# Research on Interference Mechanism of 25 Hz Phase Sensitive Track Equipment from Unbalanced Current
## 2 Cause Analysis of Abnormal Red Ribbon
### Influence of Unbalanced Current Intrusion on Relay
#### 2.4.2 Interference Analysis of Unbalanced Impulse Current on Relay Coil  
The track relay belongs to the secondary side load of the choke transformer. The interference of the impulse current will cause the magnetic saturation of the choke transformer. In order to analyze the influence of the secondary side of the transformer on the track circuit, it is necessary to simplify the choke transformer circuit. The choke transformer model studied in this paper is BE-600/25. The number of turns of the traction coil of this type of transformer is 8 turns, a total of two groups. The signal coil has 48 turns, so the circuit diagram of the choke transformer and its equivalent circuit are shown in Figure 6.  
Among them, \(Z_{1}\) represents the traction side coil impedance. \(Z_{Z}\) represents the nonlinear excitation impedance. \(Z_{2}\)\({}^{\prime}\) is the secondary winding impedance converted to the primary side. \(I_{0}\) is the current in the nonlinear excitation impedance. \(I_{1}\) is the traction coil side current. \(I_{2}\)\({}^{\prime}\) is the signal coil side current converted to the primary side. \(Z_{\mathrm{L}}\)\({}^{\prime}\) is the load. Therefore, the voltage at both ends of the load \(\dot{U}_{L}^{\prime}\) can be expressed as Equation (11). In this paper, the allowable current of the choke transformer is set to 1000 A. The primary side resistance \(R_{1}\) and the secondary side coil resistance \(R_{2}\) are both 0.2 \(\Omega\). The mutual inductance parameter is 11.09 mH. The secondary side load parameter \(R_{\mathrm{L}}\) of the choke transformer is 6\(\Omega\). \(|Z_{Z}|=2.3\ \Omega\), which can be ignored due to the small coil inductance. Equation (12) is thus obtained. \[\dot{U}_{L}\)\({}^{\prime}=\frac{\dot{U}Z_{L}\)\({}^{\prime}}{(Z_{1}+Z_{2}\)\({}^{\prime}+Z_{\mathrm{L}}\)\({}^{\prime})+\frac{Z_{\mathrm{L}}Z_{2}\)\({}^{\prime}+Z_{\mathrm{L}}Z_{\mathrm{L}}\)\({}^{\prime}}{Z_{\mathrm{Z}}}}\) (11)  
\[\dot{U}_{L}\)\({}^{\prime}=\frac{0.67\dot{U}}{0.892+\frac{0.138}{|Z_{2}|}} \tag{12}\]  
Figure 6: Choke transformer circuit and equivalent circuit. In order to simulate the situation that the choke transformer is interfered, 25 Hz signal current and 50 Hz unbalanced current are added to the traction coil, in which the effective value of the signal current is 0.3 A and the frequency is 25 Hz. At the same time, the steady-state component can be set to a certain value to simulate the unbalanced impulse current, and its frequency is set to 50 Hz. The working current in the signal coil is set to 0.8 A and the frequency is 25 Hz. The influence of DC magnetic bias on the track circuit is studied by setting different values of DC components. Therefore, the DC current values of 0 A, 0.5 A, 1.0 A, 1.5 A, 2.0 A and 2.5 A are added to the traction coil side, respectively. The excitation current generated in the choke transformer under different DC components is shown in Figure 8. It can be observed that with the increase of the DC component in the impulse current, the excitation current in the choke transformer will also increase significantly. When the DC component value reaches 2.5 A, the DC component can cause a large enough excitation current, so that the transformer core is magnetically saturated, resulting in a decrease in the secondary voltage and affecting the working state of the track circuit. Figure 9 shows the magnetic flux density nephogram under different DC components. It can be seen that, with the increase of the DC component, the magnetic density of the choke transformer core also increases, which accelerates the process of magnetic saturation. While the excitation current of the transformer is affected, the core loss of the transformer will also be affected by the unbalanced impulse current. Figure 10 shows the waveforms of the core loss of the choke transformer under different DC components. It can be seen that the DC component is positively correlated with the core loss of the choke transformer.