Integrating Autonomous Load Controllers in Power Systems

Elektriske energisystemer undergår radikale forandringer, fordi et presserende behov for at nedsætte drivhusgasudledningen forudsætter en mere effektiv udnyttelse af energiressourcerne og en overgang til mere vedvarende energi. Nye vedvarende energikilder som vind og sol har et stort potentiale, men er karakteriseret ved en fluktuerende produktion, som kun delvist er forudsigelig. Styring af forbrug er allerede brugt i begrænset omfang for at forbedre leveringssikkerhed og effektiviteten af energisystemet. I energisystemer med en høj andel fluktuerende vedvarende energikilder kan intelligent styring af forbruget spille en stor rolle i balanceringen af systemet. Det store antal og den geografiske spredning af forbruget gør koordinering af forbrugets respons en udfordring. Nye kommunikationsteknologier har reduceret omkostningerne til at forbinde apparater og lover et ”Internet of Things" (”Tingenes internet") i fremtiden, hvor apparater er fuldt forbundet til en globalt datanetværk. Strenge realtids- og pålidelighedskrav til elsystemet har motiveret forskning i nye styrings arkitekturer velegnet til sådan et stort og komplekst system. Denne afhandling har fokus på et mellemstadie i evolutionen fra dagens passive belastninger mod et ”Internet of Things". Mere præcist udgøres dette mellemstadie af autonome apparater med sensorer, aktautorer, og software til at kontrollere lokale processer, men uden et digital kommunikationsinterface. Dearkitekturer der er undersøgt i denne afhandling er ret nye, så fokus ligger på gennemførlighed og system modelleringer. Tidligere forskning har foreslået brug af frekvensfølsomme autonome belastninger til at levere primær frekvensreserve. Denne forudgående forskning har fokuseret på effekten af autonome belastninger på et højt abstraktionsniveau i store energisystemer. Analyser på dette høj niveau analyser ignorerer en væsentlig forskel mellem konventionel frekvensereserve og frekvensfølsom belastning, nemlig effekten af reduceret belastningsmangfoldighed på frekvensresponsen. For at adressere denne mangel udførte man tidsdomænemodeller af frekvensfølsomme belastninger for at tage højde for den variation i frekvens responsen, som stammer fra variationen i belastningerne. Eksperimenter og analyser har afsløret potentielle ulemper ved høj andel af frekvensfølsom belastning: tidsafhængigheder i processer, som begrænser frekvensresponsen og overskridelse af spændingskrav i elforsyningsnettet. For at håndtere disse ulemper er to strategier fremlagt, som hver for sig tilføjer værdifulde tjenester udover at de forhindrer de førnævnte problemer. Den første strategi for at håndtere tidsafhængigheder er at drive et synkront netområde på ikke-nominelle frekvenser i diskrete domæner. Det begrænser uønsket skift af tilstand i de frekvensfølsomme belastninger og fungerer som direkte kontrol af den pågældende belastning. Store synkrone maskiner kan kun langsomt ændre frekvensens setpunkt, hvilket begrænser takten, hvorved kontrol kommandoer kan blive sendt. Derimod har energikilder, der er forbundet igennem effektelektronik, mulighed for at ændre frekvenssetpunkt meget hurtigt og kan skabe en strøm af kommandoer som kan tolkes med eksisterende kommunikations protokoller. Den anden strategi er at forene en spændingsfølsom styring med en frekvensfølsom styring, og på den måde direkte undgå uønskede spændinger. Denne spændingsfølsomme styring kan også blive brugt alene, uden den frekvensfølsomme del, for at stabilisere spænding og reducere behovet for netforstærkninger alle steder hvor lavere spænding falder sammen med højere forbrug. En frekvensfølsom styring er udviklet, implementeret, og testet under realistiske forhold. Resultaterne viste en stor potentiel ressource, i nogen tilfælde større end gennemsnittet af effektforbruget. Nøjagtigheden af belastningsmodeller var verificeret ved hjælp af måledata. En spændingsfølsom styring var udviklet, implementeret og testet under laboratorieforhold, og dens opførsel var simuleret i repræsentative energisystemer. Problemerne forårsaget af udbredt anvendelse af frekvensfølsomme belastninger var simuleret, og afværgelsesstrategier anvendt. For at underbygge gennemførligheden af det fremlagte frekvensbaserede belastningskontrolsystem er analyser af eksisterende energisystemer blevet gennemført med henvisninger til tekniske standarder, specifikationer og endeligt data indsamlet fra systemer i drift. Resultaterne viser, at frekvens- og spændingsfølsomme autonome belastninger er leveringsdygtige alternativer til konventionel frekvens- og spændingsregulerende teknikker. Når de bruges sammen, komplementerer de hinanden. I systemer, hvor operatøren har mulighed for at regulere frekvensen centralt, kan de direkte kontrollere de ellers autonome frekvensfølsomme apparater. Derudover, i systemer, hvor frekvens reguleringsressourcer tillader hurtigt skift af frekvenssetpunkt, for eksempel micro-grids, kan energikilder blive brugt som sender i et lavhastigheds-envejs- kommunikationssystem.Electric energy systems stand on the brink of radical change as the