Negative Sequence Current Research Articles

Faults diagnosis in stator windings of high speed solid rotor induction motors using fuzzy neural network

The paper deals with faults diagnosis method proposed to detect the inter-turn and turn to earth short circuit in stator winding of three-phase high-speed solid rotor induction motors. This method based on negative sequence current of motor and fuzzy neural network algorithm. On the basis of analysis of 2-D electromagnet field in the solid rotor the rotor impedance has been derived to develop the solid rotor induction motor equivalent circuit. The motor equivalent circuit is simulated by MATLAB software to study and record the data for training and testing the proposed diagnosis method. The numerical results of proposed approach are evaluated using simulation of a three-phase high-speed solid-rotor induction motor of two-pole, 140 Hz. The results of simulation shows that the proposed diagnosis method is fast and efficient for detecting inter-turn and turn to earth faults in stator winding of high-speed solid-rotor induction motors with different faults conditions

A Novel Multistation Cooperative Negative Sequence Current Compensation Method for High-Speed Railway

Nowadays, the negative sequence current (NSC) issue is the most serious power quality problem for the high-speed railway power supply system, and the single substation NSC compensation methods cannot reduce the installed capacity of the compensating devices any further. A novel NSC compensation method, multistation cooperative compensation (MSCC), is proposed in this article. In order to adapt to the MSCC, a new NSC compensation principle, which is different from the traditional compensation principles, is carried out. Then, the cooperative control strategy is analyzed according to the new NSC compensation principle and the topology of the multiple traction substations. The compensating device is based on three-phase, cascaded, H-bridge converter which is suitable for the new NSC compensation principle. The capacity analysis results show that the MSCC can reduce the installed capacity of the compensating device for each single substation by 33.3% and even more under the constraint condition. A lower installed capacity means lower cost and smaller size. This advantage and the effectiveness of the MSCC are verified by both simulations and small-capacity experiments.

A Novel Co-Phase Power-Supply System Based on Modular Multilevel Converter for High-Speed Railway AT Traction Power-Supply System

The existing problems of the traction power-supply system (i.e., the existence of the neutral section and the power quality problems) limit the development of railways, especially high-speed railways, which are developing rapidly worldwide. The existence of the neutral section leads to the speed loss and traction loss as well as mechanical failures, all of which threaten the fast and safe operation of the train and the system. Meanwhile, the power quality problems (e.g., the negative sequence current, the reactive power, and the harmonic) can bring a series of problems that cannot be ignored on the three-phase grid side. In response, many researchers have proposed co-phase power-supply schemes to solve these two problems simultaneously. Given that the auto-transformer (AT) power-supply mode has become the main power-supply mode for the high-speed railway traction power-supply system, it has a bright future following the rapid development of the high-speed railway. In addition, there is no co-phase power-supply scheme designed for AT power-supply mode in the existing schemes. Therefore, the main contribution of this paper is to propose a specifically designed power-supply mode more suitable for the AT, as well as to establish the control systems for the rectifier side and the inverter side. In addition, for the proposed scheme, the operation principle is analyzed, the mathematical model is built, and the control system is created, and its functionality is verified by simulation, and its advantages are compared and summarized finally. The result proves that it can meet functional requirements. At the same time, compared with the existing co-phase power-supply scheme, it saves an auto-transformer in terms of topology, reduces the current stress by 10.9% in terms of the current stress of the switching device, and reduces the power loss by 0.25% in terms of the entire system power loss, which will result in a larger amount of electricity being saved. All of this makes it a more suitable co-phase power-supply scheme for the AT power-supply mode.

A new passive islanding detection method for hybrid distributed generation under zero power imbalance

To solve the energy consumption demand of the world and environmental problems in the future, the distributed generation is considered an alternative approach. In this paper, a new passive islanding detection technique is proposed for the hybrid distributed generation (HDG) system based on changes in negative sequence voltage (NSV) and currents during an unintentional islanding. Islanding is caused in the distributed generation system due to failures in the power grid. As per distribution energy resources interconnection standards, it should be detected within 2 sec with the equipments connected to it. It is difficult to detect islanding during zero power imbalance condition. Sequence analyser will separate the positive, negative and zero sequence components of voltages and currents from the voltages and currents obtained at the point of common coupling (PCC). During a wide range of power imbalance conditions, the change in negative sequence voltage and currents are examined for islanding detection. The simulation shows that this method is free from non-detection zone, even at zero power imbalances between load and distribution generation. The computer simulations made in MATLAB/Simulink laboratory show the effectiveness of this method.

