287 / 2018-09-25 17:00:22

Study on a Partial Discharge Detection Method for Transformers Adopting the End Shield as UHF Sensor

PD,transformer,end shield,UHF sensor,HFCT

Power transformer is one of the most important equipment in power system. It is of great significance to ensure its safe operation. However, the insulation defects occurring during manufacturing, transportation, assembly, or operation may cause the insulation degradation or even breaking down. Partial discharge (PD) detection is a sensitive method to evaluate the insulation condition of transformers. High-frequency current transformer (HFCT) has been widely used for PD detection due to its advantages of wideband and non-direct electrical connection between the sensor and transformer. Nevertheless, the upper frequency of the HFCT is usually limited, which makes it susceptible to interference such as corona in the substations. Ultra-high frequency (UHF) method has also been used to detect PD for its good anti-disturbance performance. Despite this, few transformers are equipped with internal UHF sensors while the external UHF sensors have lower sensitivity. To address the above problems, a novel PD detection method adopting the end shield as UHF sensor is developed in this paper. In capacitor bushing, the end shield should be reliably grounded to prevent from breaking down. In this paper, it is grounded through a junction box instead and L is the equivalent inductance of the ground lead. The value of L is small which can be ignored and considered as short-circuited during power frequency and overvoltage but it has high resistance under high frequency. The UHF signal acquired from the inductance is amplified with a 20 dB gain and 300 MHz~3 GHz filter through a UHF signal processing device. The PD measurement is carried out on a 80kV transformer model with three types of defects set inside, causing different type of PD such as air-gap discharge, suspended discharge and particle discharge. There is a dielectric window on the tank where the UHF antenna is placed as the internal UHF sensor. The two types of UHF and current pulse signals from HFCT (with the bandwidth of 2-30MHz) are measured simultaneously through a digital oscilloscope with 500 MHz bandwidth and 2.5 Gsamples/s sampling rate. The results show that the partial discharge inception voltage (PDIV) value of air-gap discharge and suspended discharge measured by three methods. The particle discharge inception voltage value measured by two UHF methods are same and much lower than that measured by HFCT. As the applied voltage rises, the amplitude of signals increases, and the UHF and current pulse method acquire the signals at the same time. Besides, HFCT can be easily disturbed by the interference signals coupling from the ground lead and corona discharge outside the bushing, which have little effect on two UHF sensors. Hence, the end shield UHF sensor is more sensitive and stronger anti-interference than HFCT, and can work as well as internal UHF sensor but much more convenient for installation. The proposed method can realize the internal PD UHF detection easily. It provides a new detection method for PD detection in field transformer.