Sequence Voltage Research Articles

DTCDWT-SMOTE-XGBoost-Based Islanding Detection for Distributed Generation Systems: An Approach of Class-Imbalanced Issue

This article offers an islanding detection approach based on dual-tree complex discrete wavelet transform (DTCDWT) and extreme gradient boosting method (XGBoost). Initially, the DTCDWT is used to extract the feature vector from collected negative sequence voltage and current signals at the respective distributed energy resource ends. Afterward, the nonlinear synthetic minority oversampling technique (SMOTE)-based XGBoost is used to classify the islanding (I) and nonislanding (NI) events with the following characteristics: high accuracy, good sensitivity, and faster speed. Here, the SMOTE is used as an offline prefeature processing approach to reduce the unbalancing/uncertainty of “I” and “NI” classes and helps the intelligent classifier to impart a fair classification result. This scheme is found to be effective for the detection of I and NI conditions, satisfying the IEEE islanding detection standard. Additionally, the obtained simulation results are compared with other existing works to show its usefulness.

A simplified passive islanding detection technique based on susceptible power indice with zero NDZ

Distributed energy resources (DER’s) incorporation in power system brings economical, technical and environmental benefits. Besides these benefits, excess penetration of DER’s elevates technical concerns. Islanding detection is one of main issue and is essential for personnel safety and protection of equipment. This work introduces a passive islanding detection scheme based on phase angle of positive sequence voltage (PAOPSV). The PAOPSV was selected on the basis of rigorous investigation of 13 different indices. Comparative analysis demonstrates that PAOPSV has the best sensitivity and accuracy for islanding detection among all other parameters. An extensive case study considering worst-case scenarios is accomplished in order to check the working efficacy of the suggested scheme that easily discriminates islanding events from non-islanding events such as load switching, different fault types switching, capacitor and motor switching. The suggested scheme is simple with fast execution and easy to implement that is established on IEEE 1547 generic test system in MATLAB/SIMULINK environment. Islanding is detected within 0.10 s even with a zero non-detection zone.

Analysis and Simulation of Inter-Turn Fault Of Synchronous Generator Using MATLAB

Abstract: The paper represents a comprehensive analysis of the inter-turn faults of a synchronous generator. In today's power system, we have used three phase synchronous generator to generate electric power, thus we all always try to reduce the loss in order to improve the efficiency of the alternator. We basically focus more on the stator winding faults. Stator winding has unbalanced loading, field failure and stator winding fault (including line to ground fault, line to line fault and double line to ground fault, three phase fault and inter turn fault) Alternator stator inter turn faults are considered to be rare and therefore they are not taken into that serious consideration while designing the protection system for the alternator. But we knew that there could be an inter-turn short circuit fault in the stator winding of the alternator. Early detection of inter turn faults will eliminate damage to the stator core and adjacent coils, reducing repair costs and generator outage times. Since inter turn fault causes imbalance in phase voltage, this concept is discussed in this paperfor inter turn fault detection. The negative sequence voltage of the generator is used as a fault indicator for inter turn fault detection. This new approach is done using MATLAB software. Also, this method works for external as well as internal fault and helps in keeping the generator winding healthy. Keywords: Inter Turn fault, internal and external faults, Internal Negative, Sequence Generator Voltage.

Harmonic Modeling and Analysis for Parallel 12-Pulse Rectifier under Unbalanced Voltage Condition in Frequency-Domain

This paper treats a parallel 12-pulse rectifier as the study subject. The influence of unbalanced terminal voltage, the triggering angle, as well as the commutation angle of a thyristor-controlled rectifier circuit are considered in switching functions. Then, a frequency-domain harmonic coupled admittance matrix under the effect of positive sequence and negative sequence voltages are proposed, respectively. With the integration of the above matrices, a harmonic coupled admittance matrix model of a parallel 12-pulse rectifier under unbalanced voltage is proposed. On this basis, a simplified harmonic model of a 12-pulse rectifier is established, which only considers the effect of major elements through analyzing the elements’ distribution and magnitude of the harmonic coupled admittance matrix, as well as the AC current harmonic producing mechanism. All the models proposed in this paper are verified with high accuracy through simulation in MATLAB/Simulink. The models proposed in this paper can be used to guide the selection of harmonic suppression measures and equipment.

