Michael Beltle

Application of UHF sensors for PD measurement at power transformers

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 sensors insertion depth into the tank. These results are compared to the reference GTEM cell measurement used for calibration.

Characterization of UHF PD sensors for power transformers using an oil-filled GTEM cell

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 sensors 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 cells 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 Sensors AF are taken into account.

Usability of vibration measurement for power transformer diagnosis and monitoring

This paper discusses the use of vibrations for power transformer surveillance. Therefore, basic research of transformer vibration is presented including the physical background of vibrations and methods of measurement in terms of sensor techniques comparing in-oil and outside tank wall measurement. A case study of an outside tank wall measurement performed on an aged generator step-up transformer presents first experiences in terms of long term measurement evaluating data of a 2-years period. A use case for vibration measurement is drawn for direct current (DC) detection in power transformers. DC can couple into 3 phase AC systems from nearby HVDC systems. The cores subsequent static magnetic field leads to increased core power losses and higher stress for the entire transformer.

Statistical analysis of online ultrahigh-frequency partial-discharge measurement of power transformers

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.

Radiated Emissions of an Electric Drive System Estimated on a Bench Using Disturbance Currents and Transfer Functions

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 inverters 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.

Discrimination of partial discharge sources in the UHF domain

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 transformers 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.

Analysis of UHF PD monitoring data by means of normalized cross-correlation

The paper presents a long-term monitoring example of an aged generator step-up transformer with several PD sources showing different activity. Monitoring data is recorded by the use of the electromagnetic PD measurement method, also known as the UHF-method. The phase resolved PD patterns of collected UHF measuring data are analyzed using cross-correlation. The method searches for characteristics of a typical pattern called template. PRPD patterns of the entire dataset are evaluated. The result is the time dependent occurrence of a specific pattern, respectively a specific PD source.