Research On The Control Strategies Of The Circulating Currents Elimination And Frequency Stabilization For The Parallel-connected Inverters Based On IPT Systems

The single inverter feeding based inductive power transfer(IPT)system is normally designed to meet the demand of a fixed output power rating.The capability of the IPT system is sometimes decided by the capacity of the inverter.It should be noted that it is not easy to achieve high power transfer with a single inverter in the IPT system,due to the voltage and current restriction of power switches.In order to meet the requirements of high-power applications,quite a few methods have been proposed to increase the capacity of IPT system in the past decade.One of the effective methods is applied in the IPT system is that multiply inverters are connected in parallel.Due to a slight unbalance of the control signals,the tolerance of the capacitors and inductors,the inner resistances of the inverters and other uncertainties,circulating currents exist among the parallel units which reduce the service life of the components and the reliability of IPT systems.Circulating currents can not only damage the power electronic devices and also lower the efficiency of the system by introducing some unnecessary current.And it can even reduce the reliability of the system by imposing too much current stress on the semiconductor devices.Also in order to guarantee that the load can get the power effectively,the secondary circuit of the system should be resonant.The main work in this paper is shown below.Firstly,a signal transform method is proposed in the paper.The AC signal can be decomposed into the DC signal by the proposed method in theory.Then,the analog circuit is used to represent the mathematical derivation process of each part.The equal amplitude phase shifting circuit,the low pass filter circuit,the amplitude detection circuit and the whole process of decomposing circuit are introduced in detail.Finally,the high frequency AC signal is decomposed into the DC signal and the controller can get the signal through the AD sampling.With the DC signal,the circulating elimination controller can get the amplitude and the phase of the branch current.The proposed transform method which is not easily disturbed will be verified in the next chapters 3 and 4.Secondly,in order to improve the power capacity of the IPT system,one of the available approaches is to employ parallel high-frequency inverters technology.A phasor control method based on the proposed method in chapter 2 is employed in this paper to eliminate the circulating current.The topology of parallel dual-inverter LCL-S based IPT systems and the cause of circulating current are analyzed in detail.Then,the imaginary/real part based on the current of primary coil are calculated,and then circulating current eliminating approach is provided with the knowledge of the relationship between the goal of phasor control and the imaginary/real part setting.The goal of the controller is that the real part is equal to each other,the imaginary part is zero and the primary coil current is constant.The performance of the employed approach was evaluated by the experimentation.The experimental results allow us to draw a conclusion that the circulating current between parallel connected inverters was dramatically reduced by the proposed algorithm.Thirdly,multiple inverters connected in parallel is a promising method to upgrade the power capacity of IPT systems.Due to a slight unbalance of the control signals,the inner resistances of the inverters and other uncertainties,circulating currents exist among the parallel units which reduce the reliability of IPT systems.The series-parallel resonant tank is employed in the multiple inverters based IPT system to eliminate the DC and harmonic circulating currents.The fundamental circulating currents in the paralleled inverter units are analyzed in detail.Then,for eliminating the fundamental circulating currents,a current transform method according to the chapter 2 and a control diagram are proposed to avoid acquiring the phase of the current by detecting zero cross current point which increases the accuracy of the control algorithm.A parallel-connected inverter IPT system is provided to verify the proposed approach.The experimental results show that the proposed method is effective for eliminating the fundamental circulating currents.Also the DC and harmonic circulating currents can be eliminated by the series-parallel resonant tank.Finally,it is necessary to reduce the influence of the leakage inductance in view of the loosely coupled transformer in IPT system.However,both of the leakage inductance of the coil and the circuit impedance alter against the change of working conditions and external environment.As a result,it will lead to invalid of the impedance measurement as heavy computational burden is needed because of high operation frequency.As the IPT system deviates away from the resonant condition with the change of system's parameters,the transferring power and system efficiency will degrade.A novel approaches as per maximum current tracking on secondary side and minimum current tracking on primary side are proposed to accomplish the dynamic resonant compensation.The experimental result allows us to draw a conclusion that not only the maximum transfer power of the parallel high power inverter analyzed in the chapters 3 and 4 but also the efficiency of the IPT can be improved dynamically with the proposed dynamic compensation approach.