The ever increasing electrical energy demands, limited fossil and nuclear fuel reserves, climate change, the national desire for energy independence and diversification of energy sources, thrust in the first plan distributed production of electric power from renewable sources as a key element in achieving sustainable development. Over ninety percent of the electricity generated in developed countries is consumed in homes, buildings, and industry. Greater attention must therefore be paid to end-use sectors if the promised benefits of smart grids - reduction of electricity consumption-losses, integration of renewable generation and storage, reduced use of fossil fuels, and improved grid reliability – are to be achieved.

The main problem in the usage of renewable electrical energy sources (REES) is their intermittency, which leads to problems in regulation of the power system. This problem exists both on a local production-storage-consumption level and on the power system level and becomes more pronounced with the increasing contribution of REES in the total energy production. A natural solution is to derive a coordinated and dynamic planning strategy for production-storage-consumption of electric power. With a local information- and power-connection of REES, energy storage facilities, and consumers in a system – a microgrid – one can control resulting energy flows while considering techno-economical criteria and the local energy yield forecast. Solution of such problems – via optimization for microgrid design and control – will significantly improve the integration of REES on both the local and the utility grid level.

This program includes 4 research teams with complementary competences from appropriate research fields: University of Zagreb Faculty of Electrical Engineering and Computing (FER), University of Zagreb Faculty of Mechanical Engineering and Naval Architecture (FSB), Meteorological and Hydrological Institute of Croatia (DHMZ) and Končar – Electrical Engineering Institute (KIET).

Program is in line with the HRZZ theme „Renewable Electricity Generation”. It resolves a key problem of the REES integration in the power system. Furthermore, with its multidisciplinary approach, the program promotes the establishment of an inter-institutional center of excellence in the area of distributed renewable electricity generation, thus bringing the concept of smart grids and smart cities in Croatia closer to the practice.

While the control of REES that are connected to large power systems is in the focus of the energy and control research community for few decades now, the control of smaller-sized REES such as wind turbines and PVs suitable for microgrid integration was marginalised. The implemented solutions in such systems were minimum requirements driven which led to a gap between sophisticated professional control systems of the power system connected REES and simple, basic control system of the isolated REES. Filling this gap in technically and economically acceptable manner is one of the main research areas of the energy control community in the last few years. Furthermore, significant challenges exist in the interoperation of several small intermittent REES with practically no inertia on the common microgrid power link, compared to the interoperation of predictable large inertia generators of conventional power plants in the utility grid. These challenges are addressed through properly combined power electronics control with primary regulation of power flows from REES and from/to storages and with secondary regulation of the microgrid power link electrical values (Guerrero et al. 2011).

To achieve full potential of smart grids, all the problems of local information/energy integration of renewable energy sources, electrical energy storages and loads should be addressed, and one should fully exploit all the opportunities that arise from this concept. The initial results on optimization of dynamic microgrid operation show potential for improvement of the microgrid techno-economic performance (Vašak et al. 2011) and show scalability of this concept to the scenario of virtual electricity prices negotiations of a large number of microgrids with the utility grid. It is essential to further upgrade this concept in a stochastic framework that considers REES, loads and grid intermittency, enable its real-time operation by suitable design/upgrade of the systems for weather forecasting, local load forecasting and grid utility communication.

The available options for energy storage and integration of different energy and resources flows that could help solve intermittency problems in the islanded energy systems have been proposed using the Renewislands methodology (Duić et al. 2008). The problem of storage systems is that they increase the cost of already expensive distributed REES, making them, in market terms, even less economically viable. This storage characteristic is even more highlighted in microgrids that need sophisticated and costly electronics and communication systems to manage power flows from various sources to different controllable and uncontrollable loads. Thus it is essential for further deployment of microgrids to ensure its optimal planning and sizing in order to avoid unnecessary costs on the one side and on the other to ensure acceptable level of energy supply autonomy.

The intermittent nature of REES, like wind and solar, is one of the limiting factors for their penetration in power systems, especially autonomous systems. State-of-the-art numerical weather forecasting systems for REES power prediction needs typically use a dynamical and a statistical step. The inability of the current dynamical models to forecast the site-specific local conditions is usually tackled by the statistical postprocessing methods. The dynamical forecasting system in Croatia as well as the postprocessing methods will be tailored for microgrid design and control needs through the program developments.

The most prominent research institute with practical and production experience in Croatia in the field of REES is KIET. FER and FSB have a very good track record in the optimization field, for systems design and control, and have the needed experimental facilities to support the proposed research (LARES and FSB laboratories). DHMZ has the key expertise and resources in Croatia in weather forecasting.

