Biechl, Helmuth
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Institute
This paper investigates the use of common thermal storage systems for demand side management in off-grid situations for nearly zero energy buildings. Typical parameters and characteristics were analyzed to develop mathematical models for freezers, water heaters and space heating/cooling. The models used in this work are based on simplified equations derived from differential equations. Simplified models of a battery storage and a PV-system have been added. Models for the thermal storages, PV-system and battery storage were merged to one system model. All models and simulations were designed and conducted with Matlab. Various pre-defined price based set point calculation algorithms were modified to work with the off-grid system based on the system’s voltage and available PV-power. Voltage and battery’s state of charge based algorithms are developed in this work. In a system with a freezer, water heater and space heating/cooling that is powered by a PV-system only, a possible battery storage capacity reduction of up to 50% with PV-power based and up to 36% with SOC based algorithms compared to the same system with fixed set point thermostatic control could be achieved. Additionally, the capacity could even be reduced by up to 18% by solely reacting to voltage drops.
The research work presents an approach to set-up simplified mathematical models of microgrid components based on detailed models. The verification is done by a comparison with measurement results of a real system. Using simplified models allows an accurate analysis and optimization of the dynamic behavior of existing as well as planned microgrids. The paper shows simulation and measurement results for different combinations of microgrid components in island mode operation.
Mathematical modeling and dynamic behavior of a Lithium-Ion battery system for microgrid application
(2016)
This paper deals with the analysis and simulation of a stationary battery system for microgrid application, where the system structure including battery cells, inverters, filters, transformers, control system and a simplified grid model is described and modeled mathematically. For the simulation of the whole system the software PSCADTM is used. In the first part several equivalent circuit models for Lithium-Ion cells will be compared in order to model the dynamic behavior of the battery system. Particularly the evaluation of the effect of the model's complexity on the dynamics of the entire system will be investigated. In the second part, the dependency of state of charge (SOC), temperature and aging effects of the Lithium-Ion cells on electrical system quantities will be shown. It is also investigated the fact that a high frequency battery model has to be taken into account to describe the cells' dynamics if an inverter with Pulse Width Modulation is used.
Determination methods for controller parameters of back-to-back converters in electric power grids
(2016)
The paper presents a new optimization method for PI controllers of back-to-back voltage source converters using a vector control scheme to enable the control of active and reactive power transmission between two independent grids, for example, an emulator as a load or a source between the medium voltage distribution grid and a low voltage island grid. The control principle based on three phase systems in dq-components enables an independent control of active and reactive power with a simple structure using PI controllers. The presented optimization method using pole placement (PP) technique for tuning of the controllers leads to a higher degree of freedom and therefore to better results compared to the modulus optimum (MO) optimization method discussed in [1], [2]. A cascaded control model consisting of inner current and outer power/voltage control loops is being used for the optimization of the system's transient response. The mathematical modeling of the control system as well as the evaluation of the controller parameters are described in detail. A comparison of the presented optimization method for controllers with existing methods is shown by simulation results using the software PSCAD.
Bei dem Verbundvorhaben IREN2 (Zukunftsfähige Netze für die Integration Regenerativer Energiesysteme), das im Rahmen der Förderinitiative "zukunftsfähige Netze" durchgeführt wurde, lag der Fokus auf der anwendungsorientierten Forschung und Entwicklung auf dem Gebiet "Intelligenter Verteilnetze". Es wurden Verfahren und Konzepte erarbeitet, wie Verteilnetze mit hohem Anteil an regenerativer Energieerzeugung als inselfähige Microgrids stabil und zuverlässig betrieben werden können.
Microgrids in island mode with high penetration of renewable energy sources in combination with gensets and battery storage systems need a control system for voltage and frequency. In this study the main goal is maximization of the energy feed-in by renewable sources. Therefore it is necessary to keep the State of Energy for the Battery Storage System in a range that the excess energy can be absorbed and used in a later period of the day. In this paper an approach for State of Charge scheduling based on load and generation prediction is described.
This paper presents the fundamentals of a method how to determine the state of charge (SOC) of lithium-ion batteries on the basis of two different equivalent circuit diagrams and an extended Kalman filter (EKF). It describes how to identify the parameters of these circuits by characteristic measurements. The comparison between measurement and computation results shows a good accordance. In the first step the dependency of these parameters on the temperature and on the battery age is neglected.