* [16] M. C.  
 ------- (5)
$\mathbb{E}\;indgx=\frac{\sin(4\pi n(6\cdot\mathbb{a}))}{\frac{\sqrt{2}\cos(4\pi n)}{2}}$ . 
PR values can be obtained from the load flow solution.

The main cause for the problem of voltage instability leading to voltage collapse is stressed power system characterized by excessive line loading. 

As the maximum power transfer theory restricts the amount of load that can be transferred through a line, the LF index precisely indicate the voltage stability margin for a selected operating condition. 

## 3 Methodology

Voltage Stability Margin expressed in terms of Static Voltage Collapse Proximity Indicator at a given load of a selected load bus accurately estimates the criticality of that bus from the voltage collapse point of view. Hence computation of these indicators along with voltage profiles at load buses can serve as a very good measure in assessing the criticality of a bus. In addition to line flow index, bus voltage profiles are used to identify weak buses under varying load condition. The point at which LF index is close to unity indicates the maximum possible connected load called as maximum loadability at the point of bifurcation. The line flow indices and bus voltage profiles are divided into different categories and are expressed in fuzzy set notation. The severity indices are also divided into different categories. The fuzzy rules are used to evaluate the severity of load buses. Criticality index is computed based on severity of LF index and voltage profiles. Based on this index the buses are ranked. The ranking obtained using Fuzzy approach is verified with Fast Voltage Stability Index (FVSI) [3]. 

## 3.1 Bus Voltage Profiles

The bus voltage profiles are divided into three categories using Fuzzy Set 

![0_image_0.png](0_image_0.png) notations: low voltage (LV), normal voltage (NV) and over voltage (OV). 

Figure 4 shows the correspondence between bus voltage profiles and the three linguistic variables. 

## 3.2 Line Flow Index
