Asynchronous AC Research Articles

Investigational Analysis and Experimental Study of a 3-phase Induction Motor for Electric Bike Application

With the advent of global warming and rapid incentivization of electric vehicles being manufactured, research on new and efficient models of the same becomes highly imperative to offer innovative solutions to fundamental issues that limit the market penetration. The current Indian Electric Two-wheeler (E2W) segment utilizes limited motor technologies pertaining to the need of permanent magnets. This work aims to reduce this need by investigating the use of the Asynchronous AC motor as an alternative to magnet motors and experiment with their respective drive system for feasibility of operation. The study proves the hypothesis of a more efficient and cost friendly alternative for an E2W powertrain.

Electromechanical Transient Modeling of the Low-Frequency AC System With Modular Multilevel Matrix Converter Stations

This paper studies the electromechanical transient modeling of a low-frequency AC (LFAC) system with modular multilevel matrix converter (M3C) stations. Firstly, the mathematical model of the M3C and its equivalent circuits are established. Then, an iterative power flow calculation algorithm for AC systems integrated with the M3C-LFAC system is developed. The dynamic model of the M3C for electromechanical transient simulation is studied and implemented in PSS/E. Based on a test system consisting of two asynchronous AC systems with an embedded M3C-LFAC system, comparisons between the electromechanical transient simulation results from PSS/E and the electromagnetic transient simulation results from PSCAD are conducted. The results from PSS/E and PSCAD coincide with each other well, and the validity of the established electromechanical transient model is proved. Lastly, a stability study of a modified New England 39-bus system with an embedded M3C-LFAC system is executed, and the characteristics of the M3C-LFAC system are presented.

Impact of short-duration voltage variations on VSC-HVDC performance

The growing load demand globally necessitates increasing the penetration of renewable energy sources into electrical grids as well as interconnecting grids from different countries and even continents through HVDC transmission systems. Since these applications rely on power electronics devices, several power quality issues arise, namely voltage sags and swells. This paper analyzes the response of a VSC-HVDC transmission system that interconnects two asynchronous AC grids to short-duration voltage variations like sag and swell by adjusting the voltage of the controllable AC source. The system is simulated with the help of MATLAB/Simulink. The study records the effect of the manipulated AC voltage on the active/reactive powers and AC/DC voltages at both converter stations to evaluate the system stability due to these prevalent power quality challenges. The obtained results reveal that the system hardly withstands voltage variation for a short period.

ASYNCHRONOUS ELECTRIC MOTOR WITH SINGLE-PHASE STATOR WINDING

The current standard considers the efficiency of electric motors only in terms of power efficiency and does not pay attention to the so-called economic efficiency - the power factor that affects the loss of electricity in the motor supply network and depends on its energy excellence.This ill-considered approach to establishing the energy efficiency class is a significant shortcoming of the European standard and requires its refinement. The single-phase power supply network in housing and communal services has led to the fact that modern motor compressor units of refrigerators, air conditioners, etc. are equipped with single-phase asynchronous electric motors. An innovative single-phase asynchronous AC motor for motor-compressor units of household appliances is considered in the paper. The stator winding of it is composed of two identical series-connected coils enclosed in half-slots of the stator with a spacious offset of 900.The phase shift of the currents in the coils is provided by a capacitor connected in parallel to one of the coils. The proposed motor is technological in production, has higher specific power and energy efficiency than existing models and meets the IE-3 class energy efficiency. Equivalent electrical circuit and vector diagram of currents for this motor have been developed and shown. They are intended for its mathematical simulation, calculation of static operating characteristics, analysis of dynamic properties at operation in the electric drive system, and also at the level of design work.

