Study on Electromagnetic Transient Condition of EMU Passing by Phase-separation with Electric Load in High-speed Railway *

Aiming at the complex electromagnetic transient process of EMU passing by phase-separation with electric load in high-speed railway, mechanism of overvoltage caused by switching off, overvoltage caused by switching on and impact current is analyzed systematically in this article. π-type equivalent circuit of feeding section is put forward in the analysis of overvoltage mechanism. Overvoltage and overcurrent model of passing by phase-separation with electric load are also built. Correctness of mechanism was validated by simulation. In addition, the methods to solve the influence on substations, transformers and protection devices in this process are put forward, which provides a new idea on passing by phase-separation with electric load technology.


Introduction
To make the balance of three phase load, the power supply of phase sequence rotation among substations is used in the traction power supply system.At the same time, in order to realize the electrical isolation of feeding section, the phase-separation is added in the output of the traction substation and section post in every 20-30 km [1].In order to meet the requirements of high-speed operation and security, the way of ground-switch automatic convert passing by phase-separation with electric load has been adopted in high-speed railway [2].Ground-switches are being switched frequently when EMU passing by phase-separation with electric load, and electrical parameters of "traction network -EMU -phase separation system" are changing ceaselessly in this process, which make traction network, EMU and phase separation system operate from one state to another continuously.At the same time, electromagnetic parameters in the system such as voltage and current are changing complicated in this dynamic process, which have an influence on various components of the traction power supply system [3][4][5][6][7].There are some researches about complex electromagnetic transient process of EMU passing by phase-separation with electric load.That overvoltage caused by switching off, overvoltage caused by switching on are analyzed in theory and using electronic switch to avoid overvoltage is put forward as in [8].Transient response of the system when locomotive transformer is invested is analyzed as in [9], and selecting the appropriate voltage phase angle into the locomotive transformer is to reduce the inrush current.Reference [10] put forward that secondary harmonic content of differential current is reducing because of the differences of CT transmission characteristics in both sides of transformer when closed with load.
Based on this, mechanism of all kinds of the transient phenomena is researched systematically, and overvoltage and overcurrent model of passing by phase-separation with electric load are also be built.Electromagnetic transient process having an influence on traction supply system is studied through analysis of simulation.

Scheme of Ground-switch Automatic Passing by Phase-separation
As ground-switch automatic passing by phase-separation representative Shinkansen in Japan adopts the mode of insulated overlaps device, which not only improves the quality of pantograph's current collection, but also takes a short time, usually between 0.1~0.15s[11].In this way, the overhead catenary system(OCS) has no power supply dead zone, and the main circuit breaker on EMU does not need action, therefore passing by phase-separation with electric load is achieved.Its operational principle is shown in Figure 1.The neutral section is set up at phase-separation where insulators F 1 , F 2 are installed to insulate OCS in different phase.Vacuum circuit breakers QF 1 , QF 2 are across-theline in two different phases separately.Four locomotive position sensors CG 1 ~ CG 4 are installed on both sides of the rail.The neutral section is uncharged when QF 1 , QF 2 are turned off when locomotive sails into neutral section, and sensor CG 1 can sense the position of locomotive, neutral section is supplied by A phase in the event that QF 1 is turned on.When the locomotive arrives at CG 2 but not reaches CG 3 , QF 1 is turned off and QF 2 is turned on, so neutral section is supplied by B phase.When sensor CG 4 can sense the position of locomotive, QF 1 and QF 2 are turned off, so neutral section returns to the state that the locomotive does not go through.

Analysis of Electromagnetic Transient in
EMU Passing by Phase-separation

Transient Process Equivalent Model
Electrical model in transient process mainly includes three parts, such as OCS, neutral section and EMU.OCS and neutral section are distributed parameter circuits in essence, which is the same as electric power line.OCS can be expressed as π-type equivalent circuit, and neutral section can be expressed as T-type equivalent circuit.
The main transformer on EMU is represented as RL series impedance.On the basis of the equivalent circuit of the three parts above-mentioned, equivalent power supply and equivalent impedance of the traction substation as well as mutual capacitance between OCS and neutral section are considered.Equivalent model of EMU passing by phase-separation is shown in Figure 2.
In the scheme, U A and U B represent equivalent power supplies of the traction substation.R S1 , R S2 and L S1 , L S2 represent separately equivalent resistance and equivalent inductance of the two power supplies.R 1 , R 2 and L 1 , L 2 represent separately equivalent resistance and equivalent inductance of the two feeding section.C 1 , 1 C and C 2 , C represent ground capacitance of the left and right feeding section separately.R N1 , R N2 and L N1 , L N2 are separately equivalent resistance and equivalent inductance of the neutral section.C N represents ground capacitance of the neutral section.C N1 and C N2 represent mutual capacitance between OCS and neutral section.QF 1 and QF 2 represent vacuum circuit breakers.R 3 and L 3 represent separately equivalent resistance and equivalent inductance of the locomotive's transformer.C 3 represents the ground capacitance of locomotive.

