## 2 Design and Implementation of On-Line PD Monitoring System
### Partial Discharge Detectors  
#### 2.2.1 Active Dielectric Window  
The active dielectric window (ADW), installed in the revision hole of the power transformer, was elaborated for the detection of AE and UHF pulses generated by partial discharges. The previously conducted research demonstrated that ADW assures over five-times higher AE pulses detection sensitivity compared to commonly used contact transducer type PAC R15 [33]. It results from the fact that ADW is placed inside the power transformer tank and registers acoustic signal propagating directly in oil, whereas a conventional AE sensor is mounted on the outer wall of the tank. For that reason, it only registers a small part of the acoustic wave energy generated by partial discharge. The remaining energy is lost in the result of the processes of the acoustic wave reflection and absorption by the steel wall of the transformer tank. The active dielectric window consists of a specially formed bowl-shaped ceramic element, in which all ultrasonic transducer components were built-in, i.e., piezoelectric crystal with electrodes and electric wires as well as matching and backing layer (Figure 2). The bowl-shape of the ADW enables mounting-in of the UHF antenna and detection of radio signals generated by PDs. The dielectric window was produced from high-density alumina ceramics, which is characterized by high thermal, mechanical, and chemical resistance (it does not oxidize and does not react with mineral oils and esters). The main parameters of the used ceramic material are listed in Table 1.  
Figure 2: Technical drawing of the active dielectric window. Regarding the specific environment of the ultrasonic transducer operation, which is mineral oil, the matching layer is a very important element. Its proper manufacturing assures high transducer sensitivity. So that the acoustic energy transmission from mineral oil to piezoelectric crystal could be carried out lossless (without reflections), the acoustic impedance of the matching layer should be the geometric mean of the mineral oil impedance (\(Z_{oil}\) = 1.28 MRayl at 25 \({}^{\circ}\)C) and used piezoelectric ceramics type PZT-5A (\(Z_{PZT}\) = 31 MRayl). The matching layer was made of epoxy/alumina composite with 0.3825 volume fraction of alumina, which allowed for obtaining the required value of acoustic impedance i.e., 6.3 MRayl. For the preparation of the composite, the epoxy resin of low viscosity (200 mPa:s at 25 \({}^{\circ}\)C) and high purity alumina powder with a grain thickness of 1 \(\upmu\)m was used. The backing layer was made of composite material as well. In this case, epoxy/tungsten composite with 0.08 volume fraction of tungsten of acoustic impedance 4.56 MRayl was used. The optimal values of acoustic impedance and geometric dimensions of all components of the ultrasonic transducer (PZT crystal, matching, and backing layer), which affect the shape of the frequency response curve were designed based on the simulations performed with the model of [PERSON], [PERSON], and [PERSON] (KLM) [34]. In turn, proper proportions of epoxy, alumina, and tungsten for the elaborated composite materials were selected using the Devaney and Levine model, which is based on a self-consistent formulation of multiple-scattering theory [35]. In the final effect, the transducer with relatively flat frequency response in the range between 120 and 270 kHz and with first resonance frequency totaling 134 kHz was elaborated. The transducer frequency parameters are a compromise between high PD detection sensitivity and the resistance to core magnetostriction noise. The remaining parameters of the elaborated ultrasonic transducer are listed in Table 2, whereas detailed information on the design, modeling, and production processes were discussed in [33]. Figure 3 presents a photograph of the front and back sides of the active dielectric window with visible components of the built-in ultrasonic transducer.