Luenberger Observer-Based Microgrid Control Strategy for Mixed Load Conditions

In this paper, a Luenberger observer-based microgrid control strategy is proposed to enhance the power quality of microgrids, when the grid loads are mixed and strongly non-linear. To improve performance under this condition, a Luenberger observer is designed for three phase power grids. On the basis of the observer, the components of different frequencies and sequences of voltages and currents are obtained accurately. The virtual impedance of different frequencies and sequences is designed, which makes the equivalent line impedance meet the power-sharing condition, reducing the fundamental negative sequence voltages and harmonic voltages. The active power droop equation, meanwhile, is proposed, where the bus voltage is modified. The value range of virtual impedance is discussed in the complex frequency domain. The proposed control strategy does not require any communication lines, so the hardware structure is simplified. The simulations and experiments are provided to verify the effectiveness of the proposed method.

A two‐stage IIDG fault control scheme considering the fault direction identification in active distribution networks

In traditional power systems, the positive sequence fault component (PSFC) based criteria is often used to identify the fault directions. If the PSFC impedance angle of the inverter‐interfaced distributed generator (IIDG) is between 0 and 90°, the traditional PSFC‐based criteria will remain applicable in active distribution networks (ADNs). However, due to the diversity of the IIDG fault control mode and the uncertainty of the positive sequence voltage variation caused by faults, the PSFC impedance angle of IIDG is difficult to determine. To solve this problem, this paper proposes a two‐stage fault control scheme for IIDG. At different stages, IIDGs select different power references to meet different requirements. In the first stage, the PSFC impedance angle of IIDG is fixed to a given value, thereby maintaining the applicability of the PSFC‐based criteria in ADN. The detailed power reference calculation method of IIDG is given for this stage. In the second stage, IIDGs can select different fault control modes to support the fault voltage flexibly. By using the proposed two‐stage IIDG fault control scheme, the protection devices can accurately identify the fault direction in ADNs, which is verified by the MATLAB simulation results. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Improved Sensorless Control Method and Asymmetric Phase Resistances Determination for Permanent Magnet Synchronous Machines

This article proposes an improved sensorless control method for permanent magnet synchronous machines under phase resistance asymmetry. The proposed method acquires positive sequence current and voltage signals by applying a sequence component extractor and a double synchronous reference frame current control strategy. These signals can, then, be applied to improve the extended electromotive force based sensorless observer to eliminate the undesired second-order fluctuation in position and speed estimation due to asymmetric phase resistances. In addition, the resistance deviations, which may be due to asymmetric resistances and temperature variation, could introduce dc offset error in rotor position estimation. Therefore, a model reference adaptive system based resistance estimation is employed to compensate for inaccuracy of resistances. Moreover, from the information contained in the negative sequence voltages, the specific values of all asymmetric phase resistances can be estimated as well. Experimental and simulation results are provided to validate the proposed method.

Synchronverter-Based STATCOM With Voltage Imbalance Compensation Functionality

In this work, a synchronverter-based D-STATCOM with self-synchronization and voltage imbalance compensation capability with operation independent of the power operating modes is proposed. The S-STATCOM's synchronverter base model is firstly presented in space-vector representation together with its control strategy with standard D-STATCOM control structure. Then, the Voltage Imbalance Compensation Loop (VICL) is proposed, which uses two well known resonant control structures, the Double Second-Order Generalized Integrator (DSOGI) and a Proportional plus Resonant controller (PR), in an innovative way to effectively turn the S-STATCOM into a specialized active filter for negative sequence voltage, compensating the voltage imbalance at the PCC by supplying the corresponding oscillating power locally. Simulation and experimental results on a small scale prototype are presented to validate the control strategy, which successfully reduced voltage imbalances from up to 10% to bellow the 2% acceptable levels.

Startup Control to Eliminate Inrush Current for Star-Connected Cascaded H-Bridge STATCOM

Soft startup is essential to the operation of the star-connected cascaded H-bridge (CHB) STATCOM. The conventional startup stage involves only uncontrolled rectifier process in which the dc voltages are charged by inserting startup resistors to limit the inrush current. Nevertheless, another inrush current is induced at the beginning of normal operation due to the low charged dc voltages during the startup stage. In this article, to eliminate this inrush current, a novel startup control is proposed for further charging the dc voltages after the uncontrolled rectifier process. During this controlled charging process, the startup resistors are not bypassed so as to smoothen the dc voltages and limit the inrush current. In this scenario, the overall dc voltage control with constant charging current is proposed by establishing a relationship between the overall dc voltage and the positive sequence modulation index. Besides, the cluster dc voltage balancing control based on the negative sequence voltage injection is proposed by redistributing the active power of the STATCOM. Moreover, the charging current is designed to reduce the active power of the startup resistors and increase the charging speed. Due to the flexibility of the startup control, it can be applied to any voltage-source-based ac/dc grid-connected converter. Finally, the performance of the proposed control is verified by the simulation and experimental results.

