Vulnerability Assessment of Branch Failures in an Interconnected Power System Network

Abstract

Voltage instability has been a major concern to power supply utilities and its effect has resulted into system voltage collapse and high power losses. This study employed fast voltage stability index technique (FVSI) to identify critical nodes and branches in an interconnected power system network, considering the IEEE 57-node electricity grids as case study. The modelling of the steady state characteristics of the network was carried out using load flow equations. The equations were linearized via Newton-Raphson iterative technique. Load flow modelling and simulation were carried out to determine the node voltage, phase angle and active power loss along the network lines. Simulations were done for the base case and the contingency-variation of the reactive loads in the network until FVSI value approaches one (1) to determine the maximum permissible load of each load nodes. The ranking in the system was done by sorting the maximum permissible load of the load nodes in ascending order. The smallest maximum permissible load was ranked highest implying that the node is the weakest in the system. The identified critical branches were corrected using static var compensator (SVC). Thereafter, the node voltage magnitudes and branch active power losses were computed and compared for the case networks. The results revealed that, for IEEE 57-node power system, 43 node voltages out of 57 node voltages were outside the statutory limit of ± 5%. The total active power loss was 65.303 MW. The inclusion of SVC in the system corrected the voltage limit violations on the critical nodes while the total active power loss reduced by 43.29% to 37.039 MW. These results are indications that FVSI when appropriately applied can aid the identification of critical nodes and branches in power system network while the SVC installation can minimise the power loss and improve the voltage magnitude of the system. This study established the suitability of fast voltage stability index technique for the weak nodes and branches identification in a power system for possible remedial action.