George J. Cokkinides

Smart Grid Technologies for Autonomous Operation and Control

This paper presents a new smart grid infrastructure for active distribution systems that will allow continuous and accurate monitoring of distribution system operations and customer utilization of electric power. The infrastructure allows a complete array of applications. The paper discusses four specific applications: a) protection against downed conductors; b) load levelization; c) loss minimization; and d) reliability enhancement.

PMU-based dynamic state estimation for electric power systems

This paper provides a methodology to extract the dynamic real time model of an electric power system using phasor measurement unit (PMU) data (GPS-synchronized) and other SCADA data that are available in substations. In addition to typical voltage and current measurements, PMU data include frequency and rate of change of frequency. Such data are available in raw form, as time-stamped instantaneous values, or, in the case of voltages and currents, as computed phasor data at the system fundamental frequency. A dynamic state estimator is conceptually presented that filters all available data to extract the transient swings of the system in real time. This means that the power system dynamic state estimator can be essentially used as a real time data processor, and its results can provide filtered input to many power system dynamic monitoring and control applications that are currently unavailable, such as monitoring transient stability.

Advances in the SuperCalibrator Concept - Practical Implementations

The supercalibrator concept was introduced to take advantage of the characteristics of GPS-synchronized equipment (PMUs). Specifically, GPS-synchronized equipment has the capacity to provide precise phase measurements (to 0.01 degrees accuracy) and relatively good quality magnitude measurements (up to 0.1% accuracy). However in a practical environment this precision is not achieved for a variety of reasons, such as errors from instrumentation, system unbalanced conditions, etc. The supercalibrator concept is based on a statistical estimation process that fits GPS-synchronized measurements and all other available standard data into a three-phase, breaker-oriented, instrumentation inclusive model. In this paper, this concept has been extended to provide a decentralized state estimator for power systems. The decentralized state estimator operates on substation data. The resulting substation state estimate is globally valid as long as there is a valid GPS-synchronized measurement at the substation. The paper describes the supercalibrator methodology. Presently the concept is implemented on five substations. Numerical experiments with these systems illustrate the superiority of this approach. The paper also describes preliminary implementation and performance issues. The implications of the overall approach are substantial. The supercalibrator applied to substations provides a decentralized, highly reliable and robust state estimator for large power systems

Feasibility Study: Autonomous State Estimation in Distribution Systems

We propose an autonomous state estimation based on robotic concepts and advanced state estimation methods. An autonomous, intelligent monitoring infrastructure is proposed that reliably and automatically detects devices as they are plugged-in or -out; it identifies changes in system state and automatically updates the real-time model of the system. The real-time model is used for control, operation, and optimization of the system via application software that are not addressed in this paper. The proposed infrastructure uses modern intelligent electronic devices (IEDs) named universal monitoring protection and control units (UMPCUs) that are capable of handling three key data sets for the component that they are attached to: 1) connectivity, 2) device model, and 3) measurements. The connectivity data represent the connecting points where a device is connected to the power grid, the device model data provide the mathematical model of the device as an object, and the measurements provide the numeric values of physical quantities such as voltages and currents captured by the data acquisition system. This paper describes a feasibility study of the proposed infra structure on a scaled-down three-substation power system laboratory setup. Performance metrics are provided that quantify total time latencies on IEC61850 implementation.

The supercalibrator — A fully distributed state estimator

This paper describes a new approach that led to the development of a fully distributed state estimator, named the SuperCalibrator (SC). The SC is a three-phase state estimator that operates at the substation level and requires at least one PMU at each substation. The computed substation state estimate is transferred to the control center where the overall system state is synthesized from the substation states. The SC-based distributed state estimator has been implemented on the US Virgin Islands St. Thomas and St. John power system of the USVI Water and Power Authority. The most significant performance characteristics of the system are: (a) High update rate up to four system wide state estimation solutions per second, (b) High Accuracy - typically 0.05 degrees in phase and 0.1% in magnitude, (c) Scalability, (d) No need for tuning of the state estimation process.

Distributed State Estimator Advances and Demonstration

Present state estimator performance is not 100% reliable. Specifically, there is significant probability that the state estimators may not converge to an acceptable solution. On an industry wide basis, the probability of non convergence is about 5%. The reasons for this performance have been investigated and have been reported in earlier papers. In this paper we report on a new approach that alleviates the sources of state estimator unreliability and at the same time distributes the computational procedure to each substation of the system, assuming there is at least one GPS-synchronized device (relay, PMU, recorder, meter, etc.) at each substation. This results in a true distributed estimator. The results of the distributed state estimator are communicated to the control center where the overall system state is constructed. The approach has been implemented to two subsystems of two substations each. This paper describes the overall approach and provides results from the two pilot implementations.

