# A Research of Pulse Source for EFT Test of Electrical Vehicle Piles
## 4 Electromagnetic Compatibility Protection of the System  
### Analysis for the Interference Model of Electromagnetic Pulse Conduction  
Although the pulse-generating sources were operated independently, electromagnetic interference cannot be avoided due to the common load. The interference, which would exert a severe influence on the operation of the system without protection methods, could be divided by electromagnetic radiated interference and common-ground coupling interference [16; 17]. The former was electromagnetic radiation, which was approximately antenna-generated in the connected line or the feedback line of the system and received by the other working sources of the system directly; it could be prevented by metal-shielding measures. The latter was a type of conduction interference generated by loop coupling composed of distribution parameters, such as distributed capacitance and inductance [18; 19; 20]; it was difficult to prevent because connecting the sources and elements in the same network and ground was inevitable.  
The formation of common-ground coupling interference could be simply illustrated using Figure 4, where two pulse sources are connected to the same ground; the discharge loop of hydrogen thyratron is found on the right. The loop consisting of K1-C4-R1-K1 was a typical electromagnetic pulse-generating circuit or pulse source 2, and C4 was the charging capacitance. The topology of the circuit in the left of Figure 4 was the same as that in the right. Thus, the load R1 could be supplied by two synchronous or asynchronous pulses generated by the two pulse sources through timing control. When the left pulse source discharged in the loop 1, a coupling circuit could be formed with the distributed capacitance C3 with conducted interference occurring. The coupling circuit could be simplified in another topology demonstrated in Figure 5, in which the electromagnetic pulse generating circuit acting as the interference pulse source was marked by pulse source directly. The disturbed part was indicated by the circuit diagram for the sake of clarity. C1 and C2 were the distributed capacitances in the c, d ending and d, e ending of hydrogen thyratron. C3 was the coupling capacitance between a and b points, and L1 was the equivalent distributed inductance in the circuit. The equivalent circuit of anode-cathode and grid-ground of thyratron, which was not triggered to turn on, was not illustrated.  
When switch K is turned on at \(t=0\), the equation of loop 2 is as follows:  
\[U_{R}=L_{1}\frac{di}{dt}+iR_{2}+\frac{1}{C_{3}}\int idt+\frac{1}{C_{4}}\int idt +\frac{1}{C_{1}}\int idt+\frac{1}{C_{2}}\int idt, \tag{12}\]  
where \(U_{R}\) is the output voltage of the interference pulse source.  
Supposed that the initial condition was as follows: \(u(0)=0,\ i(0)=0\), the voltage in \(d\)-\(e\) points could be obtained as follows:  
\[U_{de}=\frac{U_{0}}{C_{2}\Big{(}L_{1}P_{1}-L_{2}P_{2}+\frac{1}{C \overline{P_{1}}}-\frac{1}{C\overline{P_{2}}}\Big{)}}\cdot\bigg{(}\frac{1}{P_ {1}}e^{P_{1}t}-\frac{1}{P_{2}}e^{P_{2}t}+\frac{1}{P_{2}}-\frac{1}{P_{1}}\bigg{)}. \tag{13}\]  
In Equation (13),  
\[P_{1}=-R_{2}/2L_{1}+\sqrt{\left(R_{2}/2L_{1}\right)^{2}-1/(L_{1} C)},\] \[P_{2}=-R_{2}/2L_{1}-\sqrt{\left(R_{2}/2L_{1}\right)^{2}-1/(L_{1} C)},\] \[\frac{1}{C}=\frac{1}{C_{1}}+\frac{1}{C_{2}}+\frac{1}{C_{3}}+\frac {1}{C_{4}}.\]  
The typical simulated waveform is indicated in Figure 6 under the condition of \(R_{2}<\sqrt{L_{1}/C}\).  
The analysis of Figure 6 indicates that the grid of thyratron of source 2 was disturbed by the couple loop composed of distributed capacitance when the pulse source was discharged. The waveform was superposed by the interference pulse illustrated in Figure 3. The thyratron might be falsely triggered when the width and amplitude of the interference waveform was up to a certain threshold value.  
Figure 4: Interference diagram of mutual ground.  
Figure 5: Simplified circuit of coupling network.  
Figure 6: Simulation waveform of coupling network.