Mechanism Analysis of Overvoltage Caused
Beca s inductive load, current is sud-by Switching off use the locomotive i denly cut off before crossing zero when circuit breaker QF 1 is turned off.The residual electromagnetic energy in the inductor is converted to capacitor energy because of inductor current charging the capacitor, which results in the sharp rising of the capacitor voltage.That is why the overvoltage caused by switching off appears.overvoltage generated interceptor.The influence of locomotive resistance R 3 is ignored.The simplified circuit schematic is shown in Figure 3.
Making L = L 3 , C = C N + C 3 , QF 1 is turned off when I = I sinθ, equation o m f oscillation circuit has been got as follow.
general solution as in In ideal conditions, the magnetic energy and the electric energy are oscillating in the entire loop, and the angle frequency of the oscillation is 0 1 where, -U 0 represents initial voltage of capacitor; I 0 where, -U 0 cosω 0 represents oscillation com where, U 0 is very small and so voltag is also very large after multiply by a small current.
U m can be expressed further as in From ( 6), overvoltage is related to the ground inductance, capacitance and the angle of current when QF is turned off, and the voltage reaches its maximum v when QF 1 is turned off at θ = ±90°.
2 , the neutral section side mer of EMU and asynchronous 1 alue

Mechanism Analysis of Overvoltage Caused by Switching on
When the locomotive arrives at CG becomes uncharged in the short time when QF 1 is turned off and QF 2 is not turned on.In this case, the low voltage of the main transfor motor group in the traction invertor and the auxiliary circuit still form a closed loop [12].Current flows through the closed loop because parts of the asynchronous motor are rotating and still remain energized.It becomes coupling in the high voltage side of main transformer through auxiliary winding, which represents residual voltage of the neutral section.Due to the transient difference in voltage between the residual voltage of neutral section and the voltage of power supply, impedance of transmission line and capacitance of neutral section appear oscillation process after QF 2 is closed.Figure 4 is circuit schematic when QF 2 is closed; Figure 5 is vector diagram of the closing circuit in which c u  , L u  , R u  and B u  represent the voltage of capacitance, inductance, resistance and power supply separately, i  represents current, β represents phase angle of the power supply when switch is turned on, γ represents im edance a le.
Since L N2 is very small when compared to the total inductance of the whole loop and the ground capacitance of neutral section C N is greater than which of locomotive C 3 , the function of This is a second order linear nonhomogeneous differential equation.The same method of solving zero-state response is used in full response of the second order cuit, which can be got by bringing a non-zero initial value ristic root is cirinto equation when confirming undetermined coefficient.
Characteristic equation can be expressed as follow.represents resonance angular frequency of general solution as in (10) particular solution as in where, R turned off and QF 2 is turned on when t = 0, initial condition is Because the resistance consumes electrical energy in th enuates periodically.12) can also be expressed where, From ( 14), the voltage of neutral section relates to amplitude of power supply voltage, phase angle of the power supply when switch is turned on and residual voltage of neutral section in the moment QF 2 is turned o voltage of neutral section is superimposition of steady-state component and transient component, so the overvoltage is higher.Transient

cos( )
Where, C Z represents attenuation non-periodic c ponent.

om-
When the QF 2 is turned on, flux of locomotive as in Supposing that QF 2 is turned on in t 1 .According to the principle of flux cannot be mutation where, N represents the number of coils and R m represents total reluctance.When β meet ωt + β = β + 2kT, Φ 1 will reac maximum 2Φ s + Φ dt , and inrush current increases greatly.Impact current which up to 4-6 times as much as the ra

Simulation of EMU Passing by Phase-separation
Making use of Matlab/Simulink to establish the module of passing by phase-separation with electric load.Simulation parameters are shown as follow.R S1 = 0.6969 Ω, L S1 = 4.17 mH, R S2 = 0.3676 Ω and L S2 = 24.9mH; R .3836Ω and L 2 = 3.73 mH; 0.000325 μF and C N2 = 0.00415 μF.

Simulation Analysis of Overvoltage Caused by Switching off
When QF 1 is turned off, the simulation results of overvoltage are shown in Figure 6: Figure 6(a) shows the voltage waveform when current crossing zero plitude of current is maximum (θ = ±90°).load, the phase angle of voltage is ahead of current 90°, so the angle of Ua is 90°when θ = The simulation results in Figure 6 indicate turned off causes a large overvoltage whose maximum can reach 77.41 kV.The amplitude of the overvoltage is related to the phase of current when QF 1 is turned off, which is a good description of the correctness of the analysis of the mechanism.

Simulation Analysis of Overvo
When QF 2 is turned on in this process, the absolute value e maximum vo shown in Table 1 can be got through simulating in the condition that phase angle β is 0°-350°.
As can be seen from I, overvoltage minimum appears when β is in the vicinity of 10°or 180°; overvoltage maximum is appearing when β in the vicinity of 90°or 270°.
The waveforms of overvoltage minimum and maxi um are shown in   The protection gap on the locomotive roof discharges because of overvoltage, which results in traction substation tripping and interrupting power supply and transportation.The frequent shocks of overvoltage have an influe erter nd auxiliary motor system.The overvoltage is related to oscillation caused by the changes in circuit parameters when circuit breaker is operating and voltage phase angle when switching on or off, thereby it can be suppressed effectively by eliminating oscillation in the condition of changing the circuit parameters.In addition, using phasecontrolled rectification technology in switching on and off is an effective method.

