Biechl, Helmuth
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Institute
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 effect of network impedances on the transient stability of Low Voltage (LV) microgrids intended for islanded operation. A simulation model is developed using simplified models of 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 [1]–[3]. The paper focuses on the operation of DGs in grid forming mode using a droop based primary control. This approach is applied on a real microgrid which is set up within the pebbles research project framework. These DGs are connected through cable impedances to a resistive load bank at the point of common coupling (PCC). The effect of varying individual impedances between DGs and PCC under loading conditions on the microgrid stability is investigated. The location of load between DGs and its impact is also discussed. Finally, a control modification utilizing concept of virtual impedances (VIs) in Voltage Source Inverters (VSI) is proposed to improve the transient stability of the discussed microgrid.
In this paper, the small signal stability of a Battery Energy Storage System (BESS) used in a low voltage islanded microgrid is investigated for an ohmic load case using eigenvalue sensitivity analysis. Two approaches namely Quasi Steady State (QSS) and Dynamic Phasor Modeling (DPM) are presented and compared for a reference BESS in a real microgrid. The QSS approach is considered as a traditional method to model system dynamics assuming that they are slow enough to apply steady state rules. The DPM approach on the other hand considers the electrical dynamics in the control feedback loop and the coupling between the parallel inverters. The evaluation of the mathematical models for both approaches as well as simulation and measurement results are presented. The classical QSS stability analysis applied to the BESS does not show the dependency of stability margins on droop parameters, smoothing time constant or load parameters. This problem can be overcome by the presented DPM method. The sensitivity of the BESS and load parameters on stability limits is studied in detail.
The paper investigates the transient stability issues
in islanded microgrids with both grid forming as well as grid
following Distributed Generation (DG) units participating in
the microgrid. The focus is to identify high frequency stability
challenges due to short time transients that generally arise
from fast load changes. It is shown that the primary control
in DGs, type of load and grid impedances requires significant
considerations for transient grid stability. An extended microgrid
simulation model with two Battery Storage Systems (BSSs)
namely BSS1 and BSS2, a Back-to-back Station (B2B) as well as
a resistive load bank is modeled in this regard [1]. The models
of these DGs are based on real system components integrated
in a real microgrid demonstrator and are simplified to simulate
electrical (excluding switching) as well as control dynamics for
each DG to setup and facilitate system level simulations [2]. The
B2B and BSS1 are operated in grid forming mode (VSI inverter)
and the primary control is based on the classical droop control to
regulate output voltage and frequency. The BSS2 is operated in
grid following mode (CSI inverter) and emulates a prosumer with
a primary control that regulates BSS output active and reactive
power. The microgrid has no secondary microgrid controller and
the microgrid stability under islanded operation is exclusively
considered in this paper.
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.
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.
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.
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.