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Microgrids can be operated in on-grid mode, but also in off-grid mode (island operation). In off-grid mode, grid forming units have to ensure the grid's voltage and frequency stability. For more than one grid forming unit, the active and reactive power sharing has to be handled. This paper presents a method for voltage and reactive power control for systems without a superordinated control system or a communication link between the grid forming units. A failsafe concept is included, that means that a stable operation is given also in case that one grid forming unit is disconnected.
Microgrids with a high penetration of distributed generation (DG) in combination with energy storage systems (ESS), but also in combination with fuel-driven generation units (gensets) can be operated in on-grid mode, but also in off-grid mode (island operation). For grid restoration in island mode, a black start strategy is needed. This scientific work deals with a black start concept for island grids with a high amount of non-controllable DG units and non-controllable loads which is investigated by mathematical modeling and simulation for different scenarios. The assumed underlying control behavior of the DG units is described in the German application guide VDE-AR-N 4105. The corresponding mathematical modeling is presented and a verification by specific measurements is presented.
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.
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.
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.
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.
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.
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.
To reduce greenhouse gas emissions, volatile energy production from renewable sources is highly encouraged by international agreements. This leads to balancing challenges of demand and supply which can be addressed with smart grids or even smart city concepts. Demand side management control strategies for flexibility harvesting often include energy storage systems, like flywheel- (FESS) and battery (BESS) storages. To investigate different control strategies for a hybrid energy storage system with a flywheel and battery storage in an islanded microgrid, an existing flywheel is modernized with state-of-the-art components to support real time power hardware in the loop simulations. Testing a load levelling control strategy with this test bench showed that the cyclic lifetime of the battery storage system could be increased with peak shaving due to a reduced amount of charging and discharging operations. An excessive energy buffering control method could increase the islanded operation time by using nearly 10% of the otherwise lost energy. However, these results with the testbench showed limited use for research with the current setup due to low capacity and high self-discharge rate of the existing FESS. But due to the MATLAB-based programming interface, it is perfectly suitable as an educational setup for the demonstration of possible implementations of the European Green Deal.
The paper presents the dynamic modeling and stability analysis of Low Voltage (LV) microgrids in island operation using simplified electrical models for Distributed Generations (DGs). These simplified models are used to simulate electrical (excluding switching) as well as control dynamics for each DG to setup and facilitate system level simulations. The paper focuses on the operation of components in grid forming mode using a droop based primary control. This approach is applied on a real microgrid which is set up within the IREN2 research project framework. The demonstrator incorporates a Li-Ion based Battery Energy Storage System (BESS), a plant oil driven generator as well as a BESS emulator. First, a brief overview of the detailed model for each DG including its simplification is discussed. Next, the microgrid is set up using simplified models for transient simulations and the comparison with real measurements is shown for different microgrid topologies. Later, overall microgrid stability i.e., various instability aspects in LV island grids are discussed. In this regard, an analytical method based on Eigenvalue analysis for identification of stability limits for relevant electrical and control parameters and under various loading conditions is presented. Finally, the complete microgrid model is simulated for potential instable conditions and a comparison with the analytical solution is shown.