## 2 Modular System-Level Modeling Approach
### Model for BTM Susceptibility Prediction  
To find out the main disturbance coupling path for BTM system, the comparative experiment is taken during the train passes the neutral zone. The setup of the first experiment is shown in the Figure 5a: the disturbance received by the on-board antenna is measured directly by a spectrum analyzer. Another experiment configuration is shown in the Figure 5b: An impedance of 50 \(\Omega\) is used to take the place of BTM antenna, then the disturbance is measured by the spectrum analyzer in zero span mode.  
The comparison result is shown in Figure 6. The maximum disturbance voltage is 77 dB\(\mu\)V when the antenna connection, shown in Figure 6a. The maximum disturbance voltage is 47 dB\(\mu\)V when the antenna is disconnected and is replaced by 50 \(\Omega\), shown in Figure 6b. Thus, the main disturbance coupling path for BTM system is antenna coupling.  
Figure 5: Setup to find out the main disturbance coupling path for BTM system: (**a**) The testing with the antenna connection; (**b**) The testing with 50 \(\Omega\) connection.  
For railway system, the two most serious disturbances are generated from traction power system. One is pantograph off-line disturbance when the train passes through phase separation zones, and the other one is traction return current. In order to confirm the main disturbance source, a further field tests have been done. An \"antenna connection\" setup is taken when the train is in a normal operational situation. In this situation, traction return current is considered to be the main disturbance source. And the disturbance received by the on-board antenna is shown in Figure 7. The disturbance levels are almost the same with the result in Figure 6a which is obtained with pantograph off-line disturbance. Thus, the main disturbance does not come from the pantograph off-line disturbance, but induces from traction return current flowing on the steel rails and some other disturbance from traction power system. Moreover, according to the loop physical structure and operation frequency of BTM antenna, the main disturbance is in the form of magnetic field [23].  
Based on the analysis above, the influence of radiation disturbance and conductive disturbance could be ignored. Thus, it is assumed that the voltages of \(S_{\text{r1}}\) and \(S_{\text{r3}}\) in Figure 4 equal to zero. The modules for radiation and conducted disturbance could be ignored. For modular system-level modeling method in the paper, the module could be utilized alone according to requirements for practical problems. Thus, the BTM EMS prediction model could be simplified as induction disturbance, induction coupling, and sensitive equipment modules, as shown in Figure 8.  
Figure 6: Disturbance in BTM system when the train passes through phase separation zones: (**a**) The disturbance received by the antenna. (**b**) The disturbance in BTM system when the antenna is disconnected.  
Figure 7: Disturbance received by the antenna when the train is in a normal operational situation.  
Thus, according to the method the \(F_{s2}\), \(F_{c2}\) equal to 1 and other elements of transfer function equal to 0., the disturbance voltage \(V_{N}\) could be obtained as Equation (4).  
\[V_{N}=V_{r2}=V_{2}C_{2}, \tag{4}\]