Unbalanced Faults Research Articles

Active front‐end rectifier modelling using dynamic phasors for more‐electric aircraft applications

The more-electric aircraft (MEA) has become a dominant trend for next-generation aircraft. The electrical power system (EPS) on-board may take many forms: AC, DC, hybrid, frequency-wild, variable voltage, together with the possibility of novel connectivity topologies. To address the stability, availability and capability issues as well as to assess the performance of the power quality and transient behaviour, extensive simulation work is required to develop the EPS architectures. The study develops a fast-simulation model of active front-end rectifiers based on the dynamic phasor concept. The model is suitable for accelerated simulation studies of EPS under normal, unbalanced and line fault conditions. The performance and effectiveness of the developed model have been demonstrated by comparison against time-domain models in three-phase and synchronous space-vector representations. The experimental verification of the dynamic phasor model is also reported. The prime purpose of the model is for the simulation studies of MEA power architectures at system level; however it can be directly applied for simulation study of any other EPS interfacing with active front-end rectifiers.

A Direct Phase-coordinates Approach to Fault Ride Through of Unbalanced Faults in Large-scale Photovoltaic Power Systems

Unbalanced faults pose significant risks to photovoltaic power systems, leading to unsteady conditions in DC-link voltage and photovoltaic output power. Such photovoltaic system instabilities will, in turn, cause detrimental impact on power grid security. Therefore, the fault ride through for unbalanced faults has been an emerging and challenging area of research and development. This article presents a phase-coordinates approach for computing grid-connected current references directly from phase quantities to achieve effective fault ride through to alleviate the impact from unbalanced grid faults. The novelty lies in the development and use of the vector relationship between three-phase currents and active power, which avoids the complication in adopting negative-sequence components. The method is simple to implement and shows the advantages of high reliability and computational efficiency. Tests on a distribution grid with a large-scale photovoltaic plant have verified the effectiveness of the proposed ride through strategy in dealing with unbalanced faults.

Enhanced Control for a Direct-driven Permanent Synchronous Generator Wind-power Generation System with Flywheel Energy Storage Unit Under Unbalanced Grid Fault

—This article presents an enhanced control strategy for a direct-driven permanent synchronous generator based wind-power generation system with a flywheel energy storage unit. The behaviors of the direct-driven permanent magnet synchronous generator system with a flywheel energy storage unit under unbalanced grid fault conditions are investigated. Three control targets for the grid-side converter during network unbalance conditions are identified, including eliminating the oscillations in the total active power or reactive power or no negative-sequence current injecting into the grid. Meanwhile, the DC-link voltage oscillations can be effectively suppressed during the unbalanced grid fault by controlling the flywheel energy storage unit. Furthermore, a proportional–integral-resonant controller is designed for the flywheel motor to eliminate the oscillations in the DC-link voltage. Finally, the proposed coordinated control strategy for the direct-driven permanent magnet synchronous generator system with a flywheel energy storage unit has been validated by the simulation results of a 1-MW direct-driven permanent magnet synchronous generator wind power generation system with a flywheel energy storage unit under unbalanced grid fault.

Neural network fault diagnosis of a trolling motor based on feature reduction techniques for an unmanned surface vehicle

This article presents a novel approach to the diagnosis of unbalanced faults in a trolling motor under stationary operating conditions. The trolling motor being typically of that used as the propulsion system for an unmanned surface vehicle, the diagnosis approach is based on the use of discrete wavelet transforms as a feature extraction tool and a time-delayed neural network for fault classification. The time-delayed neural network classifies between healthy and faulty conditions of the trolling motor by analysing the stator current and vibration. To overcome feature redundancy, which affects diagnosis accuracy, several feature reduction methods have been tested, and the orthogonal fuzzy neighbourhood discriminant analysis approach is found to be the most effective method. Four faulty conditions were analysed under laboratory conditions, where one of the blades causing damage to the trolling motor is cut into 10%, 25%, half and then into full to simulate the effects of propeller blades being damaged partly or fully. The results obtained from the real-time simulation demonstrate the effectiveness and reliability of the proposed methodology in classifying the different faults faster and accurately.

Characterizing Voltage Sags and Swells Using Three-Phase Voltage Ellipse Parameters

This paper presents an algorithm for detection, classification, and characterization of voltage sags and swells in electricity networks using three-phase voltage ellipse parameters. The proposed method employs the instantaneous magnitude of three-phase voltage signals in three axes, which are separated from each other by 120 $^{\circ} $ . Thus, the resultant rotating vector, namely, the three-phase voltage vector, traces an ellipse. Then, the parameters of the ellipse, which include minor axis, major axis, and inclination angle, are used to develop the proposed algorithm for classification and characterization of voltage sags and swells. The proposed method is validated using real data of the IEEE working group and some measured real data of the Belgian transmission grid. The method is further tested for the sags and swells generated due to balanced and unbalanced faults at different buses, in test distribution networks (radial and mesh type) embedded with distributed generation and in a practical distribution network of Australia. This paper also demonstrates the proposed algorithm as a real-time sag/swell monitoring tool.

