Research On Disturbance Characteristic Evaluation And Multi-sources Coordination Control Of Off-grid Integrated Energy System

Traditional energy supply systems which are individually planned and operated have low energy efficiency,but integrated energy systems can plan and operate uniformly,providing a solution for improving energy efficiency.For an off-grid integrated energy system,due to its lack of support from the main grid,the system has low inertia and weak anti-disturbance capabilities,which brings a great challenge to its secure operation and stability of the system.When the off-grid integrated energy system has disturbances such as load switching,the voltage and frequency in the system will be greatly disturbed.Thus,how to evaluate and judge whether the load switching will have adverse impact and whether the voltage and frequency fluctuations will exceed is of great significance.In addition,in view of the voltage and frequency fluctuations during load switching,a method to reduce the voltage and frequency fluctuations during load switching or switching loads as many as possible under the premise of safe and stable is also of great significance.Therefore,this dissertation mainly focus on the above problems and the main work is summarized as followed(1)A time-domain simulation model for off-grid integrated energy systems is established.Because this paper mainly studies the fluctuation of voltage and frequency during the load is switched,that is,the dynamic characteristics of voltage and frequency.And its time constant is small,so the dynamics of thermal systems and natural gas systems can be ignored in this process.Therefore,the power system model is mainly established in the time-domain simulation model,which includes micro-turbine generation system,energy storage system and photovoltaic power generation system.According to the timedomain simulation model,the voltage and frequency fluctuations during load switching are studied.(2)A small-signal model of the off-grid integrated energy system is established,and the small-signal model is used to evaluate the disturbance characteristics of voltage and frequency.The small-signal model includes four sub-models,PQ control inverter submodel,V/f control inverter submodel,network submodel and load submodel.First,the small-signal models of each submodel are established in its own independent rotating coordinate systems,and then the small-signal models are transformed to a common rotating coordinate system using coordinate transformation equation to obtain the small-signal model of the entire system.In addition,the disturbance items is added in the smallsignal state space equation.Finally,according to the small-signal model that mentioned above,the voltage and frequency disturbance characteristics during load switching are evaluated and compared with the results of the time-domain simulation model to verify the correctness of the small-signal model.(3)A hierarchical multi-source coordinate control method is proposed to reduce the voltage and frequency fluctuations when the load is switched.The lower layer is a decentralized control layer,which mainly includes distributed power control and load control,and it is used for power balance and load management of the system.The upper layer is a coordinated control layer,it coordinates the output power of the slave microsource through an improved master-slave control method to reduce the fluctuation of voltage and frequency in when the load is switched.The core of the improved masterslave control is to reduce the output fluctuations of the main micro-source by setting the output power of the master micro-source as a reference value,and distributing the power shortage to the slave micro-source according to the capacity of the slave micro-source.Finally,the effects of hierarchical coordinated control on reducing voltage and frequency fluctuations are verified in a time domain simulation model and a small signal model.