The inductive power transmission system is applied to urban rail transit. Due to the limitations of the volume and coupling coefficient of the inductive coupling mechanism and the fact that the fluctuation of air gap in its movement will cause the fluctuation of mutual inductance value, DCDC booster link should be added to the side, rectifying side, to improve the output voltage level and stability. At present, most of the existing control strategies are based on the original side information communication. However, in the application of dynamic wireless charging in urban rail transit, the primary and secondary side coils are in the process of relative movement, so it is relatively difficult to establish reliable real-time communication, and it is easy to be interfered by electromagnetic transmission process, resulting in large errors. This paper analyzes the relationship between load and efficiency of IPT system applied to urban rail transit in detail and obtains the optimal load matching strategy of optimal efficiency. At the same time, an independent control strategy is proposed to eliminate the information communication of the primary and secondary sides and realize decoupling control. Finally, a simulation model is built to verify the effectiveness of the control strategy.
With the continuous development of urban transportation, environment-friendly urban rail transit vehicles have become an important part of the strategic framework of urban comprehensive transportation development. However, there are many disadvantages in the power supply mode of overhead contact network, such as friction electric spark, poor landscape, and so on. The inductive power transmission (IPT) technology is to transfer the power from the power supply to the power load in a noncontact way, which has the advantages of safety, reliability, and flexible power supply. Therefore, the high-power induction power transmission technology and its application in urban rail transit have become the focus of research at home and abroad in recent years [
The IPT system is applied to urban rail transit. In order to keep the vehicle running stably, its output voltage and power level are strictly limited to a predetermined range. However, the device volume and coupling coefficient of inductively coupled mechanism based on loosely coupled transformer have certain limitations, and its direct output voltage often cannot reach the present range. The variation of mutual inductance due to the fluctuation of air gap and the equivalent load due to the change of power demand will lead to the fluctuation of output voltage and affect the stability of the system. Therefore, it is necessary to add DCDC booster voltage link after diode rectification circuit of traditional IPT system to reduce output requirements of coupling mechanism [
Aimed at the dynamic IPT system applied to urban rail transit, this paper proposes a decoupling control strategy of primary and secondary sides based on the optimal efficiency. Firstly, the air gap fluctuation between the system structure and the actual running of the vehicle is analysed. Finite element method is used to simulate the variation law of mutual inductance parameters in the case of air gap fluctuation, and the resonant compensation topology with optimal output capability is selected based on mutual inductance range. In the process of dynamic operation of vehicles, mutual inductance changes caused by air gap fluctuations are inevitable. On this basis, an efficient decoupling control method of the primary and secondary sides is proposed, so that the system can maintain a high efficiency output within the range of mutual inductance parameters. Finally, simulation verifies the effectiveness of the control strategy.
100% low-floor trams in this paper are the research object; the system structure is shown in Figure
Structure of IPT system applied to urban rail transit.
According to the dynamic transmission characteristics of the system, the coil configuration mode of the primary side long and the secondary side short as shown in Figure
Configuration of primary and secondary coils.
The coil configuration with long primary coil can keep the magnetic field in the middle of the coil stable, and the magnetic field fluctuation at the coil boundary decreases with the increase of distance. Therefore, the parameter fluctuation caused by the horizontal motion of the coil is ignored, and only the mutual inductance fluctuation caused by the change of air gap in the vertical direction of the coil is considered. The air gap fluctuation of the vehicle during movement is shown in Table
Air gap fluctuation during movement.
AW0 | AW2 | AW3 | |
---|---|---|---|
Dynamic variation | 55 | 62,6 | 64,4 |
Range of air gap | 72-127 | 44,4-107 | 37,6-102 |
The variation range of air gap in the table is 37.6mm-127mm. Finite element simulation analysis was carried out on the coupling mechanism under different air gaps in ANSYS Maxwell, and the mutual inductance change curve was obtained as shown in Figure
Mutual inductance variation.
In the short action time of urban rail transit dynamic wireless charging, full reliability on the primary side/secondary side of the stability control strategy to achieve high power transmission still faces certain difficulty, so it is necessary in the process of system design to choose the appropriate harmonic compensation topology and keep the system under the range of mutual inductance high power output.
At present, the research on resonant compensation network mainly focuses on the following compensation structures: element compensation, series parallel compensation, and series parallel series compensation of three elements. Basic element compensation topology (S, P) has been studied a lot [
Four resonant compensation topologies.
