| A microgrid(MG)is a separate energy system that can operate either in parallel with or islanded from main utility grid.It can not only improve the power quality and increase the reliability of the load side in the power supply,but also can promote the consumption of renewable distributed power.In case of fault occurs in power network,microgrid will be disconnected from utility grid to operate in islanded mode,at this time to maintain its voltage and frequency stability is the core of the research.In this paper,a frequency and voltage control strategy is proposed to solve the balance problem of power supply and demand of islanded microgrid and meet the load demand to the greatest extent.When the balance of supply and demand is large,a load shedding control strategy according to load priority is proposed.Microturbine uses droop control to make the frequency and voltage stable of the islanded microgrid.Combination branch of photovoltaic and battery use PQ control to provide the required active and reactive power according to the microgrid frequency and voltage droop.The simulation results verify the correctness and validity of the control strategy.The control strategy addressed to optimally regulate both voltage and frequency in islanded network has been implemented.In particularly,by using droop control and PQ controller together for controlling different micro sources in order to regulate frequency and voltage for guaranteeing the system frequency and voltage stability in additional,the approach of persevering the storage system which is allowing to intervene in economic dispatch in order to satisfy load demand in case of imbalance between generation and load demand.Eventually storage system will participate also in controlling charge and discharge mode at the same time to increase life time of storage system in order to avoid some technical problems regarding to the voltage and frequency deeply drop.Therefore,energy storage system has some impact on microgrid such as increasing of performance,store energy at the time of surplus and redispatch and energy storage system can increase the reliability of power system.Curretly some energy storage system which are marketable are battery,supe capacitor and flywheel etc.Islanded microgrid stability is assured without use of remote communications between different points of system.So,this allows the complete management of extended system in which remote connections are not appeared or temporarily out of service.Modern electrical network operators are obliged to respond to the different challenges such as:Load demand and growth as well as changes in the geographical distribution of customers;regarding new environment policy and the usual economic growth of the marketplace on the other hand is vital and should be budgeted.Usually a microgrid is made up by an interconnected loads and distributed energy resources within clearly defined electrical boundaries that act as a single controllable entity with respect to the grid.A microgrid can connect and disconnect from the utility grid to enable it to operate in both grid-connected or islanded-mode in order to provide a customized level of power quality of high reliability,stability and resilience to grid disturbances.In any ac power system,it is vital to regulate the amplitude and frequency of the voltages via the system.Moreover,it is well known that in an interconnected ac system,the steady-state frequency must be the same throughout regulated to 60 Hz in North America and 50 Hz in some countries by respecting some standards of frequency variations as well as voltage.If the loal frequency drops due to the technical problems which lead to the loss of generation,for example,the currents throughout the system will redistribute autonomously by producing electromagnetic torques that,initially,try to maintain synchronization system between generators.If,however,the angular displacement between generators becomes too large in the ensuing transients,a loss of synchronization can occur leading to other generators tripping off line,additional transients and in the worst,case a cascading blackout and brownout phenomena.This is achieved by ensuring sufficient spinning and non-spinning reserve and having the ability to control the frequency and voltage through the scheduling of generators within the system.frequency and voltage regulation is achieved by designing micro-sources(also known as Distributed Energy Resources or abbreviated by DERS)in order to provide an output-frequency-versus power characteristic similar to the speed-versus-power(droop)characteristics of conventional turbo-and hydro-generators.Moreover,voltage regulation strategies are patterned by depending upon the output-of voltage-versus-reactive-power characteristics of the automatic voltage regulators used in conventional turbo-and hydro-generators[6-9],[94-95].By ensuring frequency and voltage stability in range of standard,with uncertainties in both structure and parameters,is far from a trivial task.Mindful of these differences,applying control strategies patterned after the conventional power grid is questionable at the very least,and in the worst case,inappropriate.This advanced,integrated distribution system addresses the need for application in locations with electric supply and/or delivery constraints,in remote sites,and for protection of critical loads and economically sensitive development.Based on different definitions microgrid should be composed by different micro sources namely PV,Microturbine,wind turbine,battery storage,supercapacitors,fuel cell etc.The micro source such as wind power and photovoltaic cells,its output power is influenced by the weather and power has obvious intermittent.This micro source only produces the constant active power or executes the maximum power point tracking Therefore,it generally uses the PQ controller but the controlling is not only PQ but also droop control as well.The droop control can guarantee microgrid voltage and frequency stability.The micro source such as micro gas turbine,fuel cell and electric accumulator etc.,its controlling is easy and It can supply power quality to a small community,which can categorize from a residential district and an isolated rural community,to academic or public communities such as universities or schools,and to industrial sites.Industrial parks can be managed as microgrids,e.g.,to decrease the energy dependency,operate as low carbon business parks and increase the economic competitiveness.Control of microgrids becomes a critical issue for ensuring reliable operation