异步电机低开关频率的模型预测优化控制研究

Medium voltage ac machines fed by high-power inverters operate at low switching frequency to restrain the switching losses of the power semiconductor devices and increase the inverters output power. An established and desired method to reduce the switching frequency without increasing current harmonics is employing the optimal pulse width modulation (PWM) patterns, but it cannot be used to closed-loop control directly since an over-current trip caused by PWM disorders during transient will be encountered. A novel suitable closed-loop control methodology that combines the optimal PWM patterns, is necessary to be investigated for high performance control at low switching frequency. Model predictive control (MPC) uses a model of the system to predict the future behavior of the variables over a time frame, and has significant advantage to include the nonlinearities and constraints easily. In MPC, the controlling quantity in a long prediction horizon is computed and repeated over a shifted horizon at the next sampling instant, thus formulating the receding horizon policy, which provides feedback and robustness to the model inaccuracies of ac machines and control delay. With its excellent merits, MPC presents a new perspectives and path for the high performance closed-loop control of ac machines operated at low switching frequency. Based on a neutral-point-clamped (NPC) three-level inverter driving induction motors, starting from several core aspects including stator flux trajectory predictive model, receding horizon optimization of pulse patterns, neutral point predictive control under optimal pulse patterns and further study regarding switching losses as optimal control target, this project are expanded through theoretical analysis, mathematical modeling, simulation assessment and experimental evaluation with the receding horizon policy within a prediction horizon of finite length and the extrapolation strategy for the trajectory prediction of system states. The goal of this project is to develop suitable high performance closed-loop control methodologies based on model predictive flux trajectory tracking control, establish an available low switching frequency control methodology, thus to provide reliable theory foundation and technical support to construct novel optimal high performance control methodologies applied to low switching frequency control for the large class of three-phase ac drives.

中高压大功率交流电机传动系统为降低开关器件损耗，减少输出波形谐波畸变，通常需要采用低开关频率的优化脉冲模式，但其不能直接用于高性能闭环控制，因为在动态会造成PWM紊乱、系统过流。模型预测控制通过对控制目标的反复优化，能够不断顾及电机模型的失配和时变、控制延迟以及各种干扰引起的不确定性并及时加以校正，为交流电机优化脉冲模式的高性能闭环控制问题提供了新的思路和途径。因此，本项目从定子磁链轨迹预测控制模型、脉冲模式滚动优化、优化脉冲模式的中点预测控制、以开关损耗为目标的优化控制等几个核心问题入手，选择电压型三电平逆变器驱动的异步电机为实验对象，结合有限预测时域的模型预测滚动优化和预测状态轨迹的外推策略，通过理论分析、数学建模、仿真研究和实验验证，研究建立异步电机低开关频率的模型预测优化控制方法，为形成适用于这类电机驱动系统的行之有效的新型高性能控制方案，提供可靠的理论依据和技术支撑。

在大容量的变频器交流电机传动应用中，鉴于系统工作的高电压和大电流，变频器装置功率开关器件的开关损耗和由此产生的热量不容忽视，降低变频器主电路的开关频率非常必要。这可使其有效降低开关损耗，提升装置效率，增大输出负载电流，并进而增加变频器出力。首先，研究了适用于低开关频率运行的三电平逆变器同步优化PWM方法，以电流低次谐波幅值方程为非线性约束条件，通过设置误差精度，增加了对低次谐波幅值的限制。对求解所需初值通过采用遗传算法求取，优化求解后的开关角具有限制特定谐波幅值和保持较小电流谐波畸变率的特点。其次，提出了采用优化脉冲模式的电机磁链轨迹跟踪建模和滚动优化脉冲模式形成方法，建立了磁链轨迹跟踪预测模型及相应控制问题的优化模型。针对系统性能要求和控制约束条件，研究和设计了模型预测脉冲模式滚动优化计算模型及其数值求解方法。在低开关频率下既能基于优化PWM获得较小谐波畸变，又基于磁链轨迹跟踪获得快速动态响应。再次，将基于定子磁链轨迹跟踪的闭环控制系统与新型中点电位平衡策略相结合，实现了对电容电压波动和偏移的有效抑制。由于所采用的方法从空间电压矢量角度分析，因此对于不同模式的优化PWM具有广泛适用性。进一步地，通过对输出矢量进行轨迹外推，形成长预测范围的输出轨迹，以逆变器的平均开关损耗为价值函数，对不同开关矢量作用的输出轨迹进行在线评估，提出了一种异步电机低开关损耗的模型预测直接转矩控制方法。基于该方法的驱动系统逆变器的开关损耗低，电机转矩和磁链动静态性能良好。最后，提出基于三电平优化矢量选择器的异步电机模型预测直接转矩控制和直接电流控制方案，根据转矩和定子磁链的给定值得到期望的优化电压矢量，只需对有限的四个电压矢量进行优化评估，显著降低了优化过程求解计算量。通过增加对开关切换次数的约束，可以形成具有开关频率限制的模型预测控制策略，在兼顾转矩和磁链偏差的同时降低逆变器开关频率。