Research On Novel Offshore Wind Farm Topologies And Their Key Technologies

The renewable energy generation represented by wind power generation and solar photovoltaic power generation has experienced a fast development,which provides a new opportunity for human beings to solve the problem of energy and environment.Compared with the onshore wind farm,the offshore wind farm owns some brilliant merits,such as:doesn’t occupy land resources,high utilization hours,rich in wind energy resources,and so on.Both the total installed capacity and the annual newly installed capacity of the offshore wind power have been increasing constantly for 20 years.However,with the continuous growth of the offshore wind farm capacity and the increase of the distance from the offshore wind farm to the coastline,the drawbacks of conventional integration techniques for the offshore wind farm are becoming more and more apparent.Therefore,it is very meaningful to research the offshore wind farm integration techniques with low cost,high efficiency and high reliability.In this dissertation,three new offshore wind farm integration topologies are proposed and studied after a literature review.The development of the DC power transmission and distribution technologies makes it possible for the offshore wind farm to integration through pure DC system.A novel all DC system based on Modular Isolated DC/DC Converter(MIDC)is proposed for the offshore wind farm collection and integration.Thorough comparisons of three candidate DC/DC converter topologies are carried out,and the LLC resonant converter is found to be the optimal Sub Module(SM)of the MIDC.The operation curve of the LLC resonant converter is deduced and then a two layer control system for the MIDC is designed,which is comprised of one top layer control system and multiple bottom layer control systems.The top layer control system is used to regulate the input current sharing among the SM groups,while ensuring the MPPT of the wind generators.The bottom layer control system is used to regulate the input voltage sharing within each SM group.In the electro-magnetic transient simulation model of the MIDC,hundreds of switching devices in the SMs operated on high frequency mean that the simulation software has to inverse the high-order admittance matrix of the simulation model in every switching period.As a result,the control system design and control parameters optimization of the MIDC will be very difficult because that the simulation model consumes too much computational time.To solve this problem,the equivalent mathematic model of the SM is built and then this mathematic model is discretized through different discretization methods.Thereafter,three accelerated simulation models of the SM are built based on these different discretization methods.The simulation speeds and simulation precisions of the three SM accelerated models are compared,and finally the fourth order Ronge-Kutta method is chosen to build the MIDC optimal simulation model.In the optimal simulation model,detailed circuits in the original simulation model is equivalent to mathematic model,in other words,all of the switching devices in the MIDC are simplied.As a result,the order of the admittance matrix is dramatically reduced,which means that the simulation speed of the simulation model will be efficiently increased.A dynamic simulation system of the offshore DC wind farm is built,within which a precision power amplifier and a input voltage 135 V/output voltage 750 V/rated power 1 kW/3 SM groups/9 SMs MIDC prototype are used to emulate a wind power generation unit in the offshore DC wind farm.The hardware structure and software structure of the MIDC prototype are designed.NI compactRIO real time controller and TMS320F28335 DSP are used as the top layer controller and bottom layer controller of the MIDC prototype control system,respectively.To ensure that the control signal generated by the top layer controller can be transmitted to the bottom layer controllers in time,the communication method between the top layer controller and bottom layer controllers is designed.Moreover,the compactRIO real time controller and precision power amplifier are used to emulate the Permanent Magnet Synchronous Generator(PMSG)and its diode-based rectifier.Experimental results verify the feasibility of the proposed offshore DC wind farm and the effectiveness of the MIDC control system.When a diode-based HVDC link is used for the integration of the offshore DFIG-based wind farm,the voltage and frequency of the offshore ac grid must be controlled by the wind generators as the diode-based converter can’t realize this target.Based on an in-depth study of this topology,the voltage expression of the offshore ac grid is deduced and the factors affecting the offshore ac-grid voltage is revealed,which proves that the offshore ac-grid voltage can be kept in a proper range automatically during normal operation.The control strategy of the DFIG rotor side converter is designed to regulate the offshore ac-grid frequency while realizing the maximum power point tracking(MPPT)control of the DFIG.Besides,the rotor side converter is also used to limit the output reactive power of the DFIG to avoid too much reactive circulating power among the DFIGs.The start-up procedure and fault ride-through control are designed to ensure that the whole system can be started up properly and to avoid overvoltage of the HVDC link during onshore PCC short circuits.The diode-based HVDC link has many advantages when it is used for the integration of the offshore wind farm,namely,low conduction losses,low installation costs,small footprint,high reliability,and so on.However,the diode-based converter can’t provide the necessary power for the DFIG during the start-up procedure.What’s more,bulky L-C filters are needed to filer the harmonics generated by the diode-based rectifier.In this dissertation,a modified diode-based HVDC link is proposed to solve these issues.The rectifier of the modified diode-based HVDC link is consisted of a diode-based converter and a small capacity modular multilevel converter(MMC)with parallel connection.The small capacity MMC has two major functions:during the start-up procedure,the small MMC can provide the necessary power to the wind farm and then help the startup of the wind generators;during normal operation,the offshore small MMC is operated as an active power filter to filter the harmonics generated by the diode-based rectifier.A two layers control system is designed for the small capacity MMC to realize the above two functions.The top layer control system is used to control the power output of the MMC while the bottom layer control system can regulate the current at the offshore Point of Common Coupling(PCC)to be sinusoidal.Moreover,the start-up procedure of the whole system is also designed,which ensures that the offshore wind farm can be started up without the help of energy storage devices.