Research On Key Technology Of Permanent Magnet Synchronous Motor Drive With Parallel Inverters

As the power of the ship, electric propulsion system has obvious advantages compared with the mechanical propulsion system, and owns broad market prospects. In low-voltage big current situation like ship electric propulsion, parallel inverters can effectively increase the system capacity and enhance the system flexibility. This thesis researched current equalization, circulating current and special problems about the inverters parallel drive system.When operated in parallel with common DC bus, circulation will be caused by zero frequency voltage which is not only a waste of the capacity of the system, resulting in additional losses, but also causing interference for the system control. This thesis analyzed the circulation loops of various harmonic of common DC bus parallel inverters with shifted carrier and discussed the method of zero sequence current suppression. In addition, due to the stray characteristics of device parameters, the parallel inverters will present uneven flow. This thesis proposed a current balance method that using double loop control to achieve equal current in both inverters.When operated in parallel with independent DC bus inverters will form a circuit loop which connects the DC capacitor to form an electric potential, causing the circulation. In this thesis, the loop circuit of the independent bus inverters in parallel with all kinds of harmonic voltage is analyzed in detail together with the factors that affect the size of the circulating current. Then the method of restraining the size of the circulation is presented in the thesis.This thesis also researched the special problems of electric propulsion system. T avoid over modulation of the inverters, the design adds voltage close loop feedback to weak the magnetic field of the PMSM, reducing the rated operating motor back electromotive force when the motor operating at normal speed with rated load, which could effectively prevent the phase current oscillation; Under power limited conditions, the suggested power loop can adjust the rotor speed reference in real time to make the motor power no more than the high power limited value under arbitrary load. Thus, the ship power system can operate reasonably and steadily with the power loop; Aiming at the zero speed with load running condition, the design improves the original three level point control algorithm. With the algorithm improvement even under the circumstance of zero speed and heavy load, the neutral point potential fluctuation can be kept in a reasonable range, and the safe and stable operation of the inverter is guaranteed.Through simulation and experiment, it is proved that the double loop control system can achieve both the average current and the amplitude of the circulation current. Field operation situations shows that under the condition of power limitation and zero speed heavy load, the system can run stably.