Efficient and Robust Power Flow Algorithm for Asynchronous Grids Coupled Through a VSC-MTDC System and Its Probability Analysis

With the interconnection of asynchronous ac grids through a voltage source converter-based multiterminal direct current (AGs/VSC-MTDC) system and the large-scale integration of renewable energy, it is important to resolve the power flow calculation problems efficiently and robustly for secure and economic operation of such system under stochastic context. Therefore, an improved sequential algorithm for deterministic power flow (DPF) of AGs/VSC-MTDC system is proposed first. This enhances the conventional manner of updating power losses of VSCs, which leads to a one-shot DPF solution of the ac/dc grid, thus boosting the efficiency; while the coupling of ac/dc systems is considered to remain high accuracy. In combination with the efficient DPF method for each sample, probabilistic power flow (PPF) analysis of the AGs/VSC-MTDC system is performed by using Zhao's three-point estimate method (Z-3PEM) with much fewer samples, thus achieving a higher efficiency with good accuracy. In addition, the PPF analysis of AGs/VSC-MTDC systems being more prone to the poor robustness in stochastic scenarios is effectively addressed, benefitting from the decreased nonconvergence of improved DPF and fewer extreme samples of Z-3PEM. Finally, the effectiveness and performance of works are verified through the results on an 11-terminal VSC-MTDC system with three asynchronous ac grids modified from different IEEE test systems.

회전형 변압기를 이용한 비동기 AC-DC 하이브리드 계통연계 시스템

Recently, research on large-scale power grid interconnection and AC-DC hybrid power gird has been issued to improve supply reliability and flexibility. The conventional power grid interconnection method is HVDC. HVDC for asynchronous AC grid interconnection has advantage of high transmission efficiency over long distances and low short-circuit current, but it has disadvantages of harmonic generation, high investment cost, and low scalability. Therefore, to solve these problems, several studies have been conducted. In particular, a variable frequency transformer(VFT) for power grid interconnection has been proposed. Asynchronous power grid interconnection using VFT has the advantages of low harmonic generation, simple control, and high scalability. In this paper, we propose a new method for interconnection two AC systems and one DC system. The proposed method has economic feasibility and convenience of control. In order to verify the performance of asynchronous power grid interconnection, simulation through Matlab Simulink was performed.

Alternating Current Asynchronous Motor Control System based on Vector Control

Alternating current asynchronous motor has the advantages of simple structure, reliable operation, and low cost. It is especially suitable for the wheel drive system of large electric drive dump truck, and the nonlinear characteristics put forward higher requirements for the control system. According to the operation characteristics of a vehicle AC asynchronous motor, the characteristic parameter description equation was obtained. Based on the principle of vector control, the control system was designed, the analysis model of the control system was built based on Simulink, and the model speed and torque control were verified. The single and double step speed, slow acceleration and load operation was selected to test, and the changes of speed, output torque and other parameters were obtained. With reference to the actual vehicle characteristics, the test bench of asynchronous AC motor control system was built, and the characteristics of the control system under different load conditions were selected for verification. The analysis results show that the motor runs well in the control system built by vector control. Under different operating conditions, the system responds quickly, the parameter fluctuation is small, and the overall stability is maintained. Under the action of the control system, when operating below the fundamental frequency, the constant speed regulation operation is realized through voltage compensation. When the fundamental frequency is exceeded and the load condition is met, the system realizes constant power speed regulation. The maximum efficiency of the motor is more than 90%, which increases with the increase of load. The control system makes the motor show good advantages. It shows the reliability of the control model and provides a reference for this kind of design research.

Energy-balancing dual-port grid-forming control for VSC-HVDC systems

This work examines energy-balancing dual port grid-forming (GFM) control for high-voltage direct current (HVDC) transmission. In contrast to the state-of-the-art, HVDC converters controlled in this way do not require assigning GFM and grid-following roles to different converters. Moreover, this control enables primary frequency control and inertia support through HVDC links. A detailed stability and steady-state analysis results in conditions on the control gains such that i) the overall hybrid dc/ac system is stable, ii) asynchronous ac areas are quasi-synchronous, and iii) circulating power in cyclic topologies is avoided. Finally, a high-fidelity case study is used to illustrate and verify the analytical results.

