Studies On Wide Area Measurements And Controls For System Protection Of Power Systems

Large disturbances have always been one of the main threats to transient stability of power systems. Wide area measurement system (WAMS) and special protection system (SPS) are adopted to observe, detect, and control such disturbances, forming the architecture of wide area protection (WAP). Phasor measurement units (PMUs) can be installed in WAMS to obtain wide area measurements detecting large disturbances. And emergency controls can be armed in SPS to take corrective action against large disturbances.There are thousands of available locations for the installation of such observers and controllers, especially in a large-scale power system. Hence it is a natural and important issue to decide where are the ideal sites for measurements to observe the system separation arising from a large disturbance and where are the ideal sites for emergency controls to act against angular instability due to a large disturbance. The issue is termed the placement of wide area observers and controllers for system protection against large disturbances, which is the main focus of the study in this dissertation.The purpose of the dissertation is to lay the theoretical foundation for the placement of wide area observers and controllers for system protection against large disturbances, to present integrated and effective placement approach for power systems, and to apply the placement approach to a specific large-scale power system.The dynamic behavior of power systems after large disturbances is studied first of all. The trajectory of severely disturbed power systems will pass by controlling unstable equilibrium point (UEP) on the verge of losing stability, which is an inference of stability region theory. Then the characteristics of controlling UEP largely reflect system separation due to large disturbances, which can be revealed via modal analysis. The concepts of modal controllability and observability are introduced into the modal analysis at the controlling UEP of a large disturbance. Rank tests are proposed to qualify the placement to ensure the unstable modes of the disturbance controllable or observable. The rank tests are the general rules imposed on the placement of observers and controllers.A placement approach based on controlling UEP is presented. Structure preserving model of power system is expanded with input and output to accommodate emergency controls and measurements. The model is linearized with respect to controlling UEP and then reduced to the network-reduction model. Finally diagonal canonical form can be obtained, and the mode observability and controllability matrices are utilized to select the suitable locations of wide area measurements and emergency controls.Another placement approach based on critical cutset is also presented. Voltage correction is proposed to obtain critical cutset more exactly. Cutset vulnerable index is corrected using bus voltages at controlling UEP instead of at SEP, which can accommodate the characteristics of system trajectory following large disturbances. Power systems losing synchronism mainly suffer from the increase of line angle differences or line flows in critical cutsets, which can be captured by measurements and be withheld by emergency controls. Then the sensitivity of line flow of the critical cutset with respect to a generator or load power can indicate whether generation tripping or load shedding is effective at the site, which can be utilized to select the suitable locations of emergency controls.The architecture of large disturbance analysis is constructed to reveal the mode of disturbance, in which unstable modes of controlling UEP, grouping of generators, and critical cutset can be integrated. The procedure of the placement of wide area observers and controllers for system protection against large disturbances is summarized and applied to China Southern Power Grid.