Control And Protection Of The Wind Turbine-based DC Microgrid

For the microgrid system dominated by renewable energy sources power generation, DC microgrid has become the new microgrid technology with its advantages. Taking a DC microgrid with wind turbine configuration as an object of study, the stable operation control scheme, that is voltage hierarchical coordinated control strategy, is proposed firstly. The DC voltage can be adjusted automaticly based on DC voltage variation. Then, based on the DC microgrid system operating characteristic under AC grid fault, various fault ride-through (FRT) strategies are proposed during two operation situations. Finally, according to the fault characteristics of voltage source converter (VSC) and system under short circuit DC fault, the current differential protection scheme as the main protection is proposed. By making full use of the DC current characteristics of each DC terminal, the protection control system can response rapidly to the line differential current value under short circuit fault, so as to realize the DC short circuit fault protection of DC microgrid system.The main research results are as follows:1. The three different operation modes of DC microgrid system are proposed, which are used to provide the basis for designing the coordinated control strategy for the DC microgrid with wind turbine. The typic structure of DC microgrid and function of each unit are described, then the dynamic model of permanent magnet wind turbine, battery energy storage system, grid-connected converter and loads are established, and the control strategy of each unit is analyzed.2. The voltage hierarchical coordinated control strategy is proposed, in order to keep the power balance of the DC microgrid under different operating conditions, such as variations of wind speed and loads. The basic principle of DC voltage droop control is analyzed, then the voltage hierarchical control strategy is proposed, according to the composition structure, work characteristics, operation mode and DC voltage working scope of wind turbine-based DC microgrid. Control methods of converters under each control layer are analyzed to ensure grid-connected converter, energy storage system, wind turbines, and loads are involved in the coordinated control system. They can adjust the DC voltage automatically according to their priorities, to meet the operation requirements under different working conditions for DC microgrid. The method does not depend on communication and the control structure can be simplified and the control flexibility is increased, and can be extended to other distributed generation power and energy storage systems-based DC microgrids.3. FRT strategies are proposed under the two situations of power surplus within the DC microgrid and power deficit within the DC microgrid. The impacts of AC grid fault on the safe operation of DC microgrid are analyzed, and according to rapid rise or fall of DC voltage as the criterion of AC grid failure, the advantages of active power control of multiple DC terminals are used to FRT control, especially the energy storage systems and wind turbines. The system has strong ability to ride through the AC grid fault, through the reasonable adjustment of charging and discharging current of storage battery and active power output of wind turbines.4. DC transmission line short-circuit fault protection scheme is proposed for the DC microgrid using ring topology. On the analysis of three kinds of topology structures of wind turbine-based DC microgrid, the general topology structure is proposed. The transient characteristics of VSC and system under DC line fault are analyzed, so as to gain the fault features of short-circuit current, and the qualitative relationship between circuit parameters and fault current and overcurrent time of diode. The protection scheme of current differential protection as the main protection and undervoltage protection as backup protection is proposed, in order to detect the fault line rapidly and open DC circuit breakers reasonably. The power supply security of the DC microgrid system is improved by detecting the input and output currents and its differential current to locate and isolate the fault lines.