• Energy Research

High-voltage direct-current (HVDC) systems play an important role in modern energy grids, whereas efficient and accurate models are often needed for system-level studies. Due to the inherent switching in HVDC converters, the detailed switch-level models are computationally expensive for the simulation of large-signal transients and hard to linearize for small-signal frequency-domain characterization. In this paper, a dynamic average-value model (AVM) of the first CIGRE HVDC benchmark system is developed in a state-variable-based simulator, such as Matlab/Simulink, and nodal-analysis-based electromagnetic transient program (EMTP), such as PSCAD/EMTDC. The 12-pulse converters in the HVDC system are modeled with a set of nonlinear algebraic functions that are extracted numerically. The results from the average-value models are compared with the results of the detailed simulation to verify the accuracy of the AVMs in predicting the large-signal time-domain transients. The developed dynamic average models are shown to have computational advantages.

Recent contributions have shown that two widely used formulations of the tap-changing transformer model are controversial, as they generate dissimilar results depending on the selected tap and operating point. In recent works, the authors proposed a new model and demonstrated its consistency to reconcile this debate. It introduces a parameter which stands for the ratio between the impedances of the nominal and tapped winding of the transformer. However, this parameter is not provided with and cannot be obtained from standard datasheets, which compels the users to rely on rough approximations. To overcome this problem, an offline state-vector-augmented parameter estimation method capable of providing accurate estimates of transformer impedance ratios is proposed in this work. It is demonstrated that their use can effectively lead state estimators to better estimates of system states. This work also contributes with the derivatives of the different measurement functions in terms of the impedance ratios, which are essential for this or any other linearized state estimator. A multi-snapshot implementation is used to obtain a twofold advantage — increased measurement redundancy and improved accuracy of the estimated parameters. A detailed formulation of the implementation and several case studies are presented to demonstrate the validity of the proposal.

Abstract The latest advances in the field of supercapacitors (SCs) have made them a feasible alternative to be used in hybrid, energy storage systems. Sizing SCs in such systems is not obvious, especially in the case of banks comprising several series-parallel configurations. In this paper, a new algorithm for sizing a SC bank in high fluctuating power demand applications is proposed. The algorithm utilizes the expected discharge power profile, usually obtained from measurement campaigns, and a set of resistance ( R ) and capacitance ( C ) values of the equivalent circuit of the SC, which are provided by the manufacturer. Instead of yielding specific values for R and C , the method determines the RC limit curve which identifies the area of the RC-plane where the range of acceptable solutions can be found for a particular application. Thus, the internal resistance can be easily included in the design process. The validity of different cell combinations, including those not regarding each series-connected element as a sole cell, can be directly checked with this method, enabling the selection of the most suitable solution from an economic point of view.

This work has been funded by the Spanish Government , Innovation Development and Research Office (MEC), under research grants DPI2017-83804-R , and DPI2017-89186-R .

This paper presents an experimental study carried out on a disc-type winding transformer. Although several industrial methods exist for the on-line and off-line monitoring of power transformers, all of them are expensive and complex, and require the use of specific electronic instrumentation. In this paper, an on-line analysis of transformer leakage flux is tested as an efficient alternative procedure for detecting insulating failures during their earliest stages. A power transformer was specifically manufactured for the study. A finite element model of the machine was designed to obtain the transient distribution of leakage flux lines in the machine’s transversal section under normal operating conditions and when shorted turns are intentionally produced. Very cheap, simple sensors, based on air core coils, were built in order to measure the leakage flux of the transformer, and non-destructive tests were also applied to the machine in order to analyse preand post-failure voltages induced in the coils. Results point to the ability to detect very early stages of failure. The asymmetry produced by the failure generates an indicator (induced EMF in the coils) that can be easily distinguished from the one obtained for the healthy machine. In fact, diagnosis can be perfectly achieved in spite of the variation of such an indicator with load.

The analysis of harmonic currents in distribution networks of industrial facilities and the associated filtering stages is essential to optimize the production of these installations. Dry-type air-core reactors are one of the main elements of harmonic filter banks. A thermal analysis of these reactors in conventional steel plants and an evaluation of the criteria used to adjust the corresponding thermal protections are proposed in this paper. Accordingly, harmonic currents through the different filter branches are assessed. The proposed methodology builds a thermal magnetic model using the finite element method (FEM) whereby electromagnetic fields, currents and losses are emulated, to allow for recreating the actual coil temperature. The study aims at increasing the reliability of filtering systems by lowering the number of unscheduled shutdowns due to conservative adjustments of thermal protections.

This paper summarises a study completely developed in for the analysis of AC-DC harmonic interaction between the power network and non-controlled three-phase rectifiers operating in discontinuous mode. The presented method is an adaptation for the case of uncontrolled three-phase rectifiers of the procedure designed by Smith and Arrillaga for the analysis of three-phase controlled rectifiers. The new algorithm has been developed in the frequency domain, and therefore introduces some important advantages for overall power system analysis. The influence of factors such as unbalance or harmonic distortion in the power supply voltage, harmonic distortion in the DC side voltage and unbalance in the AC side filtering reactances can easily be analysed. Moreover, a very flexible modelling of the circuits in both the DC and AC side can be achieved with a minimum effort

This work was supported in part by the Spanish Government, MCIN/AEI/FEDER, and the EU under Grants PID2021-122704OB-I00 and PID2019-111051RB-I00 and in part by the Government of the Principality of Asturias, FICYT, and the EU under Grant IDI/2021/000056. Paper no. TPWRS00930-2021.