Wide area dynamic monitoring and stability controls

This paper presents a new approach for wide area dynamic monitoring of the system with many possible applications. One such application is discussed to provide real time stability controls. The new approach utilizes a substation based dynamic state estimation. The substation based dynamic state estimation uses data from relays, PMUs, meters, FDRs etc in the substation only thus avoiding all issues associated with transmission of data and associated time latencies. The substation based dynamic state estimator provides accurate representation of the dynamic state of the system. The dynamic state estimator runs at rates comparable to the suggested rates in the synchrophasors standard C37.118. Presently it has been implemented to execute 10 times per second thus providing the dynamic state of the substation 10 times per second. The results of the substation based dynamic state estimator are transmitted to a central location for monitoring the dynamic state of the system. A major advantage is the fact that only the dynamic state of the substation is transmitted instead of the raw PMU data that typical wide area monitoring implementations use. Note that the data describing the dynamic state is much less that the raw data of the PMUs. This fact facilitates the speedier transmission of the data in addition to the advantage of a more accurate dynamic state as opposed to the raw data. This infrastructure can be used for a number of applications. The paper focuses on transient stability monitoring, identification of out of step conditions and control. We propose an approach that is based on accurate evaluation of the system energy function (Lyapunov indirect method) and extraction of stability properties from the energy function. Specifically, we provide a methodology for determining the required data accuracy for the reliable real time estimation of the energy function. When the data meet these requirements, the estimated energy function can be visualized and animated providing a powerful visual tool for observing the transient stability or instability of the system. The infrastructure of the substation based dynamic state estimator provides the required accuracy and the ability to predict instabilities before they occur.

Effects of Protection System Hidden Failures on Bulk Power System Reliability

Protection system hidden failures have been recognized as a contributing factor to power system cascading outages. However, in the current bulk power system reliability assessment practice, protection systems are generally assumed to be perfect, and the effects of hidden failures in protection systems are not taken into account. In this paper, the impact of protection system hidden failures on bulk power system reliability is investigated. A breaker-oriented bulk power system network model is developed to include detailed system substation configurations and associated protection system schemes. Protection system constituents, such as transducers, relays, circuit breakers, may suffer from hidden failures. Hidden failures existing in transducers and relays can be detected by the advanced system real time monitoring and analysis technologies. Thus, the major concern of this work focuses on the analysis of hidden failures in circuit breakers. The hidden failure effects analysis shows that some initial system disturbances can result in the unnecessary outages of intact power system equipment because of hidden failures in circuit breaker trip mechanisms. Contingencies resulting from hidden failure outages are further evaluated by a security-constrained adequacy evaluation methodology to obtain their influence on system reliability. The proposed analysis procedure is demonstrated with a breaker- oriented 24-substation reliability test system, which is developed based on the IEEE 24-bus reliability test system and integrates explicit substation and protection system models in the network model. Evaluation results show that protection system hidden failures downgrade the system reliability level because they lead to the outages of undamaged equipment following initial system disturbances.

Setting-Less Protection: Feasibility Study

A new protection scheme is proposed that does not require settings or the settings are simple and the need to coordinate with other protective devices has been removed. The approach can be viewed as a generalization of differential protection and it is enabled with dynamic state estimation. Specifically, the proposed protection scheme is based on continuously monitoring terminal voltages and currents of the component and other possible quantities such as tap setting, temperature, etc. as appropriate for the component under protection. The monitored data are utilized in a dynamic state estimation that continuously provides the dynamic state of the component by fitting the data to the basic model equations of the device under protection. The dynamic state is then used to determine the health of the component. Tripping or no tripping is decided on the basis of the health of the component. The basic approach, the analytics and the requirements for successful implementation of this concept are presented. Numerical experiments are presented to validate the method as well as an example comparison with conventional protection. Finally an evaluation of feasibility is provided based on present day microprocessor capabilities and it is concluded that present day microprocessors do have the computational power required by the proposed approach.

GPS-Synchronized Data Acquisition: Technology Assessment and Research Issues

GPS-synchronized equipment (PMUs) is in general higher precision equipment as compared to typical SCADA systems. Conceptually, PMU data are time tagged with accuracy of better than 1 microsecond and magnitude accuracy that is better than 0.1%. This potential performance is not achieved in an actual field installation due to errors from instrumentation channels and system imbalances. Presently, PMU data precision from substation installed devices is practically unknown. On the other hand, specific applications of PMU data require specific accuracy of data. Applications vary from simple system monitoring to wide area protection and control to voltage instability prediction. Each application may have different accuracy requirements. For example for simple system monitoring in steady state highly accurate data may not be critical while for transient instability prediction high precision may be critical. For addressing data precision requirements for a variety of applications, it is necessary to quantify the accuracy of the collected PMU data. We discuss data precision requirements for a variety of applications and we propose a methodology for characterizing data errors. In particular, we propose a new approach for improving data accuracy via estimation methods. The proposed methodology quantifies the expected error of the filtered data. Examples are provided that define the instrumentation requirements for specific applications.