Investigation of negative sequence injection capability in H-bridge multilevel STATCOM

The aim of this paper is to investigate the ability of star- and delta-connected H-bridge multilevel STATCOMs to exchange negative-sequence current with the grid. Zero-sequence injection is utilized for capacitor voltage balancing. It is shown that a singularity for these configurations exists, leading to limitations in the utilization of the compensator for asymmetry compensation purposes

Impact of Control Loops on the Passivity Properties of Grid-Forming Converters with Fault-Ride through Capability

Due to the increasing integration of renewable energy sources (RES) and a corresponding reduction of conventional generating units, there is nowadays a demand from the power-electronic converters to provide grid-forming properties through proper control of the converter systems. This paper aims to evaluate the impact of various control loops in a grid-forming control strategy equipped with a fault-ride through capability on the passivity properties of the converter system. Through the analysis of the frequency-dependent input admittance of the converter, the main factors affecting the passivity properties are identified. A simplified analytical model is derived in order to propose possible control modifications to enhance the system’s passivity at various frequencies of interest and the findings are validated through detailed time-domain simulations and experimental tests

Estimation of frequency-dependent impedances in power grids by deep lstm autoencoder and random forest

This paper proposes a deep-learning-based method for frequency-dependent grid impedance estimation. Through measurement of voltages and currents at a specific system bus, the estimate of the grid impedance was obtained by first extracting the sequences of the time-dependent features for the measured data using a long short-term memory autoencoder (LSTM-AE) followed by a random forest (RF) regression method to find the nonlinear map function between extracted features and the corresponding grid impedance for a wide range of frequencies. The method was trained via simulation by using time-series measurements (i.e., voltage and current) for different system parameters and verified through several case studies. The obtained results show that: (1) extracting the time-dependent features of the voltage/current data improves the performance of the RF regression method; (2) the RF regression method is robust and allows grid impedance estimation within 1.5 grid cycles; (3) the proposed method can effectively estimate the grid impedance both in steady state and in case of large transients like electrical faults

Coordinated control of grid-forming converters and hydro generators to enhance frequency quality of future power system

The aim of this paper is to propose a coordinated control strategy between grid-forming converters equipped with energy storage, and hydro generators to facilitate frequency support from the converters in future power systems. In this way, it is possible to take advantage of the fast dynamic properties of the converter system, and at the same time minimize the energy storage requirements associated with the converter system. The proposed tuning criterion for the frequency controller of the grid-forming converter facilitates a natural coordination between the converter system and hydro generators. The effectiveness of the proposed control strategy is compared against the conventional droop-based approach available in the literature. Finally, the analytical findings are validated using detailed time-domain simulation model in PSCAD

Frequency control and synthetic inertia in energy systems modelling

This study investigates how inclusion of frequency control constraints in electricity system modelling impacts the investment and dispatch in electricity generation and storage technologies for high-VRE futures. This is done using a linear cost-minimizing investment and dispatch model using historical load, wind and solar power conditions from Spain, Ireland, Sweden and Hungary for the year 2050. With an hourly time-resolution, constraints are added to ensure that, within each hour, sufficient inertial power and reserves are available to control the frequency. Comparing the results with and without these constraints show that nearly all impact on the results is in battery investments and operation. Furthermore, it is found that reserve requirements have a higher impact on system composition and operation than inertial power requirements

Inclusion of frequency control constraints in energy system investment modeling

This study investigates how the inclusion of frequency control constraints in electricity system modeling impacts the levels of investment and dispatch in electricity generation and storage technologies for futures that include high-level penetration of variable renewable energy. This is achieved using a linear cost-minimizing investment and dispatch model using historic load, wind and solar conditions from Spain, Ireland, Sweden and Hungary for Year 2050. With an hourly time-resolution, constraints are added so as to ensure that, within each hour, sufficient inertial power and reserves are available to control the frequency of the power grid. Comparing the results obtained with and without these constraints reveals that the main impacts on the results are from battery investments and operation. Furthermore, it is found that the reserve requirements exert a greater impact on system composition and operation than do the inertial power requirements

A Novel DTC-based Control Method of Flywheel System to Improve Fault-Ride Through Capability of the Microgrids

This paper proposes a new control method for Flywheel Energy Storage System (FESS) to guarantee a Fault-RideThrough (FRT) capability of the sensitive microgrids like big data centers. The proposed method has been developedtowards a twofold aim: regulating constant common DC-bus voltage, during serious voltage dips caused by the grid sideelectrical faults and also keeping a constant charge current during normal operation of the grid. The proposed FESSis coupled with Permanent Magnet Synchronous Machine (PMSM). The speed sensorless Direct Torque Control (DTC)the technique has been developed for PMSM control and the Extended Kalman Filter (EKF) is used to estimate the rotorposition and consequently the rotor speed. The main contributions of the overall control method are: (i) the decoupleddisturbance control at discharge mode copes with sudden load change disturbances; (ii) the DTC provides fast andprecise torque response; (iii) the real-time speed estimation by the EKF increases the speed and the accuracy of theoverall control system; (iv) the proposed FESS can be easily replaced by the Battery Energy Storage System (BESS).The proposed system and the corresponding control method are verified in the MATLAB/Simulink environment. Thesimulation results confirm the effectiveness of the proposed control method

Performance Comparison of Phase Shifted PWM and Sorting Method for Modular Multilevel Converters

Modular Multilevel Converters (MMC) are the solution of preference in HVDC applications due to modularity, scalability, low losses and low filtering requirement. Carrier-based (PWM) and carrier-less (nearest level control) modulation can be applied. By using advanced sorting methods focusing on keeping the capacitor voltage ripple under some limit, unnecessary switching events are eliminated leading to reduced switching losses. This paper presents a comparison between the steady-state performances in terms of output voltage THD and equivalent switching frequency of the Phase Shifted Carrier PWM and NLC plus sorting methods

Extended defects in 3C-SiC: Stacking faults, threading partial dislocations, and inverted domain boundaries

Abstract The presence of extended bi-dimensional defects is one of the key issues that hinder the use of wide band-gap materials hetero-epitaxially grown on silicon. In this work, we investigate, by STEM measurements and molecular dynamic simulations, the structure of two of the most important extended defect affecting the properties of cubic silicon carbide, 3C-SiC, hetero-epitaxially grown on (001) silicon substrates: (1) stacking faults (SFs) with their bounding threading dislocation arms, even along with unusual directions, and (2) inverted domain boundaries (IDBs). We found that these two defects are strictly correlated: IDBs lying in {111} planes are intrinsically coupled to one or more SFs. Moreover, we observed that threading partial dislocations (PDs), limiting the SFs, appear to have non-conventional line directions, such as [112], [123], and [134]. Molecular dynamics simulations show that [110] and [112] directions allow for stable dislocation structures, while in the unusual [123] and [134] directions, the PDs are composed of zig-zag dislocation lines in the [112] and [110] directions