Research On Grid-Connection Technology Of Self-Controlled Synchronous Motor

As the key equipment in iron and steel enterprises, metallurgy, mining or hydro-electric industries, high-voltage and high-power synchronous motor was generally used to drive large blowers, pumps, compressors, for its advantage of adjustable torque and power factor, being insensitive to grid fluctuation and very high efficiency. It is commonly considered that the self-controlled soft-start method is the best mode to start this kind of motors. To solve the difficulty of electrical grid connection technology without impact, in this paper, analysis and research are presented. It gives digital Phase-Locked-Loop to adjust motor frequency and phase, and closed-loop excitation plot to regulate Back-EMF.In the beginning of this paper, it derived from some basic mathematical relationships and working principles of this technology. Combined with three conditions, holistic approach of entire process of grid-connection was introduced. When the start process begins, under 8% of rated speed of the motor, break commutate mode is adapted. At medium and high speed, zero-crossing detection of Back-EMF is used until it approximately rises to the synchronous speed, almost the 95% of rated speed. After that, grid-connection segment activates.Secondly, to implement the closed-loop control of excitation current, a series of circuits is designed to acquire grid voltage, Back-EMF and their zero-crossing points, also to adjust excitation current and communicate with main controller. Smoothing reactor was determined based on DC bus's continuity and fluctuation condition. When starting, excitation current will be able to adjust manually and control automatic. The excitation controller has satisfactory steady state precision.At last, software PLL algorithm applies the zero-crossing-point phase-detector to compute the phase difference of grid and motor. It coordinates with speed-loop and electricity-loop and has solved PLL's narrow bandwidth problem. The PLL algorithm has smooth steady state and has no static error.A 3kW/380V/1500r/min synchronous motor is used as control plant in the experiment platform. At the end of grid-connection process, the difference of grid voltage and Back-EMF meets the following conditions: frequency difference less than 0.25Hz; voltage difference less than 5% EMF; phase difference approaches to zero. It is a very satisfactory result to realize grid-connection without impact.