- Olav B. Fosso
- Institutt for elkraftteknikk, Fakultet for informasjonsteknologi og elektroteknikk, NTNU
The objective of WP4 is to develop concepts, technologies and models for microgrids and their interaction with the distribution system.
The expected impact is a contribution to cost-efficient and robust integration of microgrids with the distribution grid, and a contribution to the integration of more distributed and renewable energy resources (DER) in the system.
- Interaction between microgrids and distribution system operators
- Microgrid energy management and protection
- Ancillary services and energy routers
- Modelling and analysis of microgrids
Our objective is to develop concepts, technologies and models for microgrids and their interaction with the distribution system.
Through our research we contribute to a cost efficient and robust integration of microgrids with the distribution grid, and to the integration of more distributed and renewable energy resources (DER) in the system.
Testing models in the Smart Grid Laboratory
In general, the microgrid system will be connected to the main system. But to preserve the security of supply they must also be operable in so-called island mode, i.e., not connected to the main grid. With an increasing number of loads and sources interfaced through Power Electronic (PE) converters, the dynamic behaviour of the system is getting more critical. The controllers have to be properly tuned to avoid oscillations and possibly unstability.
When the operating states of the (micro)grid changes frequently, the dynamic models of system components must fulfil the requirements to an acceptable response after disturbances as well as to the steady state behaviour. Therefore, we have put significant effort into this in WP4.
Testing components with Hardware-in-the-loop
To be able to verify such cases, a model of the international benchmark system from CIGRE has been implemented in the national Smart Grid Laboratory. Here we can run realistic tests using the Hardware-in-the-loop concept where physical components can be tested in a realistic environment. In such an environment, more extensive testing on different system conditions are possible and much less costly than in a real system.
However, the interaction between the microgrid and the distribution system is important from an energy balance and power flow perspective. Limited transfer capabilities must be handled in a way that makes it possible to keep the lights on in the microgrid in island mode, while dynamically exchanging power in connected state.
Using master students to develop new tools
We need tools for the diff erent modes and developing these is part of the WP4 activities. Load flow models as well as optimization and simulation tools are under development both within master projects and as tools integrated in CINELDI. They will be used jointly with other CINELDI WPs. Our close collaboration with NTNU makes it possible to test the feasibility of the new concepts.
We made quite extensive use of master students to develop prototype tools and to investigate alternative formulation for microgrid analysis. Below we have listed a few examples of Master degree topics completed in 2020:
- DC-microgrids with Stability-Preserving Plug-and- Play Features where the task was to develop a passivity-based control design of DC/DC converters for stability-preserving microgrids with plug-and play features. The importance of this is the ability to preserve the stability for changes in grid topology.
- Instantaneous Frequency Identification in Microgrids Through Adaptive Data Analysis where the objective was to explore adaptive data analysis as an alternative to present methods used for monitoring and control in the power system. The thesis demonstrated the importance of using signal analysis techniques as a supplement to classical techniques based on physical modelling of the system.
- Agent-based modelling of EV charging scheduling towards optimized operation in Smart Grids where a pricing scheme based on Locational Marginal Pricing for the charging stations was studied. The purpose was to assess its efficiency in relocating the demand in both time and space, i.e., encouraging drivers to charge during periods of higher generation thus lower prices, while distributing the load among the stations with fewer congestion and losses costs. For this purpose, a real-time cooperative simulation tool was developed, integrating an Agent-Based Model of the drivers' behaviour, and the Optimal Power Flow of the network constraints, based on a real Norwegian local network with 856 consumers.
In WP4, our objective is to develop concepts, technologies and models for microgrids and their interaction with the distribution system.
We expect our work to contribute to a cost-efficient and robust integration of microgrids with the distribution grid, and a contribution and to the integration of more distributed and renewable energy resources (DER) in the system.
In short, microgrids are groups of interconnected components that can act as a single controllable entity with respect to the grid. They may be connected to the grid or operated in isolation, although in most cases in Norway they will be connected to the grid. Microgrids can be viewed as a bottom-up approach for implementing a smart and flexible distribution grid.
Power Hardware-in-the-Loop (PHIL) for Microgrid Performance Analysis
Microgrids are complex systems consisting of components and controllers dynamically interacting in a wide range of time constants. A proper assessment of the performance of this interaction is crucial for a secure operation of the microgrid. Particularly for systems which may operate in isolated mode, it is important to assess how the components interact when limited inertia is present. For testing of such performance there are essentially four approaches:
- Full-scale microgrids: This is the most expensive way to test a microgrid. It provides the highest fidelity but has the limitation that normal operating conditions are tested in a full-scale microgrid.
- Prototypes: A scaled down version of the full-scale microgrid. It is a cheaper option compared with the full-scale but still expensive. It provides good fidelity, but it has almost the same limitation regarding the test coverage.
- Simulation: This is the most cost-effective solution for testing any physical system, microgrids included. It has the advantage of unlimited range for testing i.e. high-test coverage. Normally, it is the first step for testing any new microgrids. However, the fidelity depends on the assumptions made in the simulation models.
- Controller Hardware-in the loop (HIL) and Power Hardware-in the loop (PHIL): This is a hybrid between hardware and simulation. HIL takes the flexibility of the simulations, so it has a greater range of test coverage compared with pure hardware setups. As well, HIL takes the fidelity over the hardware that is under test.
In the National Smartgrid laboratory, it is decided to go for a flexible approach at an intermediate cost level and therefore chose the Power Hardware-in-the- Loop (PHIL) concept. A goal within CINELDI, has been to develop a Real-time Power Hardware-in-the-loop microgrid simulation platform that can accelerate the deployment of new microgrids into the Norwegian distribution system.
