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Institute
Local energy markets (LEM) represent a user-centric approach to provide direct market access to prosumers able to compensate selected inadequacies associated with the current design of central energy markets. This paper presents four LEM design options and introduces the specific configurations for both an auction-based and a central coordinator approach. While both approaches aim at maximizing welfare, they differ with regard to the role assumed for the LEM participants and coordinator. Finally, exemplary results are discussed for a test case.
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.
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 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.
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.
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 increasing share of distributed energy resources gives rise to new opportunities for deploying innovative business models and coordination schemes within sustainable energy systems. Different concepts entail different implications at socioeconomic, technical and institutional level. Hence, their thorough assessment is key to understanding their actual potential as enablers of the energy transition. Considering this background, we focus on local energy markets as an increasingly discussed approach for coordinating distributed energy systems and introduce a simulative framework for enabling a multi-regional assessment of this concept. Local energy markets bear the potential for increasing the active participation of end consumers, which could increase their acceptance for energy projects in general and their returns on investment, as well as for reducing the peak load on increasingly congested electrical grids by enhancing local energy balancing. We evaluate these hypotheses for twelve representative German regions, for which we formulate assumptions regarding the energy demand as well as the shares of distributed energy resources that are consistently aligned with an overall European energy scenario envisaging a rapid growth of electric vehicles in Germany. For this purpose, we enhance an existing framework for the assessment of local energy markets in order to be able to include the flexibility of the electric mobility sector in local trade activities. The simulation results show that local energy markets have a significant impact on energy systems: First, local trading increases the economic benefits over all participants, who would otherwise only be able to use their generation for self-consumption or direct marketing in central energy markets. Second, local energy balancing increases on average by 60%over all regions. Third, infrastructural relief of the overlaying transmission grids can be accomplished by reducing the yearly peak load at the point of common coupling by 39%on average and at the most by 97%. Furthermore, we find that including electric vehicles in local market activities does not alter but rather reinforces these effects.
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.