Daniele Colangelo

Inhomogeneity effects in HTS coated conductors used as resistive FCLs in medium voltage grids

For resistive fault current limiters (RFCLs) based on high temperature superconducting coated conductors (HTS-CCs), inhomogeneity, in terms of critical current and geometrical imperfections such as stabilizer and substrate thicknesses, plays a very important role and it may limit the penetration of such devices into the electrical market. This paper presents an electrothermal model, developed in SimPowerSystem™, able to describe the transient response of HTS-CC candidates with different degrees of inhomogeneity, both in terms of critical current and of stabilizer thickness. Critical current inhomogeneity has been modeled with Gaussian distributions. The layer thicknesses used in the simulations have been chosen by fitting the temperature dependence of real tape resistances. Our approach considers relative inhomogeneity positions as well as thermal conduction along the HTS-CC length. The model is tuned using experimental measurements made on ReBaCuO coated conductors. A new dynamical thermal calibration of the model is proposed using finite element method calculations. Inhomegeneity effects with different possible faults (e.g. three phase and single phase short-circuit) are presented.

Architecture and characterization of a calibrator for PMUs operating in power distribution systems

In recent years, the Phasor Measurement Unit (PMU) technology is rapidly evolving towards the potential deployment also in power distribution systems (DSs). In general, this specific field of applications requires PMUs whose accuracy levels are beyond those required by the IEEE Std. C37.118. Additionally, there is the need to define the architecture of an associated calibration system capable to assess the metrological performances of these devices. In this paper, we first analyse the impact of the uncertainties (in term of phase and magnitude) introduced by arbitrary PMUs on a state estimation (SE) process performed on the IEEE 13-bus distribution test feeder. The outcomes of this analysis are used to infer the most stringent steady-state performances of PMUs for DSs monitoring and, consequently, to define the requirements and the hardware architecture of a PMU calibrator presently developed at the Authors laboratories. A preliminary metrological characterization of the proposed calibrator is presented in the paper.

Analysis of the influence of the normal zone propagation velocity on the design of resistive fault current limiters

Commercial high-temperature superconducting coated conductors (HTS-CCs) have low thermal diffusivity and nonuniform critical current density. These two factors lead commercial HTS-CCs to a partial quench when they are subjected to a transport current around their average critical current (I-c.av). The consequence is the appearance of localized resistive zones, and a high risk of thermal runaway can arise when HTS-CCs are used for resistive fault current limiter (RFCL) purposes. The enhancement of the normal zone propagation velocity (NZPV) of HTS-CCs is a desirable solution for achieving sufficient thermal stability while keeping the cost of RFCLs under an acceptable threshold. Even though in recent years, several valid methods to increase the NZPV have been proposed, their impact on the design of RFCLs is not clear. For this reason, we developed a one-dimensional numerical model that enables us to simulate HTS-CCs with enhanced NZPV and to study the limitation performance of a HTS-CC-based RFCL in real operating conditions. Our preliminary re sults demonstrate that the NZPV enhancement can effectively limit the needed amount of HTS-CCs with important economic benefits for the design of RFCLs.

Definition of Accurate Reference Synchrophasors for Static and Dynamic Characterization of PMUs

The calibration of phasor measurement units (PMUs) consists of comparing coordinated universal time time-aligned phasors (synchrophasors) measured by the PMU under test, against reference synchrophasors generated through a PMU calibrator. The IEEE Standard C37.118–2011 and its latest amendment (IEEE Std) describe compliance tests for static and dynamic conditions, and indicate the relative limits in terms of accuracy. In this context, this paper focuses on the definition and accuracy assessment of the reference synchrophasors in the test conditions defined by the above IEEE Std. In the first part of this paper, we describe the characterization of a nonlinear least-squares fitting algorithm used to determine the parameters of these reference synchrophasors. For this analysis, we deploy the proposed algorithm in a PMU calibrator and characterize the algorithm performance within the actual hardware implementation for both static and dynamic test conditions. More specifically, we generate reference waveforms through a highly stable high-resolution digital-to-analog converter and evaluate how the algorithm parameters (observation interval length and sampling frequency) affect the solution accuracy. In the second part, we discuss on the appropriateness of the synchrophasor model in the evaluation of PMU performance under step test conditions. In this regard, we propose an alternative time-domain approach to assess the synchrophasor estimate during transient events.

Impact of the Normal Zone Propagation Velocity of High-Temperature Superconducting Coated Conductors on Resistive Fault Current Limiters

The engineering critical current, i.e., Ic, of high-temperature superconducting coated conductors (HTS-CCs), today available on the market, is not a uniform parameter and varies significantly along the length of the conductors. In addition, commercial HTS-CCs have a low normal zone propagation velocity (NZPV). This property, together with the Ic inhomogeneity, exposes the HTS-CCs to local thermal instabilities. A crucial challenge for the design of resistive fault current limiters (RFCLs) based on HTS-CCs is to avoid the thermal runaway of the conductors; and in this respect, the enhancement of the NZPV is a promising solution. In recent years, several methods have been proposed, and many various techniques are now available. In this paper, we are interested to quantify the impact the enhancement of NZPV will have on the design of RFCLs based on HTS-CCs whichever is the adopted technical solution. For this reason, we use numerical models to analyze the effects of the NZPV enhancement on the limitation performance of an RFCL integrated in a medium-voltage power grid.

