Research On Key Technologies Of Microgrid Converter Droop Control

The microgrid is a small-scale power distribution system that is composed of distributed generator(DG),energy conversion devices,loads,monitoring and protection devices,and can realize self-independent control,protection and management.The effective management of DG through the microgrid can reduce the impact of DG on the safe operation of the power distribution system,and also help to ensure uninterrupted power supply of important loads during grid disasters,which has important economic and social value.The types of DGs in the microgrid are different,and their roles may also be different.The corresponding converters also need to adopt different control strategies.The droop control strategy is easy to implement DG plugand-play,and it does not need to change the control strategy when the micro-grid switches operation mode,and has been widely studied and applied.In this paper,the droop control of DG converters in three microgrids of AC microgrid,DC microgrid and AC/DC hybrid microgrid is studied.The main work are as follows:(1)Aiming at the problem of reactive power sharing in the AC microgrid,the virtual impedance based droop control is adopted,the droop control of the virtual capacitor is further proposed.?Droop control based on virtual impedance: analysis of the mechanism of deviation of reactive power distribution under traditional droop control,aiming at the problem of deviation of reactive power distribution caused by the difference of line impedance,the droop control based on virtual impedance is adopted.by adding virtual impedance,the difference of line impedance is reduced,which is beneficial to achieve reactive power sharing.The example analysis proves that the droop control strategy based on virtual impedance can improve the reactive power sharing accuracy in the AC microgrid,and the reactive power sharing accuracy increases with the decrease of the line impedance difference,but it will cause the inverter output voltage amplitude to drop.?Improvement of droop control based on virtual capacitor: analyze the output characteristics of reactive power after the capacitor is connected to the output of the inverter in the AC microgrid,which proves that the reactive power can be adjusted by changing the shunt capacitor.On this basis,the improvement of droop control based on virtual capacitor is proposed.The virtual capacitor algorithm is used to simulate the characteristics of the input capacitor of the inverter output in the AC microgrid to avoid the inconvenience caused by the actual capacitor,and the calculation process of the virtual capacitor value is given.The example analysis proves that the droop control strategy based on virtual capacitor can improve the reactive power sharing accuracy only by using the voltage amplitude and reactive power output by each inverter.It does not require communication and detects other parameters,and has the characteristics of strong applicability and simple implementation;and the control strategy can raise the output voltage of the inverter while improving the reactive power sharing accuracy.(2)Aiming at the problem of active equalization in DC microgrid,an I-U droop control based on adaptively adjusting the droop coefficient is proposed.The mechanism of active equalization in DC microgrid is analyzed.It is concluded that the difference of line impedance will cause the difference of active power distribution of each DG,the increase of droop coefficient will lead to voltage drop.Aiming at these two problems,an I-U droop control based on adaptively adjusting the droop coefficient is proposed.The adaptive equalization loop of the droop coefficient is used to improve the active average accuracy,and the output voltage of the converter is raised by the voltage adjustment loop.The example analysis proves the correctness and effectiveness of the proposed control strategy.(3)Aiming at the power balance problem of AC sub-microgrid and DC submicrogrid in hybrid AC/DC microgrid,the coordinated droop control of interlinking converter(ILC)is proposed.Analyze the operation mode of ILC in the AC-DC hybrid microgrid,the ILC transmission power should be taken into account the direction and value of the power transmission.On this basis,the coordinated droop control of ILC is proposed,and its working mode is divided into three modes: rectification,idle,and inverter.The idle working mode added on the basis of the traditional droop control can prevent the frequent operation of the power electronic device when the DC bus voltage fluctuates around the rated value,reduce the generation of harmonics,and improve the power quality.The example analysis proves that the proposed control strategy can achieve the power balance of the AC sub-micro network and the DC sub-micro network.