# Optimal Design and Real Time Implementation of Autonomous Microgrid Including Active Load
## 4 Results and Discussion  
### Time Domain Simulation  
An autonomous microgrid shown in Figure 1 has been modeled. Three inverter-based DGs (10 kVA) are connected with CIL (5.8 kW) and CPL (7.3 kW) through coupling inductances, filters, and two transmission lines. Assuming an ideal source from the DG side, the dc bus dynamics can be neglected. With the realization of high switching frequencies (4-10 kHz), the switching process of the inverter may also be neglected [40]. The system parameters are given in Table 1. Based on time-domain simulation, the error in the measured active power and DC voltage will be curtailed using a weighted objective function. The optimal parameters of the proposed active load and inverter controllers, _PLL_ gains and power-sharing coefficients are presented in Table 2. Fault and step change disturbances have been applied on both ac and dc sides to explore and assess the effect of the optimal parameters on the microgrid stability. Through the time domain simulations, the microgrid performance has been examined firstly when three-phase fault occurred at active load bus. Figure 7 shows the satisfactory damping characteristics of the proposed microgrid under the fault disturbance at active load bus. Figure 7a,b depict the responses of the active power of the three DGs respectively and the DC output voltage of the DC side of the active load. To feed this fault, the output powers of the three DGs have been increased sharply after the fault immediately. Additionally, the output power of DG3 is much greater than the output powers of the DG1 & DG2 because the fault occurred at active load bus which is closed to DG3. Secondly, the microgrid performance has been examined when a three phase fault is applied at line 2 at \(t\) = 0.3 s then fault is cleared at \(t\) = 0.6 s. During this period, DG1 and DG2 respond to support the passive load by its demand while DG3 is isolated to feed the active load by its powers. Figure 8 shows the adequate damping characteristics of the proposed controller. The DGs output powers response is illustrated in Figure 8a while Figure 8b depicts the DC voltage response of the active load for this disturbance. It can be observed from the given results that the system performance has a good overshoot and settling time performance. Moreover, the results depict impressively enhanced damping characteristics. Thirdly, the controller capability is investigated and the microgrid performance is examined when the microgrid loses DG1. It means that both loads have to receive their powers from DG2 and DG3. Figure 9a,b illustrates the response of the output real power of the three DGs and the DC voltage response of the active load. Additionally, DG1 and DG2 are capable of feeding both loads after the microgrid is disturbed. After losing DG1, the results show reasonable damping characteristics of the proposed controller. From Figure 9a, at the steady state operation, the output active powers of the three DGs are 0.41, 0.43, and 0.43p.u. respectively. Therefore, the total output power is equal to 1.27p.u. While after the microgrid is cleared from the fault disturbance, the output active powers of the three DGs are 0.0, 0.37, and 0.9p.u. respectively. Therefore, the total output power is equal to 1.27p.u. This means that the total output powers of all DGs before and after fault are equal. Therefore, the two DGs (DG2 and DG3) can share the same power that the three DGs (DG1, DG2, and DG3) have shared before the microgrid lost DG1. In addition, as it was mentioned before in Section 2, each DG inverter is assumed to be connected to a constant dc power source, so there no need to regulate the dc-link voltage otherwise, a controller should be introduced to regulate the dc-link voltage [40]. Otherwise, DG3 output power could not be reached to this value since each DG source has a limited output power. For example, if DG3 represents a photovoltaic plant, the output power increase could not be available for many different reasons such as temperature and dc voltage regulations.