Stephan Thomas

The Balance of Renewable Sources and User Demands in Grids: Power Electronics for Modular Battery Energy Storage Systems

The continuously growing amount of renewable sources starts compromising the stability of electrical grids. Contradictory to fossil fuel power plants, energy production of wind and photovoltaic (PV) energy is fluctuating. Although predictions have significantly improved, an outage of multi-MW offshore wind farms poses a challenging problem. One solution could be the integration of storage systems in the grid. After a short overview, this paper focuses on two exemplary battery storage systems, including the required power electronics. The grid integration, as well as the optimal usage of volatile energy reserves, is presented for a 5- kW PV system for home application, as well as for a 100- MW medium-voltage system, intended for wind farm usage. The efficiency and cost of topologies are investigated as a key parameter for large-scale integration of renewable power at medium- and low-voltage.

Behavior and Loss Modeling of a Three-Phase Resonant Pole Inverter Operating with 120° Double FlatTop Modulation

This paper describes the loss modeling and optimization of a grid-connected resonant pole inverter operating with 120deg double flattop modulation (Rigbers et al., 2005). The proposed architecture achieves soft-switching with highly reduced current stresses compared to the conventional resonant pole inverter (Divan and Skibinski, 1987) and (Divan et al., 1988) without additional components. This concept is based on the fact that a symmetrical three-phase system delivers or draws always constant power to/from the grid with 120deg phase shifted sinusoidal phase voltages of same amplitude and coherent sinusoidal phase currents. Simple analytic behavior and loss models for all active and passive components are derived and verified using a 5 kW prototype

Design and construction of a test bench to characterize efficiency and reliability of high voltage battery energy storage systems

Stationary battery energy storage systems are widely used for uninterruptible power supply systems. Furthermore, they are able to provide grid services. This yields in rising installed power and capacity. One possibility uses high voltage batteries. This results in an improvement of the overall system efficiency. High voltage batteries may be advantageous for future medium voltage DC-grids as well. In all cases, high availability and reliability is indispensable. Investigations on the operating behavior of such systems are needed. For this purpose, a test bench for high voltage storage systems was built to analyze these processes for different battery technologies. A special safety infrastructure for the test bench was developed due to the high voltage and the storable energy of approximately 120 kWh. This paper presents the layout of the test bench for analyzing high voltage batteries with about 4,300 volts including all components, the safety requirements with the resultant safety circuit and the aim of the investigations to be performed with the test bench.

Development of a Modular High-Power Converter System for Battery Energy Storage Systems

The integration of storage systems into the grid is becoming increasingly important due to the growing amount of volatile power sources. This paper shows how to design a modular battery energy storage system (BESS) for medium voltage grids. Typically, this system is scalable in power rated from 5 MW up to 100 MW with a storage capacity of several hours. Using power electronic building blocks (PEBBs) a converter for dc grids and ac grids can be built. In this paper, the chosen topology for the ac solution is the cascaded cell converter. The focus is to determine the optimum number of levels, the modulation technique to avoid microcycles of the batteries and to present the efficiency. A formula to calculate the parasitic capacitance of lead-acid batteries is shown and verified by measurements, which is important for the design of such a converter system. Moreover, a new charging strategy for LiFePO4 — the chosen battery technology for the proposed storage system — is introduced, which prolongs the batterys life, reduces the charging time and decreases the life cycle cost. The proposed charging strategy and battery technology compared to lead-acid batteries is also economically evaluated.

Unsymmetric Control of a Matrix Converter for Two-Phase Inductive Melting Furnaces

Currently, induction motor drives and other symmetrical, three-phase inductive loads are the main applications of matrix converters. If only two loads are connected to matrix converters, the conventional control algorithms cannot be applied as usual due to the unsymmetrical load. To extend the application of matrix converters to unbalanced loads this paper derives a novel angle-modulation strategy and illustrates how to alter a carrier based PWM modulation algorithm to control the output phases of the matrix converter independently. Eventually, system level simulation results are provided to verify the feasibility.