Flexible Control Strategy for Grid-Connected Inverter Under Unbalanced Grid Faults Without PLL

Power oscillation and current quality are the important performance targets for the grid-connected inverter under unbalanced grid faults. First, the inherent reason for the current harmonic and power oscillation of the inverter is discussed with a quantitative analysis. Second, a new control strategy is proposed to achieve the coordinate control of power and current quality without the need for a phase-locked loop (PLL) or voltage/current positive/negative sequence extraction calculation. Finally, the experimental tests are conducted under unbalanced grid faults, and the results verify the effectiveness of the propose method.

Dynamic Phasor Modeling of Type 3 DFIG Wind Generators (Including SSCI Phenomenon) for Short-Circuit Calculations

Short-circuit modeling of wind generators is crucial to determine protective relay and control settings, equipment ratings, and to provide data for protection coordination. The short-circuit contribution of a Type 3 wind farm connected to a series-compensated line is affected by subsynchronous interactions, making it essential to model such behavior. Fundamental frequency models are unable to represent the majority of critical wind generator fault characteristics. The complete electromagnetic transient (EMT) models, though accurate, demand high levels of computation and modeling expertise. This paper proposes a novel modeling technique for a Type 3 wind farm based on the generalized averaging theory, where system variables are represented using time-varying Fourier coefficients known as dynamic phasors. The novelty and advantage of the proposed modeling technique is that it does not just include 60-Hz frequencies but also other dominant frequencies, such as 36 Hz, that are present due to the SSCI in the system. Methods currently used by the industry mostly rely on fundamental frequency-based analysis. Only the appropriate dynamic phasors are selected for the required fault behavior to be represented, improving computational efficiency. Once the SSCI behavior (waveforms showing resonant frequency at the point of common coupling) of a series-compensated Type 3 wind farm from real-time field data is available, the developed model could be used to simulate the scenario without necessarily having to know the exact control blocks of the wind generator controls. A 450-MW Type 3 wind farm, consisting of 150 units, was modeled using the proposed approach. The method is shown to be accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for nonfundamental frequency components present in fault currents during subsynchronous interactions.

A control structure for PWM-controlled static synchronous compensators under unbalanced conditions and grid faults

Abstract Grid connected Voltage Source Converters (VSCs) are the heart of many applications with power quality concerns due to their reactive power controllability. Among the widely used grid-connected applications of the VSCs, the Static Synchronous Compensators (STATCOMs) are commonly used for compensating the voltage quality problems that come from voltage sag and swell. In spite of superior feature of fast voltage regulation and reactive power support functionality, VSC-based STATCOMs have the major drawback of being sensitive to the grid disturbances, especially the unbalanced condition and faults. Moreover, when the STATCOMs are used in the Distributed Generation (DG) applications or reactive power support of the sensitive industrial load, the unbalanced condition becomes even more intolerable. Protection system usually trips due to over current or highly distorted current caused by negative sequence current flow under unbalanced conditions and system faults. This paper propose an alternative control structure to keep the VSC-based STATCOM online during the unbalanced condition and system faults by limiting the negative sequence current. This eliminates the need to redesign/overdesign of the STACOM power components and over rating of the semiconductor switches to operate under fault current. Converter MVA rating reduction will decrease the cost significantly. Proposed controller performance has been verified by simulation and Hardware-In-the-Loop test.

Active and Reactive Power Strategies With Peak Current Limitation for Distributed Generation Inverters During Unbalanced Grid Faults

Distributed generation inverters have become a key element to improve grid efficiency and reliability, particularly during grid faults. Under these severe perturbations, inverter-based power sources should accomplish low-voltage ride-through requirements in order to keep feeding the grid and support the grid voltage. Also, rated current can be required to better utilize reactive power provisions. This paper presents a reference generator capable to accomplish these two objectives: to keep the injected currents within safety values and to compute the power references for a better utilization of the inverter power capacity. The reference generator is fully flexible since positive and negative active and reactive powers can be simultaneously injected to improve ride-through services. Selected experimental results are reported to evaluate the performance of the proposed reference generator under different control strategies.

