-accepted technology to assess the quality of the insulation system of power transformers, there are still uncertainties about which frequency range PDs radiate and which frequency range should be evaluated in a measurement. This paper discusses both a UHF PD frequency range obtained from studies investigating laboratory experiments and a frequency range from numerous practical use cases with online and on-site measurements. The literature review reveals a frequency spectrum of ultrahigh-frequency (UHF) PD measurements in the range of 200 MHz to 1 GHz for most publications. Newer publications extend this range from 3 to 6 GHz. The use cases present UHF PD measurements at transformers with power ratings up to 1000 MVA to determine frequency ranges which are considered effective for practical applications. The "common" frequency range, where measurements from all use cases provide signal power, is from approximately 400 MHz to 900 MHz, but it is noted that the individual frequency range, as well as the peak UHF signal power, strongly varies from case to case. We conclude from the discussed laboratory experiments and practical observations that UHF PD measurements in power transformers using either valve or window antennas, according to Cigré, are feasible methods to detect PD. [ABSTRACT FROM AUTHOR]

Ultra-high-frequency (UHF) partial discharge (PD) monitoring is gaining popularity because of its advantages over electrical methods for onsite/online applications. One such advantage is the possibility of three-dimensional PD source localization. However, it is necessary to understand the signal propagation and attenuation characteristics in transformers to improve localization. Since transformers are available in a wide range of ratings and geometric sizes, it is necessary to ascertain the similarities and differences in UHF signal characteristics across the different designs. Therefore, in this contribution, the signal attenuation and propagation characteristics of two 300 MVA transformers are analyzed and compared based on experiments. The two transformers have the same rating but different internal structures. It should be noted that the oil is drained out of the transformers for these tests. Additionally, a simulation model of one of the transformers is built and validated based on the experimental results. Subsequently, a simulation model is used to analyze the electromagnetic wave propagation inside the tank. Analysis of the experimental data shows that the distance-dependent signal attenuation characteristics are similar in the case of both transformers and can be well represented by hyperbolic equations, thus indicating that transformers with the same rating have similar attenuation characteristics even if they have different internal structures. [ABSTRACT FROM AUTHOR]

Rising voltages and wide bandgap semiconductors represent the main cause for rising electomagnetic interference (EMI) emissions of dc–dc converters in electric vehicles. Conventional passive filters are the limiting factor for the improvement of the power density and conventional active EMI filters are quite complex. This article presents a method in order to compensate the EMI generated by a dc–dc converter using a simple half-bridge gate-driver IC. The compensation signal is synchronized to the converter's switching events and injected through two capacitors at the converter's input nodes. Unlike analog active EMI filters, this method is able to compensate time delays of the filter's hardware components. This article presents an approach to describe such a compensation system analytically. Therefore, bandwidth and attenuation are determined due to the synchronization error and pulse amplitude. The presented prototype system and the analytical prediction are validated by measurements at the stationary operation of the converter in a CISPR 25 test setup. Finally, the performance of the method during dynamic operation of the converter is evaluated to demonstrate the high EMI suppression effectiveness of pulsed compensation. [ABSTRACT FROM AUTHOR]

Prolonged partial discharge (PD) activities can deteriorate the insulation of a power transformer leading to its complete failure. Therefore, PD monitoring is essential to ensure that PD sources are identified early. Ultra-high Frequency (UHF) PD measurement has gained attention because of some advantages compared to the conventional electrical method, such as low sensitivity to external disturbances. CIGRÉ recommends providing dielectric windows in newly manufactured transformer tanks for installing UHF sensors. However, there is no general recommendation on the placement of UHF sensors on a transformer tank, yet. This contribution provides knowledge about this discussion based on experimental data obtained from a 300 MVA, 420-kV grid-coupling transformer. UHF signals were measured using antennas at 17 different positions on the transformer tank and were used to provide a general evaluation of signal attenuation depending on distance and different types of propagation paths. Two approaches of signal evaluation, namely, signal power and peak-to-peak amplitude, were compared and found to have a high qualitative correlation, which implies that either method can be used to draw similar conclusions. Additionally, certain sensor positions were analyzed for any inherent benefits and finally, prospective sensor positions were suggested based on their performance and relative distances. [ABSTRACT FROM AUTHOR]

