# A Contemporary Novel Classification of Voltage Stability Indices
## 6 Result and Discussion  
The generalizability of an exhaustive investigation on a comparison of voltage stability indices seems problematic due to the multiplicity of indices, and more importantly their variety in theoretical foundations and performances. However, this study discussed all the first two categories indices in Table 1. One of the main drawbacks of the remained indices in Table 1 is their high computation cost due to indices complexity and dedicated tools and system parameters requirement (the PMU-based indices simulation are relinquished). Since the investigation of all indices under this study is not applicable; therefore, a few world-wide accepted indices from each category are simulated. Whereas, those indices, which contain individual, different behavior with the rest of the indices in a class, are well detailed in Section 4. The comparison study shows that virtually the performance of the proposed indices has a high degree of accuracy for assessing the critical node and line as the results are almost close to an agreement. By relying on the theoretical formulation of both line and node indices, considering the simulation results and performances in Tables 2 and 3, as well the merits and demerits in Table 4, some findings can be noted as follows:  
* Almost all indices in a category are in agreement with identifying the weak buses and critical lines in the system. Generally, indices in a category pursue the same manner. * Despite the indices in the same category having the same theoretical foundation mechanism, the performance of some indices is in disagreement with the rest of the indices. For instance, the V\({}_{\text{cr}}\) index in the line indices category, and VSF and PSI in the node indices category do not draw the same result as other indices. The V\({}_{\text{cr}}\) from the formulation point view implies that the model analysis-based indices formulation with respect to the Jacobian matrix singularity assumption is not wholly accurate, especially at the collapse point. On the other hand, some studies without considering the generalization directly applied the quantitative results of the customary two-bus model. While for a complex system with multiple generators and control elements, it is not an adequate solution. These indices are L index, novel line stability index (NLSI), stability index (SI), voltage collapse index (VCI), voltage stability factor (VSF), line index (L), fast voltage stability index (FPSI), critical voltage (Vcr), and power transfer stability index (PTSI). * There are some indices, which are fundamentally the same, but from the driven point of view, they are different. In other works, their results are the complement of each other in a common concord. * All indices in a class are coherent to their typical theoretical bases and pursue the same performance. The range of stability for most of the indices is between 0 and 1.0. Someway, it indicates that the indices discernment characteristics of performances are in accord. * [PERSON] and [PERSON] [44] have argued that the line indices can also determine the weakest bus in the system. While, the comparison of the line and node indices in Tables 2 and 3 have negated this argument. Since mostly the line indices are driven without taking into account that the reactive power generation limits can cause misidentification. * An index with a bad ranking in compliance with other indices does not imply that the index is useless, whereas, each index is functional for a specific application. In the literature, the demerit of the sensitivity indices are pointed out as these indices alone will not be sufficient to identify a critical node, especially in an interconnected system. However, when the system is suffering from a heavy load in a stressed situation, the \(\Delta V_{i}/\Delta Q_{i}\) and \(\Delta V_{i}/\Delta P_{i}\) sensitivities indicators play a significant role in voltage collapse prediction [53].