## 3 Analysis of Power Source and Load Coordination Mechanism  
### Load Character Analysis  
Load in a microgrid consists of different types including: industrial load; commercial load; hospitals; schools; residential load, etc. Different types of load possess unique characteristics and various requirements on power supply reliability. Basically, load can be divided into three categories according to the importance grade. The classification rule of each load grade is regulated in [34], as illustrated in Table 2.  
The analysis on the power supply reliability requirement of each load grade is essential for the study on the power-source coordination mechanism. Herein, the time limit of power interruption (\(t_{\mathrm{interrupt}}\)) is used to measure the reliability requirement of load. This index describes the maximum acceptable power interruption time during a power outage failure. The \(t_{\mathrm{interrupt}}\) varies among specific types of each load grade. For instance, \(t_{\mathrm{interrupt}}\) of electrical equipment utilized in the electronics manufacturing industry is illustrated in Table 3.  
For simplification, \(t_{\mathrm{interrupt}}\) of a certain type of load is selected as the minimum value among the \(t_{\mathrm{interrupt}}\) of each type of electrical equipment. For instance, \(t_{\mathrm{interrupt}}\) of the electronics manufacturing industry is selected as 200 ms. \(t_{\mathrm{interrupt}}\) of major types of level 1 load is illustrated in Table 4.  
As illustrated above, \(t_{\mathrm{interrupt}}\) of major types of level 1 load ranges from several hundred milliseconds up to 1 min. According to the characteristics of ES devices analyzed in Section 2.2, fast responding ES devices, i.e., super capacitor, shall be considered for improving the power supply reliability. As regards level 2 and level 3 load, the time limit of power interruption is not specified in existing manuals, regulations and national standards. Hence, the power supply reliability of these loads is restricted by the annual power outage limit, as well as by considering energy shortage compensation during life cycle cost (LCC) calculation. \begin{table}
\begin{tabular}{c c c c} \hline
**Load Grade** & **Level 1** & **Level 2** & **Level 3** \\ \hline \multirow{6}{*}{Description} & Power supply interrupt may cause: & \multirow{2}{*}{Power supply interrupt may cause:} & \multirow{2}{*}{Load not belonging} \\  & extremely severe pollution; & serious pollution; & \\  & poisoning, explosion or fire; & serious political effect; & \\  & extremely severe political effect; & serious economic loss; & \\  & extremely severe economic loss; & areal social chaos. & \\  & massive social chaos. & & \\ \hline \multirow{2}{*}{Supply requirement} & Must be supplied & Should be supplied with & Can be interrupted \\  & without interruption & minimal interruption & if necessary \\ \hline \end{tabular}
\end{table} 
Table 2: Classification of load based on their importance. \begin{table}
\begin{tabular}{c l c} \hline
**Specific Type of Level 1 Load** & **Electrical Equipment** & \(t_{\mathrm{interrupt}}\) \\ \hline \multirow{6}{*}{Electronics manufacturing (Chip manufacturing)} & Emergency illumination & \(<\)1 min \\  & Fire protection facilities & \(\leq\)1 min \\  & Information Technology Computer Integrated Manufacturing (IT CIM) equipment & \(\leq\)200 ms \\  & Automatic board feeder & \(\leq\)200 ms \\  & Tin scraping machine & \(\leq\)200 ms \\  & Solder paste printer & \(\leq\)200 ms \\  & High-speed chip monuter & \(\leq\)200 ms \\  & High-speed welding furnace & \(\leq\)200 ms \\ \hline \end{tabular}
\end{table} 
Table 3: \(t_{\mathrm{interrupt}}\) of electrical equipment in the electronics manufacturing industry. A sector customer damage function (SCDF) to calculate power outage loss is proposed in [43], and the economic loss of a typical load is illustrated in Figure 3. For level 1 load, i.e. industrial load, the economic loss will rise significantly within several minutes after the outage.