Control of Flywheel Energy Storage Systems in Electrical Vehicle Charging Stations

Research output: Book/ReportPh.D. thesisResearch

Abstract

Growing environmental awareness and strong political impetus have resulted in plug-in electric vehicles (PEV) becoming ever more attractive means of transportation. They are expected to have a significant impact to the overall loading of future distribution networks. Thus, current distribution grids need to be updated in order to accommodate PEV fleets, which are recognized in smart grid (SG) objective. The prevailing concern in that sense is the combined impact of a large number of randomly connected PEVs in the distribution network. On the other hand, continually growing PEVs are likely to impose more specific and acute challenges in short term, it is also expected to expect that grid operators will impose strict demand-response requirements for the operation of charging stations (CS)s.

Accordingly, this PhD project proposed a fast charging station structure which is combined with flywheel energy storage system (FESS). The proposed PhD project supports a corresponding smart control strategy that could be termed “charging station to grid (CS2G)”. It explores the possibility of using a dedicated energy storage system (FESS) within the charging station to alleviate grid and market conditions but not compromise the PEV’s battery charging algorithms or place the daily routine of the PEV owners in jeopardy. The overall control of FCS is divided into two layers organized into a hierarchical structure with the layer being the closest to the physical equipment termed as primary layer and the one on top of it as secondary layer. Control design is hence carried out by following the common principle for management of both large interconnected and small distributed generation (DG) systems.

For the purpose of control optimization and parameter tuning of the primary layer, detailed modeling of grid ac/dc and FESS converters is built and analyzed. |Based on modeling analysis, centralized and distributed control methods are both explored to realize the coordination control of each components in the system. Specially, this project proposes a “dc voltage vs speed” droop strategy for FESS control based on distributed bus signaling (DBS) concept. Then the concept is extended to apply for control of multi-parallel FESS structure. Additionally, an adaptive dc bus voltage control for grid converter is proposed to enhance the system stability and efficiency.

Aiming at alleviating the unexpected conditions in grid-side and providing ancillary services to distributed network, multi-functional controller in secondary control layer which enables four-quadrate operation ability is proposed to cope with different scenarios, such as PEV sudden connection and disconnection, active power compensation (load shifting), reactive power compensation, loss of grid power. Moreover, Centralized and distributed secondary control methods are explored and compared; especially a dynamic consensus control concept is applied into the system for coordinating paralleled grid interfaces and FESS.

Furthermore, stability issues are discussed and analyzed based on proposed control algorithm feature. First, small-signaling model of each component are built to study the dynamic stability of system operating at different stages in details. Due to the switching modes existing in the system, stability of switching system is studied based on common Lyapunov function method when the system switches its operation behavior between two modes.

Finally, a downscaled FCS prototype with FESS is built in the intelligent MG lab, and experiments and hardware-in-loop simulation results are conducted to verify the effectiveness and feasibility with the proposed FCS concept, control schemes, modeling and stability analysis.
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Growing environmental awareness and strong political impetus have resulted in plug-in electric vehicles (PEV) becoming ever more attractive means of transportation. They are expected to have a significant impact to the overall loading of future distribution networks. Thus, current distribution grids need to be updated in order to accommodate PEV fleets, which are recognized in smart grid (SG) objective. The prevailing concern in that sense is the combined impact of a large number of randomly connected PEVs in the distribution network. On the other hand, continually growing PEVs are likely to impose more specific and acute challenges in short term, it is also expected to expect that grid operators will impose strict demand-response requirements for the operation of charging stations (CS)s.

Accordingly, this PhD project proposed a fast charging station structure which is combined with flywheel energy storage system (FESS). The proposed PhD project supports a corresponding smart control strategy that could be termed “charging station to grid (CS2G)”. It explores the possibility of using a dedicated energy storage system (FESS) within the charging station to alleviate grid and market conditions but not compromise the PEV’s battery charging algorithms or place the daily routine of the PEV owners in jeopardy. The overall control of FCS is divided into two layers organized into a hierarchical structure with the layer being the closest to the physical equipment termed as primary layer and the one on top of it as secondary layer. Control design is hence carried out by following the common principle for management of both large interconnected and small distributed generation (DG) systems.

For the purpose of control optimization and parameter tuning of the primary layer, detailed modeling of grid ac/dc and FESS converters is built and analyzed. |Based on modeling analysis, centralized and distributed control methods are both explored to realize the coordination control of each components in the system. Specially, this project proposes a “dc voltage vs speed” droop strategy for FESS control based on distributed bus signaling (DBS) concept. Then the concept is extended to apply for control of multi-parallel FESS structure. Additionally, an adaptive dc bus voltage control for grid converter is proposed to enhance the system stability and efficiency.

Aiming at alleviating the unexpected conditions in grid-side and providing ancillary services to distributed network, multi-functional controller in secondary control layer which enables four-quadrate operation ability is proposed to cope with different scenarios, such as PEV sudden connection and disconnection, active power compensation (load shifting), reactive power compensation, loss of grid power. Moreover, Centralized and distributed secondary control methods are explored and compared; especially a dynamic consensus control concept is applied into the system for coordinating paralleled grid interfaces and FESS.

Furthermore, stability issues are discussed and analyzed based on proposed control algorithm feature. First, small-signaling model of each component are built to study the dynamic stability of system operating at different stages in details. Due to the switching modes existing in the system, stability of switching system is studied based on common Lyapunov function method when the system switches its operation behavior between two modes.

Finally, a downscaled FCS prototype with FESS is built in the intelligent MG lab, and experiments and hardware-in-loop simulation results are conducted to verify the effectiveness and feasibility with the proposed FCS concept, control schemes, modeling and stability analysis.
Original languageEnglish
PublisherAalborg Universitetsforlag
Number of pages41
ISBN (Electronic)978-87-7112-832-1
DOI
Publication statusPublished - 2017
Publication categoryResearch
SeriesPh.d.-serien for Det Teknisk-Naturvidenskabelige Fakultet, Aalborg Universitet
ISSN2246-1248

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