9291_c019 - 19 State Estimation 19.1 19.2 19.3 State...

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19 State Estimation Danny Julian ABB Power T&D Company 19.1 State Estimation Problem. .............................................. 19 -1 Underly ing Assumptions . Measurement Representations . Solution Methods 19.2 State Estimation Operation. ........................................... 19 -6 Network Topology Assessment . Error Identification . Unobservability 19.3 Example State Estimation Problem . .............................. 19 -8 System Description . WLS State Estimation Process 19.4 Defining Terms . ............................................................. 19 -11 An online AC power flow is a valuable application when determining the critical elements affecting power system operation and control such as overloaded lines, credible contingencies, and unsatisfactory voltages. It is the basis for any real-time security assessment and enhancement applications. AC power flow algorithms calculate real and reactive line flows based on a multitude of inputs with generator bus voltages, real power bus injections, and reactive power bus injections being a partial list. This implies that in order to calculate the line flows using a power flow algorithm, all of the input information (voltages, real power injections, reactive power injections, etc.) must be known a priori to the algorithm being executed. An obvious way to implement an online AC power flow is to telemeter the required input information at every location in the power system. This would require not only a large number of remote terminal units (RTUs), but also an extensive communication infrastructure to telemeter the data to the SCADA system, both of which are costly. Although the generator bus voltages are usually readily available, the injection data is frequently what is lacking. This is because it is much easier and cheaper to monitor the net injection at a bus than to measure separate injections directly. Also, this approach presents weaknesses for the online AC power flow that are due to meter accuracy and communication failure. An online power flow relying on a specific set of measurements could become unusable or give erroneous results if any of the predefined measurements became unavailable due to communication failure or due to misoperation of measurement devices. This is not a desirable outcome of an online application designed to alert system operators to unsecure conditions. Given the above obstacles of utilizing an online AC power flow, work was conducted in the late 1960s and early 1970s (Schweppe and Wildes, Jan. 1970) into developing a process of performing an online power flow using not just the limited data needed for the classical AC power flow algorithm, but using all available measurements. This work led to the state estimator , which uses not only the aforemen- tioned voltages but other telemetered measurements such as real and reactive line flows, circuit breaker statuses, and transformer tap settings. 19.1 State Estimation Problem State estimators perform a statistical analysis using a set of m imperfect redundant data telemetered from the power system to determine the state of the system. The state of the system is a function of n ß 2006 by Taylor & Francis Group, LLC.
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state variables : bus voltages and relative phase angles, and tap changing transformer positions.
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9291_c019 - 19 State Estimation 19.1 19.2 19.3 State...

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