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 (