Carlo Muscas

Trade-Offs in PMU Deployment for State Estimation in Active Distribution Grids

Monitoring systems are expected to play a major role in active distribution grids, and the design of the measurement infrastructure is a critical element for an effective operation. The use of any available and newly installed, though heterogeneous, metering device providing more accurate and real-time measurement data offers a new paradigm for the distribution grid monitoring system. In this paper the authors study the meter placement problem for the measurement infrastructure of an active distribution network, where heterogeneous measurements provided by Phasor Measurement Units (PMUs) and other advanced measurement systems such as Smart Metering systems are used in addition to measurements that are typical of distribution networks, in particular substation measurements and a-priori knowledge. This work aims at defining a design approach for finding the optimal measurement infrastructure for an active distribution grid. The design problem is posed in terms of a stochastic optimization with the goal of bounding the overall uncertainty of the state estimation using heterogeneous measurements while minimizing the investment cost. The proposed method is also designed for computational efficiency so to cover a wide set of scenarios.

Impact of the Model on the Accuracy of Synchrophasor Measurement

Phasor measurement units (PMUs) are becoming one of the key issues of power network monitoring. They have to be able to perform accurate estimations of quantities of interest either under steady-state or transient conditions. Among all the sources which may contribute to the uncertainty introduced by PMUs, this paper analyzes the impact of the phasor estimation models on the accuracy of these devices, focuses on algorithms proposed in the literature for the estimation of dynamic phasors, and studies their performances under several different conditions.

A Fast and Accurate PMU Algorithm for P+M Class Measurement of Synchrophasor and Frequency

The IEEE Standard C37.118.1 defines two performance classes, P and M, for phasor measurement units (PMUs), respectively for protection and monitoring oriented applications. The goal of this paper is to define an algorithm that allows the requirements of both classes to be met simultaneously, thus avoiding an a priori selection of either the fast response time of class P or the accuracy of the class M. The designed PMU consists of two digital channels that process in parallel the acquired samples with different algorithms: the first one allows accurate measurements of steady state signals, while the second one is better suited to follow the fast signal changes. Then, a detector identifies the possible presence of dynamic situations and selects the most appropriate output for the actual operating condition. The validation of the solution, performed by means of simulations in all the static and dynamic conditions defined in the standard, confirms that the method is able to comply with all the limits indicated for both classes for synchrophasor and frequency measurement. As for the rate of change of frequency, again the compliance to P-class is fully verified, while for the M-class the only exception is represented by the tests with out-of-band disturbances.

A digital compensation method for improving current transformer accuracy

This paper deals with error compensation in current transformers. Usually the manufacturer tries to limit these errors by stating a transformer nominal ratio slightly different from the turns ratio. On the other hand techniques have been developed that allow software current compensation. The proposed method requires acquisition of the instantaneous secondary current and its adjustment with magnetizing current, taking into account hysteresis effects. To do this, a preliminary procedure is performed for core and transformer parameter identification. The compensation method has been tested on a variety of instrument transformers, with sinusoidal primary current. Test results show in all examined cases, an improvement in primary current reproduction accuracy, compared with that achieved using CTs nominal ratio, even for core partial saturation. A further development of the compensation technique is in progress, with a view to eliminating some restrictive hypothesis introduced in the present study.

A Flexible GPS-based System for Synchronized Phasor Measurement in Electric Distribution Networks

Large-scale distributed measurement systems are the object of several applications and research. The goal of this paper is to develop, by employing global positioning system (GPS) receivers, measurement techniques that are suited to the continuous monitoring of the electrical quantities in distribution networks in terms of synchronized phasors. The proposed measurement procedures, differently from commercially available phasor measurement units, are based on general-purpose acquisition hardware and processing software, thus guaranteeing the possibility of being easily reconfigured and reprogrammed according to the specific requirements of different possible fields of application and to their future developments.