Simulation Analysis of Impact Current I
C 0 MVA, variable ratio is 220/27.5 kV; equivalent resista residual voltage of neutral section and overvoltage caused gle when QF 2 is turned on.Impact current whose amplitude is 5 times higher than n of the t.Wavence on the life of traction transformers, conv a nfluence on Differential Protection apacity of traction transformer adopted in simulation is 4 nce and inductance of feeding section in both sides of phase-separation are 4.475 Ω and 19.35 mH; equivalent resistance and inductance of neutral section is 0.063 Ω and 0.198 mH; capacity of main transformer of EMU is 8MVA, variable ratio is 27.5/1.5 kV.Excitation inrush current of the EMU transformer is shown in Figure 8 when QF 2 is turned on, and inrush current of the EMU transformer closing side is shown in Figure 9.
As can be seen from the simulation results: the core of EMU main transformer is saturation under the action of by switching off, which comes into being excitation inrush current whose amplitude is high and waveform exists obvious discontinuity an that of rated current in closing side is the superpositio inrush current and low-voltage side curren form does not exist obvious discontinuity angle, however waveform generates obvious distortion because of nonperiodic component.
Making use of the CT model in PSCAD, impact current is flowing into the low-voltage side of traction transformer directly, and the secondary current of high and low sides of traction transformer can be got.CT parameter settings: CT ratio of low-voltage side k = 2000/5 A, l = 0.5 m, S = 51.2cm 2 , R = 0.13 Ω; CT ratio of high-voltage side k = 250/5 A, l = 0.7 m, S = 23.2cm 2 , R = 0.06 Ω.The impact current is imported into the primary side of CT, secondary current of CT is shown in Figure 10.
From Figure 10, due to the fact that the flux of CT core does not mutation, the waveform of secondary current does not show obvious distortion in the moment of closing.Then the waveform of secondary current is distorted because of CT core saturation, which is under the action of non-periodic component.The transmission characteristics of traction transformer between steadystate cycle component and non-periodic component is different, so there is a large difference in the non-periodic component of both sides.The non-periodic component of current in high-voltage side is much smaller than that of current in low-voltage side.Therefore, the waveform of CT secondary current of high-voltage side does not contain any distortion.
Figure 11 shows waveform of the differential current flowing in the differential relay, which indicates that the differential current distortion within the first few cycles after closing is obvious, primarily because that the nonperiodic component of impact current is not attenuated sufficiently.After a period of time, the non-periodic rier transformation, and the result is Figure 12. Figure 12 shows that in the first cycle, the second harmonic content is reducing continuously, which is related to that CT secondary current of low-voltage side of traction transformer does not appear obvious distortion in component of the current is almost attenuation finished.The second harmonic of the differential current is anazed by using Fou ly shown in  the first cycle.Subsequently, the second harmonic content increases with the distortion of the waveform.The second harmonic content is less than 15% below its set value (15% to 20%) in 0.15s after closing, so protection cannot latch and differential protection of traction transformer is misoperation.
According to the analysis above, the impact current can cause traction transformer differential protection misoperation.For the misoperation caused by that second harmonic content is low, using dual scheme in different tial p e waveform recognition atresia criterion.

Conclusions
Mechanism of overvoltage and impact current in the electromagnetic transient process of passing by phase-separation with electric load is studied in the round in this article, and the equivalent model is built for each of the transient process, which can be used to analyze overvoltage and impact current.Suppressing overvoltage by changing the parameters of the circuit and using isoperation caused by impact current EFERENCES igure 12. Second harmonic amplitude percentage in difrent.
rotection can solve the problem by increasing th phase-controlled rectifier technology is proposed; differential protection m is solved by increasing the waveform recognition atresia criterion.So the article provides a reference for subsequent studies, and the resonance overvoltage in this transient process is more complicated, which needs further analysis in later period.

Figure 4 .Figure 5 .
Figure 4. Schematic diagram of equivalent circuit when QF 2 is turned on.
is made up of steady-state flux and flux generated b component when QF 2 is turned on.The ltage of where, Φ s represents steady-state flux, Φ d represents steady-state flux generated by DC component and Φ dt represents damped flux generated by DC component.

4 .
Ω, L 1 = 4.76 mH, R 2 = 0 R N = 0.151 Ω and L N = 1.47 mH; C N = 0.01155 μF, C N1 = (θ = 0°); Figure 6(b) shows the voltage waveform when the am-operating conditions when electromagnetic transient process och the ted current in closing side is the superposition of the inrush current and low-voltage side current.
CT secondary current in low voltage side of traction transformer C T s econ dary cu rrent/CT secondary current in high voltage side of traction transformer
QF 2 is turned on when the phase angle of Ua is 270°