Double-line Frequency Ripple Power Suppression Technique Based On Bidirectional AC/DC Converters

As the essential connection between the grid and the EV power battery in a Vehicle-to-Grid(V2G)scenario,the high power density and operation life of the converter is severely limited by the double-line frequency ripple power(DFRP)in a two-stage structure of on-board charger(OBC).Active power decoupling(APD)technology allows active decoupling control by power electronic switching devices to suppress or eliminate the DFRP.With active power decoupling,it is possible not only to significantly reduce the capacitance on the dc bus,but also to replace electrolytic capacitors with film capacitors,therefore extending the lifetime of the entire system.These APD techniques have significant advantages in terms of size,cost and reliability,which are increasingly being investigated in the field of AC/DC converters.In this paper,the need for miniaturization and longevity in OBC applications is taken as a guideline,and the technique of the DFRP suppression in grid-connected AC/DC converters is investigated in depth.Starting from the topology and control methods,the theories and methods related to the APD topology construction and APD control of single/three-phase AC/DC converters are studied,covering single/three-phase decoupling topology,intrinsic topology correlation,multiplexing modulation strategies,mathematical modelling and decoupling control algorithms.The aim of this paper is to enrich and develop the basic theories and methods for APD topology construction of AC/DC converters and their optimal control for the different needs of single-phase and three-phase APD topology,with the main contents as follows:First,a unified analysis method for the DFRP in the single/three-phase AC/DC converters is established.Based on a thorough investigation of existing single/three-phase APD topologies,the construction ideas and evolution rules of single/three-phase APD topologies are revealed.Then,the intrinsic links between different decoupling topologies are explored,and the DFRP generation mechanism of single-phase and three-phase topologies is inducted through theoretical derivation.Unlike single-phase APD systems,APD in three-phase is only necessary during grid unbalance faults,so the focus of the APD function differs between these two and the way they are implemented.In this paper,the different APD requirements of single-phase and three-phase systems and their current research status are described separately to provide a theoretical basis for subsequent research.Second,a power decoupling method based on single-phase Buck-type Totem-pole PFC topology is proposed.To address the problem of reduced power density of the decoupling topology brought by the introduction of additional switching devices,a multifunctional modulation strategy is proposed to achieve power decoupling by reusing the switches of the original topology.Then,a self-adaptive power decoupling controller is designed,without adding additional decoupling sample circuits.Considering the zero-cross distortion and positive-negative half-period asymmetry of the decoupling voltage under undetected controller,a duty cycle feed-forward compensation strategy is proposed to obtain good decoupling performance.Based on single-phase solution,the APD strategy is extended to an interleaved Totem-pole PFC topology,which proves the generality of the proposed APD strategy.The above methods are both simulated and experimentally verified to demonstrate the steady-state performance of the single-phase APD strategy in both rectifier and inverter modes.Third,a dc-bus voltage surge suppression strategy based on a single-phase APD topology is proposed under the rectifier-inverter bidirectional switching scenario.This strategy aims at solving the problem of system transient performance degradation due to the reduction of dc-bus capacitance in the APD topology.By using the passive characteristic of the added decouping branch as a transient buffer,it can achieve the maximum energy utilisation of the decouping branch.In order to verify the robustness of the APD control during rectifier-inverter mode switching,a state-space model of the applied disturbance signal is established.The quadratic stability problem of the system during bidirectional switching is transformed into a solvable linear matrix inequality(LMI).The above methods are validated by simulation and experiment to demonstrate the transient performance of the single-phase APD topology under bidirectional switching,grid drops and load perturbations conditions.Finally,an active power decoupling strategy based on a three-phase three-wire three-level T-type topology is proposed,in order to provide a technical means for the DFRP suppression under three-phase unbalanced conditions.By introducing a compensation current coefficient into the controller,the dc-side DFRP suppression is achieved and the relationship between the ripple suppression system and the grid unbalance is quantitatively analysed.As the DFRP only exists under three-phase fault conditions,the decoupling strategy can be automatically removed after the fault is over,without additional hardware costs.The proposed decoupling strategy also eliminates the need to detect and control the positive and negative sequence components under three-phase unbalance conditions,enabling uniform regulation under both normal and fault conditions.The proposed decoupling strategy is demonstrated by simulations and experiments,which illustrate its effectiveness on the T-type three-level topology.In this paper,the front-stage AC/DC converter of the OBC is used as the research object to investigate the problem of the DFRP.For the inherent DFRP existing in single-phase topology,the optimal decoupling strategy is to reuse the original topology and add a decoupling branch for power ripple absorption as far as possible;for the DFRP in three-phase topology which only exists under unbalanced conditions,the optimal solution is to make the power ripple be absorbed inside the converter by injecting harmonic currents,so that the DFRP is not directly reflected on the dc-bus voltage.The single/three-phase decoupling strategies proposed in this paper are completely independent of the original control loop.They are applicable in both rectifier and inverter mode,fully responding to the bi-directional needs of OBC applications in V2 G scenarios.The proposed single/three-phase APD strategies are also simulated and experimentally verified in the PLECS simulation platform and experimental platforms.