Cybersecurity Enhancement for Multi-Infeed High-Voltage DC Systems

Composed of multiple two-terminal high-voltage DC (HVDC) transmission systems, a multi-infeed HVDC (MIDC) system exchanges massive power among multiple asynchronous AC systems. However, as an intrinsically cyber-physical system, an MIDC system could suffer from cyber-attacks, leading to massive power mismatches in multiple AC systems, and resulting in catastrophic consequences. Since the sequential responses of an MIDC system and interconnected AC systems are in different timescales, this paper first establishes a two-timescale model to evaluate the sequential impacts caused by cyber-attacks. Then, an event-triggered cyber-defense strategy is proposed to enhance the cybersecurity of an MIDC system by mitigating multiple non-simultaneous cyber-attacks. Whenever new cyber-attack events occur, the proposed cyber-defense strategy, which is mathematically modeled as a mixed-integer quadratic programming problem, is executed on-line and updated in an event-triggered manner. Simulation results on an MIDC system demonstrate that the low-cost and almost blind cyber-attacks can cause severe frequency deviations, and the proposed strategy can mitigate multi-cyber-attacks effectively.

Implementation and Validation of a Model Predictive Controller on a Lab-Scale Three-Terminal MTDC Grid

In this paper, a reliable methodology is proposed in order to implement and validate a Model Predictive Control (MPC) scheme on an actual Voltage Source Converter (VSC) integrated in a scale-down multi-terminal DC grid. The objective of the investigated MPC controller is to enable AC frequency support among two asynchronous AC areas through a High Voltage Direct Current (HVDC) grid, while considering physical constraints, such as maximum and minimum DC voltage. A systematic and accurate implementation strategy is proposed, based mainly on the Hardware In the Loop (HIL) and Power Hardware In the Loop (PHIL), leading to the real-life testing on VSC, controlled by a classical microcontroller. The technical problems during the implementation process, as well as the proposed solutions, are described in detail through this paper. This procedure is deemed valuable to bridge the gap between offline simulation and the actual implementation of such advanced control scheme on experimental test rig.

Inertia Emulation and Fast Frequency-Droop Control Strategy of a Point-to-Point VSC-HVdc Transmission System for Asynchronous Grid Interconnection

This article proposes a novel inertia emulation and fast frequency-droop (IEFF) control strategy for a point-to-point voltage source converter-based high voltage direct current (VSC-HVdc) transmission system. It allows asynchronous ac grid synchronization in forms of inertial and fast frequency-droop responses provided to “both” sides of the VSC-HVdc interconnected grids, different from the historically reported schemes only providing these responses to one grid. Under the IEFF scheme, the power-regulating VSC (PR-VSC) station emulates a synchronous generator directly through its active power control, whereas the dc-voltage-regulating VSC (DR-VSC) station employs a dc voltage/frequency droop control to convey its connected grid frequency communication-free to the remote PR-VSC for inertial and fast frequency-droop oriented power variations. To avoid the clash of the PR-VSC IEFF and DR-VSC IEFF control under simultaneous frequency events in the asynchronous grids, a novel frequency support mode selection scheme is designed to effectively prioritize the IEFF support with a higher frequency gradient. The key parameters of the IEFF controller are optimized through small signal stability analysis. The effectiveness of the proposed IEFF scheme is verified through controller hardware-in-the-loop experiments. The results show that the proposed IEFF scheme effectively promotes synchronization and enhances overall frequency stability for the two interconnected asynchronous grids.

An Improved Frequency Support Algorithm for MT-HVDC Systems

This paper develops a new control strategy for voltage source converter (VSC) based multi-terminal high voltage dc (MT-HVDC) grids for enhancing the mutual frequency support between MT-HVDC interconnected asynchronous ac systems. The existing droop control structure of VSCs is augmented with an additional feedback voltage-based loop that generates a voltage shifting term to be added to the original dc voltage reference. The addition of this integrator-based controller nullifies the difference between voltage errors seen at adjacent converter stations. This action helps to overcome inaccuracies in the mutual frequency support caused by the uneven voltages across a dc system. The proposed strategy uses only local variables for the continuous-time control; thus, it does not rely on a high-bandwidth communication network, and its performance is not affected by the change of ac systems’ parameters. Modal analysis is carried out to investigate the stability of the proposed controller. Finally, comprehensive simulation studies are used to verify the proposed strategy’s performance and compare it with recently reported alternative controllers.