A new approach to improving the sensitivity of earthing relay and reducing the over-voltage in 6 kV grid of open-pit mines

Purpose. Earthing relays are utilized very commonly in open-pit mines for disconnecting the failure part of 6 kV electric system from earthing incident. Much previous research is mentioned on searching either an optimal method for improving the effect of relays or improvements of reducing the overvoltage caused by grounding separately. The paper presents a new approach to not only improving the relays sensitivity but also to limiting the over-voltage. A new algorithm is also proposed to control automatic connection of an additional resistor in order to achieve both above mentioned purposes of 6 kV earthing relay. Methodology. The propose controlling diagram is implemented on PROTEUS to get 3U0 signals. The main components appeared when 6 kV feeders were faulted to earth. Afterward, the detecting signal is utilized as input for simulation diagram in MATLAB to get and analyze the wave forms of responded zero sequence voltage. Findings. The paper presents a new algorithm utilizing a resistor that is auto-controlled to auto-connect the open-triangle coil of 6 kV measurement transformer at the moment of earth fault. This automatic connection aims to increase the relay sensitivity as well as reduce the overvoltage. The finding results are sensitivity factors of relay and voltage magnitudes of healthy phases, which will be compared with corresponding values when there is no connection of resistor to prove the effectiveness of the method. Originality. A useful method is proposed for increasing the earthing current to enhance the sensitivity of earthing relay. Practical value. The proposed control diagram could be used to design a device aiming to improve the safety of 6 kV grid in open pit mines when earth faults occur.

Stator winding fault detection of an Alternator using MATLAB

The paper represents the comprehensive analysis of stator winding faults of an alternator. An alternator is main component of power system which plays a big role in whole system for security and stability. The conventional system only detect symmetrical and unsymmetrical faults, they will not detect inter turn fault. The fault detection of stator winding is an effective and important measure to ensure the safe operation of generator. Early detection of stator winding fault would eliminate damage to stator core & adjacent coils, also reduce repairing cost and outage time of generator. As stator winding fault causes unbalance in phase voltage, this concept is discussed in this paper for the detection of stator winding fault. The negative sequence voltage of generator is used as fault indicator for fault detection. This new method is done using MATLAB software. Also, this method operate for external as well as internal fault and helps the generator winding keep healthy. Keywords— stator winding fault, internal negative sequence voltage, internal and external faults.

Grid-Forming and Grid-Following Based Microgrid Inverters Control

Microgrids (ℳ-grids) can be thought of as a small-scale electrical network comprised of a mix of Distributed Generation (DG) resources, storage devices, and a variety of load species. It provides communities with a stable, secure, and renewable energy supply in either off-grid (grid-forming) or on-grid (grid-following) mode. In this work, a control strategy of coordinated power management for a Low Voltage (LV) ℳ-grid with integration of solar Photovoltaic (PV), Battery Energy Storage System (BESS) and three phase loads operated autonomously or connected to the utility grid has been created and analyzed in the Matlab Simulink environment. The main goal expressed here is to achieve the following points: (i) grid following, grid forming modes, and resynchronization mode between them, (ii) Maximum Power Point Tracking (MPPT) from solar PV using fuzzy logic technique, and active power regulator based boost converter using a Proportional Integral (PI) controller is activated when a curtailment operation is required, (iii) ℳ-grid imbalance compensation (negative sequence) due to large single-phase load is activated, and (iv) detection and diagnosis the fault types using Discrete Wavelet Transform (DWT). Under the influence of irradiance fluctuation on solar plant, the proposed control technique demonstrates how the adopted system works in grid-following mode (PQ control), grid- formation, and grid resynchronization to seamlessly connect the ℳ-grid with the main distribution system. In this system, a power curtailment management system is introduced in the event of a significant reduction in load, allowing the control strategy to be switched from MPPT to PQ control, permitting the BESS to absorb excess power. Also, in grid-following mode, the BESS’s imbalance compensation mechanism helps to reduce the negative sequence voltage that occurs at the Point of Common Coupling (PCC) bus as a result of an imbalance in the grid’s power supply. In addition to the features described above, this system made use of DWT to detect and diagnose various fault conditions.

