Fixed Switching Frequency Sliding-Mode Predictive Current Control of a PMSM Variable-Frequency Drive System with Long Cables

To realize efficient and reliable production and transportation for underground coal mine mining applications, the inverters are generally placed above the mine, and the permanent magnet synchronous motors (PMSMs) are deployed underground and connected by long cables. Long cables will aggravate the over-voltage and the insulation aging of PMSMs. Thus, an output LC filter is necessary. Nevertheless, introducing LC filters increases the system order, leading to complicated control structures, time-consuming parameter-tuning efforts, and unsatisfactory steady-state performance. Therefore, this paper proposes a fixed-switching frequency sliding mode predictive current control (FSF-SPCC) scheme for PMSM long-cable variable frequency drive systems to improve the steady-state performance and robustness of the system. First, a linear sliding-mode switching function for stator-current tracking is designed, and a sliding-mode prediction model is built. Then, based on the ‘equivalent control’, a cost function based on sliding-mode surface tracking is designed. Since the sliding-mode surface inherently realizes the multi-variable control, the resonance is effectively suppressed, and the system robustness is significantly enhanced. Also, the delay compensation of the controller is considered to eliminate the performance degradation caused by the one-step computational delay of the actual digital controller. In addition, a three-vector FSF-SPCC controller is further proposed to improve the system’s steady-state performance and facilitate the LC-filter design. By simulating the principle of space vector modulation, two active vectors and one zero vector are employed simultaneously to synthesize the dead-beat solution based sliding mode voltage reference. Therein, the optimal voltage-vector combinations and corresponding duty cycles are determined by minimizing the weighted tracking errors of the reference voltage vector, which improves steady-state performance with a fixed switching frequency. Experiments are carried out based on an actual low-power PMSM long-cable drive system. Experimental results show that compared with the conventional PCC method, the proposed FSF-SPCC scheme can achieve lower stator-current ripples and total harmonic distortion (nearly five-fold decrease) at the rotor speed of 1 000 r/min in the steady state. Since the proposed FSF-SPCC inherently controls two state variables, the resonance is suppressed, and the steady-state performance is enhanced. Moreover, the harmonic spectrum of the proposed method is concentrated at the switching frequency and its integer multiples, resulting in a constant switching frequency. Besides, speed regulation (from 500 r/min to 1 000 r/min) shows that the proposed method can obtain a similar dynamic response as the conventional PCC method. Changing ±50 % stator-inductance parameters shows that the proposed FSF-SPCC scheme is more robust to model mismatches than the conventional PCC method. The following conclusions can be drawn from the experimental analysis: (1) The proposed method designs a cost function based on sliding-mode surface tracking, which effectively simplifies the control structure, suppresses the resonance, and enhances the robustness. (2) A three-vector FSF-SPCC controller is further proposed, which employs three voltage vectors to synthesize the sliding-mode voltage reference. Hence, a fixed switching frequency is achieved, and the proposed method has a more compact output-filter design and better steady-state performance than the conventional PCC method. © 2023 Chinese Machine Press. All rights reserved.