| With years of industrial development, the world's energy structure is now changing from traditional energy to renewable energy.Grid voltage harmonics would be significantly increased if power generated from wind turbine or PV panels is directly injected into the main grid, and it would also shorten the electrical equipment's time of service or even lead to regional power system instability. Besides, since digital system and electronic instruments which demand higher power quality are more widely used than before, more stable grid voltage is required. Being able to convert DC power to AC power, inverters play a vital role in UPS. Therefore, effective control methods towards inverters are key to UPS system. In order to enlarge the capacity of UPS so as to supply more load, inverters are usually connected in parallel. The technology is to achieve proper current circulation and balanced power distribution.Firstly an equivalent model of single phase full bridge inverter with single-polarity doubled-frequency SPWM modulation is established. After that, LC filter parameters are chosen with reference to IGBT switching frequency. A double loop control model independent of load type is demonstrated, and PI controllers are adopted in both inner current loop and outer voltage loop. The control parameters are determined by pole-placement method. Relevant simulation model is established in Matlab/Simulink, the steady-state inverter performance is examined with linear and nonlinear load. Besides, dynamic performance of the system is also tested.In order to obtain higher steady-state voltage accuracy, another model is proposed based on quasi-PR control strategy. Similarly, a double-loop control model with feed-forward load current is adopted. The control parameters are determined in the following sequence:firstly inner and outer proportional coefficients, secondly inner and outer loop quasi-resonant component in fundamental frequency and lastly inner-loop component in harmonic frequency. With the help of Matlab/Simulink, a model is established, steady-state as well as dynamic performances of the system are tested, and its output voltage characteristics are compared with double-loop PI control model.In the design of parallelled inverter system, an equivalent impedance model of the system is firstly established, causes for circulating current as well as power characteristics of the system are analyzed, and comparison between the equivalent output impedance of inverters with and without feedforward of load current is conducted. Besides, the principle of drooping control strategy is stated based on which active and reactive power could be divided equally, and reference conditions for selection of droop parameters are inferred. After that, resistive and inductive virtual impedance are introduced to decrease circulation. To further reduce the harmonic current's impact on output voltage, virtual impedance based on second-order integration is proposed. Finally, two sets of parallel-connected inverter model based respectively on PI control and quasi-PR control are established in Matlab/Simulink, simulation results show that both resistive and inductive virtual impendances could help restrain circulating current. Larger droop parameters lead to better current performances, but system stability and output voltage error should also be taken into consideration. Complex virtual impendance which makes the equivalent output impendance resistive is adopted, and steady-state performance as well as dynamic performance is separately examined. Furthermore, quasi-PR virtual impedance helps to decrease voltage THD when carrying nonlinear load. Contrary to charateristics of single inverter, output voltage accuracy is higher with double-loop PI contral stategy. |
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