Research On Four-quadrant Converter Nonlinear Oscillation In High-speed Trains

High-speed train is supplied by alternating current (AC) power and base on single-phase four-quadrant (4Q) converter as one necessary part of the’AC-DC(direct current)-AC transform process. This paper considers digital control sampling delay effects in4Q converter circuit analysis model. Focusing on the stability characteristics of the system, it analyzed the system stability of different circuit parameters and control parameters. It also explained the nonlinear oscillation mechanism at different frequency areas and harmonics increase of the phenomenon. The paper presented a mathematical model for the single-phase4Q converter and used Hopf bifurcation theory of nonlinear systems, small disturbance method and the root-locus method to study the typical drive system stability. Using describing function method, the PWM saturation characteristic has been modeled and analyzed, and a limit cycle exists when the current controller proportional coefficient is beyond the bifurcation value. It also established a discrete-time mapping model for the digital controlled system to observe a period-doubling bifurcation, a border-collision bifurcation, and the oscillation development process from a certain phase zone to the entire cycle range.According to the equivalent impedance characteristics of traction power supply system, this paper has built a state-space average model of the traction network-4Q converter coupling system. Using Hopf bifurcation theory, it predicted a bifurcation phenomenon and a new unstable boundary. It also derived a frequency calculation method when a4Q converter harmonic oscillation appeared in high speed trains.Taking into account the difficulty in handling high power converters for experimental analysis, a computer simulation has been implemented to deliver a validated tool. However, a small scale prototype has been developed to confirm the simulation results.