# PSO-Based Voltage Control Strategy for Loadability Enhancement in Smart Power Grids
## 6 Numerical Examples and Results
### Ieee 9-Bus  
The first test system is concerned with IEEE 9-bus system that consists of three generators, three loads, and nine branches, as shown in Figure 4. Furthermore, it is simply assumed that the phasor measurement units (PMUs) are installed at bus 5 and bus 6 (i.e., bus 5 and bus 6 are chosen as pilot buses). In other words, only the voltage magnitudes at bus 5 and bus 6 are used as control inputs.  
To confirm the effectiveness of the proposed control algorithm, comprehensive simulations including different load bus variations, different load levels, and different branch outage contingencies have been studied. Among those experiments, only the results listed below are discussed.  
* Case 1: All load bus variations and light load conditions.
* Case 2: Single load bus variation and peak load condition.
* Case 3: Several load bus variations, peak load conditions, and line outage contingencies.  
Case 1  
In the first test case, all load bus variations are simulated. The test system is operated at a light load condition, where the voltage magnitudes obtained from the PMUs are \(V_{5}\) = 0.8866 p.u. and \(V_{6}\) = 0.9902 p.u.  
In this case, low voltage violation occurs at bus 5 since \(V_{5}\) = 0.8866 p.u. is less than the predetermined value of 0.9 p.u. This will activate the proposed method, and the effect of the control strategy is shown in Figure 5. From the shown figure, one can see that the voltage magnitude of bus 5 is improved from 0.8866 to 0.9974 p.u. Also, the maximum load power is increased from 3.94 to 4.47 p.u.  
Case 2  
Single load bus variation is considered in this case. During the simulation, we raise load disturbance at bus 6. The test system is investigated under peak load conditions and is operated  
Figure 4: One-line diagram of IEEE 9-bus system with phasor measurement units (PMUs). PV: PV generaotr bus (voltage-controlled bus).  
Figure 5: Simulation result of Case 1 for IEEE 9-bus.  
in the dangerous region. The measured voltages without control actions are \(V_{5}\) = 0.9801 p.u. and \(V_{6}\) = 0.8707 p.u.  
Figure 6 illustrates the simulation result, indicating that the maximum load power is greatly enhanced from 3.77 to 4.29 p.u., and \(V_{6}\) improved from 0.8707 to 1.0404 p.u.  
Case 3  
In the third simulation, several load bus variations are simulated. The test system is investigated under stressed conditions, and the line #7-8 is studied under branch outage contingency condition. The system is operating in the dangerous region, which could result in \(V_{5}\) = 0.8773 p.u. and \(V_{6}\) = 0.8687 p.u. before control actions performed. Since the system has poor voltage level, the proposed methodology will be activated to prevent imminent voltage collapse. Figure 7 shows the simulation result. In Figure 7, it is clearly seen that system voltage stability is considerably improved after the proposed methodology.  
Figure 6: Simulation result of Case 2 for IEEE 9-bus.  
Figure 7: Simulation result of Case 3 for IEEE 9-bus.  
The results in terms of \(y^{RMS}\) and \(VSM\) for the above three cases are summarized in Table 1, showing that the performance of the power grid is significantly enhanced. This means that the control decisions provided by the proposed scheme are efficient and effective for the improvement of overall power grid voltage stability.