The study of emission current of three terminals is presented.  Emission current was measured using switching impulse voltage applied to a metal screen above a terminal, Fig. 1. Switching impulse voltage simulates the increase of voltage stress near the terminal during lightning discharge. A metal screen was used to provide simulation of a charged cloud.  Measurements from a Franklin Rod were compared to measurements from two types of point discharge terminals.

Switching Impulse was obtained from a 20-stage, 57 kJ impulse generator.  The generated switching impulse in this test had a time-to-peak of 250 μs and a time-to-half-value of 2000 μs, both positive and negative polarity.  Emission current was recorded across a small grounding resistor of 1000 Ω that connects to the terminal under test.  Output from the resistor emission current was connected to the TDS-7104 in order to observe both voltage and current waveforms simultaneously.

Measurements were taken at 3 m and 4 m air gap spacing from the metal screen to the test terminal. A set of three impulse voltages was applied for the same voltage magnitude and polarity.  For each of the three terminals, four different levels of voltages were applied from 800 kV to 1100 kV at positive and negative polarity. The measurements were conducted for no rain, no wind, with low and high wind, with light and heavy rain. From the conducted study of the emission current of three samples, several conclusions were stated.  The study shows emission current is higher for the heavy rain condition.  The wind at the applied wind speeds did not have an impact on emission current, Fig. 2. The lowest emission current was obtained for the Franklin Rod.

Fig. 1

Fig. 2