Grid Converters for Stationary Battery Energy Storage Systems

Research output: Book/ReportPh.D. thesis

Abstract

The integration of renewable energy sources in the power system, with high percentage, is a well known challenge nowadays. Power sources like wind and solar are highly volatile, with uctuations on various time scales. One long term solution is to build a continentwide or worldwide supergrid. Another solution is to use distributed energy storage units, and create virtual power plants. Stationary energy storage is a complementary solution, which can postpone the network expansion and can be optimized for dierent kind of grid services. As an energy storage solution with timing for few seconds to hours, rated at MW and MWh, battery energy storage systems are suitable and ecient solutions.
Grid connection of the storage system can be done at dierent voltage levels, depending on the location and application scenario. For high power and energy ratings, increase in the battery and converter voltage ratings can enhance the overall system eciency.
This work is divided in two parts, "Control of DC-AC Grid Converters" and "Medium Voltage Grid Converters for Energy Storage". The rst part starts with a brief review of control strategies applied to grid connected DC-AC converters. A control implementation was realized for a 100 kW active rectier to be used in a 6 kV battery energy storage test bench. In the second part, dierent solutions for power converters to interface energy storage units to medium voltage grid are given. A new modular multilevel converter concept is introduced, where the energy storage units are integrated in each converter cell.
The control of DC-AC grid converters has been a research subject for more than a century, and there is still place for improvements. A review of the main control principles is given in the rst part. The stationary frame control was implemented for a low-voltage 100 kW bidirectional grid converter, to be used in a high voltage battery energy storage test bench. The control structure proved to be stable without damping. The converter was tested in the test bench and the experimental results are presented.
Multilevel converters are replacing the classical two-level converters more and more, on a large variety of applications. For medium voltage applications, multilevel converters are a necessity. The second part presents a review of hard-switched and soft-switched multilevel converter topologies for medium voltage. Four converter topologies were chosen as potential solutions for direct connection of battery energy storage systems to the grid. An evaluation is done, in terms of semiconductors requirements and losses, output voltage quality and common mode voltage.
The main advantage of batteries direct connection to the grid is the high efficiency potential. However, this solution is suitable only for battery technologies with low voltage variation. It is also necessary to build a battery system with high amount of serial connected cells, and the knowledge in this eld is still limited nowadays. Therefore, twostage converters solutions were introduced to overcome these disadvantages. Modular multilevel converters can make use of battery voltage technologies where the maturity and reliability is well proven in industry.
Cascaded H-bridge topology with bidirectional boost converters is proposed to interface low voltage batteries to the medium voltage grid. A control structure based on single phase control is proposed. It balances the capacitor voltages and the state of charge of batteries from dierent cells. A semiconductor loss analysis is performed and it shows the loss distribution in the converter cell and the eciency over a wide battery voltage variation.
A new modular multilevel converter structure with integrated energy storage is introduced.
This converter structure is suitable to interface low and medium voltage energy storage units to medium and high voltage grids. It can also interconnect a DC and AC grid with bidirectional power ow, were both can be backed-up for the distributed energy storage units installed in each converter cell. The converter operation and control methods are presented, and the energy storage system construction concept and challenges are addressed.
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The integration of renewable energy sources in the power system, with high percentage, is a well known challenge nowadays. Power sources like wind and solar are highly volatile, with uctuations on various time scales. One long term solution is to build a continentwide or worldwide supergrid. Another solution is to use distributed energy storage units, and create virtual power plants. Stationary energy storage is a complementary solution, which can postpone the network expansion and can be optimized for dierent kind of grid services. As an energy storage solution with timing for few seconds to hours, rated at MW and MWh, battery energy storage systems are suitable and ecient solutions.
Grid connection of the storage system can be done at dierent voltage levels, depending on the location and application scenario. For high power and energy ratings, increase in the battery and converter voltage ratings can enhance the overall system eciency.
This work is divided in two parts, "Control of DC-AC Grid Converters" and "Medium Voltage Grid Converters for Energy Storage". The rst part starts with a brief review of control strategies applied to grid connected DC-AC converters. A control implementation was realized for a 100 kW active rectier to be used in a 6 kV battery energy storage test bench. In the second part, dierent solutions for power converters to interface energy storage units to medium voltage grid are given. A new modular multilevel converter concept is introduced, where the energy storage units are integrated in each converter cell.
The control of DC-AC grid converters has been a research subject for more than a century, and there is still place for improvements. A review of the main control principles is given in the rst part. The stationary frame control was implemented for a low-voltage 100 kW bidirectional grid converter, to be used in a high voltage battery energy storage test bench. The control structure proved to be stable without damping. The converter was tested in the test bench and the experimental results are presented.
Multilevel converters are replacing the classical two-level converters more and more, on a large variety of applications. For medium voltage applications, multilevel converters are a necessity. The second part presents a review of hard-switched and soft-switched multilevel converter topologies for medium voltage. Four converter topologies were chosen as potential solutions for direct connection of battery energy storage systems to the grid. An evaluation is done, in terms of semiconductors requirements and losses, output voltage quality and common mode voltage.
The main advantage of batteries direct connection to the grid is the high efficiency potential. However, this solution is suitable only for battery technologies with low voltage variation. It is also necessary to build a battery system with high amount of serial connected cells, and the knowledge in this eld is still limited nowadays. Therefore, twostage converters solutions were introduced to overcome these disadvantages. Modular multilevel converters can make use of battery voltage technologies where the maturity and reliability is well proven in industry.
Cascaded H-bridge topology with bidirectional boost converters is proposed to interface low voltage batteries to the medium voltage grid. A control structure based on single phase control is proposed. It balances the capacitor voltages and the state of charge of batteries from dierent cells. A semiconductor loss analysis is performed and it shows the loss distribution in the converter cell and the eciency over a wide battery voltage variation.
A new modular multilevel converter structure with integrated energy storage is introduced.
This converter structure is suitable to interface low and medium voltage energy storage units to medium and high voltage grids. It can also interconnect a DC and AC grid with bidirectional power ow, were both can be backed-up for the distributed energy storage units installed in each converter cell. The converter operation and control methods are presented, and the energy storage system construction concept and challenges are addressed.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages147
ISBN (Electronic)978-87-89179-83-4
StatePublished - 2011
Publication categoryResearch

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