Electrical and Thermal Characterization of Commercial Superconducting YBCO Coated Conductors

This paper reports on experimental and modeling activities aimed to identify E(I, T) characteristics of commercial YBCO high-temperature superconductor coated conductors (HTS-CCs). The aforesaid approach is applied to investigate the HTS-CCs behavior in the different regimes: superconducting state, flux-creep, flux-flow, and normal state. In order to thoroughly represent the HTS-CCs behavior, parameterized analytical formulations of E(I, T) characteristics have been assumed and then validated through fitting methodologies. Numerical calculation of the electric field as a function of the temperature and of the current density in the HTS-CC, is fundamental to the development of models suitable to design devices based on HTS-CCs. In fact it allows improving the dedicated-software calculation accuracy. This is particularly important for the simulation of superconducting fault current limiter behavior against short-circuits and/or overloads events, because during the superconducting fault current limiter limitation, the HTS-CCs performance strongly depends on the different above-mentioned regimes. As matter of fact, numerical simulations are somewhat simpler for high prospective currents as compared to intermediate conditions as for overload or low short-circuit currents.

MV Power Grids Integration of a Resistive Fault Current Limiter Based on HTS-CCs

Due to the energy demand growth and distributed generation units penetration, a substantial increase of the rated short-circuit current of the electrical lines is expected. As a consequence, the electrical grid infrastructure needs to be extended or drastically renovated. In this context, resistive superconducting fault current limiters (RFCLs) based on high-temperature superconducting coated conductors (HTS-CCs) represent a promising technology to limit the upgrading costs. Thanks to recent improvements on HTS-CCs performances, RFCLs are now close to commercial applications. However, because they are novel devices, their real impact on the electricity network remains an open issue. In particular, the subject of this research is to study the grid integration of the RFCL designed within the European project ECCOFLOW. The device has been simulated in two applications in two different typologies of existing medium voltage grids: RFCL used as busbars coupler and RFCL used as transformer feeder. This contribution is the continuation of previous works, where the effects of symmetrical and unsymmetrical short-circuits on inhomogeneous HTS-CCs have been extensively analyzed.

Definition and assessment of reference values for PMU calibration in static and transient conditions

The calibration of Phasor Measurement Units (PMUs) consists of comparing Coordinated Universal Time (UTC) time-stamped phasors (synchrophasors) estimated by the PMU under test, against reference synchrophasors generated through a PMU calibrator. The IEEE Standard C37.118-2011 and its amendment (IEEE Std) describe compliance tests for static and dynamic conditions, and indicate the relative limits in terms of accuracy. In this context, the paper focuses on the definition and accuracy assessment of the reference synchrophasors in the test conditions dictated by the above IEEE Std. In the first part of the paper, we describe the characterization of a nonlinear least-squares (NL-LSQ) fitting algorithm used to determine the parameters of the reference synchrophasors. We analyse the uniqueness and robustness of the solution provided by the algorithm. We assess its accuracy within the whole range of static tests required by the IEEE Std. In the second part, we discuss the appropriateness of synchrophasor model to evaluate the PMU performance in step test conditions. We compare the proposed algorithm against two synchrophasor estimation algorithms. Finally, we propose a time domain process for the better evaluation of PMU performances in transient conditions.

Impact of synchrophasor measurement types and uncertainties on the accuracy of distribution system linear state estimators

The paper aims at assessing the effects of combined voltage and/or current synchrophasor measurements, and their associated uncertainties, on the accuracy of state estimators adopted in distribution systems. Such an assessment is first carried out with respect to a generic transmission line with the purpose of determining the combination of voltage and/or current synchrophasor measurements that provides the best accuracy of the estimated quantities. A comprehensive analysis on the impact of different measurement uncertainties and operating conditions is included for this specific case. In order to derive general conclusions, the study is then extended to a distribution system composed of the IEEE 13-bus test feeder. For this case, we perform an a-posteriori assessment of the probability distributions of the estimation errors by using a discrete Kalman filter state estimator fed with noisy voltage and/or injected current synchrophasor measurements.

Indirect cooling of superconducting fault current limiter

Superconducting fault current limiters based on high temperature superconducting coated conductors are novel devices which enable us further development of the medium voltage electricity network. In the current manuscript we present a short overview of the most important parameters and features of these devices. A special attention is paid to the thermal analysis and the cooling conditions. We propose an optimization of the insulator thickness of the superconducting tapes to reduce the temperature increase of the superconductor at rated current and given time of limitation. With our approach it is possible to reduce the temperature increase of 70 K.