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Stability Analysis of MicroGrids

MicroGrid concept corresponds to the latter approach. This is a new concept to gather or aggregate distributed energy resources and loads in a small area. This concept has been proposed to achieve effective use of distributed energy resources, provide a high-quality energy services and avoid giving bad impact to the connected distribution grid. These major advantages of MicroGrid can be summarized as follows:

• a) MicroGrid can provide an efficient way to integrate distributed energy resources and loads for taking full advantage of them (including CHP).
• b) MicroGrid can be a “grid-friendly” entity and does not give undesirable influences to the connecting distribution grid. That means the operation policy of distribution grid does not have to be modified.
• c) MicroGrid can achieve a flexible way for distributed energy resources to connect and disconnect as they like, which is called as “plug-and-play” feature.
• d) MicroGrid can independently operate without connecting to the upper distribution grid when a fault occurs (islanding mode).

The last item d) is one of the most important features of MicroGrid but there might be a control problem, and this research is focusing on this issue. When operating independently as islanding mode, all loads have to be supplied and shared only by distributed energy resources. For bulk power systems, there are many large generators, which have big inertia, and they always store much kinetic energy in its rotating body. The stored energy can contribute to compensate the power disturbance with slight frequency drop. MicroGrid, however, does not have a large, central generator and there is possibility that the balance of demand and supply cannot be maintained when a large disturbance occurs. Moreover, MicroGrid can include various types of distributed energy resources and undesirable oscillation continues even if the energy balance being maintained. The characteristics of these distributed energy resources are totally different each other, and their response times and the response itself are also different. This may cause the interaction problem (power oscillation here) when entering the islanding mode operation conditions.

In this study, we have developed the MicroGrid model that includes diesel engines, microturbines and fuel cells as typical examples. These distributed generators are not affected by natural phenomena like wind or sunshine, which are suitable to identify what kind of phenomena would occur when entering the islanding mode. Now, an example of MicroGrid implementation and its modeling are being developed and a typical case of the interaction problem is being analyzed through numerical simulations.