Comparison between separated and not separated positive and negative sequence control in a high voltage direct current transmission system during unbalanced grid faults

Especially during unbalanced grid faults the behavior of voltage source converter based generation or transmission units is highly dependent on the implemented control of the converter. For the control strategy of the converter the two general alternatives are on the one hand a separated control of positive and negative sequence quantities and on the other hand a not separated control in which voltages and currents are not decomposed in the symmetrical components.Furthermore the quickness of the converter control allows the injection of reactive current during grid faults in order to stabilize the voltage locally. The possibilities and limitations of this so called dynamic voltage control depend highly on the implemented control strategy as well.In this publication the advantages and disadvantages of the two control strategies with separated and not separated positive and negative sequence are explained and demonstrated during an unbalanced grid fault. For this an AC-DC hybrid transmission test system in which the converter control of the high voltage direct current transmission system is equipped with both control strategies forms the basis for this comparison.

A Compound fault diagnosis for rolling bearings method based on blind source separation and ensemble empirical mode decomposition.

A Compound fault signal usually contains multiple characteristic signals and strong confusion noise, which makes it difficult to separate week fault signals from them through conventional ways, such as FFT-based envelope detection, wavelet transform or empirical mode decomposition individually. In order to improve the compound faults diagnose of rolling bearings via signals’ separation, the present paper proposes a new method to identify compound faults from measured mixed-signals, which is based on ensemble empirical mode decomposition (EEMD) method and independent component analysis (ICA) technique. With the approach, a vibration signal is firstly decomposed into intrinsic mode functions (IMF) by EEMD method to obtain multichannel signals. Then, according to a cross correlation criterion, the corresponding IMF is selected as the input matrix of ICA. Finally, the compound faults can be separated effectively by executing ICA method, which makes the fault features more easily extracted and more clearly identified. Experimental results validate the effectiveness of the proposed method in compound fault separating, which works not only for the outer race defect, but also for the rollers defect and the unbalance fault of the experimental system.

Transducer invariant multi-class fault classification in a rotor-bearing system using support vector machines

Faults in a rotor-bearing system due to bearings and unbalance have been classified using support vector machines (SVMs). Vibration signals on a rotor-bearing system were measured simultaneously at five different rotating speeds using seven transducers. The most sensitive feature of the vibration signals has been determined using compensation distance evaluation technique. Multi-class SVMs classification algorithm was then implemented for classification of the faults by considering SVMs created by the possible combinations of the most two sensitive features for each type of fault. By using optimal SVM parameters, the effective location of transducer among seven transducers for best classification of the faults has been investigated and found that any transducer provides a classification of 75% or better and this classification rate increases when more transducers are considered. This paper provides a robust SVM based technique using only time domain data without any additional preprocessing for classifying bearing and unbalance faults.

An Application of Proportional-Resonant Controller in MMC-HVDC System under Unbalanced Voltage Conditions

This paper presents an application of proportional-resonant (PR) current controllers in modular multilevel converter-high voltage direct current (MMC-HVDC) system under unbalanced voltage conditions. The ac currents are transformed and controlled in the stationary reference frame (αβ-frame). Thus, the complex analysis of the positive and negative sequence components in the synchronous rotating reference frame (dq-frame) is not necessary. With this control method, the ac currents are kept balanced and the dc-link voltage is constant under the unbalanced voltage fault conditions. The simulation results based on a detailed PSCAD/EMTDC model confirm the effectiveness of the proposed control method.

Novel Configuration and Transient Management Control Strategy for VSC-HVDC

This paper presents a novel system configuration for voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission connected to a large-scale offshore wind power plant (WPP). The proposed scheme is reconfigured at the onshore end to achieve shunt and series compensation, which is named as `Unified-VSC-HVDC' (U-VSC-HVDC). A mathematical model of the proposed configuration is derived to determine the rating of the employed series and shunt converters. To achieve a flexible control strategy for balanced and unbalanced fault conditions, the proposed transient management scheme employs positive and negative sequence controllers for the series compensation. The negative sequence voltage components are determined in such a way as to minimize power oscillations caused by asymmetrical faults, and hence to reduce DC link voltage overshoots. A test system comprised of a detailed representation of the proposed configuration is simulated and evaluated using PSCAD/EMTDC. A comprehensive study validates the capability of the proposed configuration and transient management scheme for achieving smooth power transfer and superior transient performance of the electrical grid. Also, it minimizes the possibilities of electrical network propagations in response to symmetrical and asymmetrical grid faults.