A new passive islanding detection method for hybrid distributed generation under zero power imbalance

To solve the energy consumption demand of the world and environmental problems in the future, the distributed generation is considered an alternative approach. In this paper, a new passive islanding detection technique is proposed for the hybrid distributed generation (HDG) system based on changes in negative sequence voltage (NSV) and currents during an unintentional islanding. Islanding is caused in the distributed generation system due to failures in the power grid. As per distribution energy resources interconnection standards, it should be detected within 2 sec with the equipments connected to it. It is difficult to detect islanding during zero power imbalance condition. Sequence analyser will separate the positive, negative and zero sequence components of voltages and currents from the voltages and currents obtained at the point of common coupling (PCC). During a wide range of power imbalance conditions, the change in negative sequence voltage and currents are examined for islanding detection. The simulation shows that this method is free from non-detection zone, even at zero power imbalances between load and distribution generation. The computer simulations made in MATLAB/Simulink laboratory show the effectiveness of this method.

Identification of Islanding Events in Utility Grid With Renewable Energy Penetration Using Current Based Passive Method

Renewable energy (RE) generation levels are increasing in modern power systems at a fast rate due to their advantages of clean and non-exhaustible nature of energy. However, this type of generation creates technical challenges in terms of operation and control due to uncertain and un-predictable nature of generation. Islanding is an operational scenario where there is a loss of grid and RE generators continue to feed power to the local load. This has harmful effects on the RE generators and operating personal. Hence, it is expected that islanding scenario is identified in minimum time and RE generators are disconnected within 2 s duration after island formation. This paper designed an islanding identification scheme (IDS) by designing a current islanding detection indicator (CIDI) that combines the features computed by processing the current signals, negative sequence current (NSC) and negative sequence voltage (NSV) using the Stockwell transform (ST) and the Hilbert transform (HT). Information contained by the total harmonic distortions of voltage ( THD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> ) and current ( THD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> ) is also used while designing the CIDI. Islanding and non-islanding events of category-I & II are identified and discriminated from each other by comparison of peak magnitude of CIDI with the first threshold value (FTV) and second threshold value (STV). This IDS effectively recognizes the islanding events even in the noisy environment with minimum non-detection zone (NDZ) and minimum time. The efficiency is greater than 98% even with the noise of 20dB SNR (signal to noise ratio). The performance of proposed IDS is better compared to IDS using discrete wavelet transform (DWT), Empirical mode decomposition (EMD), Slantlet transform & Ridgelet probabilistic neural network (RPNN), and artificial neural network (ANN). The effectiveness of IDS is validated on IEEE-13 nodes test system using MATLAB software, practical distribution network and in real time scenario by use of real time digital simulator (RTDS).

An Improved Three-Stages Cascading Passivity-Based Control of Grid-Connected LCL Converter in Unbalanced Weak Grid Condition

This paper proposes an improved three-stages cascading passivity-based control (PBC) for a grid -connected LCL converter in unbalanced weak grid condition. In general, the traditional double-loop control based on positive and negative sequence transformations is used in grid-connected converter control in unbalanced weak grid condition. However, it is time consuming for second harmonic filtering, and positive and negative sequence currents need to be controlled separately. The PBC has strong robustness to interference, and the line voltage based PBC can deal with the voltage unbalance effectively and easily without negative sequence transformation. But the traditional PBC needs six variables and three damping coefficients for the grid-connected LCL converter in unbalanced grid condition, and it has the disadvantage of difficult implementation. The improved PBC can realize the same control effect with three-stages cascading PBCs of two variables and one damping coefficient, and it has the advantages of easy implementation, good performance and high stability. First, the modeling and controller design are detailed described. Then the SIMULINK simulation results demonstrate the benefits of the improved control strategy. Finally, a grid-connected LCL converter prototype of 5kW is built and the experimental results verify the correctness and effectivity of the improved three-stages PBC strategy.

