Modelling, Control and Integration of Distributed Generators for Enhanced Ancillary Services [article]

Pietro Raboni
2016 Ph.d.-serien for Det Teknisk-Naturvidenskabelige Fakultet, Aalborg Universitet  
he was R&D engineer in the Central PhotoVoltaic Inverter team of ABB DMPC PG Solar, formerly Power-One, in Valdarno (IT). During this time he was involved in the collaborations with Corporate Research Centres (CH-SE-US-CN) and with onsite researches in the field of control and integration of grid connected PV inverters. Moreover he took responsibility for development of PhotoVoltaic Inverter model portfolio (using DIgSILENT and Simulink), for grid code and patent reviewing and for the
more » ... n and assessment of Grid Impact Studies for utility scale plants (approx. 110 MW installed in the EMEA markets). He developed the prototype of a controller for "hybrid systems" and participated to the commissioning of Gasfinolhu (MAL) microgrid. Since December 2015 he has been PowerStore specialist in ABB PG, formerly Power System Power Generation, just for its integration in microgrids. He is co-author of 5 ABB Invention Disclosures and he has 2 pending patents. The author has interests in the modelling and control of inverters, for both grid connected and islanded operations, and in the optimized operation of renewables considering weather forecasts. III ENGLISH SUMMARY The increasing share of Distributed Generators (DGs) poses concerns about network stability and their fair participation to the electricity market. Their aggregation in Virtual Power Plants, sometimes in combination with load prosumers, could represent an answer to these issues. Nevertheless Technical Virtual Power Plant still offer limited ancillary services, as reactive power management and spinning reserve. Focus of this thesis is on the control of DGs for providing further and higher quality ancillary services. In particular LV dispatchable DGs, hereinafter called flexible DGs, as stationary battery units (SBUs), microturbines (MTs) and Diesel Engines (DEs) appear promising solutions. The models of a SBU derived from a commercial Electric Vehicle battery and a MT are proposed. Both units are Voltage Source Converter (VSC) interfaced and, following recent Power Electronics state of the art, they are equipped with an LCL output filter. The thesis starts deepening the design of current control scheme for a stable operation in grid connected mode. Such flexible units offer on the other hand the possibility to survive a LV feeder in the future case of intentional islanding or for black start service. Therefore the design of the voltage regulator is developed proposing the compensation of the voltage drop on to grid side inductance. Moreover considering the hierarchical control a comparison between direct and reverse droop configurations is proposed. The participation to local reactive power market implies the precise control of reactive power through DGs. While reactive power regulator tuning is straightforward in case of current controlled VSCs, it is more challenging in case of synchronous generator interfaced DGs, as DEs and small CHP units, due to the high system order introduced considering the electrical machines, real exciter and network equations. The analytical tuning of the same regulator is here developed starting from small signal analyses of the plant, with an approach fitting to any reactance to resistance ratio of the equivalent Thevenin model regarded for representing the network. Such time consuming tuning procedure is then compared with an automated regulator design based on multiple time domain runs driven by a Simplex Optimisation. In particular some novel objective functions relying on typical step response indexes are proposed and assessed. Modelling, Control and Integration of Distributed Generators for Enhanced Ancillary Services IV The last chapter moves the research focus from DG controllers to the aggregated response of a Distribution Network (DN), highly populated by DGs, for large voltage stability system studies. Indeed while DGs dynamic reactive support and disconnection thresholds are well defined by grid codes, the overall response of the DN where they are connected is almost unknown. Just for this reason Transmission System Operators (TSOs) are used to represent a DN with an equivalent MV busbar where a handful of typical DGs and lumped loads are connected. In this chapter differently it is proposed to model part of the DN with an equivalent adaptive node, whose active and reactive power response is dependent by the voltage at the interconnection