Analysis on the Effect of the Nonlinear Resistance on the Locomotive Operating Overvoltages

With the application of the articulated phase insulator, and the speed of electric locomotive rising, it is inevitable for the electric locomotive to adopt the technology automatic passing through the electric phase separation. However, when the locomotive passes the electric phase separation, a variety of overvoltages will be generated, such as the cut-off overvoltage and the closing overvoltage. In this paper, the causes of the two overvoltages above are analyzed theoretically and simulated in Simulink. Then this paper discusses the suppression effects on the cut-off overvoltage and the closing overvoltage by paralleling the nonlinear resistance and the main breaker, or parallelling the nonlinear resistance and the locomotive transformer. The simulation results show that parallelling the nonlinear resistance and the locomotive transformer has suppressive effects on the two overvoltages mentioned above.


Introduction
The application of the articulated phase insulator can eliminate the mechanical hard point problem caused by the device phase separation, and increase the reliability of the electric locomotive.With the speed rising, the time interval of passing through the phase separation is reduced.When the electric locomotive passes through the electric phase separation, the traditional approach relies on the driver's operation, demoting and closing auxiliary motor, and disconnecting main breakers in order.After the locomotive passes, it is then operated in the reverse order, so the pantograph can pass through phase separation in the non-current way, ensuring the service life of the catenary and pantograph [1].The method is not adopted in the modern railway system because of its frequent operation and low safety factor, so it becomes a necessity to use the technology of automatic passing through the electric phase separation (including the vehicle auto-passing phase separation, column auto-passing phase separation, ground switching auto-passing phase separation [2]).When the locomotive passes automatically through the phase separation, the operation of main breaker will cause the cut-off overvoltage and the closing overvoltage, which are analyzed in literature [3][4][5].As the generated overvoltage threatens the running safety of locomotive, it is important to find the corresponding suppressive measures to ensure the safe operation of the electric locomotive.
This paper analyzes the causes of the cut-off over-voltage and the closing overvoltage when passing through the phase separation with adopting the combination of the ground installations and the column switch.
Literature [1], [6] mentioned the suppression of nonlinear resistances, but both were with no simulations.This paper discusses the suppressive effects of the operating overvoltage by using Matlab.

The Cut-off Overvoltage
The Figure 1 that can found in [7] shows that the locomotive passes through the phase separation with the combination of the ground installations and the column switch.
When the electric locomotive passes point A, the main breaker is automatically disconnected, with the huge current being cut off which generates arcs.If the interrupters are not strong enough, the electric locomotive will rush through the phase separation charged, leading to the damage of the pantograph and catenary.In severe

The Closing Overvoltage
the phase separation According to t racteristics of the traction network und in [1], an equation ca cases, it will result in the traction substation tripped accident.Otherwise, it will generate high cut-off overvoltage.
After the locomotive passes through to point B, the breaker is required to be closing to resume power.This operation of the breaker may generate the closing overvoltage whose peak is related to the catenary voltage.And locomotive auxiliary winding and asynchronous auxiliary cluster are still in the working state.Some motors act as the electromotor, while others act as the generator, so the auxiliary systems can be seen as a power.The voltage of auxiliary, which is called the residual voltage, can be coupled to the primary side of the locomotive transformer, whose peak is related to the number of auxiliary motors.Due to the presence of residual voltage, the complete response of the closing circuit is the superposition of the zero-state response and the zero-input response, which may increase the peak of overvoltage, and even lead to the tripped accident of the traction substation.

The Equivalent Circuit of Cut-off Overvoltage
he cha that the line reactance is much bigger than the resistance, the intercepting overvoltage equivalent circuit is set up as shown in Figure 2. L1 is the sum of the equivalent reactance of the traction substation and the contact line.C1 is the circuit equivalent capacitance of the circuit ground.L2 is the locomotive's main circuit, while C2 is its equivalent capacitance to earth.
As seen in Figure 2 that can fo n be drawn as follows.
1 0 Assuming that the two initial values are 0 u and ' 0 u , the Equation ( 1) can be simplified by using Laplace transform.
Using the inverse transformation of La pli place to simfy the Equation ( 2), is the ratio of intercepting overvoltage to the tractage.
Because of From Equation ( 6), it could be obt cep quivalent Circuit of Closing paration, ained that the interting overvoltage is proportional to the circuit resonance frequency and the contact line when the current is cut down.