Fuzzy Fault Detection for Permanent Magnet Synchronous Motor

In the transition to automated and automatic manufacturing an urgent problem is to increase the reliability of mobile robots (MR) and their drives, creation of devices to monitor the technical characteristics of MR, diagnose and predict the remaining resource. Inspite of the high relevance of the diagnosing MR drives problem, there are no generally accepted methodology for diagnosing MR drives, criteria for selecting methods, parameters and volumes of diagnostics at present. An unsolved problem, related to the diagnosis of MR drives and the prediction of their residual life remains, is the development of methods that allow to carry out of automatic complex multiparametric diagnostics and prediction of the residual life using artificial intelligence methods. Effective fault detection and diagnosis can improve the reliability of the MR drive and avoid costly maintenance. In this paper a fault detection scheme for synchronous motors with permanent magnets based on a fuzzy system is proposed. The sequence current components (positive and negative sequence currents) are used as fault indicators and are set as input to the fuzzy fault detector. The expediency of the proposed scheme for determining of various types of faults for a synchronous motor with permanent magnets under various operating conditions is simulated using the SimInTech software.

The Flexible Smart Traction Power Supply System and Its Hierarchical Energy Management Strategy

With the increasing requirements of the railway sector for electrified railways and the development of society, the traction power supply system needs to become more flexible, economic and reliable. The traditional traction power supply system has many problems due to its own topology, such as negative sequence current, load fluctuation, low utilization rate of regenerative braking energy, and inability to use the distributed generation along the railway. In order to solve these problems and make the traction power supply system (TPSS) more flexible, efficient and reliable, a novel TPSS called flexible smart traction power supply system (FSTPSS) is proposed. This traction power supply system consists of AC-DC-AC traction substation and the microgrid along the railway. It can effectively mitigate the problems of negative sequence current, improve regenerative braking energy utilization and distributed generation utilization. A hierarchical energy management strategy is also proposed here, which can effectively improve the operating economy of FSTPSS and reduce the load fluctuation of FSTPSS. And based on the actual load measurement power of a traction substation of China’s Beijing-Shanghai high-speed railway, the proposed energy management strategy has been verified for the economic improvement and load fluctuation reduction of TPSS.

Grid-Following Voltage Source Converters: Basic Schemes and Current Control Techniques to Operate With Unbalanced Voltage Conditions

The growing relevance of voltage source converters (VSCs), and the deep impact they have on the development and maintenance of the electrical grid, increase the necessity of further research on how to deal with nonideal grid conditions from the VSCs control. This paper is aimed to summarize the basic techniques and schemes that might be required for a grid-connected VSC to work under these conditions: grid synchronization schemes, sequence decomposition, current reference generation, and current controllers. At the same time, some alternative schemes that improves the basic ones are cited. Modelling and the two typical current controllers design and tuning under stationary and synchronous reference frames are also exhibited. Given the importance of the current control stage in the VSC behaviour, five control schemes, designed to track negative sequence currents, are shown and tested in simulation and experiments. According to the experiments, it is shown that the standard proportional-resonant controller achieves the best performance in negative sequence tracking due to the robustness of its non-ideal version, the improved implementation thanks to the delta operator, and the non-dependence on grid-synchronization schemes. Alternatively, one approach based on dual synchronous reference frame is also highlighted for easiness of implementation and good performance.

Phasor-domain Dynamic Model of Asymmetric Current Injection Controller for Converter-interfaced Generator

This paper presents a controller model of asymmetric current injection for converter-interfaced generators suitable for root-mean-square (RMS) phasor-domain, fundamental-frequency, three-phase, and dynamic simulation tools. The effectiveness of the proposed controller is assessed with simulations in test systems with high percentage of converter-interfaced generation. The simulations focus on the operation of protection relays that use negative-sequence quantities in their directional elements. This paper also presents and compares two strategies to limit reactive negative-sequence currents, and active and reactive positive-sequence currents. A tutorial test system and a regional system part of the actual Brazilian Interconnected Power System are used to assess the correctness of the proposed controller in three-phase fundamental-frequency RMS dynamic simulations.

Model predictive flexible power control for grid‐connected inverter under unbalanced network

Model predictive power control (MPPC) is considered as a promising algorithm utilised in grid-connected inverter due to its fast dynamic response, simple control structure and multi-objective optimisation. However, under unbalanced network, the grid current with excessive distortion troubles the application of MPPC. Based on the conventional MPPC, this study proposes a model predictive flexible power control (MPFPC) to reduce the current total harmonic distortion and achieve three flexible targets, including the cancellation of the active- and reactive-power oscillation and the negative-sequence grid current. In this algorithm, the power references are divided into two parts, which are used as reference values for the instantaneous active power control (IAPC) and the instantaneous reactive power control (IRPC), respectively. The three flexible targets are achieved by regulating the ratio of the two parts of power reference. Meanwhile, the grid current of MPFPC is the sum of IAPC and IRPC; therefore, the grid current distortion can be suppressed effectively. Simulation and experimental platform constructed by a three-level inverter are established to validate the effectiveness of MPFPC, and the results exhibit that the MPFPC meets the above expectations.

