مدل سازی ذخیره انرژی برای برنامه ریزی توزیع Energy Storage Modeling for Distribution Planning

I. INTRODUCTION nergy storage devices are being proposed as the solution to various operational and reliability problems on power systems mainly due to large amounts of variable and uncertain power sources such as wind and solar generation. Some locations such as the US state of California and the Canadian province of Ontario have been particularly aggressive in adding large amounts of storage to the grid (100’s of MW of capacity) to counter the anticipated problems from these sources of generation.. A certain amount of this storage will undoubtedly be installed on distribution systems. Recent storms in the US such as the occurrence of the historic derecho (straight-line winds) and Storm Sandy in June R. C. Dugan is with the EPRI, Knoxville, TN 37923 USA (e-mail: r.dugan@ieee.org). J. A. Taylor is with the EPRI, Knoxville, TN 37923 USA (e-mail: jtaylor@epri.com). D. Montenegro was formerly with Universidad de Los Andes, Bogota, Colombia and has recently joined EPRI, Knoxville, TN (e-mail: dmmartinez@epri.com). and October of 2012, respectively, drew considerable attention to the topic of resiliency of the grid. Many customers were without electric power for several days and some for several weeks. This prompted calls for microgrids to be built so that parts of the distribution system can be operated in islands until power lines damaged by the storms can be repaired to reestablish grid connection. Energy storage is a key part of making such efforts successful. Although a great deal of the storage will likely be installed on the distribution system, much of it will be controlled by the area grid operator for the benefit of the transmission grid. The distribution system is simply a host for the storage. If the grid frequency begins to droop due to loss of wind generation, the distributed storage will be called upon to provide power to counter the downward ramping of the wind turbines. If the frequency increases above nominal, the storage elements would switch to charging mode and absorb power up to maximum storage capacity to slow the frequency. This gives time for more conventional generation sources to re-dispatch to meet the load. Storage is energy storage, measured in kWh or MWh, while most distribution planning studies are focused on the capacity to deliver power, measured in kW or MW. Energy is the time integral of power, so modeling storage naturally adds the time dimension to the planning problem, which is both useful and challenging at the same time. One ramification is that static power flow solutions will no longer provide adequate insight into many of the planning problems that planners will face. Planners must simulate the system over a reasonable amount of time to get the correct answer when there are time-variant resources such as solar PV and wind generation. II. APPLICATIONS OF STORAGE ON DISTRIBUTION SYSTEMS Some of the applications proposed for storage installed at the Distribution primary (MV) or secondary (LV) level include: Compensating for, or smoothing, solar PV power output ramping. Extending the power output from solar PV to meet the early evening peak demand. On many distribution systems the peak load occurs after the sun has gone down. Support of Transmission grid: Compensating for the loss of solar power at the end of the day to reduce the need to for fast ramping of conventional sources; stabilizing the grid during periods of high variability of renewable resources. Extend the capacity of an existing distribution substation or feeder. Supporting alternate feeds for temporary reconfiguration. Controlling the frequency of a microgrid. Increase the available short-circuit current of a microgrid so that it is more capable of operating distribution system and customer protective devices. Reducing the cost of electricity to a given power purchaser by charging off-peak when the energy is cheaper and discharging to supply load during peak demand periods. There are undoubtedly many more potential applications for storage, but this list gives an idea of why there is much interest in storage on utility power distribution systems. III. DISTRIBUTION PLANNING ISSUES INTRODUCED BY ENERGY STORAGE Installing energy storage devices on the power distribution system introduces several issues to be considered by planners. These issues include: Overvoltages while discharging. The impact depends on the location and capacity of the storage devices and is similar to the impact which is evaluated for hosting capacity analysis of inverter-based solar PV systems. This could be a particular problem when storage is dispatched for purposes other than the benefit of the local feeder. The storage dispatch could occur at night or any other light loading time. Low voltages while charging. To reach the goals of several 100 MW of storage across a utility service area, one can easily imagine that the capacity of storage devices on a given distribution feeder could be several hundred kW to a few MW. Voltage regulation while compensating for transmission grid support. The power produced or consumed may very well have no relevance to the behavior of the load on the distribution system. Interference with overcurrent protection practices. All distributed power sources have the potential to disrupt long-standing utility practices during fault clearing operations. Since most storage devices currently being considered have inverter-based interfaces to the utility grid, short-circuit current contributions are expected to be 110-120% of rated current. With several large devices this is sufficient to disrupt fast-tripping/fuse-saving coordination. On the other hand, it is insufficient to operate conventional overcurrent devices such as fuses, reclosers, and circuit breakers (see next bullet). Sufficient short circuit capacity to operate overcurrent protective devices when operating as a microgrid. This is a common problem with all smaller resources and calls for a different approach to distribution system protection based more on voltage quantities and impedance relaying. Even if the protection on the utility-owned distribution system is modified to accommodate low-capacity sources, the vast majority of consumers will still have conventional overcurrent breakers that require a strong short-circuit current to operate.