# Power Management Strategy for an Autonomous DC Microgrid
## 3 Power Management Strategy
### Case 5, 6, 7, 8, 9  
If there is not enough PV power to meet load demand, other sources are required to complement the deficient power.  
The electrochemical battery bank is set to provide the energy in the first place. Simultaneously, the energy in the SC decreases on account of self-discharging as time goes by. The SC could therefore be empty before the battery bank energy is depleted. As the SC is essential for DG start-up, draining of SC energy should be avoided. Hence, when \(SOC_{SC}\) reaches \(SOC_{SC\_MIN\_MAX}\), the electrochemical battery bank should provide additional power to compensate the self-discharging power of the SC, so that the start-up of the DG can always be supported by SC power. Consequently, the role of the battery bank is not only to supply the load, but also to recharge the SC. Nevertheless, it is possible that the required power could be greater than the electrochemical battery bank's maximal discharging power, so the load power is limited and the system is driven as given in case 5 of Figure 3. Otherwise, the electrochemical battery bank's power is enough, corresponding to case 6. On the other hand, if \(SOC_{SC}\) is greater than \(SOC_{SC\_MIN\_MAX}\), the electrochemical battery bank only supplies load demand. There are also two branches depending on whether or not electrochemical battery bank power is insufficient, represented by the cases 7 and 8, respectively.  
The last scenario is the case 9, where, in the case of insufficient PV power, the parallel supply mode of the PV and DG power sources is considered. During the duty-cycle noted \(D\), the DG supplies power in parallel with the PV source. Once the PV source and storage can satisfy load power demand or the duty-cycle time is ended, the DG is stopped to save fuel and reduce greenhouse gas emissions. To start up the DG, there are two conditions that can be met. If either is met, the DG will be started up. The first condition is that load power-shedding demand is more than 20% of total load power demand. The second is that \(SOC_{B}\) reaches its lower limit. During the period of DG start-up, the SC is used to stabilize DC bus voltage and balance the DC bus power for a short period. When the DG reaches its stabilized output state, it will be connected to the DC bus. The DG then operates not only to compensate the shortage of PV power, but also to recharge the two storage units, the electrochemical battery bank, and the SC. In this case, the power reference of the DG is set to be the sum of the maximal battery bank recharging power and the difference between PV power and load power. According to the recharging priority, the SC is first recharged to \(SOC_{SC\_MAX\_MAX}\), and then all excess power is injected into the electrochemical battery bank. The DG will then be stopped if either of the following conditions is met: (i) The electrochemical battery bank is fully recharged; or (ii) The DG has run for more than the chosen duty-cycle time period. The maximal running time limitation is for economic purposes. A short limit can save fuel but may lead to frequent start-up of the DG as the battery bank is not fully recharged, which can reduce DG lifespan. For a small-scale power DG, the duty-cycle time period has to be determined as a good trade-off between fuel consumption and start-up frequency. Case 9 is the only case in which the DG is started and stopped.