Operation and Control of Hybrid Power Plants in LV Isolated Microgrid
Authors
Pedersen, Morten Virklund ; Sørensen, Sebastian Bille ; Expethit, Adrian
Term
4. term
Education
Publication year
2018
Submitted on
2018-06-01
Pages
186
Abstract
This thesis addresses how low-voltage isolated microgrids with high shares of renewables can be operated stably using simple, practical control principles. Focusing on a system comprising wind, PV, diesel generator sets (DGS), and battery energy storage (BESS), it targets the core question: how to design and tune device- and network-level control so that voltage and frequency remain stable across operating conditions? A review of 56 papers reveals a lack of practical guidance in the literature, which often favors complex algorithms over usable design steps, motivating a stepwise approach. The work characterizes component capabilities for V/f control and proposes an architecture where BESS and DGS act as grid-forming units when co-located (DGS regulates P/f and BESS Q/V via droops), with BESS taking over both when the DGS is off, while PV and WT operate as grid-following units. Each unit and the grid are modeled modularly in state space, and small-signal analysis using eigenvalues and participation factors identifies critical dynamics; the PLL and BESS voltage loop gains are shown to strongly influence stability, requiring careful tuning, particularly when the DGS is off. A voltage sensitivity analysis indicates that active power affects voltage more than reactive power at the load bus. At the network level, outer control loops are designed so BESS active and reactive power correct frequency and voltage errors, respectively, eliminating steady-state errors after large load steps. Dynamic results demonstrate stable operation under significant load changes, and the thesis concludes with a step-by-step guideline for designing device- and network-level control in isolated microgrids.
Denne afhandling undersøger, hvordan lavspændte, isolerede mikronet med høj andel af vedvarende energi kan drives stabilt ved hjælp af enkle, praktiske kontrolprincipper. Med udgangspunkt i et system bestående af vind, PV, dieseldrevne generatorer (DGS) og batterienergilagring (BESS) adresseres det centrale spørgsmål: hvordan udformes og tunes enheds- og netværksniveaukontrol, så spænding og frekvens holdes stabile under varierende driftsforhold? En gennemgang af 56 artikler viser, at litteraturen ofte prioriterer komplekse metoder frem for praktiske retningslinjer, hvilket motiverer en trinvis designprocedure. Afhandlingen karakteriserer komponenternes evner til V/f-kontrol og foreslår en kontrolarkitektur, hvor BESS og DGS agerer grid-forming, når de er samlokaliseret (DGS håndterer P/f og BESS Q/V via droops), mens BESS overtager begge ved slukket DGS, og PV/WT er grid-following. Hver enhed og nettet modelleres modulært i tilstandsrum, og småsignalanalyse med egenværdier og deltagelsesmatricer anvendes til at identificere kritiske dynamikker; især påvises stærk indflydelse fra PLL og BESS’ spændingssløjfe, som kræver omhyggelig tuning for at undgå ustabilitet, især når DGS er ude af drift. En spændingsfølsomhedsanalyse viser, at aktiv effekt påvirker spænding mere end reaktiv effekt ved lastbussen. På netværksniveau designes ydre sløjfer, hvor BESS’ P og Q korrigerer henholdsvis frekvens- og spændingsfejl, hvilket eliminerer stationære fejl efter store lasttrin. Dynamiske resultater demonstrerer stabil drift under betydelige lastændringer, og afhandlingen afslutter med en step-by-step guide til design af enheds- og netværkskontrol for isolerede mikronet.
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