Droop control method for DC microgrid in off-grid state

Abstract

The invention relates to a droop control method for a DC microgrid in the off-grid state. The k initial virtual impedance, the N acceleration factor, the capacity of the storage battery, the state ofcharge and the depth of discharge are applied to adaptively regulate the virtual impedance. The lambda factor is introduced to eliminate the influence of mismatch line resistance, and the voltage recovery term is introduced to recover the bus reference voltage. Compared with the methods in the prior art, the influence of the mismatch line impedance on the current distribution of the energy storageunit can be eliminated and the current is distributed according to the capacity in the energy storage unit so that that the state of charge of each energy storage unit is gradually balanced, excessive charging and discharging of part of the energy storage units is avoided, the line impedance information does not need to be measured, only the compensation term which eliminates the influence of themismatch line resistance is added so as to realize balancing of the state of charge, and the voltage recovery term is added to ensure that the voltage is restored to the reference value.

Description

technical field [0001] The invention relates to the field of DC microgrids, in particular to a droop control method in the off-grid state of DC microgrids. Background technique [0002] In order to reduce environmental pollution related to fossil fuels, distributed generators (DGs) technology based on renewable energy (renewable energy source, RES) has been widely developed in the past ten years. Solutions for renewable energy and loads. Microgrid has various DC sources (such as photovoltaic (PV)), AC sources (such as wind turbine generator (WTG)) and loads (such as electric vehicles). Compared with AC microgrid, DC microgrid will not There are problems such as harmonics and reactive power. Therefore, DC microgrids have been widely studied and paid attention to. [0003] In order to ensure the reliable operation and power quality of the microgrid, how to reduce the power fluctuations caused by RES such as solar energy and wind energy is the focus of research on the microgr...

AI Technical Summary

Problems solved by technology

[0004] In a DC microgrid, I-U droop is usually used to control the DC / DC converter on the bus, but in traditional droop control, each energy storage unit can only distribute power in proportion to its output current, and does not take into account the initial Is the SoC the same, so the SoC can't be balanced between DESUs

In order to make the SoC reach equilibrium quickly, many scholars at home and abroad have proposed different solutions. One method is the power function SoC n Adaptively adjust the droop coefficient of each energy storage unit, where n is the convergence factor. This control method has shown effectiveness in the SoC balance experiment of the energy storage unit, but it does not consider the voltage deviation caused by the droop control of the DC microgrid; Some scholars have proposed an adaptive hierarchical coordination control method. First, the power allocation level is used to determine the dominant energy storage unit of the energy storage system, and then the power balance level control is used to achieve SoC balance. In addition, the fuzzy Algorithms are proposed to realize the automatic distribution of power according to the state of charge of the energy storage, but the final power balance needs to be achieved by constant voltage charging of the energy storage. During the discharge period, it is impossible to determine whether the state of charge of each energy storage unit can reach balance, and it is complicated. The controller design is not suitable for practical engineering

[0005] In the above studies, even though the SoC balance is achieved by adjusting the virtual impedance of each DESU, in the energy storage unit, there is inevitably a mismatched line resistance, so the accurate current or power distribution among the DESUs cannot be guaranteed.

Using the voltage change rate to replace the voltage in the droop control can effectively reduce the influence of the line impedance on the load distribution accuracy, but this method does not consider the information of the state of charge, and cannot perform power distribution according to the capacity when used for the SoC balance control of the energy storage unit; Another method proposes to use the SoC-related e-exponential droop control to analyze the control parameters in the droop coefficient, and to reduce the influence of line impedance by selecting appropriate parameters to achieve SoC balance, but this method cannot completely eliminate the impact of line impedance. The impact of current load distribution, and the set control parameters must be large enough, which will inevitably cause a certain degree of DC bus voltage deviation

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