HVDC Transmission and its Potential Application in Remote Communities: Current Practice and Future Trend

High voltage direct current (HVdc) transmission systems play an essential role in our modern power grids, not only providing bulk power transmission and asynchronous ac connection, but also enabling renewable energy integration into power grids. Renewable energy integration through HVdc transmission systems leads to the creation of multiterminal HVdc (MT-HVdc) grids. Recently, the idea of electrifying remote communities by HVdc transmission systems has emerged, and multi-terminal small-scale HVdc grids have been investigated in such applications. Despite numerous applications and significant potential of HVdc transmission systems, various technical and economic challenges in utilizing these systems still persist. In this article, an extensive literature review is conducted on HVdc transmission systems, particularly focusing on their role in renewable energy integration, and their potential application in remote communities. Future research directions are also recommended.

Towards Optimal Coordination Between Regional Groups: HVDC Supplementary Power Control

With Europe dedicated to limiting climate change and greenhouse gas emissions, large shares of Renewable Energy Sources (RES) are being integrated in the national grids, phasing out conventional generation. The new challenges arising from the energy transition will require a better coordination between neighboring system operators to maintain system security. To this end, this paper studies the benefit of exchanging primary frequency reserves between asynchronous areas using the Supplementary Power Control (SPC) functionality of High-Voltage Direct-Current (HVDC) lines. First, we focus on the derivation of frequency metrics for asynchronous AC systems coupled by HVDC interconnectors. We compare two different control schemes for HVDC converters, which allow for unilateral or bilateral exchanges of reserves between neighboring systems. Second, we formulate frequency constraints and include them in a unit commitment problem to ensure the N-1 security criterion. A data-driven approach is proposed to better represent the frequency nadir constraint by means of cutting hyperplanes. Our results suggest that the exchange of primary reserves through HVDC can reduce up to 10% the cost of reserve procurement while maintaining the system N-1 secure.

Comparative study of frequency converters of three-phase electric motors in automatic control of technological parameter

Frequency converters of asynchronous three-phase AC motors are widely used in industry, including food production. A thorough understanding of the technical capabilities of frequency converters contributes to their efficient use. This article discusses a technical comparison of two frequency converters of three-phase electric motors of the Russian company "OWEN" and the Danish company "DANFOSS" according to the criterion of the quality of regulation of the technological parameter. For comparison, a test bench was created, in which the devices were tested in turn, under the same conditions. During the preparation and implementation of testing, it turned out that the hardware capabilities of frequency converters for their use in regulating technological parameters are the same, and the parametric capabilities are different. The frequency converter of the "OWEN" company is equipped with a PI controller, and the frequency converter from "DANFOSS" is equipped with a PID controller. As the results of testing the devices show, despite

Comprehensive performance analysis of flexible asynchronous AC link under various unbalanced grid voltage conditions

A flexible asynchronous AC link (FASAL) system is a technology that interconnects two power system networks operating at the same or different frequencies. The FASAL system is a newly developed technology; therefore, it requires a thorough investigation under different unbalanced grid voltage conditions for better behavioral understanding and appropriate future controller design. In this paper, the FASAL system is analyzed under various symmetrical and asymmetrical unbalanced grid voltage conditions. Moreover, the performance of the back-to-back voltage source converter (VSC) based HVDC link is also analyzed to establish its comparison with the FASAL system under the same conditions. The FASAL and VSC-HVDC systems are modeled in PSCAD/EMTDC software environment. The simulation results illustrate that the FASAL system performance is better under transient as well as steady-state conditions. Furthermore, the reduction in power at the receiving end side is also lesser in the FASAL system. For example, a 10% sag contributes only a 15% dip in power in the FASAL system, whereas VSC-HVDC reports a 30% power dip for the same sag condition.

