Advanced Current-limiting Control of Inverter-interfaced Distributed Energy Resources to Develop Self-Protected Microgrids

In the upcoming “smart grid” era, advanced control schemes are required for inverter- interfaced DERs to guarantee stability of inverter-dominated feeders and microgrids. Nevertheless, in many of the recently proposed methods, the safe and stable operation of inverters can not be analytically guaranteed under normal and abnormal grid conditions. In this thesis, single-phase grid-connected inverters are initially considered and an enhanced Current-Limiting Droop (CLD) controller is proposed. In contrast to the original CLD, which limits the inverter current under a lower value than its maximum during faults, the proposed controller fully utilizes the inverter capacity. An inherent current limitation is proven through nonlinear ultimate boundedness theory and is shown to facilitate the operation of Fault-Ride-Through (FRT) schemes. Furthermore, conditions for asymptotic stability of the closed-loop system are derived. Additionally, a new CLD scheme is proposed, which operates without the need of a PLL and introduces a virtual inertia property to DERs. In the sequel, three-phase grid-connected inverters are investigated and a new controller in the dq-frame is proposed to deal with FRT in three-phase systems. Initially, a novel method to divide the current into its symmetrical components during unbalanced faults is proposed. Hence, based on an adaptive bounded integral controller, the proposed scheme provides voltage support to both positive and negative sequences, while ensuring the current boundedness and asymptotic stability of the closed-loop system. In the final part of this thesis, the safe and stable operation of three-phase inverter-based microgrids is investigated. Particularly, an advanced controller is proposed to deal with extreme load conditions. Through the proposed scheme, the limitation of the inverter current during transients is guaranteed, without the need of online adaptation techniques. Furthermore, the proposed approach significantly simplifies the stability analysis of microgrids, since it can be investigated through a Jacobian matrix of reduced size

Voltage support under grid faults with inherent current limitation for three-phase droop-controlled inverters

A novel nonlinear current-limiting controller for three-phase grid-tied droop-controlled inverters that is capable of offering voltage support during balanced and unbalanced grid voltage drops is proposed in this paper. The proposed controller introduces a unified structure under both normal and abnormal grid conditions operating as a droop controller or following the recent fault-ride-through requirement to provide voltage support. In the case of unbalanced faults, the inverter can further inject or absorb the required negative sequence real and reactive power to eliminate the negative sequence voltage at the PCC whilst ensuring at all times boundedness for the grid current. To accomplish this task, a novel and easily implementable method for dividing the available current into the two sequences (positive and negative) is proposed, suitably adapting the proposed controller parameters. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the total grid current remains limited below its given maximum value under both normal and abnormal grid conditions. Asymptotic stability for any equilibrium point of the closed-loop system in the bounded operating range is also analytically proven for first time using interconnected-systems stability analysis irrespective of the system parameters. The proposed control concept is verified using an OPAL-RT real-time digital simulation system for a three-phase inverter connected to the grid

Educational programme management methodology for research projects

Smart grids and intelligent energy systems play a pivotal role in fostering the sustainable advancement of our civilization. Over the past few decades, power systems, like many other sectors, have undergone a rapid digital transformation. This rapid development necessitates a proactive response from universities, research institutes, industry stakeholders in relation to educational programmes. Educators must rapidly adapt their curricula and teaching methodologies to effectively train the next generation of engineering professionals. While curriculum crafting for new educational programs is inherently challenging, another layer of complexity arises when research collaborations in large consortia are tasked with delivering high-quality education within a given project scope and time frame. This paper outlines a methodology for establishing an educational strategy for such research projects. This approach takes into account the available resources and expertise of the project participants, while embracing modern, learner-centric educational methodologies. It also ensures alignment with broader objectives or frameworks. Furthermore, the strategy incorporates a dynamic evaluation process that runs concurrently with the educational activities. Finally, the ERIGrid 2.0 H2020 project upon which the proposed methodology was developed, is presented as a case study

Virtual shifting impedance method for extended range high-fidelity PHIL testing

A novel power hardware-in-the-loop interface algorithm, the Virtual Shifting Impedance, is developed, validated and demonstrated in this paper. Building on existing interface algorithms, this method involves shifting a part of the software impedance to the hardware side to improve the stability and accuracy of power hardware-in-the-loop setups. However, compared to existing approaches, this impedance shifting is realized by modifying the command signals of the power amplifier controller, thus avoiding the requirement for hardware passive components. The mathematical derivation of the Virtual Shifting Impedance interface algorithm is realized step-by-step, while its stability and accuracy properties are thoroughly examined. Finally, the applicability of the proposed method is verified through power hardware-in-the-loop simulation results

Energy Systems Test Case Discovery Enabled by Test Case Profile and Repository

Smart energy systems comprise multiple domains like power, thermal, control, information, and communication technology, which increases the complexity of research and development studies. This expansion also requires larger and ever so complex experimental pilot environments driving the demand for geographically distributed multi-research infrastructure tests. The Holistic Test Description approach supports the design of multi-domain and multi-research infrastructure tests by orga-nizing the test cases into comprehensive segments, ensuring all relevant items for testing are covered. These test cases eventually form a pool, which to understand holistically would require studying and reading all the descriptions. This work proposes therefore the concept of Test Case Profiles to improve test case discovery and the structured creation of them. Test Case Profiles add further structure to the indexing in test case repositories. Along with the proposed indexing method, four different use cases are introduced to motivate additional applications of the proposed concept.Energy Systems Test Case Discovery Enabled by Test Case Profile and RepositoryacceptedVersio