point and by main external factors affecting the response (i.e. the irradiance for a photovoltaic plant and the wind speed for a wind power plant). The proposed method is time independent and it relies on the assumption of the absence of slow dynamics in the DN to be reduced. Results are validated comparing the response of the reduced DN to the corresponding detailed model. The overall research is carried with PSCAD®, MATLAB® and its toolboxes. This thesis was financed within the project Development of a Secure, Economic and Environmentally-friendly Modern Power System (SEEMPS) supported by Danish Council for Strategic Research (DSF 09-067225) V DANSK RESUME Den øgede maengde decentrale anlaeg udgør en udfordring for systemstabilitet og deltagelse på en fair måde i elmarkedet. Aggregering i virtuelle kraftvaerker, undertiden også kombineret med kunder, der producerer el, kan vaere et svar på disse udfordringer. På den anden side tilbyder sådanne virtuelle kraftvaerker kun systemydelser i begraenset omfang, her taenkes på eksempelvis reaktiv effektkontrol og roterende reserver. Denne afhandling har fokus på kontrol af decentrale anlaeg med henblik på at yde en bedre kvalitet systemydelser. Saerligt fleksible anlaeg såsom stationaere batterier (SBU), mikroturbiner og dieselmaskiner fremtraeder som attraktive løsninger. Modeller af SBU er afledt fra kommercielle elbil batterier og en mikroturbine er udviklet. Begge enheder er udstyret med netinterface med voltage source converter VSC og i henhold til nutidige standarder for effektelektronik, herunder udstyret med LCL-filter. Afhandlingen laegger ud med at uddybe strøm regulering med henblik på stabil drift i nettilsluttet tilstand. Fleksible enheder kan bruges til opstart af lavspaendingsnet under Ø-drift og ved opstart fra dødt net. Design af spaendingsregulering er derfor udviklet til at kompensere for spaendingsfald henover netinduktans. Yderligere er det overvejet at udføre en hierarkisk regulering der foretager en sammenligning med både direkte og revers droop konfigurationer. Deltagelse i local reaktiv effektkompensering i elmarkedet nødvendiggør en praecis regulering af reaktiv effect fra decentrale anlaeg. Reaktiv effektregulering er ganske ligetil når der anvendes VSC netinterface, hvorimod det er mere udfordrende når der anvendes direkte tilkoblede synkrongeneratorer som i små kraftvarmevaerker. Dette skyldes det samlede systems orden er højere på grund af de elektriske maskiners dynamik, deres magnetiseringsmaskiner og netmodellering. Analytisk tuning af regulatorer er udviklet ud fra en småsignal analyse af anlaeggene under anvendelse af en passende fitting (kurvetilpasning) af R/X forholdet for den aekvivalente Thevenin model, der repraesenterer nettet. Dette er tidskraevende og vil blive sammenlignet med et automatiseret regulator design baseret på multiple tidsdomaene simuleringer under anvendelse af Simplex optimering. Saerlige nye objektfunktioner der baserer sig på typiske enhedsstep svar er foreslået og vurderet. Sidste kapitel flytter det videnskabelige fokus fra de decentrale anlaeg og disses regulering som enkeltenheder og til en aggregeret respons af et samlet dis-Modelling, Control and Integration of Distributed Generators for Enhanced Ancillary Services VI tributionsnet DN, indeholdende mange decentrale enheder. Der udføres studier af spaendingsstabilitet. Upåagtet nettilslutningsbestemmelser specificerer reaktiv effektregulering og udkoblinkskriterier for de enkelte anlaeg er den samlede respons af sammensatte net/anlaeg som en aggregeret enhed ikke velkendt. Derfor anvender TSO (transmissions system operatør) i dag DN som en standard samleskinne model indeholdende en standard maengde DG og belastninger. I dette kapitel foreslås at modeldanne dele af DN som et aekvivalent knudepunkt, hvis aktiv-og reaktiv effektrespons afhaenger af spaendingsprofilen i tilslutningspunktet og eksterne faktorer som for eksempel solintensitet og vindhastighed. Den foreslåede model er tidsuafhaengig og bygger på fravaer af langsom dynamik i DN, der reduceres til et aekvivalent knudepunkt. Resultaterne er valideret ved sammenligning med simuleringer af detaljerede modeller. Resultaterne er udført under anvendelse af PSCAD, MATLAB og dennes vaerktøjskasser. Afhandlingen er finansieret af den Danske stat (DSF) og industrielle partnere som en del af SEEMPS projektet. VII ACKNOWLEDGEMENTS I express my sincere gratitude to my supervisor in this work, Prof. Zhe Chen, for the PhD opportunity, his kind guidance and patience.
doi:10.5278/vbn.phd.engsci.00051 fatcat:64gj73paizatzj7cjstxuuaf3y