Research On Dual-loop Control And Parallel Operation Technique For High Frequency UPS

Uninterrupted Power Supply's (UPS) paralleling operation is the main technique to enhance the reliability and enlarge the capacity of the power supply. Based on the research and design for a paralleling operation system of 3 Three-phase 40KVA online UPSs, several key factors concerning paralleling operation are studied in detail in some respects, such as dual-loop control based waveform correction techniques, the parallel model and circumfluence characteristics of the paralleling operation system based on dual-loop control, Digital Phase Lock Loop (DPLL)'s linear mathematical model and characteristics, distributed logic based load-sharing control method, etc. With the load current treated as a measurable disturbance input, a linear mathematical model is established for a PWM inverter, and a electric model with equivalent output impedance for the PWM inverter is presented. A dual-loop control method with inductor current feedback as inner loop is introduced, and the control structure is analyzed in detail. While the output current feedforward is adopted, the inverter has a high dynamic performance when supplying a non-linear load, the distortion of the output waveform is restrained. The mechanism that how does this control method restrain the load current disturbance is analyzed from the aspect of output impedance, and the influence on the output impedance by various controller parameters is explained. Experiment results indicate that inductor current feedback based dual-loop control method is feasible. A parallel equivalent electric model with output impedance is established, on the basis of this model, circumfluence characteristics are analyzed in detail. Considering the advantage of dual-loop control, load average current feedforward is introduced, by which the impedance between two inverters connected is enlarged without losing the dynamic characteristic, so the circumfluence is reduced. The transfer function of the circumfluence based on dual-loop controlled inverters'parallel system is brought out, and by which the circumfluence characteristic is analyzed. In order to reduce the circumfluence further, an instantaneous circumfluence droop method is introduced. Furthermore the simulation has been carried out, and the analysis above is coincident with the simulation results. Phase Lock is a indispensable part in parallel operation system based on distributed logic control. Digital PLL's linear mathematical model is established, and it's stability is analyzed, then the DPLL's stability conditional expression is given. Whereas the phase lock in UPS is very particular, this dissertation emphasizes on the dynamic performance with disturbance of the utility source's frequency noise and the reference phase. Simulation results indicate that lower phase erro and slower output frequency alteration are not coexistent. In order to solve this conflict in parallel operation system, a 2-level PLL and a re-modulation technique are employed. On the other hand, a synchronization phase lock method with handshake is realized, minimizing the possibility of the UPS's losing lock with each other when the utility source is unstable. On the basis of the circumfluence characteristic of dual-loop controlled inverter's paralleling system, the circumfluence power control strategy is given. To get more precise voltage steady magnitude and faster circumfluence convergence speed, some measurement is adopted in experiment. The experiment has been done, the results tell the control strategy adopted can restrain the circumfluence effectively.