Coordinated Control And Optimization Strategies Of Distributed Generation In Smart Grids

Due to increasing concern about the environment,skyrocketing cost for fossil fuels,and de-sire to increase the diversity in energy supply,the focus of electrical power industry is gradually moving from conventional energy resources(e.g.,fossil fuels)to renewable energy resources in the past decades.A high penetration rate of renewable energy is regarded as one of the main fea-tures of future smart grid.However,since distributed generators(DGs)cannot provide enough system inertia to sustain stability of the grid,and their power output is always fluctuating and un-predictable,they are not considered to be grid-friendly,which limits further integration of DGs to a great extent and poses serious challenges to power system operation,dispatch and control.This dissertation mainly focuses on distribution network and micro-grid with a high penetration of DGs,and aims to increase the grid-friendliness of these DGs by actively controlling their power output,which enables them to participate in power system economic dispatch as well as to provide ancillary services necessary to the grid.The main contributions of the dissertation are threefold.Firstly,with respect to decentralized control of a single DG,the dissertation proposes active power control algorithms of photovoltaic systems(PVs)based on Newton quadratic interpolation method and perturbation and observation method,which enable PVs to provide ancillary services to the grid.Specifically,when a given reference power is less than the maximal available power,the PV operates in a power dispatch mode,with its actual power output equaling the reference value.Otherwise,the PV can adaptively switch to the maximal power point tracking(MPPT)mode.Based on the active power control algorithms,PVs can flexibly adjust their power output to provide frequency support to the grid on condition that the capacity of energy storage is not sufficient:when the power perturbation is relatively small,PVs operate in the power-frequency droop mode,alleviating frequency deviation in the steady state;when the power perturbation is severe,PVs operate in an emergency mode,restraining the transient fluctuation of system frequency by sharply curtailing their power output.Simulation results of a micro-grid verify the effectiveness of the proposed method.Secondly,regarding coordination of multiple DGs,this dissertation proposes virtual power plant(VPP)control methods in centralized and distributed manners,the objective of which are to make the aggregated behavior of the VPP meet the requirement of power systems,while each of its components including DGs and controllable loads is self-optimized.This problem is studied from three perspectives.1.A coordinated control method of virtual power plant(VPP),which includes PVs and con-trollable loads,is proposed so that the aggregated power output of the VPP can be flexibly adjusted in a wide range.To achieve this,power output of the PVs and operational modes of the control-lable loads are coordinated by solving a mixed integer programming(MIP)problem.Meanwhile,with a quadratic interpolation based active power control strategy,each PV can operate in a power dispatch mode and simultaneously estimate its maximal available power,which is an input to the MIP problem.Externally,the VPP can quickly adjust the aggregated power and achieve functions important to power systems with high penetration of distributed energy resources,such as primary frequency regulation.Simulation results validate the effectiveness of VPP in providing frequency support to an island micro-grid.2.When there is a considerable number of control variables,centralized control scheme may result in high investment in the communication infrastructure,and cannot meet the requirement of real-time control and plug-and-play of DGs.For this reason,a distributed control and generation estimation(DCAGE)approach is proposed based upon DGs in smart homes including roof-top PVs and controllable loads,which enables all the DGs to operate at the same ratio of its maximal available power,while their aggregated power meets the VPP dispatch command.This method only requires that a small number of DGs know the VPP dispatch command,and the communi-cation topology among the DGs is strongly connected.Simulations in the IEEE standard 34-bus distribution network verify the effectiveness of the proposed approach under various scenarios.3.Despite the advantages mentioned above,distributed scheme is more susceptible to cyber-attacks due to the lack of a central authority and relatively low security level.To address this potential risk,an attack-resilient cooperative control strategy is proposed.With properly designed observation network,each DG can monitor the behaviors of all its in-neighbors,and gradually isolate the misbehaving DGs(when present)from the network as long as they do not collude with each other.Consequently,even certain DGs misbehave,the rest of them can together accomplish the control objective provided that the remaining communication network is still connected.Simu-lations of the IEEE standard 34-bus test feeder demonstrate effectiveness of the proposed strategy.Thirdly,regarding optimal operation of a smart grid,a distributed real-time optimal power flow control approach is proposed,which integrates the conventional hierarchical control includ-ing primary,secondary and tertiary controls,and considers power balance,power flow limit and generation limit constraints at the same time.The local communication among neighboring gen-erators and measuring systems enable the proposed approach to maintain the power output of the generators at the optimal operational condition in the real-time,while stabilize the system fre-quency to the nominal value.Simulation results in a micro-grid modified from the IEEE 34-bus test feeder and the IEEE 39-bus bulk grid validate the effectiveness and efficiency of the proposed approach.