S. Coenen

Sensitivity of UHF PD measurements in power transformers

The reliability of electrical energy networks depends on the quality and availability of electrical equipment like power transformers. Local failures inside their insulation may lead to catastrophic breakdowns and might cause high outage and penalty costs. To prevent these destructive events power transformers are e.g. tested for partial discharge (PD) activity before commissioning and currently also during service. The current work deals with the electromagnetic PD detection method, also known as UHF method. The disadvantage of the UHF method is still the missing possibility for a calibration or at least a verification of the sensitivity. The so-called Sensitivity Check might show in future a relation between unconventionally measured UHF quantities to the apparent charge level in pico Coulomb. An important aspect is the attenuation of UHF signals within power transformers which is investigated in this paper.

Location of PD sources in power transformers by UHF and acoustic measurements

The paper presents the experience of unconventional methods for partial discharge (PD) measurement to detect and localise PD sources in power transformers. The UHF PD measurement method is usable as stand-alone measurement and as a supporting measurement for off- and on-line PD detection. The sensitivity of UHF PD measurements is sufficient and is normally not affected by external disturbances. Especially in noisy surrounding it might be a very helpful method to support other PD measurement techniques for example dissolved gas analysis and acoustic location of PD. Propagation time of UHF signals can be used for geometrical PD location. The accuracy seems to be adequate to determine the phase limb where the PD is located. Additionally, different measurable UHF amplitudes support an estimation of the PD location. However, since transformers rarely possess more than three oil valves for installation of UHF probes, an additional acoustic measurement is usually required for location. Using the knowledge gained from the UHF location, acoustic sensors can be placed near to the PD source at the transformer tank. The paper explains first the fundamentals of PD measurements and PD source location and presents two case studies.

On-site experiences with multi-terminal IEC PD measurements, UHF PD measurements and acoustic PD localisation

The paper presents the experience using a combination of different partial discharge (PD) measurement methods both to detect PD in power transformers and to improve interpretation and localisation of their sources.

Fundamental differences of the PD measurement according to IEC 60270 and in UHF range

PD measurements in the UHF frequency range are an important tool for diagnostic at GIS. A general correlation between the measurement according to IEC standard 60270 and measurements in the UHF range is not possible. By the use of a ldquosensitivity checkrdquo the quality of the UHF measuring system is verified at GIS. In this contribution the usual procedure is pointed out and the boundary conditions and simplifications are shown. The knowledge of this is important for the interpretation of the result of the sensitivity check.

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.

Attenuation of UHF signals regarding the sensitivity verification for UHF PD measurements on power transformers

The reliability of electrical energy networks depends on the quality and availability of electrical equipment like power transformers. Local failures inside their insulation may lead to catastrophic breakdowns and might cause high outage and penalty costs. To prevent these destroying events power transformers are e.g. tested on partial discharge (PD) activity before commissioning and currently also during service. The current work deals with the electromagnetic PD detection method, also known as the UHF-Method. The disadvantage of the UHF-Method is still the missing possibility for a calibration or at least a verification of the sensitivity. The so-called Sensitivity Check might show in future a relation between unconventionally measured UHF quantities to the PD apparent charge level in pC. An import aspect is the attenuation of UHF signals within power transformers which is investigated in this paper.