urgent need to reduce greenhouse gas emissions pushes more efficient utilization of energy resources and the adoption of renewable energy sources. New renewable sources such as wind and solar have a large potential, but they are characterized by variable generation that is only partly predictable. Managing loads is already used in limited circumstances to improve security and efficiency of the power system. In power systems with a large penetration of variable generation, load management has large role to play in adapting consumption to the fluctuating production. The large number and geographic dispersion of loads make coordinating their behavior challenging. New telecommunication technology has reduced the cost of linking devices, promising a future "Internet of Things" where loads are fully networked. Strict real-time constraints and reliability constraints in power systems are motivating research into new control architectures suitable for such a large and complex system. The focus of this thesis is on an intermediate stage of evolution between today's largely passive loads and a future "Internet of Things". Specifically, this intermediate stage is autonomous devices with sensors, actuators, and software to control local processes but without digital communications interfaces. The architectures explored in this thesis are newly emergent, so the focus is on feasibility and system modeling. Earlier research has proposed using autonomous load controllers to provide primary frequency reserves. This previous research has mainly focused on the effect of autonomous loads at a high level of abstraction, in large-scale power systems. High-level analysis ignores a significant difference between conventional frequency reserves and frequency-sensitive loads, namely the effects of reduced load diversity on the frequency response. To address this shortfall, time-domain models of the frequency-sensitive loads were constructed that include the variation of frequency response resulting from changes in load diversity. Experiments and analysis have revealed potential drawbacks of high penetrations of autonomous frequency-sensitive loads: time constraints on the underlying processes which reduce the frequency response, and violations of voltage constraints in the distribution systems arising from synchronized loads. Addressing these drawbacks, two mitigation strategies are proposed, each of which add valuable services in addition to preventing the above mentioned problems. The first strategy to address time constraints is to operate a synchronous power system at off-nominal frequencies in discrete domains, thus limiting unintended state changes of frequency-sensitive loads. The effect of operating in discrete frequency domains is to dispatch frequency-sensitive loads. Large synchronous machines can only change their frequency setpoint slowly, greatly limiting the rate of change of dispatch symbols. However, energy sources interfaced with power electronics can change their frequency setpoint very rapidly, creating a stream of symbols that can be decoded with conventional telecommunication protocols. The second strategy is to merge a voltage-sensitive control loop into the frequency-sensitive controller to directly avoid violations of voltage constraints. This voltage-sensitive controller can also operate alone, without the frequency-sensitive controller, to provide voltage regulation service and increase load diversity in any distribution network where lower voltage level corresponds to higher load.The frequency-sensitive load controller has been designed, implemented, and tested in real-life settings. Its performance demonstrated a large potential resource, in some cases greater than the average power consumption. The accuracy of load models was validated by comparison with field data. A voltage-sensitive controller was designed, implemented in an embedded system, and tested in laboratory settings. The voltage-sensitive controller was also implemented in a software simulation environment and tested in representative distribution systems. The problems anticipated by large-scale deployment of frequency-sensitive loads were simulated, and mitigation strategies were applied. To support the feasibility of the proposed frequency dispatch system, analysis of existing power systems was conducted using existing technical norms, specifications, and data collected from operating power systems. The results shows that frequency-sensitive and voltage-sensitive autonomous load are viable alternatives to conventional frequency and voltage control devices. When used in combination, they complement each other. In systems where the operator has centrally dispatchable resources to regulate frequency, these resources can be used to dispatch otherwise autonomous frequency-sensitive loads. Moreover, where centrally dispatchable frequency regulation resources can rapidly change operating points, such as in a micro-grid, the energy sources can be used as transmitters for a ultra-low-bandwidth uni-directional power line communication system

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