Joint Operation Optimization Of Sending-side System For Wide Area Renewable Energy Transmitted By DC

In order to optimize the energy structure and promote green development,the construction of renewable energy is constantly being intensified in the northwest region of China in recent years.However,the local load cannot meet the consumption demand of large-scale new energy because of the relatively low economic volume of Northwest China,and the renewable energy needs to be collected and delivered to the load centers through long-distance transmission system.High voltage direct current(HVDC)transmission technology does not have the stability problem of synchronous operation,and it eliminates the limitations of transmission distance.Therefore,relying on the HVDC transmission channel and through the multi energy complementary mode,it is an effective method to bundle the output of wind power,photovoltaic power,thermal power and other multi-types of power supply in the sending-side system to the load center for consumption,which can realize the intensive development and optimal allocation of the overall resources.Under the background of wide area renewable energy transmitted by DC system,it is of practical significance to study the influence of DC operation characteristics on the optimal dispatch of power system.This paper gives full consideration to the operation and regulation characteristics of the DC system,and studies the operation optimization of the wide-area new energy under the scenarios of FDC grid collection and conventional DC delivery.The specific contents are as follows:Firstly,from the aspect of mathematical model and governing equation,the basic converter principle and control principle of Line-Commutated Converter-HVDC(LCC-HVDC)and Voltage-Source Converter-HVDC(VSC-HVDC)are studied respectively.Then,based on the control principle of converter station and the coordinated control mode between stations of Flexible DC grid,the access and control strategies for different types of nodes of the transmission system under the condition of wide area new energy passing through Flexible DC grid are proposed.Moreover,based on Newton-Raphson method,the method to calculate voltage and active power injection of different types of nodes in flexible DC grid is derived,and the validity of the method is verified by an example,which lays a foundation for the research of operation optimization of the terminal system containing flexible DC grid.Secondly,based upon the operation characteristics of the VSC stations and the inter-station coordination control strategy of flexible DC grid,an optimization model of the source-grid coordinated dispatch of the flexible DC system is proposed.The objective of the model is to minimize the daily operation cost,and the curtailment of wind and photovoltaic power is included in the form of penalty cost.The operation constraints of the flexible DC power grid and converter station are under consideration,and then based on the decomposition and coordination idea,the model is divided into the main problem of economic dispatch and the sub-problem of power flow calculation.Through continuous iteration,the reference active power and reference voltage of the flexible converter station are modified.Based on the above,a case study for a 6-bus system shows the effectiveness of the proposed model and method.Finally,based on the operation characteristics and constraints of conventional DC converter stations,comprehensively considering thermal power generation costs,renewable energy curtailment rate and load demand characteristic of the receiving side,a joint operation optimization model of renewable energy sent out by DC transmission system is established,and the power curve is discretized with adequate consideration to the actual scheduling requirements;based on fuzzy theory,the multi-objective optimization problem is transformed into a single-objective optimization problem aiming at maximizing the satisfaction of dispatch staff.Through a specific case study,the effects of different optimization objective schemes and different limitations of the day-adjusting times on each evaluation index are analyzed respectively,which verifies the effectiveness and practicability of the model proposed in this paper.