Connection and Control Strategy of PV Converter Integrated into Railway Traction Power Supply System

In order to supply the single-phase locomotive load and mitigate the negative sequence current, this paper develops a V/V transformer-based connection and control strategy of three-phase photovoltaic (PV) converters integrated into railway traction power supply systems. In this V/V transformer-based connection, the two-phase traction voltage is converted into the three-phase voltage. This approach can offer a common low voltage AC bus, which is more convenient for more access to three-phase PV converters. Based on this V/V transformer-based connection, an individual phase current control strategy with the hybrid current reference is fully designed. In this control strategy, the current reference, containing two parts, is generated. One is the asymmetrical part for powering the single-phase locomotive load and mitigating the negative sequence current. The other is the symmetrical part for feeding the surplus power back to the utility grid. Then, each phase current replaces the dual-sequence current to be controlled to track the corresponding phase current reference. Consequently, PV converters can flexibly inject the symmetrical and asymmetrical currents without the dual-sequence extraction for a simpler implementation. Finally, the effectiveness of the developed connection and control strategy is validated by the simulation studies.

Model predictive DC-component power control for grid-connected inverters under unbalanced network

Under an unbalanced network, model predictive control (MPPC) with a new definition of the instantaneous reactive power has attracted a great deal of attention due to its simple control structure and outstanding steady-state performance. However, the reactive power cannot precisely trace the nonzero reference. In this paper, a model predictive DC-component power control (MPDCPC) is proposed to tackle the above-mentioned problem. Additionally, the MPDCPC can eliminate oscillations on the reactive power and the negative-sequence current. Then, the corresponding mathematical formulas are derived to extract the DC-component powers, to calculate the DC-component power derivative and to modify the power reference. By regulating the DC-component power to trace the modified reference, the MPDCPC can reduce the current harmonic and remove the oscillations on the active power, reactive power or negative-sequence current. Simulation and experimental platforms are established to demonstrate the validity of MPDCPC. Results demonstrate that the MPDCPC can effectively suppress current distortion and remove oscillations on active power, reactive power or negative-sequence current. In addition, the MPDCPC can successfully break through the restrictions of the MPPC with a new definition of instantaneous reactive power.

OCNOP Control of Grid-Connected Converters for Single-Phase Open Circuit Fault

An open circuit fault-tolerant operation of a grid-connected inverter requires switching reconfigurations or redundant inverter leg to bypass the faulty device. Such measures led to increased cost and design complexities. In the absence of such reconfiguration mechanisms, a grid-connected inverter with an open circuit fault in one of the inverter legs injects negative sequence current into the grid. The negative sequence current causes oscillation in dc-link voltage and thereby injects lower order harmonic into the grid. The present work proposes a novel control strategy (open circuit nonoscillatory power) to deliver a steady power into a balanced three-phase grid with only two active legs of the grid-connected inverter. The current of these two active phases are controlled to be equal in magnitude and 60° phase apart. This inhibits the flow of negative sequence current and in turn limits the lower order harmonic injection within the permissible limit. A control strategy is also proposed to modify the power references during postfault operation so that the postfault current in two active phases are limited within the converter current rating. The performance of the proposed scheme is compared with the conventional vector control scheme through a simulation study. The simulation results for the proposed control scheme are verified through experimental study in a laboratory prototype.

A Novel Direct Power Control for DFIG With Parallel Compensator Under Unbalanced Grid Condition

This article presents a voltage-modulated direct power control (VM-DPC) with an additional parallel compensator for the doubly fed induction generator under unbalanced grid conditions. The proposed method not only guarantees a satisfying steady-state performance but provides also a regulating property of the negative-sequence output currents through designed negative-sequence parallel compensator. It can provide symmetrical stator currents and suppress the ripples in both active and reactive powers under unbalanced grid conditions. The performance of the proposed method are verified by comparing it with three different control strategies in simulations carried out in MATLAB/Simulink SimScape Power System. Finally, the effectiveness of the proposed method is evaluated in an experimental prototype, which proves the proposed VM-DPC with the additional compensator has a satisfactory steady-state performance and a fast power transient response under unbalanced grid conditions.

