33 Tieline Controls in Microgrid Applications 1. Introduction As electric distribution technology moves into the next century, many trends are becoming apparent that will change the requirements of energy delivery. These changes are being driven from both the demand side where higher energy availability and eficiency are desired, and from the supply side where the integration of distributed generation and peak-shaving technologies must be accommodated. Distribution systems possessing distributed generation and controllable loads with the ability to operate in both grid-connected and standalone modes are an important class of the so-called Microgrid power system (Figure 1). Figure 1. Microgrid power system This class of Microgrid strives for optimized operation of the aggregated distribution system by coordinating the distributed generation and load resources - not only when connected to the main grid but also in a stand-alone mode. In either mode of operation, advanced local controls, energy management and protection technologies are required for robustness and reliability. While the energy management optimization objective function can be tailored to the needs of each application, in general the overall objective is to optimize operating performance and cost in the normally grid-connected mode, while ensuring that the system is capable of meeting the performance requirements in stand- alone mode. One very appealing technology for grid connected operation is tieline controls, which will regulate the active and reactive power low between the Microgrid and the bulk grid at the point of interconnection. These controls essentially allow the Microgrid to behave as an aggregated power entity that can be made dispatchable by the utility. Particularly beneicial to the utility is the fact that this feature can be designed to compensate for intermittency associated with renewable energy resources such as wind energy and solar energy, essentially pushing the management burden inside the Microgrid. This paper reviews the overall architecture of the Microgrid concept, and presents details associated with the tieline control features. 2. Microgrid Concept and Architecture A report by Navigant Consulting [1] prepared for DOE’s Ofice of Electricity Delivery and Energy Reliability identiies four classes of Microgrids: Single Facility Microgrids These Microgrids include installations such as industrial and commercial buildings, residential buildings, and hospitals, with loads typically under 2 MW. These systems typically have low inertia and require backup generation for off-grid operation. Microgrids for these applications will be designed to have improved power availability and quality, and a subset of them, such as hospitals, will require a seamless transition between grid- connected and island operation. Multiple Facility Microgrids This category includes Microgrids spanning multiple buildings or structures, with loads typically ranging between 2 and 5 MW. Examples include campuses (medical, academic, municipal, etc), military bases, industrial and commercial complexes, and building residential developments. As with single facility Microgrids, the design of multiple facility Microgrids will be driven by the need for high availability as well as improved power quality. Tieline Controls in Microgrid Applications M. Adamiak GE Digital Energy, Multilin S.Bose, GE Global Research, Y.Liu, GE Global Research, K.Bahei-Eldin GE Global Research, J.deBedout GE Global Research,