## 2 Design and Implementation of On-Line PD Monitoring System
### Partial Discharge Detectors
#### 2.2.3 High-Frequency Current Transformer
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ferrite Type** & **Parameter** & **Symbol** & **Value** & **Unit** & **Conditions** \\ \hline \multirow{8}{*}{MnZn} & Initial permeability & \(\mu_{i}\) & 900 \(\pm\) 20\% & – & 25 \({}^{\circ}\)C; \(\leq\) 10 kHz; 0.25 mT \\  & Amplitude permeability & \(\mu_{a}\) & \(\approx\) 1700 & – & 100 \({}^{\circ}\)C; 25 kHz; 200 mT \\  & Magnetic flux density & \(B\) & \(\approx\) 410 & mT & 25 \({}^{\circ}\)C; 10 kHz; 1200 A/m \\  & Specific power loss & \(P_{\pi}\) & \(\approx\) 130 & kW/m\({}^{3}\) & 100 \({}^{\circ}\)C; 1 MHz; 30 mT \\  & Loss factor & _tan_ & n/a & – & – \\  & Resistivity & \(\rho\) & \(\approx\) 10 & \(\Omega\)m & DC; 25 \({}^{\circ}\)C \\  & Curie temperature & \(T_{c}\) & \(\geq\) 220 & \({}^{\circ}\)C & – \\  & Density & \(D\) & \(\approx\) 4700 & kg/m\({}^{3}\) & – \\ \hline \multirow{8}{*}{NiZn} & Initial permeability & \(\mu_{i}\) & 125 \(\pm\) 20\% & – & 25 \({}^{\circ}\)C; \(\leq\) 10 kHz; 0.25 mT \\  & Amplitude permeability & \(\mu_{a}\) & n/a & – & – \\  & Magnetic flux density & \(B\) & \(\approx\) 380 & mT & 25 \({}^{\circ}\)C; 10 kHz; 3000 A/m \\  & Specific power loss & \(P_{\pi}\) & n/a & – & – \\  & Loss factor & _tan_ & \(\leq\) 80 \(\times\) 10\({}^{6}\) & – & 25 \({}^{\circ}\)C; 3 MHz; 0.25 mT \\  & Resistivity & \(\rho\) & \(\approx\) 10\({}^{5}\) & f\(\Omega\)m & DC; 25 \({}^{\circ}\)C \\  & Curie temperature & \(T_{c}\) & \(\geq\) 350 & \({}^{\circ}\)C & – \\  & Density & \(D\) & \(\approx\) 4500 & kg/m\({}^{3}\) & – \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Specifications of soft ferrite materials used in the study. Litz-wire consists of 920 strands insulated electrically from each other, wherein a diameter of a single wire is 0.016 mm (AWG54). The cores were split in half using a diamond cutting disc. The secondary winding with a counter-wound compensation turn was applied to one of the core halves. To reduce the stray capacitance (or more precisely, the turn-to-turn capacitances), the distance between individual turns was maximized. The enclosure of HFCT sensor was made from aluminum with a thickness of 2 mm to attenuate radiofrequency electromagnetic interferences and improved performance in an electrically noisy substation environment. It was equipped with a 1 mm slit for better magnetic coupling between primary and secondary winding [97]. The sensor HFCT housing and the housing of the conditioning and A/D processing module were integrated and jointly are a complete sensor ISO-002-HF with the function of fiber optic signal transmission (Figure 13). The investigation on the influence of the number of turns \(N\) and material of the used core on the frequency response of the prototype HFCT sensors was performed using the measurement setup presented in Figure 14. The measurement setup consisted of the oscilloscope Tektronix MDO 3104 with the sampling frequency of 5 GS/s, 1 GHz bandwidth, and built-in signal generator. The sinusoidal signal was generated in the frequency range from 100 kHz to 30 MHz with a step frequency of 100 kHz. It allowed to cover the full range of high-frequency (HF) band, i.e., 3-30 MHz. The current flow in the primary winding was forced by using a 50 \(\Omega\) noninductive resistor \(R_{L}\) in series. The same resistor was connected in parallel with the secondary winding of the HFCT sensor. Figure 14: Schematic diagram of the measurement setup for measuring the frequency response of the HFCT sensor; OSC = oscilloscope type Tektronix MDO3104, GEN = built-in arbitrary function generator, PC = computer, HFCT = high-frequency current transformer under test. Figure 13: The components of sensor ISO-002-HF: 1 = MnZn ferrite core with secondary winding, 2 = insulation, 3 = N-type connector, 4 = high-frequency current transformer (HFCT) sensor housing, 5 = slit in metallic housing, 6 = adjustable latch, 7 = waterproof enclosure for electronics, 8 = cable gland for power supply, 9 = fiber optic cable gland.