## 3 Research on the Characteristics of High-Voltage Switching with Three-Electrode Plane Spark Gap
### Dynamic Operating Characteristics  
#### 3.3.1 Dynamic Minimum Trigger Voltage  
In order to determine the conduction condition of the three-electrode planar spark gap high voltage switch with seven kinds of main electrode gaps, the three-electrode planar spark gap high voltage switch with main electrode gaps of 0.8 mm, 1.0 mm, 1.2 mm, 1.8 mm, 2.0 mm, 2.2 mm, and 2.6 mm are designed, and the trigger electrode widths are 0.6 mm and 0.8 mm, respectively. With the charging voltage of 2.0 kV, the minimum trigger voltage curves of various switch parameters are shown in Figure 7.  
As shown in Figure 7, under the condition of charging voltage of 2.0 kV, as the main electrode gap increases from 0.8 mm to 2.6 mm, the minimum trigger voltage value of the planar spark gap switch increases from 677 V to 1783 V (\(a\) = 0.6 mm), 685 V rises to 1766 V (\(a\) = 0.8 mm). This shows that with the continuous increase of the main electrode gap, the minimum trigger voltage of the switch increases. With the same gap, the width of the trigger electrode is wider, the minimum trigger voltage becomes lower, for the reason that the conduction principle of the three-electrode planar spark gap high voltage switch is to apply pulse trigger voltage to the trigger electrode. Herein, the gap electric field is distorted and the air breakdown effect occurs between the gaps, making the two poles of the switch conduct. When the three-electrode planar spark gap high voltage switch is in the triggering state, the average electric field strength between the two poles is calculated as follows [26; 27]:  
\[E=\frac{U_{SB}}{b-a} \tag{1}\]  
\(E\): Average electric field, V/m;  
\(U_{SB}\): Working voltage, V;  
\(b\): Gap, mm;  
\(a\): Trigger electrode width.  
As shown in Formula (2), under the condition of the same trigger electrode width, when the applied voltage \(U_{SB}\) between the two poles is constant, the average electric field strength \(E\) increases with the decrease of gap \(b\), meaning that the smaller the gap between the two poles of the switch is, the greater the average electric field strength becomes,  
Figure 7: The curves of minimum trigger voltage varying with gap.  
and the easier the air gas breakdown effect occurs, so the energy required for the trigger electrode is lower. On the contrary, the average electric field strength between the two poles of the switch is stronger and the energy required for the trigger is higher, when the switch is on. As shown in Formula (2), under the condition of the same gap, when the applied voltage \(U_{SB}\) between the two poles is fixed, the width of the trigger electrode is increased, the average electric field strength increases, and the air breakdown effect occurs easily, resulting in the lower energy required for the trigger electrode.  
The matching test between the working voltage and the trigger voltage was carried out. The test results show that the relationship between the trigger voltage and the working voltage is inversely proportional. When the working voltage was high, the switch turned on easily, and the minimum trigger voltage was reduced. That is because when a relatively high voltage is applied between the two poles of the switch, the average electric field strength increases, and it is easier for breakdown effect to take place between the two poles of the switches, the trigger pole and air interface. Therefore, the required trigger energy will become low. The fitting curve of the relationship between the working voltage of the switch and the trigger voltage was shown in Figure 8.