Hybrid virtual impedance‐based control strategy for DFIG in hybrid wind farm to disperse negative sequence current during network unbalance

When a hybrid wind farm based on doubly fed induction generator (DFIG) and fixed speed induction generator (FSIG) is connected to an unbalanced network, most of the negative sequence current caused by the unbalanced grid will be centralised at the FSIG for its inherent small negative sequence output impedance (NSOI). The centralised negative sequence current can cause significant torque pulsation in the drive train of FSIG, which may go beyond the pressure endurance of the drive train and cause some damages. Meanwhile, losses and risks of overcurrent and grid code violation can also be increased. This study presents a control strategy for DFIG in hybrid wind farm during network unbalance, which can disperse the negative sequence current and the consequent pressure among the three parallel power sources without communication, i.e. among the DFIG stator, the grid side converter (GSC), and the FSIG, rather than centralising the pressure at any of them. In order to achieve this target, a novel hybrid virtual impedance method is proposed to flexibly control the NSOI of the DFIG stator and the GSC from zero to infinity. Theoretical analysis and simulation results are provided to verify the operation performance of the proposed strategy.

Extended Park's vector method in early inter‐turn short circuit fault detection for the stator windings of offshore wind doubly‐fed induction generators

To avoid dramatic economic losses and unrecoverable failures in offshore wind farms, it is necessary to accurately detect faults at the early stage of doubly-fed induction generators. The new extended Park's vector method is proposed in the study. The amplitude of the twice fundamental frequency component of the square of the Park's vector module is selected as the new fault feature to realise the early fault detection. The relationship between the extended Park's vector method and negative sequence current of the stator is also analysed in the study. The Fryze–Buchholz–Dpenbrock algorithm and the non-linear least squares method are applied to estimate the mathematic relationship among unbalanced stator voltage, load change and the negative sequence current of the stator. The experiments were conducted and the results showed that the proposed method is robust to the effects of the unbalanced stator voltage and load change. Therefore, the proposed method can accurately detect faults at the early stage with high sensitivity and reliability.

An improved power regulation method for a three-terminal hybrid AC/DC microgrid during module failure

To reduce the number of power conversion stages and achieve the direct connection to medium-voltage (MV) grid without line-frequency transformer integration, a three-terminal hybrid AC/DC microgrid with one AC port and two DC ports has attracted great interests. It is composed of a cascaded H-bridge (CHB) converter based AC grid port and two dual active bridge (DAB) converters based DC microgrid ports. The large number of power modules increases the probability of module failure, which may cause unbalanced capacitor voltages of CHB converter and unstable DC microgrid voltages. This paper proposes an improved power regulation method with a combination of zero-sequence voltage (ZSV) and negative-sequence current (NSC) injection. When module failure occurs, first, ZSV is injected to adjust the modulation margin in the faulty phase; then, based on three-phase power analysis, NSC injection is activated to realize DC voltage balancing control in three clusters. In addition, the constraints ofZSV and NSC injection are respectively investigated, and the selection of injected ZSV and the derived NSC are analyzed. In a three-terminal hybrid AC/DC microgrid, the simulation waveforms validate the effectiveness of the proposed fault-tolerant operation scheme with the stable DC microgrid voltages and balanced DC capacitor voltages. Experimental results on a CHB converter also verify the effectiveness of the proposed scheme.

Design and Analysis of Adaptive Impedance Structure for Cophase Railway Traction Supply Power Quality Conditioner

The electrical locomotives are treated as single-phase nonlinear loads, which can cause the low power factor, high contents of harmonic currents, and negative-sequence current. The power quality conditioners are widely used for solving the power quality issues in the railway power system. In power quality conditioners, the coupled impedance can affect the operation voltage, compensation range, and performance. Therefore, this article discusses the effect of different coupled impedances in conditioners, including fixed inductive impedance, fixed capacitive impedance, and adaptive impedance. Moreover, an adaptive impedance-coupled hybrid power quality conditioner (AI-HPQC) is proposed in comparison with inductive-coupled railway power quality compensator (RPC) and capacitive-coupled hybrid power quality conditioner (HPQC). Furthermore, a control method for the AI-HPQC is proposed for the coordination control of the adaptive impedance part and the active inverter part. The validities of the RPC, HPQC, and the proposed AI-HPQC are verified by both simulation and experimental results.