New Algorithm for 2-terminal Transmission Line Fault Location Integrating Voltage Phasor Feature and Phase Angle Jump Checking

The low precision and instability of fault location results of power system transmission lines is a major problem that affects the safe operation of power systems, which needs to be solved more effectively. With the increasing coverage of the synchronic Phasor Measurement Unit (PMU) in power systems, it has laid a good foundation for the research and application of high-precision fault location method based on 2-termnal synchronous measurement of phasor. Based on this, a new high-precision fault location algorithm integrating voltage phasor feature and phase-angle jump checking is proposed in this paper. Firstly, the synchronous fundamental positive sequence voltage and current phasors of the fault line are extracted; and then all the roots(including real and virtual roots) are calculated by using the condition that the fundamental positive sequence voltage phasor at the fault point is equal; finally the unique real root represents the actual fault point can be determined by using the feature that the check function of phase-angle will step-jump when crossing the fault point from left to right, and then the purpose of high precision fault location is realized. This algorithm has the characteristics of strong versatility, high precision and immunity to transition resistance. The results of simulation and example show the feasibility and effectiveness of the fault location algorithm the fault location algorithm proposed in this paper.

Control Design of a 99% Efficiency Transformerless EV Charger Providing Standardized Grid Services

A nonisolated DC charger is proposed for electric vehicle (EV) battery system. The leakage current can be reduced by the novel charging system without bulky transformer. A zero sequence voltage control method is developed to stabilize the common-mode voltage, thus, reduce the leakage current. The proposed fast charger includes two energy conversion stages: DC–DC converter for battery side control; DC–AC converter for grid interface and common-mode voltage control. The parameters of the system with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter are analyzed and optimized for a better performance of grid interface. Grid service functions are designed for the EV charger to provide grid-voltage/frequency compensations. High power (22 kW) and high efficiency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 99%) are achieved with low-leakage current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt;$</tex-math></inline-formula> 20 mA). The experiments are implemented to verify the proposed EV charger system.

Research on Harmonic Suppression of New Controller in Islanding Mode of Microgrid

The proportional complex integral (PCI) controller can realize zero steady‐state error control of AC signals with specific frequencies in a static coordinate system. The frequency of the traditional power grid is relatively fixed, so the zero steady‐state error control of the AC signal is realized by adopting the PCI controller. With the increasing proportion of renewable energy generation, the frequency characteristics in the power system become uncertain. When the microgrid operates in island mode, the frequency of the fundamental wave and harmonics fluctuates greatly, which causes the PCI controller to fail to achieve zero steady‐state error control. This paper proposes two controllers: positive and negative sequence decoupling quasi‐proportional complex integral (PNDQPCI) controller and high‐order positive and negative sequence decoupling quasi‐proportional complex integral (HPNDQPCI) controller. The PNDQPCI controller can effectively control the positive and negative sequence voltage when the system frequency drifts; The HPNDQPCI controller can effectively suppress multiple harmonics. Finally, simulation and experiment are presented to verify the theoretical analysis as well as the feasibility and the superiority of the proposed controller. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Estimating Parallel Transmission Line Fault Using Phasor Measurement Unit based Artificial Neural Network

In a parallel transmission line, fault line, fault location, and classification have been identified separately. Since fault location takes more calculation time, it is unfit for protection purposes. Thus, this paper presented a new scheme that estimates the faulted line, fault location, and type of fault in a parallel transmission line, with the help of Phasor Measurement Units (PMU) and the Artificial Intelligence Technique. The proposed scheme uses phasors of Positive Sequence Voltage (PSV) and Positive Sequence Current (PSC) to detect the faulted line in a parallel transmission line. Further, the Artificial Neural Network (ANN) models have been designed to estimate the fault distance on a faulted line and classify the fault types. The PSV and PSC obtained from PMUs are selected as inputs because they have a negligible mutual coupling effect on the parallel transmission lines. The IEEE 9 bus system and the IEEE 30 bus system have been considered test cases to validate the proposed scheme. The proposed scheme is also validated by hardware in the loop on an OPAL-RT real-time simulator (OP RTS 5700). The results show that the proposed scheme identifies the fault line, fault distance, and type of fault regardless of its location on a parallel transmission line. Besides this, the proposed scheme has a quick response time, making it suitable for wide-area backup protection applications.

