80 10 th International Symposium „Topical Problems in the Field of Electrical and Power Engineering“ Pärnu, Estonia, January 10-15, 2011 Control of an Uninterruptible Power Supply in a DC Microgrid System Andriy Palamar 1 , Mykola Karpinskyy 2 1 Tallinn University of Technology, Department of Electrical Drives and Power Electronics 2 University of Bielsko-Biala, Department of Electrical Engineering and Automatic andrij777@mail.ru , mkarpinski@ath.bielsko.pl Abstract In this paper DC microgrid for telecommunication loads is presented. A digital control system of an UPS is proposed which coordinates the management of converter, battery, and switches in the DC microgrid. The hardware and software of the system is developed. Moreover, a friendly graphical-user interface is provided. Control strategy of an AC/DC UPS system in different operation modes is also described. The proposed control and monitoring system allows to realize the complex algorithms to increase the reliability and efficiency of the UPS and this way improve the power quality for the local loads of the DC microgrid. Keywords Control system, UPS, DC microgrid Introduction The interest in the continuity of power supply in the local distributed areas has increased significantly in recent years. Today more and more consumers, such as computer servers, medical equipment, telecommunication systems, require constant availability of electrical energy. For this purpose, the installation of small distributed energy resources (DER) is a practical solution. DERs encompass a wide range of prime technologies, such as diesel generators, gas turbines, microturbines, photovoltaic, fuel cells, and windpower [1]. However, it is well known that if many DERs are installed into a main grid, they can cause problems such as voltage rise and protection problem. Microgrid concept was presented [4] in order to introduce distributed energy resources into the main grid seamlessly and obtain maximum benefits from the emergent potential of DERs. It is a new energy supply and network management technology. A microgrid could be defined [4], [5] as a low voltage distribution network with distributed energy sources (micro turbines, fuel cells, photovoltaic, diesel generators, etc.) altogether with storage devices (flywheel, batteries, etc.) and loads. The main benefits of a microgrid lie in that it supply power locally, reduce grid investment, operation costs and transmission line losses, decrease network capacity, shave the load peak, and increase reliability. A microgrid can be built using AC or DC current network [6]. The AC microgrid system has been proposed and described in detail in the previous papers of the authors [7] – [9]. Despite the fact that the AC microgrid system has a benefit to utilize existing AC grid technologies, protections and standards its application involves some problem of low efficiency due to the number of power conversions required within the crucial current path from the main grid to the loads. One solution of this problem is the application of a DC microgrid as an efficient method to combine high reliability and the possibility to reduce the losses. It can eliminate DC/AC or AC/DC power conversion stage and thus has advantages in the stand of efficiency, cost and system size. DC microgrid is inspired by the absence of reactive power, the possibility of the efficient integration of small-distributed generation units and the fact that, internally, all the loads operate using a DC voltage. This paper presents a DC microgrid system based on a diesel generator as a DER and battery bank as a distributed energy storage device. The best way to ensure correct control process of the battery set is to use an uninterruptible power supply. UPS systems provide continuous, reliable, and high-quality power for important loads in order to prevent expensive hardware failure, data loss or corruption, cessation of the business process in cases of disturbance in the main electrical grid [10]. Uninterruptible power supply systems are still in their research stage. Although UPS has applications in many various fields, it requires improvement of many technologies for customer needs. Availability of low cost, user friendly and compact control and monitoring system is one of them. Monitoring of input and output electrical parameters of an UPS, digital control of charging and discharging of a battery, and the archiving of events in real time are necessary parts of such management systems. In response to the requirements above, a digital control and monitoring system for AC/DC UPS in the DC microgrid is developed in this work. A hardware and software of the system is designed. The main goal of the paper is to realize the control strategy of the management system in different operation modes of the studied uninterruptible power supply in the DC microgrid.