Absolute calibration of radiometric partial discharge sensors for insulation condition monitoring in electrical substations

Measurement of partial discharge (PD) is an important tool in the monitoring of insulation
integrity in high voltage (HV) equipment. Partial discharge is measured traditionally using
galvanic contact techniques based on IEC 60270 standard or near field coupling [1]. Freespace
radiometric (FSR) detection of PD is a relatively new technique. This work advances
calibration method for FSR measurements and proposer a methodology for FSR
measurement of absolute PD intensity. Until now, it has been believed that absolute
measurement of partial discharge intensity using radiometric method is not possible. In this
thesis it is demonstrated that such measurement is possible and the first ever such absolute
measurements are presented. Partial discharge sources have been specially constructed.
These included a floating electrode PD emulator, an acrylic cylinder internal PD emulator
and an epoxy dielectric internal PD emulator. Radiated signals are captured using a
wideband biconical antenna [1]. Free-space radiometric and galvanic contact measurement
techniques are compared. Discharge pulse shape and PD characteristics under high voltage
DC and AC conditions are obtained. A comparison shows greater similarity between the
two measurements than was expected. It is inferred that the dominant mechanism in
shaping the spectrum is the band-limiting effect of the radiating structure rather than band
limiting by the receiving antenna. The cumulative energies of PD pulses in both time and
frequency domains are also considered [2]. The frequency spectrum is obtained by FFT
analysis of time-domain pulses. The relative spectral densities in the frequency bands 50
MHz – 290 MHz, 290 MHz – 470 MHz and 470 MHz – 800 MHz are determined. The
calibration of the PD sources for used in the development of Wireless Sensor Network
(WSN) is presented. A method of estimating absolute PD activity level from a radiometric
measurement by relating effective radiated power (ERP) to PD intensity using a PD
calibration device is proposed and demonstrated. The PD sources have been simulated
using CST Microwave Studio. The simulations are used to establish a relationship between
radiated PD signals and PD intensity as defined by apparent charge transfer. To this end,
the radiated fields predicted in the simulations are compared with measurements. There is
sufficient agreement between simulations and measurements to suggest the simulations
could be used to investigate the relationship between PD intensity and the field strength of
radiated signals [3].