Hardware Implementation of Hybrid AC-DC Power System Laboratory Involving Renewable Energy Sources

Future power system will highly rely on renewable energy sources. The need for expert designers and operators for such systems is increasing. The concept of renewable energy sources have been discussed by evolving new platforms for design, development and integration of renewable energy sources such as photovoltaic and wind for research and educational efforts. This includes the implementation of energy source emulators and related control issues. Laboratory experiments and their analysis prepare a suitable environment for students to achieve a high level of knowledge and deep understanding of the fundamentals of renewable energy concepts in the real world. This research and education environment involves techniques such as ac-dc power system design, real time operation, analysis and control that provide a state of the art platform to solve most challenges of actual renewable sources in modern power systems. Introduction During the past 20 years, energy costs have increased annually. In addition, the need for an efficient, less environmentally damaging, renewable energy resource is significantly growing. Generating electricity from sources such as fossil fuels, biomass and nuclear power requires a lot of water which may intensify the drying of more lands and warming of earth. However, renewable energy resources such as wind and solar energy can provide electricity with the lowest pollution and solve many of the problems leading to global warming and impacts on energy demands in the near future. This is especially problematic for developed and industrial nations. A renewable energy education platform in lectures that include both theory and practice will be the key point to providing effective training for future job demands in the energy industry. The objective of this paper is to apply the concepts of integrating two main renewable energy sources into power grids with implementation examples in laboratory based test bed. The smart grid test bed developed at the energy systems research laboratory at Florida International University is presented. The techniques for integrating and emulating renewable energy sources, such as wind and solar energy generation, identifying the requirements and educational initiatives for educating and training students are explained in the following sections. An overview of developed hybrid smart power system including renewable energy resources is explained in section 4. The development of software and hardware implementation processes in ac/dc grid of the emulated power system will be identified. Section 5 discusses the ac/dc grid architecture, operation and control strategies. Section 6 highlights some of the educational efforts being developed on the smart grid test bed. This will be followed by conclusions. Need, implementation and concerns of renewable energy sources Utilizing renewable energy sources will reduce the consumption of fossil resources for energy generation and transportation for the reduction carbon emission and air pollutants. They can be used to enhance the reliability, resiliency, efficiency and finally security of microgrids. In the customer’s side, resources for energy generations can cut the peak loads of customers and consequently the costs of electricity. This will be improved when the renewable energy sources are used along with energy storage systems. It can be mentioned that in the area of integration of renewable energy sources into interconnected power systems, energy storage systems are important enabling factor. This is due to uncertainty of solar and wind energy resources. The stored energy is used when it is needed. This can improve the transmission capacity; reduce the requirements for generation units and maximize the use of variable sources such as PV and wind. The negative aspects of energy storage systems are losses due to multiple conversions in ac/dc or dc/ac stages, cost of energy storage devices and the complexity of the conversion process. Large penetration of distributed generations and implementation of HEV into the grid provide a great opportunity of using idle battery storage of electric cars to be used as distributed energy storage systems. From a technical point of view, large implementation of variable resources will make the grid more complex to operate. Future power system engineers should consider these distributed generations and should be greatly familiar with the many technical aspects. Utilizing renewable energy sources not only provides an excellent opportunity for students to have a deep and realistic understanding on this subject, but also prepares and train them to test their ideas in a highly technical research and development atmosphere. The implementation of operational regimes and conducting verification experiments will be necessary for future students to get into this area: a) Introduction to hybrid power grids b) Emulation and development of renewable energy sources (in particular wind and solar) c) Integrating techniques and approaches for optimally control and operation of PV and wind emulators along with energy storage system d) Real time energy transfer techniques, converters and tools in hybrid power systems with emphasis in dc micro grids connectivity Overview of integrated hybrid power system platform A. AC Power Grid Energy transfer systems are relied on the basis of ac network along with bulk energy generations. Distributed generations will be added to this system through the micro grids. It is required to have the ac infrastructure as a basis for integrating any micro grids or for grid connected application of renewable energy resources, and also to analyze their performance and evaluate innovative ideas. In the emulated ac grid system, the cost, flexibility of operation and safety (compared to actual high voltage systems) is the main advantage. The developed ac grid consists of generating stations, various programmable dynamic load modules, overhead transmission lines or cable models, synchronizers, ac measurement buses and feeders also in addition to wide area control, monitoring and operation package 1 . This particular ac system has the flexibility to work in either 50 or 60 Hertz in three phases four wires connectivity basis. Complimentary information is described in details and available in 2 . B. PV Emulation One of the main research topics studied in our hybrid ac-dc power system laboratory is development of techniques for implementation and energy management of large and small scale photovoltaic power plant. Photovoltaic arrays have a nonlinear voltage-current characteristic that its output power varies with solar radiation and cell temperature. Due to the limitation on solar energy availability in a laboratory environment, an alternative photovoltaic emulator was used. The emulator is a 6 kW Magna Power Electronics dc programmable power supply (Figure 1) and its maximum output voltage and current are 375 V and 15.9 A, respectively. The Photovoltaic Power Profile Emulator (PPPE) software is used to calculate voltage and current profiles of a specific solar array based on predefined parameters. These profiles are sequentially sent to the power supply via RS232 serial communication interface. A power profile can be generated manually based on voltage-current curve of a photovoltaic cell or it can be defined based on short circuit current, open circuit voltage and maximum power point (MPP) of a cell. Figure 1. Programmable power supplies. Figure 2 shows typical voltage-current and voltage-power characteristic of a PV module. As it can be seen, the power curve has only a single maximum point and it is always desired that the PV module operate close to this point. Different maximum power point tracking (MPPT) techniques can be used to maintain the operation point at the MPP under varying conditions such as load, temperature, and insolation. The MPPT algorithms can be classified into direct and indirect categories. The indirect methods are the “open-circuit voltage method”, the “shortcircuit method”, the “look-up table method” and the “curve-fitting method”. These methods are based on the data which show the characteristics of the PV panel at different environmental and working conditions. The direct methods include “artificial intelligence method”, “differentiation method”, “P&Q method”, and so on. The direct methods are more robust i.e. prior knowledge of the PV parameters is not required. However, voltage or current measurement and a feedback loop are necessary for the converter control circuit 2 . Figure 3 shows the schematic diagram of photovoltaic system with MPPT controller. The PV module and the dc bus are interfaced with each other through a dc/dc converter, which can be a step-down or step-up converter depending on the dc bus voltage and the panel size. The controller circuit samples the voltage and the current at the output of the photovoltaic panel. Then the switching signal is sent to the converter to regulate the output current and to maximize the power transfer. Figure 2. Voltage-current and voltage-power curve of a photovoltaic module. DC-DC converter (buck-boost)