Analytical Model of Electromagnetic Performance for Permanent-Magnet Vernier Machines Using Nonlinear Exact Conformal Model

This article investigates the air-gap field distribution of the permanent-magnet Vernier machine (PMVM) using a nonlinear exact conformal model (NECM) to account for slotting effect, flux modulation effect, and iron nonlinearity. The exact conformal model (ECM) based on the region of one-slot and one-flux-modulation-pole (OSECM) are introduced to show the effectiveness of the linear analytical model for PMVM. It can keep high calculation accuracy and significantly reduce the computational burden. Then, the NECM is developed from OSECM by introducing the equivalent saturation current into the air region and coil region. The lumped parameter magnetic circuit model (LPMCM) model is used to obtain the magnetic potential of the iron region and therefore calculate the equivalent saturation current. The NECM which combines LPMCM and OSECM can essentially improve the accuracy of the linear analytical model. The harmonic analysis of the air-gap field is performed to theoretically explain the component of electromagnetic torque. Both finite element model (FEM) simulation and test results are presented to validate the NECM

Comparative analysis of partitioned stator flux reversal PM machine and magnetically geared machine operating in Stator-PM and Rotor-PM modes

In this paper, the partitioned stator flux reversal permanent magnet (PM) (PS-FRPM) machine and the conventional magnetically geared (MG) machine operating in both stator-PM (SPM) and rotor-PM (RPM) modes are comparatively analyzed in terms of electromagnetic performance to provide design guides for a MG machine regarding: (a) a SPM or RPM type machine and (b) a higher or lower gear ratio machine. It is found that a SPM type machine is recommended, since both PS-FRPM and MG machines operating in SPM modes have a higher phase back-EMF and hence torque than their RPM counterparts, respectively, as a result of a similar phase flux-linkage but a higher electric frequency since the iron piece number is larger than the PM pole-pair number. Moreover, a smaller gear ratio machine is preferred from the perspective of a higher power factor and hence a lower inverter power rating, as the conventional MG machines with higher gear ratios suffer from larger flux-leakage, higher synchronous reactance and hence lower power factors, as well as higher iron losses, than the PS-FRPM machines. However, higher gear ratio machines feature lower cogging torques and torque ripples due to the smaller difference between the PM pole-pair number and iron piece number. Both prototypes of PS-FRPM machine operating in SPM mode and MG machine operating in RPM mode are built and tested to verify the FE predicted results

Systematic evaluation for multi-rate simulation of DC Grids

With wide applications of power electronic devices in modern power systems, simulation using traditional electromechanical and electromagnetic tools suffers low speed and imprecision. Multi-rate methods can enhance efficiency of simulation by decreasing the scale of systems in small time-steps. However, the existing traditional methods for multi-rate simulation suffer the problems of instability and simulation errors. These have hindered the application of multi-rate simulation in power industry. Therefore theoretical evaluation on different multi-rate simulation methods is crucial to understand the feasibility and limitation of the methods, and to contribute to overcome the drawbacks of the traditional methods. In this paper, the multi-rate simulation performance based on two traditional technologies and a Modified Thevenin Interface are evaluated to provide an overall feasibility of multi-rate algorithms in the power simulation. The Modified Thevenin Interface is proposed to overcome the drawbacks in synchronization. Three theorems are proposed and proved for theoretically analyzing the stability of the simulation methods. Error analyses of the multi-rate methods are performed to identify the relationships between errors and simulation conditions. Besides, the accuracy and efficiency performance in a practical project of VSC-MTDC shows the feasibility and necessity by using multi-rate simulation. Through the theoretical analysis, the issues of stability and accuracy of multi-rate simulation for the DC grids have been better understood, based on which an improved simulation algorithm has been proposed to overcome these issues. Long-term system dynamics of large-scale systems containing DC grids and fast transients of HVDC converters can be investigated simultaneously with high speed and sufficient accuracy