Reliable operation of high-voltage equipment depends on defect-free HV insulation systems. However, defects in the insulation can lead to partial discharge (PD) activity. PD measurement has been shown to be a useful tool for evaluating the condition of high voltage components including gas-insulated switchgear (GIS). This work focuses on characterizing the motion and PD signal behavior of free-moving particles in an experimental GIS arrangement under high dc field stress. For this purpose, the particle geometry, material composition, and amplitude of the applied negative dc voltage are varied. PD signals are measured using a broadband ultra-high-frequency (UHF) sensor and recorded in the time domain. The UHF signal is used to trigger a synchronized high-speed camera in order to observe the correlation between particle motion and UHF PD pulse activity. Certain unique combinations of UHF pulse repetition rates and signal amplitudes in the time domain are observed that depended on and were correlated with particles’ motion. These are evaluated by means of pulse sequence analysis, whose significant clusters enable estimation of particle motion. In particular, particle motion with small hopping amplitudes on the negative electrode—known as “firefly”—is differentiated with high accuracy. [ABSTRACT FROM AUTHOR]

The reliability of electrical energy networks depends on both, the quality and reliability of its electrical equipment, e.g. power transformers. Local failures inside their insulation may lead to breakdowns and hence to high outage and penalty costs. Power transformers can be tested on partial discharge (PD) activity before commissioning and monitored during service in order to prevent these events. In the first part, this contribution presents different types of ultra-high frequency (UHF) sensors for PD measurement. Various applications of UHF sensors and proper sensor installation are discussed. The second part of the contribution is about the necessity of UHF measurement comparability and reproducibility. Therefore, a new calibration procedure for the UHF method is proposed and discussed in respect of the procedure for the IEC 60270 compliant conventional electric method. The characterization of UHF sensors is a key precondition for the UHF calibration process in order to obtain calibration for the full measurement path. Sensor characteristics are described by the antenna factor (AF) which is determined under inside transformer conditions in an oil-filled Gigahertz Transversal Electromagnetic cell (GTEM cell). In addition to the calibration procedure, the performance of the installed sensor has to be determined. The evaluation is based on the concept of transmitting electromagnetic waves through the transformer tank from one UHF sensor to another. This performance check procedure is used in this contribution for the examination of the influence of the sensor's insertion depth into the tank. These results are compared to the reference GTEM cell measurement used for calibration. [ABSTRACT FROM AUTHOR]

Common reasons for local failures in power transformers are partial discharges (PD) in the electric insulation. The ultra-high frequency (UHF) PD measurement method gained in importance for transformer diagnosis and monitoring within the last years. Due to its robustness against external noise, it is suitable for both, factory acceptance tests (FAT) and site acceptance tests (SAT). For the application at acceptance tests, it is necessary that the UHF technique is a reliable testing method and also comparable between different UHF measurement systems. Therefore, the UHF sensor's characteristic has to be included in a calibration procedure by using its antenna factor (AF). This contribution implements a custom-built oil-filled Gigahertz-Transversal-Electromagnetic cell (GTEM cell) for the AF evaluation. In the first part, the GTEM cell's design and dimensioning are illustrated and the cell is evaluated in terms of its high frequency behavior. The second part determines the AF of two different UHF sensor types: UHF Drain Valve Sensors and UHF Plate Sensors using the GTEM cell. Additionally, the influences of insertion depths and standardized gate valves on the Drain Valve Sensor's AF are taken into account. [ABSTRACT FROM PUBLISHER]

