## 3 Analysis of Power Source and Load Coordination Mechanism
### Power Source Capacity Planning Strategy Considering Source-Load Coordination  
According to the discussion in Section 3.1, the energy shortage of level 1 load shall be fully compensated at all costs. In order to achieve uninterrupted power supply even if power generation is not sufficient, ES devices and backup sources shall be coordinated according to their response speed, discharge durations and other characteristics. Hence, super capacitors with fast response speed are utilized for smoothing short-time fluctuation or power supply during the initial stage of a long-term energy shortage, while a battery energy storage system (BESS) will successively be utilized for subsequent supply. In extreme cases with intensively long shortage duration, backup sources such as a diesel generator will operate as a stable power source until renewable energy generation is restored. As for level 2 load, the requirement on power supply reliability is not as strict compared to level 1 load; thus, a short-time outage at the initial stage of a shortage period is acceptable in most cases and investment on SC can then be reduced. When it comes to level 3 load, BESS capacity can also be reduced since level 3 load basically has no requirement on power supply reliability. The principle of source-load coordination is illustrated in Figure 4. \begin{table}
\begin{tabular}{l l l} \hline \hline \multicolumn{2}{c}{**Types of Level 1 Load**} & \multicolumn{1}{c}{\(\boldsymbol{t_{\text{interrupt}}}\)} \\ \hline \multirow{4}{*}{Industrial load} & Mining & \(\leq\)200 ms \\  & Chemical industry & \(\leq\)200 ms \\  & Metallurgic industry & \(\leq\)1 s \\  & Electronics manufacturing & \(\leq\)200 ms \\ \hline \multirow{4}{*}{Social load} & Communication & \(\leq\)800 ms \\  & Radio and television & \(\leq\)800 ms \\  & Information safety & \(\leq\)800 ms \\ \cline{1-1}  & Public services & \(\leq\)1 min \\ \cline{1-1}  & Transportation & \(\leq\)800 ms \\ \cline{1-1}  & Medical services & \(\leq\)0.5 s \\ \cline{1-1}  & Assembly occupancies & \(\leq\)1 min \\ \hline \hline \end{tabular}
\end{table} 
Table 4: \(t_{\text{interrupt}}\) of major types of level 1 load. Figure 3: Economic loss curve of typical load after power outage. BESS in this research consists of a Li-ion battery and a lead-acid battery. Compared to a lead-acid battery, a Li-ion battery has advantages including: (a) relatively high energy density; (b) longer lifetime at high depth of discharge (DOD); (c) high power receiving capability to realize high current charging in a short time, while the charging current of a lead-acid battery is usually limited within 0.25 C to avoid capacity degradation. Nevertheless, the unit cost of a Li-ion battery can be 5-7 times higher than a lead-acid battery, which is the major factor restricting its application. A reasonable portion of a Li-ion battery in BESS, which is charged and discharged with priority, will significantly reduce the utilization frequency of a lead-acid battery and extend the overall lifetime of BESS. Through surveying typical daily load and climate parameters including temperature, sunlight intensity and wind speed, the curves of load and generated power of wind turbine (WT) and photovoltaic (PV) can be obtained. The difference of generated power and load is then defined as \(\Delta P\), and the area enclosed by the \(\Delta P\) curve and the axis is \(\Delta E_{i}\). Whether \(\Delta E_{i}\) is a positive or negative value determines the operation status of ES devices (charging or discharging) during this time interval. \(t_{\text{B-switch}}\) and \(t_{\text{D-switch}}\) represents the response speed, or switching time, for BESS and diesel generator respectively. The overall compensation strategy is arranged as follows:  
1. Charging process  
For a certain time interval with a duration of \(t_{i}\), if \(\Delta E_{i}>0\), ES devices will be charged. The redundant energy generated by WT and PV shall firstly be used to charge the SC in order to guarantee energy reserve of fast response ES for uninterrupted supply of level 1 load.