Hysteresis and eddy currents compensation in current transformers

In the paper a digital technique for improving the accuracy of instrument current transformers is presented. Since the exciting current can be considered as the main error source, its evaluation can allow the compensation of its detrimental effects to be obtained. The exciting current required by the transformer in every kind of steady state operation can be determined by simply acquiring the secondary current, provided that the examined CT has been preliminarily identified. A simple scalar model for the CTs magnetic core, taking into account saturation as well as hysteresis and eddy currents phenomena, has been implemented in a software compensation routine. This allows us to improve the accuracy in the reproduction of the primary current, in the case of both sinusoidal and distorted current waveforms (provided that DC components are not present). Many experimental tests, under different practical situations, have been performed. The results clearly show that the proposed technique is able to significantly reduce, in comparison with traditional methods, the errors introduced by current transformers.

Optimal Allocation of Multichannel Measurement Devices for Distribution State Estimation

This paper proposes an optimization algorithm that is suitable for choosing the optimal number and position of the measurement devices in distribution state estimation (DSE) procedures used in modern electric distribution networks. The algorithm is based on the techniques of dynamic programming, and its goal is to guarantee both the minimum cost and the accuracy required for the measured data needed to operate management and control issues, such as energy dispatch and protection coordination. Both the uncertainty introduced by the measurement devices and the tolerance in the knowledge of the network parameters (line impedances) are taken into account in the proposed approach. The aggregation of the quantities to be measured in a few measurement points has been favored to reduce the overall cost of the measurement system. Random changes in the loads are considered to establish adequate reference conditions for the tests. Tests relevant to real distribution networks are presented to show the validity of the proposed approach. The results emphasize how both the influence of the tolerance on the network parameters and the cost of the measurement system can dramatically be minimized by suitably choosing the algorithm to be implemented to solve the DSE problem.

Optimal Meter Placement for Robust Measurement Systems in Active Distribution Grids

Future active distribution grids are characterized by rapid and significant changes of operation and behavior due to, for example, intermittent power injections from renewable sources and the load-generation characteristic of the so-called prosumers. The design of a robust measurement infrastructure is critical for safe and effective grid control and operation. We had earlier proposed a placement procedure that allows finding an optimal robust measurement location incorporating phasor measurement units and smart metering devices for distribution system state estimation. In this paper, the lack of detailed information on distributed generation is also considered in the optimal meter placement procedure, so that the distributed measurement system can provide accurate estimates even with limited knowledge of the profile of the injected power. Possible non-Gaussian distribution of the distributed power generation has been taken into account. With this aim, the Gaussian mixture model has been incorporated into the placement optimization by means of the so-called Gaussian component combination method. The occurrence of either loss of data or degradation of metrological performance of the measurement devices is also considered. Tests performed on a UKGDS 16-bus distribution network are presented and discussed.

A New IED With PMU Functionalities for Electrical Substations

This paper proposes a new distributed architecture to measure synchrophasors in power substations, inspired by the standards IEC 61850. The new scheme is implemented in the process bus, where a high performance merging unit (MU), synchronized by precision time protocol (PTP), according to the standard IEEE 1588-2008, sends the sampled values (SVs) to an intelligent electronic device (IED) enabled to behave as a phasor measurement unit (PMU). The synchrophasors are evaluated in the IED through an algorithm based on the Taylor Fourier Transform and suitably modified to have high performance also in terms of response time in presence of step change conditions. Different tests and considerations are presented to evaluate the various elements of uncertainty that a distributed architecture can introduce and, finally, to ensure the feasibility of the proposed approach.

Investigation on Multipoint Measurement Techniques for PQ Monitoring

The problem of locating the sources of harmonic distortion in power networks is still a critical task. Several methods based on multipoint measurement techniques have been recently proposed in the literature for harmonic-pollution monitoring. The main goal of this paper is to compare the two most powerful tools designed for this purpose. In this paper, the results of both computer simulations on an IEEE test network and experimental tests performed by means of simultaneous measurements on an ad hoc benchmark power system are presented along with a discussion on the practical usability of such monitoring techniques