SS
PS
LCLS
LCCS
In the figures,
The circuits in Figures
Based on this, four resonant compensation topological parameter relations can be analyzed as shown in Table
Four topological parameter relationships.
Parameter | SS | PS | LCLS | LCCS |
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In the range of mutual inductance that can be achieved by the system in the previous section, the variation trend of output power and efficiency of the four topologies with mutual inductance under the same parameter is obtained, as shown in Figures
Output power and efficiency vary with mutual inductance.
SS
PS
LCL
LCC
As can be seen from the image, the output power of the four topologies increases with the mutual inductance. However, compared with the other three compensation topologies, the primary S-type resonant compensation topology has a very strong power output capacity and is suitable for high-power applications. As can be seen from Figures
Therefore, it can be concluded that, under the same system parameters, the output capacity of SS resonant topology is obviously better than the other three, so SS resonant network is selected as the system compensation topology. The following will focus on the efficiency improvement strategy of the system.
According to the analysis in the previous section, the system has a high output capacity under SS resonant topology, which is suitable for the application of high-power urban rail transit. However, as can be seen from Figure
From the formula in Table
It can be seen that the system efficiency is related to the factors in the formula. When the system hardware is determined, the impedances
Efficiency varies with mutual inductance and load.
Taking the derivative of the efficiency with respect to the load
According to the topology shown in Figure
Combining (
So duty cycle D is
Therefore, the system can track the load with the optimal efficiency through (
Therefore, if the duty cycle of boost DCDC remains in (
In the application of dynamic wireless charging in urban rail transit, the primary and secondary side coils are in the process of relative movement, so it is relatively difficult to establish reliable real-time communication, and it is easy to be interfered by electromagnetic transmission process, resulting in large errors. And during dynamic power supply, in order to ensure the supply of load and battery energy storage, the inductive power transmission system should always work with the maximum rated transmission power that can be obtained [
Decoupling control block diagram.
The primary side adjusts the output voltage pulse width of the inverter through PI closed-loop to realize constant current control of the current of the primary side coil. The transmitting coil current
In order to verify the original side decoupling control strategy based on optimal efficiency load tracking proposed in this paper, the MATLAB/Simulink simulation tool was used to build the system model. The system parameters were based on the high-power inductive power transmission system applied to urban rail transit. The simulation parameters were set as shown in Table
System simulation parameters.
Parameters | Value |
---|---|
DC voltage | 750 |
Maximum power | 100 |
Frequency f/kHz | 50 |
Primary coil self-induction | 160,8 |
Secondary coil self-induction | 91 |
Primary coil self-resistance | 13,10 |
Secondary coil self-resistance | 3,12 |
Primary compensation capacitance | 63,07 |
Secondary compensation capacitance | 111,45 |
Load resistance | 5 |
Mutual inductance | 4,5-9,0 |
Simulation waveform.
Voltage and current of the primary and secondary sides
Output voltage and current
Under this control strategy, the equivalent load of the system keeps track of mutual inductance changes, so that the system always maintains high-efficiency output, and the primary and secondary sides are decoupled. The theoretical calculation value and simulation value of system output power and transmission efficiency changing with mutual inductance are compared as shown in Figures
The output power of theoretical calculation and simulation value.
The transmission efficiency of theoretical calculation and simulation value.
As can be seen from the image, the simulation results are roughly consistent with the theoretical calculation values. The output power varies linearly with the mutual inductance value. The higher mutual inductance value corresponds to the higher output power. In addition, the transfer efficiency of the coupling mechanism can be maintained at more than 90% within the range of mutual inductance that can be achieved by the coupling mechanism, which verifies the effectiveness of the control strategy.
The inductive power transmission system is applied to the dynamic situation of urban rail transit, which puts forward strict requirements on its energy output capacity and stability. Moreover, it is relatively difficult to establish reliable real-time communication due to the short time interaction. Based on this, this paper proposes the decoupling control strategy of the primary and secondary sides based on the optimal efficiency load tracking for the high-power dynamic inductive power system, so that the primary and secondary side communication can be cancelled in the inductive power transmission system and independent decoupling control can be realized. Simulation results show that the control method proposed in this paper can realize decoupling control of the system, the control of output power of the primary side through a closed loop, and the load tracking of the optimal efficiency of the secondary side. The results show that the control method is feasible and effective and has high engineering application value.
The Control Ideas of reference ([
The authors declare that they have no conflicts of interest.
The project presented in this article is supported by the National Key Research and Development Program (2017YFB1201003-014).