| As the indicator of the balance between power supply and demand,the system frequency needs to be kept within certain ranges in power system operation.To guarantee this,the system operator is supposed to maintain sufficient frequency reserve.Conventionally,this part of reserve capacity is provided by generation side.However,in recent years,the growing penetration of renewable energy sources changes the power-supply structure significantly.On the one hand,the renewable energy sources have brought more uncertainties to system operation,which improves demand of the reserve capacity in the power system.On the other hand,more and more conventional power plants are displaced by renewable energy sources with poor controllability,which exacerbates the shortage of the reserve capacity.At the same time,the demand response and VSC-HVDC technology make it possible to adjust the power consumption of flexible load and provide power support through VSC-HVDC link.A feasible way is to make demand side resource and VSC-HVDC link participate in system regulation.Aluminum smelter load(ASL)is a typical flexible load with energy-intensive characteristic,large thermal inertia and high controllability;VSC-HVDC system has fast power regulation ability.In this paper,both ASL and VSC-HVDC system are taken as the regulation resource in frequency control of power system.The work and achievements are as follows:(1)Dynamic model of ASL.This thesis analyses the properties and functions of the main components of the aluminum smelter loads in detail.The principles and control mechanism of self-saturable reactor(SR)and dc current control scheme of the aluminum smelter loads are also analyzed.Based on the analysis,the element model of ASL is built.Then,the mathematical models of potline,rectifier with SRs,and the internal circuit of SRs are built respectively.Based on state space theory,the mathematical models are integrated into a state-space model,which can characterize the dynamics of ASL.This model is the basis for the subsequent sections.(2)Hierarchical control strategy of ASLs to provide primary frequency support.This section proposes a hierarchical control strategy of ASLs to provide primary frequency support,which consists of optimal control at the higher level and frequency response scheme at the lower level.At the higher level,the supervisory computer sets constraints to the lower level periodically to minimize the control cost by optimal reserve allocation,which is formulated as a mixed-integer linear programming.The lower level responds to frequency deviations automatically by a local frequency control scheme.The proposed MPC controller improves the transient performence of the local control.The dynamic frequency threshold approach guarantees the security of ASLs in serious anode effect situations.(3)Coordinate "source-network-load" secondary frequency control considering power support through VSC-HVDC link.This section proposed a secondary frequency control strategy allowing industrial power grids to provide emergency HVDC power support(EDCPS)to the system requiring power support(RPS system)through VSC-HVDC link.An architecture involving multi MPC controllers with periodic communication is designed to simultaneously obtain optimized EDCPS capacity and minimize the adverse effect on the system providing power support(PPS system).Besides,a model of virtual power plant(VPP)containing aluminum smelter loads(ASLs)and high penetration wind power is established in PPS system,where the flexibility of VPP is improved by ASLs providing demand response and the uncertainty of wind power is handled by the incorporation of chance constraints. |
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