Research Of High Frequency Modularized UPS & Parallel Control Technology

Realization of modularization, intelligence, greenness and high efficiency is the development direction of UPS systems. The key technology to implement modular operation of UPS is the control strategy of paralleled inverters. According to the modularization concept, lots of control problems must be solved to design of UPS power system and achieve high input power factor and realize the hot plug parallel operation. Technical problems in real applications need to be solved too. To some extend, engineering problems-solving is important for theory research results turning into practical applications. In this dissertation, research work on the realization of modularization, intelligence, digital control, high efficiency and clean power of UPS system is carried out. Based on distributed-logical-control method, theory analysis and experimental research on parallel operation of high frequency modularized UPS are presented in this paper. A novel dual Boost-PFC converter is proposed to solve the DC bus voltage unbalance caused by the coupling between two Boost-PFC converters. Based on the study of the operation modes of the new dual Boost-PFC converter, it is found that the novel topology can be taken as two separate Boost-PFC converters. These two converters work during positive and negative half cycles respectively, and the two independent Boost converters settle the base for realizing DC bus voltage decoupling control, achieving high input power factor as well as stable and symmetrical dual DC bus voltage outputs, and it can be used to supply the symmetrical DC bus voltage for the cascade half-bridge inverter. Mathematic models of Boost converter and paralleled operation system with multi-UPS are built. Based on the study of the mathematic model of paralleled multi-UPS system, complex coupling relations between real, reactive circulating currents of the paralleled inverters and the inverters output impedance, output voltage amplitude, phase angle are presented. It is pointed out that the paralleled static inverters are far more different from the traditional generators in parallel, so power regulating methods of the generators in the power system can not be applied to current sharing control scheme for paralleled inverters. Based on the research of the novel double Boost-PFC converter topology, a control strategy is proposed in the dissertation. The control scheme adopts double loop control strategy, and hysteresis loop control is applied for the inner current loop while PI controller is applied for the outer voltage loop. Theoretical analysis indicates that the two independent double loop control systems control the two DC buses separately, thereby solving effectively the coupling problems of DC bus voltage for Dual Boost-PFC converter. Experimental works are carried out, and the results indicate that better control effects are acquired with the proposed control technique. A novel control method employing current feedforward technique for Boost-PFC converter is researched to solve the problems in traditional method, such as input current distortion and non-unit input power factor for Boost-PFC. Simulation works indicate that the current feedforward control can compensate for zero-crossing distortion. The control method for modularized UPS based on the newtopology of high-frequency link to realize parallel operation is proposed. The control method includes the control on high-frequency inverter, which are load sharing in parallel, inner/outer synchronization control, switching logic control required by hot plug operation, bypass control, synthetically switching logic control on protection of modularized parallel UPS against shock, short-circuit & overload etc. The inverter according to the design of modular concept requires corresponding control method and switching logic required in parallel for hot plug operation. Switching inner/outer synchronization control, load sharing in parallel, switching logic control required in parallel for hot plug operation,bypass control are researched in detail in this dissertation which settle the basis for realization of synchronous operation, switching logic in parallel, and load sharing technique, ensure the reliability and stability of the paralleled inverter system. Based on theoretical research, six single-phase 3KVA high frequency modularized UPS have been built as experimental prototypes. The experimental research works for UPS which adopts the above control method are carried out. The experimental results show that stability of high frequency modularized UPS in parallel, load sharing during the paralleled modules, stability of hot plug ,stability of dynamic switching logic for multi-modules in parallel system are acquired better. Parallel operation control method based on SMC is proposed and parallel control method combining SMC with traditional PID controller is proposed, too. Through theoretical research for SMC applied in paralleled inverter system, switchover function of discrete SMC is given and control equation of the switchover function is derived. The relative simulation is carried out. As shown in the results, faster response is acquired using SMC, but high frequency oscillation existing in steady state. Novel control method combined SMC with traditional PID is used to solve the high frequency oscillation problem. Simulation research proves the control performance used by the control method combinedSMC with PI controller and results show that better stability, anti-perturbation ability, and load sharing control capability are achieved by using the combined control scheme. Based on the demand of the practical application and the research of the novel topology of dual Boost-PFC converter, the main circuit is studied which is used in the commercial products with switching functions between utility power and batteries as well as pre-charge function for dc bus and the circuits of PLL, output power detection, the circuits to protect against overload,shock,short-circuit are given, the demand is satisfied to protect the inverters in parallel system against overload, shock and short circuit. These protection functions for paralleled multi-UPS system are tested by experimental works in detail and the results show that these protection functions can protect the paralleled inverter against the overload, shock, short circuit and they play a very important role in improving the safety operation of the paralleled UPS system.