| The problems of traditional energy shortage and environment deterioration are more and more serious, all countries are focusing on the development and utilization of wind, solar, water, tidal power and other new renewable energy. As the rapid development of the power electronic technology, it become relatively easy to control and utilize the wind power. The current development situation of wind power on land is basely saturated, offshore wind have a lot of advantages compared to the terrestrial wind, such as, abundant resources, stable wind speed, large single capacity, wide development space, no occupation of land, and small impaction to environmental, these advantages make the offshore wind power become a new trend of global wind power development in the future. The way of alternating current transmission need to reactive compensation in offshore wind power transmission, and the compensation capacity will increase with the cable length and the bus voltage. Skin effect and proximity effect exist in the alternating current transmission, so the conductor cross sectional area is not fully used, which affect the conductive efficiency. Alternating current transmission has a lot of other issues, there are capacitive charging current, the power grid voltage is unstable, the transmission loss is big, and the power quality is poor. In recent years, with the development of the high power electronic devices, the control technology of power system, and the need of special transmission technology, the technology of VSC- HVDC become a research hotspot. The voltage source inverters in VSC- HVDC adopt the controllable shut off type electronic devices(such as IGBT and IGCT), use pulse width modulation(PWM) technology to determine the opened and shut off time of each bridge arm. VSC- MTDC system develops by VSC- HVDC system, it contains more than three converter stations, and the connection type between the converter station are varied, it can supply power in different point, and achieve power in different point. Integrated all its advantage, it become the most ideal offshore wind farm access and transmission mode, and get big attention in scholars and engineers, in recent years, a number of VSC- MTDC project put into operation or under construction around the world. VSC- MTDC transmission system can operate safely and economically which depend on the efficient control system. The main research content of this article is as follows:1. Because there is no additional energy storage device in addition to the traditional dc container in the VSC- MTDC system, the rated of active power which the system can transmit is limited within a certain range, if the voltage of dc bus is too high, the transducer and cable will be damaged, and if the voltage of dc bus is too low, the converters will out of control. So, the main task of the VSC- MTDC system is about controlling the dc bus voltage. This paper mainly research how to coordinately control the VSC- MTDC transmission system which has five ports with a ring wind farm under different wind conditions. Coordinated control strategy is about how to realize the inverters run at the corresponding balance point under various wind conditions. It can ensure the dc bus voltage. First, establishing mathematical model under the d- q synchronous rotating coordinate system and research the vector inverter control strategy, then introducing the coordinate converter control strategy briefly in recent years, and analyzing the V- I characteristics of stroke side rectifier and the grid side inverter in the voltage droop control strategy of multiterminal HVDC system in detail. At last, listing power flow equations to find out the droop control system balance point in different conditions, such as, when the system with various wind power, ac voltage sag and absence of side converter, by using Matlab programming at the same time.2. The coordinate droop control strategy of VSC- MTDC system is based on the local controller, we can acquire a voltage deviation after comparing the converter station voltage deviation with the voltage reference, then utilize proportional controller to narrowing the voltage deviation in order to realize the voltage control. It has good robustness, and can satisfy the different requirements of VSC- MTDC transmission system in the various operation conditions, and the control strategy does not need communication equipment between each converter station. However, in the control strategy, the slope value, the change of the active and the reference value of reactive power will affect the dc current distribution in the network. The experiment analysis shows that droop control strategy can coordinately control the system with multiple converter stations, but it can not guarantee that the dc network always work in the minimum loss value, that is to say its economy benefits is less than optimal, but in theory, when the communication equipments in the system operation normally, these two goals that dc system control coordinately and the loss of dc network is minimum can be realized at the same time. If there are communication equipments between different converter stations, can study the upper control strategy, which can achieve optimal power flow. I respectively put forward two different kinds of VSC-MTDC system optimal power flow control strategy, the first strategy is based on interior-point method, this strategy act on the central controller, with the purpose of providing suitable bus voltage reference to network side inverter, allowing all of the available wind power to transform from wind farm side rectifier to high-voltage dc network at the same time. The second strategy is based on sequential quadratic programming method, in order to realize the optimal power flow of MTDC system, the converter station can get the real- time control parameters in the strategy. The real- time control parameters include the droop gain factor and the dc voltage reference. In this paper, I respectively list the objective function, inequality and equality constraint conditions of expression of two strategies in the structure which has five ports with a ring wind farm, and verify the validity of the algorithm by Matlab program.3. It is the main task of MTDC system that controlling the dc bus voltage to guarantee that the system operates safely and economically in various working condition, but it also need to transmit the energy which collected by the wind farm to grid land, according to predefined power dispatching standard. The control strategy which acts on the top level of the control system of VSC- MTDC system, dc system operation control strategy can highlight the flexibility of electric power dispatching, when system operates under different running condition, or distribute of wind farm produce electricity in different ways. This paper studies three kinds of optimal control of electric power dispatching mode considering wind power fluctuations and the different requirements of VSC- MTDC system. In this paper, I respectively list three kinds of trend of electric power dispatching mode equation, and the correctness of the algorithm is validated by Matlab programming. |
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