A New Voltage Stability Index For Predicting Voltage Collapse In Electrical Power System Networks

ABSTRACT

Power system voltage instability often results in voltage collapse and/or system blackout which is a source of concern for power network operators and consumers. This work proposes a new line stability index that is suitable for investigating the voltage stability condition of Power System Networks (PSNs). This index, which is called the New Line Stability Index-1 (NLSI_1), is derived from first principles and shown to incorporate the Line Stability Index (Lmn) and the Fast Voltage Stability Index (FVSI), with an associated switching logic based on the voltage angle difference since it can indicate the incidence of voltage collapse. The NLSI_1 aims at improving the accuracy and speed of identifying the weakest bus associated critical lines with respect to a bus for purposes of optimally placing compensation devices as well as investigating the effect of increasing reactive power loading on the PSN. The developed index (NLSI_1) was tested on the IEEE 14-bus system and the present 28-bus, 330-kV Nigeria National Grid (NNG) using a program coded in the MATLAB environment. The three indices were then simulated for the base case and the contingency – variation of the reactive loads in the network. For the base case, the IEEE 14-bus test system was stable with all the three indices approximately equal and < 1 for all the lines. Contingency simulations were carried out revealing that bus 14 ranks as the weakest bus of the system, with the smallest reactive load of 74.6 MVAr among the load buses. The values of the indices, Lmn, FVSI and NSLI_1 are approximately equal for the IEEE 14-bus system thereby validating the efficacy of the new line stability index-1 (NLSI_1). For the NNG system, the power flow solution showed that the voltage profiles for load buses 9, 13,14,16,19 and 22 (Kano, Gombe, New Haven, Jos, Ayede and Onitsha, respectively) have voltage magnitudes 0.932, 0.905, 0.949, 0.844. 0.93, and 0.818 p.u, respectively against the voltage criterion of 0.95 p.u. These low voltages are indication that the network buses are prone to voltage instability. The base case of the NNG simulation values for all three indices (Lmn, FVSI and NLSI_1) were less than unity (