The reliable operation of power transformers is an essential factor for the high availability of electric energy. Reliability demands can be enforced using condition based asset management which relies on different diagnosis measurements and continuous monitoring. This contribution determines the abilities of asset monitoring by regarding transformers' mechanical oscillations during operation. The physical origins of oscillation sources are introduced as well as the common measurement technique using accelerometers. The contribution evaluates the method's principle diagnostic abilities by correlating the actual mechanical status of an active part and resulting mechanical vibrations in a laboratory setup. The practical usability is determined by long-term comparison of measurements on different power transformers in service. Several influencing factors are evaluated: the sensor position, the changing transformer's load and operating temperature during service as well as the position of the on-load tap changer. Additionally, mechanical oscillations are used to determine the influence of superimposed, undesirable DC components on 3-limb power transformers originated by coupling between nearby AC and HVDC lines. [ABSTRACT FROM PUBLISHER]

Partial discharges (PD) are a significant contributor to insulation failure within power transformers, and as such, the localizing of a PD source can be of critical importance when it comes to a transformer's assessment and repair. One PD localization technique which is acquiring increased attention is the application of an ultra high frequency (UHF) sensor to trigger an acoustic array. This hybrid approach helps to overcome the poor signal to noise sensitivity issues commonly associated with externally mounted acoustic sensors by leveraging the benefits offered by an internally mounted UHF sensor to trigger PD events. The internally mounted UHF sensor is insensitive to external corona discharges which typically are the largest disturbance source associated with conventional measurements. A problem with this hybrid methodology is when there are multiple PD. The research presented in this paper demonstrates that UHF PD signals are primarily independent of the type of PD source. The research also demonstrates that UHF PD signals inherit signal characteristics which are particular to their location within a transformer structure, and inherit distortion characteristics which are particular to their journey from source to destination. As a result it is proposed that the correlation of UHF PD signatures can be used for discrimination purposes in order to identify individual PD sources within a transformer using a single UHF sensor. This could then be used in conjunction with acoustic measurement techniques to sequentially localize multiple PD within a power transformer which formerly has proven to be a technical challenge. [ABSTRACT FROM AUTHOR]

Electric drive systems are going to be the future of the automotive industry. Electromagnetic compatibility (EMC) issues and challenges arise with its growing entry into modern cars. This contribution provides an overview of the field dominating disturbance currents of an automotive high-voltage (HV) power inverter within an electric drive system. In order to minimize the cost and time needed for EMC measurements and engineering, a precompliance measurement method is introduced and applied to the inverter's setup. The method presented allows the calculation of radiated emissions, according to CISPR 25, needed during the development process without the need for an expensive anechoic chamber. Transfer functions in combination with measured disturbance currents on the attached HV harnesses are used to obtain a cost-efficient estimation of the radiated emissions occurring during a component test. Its theory is discussed and demonstrated using a derived model. The method is verified using a rudimentary test setup, which represents a distributed system, and is later validated by the active electric drive system test setup introduced. Examinations of repeatability and reproducibility of disturbance current measurements as well as applied EMC counter measures complete the contribution. [ABSTRACT FROM PUBLISHER]

The measurement of partial discharge (PD) has been established as a common method for transformer testing using electrical measurements according to IEC 60270 [1]. However, in this article, an UHF approach is presented. The UHF method was first introduced on gas-insulated switchgear but is also applicable to power transformers [2]. In the technique, an antenna is inserted into the transformer tank through an oil valve. The method detects PD by measuring electromagnetic emissions originating from transient currents of PD [3], [4]. Because the tank is grounded, it represents a Faraday cage shielding external noise, which makes the UHF method less sensitive to external PD such as corona. Compared to electrical PD measurements, the UHF method has to be treated differently. Unlike electrical PD measurements, UHF measurements cannot be calibrated in terms of apparent charge in picocoulombs because the strength of the electromagnetic field is measured. However, like electrical measurements, it is possible to correlate PD to the phase angle of the applied voltage using the phase-resolved PD (PRPD) pattern. This article presents an overview of UHF PD measurements of a generator step-up transformer over a three-year period. Several PD sources are active in the transformer, and fast changes in the PD pattern and levels are observed. An approach for the evaluation of measurement data is introduced, and the long-term development of PD is discussed in the article. The aim is to discriminate different PD sources from the PRPD pattern and classify them in terms of their level and frequency of occurrence on a long-term basis. Thus, a classification of different sources and their severity is attempted. [ABSTRACT FROM PUBLISHER]