Low Switching Frequency SPWM Strategies for Open-Winding Machine With Low Current Harmonics

In this article, two novel sinusoidal pulsewidth modulation (SPWM) strategies with simple implementation are proposed for an open-winding machine fed by isolated dc-bus dual two-level three-phase inverters to reduce switching frequency and current harmonics. The proposed strategies utilize zero sequence voltage injection and unbalanced reference voltage distribution to generate discontinuous PWM with low current harmonics and less switching action. Two different zero sequence voltages are introduced in this article corresponding to two proposed PWM strategies with different switching actions in a PWM cycle. The novelty of this article is low current harmonics and less switching actions in a PWM cycle compared with the conventional SPWM strategy. Moreover, compared with the subhexagonal center PWM strategy, the proposed SPWM strategies have advantages of easy implementation and half switching commutations with the same current harmonic characteristics. The superiority of the proposed SPWM strategies is validated by simulation and experiment results.

Distribution Network Voltage Arc Suppression Method Based on Flexible Regulation of Neutral Point Potential of the New Grounding Transformer

Aiming at the problems that the existing arc suppression cabinet technology cannot adjust the zero sequence voltage flexibly and the control and hardware implementation of active arc suppression equipment based on power electronics are difficult, a new method of voltage arc suppression in the distribution network based on the flexible regulation of the neutral point potential of the new grounding transformer is proposed. From the perspective of the sequence component, the variation law of the neutral point voltage before and after the closing of the tapped grounding branch of the new grounding transformer is analyzed. By selecting the appropriate initial tap position of the tap-to-ground switch, the fault phase voltage is flexibly controlled so that the fault phase voltage is reduced to below the reignition voltage to quickly solve the influence of the single-phase grounding fault, and the risk of non-fault phase insulation breakdown is reduced to ensure the stable operation of the distribution network. According to the ratio of the fault phase current to the zero sequence voltage before and after the gear inputs, whether the arc suppression of the single-phase grounding fault is completed is judged. Compared with power electronic products, the new grounding transformer has the advantages of high reliability, long service life, and low cost and has strong economy and applicability in engineering. The single-phase grounding fault model of the 10kV distribution network is built in the PSCAD/EMTDC simulation environment, and the new method of the distribution network voltage arc suppression based on the flexible regulation of neutral point potential of the new grounding transformer is verified. The simulation results show that the proposed method can effectively reduce the fault phase voltage to achieve the reliable arc suppression of the single-phase grounding fault.

Wavelet packet coefficient of transient first half wave line selection method for single-phase grounding fault

Overvoltage will occur after single-phase grounding fault in distribution network, endangering system insulation. It is of great significance to find out the fault line as soon as possible to ensure the safe operation of the system. Wavelet modulus maxima can be used to detect the sudden change singularity of transient current. However, the judgement of single mode maximum value is easily influenced by the form of fault and interference, leading to misjudgement. In this paper, the transient characteristics of single-phase grounding fault are analyzed, and the magnitude and phase characteristics of the first half wave are given; Based on wavelet packet algorithm, several key problems such as boundary effect, wavelet selection and decomposition scale are explored; Taking the zero sequence voltage as the reference to determine the fault interval, the zero sequence current of each line is decomposed by three-scale wavelet packet using db6 wavelet, and the wavelet coefficients of the characteristic frequency band are extracted for comparison. The fault line is the one with the largest amplitude and different phase from others. The distribution system model of neutral grounding through arc suppression coil is built. Simulation is carried out for several cases with great difficulty in line selection, such as short circuit when phase voltage crosses zero, high resistance grounding and long line fault. The experimental results prove the reliability of this method.

An optimal phase control method for the suppression of phase sequence exchange impulse current and voltage

Phase sequence exchange (PSE) is an emerging emergency control method for power systems. However, PSE will generate impulse current and voltage. In this research, to suppress the impulse of PSE, a phase control method for zero-current opening and equal-voltage closing is adopted, and verified via both physics and experiments. First, an equivalent circuit is established, and the differential equations are solved to validate that this phase control method is the optimal method for the suppression of the impulse current and voltage in PSE. Then, the action sequence scheme and calculation method of the phase control method are advanced. Finally, based on the real-time digital simulation system (RTDS), a PSE prototype was used to carry out impact experiments. The results verify that the performance of the optimal method is the best compared to those of other control methods, the requirement for the sampling frequency is low, and there is almost no additional cost. In the two experiments, the first peak of the impulse current was found to be reduced by 47.49% and 33.28% on average, respectively, and the first peak of the impulse voltage was found to be reduced by 14.89% and 44.10% on average, respectively.