## 2 Design and Implementation of On-Line PD Monitoring System
### Partial Discharge Detectors
#### 2.2.3 High-Frequency Current Transformer
The measurement of the frequency response was automatically realized by the application implemented in LabVIEW environment and launched on a computer connected to the oscilloscope. The individual points of the frequency response curve \(FR_{HFT}\)_(f)_ were calculated as  
\[FR_{HFT}(f)=20\cdot\log\frac{V_{R}(f)}{V_{\text{GEN}}(f)}, \tag{6}\]  
where \(V_{R}(f)\) is the RMS value of voltage measured on the output of the HFCT sensor at frequency \(f\), \(V_{GEN}(f)\) is the RMS value of voltage measured on the output of signal generator at frequency \(f\). Figure 15 presents the frequency response curves of the prototype HFCT sensors. As one may notice, simultaneously with the increase in the number of turns, the sensor sensitivity decreases. It results from the fact that the values of that parameter depend directly proportional to the load resistance \(R\) (which in this case was constant and equal to 50 \(\Omega\)) and inversely proportional to the number of turns \(N\). Increasing the number of turns raises the magnetizing inductance \(L_{m}\), which causes the reduction of lower \(-3\) dB cutoff frequency \(f_{L}\) of the HFCT sensor. This effect is even more visible for sensors with MnZn soft ferrite core, whose magnetizing inductance is high regarding the high value of permeability \(\mu\) at low frequencies. For comparison, the value \(f_{L}\) for current transformer with MnZn ferrite core and 10 turns is lower than 100 kHz, in turn, for a current transformer with NiZn ferrite core with the same number of turns, \(f_{L}\) equals 450 kHz. For a lower number of turns, the differences are even more clear. For instance, \(f_{L}\) for HFCT sensor with MnZn ferrite core and \(N=3\) is also very low and equals 250 kHz. The current transformer of the same construction, but with the ferrite core NiZn, has \(f_{L}\) equal to even 3 MHz. It should be emphasized that for the HFCT sensors devoted to PD detection, the more important parameter than the \(f_{L}\) is higher \(-3\) dB cutoff frequency \(f_{H}\), since the discharges in oil-paper insulation emit high-frequency waves mainly in the range between approx. 2 and 20 MHz [98]. In this case, it is more favorable to apply a lower number of turns. It allows to obtain high sensitivity \(S\) of the HFCT sensor and high value of the parameter \(f_{H}\), which improves the performance of the sensor in the range of higher frequencies (\(>1\) MHz). One may notice in Figure 14, that the positive effect resulting from reduction of the number of turns is more for sensors with the core made from NiZn soft ferrite since permeability \(\mu\) of the applied material is a practically constant value in the wide range between \(1\) MHz and approx. 50 MHz. Based on the analysis of the measured frequency response curves, for further research, the two prototype HFCT sensors were selected. The first of the chosen sensors was manufactured from ferrite  
Figure 15: Frequency response of the prototype HFCT sensors depending on the number of turns \(N\) and the core material used: (**a**) MnZn soft ferrite, (**b**) NiZn soft ferrite, compared with frequency response of commercial HFCT sensors (RFCT-4 and HFCT 140/100). core MnZn with the three-turn secondary winding. The main advantages of that construction are a very high sensitivity (higher than popular commercial sensors), wide bandwidth (250 kHz-25.5 MHz), and relatively flat frequency response curve. The second from the selected constructions was built from ferrite core NiZn with four turns. It is characterized by slightly lower sensitivity (\(\leq\)2.67 dB) than the transformer with MnZn core, but its frequency response curve is almost ideally flat in a wide band between 1.8 and 30 MHz. In the first stage of the laboratory tests, measurements of the impulse response of the selected prototype HFCT sensors were conducted. For this purpose, a standard LDC-5 PD calibrator (dDoble Engineering Company, Marlborough, USA) was used, which allowed to inject pulses with an apparent charge of 500 pC and duration of 50 ns into the primary circuit.