Research On Control Techniques Of Charge-Swap In Electric Vehicles Considering The Influence Of Battery And Grid

Electric vehicles (EVs) use electrical energy instead of petroleum, featuring with the advantages of low emission and noise. It has attracted more and more attentions around the world and become one of the effective means to alleviate the energy crisis and environmental pollution. During the EVs industrialization, the interaction between the EVs, battery energy storage system (BESS) and distribution network are the major problems. For this important issue, researchers use classical linear control theory to analyze the power electronics interfaces. Unfortunately, the general intrinsic mechanism of their interaction is still unclear and the systematic analysis methodology by synthesizing the detailed characteristic of the EVs, power electronics interfaces and distribution network are not effectively discovered. In order to solve the above-mentioned problems, the unbalance voltage of distribution networks, the characteristics of EVs battery and the impact of nonlinear and digital control of power electronics interfaces are analyzed. Based on the computer techniques and modern control theory, the composite control techniques of power electronics interfaces in EVs charge/swap system are researched deeply from both aspects of parameter optimization and controller design. The main contents are expressed as follows.In EVs fast charging system, high power rectifier is applied to provide DC bus for EVs chargers. However, slightly unbalanced voltage of distribution network can cause large unbalanced current and non-characteristic harmonics in three-phase uncontrolled rectifier with large capacitor. This dissertation proposes an accurate model to analyze the performance of uncontrolled rectifier in slightly unbalanced voltage. The interaction between unbalanced voltage and rectifier are expressed and the curves of current unbalance,3rd harmonic current, output average voltage, output voltage droop and output voltage harmonic as function of loads under the condition of2%voltage unbalance are illustrated. Moreover, the design procedure of capacitor is given considering the influence of unbalanced voltage. The current unbalance decreased to6.15%from12.64%and3rd harmonic current decreased to0.59A from1.83A.The battery voltage and current are nonlinear and vary with the charging profiles, making the optimal design of battery charger more difficult and complicated. This dissertation presents an accurate steady-state model and loss model of LLC resonant converter. And a new time-weighted average efficiency (TWAE) index is proposed, which represents the average weight of conversion efficiency during battery charging period. Then an optimal parameters design method is proposed. The TWAE is achieved serving as the objective function and a variable step exhaustive search algorithm is applied considering the constraints of operation and variable range to reduce the search space and speed up the search algorithm. A3kW LLC resonant charging converter from front to end is designed with maximum efficiency of95.4%and TWAE of95.07%.Due to the influence of nonlinear characteristics of the LLC resonant converter, it’s difficult to analyze the stability using the classical linear control theory. Using the describing function method, this dissertation proposed a simple equel model of LLC resonant converter. Based on this model, the small-signal stability of LLC resonant converter is researched by using Lyapunov linearization method. Moreover, this dissertation creates Lyapunov energy function by using Brayton-Moser’s mixed potential function potential theory. Then the large-signal stability is analyzed using Lyapunov direct method. The small-signal and large-signal stability criterion are achieved. In order to make the LLC resonant converter work stable during changing operation mode, a nonlinear control strategy with resonant current polarity detection is applied in LLC resonant converter. Simulation results verify the effectiveness of the control strategy.In the EVs battery energy storage system, digital control strategy could deteriorate the performances of output voltage and even make the system unstable in some serious condition, as the battery energy storage inverter has high power and low switching frequency. This dissertation presents the digital model of BESS. And the predict control strategy is used to eliminate the influence of time consumption. The formation mechanism of prediction control error is given. The influence of prediction error on digital control strategy for inverter is analyzed. The functions between model error, input change and prediction error are calculated. Then a full-state feedback control strategy based on integral of output error is given. The outer loop uses the integral of output voltage error to suppress the prediction error and the digital controller selects the integral of backward difference to reduce the degrees of freedom. Experiment results verify the effectiveness of the proposed strategy, the static error is reduced to1%from9%in the experiments.