Wide area voltage protection

Abstract

Two very promising wide-area voltage protection (V-WAP) solutions, able to face stability and security problems in the transmission grid, are presented. Both require real-time effective risk indicators whose timely computing is a crucial issue. The first method makes use of a voltage instability index based on real-time computing of the Thevenin equivalent circuit seen by a given high voltage (HV) bus. The innovating algorithm utilised for the real-time adaptive identification of the Thevenin voltage and impedance equivalent values requires local phasor measurements at fast sampling rate. The unique ability of the second method is mainly because of an effective coordination with a supposed operating wide-area voltage regulation (V-WAR) modern system where the secondary and tertiary voltage regulations (SVR and TVR, respectively) operate according to their hierarchies. Evidence for the feasibility and simplicity in defining and developing an effective V-WAP solution once reliable real-time risk indicators are available is provided. The feasibility of the first method comes from the high computing speed of the local risk indicator at each area bus where a phasor measurement unit (PMU) is installed. The main simplification of the second method comes from the already existing and appropriate subdivision of the network into areas provided by the operating SVR, the same areas as those of the protection scheme intervention. From this starting point, a voltage stability index is proposed operating the area load shedding (LS) according to a simple and incontrovertible logic based on the real-time area control effort. Dynamic simulation results on the North Italy power system show the proposed protection scheme's effectiveness and ability to achieve the continuity and stability of the controlled power system in front of a ramping load increase, by also containing the LS amount.