Introduction

Figure 1 provides a schematic illustration of the traditional state estimation procedure.  In the lower part of the figure the data acquisition systems at the substation level are illustrated. A communications system brings all data to a central location, the control center. At the control center the data are utilized to extract the system model in real time using state estimation techniques. This centralized approach has served the industry with reasonable success. However, the statistical performance of the centralized approach is not totally satisfactory.  Surveys have shown that on average the reliability of the centralized state estimation is about 95% for the US utilities.

Another issue is the speed of the state estimators. The present centralized approach requires that all the data be brought into the control center and then be processed simultaneously using the overall model of the system. This approach results in relatively slow response time, in the order of minutes.  There is a need for state estimators with faster response.  Both of the stated issues associated with the traditional state estimation approach are addressed with the development of a SuperCalibrator based state estimator, which is described next.

Supercalibrator Description

The functional description of the distributed state estimator is illustrated in Figure 2.   All locally available data (within each substation) are utilized by the substation level state estimator.  These data include measurements generated by traditional SCADA equipment, PMUs, Digital Relays, and Digital Fault Recorders.  These measurements are therefore a mix of both scalars and phasors. 

It is important to recognize that present day modern substations have higher level of automation and employ standards such as the IEC 61850, which makes available all the data from relays, PMUs, SCADA, meters, etc. on a common bus accessible from any other device. In this case the SuperCalibrator is simply an application on a substation computer that simply accesses the IEC 61850 bus to retrieve the data and perform the state estimation.

The substation level state estimator uses these data along with a detailed substation model to generate the local state estimate.  The substation model is a breaker oriented, instrumentation inclusive three phase model.  This approach allows sharp bad data detection and identification, and alarm analysis and root cause identification. The advantage comes from the fact that at the substation level, there is greater redundancy of data than a typical centralized state estimator based on SCADA data alone. This redundancy facilitates the detection of bad data and system topology errors. In addition, the state estimator problem is much smaller in size and therefore powerful hypothesis testing methods are applied for both bad data and topology errors without substantial deterioration of the computational efficiency. Note that comprehensive hypothesis testing in centralized state estimators is a practical impossibility because of the large number of hypotheses associated with a large system.  The use of the three-phase breaker-oriented model facilitates the identification of symmetric and asymmetric topology errors (one pole stuck, etc.).  Traditional symmetric state estimators cannot identify asymmetric root cause events. The overall approach of the SuperCalibrator is illustrated in Figure 3. The substation level State Estimation results are transmitted to the control center where the system wide state is synthesized. The substation state estimates and/or the system wide state is used for displays and other applications.  Note that after the system has been successfully installed and tested, no further data processing is required at the control center.  However, to verify the correct system installation, the state estimation coordinating algorithm is exercised.  This algorithm is exercised only at the commissioning time and whenever GPS synchronization has been lost in one or more substations.  The coordination algorithm checks the consistency of the estimated line flows obtained from the terminating substations. The estimates must be identical within the accuracy of the distributed state estimator.

In subsequent paragraphs brief descriptions of the algorithms involved are provided.