Mihaela Albu

Embedding remote experimentation in power engineering education

Engineering education by its nature is a costly program in university environments. Perhaps the most costly component is the laboratory facility, usually consisting of specialized equipment. Effective instruction of some topics in power engineering education requires experience with actual equipment, rather than small-scale replicas or simulation. In this paper, a new laboratory approach is described, as implemented in a virtual, Internet-based, experimentation platform. This virtual laboratory (VLab) utilizes real equipment distributed among multiple universities from which remotely located students can perform experiments. The software solution is a multiuser, client-server architecture developed in the LabVIEW environment. Implementation details including video, chat, archiving, and the hardware and software platforms are presented in the paper. An example presented herein is the study of current and voltage waveforms while controlling relays and low-voltage contactors. The applications have been tested with student teams enrolled in the electrical engineering department of Politehnica University of Bucharest and the power engineering program at Arizona State University.

A power quality index based on equipment sensitivity, cost, and network vulnerability

This paper focuses on voltage sag phenomena and their impact on customer satisfaction. In order to derive a unique power quality of service index, information from both the supply network (according to standards in use) and the customer (defined in terms of load sensitivity and interruption cost) are merged. The well-known CBEMA power acceptability curve is revisited with definitions from the IEEE Standard 1159 superimposed. This depiction suggests a way to assign a cost or index of power quality events. Although individual cost assignments used in the calculation are subject to question, once the assignments are made, the calculation is consistent and gives a useful measure of quality of service. The method is illustrated factoring severity of the events based on the CBEMA curve and IEEE 1159 classified power quality events. The paper addresses the elusive issue of the cost of power quality disruption.

On the use of RMS values in power quality assessment

The root mean square mathematical operation is widely used in power engineering. The process has a frequency response characteristic and an associated time constant which is important especially for short term signals. Potential problem areas in using RMS values in power quality assessment are identified and discussed.

Uncertainty in Power System State Variables Obtained Through Synchronized Measurements

This paper computes the uncertainties associated with the power system state variables obtained with the help of phasor measurement units (PMUs). An integer-quadratic-programming-based method is used to determine the minimum number and the optimal locations of the PMUs to ensure complete topological observability of the system. Three approaches are used to estimate the uncertainties in the state variables: the use of the classical uncertainty propagation theory, the Monte Carlo method, and the random fuzzy variables (RFVs). The methods are applied on a test system, and the results are presented. The choice of method depends on the application; suggestions are offered based on the conclusions of the work in this paper.

Virtual synchronous generator: An element of future grids

The future power system integrated a large share of distributed generation (DG) may causes a reduction of the system rotational inertia, resulting in high frequency variations with any disturbances. An additional rotational inertia will be provided to the system if many DG units combined with relative small storage systems operate like virtual synchronous generators (VSG). This paper presents the demonstration approach for the VSGs in order to bring the VSG models developed in the laboratory into practice at two distribution test sites for both single- and three-phase applications. The results show promising results where changes in frequency and voltage are clearly counteracted by power flows from the VSG. In addition, VSG may provide a solution for hosting a large share of DG in future grids while maintaining system stability.

Virtual synchronous generator: Laboratory scale results and field demonstration

This paper describes an approach to demonstrate the operation of virtual synchronous generators (VSG) in practical distribution systems. After an extensive simulation phase at laboratory scale, the field demonstration is planned at two sites located in the Netherlands and in Romania, for small VSGs of 10 × 5 kW and one large VSG of 100 kW respectively.

Measurement Uncertainty Considerations in Optimal Sensor Deployment for State Estimation

This paper proposes a method for optimal placement of phasor measurement units (PMUs) for measuring the states of a power system. A method to compute the measurement uncertainty associated with the estimated states is also illustrated in the paper. The PMU placement strategy ensures complete observability of the power system states for normal operating conditions, as well as under the loss of a single transmission line or even a single measurement unit. An integer quadratic programming approach is used to minimize the total number of PMUs required to make the system completely observable, and to maximize the measurement redundancy at the power system busses. The goal of the research is to take into account the measurement uncertainty while determining the optimal number and locations of the PMUs for state estimation. Simulation results on the IEEE 14-bus test system are presented in this paper.

The Effect of Variable Weights in a WLS State Estimator Considering Instrument Transformer Uncertainties

The state estimator constitutes the cornerstone of the supervisory control and data acquisition system since it provides the power system operating situation in consecutive time intervals. Furthermore, the output of the state estimator is used by other tools responsible for the monitoring and control of the power system. Therefore, there is a need for the power system state estimator to be as accurate and reliable as possible. The main source of uncertainty that may deteriorate the accuracy of a weighted least squares (WLS) state estimator, provided that the network parameters are perfectly known, is the uncertainty that is encompassed in the measurements. It is well known that the measurement chain is not ideal and this information is passed to the state estimator through the measurement weights. In this paper, the effect of the measurement weights, which are calculated by considering both the standard uncertainties associated with the measurement devices and the instrument transformers, on the WLS state estimator is examined.

Online Monitoring of the Power Transfer in a DC Test Grid

One solution for increasing the use of renewables is to find new strategies to promote the connection of distributed energy resources (DERs) within the existing power system. A solution that allows a flexible integration of dispersed generation (DG) into energy networks is the use of dc at the user layer. This seems a reasonable choice since several renewable sources and the presently available storage systems deliver electricity in dc form (e.g., fuel cells and solar cells). Furthermore, other generators (wind turbines and microhydro or microgas turbines) deliver electricity in ac form but mostly at variable or nonstandard frequencies. Consequently, the output of these generators must be rectified, converted again into ac, and conditioned to meet the nominal grid parameters. Anticipating the extensive use of low-power DC-based intelligent devices, one can avoid losses in energy transfer by using a dc layer within the distribution networks, at least where the energy is produced in dc form. DC grids in buildings are expected to be one of the main applications and challenges in the future. The development of a dc test network, including measurement and communication, is presented in this paper. The development of promising applications showing the effective integration of DG into intelligent networks with low-power loads is illustrated and discussed. Specific indicators are formulated to characterize power quality issues of dc systems.

Storage selection for DG applications containing virtual synchronous generators

The paper is presenting an algorithm for establishing the best types of electrical storages for the virtual synchronous generator (VSG), depending on the application case and desired nominal power of the VSG. The resulting application provides a description of available technologies in terms of characteristics matching the desired properties of the storage in accordance with the scenarios described by the user.