## 2 Design and Implementation of On-Line PD Monitoring System
### Partial Discharge Detectors
#### 2.2.3 High-Frequency Current Transformer
The waveforms of the calibration pulses induced in the secondary circuit of the HFCT sensor were registered using an oscilloscope (Figure 16). Figure 17 shows the recorded waveforms of the impulse response of the tested HFCT sensors. The highest detection sensitivity of the pulse of apparent charge 500 pC was shown by the prototype current transformer made from MnZn ferrite core with three turns. At the output of this current transformer, the pulses of amplitude \(A_{max}\) = 146 mV were registered, which means that its sensitivity was by about 70% higher than the prototype sensor made from NiZn ferrite core (\(A_{max}\) = 86 mV) and commercial HFCT sensor type RFCT-4 (\(A_{max}\) = 88 mV). Figure 16: Examination of HFCT sensors: (**a**) Schematic diagram of the measurement setup for measuring the impulse response of the HFCT sensor using standard PD calibrator: OSC = oscilloscope Tektronix MDO3104, HFCT = high-frequency current transformer under test, CAL = partial discharge calibrator type Doble LDC-5; (**b**) time waveform of PD calibration pulse with an apparent charge of 500 pC. Figure 17: Impulse response of the prototype (MnZn ferrite core with three turns, NiZn ferrite core with four turns) and commercial HFCT sensors to partial discharge calibration impulse with an apparent charge of 500 pC. Time parameters of the pulses generated by standard PD calibrator are optimized for the conventional method IEC 60270, in which case the registration of PD pulses is performed in a relatively low frequency band (100-500 kHz). For that reason, the tests with the use of the PD calibrator do not allow unambiguous statement that the prototype HFCT sensor with core MnZn is an optimal construction of the detector of high-frequency PD pulses. Nevertheless, measurements with the use of calibrator showed that due to the lowest lower \(-3\) dB cutoff frequency (\(f_{L}\) = 250 kHz) among the tested HFCT sensors, it may be successfully employed both in the standardized frequency range according to IEC 60270 as well as at higher frequencies up to 25.5 MHz. In the second stage of the laboratory research, the detection sensitivity of PD pulses generated in oil-paper insulation was tested, which were simultaneously registered by four HFCT sensors (two prototype and two commercial). The research was conducted with the use of a transformer tank filled with mineral oil. The electrode systems devoted to generating PD in oil and surface PD on the pressboard sample were put inside the tank. Detailed information concerning the geometry of the applied electrode systems was presented in [24]. The HFCT sensors were installed on grounding wire going out of the tank through the ceramic bushing insulator. The PD pulses were registered with the use of a four-channel oscilloscope Tektronix MDO3104 with the sampling frequency of 5 GS/s. Additionally, the standard IEC 60270 measurement setup was applied to control the level of the PD apparent charge (Figure 18). Discharges in oil of apparent charge in the range between about 200 and 850 pC were generated at 29 kV. In turn, the surface discharges had a significantly higher intensity and apparent charge (\(q\) = 450-5100 pC). This type of discharge was generated by a voltage of 15.4 kV. Figure 19 presents typical partial discharge time waveforms and power spectral density (PSD) characteristics registered with the use of HFCT sensors. The results of frequency analysis showed that PDs in oil generated pulses of high frequency in the band between 12.4 and 18.3 MHz, whereas surface PD generated pulses in a narrow band between 10.8 and 11.6 MHz. The PD detection sensitivity of both prototypes was higher than commercial sensors. In the case of discharges in oil, the highest sensitivity was observed for the prototype HFCT sensor with four turns and NiZn soft ferrite core. Its sensitivity was 36% higher on average than the sensors from MnZn soft ferrite with three turns.