Optimal Energy Management,Simulation Modeling,Control And Protection Of Microgrids

Hybrid energy systems containing distributed generator resources(DERs),powered by micro-sources such as photovoltaic(PV)power system,microturbine,wind power system,mini-hydro,fuel cell,etc.,have been becoming prominent among the power industries and utilities because of their easy accessibility(the renewables:especially wind and solar),reduced-emission(environmental friendly clean energy),simple(less complex structure),enhanced operational efficiency and higher reliability.The growing penetration of distributed generations(DGs)and the existence of hybrid decentralized energy systems with several DGs in electrical proximity to each other have resulted in the idea of microgrid.A microgrid is a set of interconnected loads,DGs and energy storage systems(ESSs)at a distribution level with distinct electrical boundaries.It has black start capability and can operate either in islanded mode independent from the main grid or grid-connected mode in parallel with other microgrids or main utility grid.Microgrids provide improved quality,efficient,reliable,secure,economical and sustainable electrical power supply to end-users such as institution/campus sites,hospitals,universities,commercial/industrial facilities,remote off-grid communities,military bases,data centers,municipalities and etc.However,the integration of RESs and ESSs into a microgrid system and its dual mode of operation can cause challenges and impact on microgrid planning,modeling,analysis,energy management,control and protection.Motivated by this,this thesis studied the optimal energy management,simulation model,control and protection of multi-DER microgrids,operating either in the islanded mode or grid-connected mode of operation,considering high penetration of renewable energy sources.Firstly,the thesis presents a day-ahead optimal energy management strategy for the operation of microgrids with high penetration of renewables operating either in the islanded mode or grid-connected mode of operation.The performance objective of the strategy includes:minimizations of fuel cost,operation and maintenance costs and energy purchasing expenses from the main grid,and maximization of renewable energy utilization and financial profit from energy selling revenues to the main grid.The energy management optimization problem established,in this thesis,is actually a multi-parametric objective function,which belongs to the class of constrained nonlinear problems.The objective problem is formulated considering the uncertainties in renewable generations and load demand,grid electricity prices and storage options.Accurate forecasting of the renewable energy generations(wind and solar),load demands and electricity prices plays a vital role in the performance of the proposed energy management system(EMS)model.The regrouping particle swarm optimization(RegPSO)approach combining the conventional particle swarm optimization(PSO)algorithm and a mechanism to prevent the algorithm from stagnation when the particles face prematurely convergence is used to solve the EMS optimization problem.The RegPSO solutions for the EMS objective problem are then:the amount of power to be generated by the dispatchable generation units(microturbine,diesel generator,etc.),the charging/discharging power of the energy storage system(ESS)units and the quantity of power exchange with the main grid,in each hour of the next/future day.Simulation results have demonstrated the effectiveness of various aspects of the proposed EMS strategy in different scenarios.Secondly,in this thesis,control strategy and simulation model of a microgrid has been developed based on a practical microgrid information,Goldwind Smart Microgrid System(in Beijing,China)in an electromagnetic transient software environment,PSCAD/EMTDC.Developing a complete microgrid simulation model in the form of either software or hardware system is very important to study microgrid issues such as:islanding and synchronization phenomena,transient,dynamic and steady state control and stability problems,power flow analysis,harmonic analysis,and fault detections and protections.The case study microgrid system model consists of basic power system component models,two renewable energy source models(wind energy system(WES)and photovoltaic(PV)solar system),two dispatchable energy source models(CCHP microturbine and diesel generator),three energy storage source models(vanadium redox flow battery(VRB),lithium-ion battery and supercapacitor),demand side(load)model and utility grid equivalent model.Each source model,except the diesel generator,is interfaced with power electronic converters with different combinations,configurations and control strategies.The models for the individual microgrid components have been developed first,and then the complete microgrid system simulation model has been built and simulated by integrating all the individual component models.The complete system model is able to automatically switch to islanded mode when there is a fault in outside utility grid or intentional islanding is required and back to grid-connected mode when the fault is cleared or resynchronization is needed.All the source models operate in PQ control mode when the microgrid is grid-connected;however the ESS model or the other dispatchable source models should be able to operate in VF mode when the microgrid is islanded to set the voltage and frequency reference values of the microgrid system.Several simulation case studies were conducted on the developed model.The obtained results demonstrated that the developed microgrid system in PSCAD has acceptable performances under various scenarios with distributed renewable energy sources and the proposed control strategies.At the last,a comprehensive microgrid protection scheme that applies for three phase to ground,double line to ground,single line to ground and line to line solid and high impedance faults is proposed in this thesis.Deployment of distributed generations(DGs)causes a microgrid or conventional distribution feeder to tackle several challenges,regarding control and protection issues.The major ones are:bi-directional flow of power,changes in fault current magnitude,and continuous changes in operational configuration due to plug-and-play of DGs and loads and intermittency of the renewable DGs.This issue extravagates when the microgrid contains several converter-interfaced DGs and operates in the islanded mode of operation.Hence,the conventional protection strategies and relaying techniques will no longer be sufficient to protect islanded microgrids against network faults and disturbance conditions.This thesis proposes a fast and reliable communication-supported protection strategy for insuring safe operation of converter-interfaced microgrids.The strategy is implementable by commercially accessible microprocessor based digital relays and applicable for the protection of small-scale low-voltage and large-scale medium-voltage microgrids.It provides a backup protection to handle communication failures and malfunction of protective devices.The thesis also presents the detailed structural layout of the digital relay which executes the proposed protection strategy.A number of improvements are proposed to find an alternative way for conventional overcurrent relays to reliably detect small-magnitude fault currents and high impedance faults,commonly encountered in converter-interfaced microgrids.Simple and economical bus protection method is also proposed.Several simulations have been conducted on a comprehensive model of the realistic operational industrial microgrid in PSCAD,for different case studies and fault scenarios,and verified the effectiveness of the proposed strategy and its digital relay.