Research Of Control Strategy Of Neutral-Point-Clamping Three-Level(NPC-3L)Inverters Fed Dual-Three PMSM Drives

In this paper, the vector control strategies under normal and fault states of neutral-point-clamped three-level (NPC-3L) six-phase inverters fed dual-three permanent magnet synchronous motor (PMSM) were analyzed and studied deeply.Magnetic motive force (MMF) analysis reveals the superior performance of dual-three motor in eliminating air-gap harmonic MMF. The dynamic mathematical models and equivalent circuits of dual-three PMSM were modeled based on stationary coordinate frame, dual synchronous coordinate farme and vector space decomposition (VSD) frame. Three efficient pulse width modulation (PWM) strategies of NPC-3L six-phase inverters were designed in detail:carrier deposition based PWM (CDB-PWM), dual three phase space vector modulation strategy (DT-SVM) and the six-phase space vector modulation strategy based on VSD (VSD-SVM). On account the flexibility of dual-three PMSM Control, the first vector control strategy based dual synchronous model and DT-SVM achieves the full decoupling control of dual-three PMSM based on full decoupling feedforward compensation proportional-integral (PI) controller, in addition, torque average control optimizes the copper loss of dual-three PMSM. The second vector control strategy based VSD model and VSD-SVM also achieves decoupling control of d-q equivalent circuit of.dual-three PMSM in VSD model. The unnecessary currents in harmonic subspace were suppressed effectively in open-loop control. Compared with the first vector control system, the second vector control strategy greatly reduces the number of current loop controller and feedforward compensation term, simplifying the design of current loop. In addition, a series of non-ideal factors were considered which contribute current component in harmonic subplane. In terms of the frequency of current component in harmonic subplane, the non-ideal factors were divided into three categories:the structure asymmetry of dual-three PMSM, harmonic electromotive force and other unmodelled factors. The proportional multi-resonant (PRs) closed-loop current adjust strategy based on harmonic voltage injection was proposed for non-ideal harmonic current factors.Research on fault-tolerant operation is currently mostly used in two-level three-phase AC motor system, NPC-3L six-phase inverters fed dual-three PMSM fault-tolerant operation research has not been reported. The existing current regulating fault tolerant strategy makes a mistake in distinguish optimal conditions between minimum current stress and minimum loss, and lack of evaluation indexes. In addition, the existing fault-tolerant strategy based on model reconstruction lacked explicitly physical meaning in dividing torque subplane and harmonic subplane. This paper proposes two kinds of current regulation optimal objectives, minimum loss (ML) and minimum variance (MV). Meanwhile, a series of index, such as copper consumption ratio, variance and current stress ratio, were used for evaluating the performance of different current fault tolerant optimal conditions. In this paper, the postfaulted models and equivalent circuits of dual-three PMSM were reconstructed in torque subplane and loss subplane based on the principle of VSD. The current in loss subplane determines the copper loss of dual-three PMSM in fault state. The specific forms of the current were injected into loss subplane for minimizing copper loss or current magnitude variance of dual-three PMSM. In addition, the paper also presents a fault-tolerant bridge circuit composed of a series of bidirectional thyristor, which using the dc-link capacitors of NPC type inverters and fast fuses to achieve full active fault isolation. A detailed analysis of the midpoint voltage fluctuation behavior under fault is based on the mathematical model of midpoint voltage deviation. Meanwhile, an approximate zero-sequence voltage superposition method is proposed for suppressing midpoint voltage deviation exactly.Finally, convincing simulation and experiment results verify the theoretical analysis and research in this paper.