As a base case, it was decided to simulate an internationally recognized benchmark low voltage microgrid network. This is an implementation where one or two converters exchange power with a simulated distribution grid following the PHIL approach. The PHIL uses a real-time simulator to generate the simulation environment that is compatible with real hardware, in this case a power converter. The Real-time simulator interacts with a power amplifier to exchange energy with the hardware under test. This diagram (right) visualises the process, while the image below shows the schematic and physical converter implementation. The simulation environment is provided with a user-interface as illustrated in the figure below.
Simulation studies proved both the feasibility of the model and the facilities in the National SmartGrid laboratory to simulate microgrid models using the PHIL approach. There is great potential in applying this technique to microgrid research and development since it can be applied to a wide range of system models with a flexibility in designing test scenarios. For the further work, a more sophisticated energy management system will be developed to demonstrate the potential in a more complex microgrid.
Activities related to assess the state of the art internationally and relate this to challenges within the setting of CINELDI were initiated in 2017. These activities were concluded in 2018 in the memos:
• Mikronett i Norge – Muligheter og utfordringer
• Microgrid protection – challenges and solutions
• Fault responses of inverter-interfaced DER – literature review
Development of laboratory infrastructure
An essential part of the activities within WP4 in 2018, has been the development of a laboratory infrastructure to be used to facilitate research and development activities within CINELDI as well as to do externally funded projects. A setup in the smart grid lab that is a building block for the proposed "Real-time power hardware-in-the-loop simulation platform to evaluate ancillary services in microgrids" has therefore been implemented. The setup is two parallel inverters operated in island mode for feeding a linear load. The load is emulated using the grid emulator and the real time simulator for providing more flexibility in terms of the feeder impedance/load that can be connected to the system. At the moment, the setup is able to operate one converter with an emulated load, a synchronisation mechanism is needed to connect the other converter. A figure is provided for illustration.
PhD/Post Docs – internally and externally funded
WP4 currently has two PhD-students funded from CINELDI where one is working on the task Microgrid Protection with the perspective of communication and use of 5G technologies, while the other is in the task on Ancillary Services and Energy Routers. The candidates are progressing well and have publications accepted and under review. To strengthen the activities within WP4, externally funded PhD and Post docs have been recruited to work with Microgrid related topics. One PhD-student and one Post doc are working within dynamic interaction in systems with high penetration of Power Electronic converters. Both positions are on strategic funding from NTNU. One PhD-student funded through SINTEF Energy Research/ Norwegian Research Council is working within planning in systems with distributed intermittent resources and storage devices. The experience with techniques developed for large-scale hydro scheduling is a core for the activities. An exchange PhD-student funded by the Norwegian Research Council coming from Shanghai Jiao Tong University, China, working within planning and operation of microgrid systems is on a one year stay at NTNU while a PhD-student from Universidad Tecnica de Pereira, Colombia, is working on control strategies for non-linear systems during his seven-month stay at NTNU. The external resources have made it possible to work on a broader scope and to build a more solid foundation for the future work within WP4 Microgrids.
MSc and project students – WP4 relevant activities
A priority within WP4 has been to have a close collaboration with MSc-students at NTNU to provide interesting and challenging research activities within microgrids. The activity of one student is equivalent to eight months full-time work. Five MSc-projected were concluded in the spring 2018. In the autumn 2018, seven students were recruited for the project continuing in a MSc-project finishing in 2019. These projects cover all major activities of WP4.
Dissemination and publications
Several conference and journal papers have been published related to the WP4 from researchers and internally and externally funded PhD-students and Post docs. Additionally, a significant number of papers are in the review process in internationally recognised journals. Five MSc theses were completed in June 2018 and seven student projects completed in December 2018. The student projects are continued in the spring 2019 and will be completed as MSc-thesis in June 2019.
Demonstrating simplified reference system for microgrids on the smart grid laboratory
A simplified reference system for studying isolated microgrids has been defined. The reference system has been used for developing and testing primary control strategies, a droop control that ensures cooperative power sharing and harmonic sharing. One of the challenges is deriving a droop controller that works when the line feeder impedances change dynamically and varies from largely resistive to more inductive characteristics, and still obtain proper sharing of power and current harmonics.
This research has been driven forward through collaboration between PhD candidate Fredrik Göthner and two master students. The simplified reference converter system and associated cascaded controllers have been successfully implemented in simulation environment, supporting master thesis and PhD thesis work. A related paper entitled Considerations of Virtual Impedance Implementation in the Synchronous Reference Frame was authored by Fredrik Göthner and Raymundo E. Torres-Olguin, and accepted for IEEE Environment and Electrical Engineering conference.
Two contributing master projects are Improved Power Sharing in AC Microgrids by Using Decentralised Virtual Impedance Control, and Advanced Harmonic Sharing Techniques for Microgrid Applications. The research is in collaboration and supervision by Professor Dr. Olimpo Anaya-Lara from University of Strathclyde. Extensive laboratory activities are planned starting March 2018, the previous simulation work and promising results are input for a hardware-in-the-loop implementation at the Norwegian Smart Grid Laboratory. The plan is to utilize and demonstrate the advanced facilities available at the laboratory, with the help from research partners at NTNU and SINTEF Energy Research.
State-of-the-art review of protection in microgrids
Through the work WP4 is creating high-level use cases identifying important challenges for relay protection custom to microgrids. One use case description has been proposed Adaptive microgrid protection, and a state-of-the-art review on protection in microgrids has been started, forming the basis for further research. The WP4 microgrid work package will partly run in cooperation with WP2 supporting research of a PhD candidate focusing on 5G for Low-Latency, Secure and Dependable Communication Services for Fault Handlings.