High Efficiency DC Drive System For Ultra-capacitor Electric Bus

As the energy storage equipment, ultra-capacitor with the outstanding performance of high power density is suitable for city bus with the characteristics of frequent start-stop and fixed line. It is the key problem of promoting ultra-capacitor electric bus (UCEB) development that how to improve the utilization rate of the limited on-vehicle energy by driving the vehicle and recovering the braking energy efficiently. Hereby, the high efficient DC drive system of UCEB is studied in this dissertation by decreasing the loss of ultra-capacitor, converter, enhanced magnetism DC motor for the UCEB drive system and recycling the braking energy efficiently.The characteristics of the drive system converter play a decisive role in the vehicle performance. The on-vehicle energy of UCEB is very limited, so, a current two-quadrant converter is chosen by analyzing and comparing the efficiency, reliability and cost of basic bi-directional DC converters; and then the drive system scheme with the functions of buck chopping driving, boost chopping braking energy regeneration and on-vehicle charging is proposed. Based on the method of feedback energy by coupled inductors, a snubber circuit that transfers the turning-on snubber energy to the ultra-capacitor is designed to achieve ZCS, ZVS of switches and reduce the switching loss of high-power converter. Considering the demands of dynamic and comfort performance, a control circuit suppressing current overshoot is designed to regulate the duty cycle by following the given reference and limiting the amplitude real-timely. The aforementioned converter is also used as the ultra-capacitor on-vehicle charging power by changing the circuit connection, which is helpful to simplify the charging station devices and save the cost of the UCEB system.In order to optimize the performance of braking energy regeneration, the influence laws that how excitation current affects the efficiency and power of recycling braking energy for the enhanced magnetism DC generator are analyzed. Then, the optimal braking control strategy combining the electric braking with mechanical braking is put forward under the conditions of stable motor speed and armature current. According to the condition of getting the maximum generation efficiency, the efficiency optimal control strategy of recycling braking energy or the enhanced magnetism DC generator is designed when electric braking works independently. Then, in order to obtain the maximum power of recycling braking energy under vehicle composite braking, the energy optimal control strategy is designed to recycle much more braking energy during once braking process. Homoplastically, the optimal driving control strategy is designed to improve the performance of energy transformation for the enhanced magnetism DC motor. The experimental results verify the above-mentioned typical characteristics of the drive system converter and enhanced magnetism DC motor, which provides necessary conditions for the further study about the vehicle performance and its improvement scheme.The platform experimental results indicate that the efficiency of drive system converter is low under light load condition and the high frequency chopping pulse current causes the unfavorable influence to the ultra-capacitor service life and internal resistance power loss. Specifically, in order to improve the conversion efficiency of limited on-vehicle energy for the UCEB drive system, the multiple DC conversion scheme is proposed. According to the different output power and current of the drive system converter, the regulation scheme of working unit number for multiple converter is employed to increase the converter efficiency under low power transmission, which widens the power range of high efficiency for the drive system converter. Then, the variation amplitude of the high frequency pulse current following through the ultra-capacitor is decreased when multiple circuit units work together under heavy load road condition, which reduces the power loss and improves the work environment for the ultra-capacitor. Furthermore, the decreasing of current ripple amplitude and the increasing of current ripple frequency improve the dynamic performance of the enhanced magnetism DC motor for the UCEB.The simulation results show that it is difficult to realize the soft-switching for the conventional multiple converter. Hereby, the parallel interleaved converter under phase staggers PWM control is proposed. The switches of interleaved unit internal realize ZCS and the reverse recovery loss of freewheeling diode is reduced by coupled filter inductors. The parallel connection of interleaved units decreases the current capacity demand for the high power converter. The phase staggers PWM control increases the equivalent switch frequency and retains advantages of multiple converter for the parallel interleaved converter. The current sharing characteristic of interleaved unit internal is presented with combination of theory analysis, simulation and experimental study. Based on the datasheet estimation combined with polynomial fitting method, quantitative analysis about power loss indicates that the main power device loss of parallel interleaved converter is lower than that of the current two-quadrant converter under the same power grade, which verifies it is feasible that parallel interleaved converter is applied to UCEB from the power loss perspective. It provides technical reserve for improving the performance of the UCEB drive system.