Ultra-high frequency (UHF) partial discharge (PD) measurements in power transformers are becoming popular because of the advantages of the method. Therefore, it is necessary to improve the basic understanding of the propagation of signals inside the transformer tank and the factors that influence the sensitivity of the measurement. Since the winding represents a major obstacle to the propagation of the UHF signals, it is necessary to study the effect of winding design on signal propagation. Previous research activities have studied these effects using simplified models, and it is essential to consider the complexity of propagation in a complete transformer tank. Additionally, the quality of UHF PD measurements depends, to a large extent, on the sensitivity of the UHF sensors. In this contribution, a simulation model consisting of a simple, grounded enclosure with multiple winding designs is used to study the propagation characteristics of UHF signals when an artificial PD source is placed inside the winding. After analysis of the results, the winding designs are incorporated in an existing and validated simulation model of a 420 kV power transformer and analyzed to observe the influence in a more complex structure. Two commonly used sensor designs are also used in the simulation model to receive the signals. In all cases, the propagation and signal characteristics are analyzed and compared to determine the influence of the winding and sensor design on the UHF signals. It is found that the level of detail of winding design has a significant impact on the propagation characteristics. However, the attenuation characteristics of the UHF signals received by the two sensor designs are similar, with the electric field distribution around the sensor being the key difference. [ABSTRACT FROM AUTHOR]

With an increasing interest in ultra-high frequency (UHF) partial discharge (PD) measurements for the continuous monitoring of power transformers, it is necessary to know where to place the UHF sensors on the tank wall. Placing a sensor in an area with many obstructions may lead to a decrease in sensitivity to the UHF signals. In this contribution, a previously validated simulation model of a three-phase 300 MVA, 420 kV power transformer is used to perform a sensitivity analysis to determine the most sensitive sensor positions on the tank wall when PD activity occurs inside the windings. A matrix of UHF sensors located on the transformer tank is used to perform the sensitivity analysis. Some of the windings are designed as layer windings, thus preventing the UHF signals from traveling through them and creating a realistic situation with very indirect propagation from source to sensor. Based on these findings, sensor configurations optimized for UHF signal sensitivity, which is also required for PD source localization, are recommended for localization purposes. Additionally, the propagation and attenuation of the UHF signals inside the windings and the tank are discussed in both oil and air. [ABSTRACT FROM AUTHOR]

The continuous, non-intermitted service of electrical grids relies on the reliability of their assets, e.g., power transformers. Local insulation defects can result in serve failures such as breakdowns with severe subsequent costs. The prevention of such events is crucial. Hence, partial discharge (PD) activity at power transformers is evaluated directly in the factory before shipment. Additionally, PD activity can be monitored during service using the ultra-high frequency (UHF) method. In this contribution, a calibration procedure is proposed for the UHF method. The calibration process is required to ensure both, reproducibility and comparability of UHF measurements: Only a calibrated UHF measurement procedure can be introduced supplementary to IEC 60270 in acceptance tests of power transformers. The proposed calibration method considers two factors: The influence of the UHF-antenna's sensitivity and the PD recorder characteristics including accessories such as cable damping, pre-amplifier, etc. The former is addressed by a characterization of UHF sensors using the standard antenna factor (AF) in a gigahertz transverse electromagnetic (GTEM) cell. The PD recorder's influence is corrected by using a defined, invariable test signal as reference for all recording devices. A practical evaluation of the proposed calibration procedure is performed in a laboratory setup using different UHF recording devices and UHF sensors using artificial PD signals and real voltage-driven PD sources. [ABSTRACT FROM AUTHOR]