Current signature analysis for unbalance fault detection in a rotor supported by active magnetic bearings

Current signature analysis for unbalance fault detection in a rotor supported by active magnetic bearings

A novel negative sequence reactance based high speed directional relay algorithm

Summary Negative sequence (NS) directional protection may not provide desirable results when the fault component magnitude is quite small as a result of the existence of high resistance faults or remote forward faults with a strong source. Full cycle (FC) Fourier-based algorithms are widely used in digital relaying devices in order to estimate phasors. The common required time for fault detection in these relays is approximately one power frequency cycle. This paper presents a novel NS reactance-based high speed directional relay algorithm. General park transformation (GPT) is applied to calculate NS components within two to three sample intervals, which greatly increases the protection's operation speed. The fast response is comparable to transient component-based protection but with improved reliability. The NS reactance-based directional elements provide high sensitivity with no need to set the voltage or current threshold. The principle can correctly detect the fault direction even in the presence of low voltage and current. Extensive simulation results verified the effectiveness of the proposed algorithm and show that it has adequate selectivity, sensitivity and reliability, a fast operating response and immunity to fault resistance under a variety of system and unbalanced fault conditions. These excellent properties make it a good prospect for engineering applications. Copyright © 2014 John Wiley & Sons, Ltd.

Damping of Power System Oscillations by using ANFIS based UPFC supplementary controller

The main purpose of this paper is to investigate an Adaptive Neuron- Fuzzy Inference System (ANFIS) based supplementary Unified Power Flow Controller (UPFC) in superimposing a damping function on the control signal of UPFC who’s incorporated in multi-machines power system. By using a hybrid learning procedure, the proposed ANFIS construct an input –output mapping based on stipulated input-output data pairs.This control scheme designed to produce supplementary signal for damping oscillations in the interconnected power systems. Oscillations in power systems have to be taken a serious note when the fault takes place in any part of the system; else this might lead to the instability mode and shutting down of the power system. The UPFC device is modelled using dynamic phasor method and interfaced with an interconnected power system. The improved damping performance was studied and it is validated on a benchmark power system example taken from literature. The systems model is evaluated via simulations during unbalanced faults. The control actions are formulated as a set of local controllers, namely the generator excitation controller, power system stabilizer (PSS) and unified power flow controller, which are coordinated to maintain transient angle stability, transient voltage stability and damping of inter-area and local oscillations following a disturbance.

Design of Mine Ventilator Fault Diagnosis System

The common faults of mine ventilator are researched in this paper, and rotor misalignment, unbalance, oil whirl, surge and other faults and fault characterization of the generation mechanism are analyzed. The faults diagnosis system is designed based on rough neural network. First, the characteristics of the type of fault for fan failure data collection, including vibration and temperature signals. Then, the pretreated sample data using rough set attribute reduction method to delete redundant attributes. Finally, the sample data is divided into training and testing samples, were used to train and test the neural network classifier. Experiments show that the system is reliable, diagnostic yield, improved ventilator system security, expanding the scope of application of rough sets.

지중송전선로의 대칭분 임피던스 모델링에 관한 연구

Power system fault analysis is commonly based on well-known symmetrical component method, which describes power system elements by positive, negative and zero sequence impedance. The majority of fault in transmission lines is unbalanced fault, such as line-to-ground faults, so that both positive and zero sequence impedance is required for fault analysis. When unbalanced fault occurs, zero sequence current flows through earth and groundwires in overhead transmission systems and through cable sheaths and earth in underground transmission systems. Since zero sequence current distribution between cable sheath and earth is dependent on both sheath bondings and grounding configurations, care must be taken to calculate zero sequence impedance of underground cable transmission lines. In this paper, EMTP-based sequence impedance calculation method was described and applied to 345㎸ cable transmission systems. Calculation results showed that detailed circuit analysis is desirable to avoid possible errors of sequence impedance calculation resulted from various configuration of cable sheath bonding and grounding in underground cable transmission systems.

Fault diagnosis of rotating machinery with a novel statistical feature extraction and evaluation method

Fault diagnosis of rotating machinery is receiving more and more attentions. Vibration signals of rotating machinery are commonly analyzed to extract features of faults, and the features are identified with classifiers, e.g. artificial neural networks (ANNs) and support vector machines (SVMs). Due to nonlinear behaviors and unknown noises in machinery, the extracted features are varying from sample to sample, which may result in false classifications. It is also difficult to analytically ensure the accuracy of fault diagnosis. In this paper, a feature extraction and evaluation method is proposed for fault diagnosis of rotating machinery. Based on the central limit theory, an extraction procedure is given to obtain the statistical features with the help of existing signal processing tools. The obtained statistical features approximately obey normal distributions. They can significantly improve the performance of fault classification, and it is verified by taking ANN and SVM classifiers as examples. Then the statistical features are evaluated with a decoupling technique and compared with thresholds to make the decision on fault classification. The proposed evaluation method only requires simple algebraic computation, and the accuracy of fault classification can be analytically guaranteed in terms of the so-called false classification rate (FCR). An experiment is carried out to verify the effectiveness of the proposed method, where the unbalanced fault of rotor, inner race fault, outer race fault and ball fault of bearings are considered.