Comparative analysis of the reliability of electric locomotives based on semi-Markov models of restorable systems

Based on the general theory of semi-Markov recovery processes and their application to Markov recovery processes, the applicability of stationary reliability indicators to assessing the reliability of electric locomotives with an asynchronous motor, the functioning of which is considered as a superposition of independent alternating semi-Markov processes, is shown. By calculating the main indicators of system reliability in a steady state, a comparative analysis of the operational reliability of electric locomotives with an asynchronous DC and AC motors, used respectively on the Azerbaijan and Kazakhstan railways, was carried out.

Coordinated frequency regulation among asynchronous AC grids with an MTDC system

With the increasing penetration level of renewable power, the power systems’ inertias are decreasing. To ensure that the frequency is above the threshold of the under frequency load shedding, one effective way is to share the spinning reserves among asynchronous AC grids through HVDC systems. In this paper, a coordinated frequency regulation scheme that enables spinning reserves sharing is proposed for such hybrid AC/DC grids. This scheme is a communication-free method that consists of the droop control, the inertia emulation control and the frequency safety control. The combined response of the three controls can increase not only the frequency nadir but also the settled frequency of the disturbed grid. In addition, the hysteresis comparators and the power injection limits are adopted in the frequency regulation scheme to reduce the coupling effect among AC grids. Thus, under small and harmless disturbance, the frequency regulation scheme will be inactive; the disturbance in strong AC grids would not affect the weak AC systems. The frequency regulation scheme coordinates well with the automatic generation control of the grids. The feasibility and effectiveness of the proposed frequency regulation scheme are verified through simulation studies.

Coordinated Flexible Damping Mechanism With Inertia Emulation Capability for MMC-MTDC Transmission Systems

The rapid growth in penetration levels of power-electronics-interfaced renewable power generation into grids has been reducing system kinetic inertia, which results in degraded frequency stability and oscillation damping. In an attempt to address this challenge, this article proposes a control scheme that realizes a coordinated flexible damping mechanism (CFDM) with inertia emulation capability for modular multilevel converter-based multiterminal dc (MMC-MTDC) transmission systems. The proposed CFDM scheme allows the MMC-MTDC system to autonomously provide an emulated inertial response with a flexible damping effect to ac systems in a similar fashion of synchronous generator, without the need for communication. To ensure the precision of the total provided damping power, the power losses of the converter stations and MTDC networks are also compensated in the algorithm. The CFDM controller parameters are optimized by the small-signal stability analysis. The effectiveness of the proposed CFDM scheme is demonstrated in a power system model simulated in MATLAB/Simulink, which consists of a three-terminal MMC-MTDC transmission system and three asynchronous ac girds. It is verified that the proposed CFDM scheme can effectively minimize grid frequency deviation and damp angular oscillation in grid events of load changes and faults.

Frequency and Voltage Regulation in Hybrid AC/DC Networks

Hybrid ac/dc networks are a key technology for sustainable electrical power systems, due to the increasing number of converter-based distributed energy resources such as solar or wind. In this article, we consider the design of control schemes for hybrid ac/dc networks, focusing especially on the control of the interlinking converters (ILCs). We present two control schemes: first for decentralized primary control and second a distributed controller to achieve secondary control objectives as well. In the primary case, the stability of the controlled system is proven in a general hybrid ac/dc network, which may include asynchronous ac subsystems. Furt hermore, it is demonstrated that power sharing across the ac/dc network is significantly improved compared to previously proposed dual-droop control. The proposed scheme for secondary control guarantees the convergence of the ac system frequencies and the average dc voltage of each dc subsystem to their nominal values. An optimal power allocation is also achieved at steady state. The applicability and effectiveness of the proposed algorithms are verified by advanced simulations on a test hybrid ac/dc network in Simscape Power Systems.