Mitigating the Impact of an Official PEV Charger Deployment Plan on an Urban Grid

Plug-in electric vehicles (PEVs) are rapidly increasing all over the world as electromobility is being promoted to reduce the greenhouse gas (GHG) emissions. PEVs locally produce zero emissions and they are more efficient in contrast to internal combustion engine vehicles. On the other hand, the impact of the uncoordinated charging of PEVs is expected to create new challenges for the established distribution grid. This article initially evaluates the impact of uncoordinated charging on the voltage regulation and transformer loading in the city of Heraklion, Greece. Then, a coordinated charging strategy is proposed to deal with the issue of transformer capacity violation by PEVs. Two cases were considered when the transformer overloading from PEV charging was taken into consideration: (a) keeping the transformer below its nominal capacity limit and (b) keeping the transformer below 90% of its nominal capacity limit. This was achieved by distributing the available capacity (nominal capacity minus the residential loads) of the transformer to the PEV chargers in a way that the aforementioned limits were satisfied. Real-world data of the distribution grid and the hourly power consumption of the city were used to validate the proposed method. Results show that possible transformer capacity violation can be tackled by the proposed coordinated charging, without exceeding the voltage regulation limits

Black-start of microgrids: Insights based on demonstration sites in Europe and India

This paper provides an insight into power system restoration on a small scale, where the distributed generation in microgrids is used to facilitate black-start strategies to provide faster and efficient power restoration. The idea was to employ non-conventional and renewable generation for black-start provision in microgrids with implementation of grid-forming strategies and control coordination. Both AC and DC microgrids, operating in grid-connected and islanded modes, were considered in this work and exemplified on three study cases based on the demonstration sites of the Re-Empowered project (locations in Denmark, Greece and India). Case studies include a larger system in Bornholm, where the network was sectionalized into two smaller microgrids and subsequently established simultaneously, before being synchronized and interconnected. Two additional studies are based on a smaller scale AC and DC islanded micro-grids, where different methods for black-start were considered

Mitigating the Impact of an Official PEV Charger Deployment Plan on an Urban Grid

Plug-in electric vehicles (PEVs) are rapidly increasing all over the world as electromobility is being promoted to reduce the greenhouse gas (GHG) emissions. PEVs locally produce zero emissions and they are more efficient in contrast to internal combustion engine vehicles. On the other hand, the impact of the uncoordinated charging of PEVs is expected to create new challenges for the established distribution grid. This article initially evaluates the impact of uncoordinated charging on the voltage regulation and transformer loading in the city of Heraklion, Greece. Then, a coordinated charging strategy is proposed to deal with the issue of transformer capacity violation by PEVs. Two cases were considered when the transformer overloading from PEV charging was taken into consideration: (a) keeping the transformer below its nominal capacity limit and (b) keeping the transformer below 90% of its nominal capacity limit. This was achieved by distributing the available capacity (nominal capacity minus the residential loads) of the transformer to the PEV chargers in a way that the aforementioned limits were satisfied. Real-world data of the distribution grid and the hourly power consumption of the city were used to validate the proposed method. Results show that possible transformer capacity violation can be tackled by the proposed coordinated charging, without exceeding the voltage regulation limits

Smart Campus Microgrids towards a Sustainable Energy Transition—The Case Study of the Hellenic Mediterranean University in Crete

Smart campus microgrids are considered in this paper, with the aim of highlighting their applicability in the framework of the sustainable energy transition. In particular, the campus of the Hellenic Mediterranean University (HMU) in Heraklion, Crete, Greece, is selected as a case study to highlight the multiple campus microgrids’ advantages. Crete represents an interesting insular power system case, due to the high renewable energy sources capacity and the large summer tourism industry. There is also a high density of university and research campuses, making the campus microgrid concept a promising solution for the energy transition and decarbonization of the island. In this sense, policy directions that could facilitate the development of the smart campus microgrid are also given, to motivate areas with similar characteristics. For the performed case study, the HMU microgrid is assumed to consist of PV systems, wind turbines, battery energy storage systems and EV chargers. The analysis explores the financial feasibility and environmental impact of such an investment through the optimal sizing of the systems under investigation, while a sensitivity analysis regarding the battery system cost is also performed. Apart from the financial benefits of the investment, it is evident that the main grid experiences a significant load reduction, with the microgrid acting as a RES producer for many hours, hence improving system adequacy. Moreover, it is shown that the location of HMU makes the investment more sustainable compared to other locations in northern Europe, such as Stockholm and London. The methodology and the derived results are expected to motivate such investments, especially in areas with high RES capacity and a high density of university and research campuses

Modeling of DC-Side Dynamics in PV/Battery Grid-forming Inverter Systems

The DC-side dynamics of two-stage grid-forming (GFM) inverters are often neglected or oversimplified in power system studies, although they play a vital role in stability. Detailed models for the primary source and power converter are not practical for complexity reasons, leading usually to DC side representations that omit critical aspects such as the operational limits. To bridge this gap between accuracy and simplicity, this paper proposes for the first time an equivalent model for the DC side dynamics of GFM systems that captures all important dynamics and limits in a simple formulation. The model supports either batteries or photovoltaic arrays as a primary source and is easily parameterizable with minimal information, thus proving a useful tool for grid stability studies. Time-domain simulations validate the proposed equivalent model