## 2 Design and Implementation of On-Line PD Monitoring System
### Partial Discharge Detectors
#### 2.2.3 High-Frequency Current Transformer  
The high-frequency current transformer is an inductive PD detector, which is usually installed on the wire grounding the investigated high voltage device. That wire is simultaneously a primary winding of the current transformer. A typical sensor HFCT consists of toroidal ferrite core, on which from several to over a dozen turns (secondary winding) are wound-up. This winding is terminated with a low-inductance sense resistor. A current pulse of high frequency flowing through the grounding wire (primary winding) causes the occurrence of a time-varying magnetic field, which passes through the secondary winding and induces a voltage across the sense resistor. Figure 12 presents a model and  
Figure 11: The comparison of the exemplary UHF pulses registered by the commercial UHF disk antenna and prototype meandered PIFA for (**a**) surface PD of an apparent charge of \(q=625\) pC, (**b**) creeping spark of apparent charge of \(q=4670\) pC. Figure 10: Schematic diagram of the measurement setup in which PD detection sensitivity of the prototype UHF antenna was tested: U = high-voltage supply, Z = short-circuit current limiting resistor, OSC = four-channel oscilloscope, TT = oil-filled transformer tank, PD = electrode system for surface partial discharge generation, PA = prototype meandered PIFA, CA = commercial UHF disk antenna as reference, C\({}_{\text{K}}\) = coupling capacitor, CD = coupling device (measuring impedance), CC = connecting cable, M = conventional partial discharge detector, PC = computer. equivalent circuit of the HFCT sensor, where \(i\) is the current flowing through the primary winding, \(R\) is the sense resistor, \(V_{R}\) an induced voltage in the secondary winding, \(L_{m}\) is the magnetizing inductance of the current transformer, \(R_{c}\) is the core parallel equivalent resistance, \(L_{l}\) and \(R_{s}\) are the leakage inductance and series resistance of the secondary winding, respectively. The remaining parasitic components included in the model are stray capacitance \(C\) and inductance \(L_{R}\) of the sense resistor [91]. The HFCT sensors are applied in the diagnostics of high-voltage power cables, power generators, metal-enclosed switchgears, and gas-insulated systems (GIS) [92; 93; 94]. In the case of power transformers, the HFCT sensor may be installed on a grounding wire of the tank, on a grounded measuring tap of the bushing insulator, or a grounding of the neutral point of the transformer winding. The last option assures the highest detection sensitivity of the PDs occurring in the power transformer main insulation system. The toroidal HFCT sensor core is most commonly made from soft ferrite. This material is ferromagnetic and consists of iron oxides mixed with oxides of manganese and zinc (MnZnFe\({}_{2}\)O\({}_{4}\)), or nickel and zinc (NiZnFe\({}_{2}\)O\({}_{4}\)). Manganese-zinc soft ferrites have high relative permeability (\(\mu_{r}\approx\) 350-20,000 at 10 kHz), medium saturation flux density (\(B_{s}\approx\) 320-545 mT at 1200 A/m), operating frequency range from a few kilohertz to 4 MHz, and low resistivity \(\rho\) (from 10 \(\Omega\)m at 10 kHz to 0.01 \(\Omega\)m at 100 MHz). In turn, nickel-zinc soft ferrites are characterized by low relative permeability (\(\mu_{r}\approx\) 15-2000 at 10 kHz), slightly lower saturation flux density (\(B_{s}\approx\) 220-380 mT at 1200 A/m), much higher maximum operating frequency (up to about 200 MHz), and much higher resistivity \(\rho\) (from \(10^{5}\)\(\Omega\)m at 10 kHz to \(10^{3}\)\(\Omega\)m at 100 MHz) [95]. The clamp-type HFCT sensor devoted to PD monitoring in power transformers should have high sensitivity, wide bandwidth, closer to the constant line frequency response characteristics, and be equipped in enclosure shielding from external electromagnetic interferences. The sensitivity \(S\) of the HFCT sensor depends on resistance value \(R\) and the number of windings \(N\), as shown in Equation (2).