Series-connected DC Based Offshore Wind Farm And Its Control

At present, offshore wind energy has been taken care more than onshore wind energy,so that large-scale offshore wind farm is being planned by many countries. Since fewengineering of such kind have been implemented, there is many issues needed to beresearched. By the conclusion of its research achievements globally, it become a maintrend to analysis of DC wind farm in detail and propose innovative solution in offshorewind farm research.The paper takes the wind farm based on series DC and its integration by use ofModular Multilevel Converter (MMC) as subjects, which is intended to the control of windpower generating system, wind farm and its integration converter. Comprehensive,thorough and detailed studies with respect to theoretical analysis，model design andsimulation verification, are carried out. Some important conclusions andindependent-innovative achievements are made and obtained.⑴Based on the operation principle of variable speed wind power generating system,the control strategy is designed for series-connected wind turbine. It consists of MaximumPower Point Tracking (MPPT) and the voltage limitation control, which can grantee thewind turbine running normally in the wind farm based on series-connected DC. In case ofpermanent magnetic synchronous generator, control diagram have been made up for eachtarget and the controller have been designed according to the Optimum Symmetric Method(OSM). The simulation verifies the control system can deal with each status and showsgood performance.⑵In accordance with the theory of wind generator and converter and their controlstrategy, the simplify control model for DC series-connected wind turbine is get. It,combined with the physical simplify model, can be kept as simplify model of wind powergenerating system. By simulation, it has been compared with detailed model and used toconstruct wind farm for testing its characteristics. The results show it can response thewind turbine closely and has likely static and dynamic performance.⑶If a grid voltage sag happens in the receiving grid, the wind farm must reduce the wind energy absorbed to keep the DC transmission voltage stable. Based on thecharacteristic of series-connection, the paper proposes a method to adjust the voltagelimitation in real time according the wind power generating system’s output power. It canmake the clsters’ voltage limitation equal to the limitation of DC transmission voltage. Thesimulation verifies the method can make wind farm ride through low voltage successfully.⑷The integration side of wind farm is of MMC. By the requirement of the grid andwind farm, the principle and method are demonstrated for the parameters calculating ofMMC. For its step modulation, an algorithm based on Langrage has been provided whichwouldn’t increase the calculate burden even as the number of levels increases. Furthermore,the hybrid modulation with step and multi carrier has been proposed which can enlarge themodulating range. Such modulation would improve the ability to handle with the fault ofgrid.⑸MMC, with step modulation, has to take long sampling period and is lack ofdynamic response speed. To solve it, flatness based control is applied with the convertercurrent. Simultaneously, the range of current reference is predicted in real time accordingto the converter maximum output voltage, the grid voltage and the current measurement.The two methods decrease the interaction between the control loops, and the simulationverifies they can enhance the dynamic performance of converter obviously.