## 2 Layered Fault Tree Model for Smart Grids
### The Layered Fault Tree  
With all the new concepts proposed with smart grids, there are three main aspects of differences between the future distribution systems and the conventional:  
* With the introduction of DGs, new distribution systems become multi-power resources served networks, instead of traditional radial construction served by a single source, so the structures of the new distribution systems will be improved, in order to gain a better architecture to have DGs access. * The uncertainty of the operating state of the new distribution systems increases greatly. The output power of renewable primary energy sources has great randomness, and no longer depends on users' loads. Moreover, new distribution systems may be operating in islanded mode or grid-connected mode, which can reconfigure the new distribution systems to be bidirectional networks, with many small-scale DGs integrated. * Control techniques and methods for the envisioned distribution systems are undergoing significant changes. Different from conventional distribution systems, it is difficult for the smart grids to use a single control center to regulate the whole system rapidly and efficiently. Control of the new distribution systems should base on local information as much as possible [1, 18, 19]. Therefore, the theory of distributed control for new distribution systems can be far more complicated than traditional control theory. Figure 1: Common event representations and logic operations of fault tree. Consequently, enhanced techniques and approaches for reliability evaluation of smart grids are of great necessity. The fault tree model can be both versatile and easy to use among the techniques available. Due to the differences between smart grids and the conventional distribution systems, the fault tree model needs to be revised in order to reflect the characteristics of smart grids. The running mechanisms should also be reflected in the modeling and evaluating process. Before the procedure is given, key assumptions were made about the research object in this paper:  
1 The islanding strategy of a smart grid is intentional. It gives the possibility to imply the state changing process in the structure of the fault tree model and in the simulation process. Based on a specific architecture of smart grid, the research object should also be an integral potential islanded local framework, with one or more intelligent substations in it. 2 In the local smart grid framework, power can be dispatched freely, and loads of low priority can be cut off from the system in order to guarantee power supply for more important loads if needed. The lines or buses in the distributed smart grid infrastructure also have enough capacity and won't be overloaded when transmitting electricity. This assumption conforms to the designing and operational feature of smart grids, which increases the power distribution reliability. 3 The intelligent control and protection systems of smart grids are not further analyzed and decomposed in this paper. These functions are usually incorporated in the intelligent substations in a specific smart grids architecture and will be achieved not only by devices but also through the intelligent control software. The consideration of these systems can make the models improved, but will increase the complexity additionally of the overall procedure. For a smart grid, there are two operation modes, each mode has its own proper conditions and the two modes can mutually transform. In fact, grid-connected running is the normal running state, and islanded mode can be regarded as its sub-procedure. It provides a condition for hierarchical description of the fault tree. The fault tree can be layered by distinguishing and separating two different power supply modes, which are utility supply and local supply. When the main grid works properly, the power supply for the concerned load can be fully guaranteed.