Energy Storage Power Conversion System in the Micro-grid

Abstract

With the development of new energy sources such as wind power and solar power generation technology, distributed generation and micro-grid technology attract increasingly great attention. However, the renewable energy resources are intermittent in nature under the influence of meteorological fluctuations, and produce fluctuating active power. An energy storage system is indispensable for compensation of the active-power fluctuation, it can mitigate the disturbance and maintain the stability of voltage and frequency. Power conversion system(PCS), as an interface between storage system and public grid, plays an great role in achieving the power transfer between storage system and public grid. This paper summarize the current topologies and the control strategies commonly used in engineering under different working situations and requirements, and analyze their differences and characters, which will helps in choosing the PCS structures and control strategies. Introduction With the rapid development of global economy, the global electricity consumption surges which gives rise to the serious power supply shortage. Renewable energy sources such as wind turbine generators and photovoltaics provides a new way to resolve the problem. However, the distributed power such as photovoltaic power and wind power are intermittent and produce fluctuating active power. Interconnecting these intermittent sources to the utility grid at a large scale may affect the voltage and frequency control of the grid, and may lead to severe power quality issues[1]. Therefor , Energy storage system is very important for the micro-grid, it can store the excess power from the grid when the power supply is greater than the power consumption. On the other hand, it releases the shortage of power to the grid when the generation system produces a smaller power. The energy storage system brings a significant enhancement in power quality, stability and reliability to the grid. Therefor, the energy storage system is more and more widely used in distributed generation system and micro grid. Energy storage system consist of two parts, storage facility and PCS. PCS, the interface between storage facility and micro grid(utility grid), operates the function of transferring and swapping energy between the storage system and the micro-grid bidirectionally. The structure and control strategy are significant for the stability, efficiency and performance of the PCS. This paper summarizes the commonly used topology structures, and analyzes their differences and characters. Then the control strategy is presented and summarized. Topologies and Control Strategies of PCS The storage facility in this paper is referred to storage battery units. PCS is a power electronic converter in nature used to regulate the power transfer between the storage facility and micro-grid. Figure 1 show the structure of the whole energy storage system. International Conference on Information Sciences, Machinery, Materials and Energy (ICISMME 2015) © 2015. The authors Published by Atlantis Press 261 Storage Facility PCS Release Power Store Power Micro-grid Fig.1 Structure of energy storage power conversion system PCS has many topological structures, the commonly used structures such as single-stage PCS, two-stage PCS and cascaded PCS will be presented. A. Single-stage PCS Figure 2 shows the single-stage PCS topology. From the figure we can conclude that the PCS is essentially a bidirectional dc-ac converter. This converter can not only absorb superfluous energy from the grid and store the power in battery units, but also release energy stored in the battery banks to the grid and convert the dc voltage to ac voltage connected to the grid in order to keep the balance of the grid. Moreover, a filter could be installed in the output side of the battery unit to filter out the harmonics and mitigate the ripple in the currents injected into the grid. DC-AC Battery Filter Transformer Grid