Enhancement of Power System State Estimation

Power Utility companies use the state estimator to provide system operating status to the operators of their control center to allow them to manage and to take appropriate measures to prevent the loss of electricity. The unavailability of state estimation solution may cause the occurrence of cascading failures or blackouts in local and/or regional areas for considerable time periods, if disturbance occurs during the period of unavailability and thus can not be closely monitored. The robustness and reliability of state estimation is a critical issue and concern of power utilities. The Weighted Least Square (WLS) method is the commonly used state estimation methodological approach in power industry. If one or more gross errors are contained in the measurements the WLS state estimator may not reach a solution and diverge. A well-known example when the WLS did not converge due to the existence of a topology error was a indirect contributing factor to the August Blackout in Northeastern U.S. in 2003. According to the President’s Task Force the operator could not determine the status of the system because of a computer program ‘glitch’. This ‘glitch’ was a failure of the WLS method to converge and give a solution to the State Estimation. Task Force comments noted the ‘unacceptability’ of such computer program errors when the economic impact of the consequential blackout was so dramatic. The economic damage of the 2003 blackout was reported to be in excess of $10 Billion dollars. The following figure shows the convergence property of WLS state estimation. This figure was obtained on IEEE-118 bus system. WLS state estimation has been simulated on 5000 different patterns of load levels for IEEE 118-bus system. It is clear to see that WLS state estimation will be completely unfunctional after the load level reaches a specific amount. Details of this simulation will be explained later in the chapter. The need to detect the gross errors is a critical and challenging issue for WLS state estimation. Many researchers have tried to develop algorithms to detect gross errors for WSL state estimation without dramatic success. Most of the detection techniques proposed so far are based on a solution of WLS state estimation. The dilemma is that detecting gross errors requires a solution of state estimation under the presence of gross errors that solution may not occur. Topology errors are classified in two categories: branch status errors and substation configuration errors (Abur and A.G. Exposito, 2004). The analysis of conditions upon which topology errors can be detected was presented in (K. A. Clements and A. Simoes-Costa, 1988