A state estimator is a software program that describes the current state of an electrical transmission or distribution system. The estimator inputs data available from the network, filters it, and uses it for calculating and predicting unknown values. Specific conditions the state estimator provides are voltage magnitude and voltage phase angles at each bus on the system as well as active and reactive power flows.
The state estimator is a component of the energy management system (EMS) used to operate an electrical grid. The results of the estimator are used by various other operational systems including operator displays, condition alarming, operational power flow, generator dispatch, contingency analysis, and locational marginal price calculations.
The process of state estimation begins with collection of measurements from sensors placed around the network and from phasor measurement units (PMUs). Data collected is then transferred to the operations center from the SCADA system. Because data collected from the field is imperfect and at times inaccurate due to faulty equipment or data communication issues, state estimation uses statistical techniques to estimate actual values based on the imperfect data. This includes approximating unknown values and filtering errors. This is important because the failure to remove faulty data would likely result in faulty assumptions about the state of the system. Once the data has been processed, a network model is used to calculate the assumed “true” state of the system. Given this, estimators can also be used to predict the future state at a given time. This allows system operators to plan ahead and prepare for changing system conditions.