Partial discharge monitoring of power transformers by UHF sensors
PD measurement is suitable to detect damages in the insulation of power transformers at an early stage which helps minimising the risk of failure
Power transformers are essential to the reliability of the electrical grid. Hence, the reliable operation of power transformers is vital for supply security. Transformer failures regularly lead to significant damage and associated costs. All critical internal faults should be detected at first appearance. Therefore, different diagnostic methods have been developed to meet the increasing demands for on- and off-site measurements . In particular, partial discharges (PD) measurement has been established to detect local defects in the paper / oil insulation. Defects can be both initiated and subsequently enlarged by the destructive nature of PD.
There are three different ways of PD monitoring: indirect detection by dissolved gas analysis (DGA) and direct detection by either electrical PD measurements according to IEC 60270 , or by electromagnetic measurements in the ultra-high frequency range (UHF: 300 MHz – 3 GHz) . DGA only provides an indication of the presence of PD. An increasing number of transformers are monitored using direct methods. PD measurement is suitable to detect damages in the insulation of power transformers at an early stage and thereby helps minimising the risk of failure . Its significance is emphasised by a standardised electrical measurement according to IEC 60270, which is required for all acceptance certificates at routine testing. The apparent charge QIEC is an indicator of transformer quality. The electromagnetic UHF method gains importance in terms of monitoring and on-site diagnostics . The electromagnetic emissions of PD are recorded using a UHF antenna which is installed into the transformer tank. The principle propagation paths of the methods are shown in Fig. 1.
Electrical signals propagate by the galvanic coupling of the winding and are decoupled by the measurement capacity of the bushing for online monitoring or with an external coupling capacitor (not shown). Electromagnetic signals travel directly through the oil-filled volume of the transformer. Usually, the Faraday shielding of the transformer tank and low-pass filters provided by high voltage bushings shield UHF PD measurements against external disturbances . Therefore, it is less susceptible to external interferences compared to the electrical method. This is advantageous for measurements in noisy environments for on-site / online measurements and for monitoring. CIGRÉ Working Group WG A2-27 recommends in brochure 343 to provide DN50 valves to all transformers for the later fitting of UHF probes. Alternatively, dielectric windows can be used for UHF sensors . Both UHF sensors types are presented in the next chapter. Then a recommendation for the placement of window type UHF sensors at new power transformers is given. For the UHF method, a calibration procedure is now available [9, 10]. 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 the acceptance tests of power transformers.