Guerrero, J.M.; Vasquez, J.C.; Matas, J.; de Vicuna, L.G.; Castilla, M. "Hierarchical Control of Droop-Controlled AC and DC Microgrids – A General Aproach Towards Standardization," IEEE Transactions on Industrial Electronics, 58(1), 2011; pp. 158-172.

Vašak, M.; Baotić, M.; Perić, N. "Deterministic Power Flow Optimization in an Experimental Microgrid," Fifth Global Conference on Power Control and Optimization, Dubai, United Arab Emirates, June 2011.

Duić, N.; Krajačić, G.; Carvalho, M.G. "RenewIslands methodology for sustainable energy and resource planning for islands," Renewable and Sustainable Energy Reviews 12(4), 2008; pp. 1032-1062.

The main aim of proposed action is to make stronger bonds between 4 research groups from 4 institutions by participation on internationally competitive programme that includes joint research of microgrids, a novel topic and one of important prerequisites for development of distributed energy generation from renewable energy sources.  Developed mathematical models, tools and conducted experiments will set the microgrids as important scientific field in power engineering research.  The project represents the innovative approach to development, analysis and optimization of microgrid as the main core of future energy systems based on smart grids, virtual power plants and liberalized energy market. Additional aim within this programme is to upgrade, improve and connect three laboratories and several measurements stations in order to conduct all experiments necessary for qualitative validation of modelling results and proving the hypothesis. Some tasks within the project are specially formulated to foster training of young researchers and enhance knowledge transfer between the participants.

The most important outcome of cooperation on the project activities are  strong foundations for development of microgrid-DES Centre of Excellence, supported by three advanced  laboratories and capable to address the most challenging problems rising from advanced control and smart use of microgrids, renewable energy sources and energy storages.

Methods of mathematical modeling of systems and processes, computer programming, optimization, simulation and experimental testing, will be used by partners in achieving the following objectives.

Develop hierarchical optimal control system for dynamic operation of REES microgrid:

• control of the microgrid power electronics interface to REES and storage systems;
• coordinated control of power flows;
• control of the voltage-current conditions;
• control of the active and reactive grid power import/export;
• predictive control of the power flows from/to energy storage units based on the (stochastic) weather forecast and on the microgrid state;
• experimental testing in the Laboratory for Renewable Energy Sources (LARES) at FER.

Develop the corresponding communication system:

• control system for microgrid with a „virtual market“ of electric power – the optimal adjustment of the microgrid production/consumption profile based on the prices declared dynamically by the grid operator;
• forecast service control system;
• microgrid local load profile prediction based on available past consumption data and other correlated data (date, time, weather forecast), while considering uncertainties;
• communication system integration at the LARES.

Develop algorithms for planning and optimization of microgrid components, including power producing units and availability of electric energy storage elements. Develop the methodology for the micro-windturbine selection and develop guidelines for their construction and placement.

Develop detailed mathematical model for analysis and optimization of PV array energy production under different constrains given by available technology, space, location, storage capacity and costs. PV model will be compatible and integrated with other developed tools. Formulation of microgrid energy system integrated in typical building. Researchers at FSB have certain experience and knowledge in development of energy planning tools, but participation on the program will enable them direct contact with different research groups that have specific knowledge and expertise thus proposed program will further improve their modeling skills which will result in building of more accurate and more realistic energy planning models.

Develop modeling and control methods for the energy storage system that combines complementary technologies (high power density – supercondensators/flywheels and high energy density – accumulator batteries, hydrogen storage) at diverse dynamic conditions for storage and production of energy:

• experimental identification of the energy storage components;
• derive experimentally tested, control-oriented models for energy storage;
• on-line estimation of the energy storage state (current integral and voltage in no-load operation for batteries, fill level for hydrogen storages)
• combined control of energy storages based on long-term predictive strategy with fast controlled reactions in cases of sudden power peaks/sags.

Up to date, DHMZ’s forecast system was a combination of the full-physics model at 8 km grid spacing and a simplified model, so-called dynamical adaptation, at 2 km grid spacing. Through project activities, DHMZ will:

• evaluate the accuracy of the current forecasting system and estimate the related forecast uncertainties
• develop and implement the advanced numerical weather prediction model at 2 km grid spacing
• develop statistical postprocessing module using Kalman filter for site-specific weather forecast
• evaluate the accuracy and added value of the improved forecasting system