# Analysis of the Possibility of Using Drill-Type Electrodes for Air Ionization
## 2 Measuring System, Characteristics of Drill-Typed Electrodes, Research Programme  
Measurements of characteristic voltages of partial discharges between drill-type electrodes were taken in a typical high voltage coil in the High Voltage Laboratory. The basic elements of the high voltage test system were a control panel with protections and a test transformer with an upper voltage of 110 kV (Figure 1). A single-phase test transformer (TP110) with a rated gear ratio of 220/110,000 (V/V) was used, supplied with 50 Hz alternating voltage. The supply voltage to the electrode system was measured on the high voltage side by an electrostatic kilovoltmeter. Additional measurement on the low voltage part was carried out using a voltmeter that was built into the control panel. The control panel allows automatic adjustment of the high voltage rise, at a speed of 1 kV/s. The drill-type needle spark gaps were tested in the test system and constituted a load of a capacitive nature. The voltage increase was manually controlled in about 1 kV steps every 10 s. During the breakdown of the air layer between the drill-type electrodes, the test transformer changed its operation from an idle state to a short-circuit state; it must therefore be switched off immediately. The system was switched off by an immediate circuit breaker or by overcurrent protection. At the moment of the flashover, breakdowns were generated in the whole system. They threatened the insulation of the test transformer and measuring apparatus--including computers near the high voltage coil. To limit and suppress them, a water resistor was installed directly on the test transformer which limited the current to mA range (max. 100 mA for this system). The metrological conditions for all tested electrode systems were identical: ambient temperature 20 \({}^{\circ}\)C, air humidity 52%. Due to the basic and experimental scope of research, the impact of metrological conditions at this stage was not analyzed. The standard needle electrodes used in the High Voltage Laboratory were conical electrodes with a 45\({}^{\circ}\) cone angle. The term \"needle electrode\" is extensively discussed in the literature and the theoretical approach to this issue is contained, among others, in the papers [20; 21]. Based on this literature, the maximum electrical field strengths in the needle--needle system can be calculated after modelling the electrodes with hyperboloids. It is made possible by the following formulas (1)-(3). \[E_{max}=E_{mean}\times\beta=\frac{U}{s}\times\frac{cos\alpha}{sin^{2}\alpha \times lnctg^{\frac{\alpha}{2}}_{\beta}} \tag{1}\]  
Figure 1: Diagram of the high voltage test system: U—supply voltage (230 V); SW—voltage switch; L1, L2—operating status indicator lights; V—voltmeter; kV—kilovoltmeter; Ro—water resistor (1.1 M\(\Omega\)). \[\beta=\frac{cos\alpha}{sin^{2}\alpha\times lnctg\frac{\alpha}{2}} \tag{2}\]  
For the small angle (\(\alpha<7^{\circ}\)), formula (1) can be simplified to formula (3):  
\[E_{max}=\frac{U}{s}\times\frac{1}{\alpha^{2}\times ln\frac{2}{\alpha}} \tag{3}\]  
Eleven different diameters of the drill-type needle electrodes were selected for the test: from 0.5 mm to 1.5 mm, with a change of 0.1 mm. Drill-type electrodes based on PCB (Printed Circuit Boards) drill technology made of HSS (High Speed Steel) steel were used. They are characterized by high hardness and, due to their being doped with chromium, increased chemical resistance. Figure 2 shows a sketch of a drill-type electrode with a diameter of 1 mm. Figure 3 shows an image of 11 pairs of drill-type electrodes used in measurements. Figure 4 shows an image of a measuring spark gap with mounted drill-type electrodes, prepared for measurements. The main measurement criterion is the visual observation of the appearance of the corona phenomenon on the HV electrode and the determination of the voltage value at which the phenomenon occurred. Measurements included three characteristic voltages: corona voltage U\({}_{\rm o}\), sparking voltage U\({}_{\rm s}\) and breakdown voltage U\({}_{\rm b}\).