The E Overvoltage
After the locomotive passes through the phase se the circuit can be equivalent as shown in Figure 3 for the moment the switch is on.The parameters in Figure 3 [8] are basically the same with the ones in the Copyright © 2013 SciRes.EPE The zero-input response The zero-input responses of the formula ( 8) is And the eigenvalues are The initial value can be defined as aand a.
E f on (9) can be simplified by using Laplace tr as follows: denotes the residual peak, the equati ansform The zero-state response of circuit is The complete response of the system both the zero-input response and the zero Based on the theoretical analysis, a large number of siied out by using the model in Simulink.The simulation parameters are as below: voltage of cate-voltages will not be generated voltage the voltage of catenary reaches its pe opeak when hile the phase of is the sum of -state response.

Simulation of the Cut-off Overvoltage
Figure 4(a) shows high when disconnecting the main breaker while the of catenary reaches 0.
Figure 4(c) shows it will produce an overvoltage with the peak value reaching 129.29 kV when disconnecting the main breaker while ak value.The value 129.9 kV is 3.34 times of the normal peak value, and far beyond the regulated voltage level of the safe operation of the electric locomotive.

Simulation of the Closing Overvoltage
The closing operation needs to be done after the locom tive passed through the phase separation.Different values of the closing overvoltage will be generated the catenary voltage is in different phases.
The simulation results Figure 5 (a) show that higher overvoltage will not be caused when the phase of the catenary voltage is around 0°or 180°.W the catenary voltage is around 90°or 180°, the ) [9].The arrester itself is a kind of nonline .Fi small, its resistance value is infinity, and w

Model of the Non
The overvoltage has a serious in eration of the electric locomotive.The traditio ing operation aims to decrease the levels of th ages, but it is no longer suitable for the high-speed train, so better suppression measures involving hardwares need to be found.
Usually, in power systems, transient voltages caused by direct lightning strikes are prevented by installing arrester (MOA ar resistance (MOV), composed of ZnO (the main ingredient) and a few other metal oxygen content additives that are used to constitute grain boundaries phase and improve some properties of the arrester [10].If the device is specially processed to lower its voltage level, the arrester can be installed in electric locomotive to suppress the operating overvoltage.The following is the analysis of the arrester's characteristics and the suppression effects on the two overvoltages mentioned above.
According to the characteristics of the nonlinear resistance, its model is built by using Matlab/Simulink platform, and its features are verified through simulation Figure 6(e) presents the properties of nonlinear resistance when its terminal voltage changes.When the terminal voltage is hen its terminal voltage is up to a certain extent, its resistance value reduces to zero quickly.These properties are called as the clamping voltage effect which can be used to suppress the cut-off overvoltage and the closing overvoltage.

Suppression Simulation of the Overvoltage
Parallelling the nonlinear resistance and the main breaker, the suppressive effects on the two overvoltages are shown in Figure 7 (disconnecting or closing main breaker when the catenary voltage reaches its peak value).When paral nce a he main transformer o uppressive effects lelling the nonlinear resista f the locomotive, the s nd t on the two overvoltages are shown in Figure 8.

Conclusions
Through theoretical analysis and simulations, the conclusions can be drawn as follows: 1) If the main circuit breaker is off when the phase of the catenary voltage is close to its peak phase, the cut-off overvoltage will be very high.Otherwise, if the main circuit breaker is on when the phase of the catenary voltage is close to its peak phase, the closing overvoltage will be very high.

NCES
s of Ground's Auto-passing Neutral Section at Switching Time," Transactions of China Electro Technical Soc 1, pp. 150-154.
[4] X. J. Wei, H. he suppressive effects on the cut-off overv and the switching overvoltage by placing the nonlinear resistances in different positions are obtained.When parallelling the nonlinear resistance and the main circuit breaker, the cut-off overvoltage can be inhibited efficiently, while the closing overvoltage cannot be inhibited; when parallelling the nonlinear resistance and the main transformer of the locomotive, both the cut-off overvoltage and the closing overvoltage can be inhibited efficiently.

Figure 1 .
Figure 1.The schematic diagram of electric locomotive passing the articulated phase insulator.

Figure 2 .
Figure 2. The equivalent circuit of the intercepti overvoltage.ng influence of L , C can be ignored, the differen 1 tial equation can be lis ollows: 2 ted as f ( )

Figure 3 .
Figure 3.The equivalent circuit of the closing overvoltage.

Fig- ure 5 Figure 4 .Figure 5 .
Figure 4.The cut-off overvoltage caused by differen phases of the catenary voltage.t gure 6(a) shows the structure of the nonlinear resistance model, including CCS (Controlled current source), Fcn (User-defined function: I0*(u/V0) ^alap), V meter (voltmeter), and a first-order transfer function [11].Fig- ure 6(b) shows the characteristics of the simulation circuit.Figures 6(c), (d) and (e) respectively show the voltage waveform, the current waveform and the volt-ampere characteristics.

Figure 6 .
Figure 6.Model and performances of the MOV.

Figure 7 .
Figure 7. Parallelling the nonlinear resistance and the main breaker.