IEES - Institut für Elektrische Energiesysteme
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In the future more and more conventional power plants, which provide ancillary services such as provision of reactive power for voltage control and primary control power (active power) for frequency control to the transmission system and thus secure the energy supply, are going to be replaced by renewable energy sources. Due to this fact new concepts for providing these services by renewable power sources will be necessary in the future to maintain stability of the network operation. This refers to the delivery of active and reactive power by distributed generation (DG) and distributed storage (DS). Beyond that DG and DS can be found nowadays in households or also called nanogrids. This research work presents a concept of a nanogrid that can provide ancillary services to the distribution grid in the low voltage level and transfer reactive power as well as primary control power to higher voltage levels by upscaling, which means the connection of many nanogrids. The implementation of the concept is done in a real system and also in a simulation environment that uses simplified mathematical models.
This research work presents an operation mechanism for supplying a scheduled value of reactive power at the medium voltage (MV) side of the distribution transformer by a group of interlinked nanogrids in the low voltage level (LV), which are part of a topological power plant (TPP). The operation mechanism takes into account the self-consumption of the nanogrid and network constraints, such as the permissible voltage band for each node and the loading of the transformer as well as cables. Furthermore, the minimization of active power losses within the TPP is taken into consideration while the scheduled reactive power at the MV side should be accomplished.
Due to the increasing share of volatile renewable energy sources, like photovoltaics (PV) and wind energy in nearly Zero Energy Buildings (nZEB), there is an increasing need for demand-side management (DSM) or demand response (DR) programs to balance the production and consumption in the grid. The flexibility that can be obtained for smart grids from such DR methods is not limited to appliances like water heaters or dishwashers but can also be achieved with space heating and air-conditioning. In such an interdisciplinary investigation, often one part is simplified, in this case, typically either the thermal models or the implemented DR strategy are very detailed. In this work, a detailed thermal model of a control center is obtained and calibrated in IDA ICE building-modelling software with measurements from a test site in Germany. Afterward, several price-based load matching algorithms are applied to the model to see the possible flexibility exploitation with the thermal capacity of this small building. Not all investigated algorithms show good performance but some of them show promising results. Thus, this model can be used for DR methods and should be extended to work with more DSM strategies and provide ancillary services.
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.
The massive development and implementation of small- and medium-scale generation units within the distribution grids result in novel needs and options for ancillary services. The project DeCAS develops solutions for the coordinated activation of ancillary cross-voltage-level services considering the different objective functions of individual voltage levels. This study aims to analyse, define and describe common requirements focusing on a technical, stakeholder and market perspective.
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.
The increasing decentralization of the power supply poses new challenges to the market integration of renewable energies. One concept for the market integration of distributed energy resources (DER) is the virtual power plant (VPP). A VPP aggregates a portfolio of smaller DERs and operates them as a unified and flexible resource on the energy and balancing power markets. The operation planning of the market participation is, however, characterized by uncertainties, mainly due to the forecasting errors of renewable energy resources. Thus, this paper presents a model for the multi-market operation planning plan of a VPP with regard to the forecasting errors of the associated DERs. The spot- and intraday markets and the balancing power markets are modeled individually to ensure a valid market participation of the VPP. The forecasting errors are integrated in the planning process through a two-stage stochastic optimization. Exemplary results show the impact of controllable DERs in the planning process of a VPP consisting of a storage system and several PV power plants.