| Researches on microgrid which use the distributed resource as the power unit have become one of the hot topics in the study of new energy. Developed countries and regions like US, Europe and Japan have already set several microgrid models. Microgrid can be classified as DC microgrid, AC microgrid and hybrid microgrid, the microgrid hereby refers the AC microgrid. With the purpose of operation stably and efficiently, the control of inverter of microgrid is critical, the interface converter can equivalent to the voltage source with internal impedance and the droop control strategy is common strategy which can make the microgrid operate in different mode.In the grid-connected mode, microgrid connects to the grid through the static switch and run in parallel. With the support of gird, the output voltage of inverter wouldn’t appear magnificent unbalanced or distorted phenomenon, however, the load current, of which5th and7th harmonic current make up a high proportion, may be obvious. Therefore, with the control of5th and7th harmonic current, they can be reduced to an accepted range and the power supply environment is improved as well. The paper treats5th and7th harmonic current as control objects, implements the control of amplitude and phase of harmonic current in synchronous harmonic dq frame and embeds it into the traditional droop control, which let the inverter can not only implement the fundamental power control but also output the harmonic current precisely. This kind of control strategy mixes control of fundamental and harmonic, so the inverter can carry on the power control at the same time it has the ability to absorb or emit harmonic current which can make the inverter have the function of both power output and active power filter.In the island mode, the paper analyzes the reason why harmonic voltage appears is that the output harmonic impedance of inverter is large. The unbalanced voltage is caused by2nd harmonic in synchronous dq frame and the distorted voltage is mainly caused by6th and12th harmonic in synchronous dq frame. Then, the paper analyzes that harmonic voltage interacting with harmonic current result in the harmonic power. Afterward, assuming the output voltage is balanced and distortionless, the paper focus on the relationship between the harmonic current and harmonic power. The H-G control strategy is the use of this relation, make the output voltage feedback to the reference current through the conductance and reduce the harmonic voltage. In addition, resonant control can change the response near the resonant frequency, this kind of control can be mixed with H-G control and multi-resonant H-G control appears. The improved control can decrease the output impedance of specify harmonic frequency, so the harmonic voltage is decreased too. Comparing to the H-G contol, the improved control is more targeted to different frequency harmonic and according to the propotion of different frequency harmonic the control is implemented precisely with different conductivity value. Therefore, even thought the inverter runs with unbalanced load or nonlinear load, the improved control can maintain the output voltage with a low rate of unbalanced degree and low distortion.Followed the theory, the paper has carried on the simulation and the experimental validation. The whole experiments are carried on with the three phase voltage source inverter which based on the RT-LAB. |
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