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中文摘要：
既有的最近电平逼近调制算法（简称NLM传统算法）用于级联模块数相对较少（约为10个左右）的铁路牵引供电地面过电分相系统级联H桥多电平变流器（简称变流器）时,各子模块尤其是最后一级子模块会导致变流器输出电压波的等效面积误差较大（最大约为6%）,而且其电压分辨率也会引起变流器输出电压波的等效面积误差,这些误差导致变流器调制精度降低、控制性能变差。为了解决这一问题,提出NLM改进算法,即基于输出电压波面积等效的基本思想,通过对变流器输出电压幅值附近正弦调制波的面积计算,在确定最后一级子模块投入工作的时长后再确定各子模块开始投入工作的时刻,从而实现变流器输出电压的高精度跟踪调制,并且对给定的输出功率跟踪更准确。利用Matlab/Simulink软件对NLM改进算法进行仿真验证,结果表明：NLM改进算法能够提高变流器输出电压的调制精度,其误差仅约为传统算法的20.93%,而输出电压的谐波含量仅比传统算法的增加约3.2%,使得地面过电分相系统的性能得到显著提高。
英文摘要：
When the existing nearest level modulation（NLM for short）algorithm was adopted in the cascaded H-bridge multilevel converter（CHMC for short）of the ground＇s auto-passing neutral section in railway traction power supply,the equivalent area error of converter output voltage caused by each submodule,especially the last level submodule was larger（the maximum about 6%）.It was because the CHMC consisted of a relatively small number of submodules,ten or so.Moreover the resolution of NLM could also cause equivalent area error.These errors would lead to lower modulation precision and worse control performance.To solve these problems,an improved NLM algorithm was proposed based on the thoughts of equivalent output voltage wave area.High precision modulation of the CHMC output voltage could be obtained by calculating the sinusoidal modulation wave area near converter output voltage amplitude to determine the function time of the last level submodule and the time for each submodule to function.Thus,high precision tracking and modulation for converter output voltage could be achieved,and the tracking for given output power would be more precise.The improved NLM algorithm was verified by Matlab/Simulink software,and results show that the improved NLM algorithm can improve the modulation precision of converter output voltage and its error is only 20.93% that of the existing NLM algorithm and the harmonic of the converter output voltage only increases by 3.2%.The performance of ground＇s passing neutral section system has been significantly improved.
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