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More than one metric is used to evaluate the accuracy of the results of the state estimator solution:

 Cost index is also referred to as “performance index” or “quadratic cost.” In general, it measures the sum of the squares of the normalized estimate errors (residuals). Increasing cost index values could indicate deteriorating state estimator solution quality. This is the most commonly used indicator, whose values range between 45 and 58%.

 Chi‐squared criterion is the second most used, and its value ranges between 36 and 42%.

 Measurement error/bias analysis is used as a performance indicator.

 Average residual value is used as a performance indicator.

The reliability entity should track the selected metric over time to establish the pattern and determine what indicates a problem with state estimator solution quality. Deviation from the “normal range” of these metrics should trigger state estimator maintenance and support. These metrics are important because they could affect the CA solution.

Many factors affect SE solution‐quality metrics such as:

1 Electrical device modeling, connectivity, and telemetry data mapping. If the topology is incorrect, the state estimator may not converge or may yield grossly incorrect results. A topology error may be caused by either inaccurate status of breakers and switching devices or errors in the network model.

2 Availability and quality of telemetry data. Telemetry data are essential components of the state estimation process.

3 Inadequate observability. State estimation is extended to the unobservable parts of the network through the addition of pseudo‐measurements that are computed based on load prediction using load distribution factors, or they can represent non‐telemetered generation assumed to operate at a base‐case output level. The quality of pseudo‐measurements may be bad if they are not updated regularly to reflect current conditions.

4 Measurement redundancy of the network is defined as the ratio of the number of measurements to the number of state variables in the observable area of the network.