大功率并网变流器离散域电流控制策略

doi:10.19595/j.cnki.1000-6753.tces.222309

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摘要

MW级大功率并网变流器的开关频率较低,延迟和离散化误差对控制性能影响较大,导致动态响应慢、电网背景谐波影响大等问题。因此,直接离散域控制系统设计成为必要。该文针对LCL滤波并网变流器建立离散域数学模型,并基于此提出桥臂侧电流和网侧电流融合的离散域电流控制器。同时,为了在不额外增加电流传感器和检测电路的情况下获取并网电流信息,设计电容电压微分器,实现网侧电流的估算,降低硬件复杂性和控制成本。研究表明,该文设计的离散域电流控制器,有着较好的动态性能和电网背景谐波抑制能力。最后,通过硬件在环半实物仿真系统对该文的分析和设计进行了实验验证。

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关键词 ： LCL型并网变流器, 低开关频率, 离散域控制, 背景谐波

Abstract：

For high power grid-connected converters, the switching frequency usually is low. Consequently, the negative effects enforced by the control delay and discretizing errors are heavy. However, traditional current control strategies generally ignored these negative effects and, hence, reducing the performance of the designed controller. Recently, some strategies were presented to compensate the control delay and the discretizing errors, but most of them suffered from the low dynamic response and the high sensitivity to the power grid harmonics. To address these issues, a current control strategy combining the bridge arm-side and the grid-side currents is designed directly in the discrete-time domain and pole-zero placement is used in synchronous coordinates. Meanwhile, a differentiator to the capacitor voltage is introduced to make estimation of the grid-side currents. Consequently, hardware complexity as well as the corresponding control cost is reduced.
Discrete-time model of the converter with LCL as output filter is built in synchronous coordinates, including the computational delay, firstly. Following that, the corresponding structure of the combining state-space control scheme is as follows: the harmonic control loop of the grid-side currents is constructed and combined with the fundamental current control loop on the bridge arm-side, which uses the full state-feedback control. And then, the integral controller is introduced for improved disturbance rejection and the reference-feedforward controller is designed to increase the reference-tracking dynamic performance. Consequently, the closed-loop transfer function of the system is obtained based on the discrete-time model and the control scheme, and then the state feedback function matrix and the feedforward module are optimized based on the pole-zero placement. Finally, a differentiator is introduced to make estimation of the grid-side currents, reducing the hardware cost.
Experiment results based on a hardware-in-loop simulator show: the response time of the bridge arm-side currents to a 1000A step-up change is about 4.5ms; the recovery time of the bridge arm-side currents facing a grid-voltage dip of 0.4 p.u.is about 10ms; and the total harmonic distortion of the grid-side currents is 2.28%, below the 5% limit given in standards, in the circumstances of 6% fifth and seventh harmonic components appearing in the grid voltage. Finally, to verify the robustness of the proposed strategy to the parameter deviations, the actual system parameters are deviated artificially from their nominal values. The experimental results show that the proposed strategy can operate satisfactorily in the range of ±40% parameter deviations and the influence of the parameter deviations is low.
The following conclusions can be drawn from the experiment results: (1) compared with the typical discrete-time state-space control strategy, the proposed strategy keeps the same great dynamic performance but achieves an improved capability of rejecting the grid harmonic-disturbance. (2) the proposed control strategy simplifies the design complexity based on the direct pole-zero placement and increases the degrees of freedom based on the combining design of fundamental current and harmonic control loops. (3) the differentiator to the capacitor voltage is designed to get knowledge of grid-side currents without additional current sensors and the corresponding sampling circuit required, reducing the hardware complexity and the cost.

Key words： Grid-connected converter with LCL as output filter low switching frequency discrete-time control grid harmonics

收稿日期: 2022-12-13

PACS:

TM464

基金资助:

安徽省科技重大专项资助项目（202003a05020029）

通讯作者: 杨淑英男,1980年生,教授,博士生导师,研究方向为风力发电系统、电驱动系统。E-mail：yangsyhfah@163.com

作者简介: 王旭男,1998年生,硕士研究生,研究方向为大功率风电变流器电流控制。E-mail：wx1998_wangxu@163.com

引用本文:

王旭, 杨淑英, 李典, 张兴. 大功率并网变流器离散域电流控制策略[J]. 电工技术学报, 0, (): 133-133. Wang Xu, Yang Shuying, Li Dian, Zhang Xing. Discrete-time domain Current Control Strategy for High Power Grid-connected Converters. Transactions of China Electrotechnical Society, 0, (): 133-133.

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