Study on LCL type IPT system power bi-directional push mode

. . Aiming at the working principle of inductive power transfer (IPT) technology, a bi-directional power conversion topology based on LCL type composite resonance is proposed. This topology introduces the advantages of LCL type resonance into the bi-directional power topology which has a resonance capacity larger than the single stage LC, it has a strong load adaptability, a smaller switching device voltage and current stress. Aiming at the LCL bi-directional power topology proposed ， its working principle is introduced in detail ， and the AC impedance model and mathematical model of the system are established. On this basis, the feasibility is verified by using the simulation experiment results.


Introduction
Inductive power transfer technology, as a brand-new research field, has received extensive attention in recent years and it is widely used in many fields like electric vehicle power supply, special environment of underwater mine. However, due to the one-way transmission characteristic of conventional IPT, a series of problems exist in the application of high-power types (materials handling equipment, urban electrified transportation etc.) such as power traction. These include the difficulty of electric power feedback resulted by the one-way transmission characteristic and a lower power transmission efficiency resulted by the open loop of electric power transmission.
In order to solve the problems above, a bi-directional high frequency electric power push mode is proposed in document [1], which is used to realize the bi-directional transmission of power electric transfer between the transmitting end and the receiving end. The characteristics of the mode lies in: The transmitting and receiving end of the system both use peer-to-peer topology, which works in magnetic field excitation and receiving mode to satisfy the bi-directional transmission of energy. Meanwhile, according to the working state and need, the direction and size of power electric push can be changed dynamically, and then the operation efficiency of the system is increased.
The bi-directional electric energy topology proposed in document [2] is a symmetrical SS type. The topology has a characteristic: the current in the resonance coil all passes through the high-frequency inverter circuit or high frequency rectifier circuit, so the switching device bears rather big current stress. Especially in high-power application, the heating problem of the switching device is very serious. To deal with this problem, bi-directional power conversion topology based on LCL type composite resonance is proposed. This topology introduces the advantages of LCL type resonance into the bi-directional power topology. It has characteristics as follows: a larger resonance capacity than single-level LC resonance; a strong load adaptability with lower resonance frequency drift; a lower voltage and current stress of switching device at the same power level.
Aiming at the LCL bi-directional power topology proposed，its working principle is introduced in detail， and the AC impedance model and mathematical model of the system are established. On this basis, the feasibility is verified by using the simulation experiment results.  Fig.1 is the LCL type bi-directional high frequency AC power topology. Taking the P end topology as an example, its high frequency conversion is formed by full-bridge circuit which consists of four switch tubes (P1~P4) and anti-parallel diode(PD1~PD4).The full-bridge circuit can operate in rectification and inverter modes. LrP, CP and LP form the LCL type composite resonance network, which is used to generate high frequency resonance in order that the S end receives magnetic field energy. Document [3] has proposed that without extra adding power conditioning link, dynamical adjustment of power transmission can be realized and the system can operate in energy injection mode and energy free oscillation mode. Taking the P end as the energy transmitting end and S end as the receiving end as an example to illustrate. They are shown in Fig.2. Working mode 1 is the energy injection mode, where (P1，P4)and(P2，P3) alternatively conduct and convert the DC input EVP into high frequency square wave voltage, which is injected into the resonant network made up of LrP, CP and LP. So, high frequency sine wave energy output is generated in transmit coil. By detecting the current in the resonant inductor Lr and controlling the alternative conduction of two sets of switch tubes (P1，P4) and(P2，P3), the switch of switch tubes can always conduct at the zero-crossing point of resonant current, where the loss of switch tubes is zero. Then, the operation of ZCS (zero current switching) is realized.
Working mode 2 is the energy free oscillation mode, where ( P2 ， PD4 ) and ( P4 ， PD2 ) form the bidirectional freewheel switch pair to maintain the bidirectional energy flow of resonant link. In this case, the systems completely rely on energy storage of resonant link to maintain autonomous oscillation. Similarly, these two pairs of switch tubes can detect the zero-crossing point of resonant current to realize the operation of ZCS.
By dynamically controlling two working modes of energy emission, dynamic adjustment of input power and output voltage can be realized. In these two modes, the S end is always in energy receiving state. (S1，S2， S3，S4) all turn off.(SD1，SD4) and(SD2， SD3 ) form the full-bridge rectifier circuit, which completes the bi-directional rectifier of high frequency sine wave and output to load RVS.

AC Impedance analysis
As the LCL type bi-directional power conversion topology is a symmetrical structure, so the circuit parameter of P end and Send must be the same, which means the parameters of resonant inductors ( When system is in complete resonance, the imaginary part of total impedance is t Z zero and the phase angle is zero. That is： ( ) Inherent resonance frequency (zero phase angles) of system can be solved by combining formulas (1) to (4).

LCL type bi-directional high frequency AC power conversion mathematical model
In order to analyze the topology of LCL type directional high frequency AC power conversion, mathematical models needs to be established for two working modes. Aiming at the working mode 1 shown in Fig.2 (a), its correspondent equivalent circuit model can be established, shown in Fig.4.
For working mode 2 shown in Fig. 2(b), the correspondent equivalent circuit model can be established, shown in   The power push mode is designed to push from P end to S end (forward push). The time of system simulation is set as 0~60ms. When the system enters a steady state, waveforms of P end and S end resonant capacitance voltage(30ms~30.1ms) are shown in Fig.6.

Conclusion
It can be seen from simulation results that, the proposed LCL type power bi-directional push topology successfully overcomes the weaknesses of the singlelevel LC type bi-directional power conversion topology device, where it bears a larger electronic stress. The bidirectional push of energy is also realized. For LCL type bi-directional circuit topology proposed, working methods are introduced in detail. Meanwhile, the AC impedance model and mathematical model are established. On this basis, the feasibility is verified by using simulation.