of the microgrid.A variety of research has been carried out particularly on the subject of the regulation,control system and power management in order to improve the system stability.Each AC and DC microgrid has different hierarchical controls,but those could be generalize into three levels in reference to the hierarchical control standard of international society of automation(1)the primary control is based on the droop method,including an output-impedance virtual loop;(2)the secondary control allows the restoration of the deviations produced by the primary control;and(3)the tertiary control manages the power flow between the microgrid and the external electrical distribution system.The PQ controller and droop control should be able to control and manage power properly in both operating mode,grid-connected mode and stand-alone mode[2].The operating of microgrid in islanded mode would lead to more challenges,particularly when the imbalance of generation and consumption happen because of flexible load and DERs.An integration of energy storage system into the PQ controller and droop control will improve the controller performance;meanwhile,DC link capacitors support the voltage regulation,as in energy management which has a strong impact on power system stability and that will lead to the coordination of charging and discharging constraints against overcharging and deep charging and increase lifetime of some storage devices such as battery and so on,sometimes the storage devices aforementioned can be used as back-up system in case of the main micro sources becomes incapable to reach load demand according to the structure of microgrid.The integration of distributed generation(DG)at medium and low voltage,both in utility grid and downstream of the meter,is increasing in developed countries worldwide as well in poor countries.Usually one key economic potential of distributed generation application at customer premises lies in the opportunity to locally utilize photovoltaic array,battery storage system,fuel cell and biomass to electricity by reciprocating engine generators and some micro sources can produce heat and power at the same time.Moreover,some micro sources such as PV,biogas and another small renewable energy and controllable loads are expected to play a significant positive impact in future electricity and on environment sustainability.The use of microgrid which is composed by micro sources which cannot pollute an atmosphere surely,they will substantially reduce carbon emission,thereby contributing to the commitment of most developed countries as well as in cases of regional and government to achieve their greenhouse gas emissions reduction purposes,or otherwise substantially reduce their carbon footprints.Also,when the generation is approximately close to demand can facilitate to distribute power quality and reliability as well as stability of electricity delivered to sensitive endures.Which are main pillar of power system stability legislators.Indeed,distributed energy generations can be actively used to enhance power quality.Usually in power system stability and control frequency and voltage are very vital and ubiquitous meanwhile frequency and voltage of microgrid in islanded mode are very vulnerable due to the variation of load and microgrid subject to the main challenges namely:voltage oscillations,frequency oscillations and power quality issues but the frequency and voltage should vary between the accepted according to the standard which is very stringent.Regarding global warming the legislators has been obliged to provide some adequate methods and more incentive for renewable energy sources.As the output,it is expected that the future low voltage network will see and use array.Microturbine and battery energy storage system in abundance in order to ensure a balance between power system and consumption under all conditions and the facts which are regarding islanded operation mode.It has been discussed in different papers and in books that most of time generation and consumption are not balanced which provoke frequency and voltage fluctuation.The integration of photovoltaic array,battery and microturbine in islanded microgrid mode with efficient converters and their controllers and algorithm designed in state flow will make frequency and voltage to be regulated and remain in constraint.A single droop frequency control with microturbine has been used to control the frequency and voltage and PQ controller accompanied by an algorithm has been used by considering allowable limit of SOC of the storage battery system for the isolated micro grid.Microgrid in the islanded mode the frequency and voltage are very important and wherever are not controlled by the utility grid meanwhile frequency and voltage are depending upon active and reactive power although the dropping levels of frequency and voltage will provoke some technical problems namely blackout,brownout,harmonic distortion,voltage flicker,voltage unbalance and short circuit level and the current control techniques used in the grid-connected mode cannot assure the sustainable operation of the islanded mode.Therefore,some micro sources with ac output may have different frequency so that main bus need to use some devices in order to be integrated into an ac microgrid by respecting some standards.The implementation of droop control,the P-co and Q-E droop coefficients are used to emulate the inertia of an ac machine.The main challenges of implementing conventional droop control in ac systems with inverters are frequency and voltage deviations leading to tradeoffs inherent to droop control in islanded mode but Voltage and frequency are dependent upon load levels which may convey to the loss of synchronization.According to the principal of electrical power network analysis microgrid is not a robust system by comparing to a utility grid system.Hence,additional control strategies should be implemented for achieving a microgrid with stability,reliability and sharing performance of power quality.Most of distributed generation units cannot produce reactive power,thus,they cannot support voltage stability during dynamic state so that proper voltage and frequency controllers could be designed to maintain the voltage and frequency stability of the microgrid.Eventually,it is mandatory and vital to consider voltage regulation for planning and operation of distribution systems.In addition,this can be achieved by studying islanded microgrid system to see and analyze the influence of distributed generation on the system. |
PDF Full Text Request