Zero Sequence Research Articles

A positive, negative and zero sequences electric power, to improve upon the standard IEEE 1459-2010

PurposesThe purpose of this paper is to identify on the instantaneous electrical power basis of a nonsinusoidal periodic current three-phase asymmetric system, active and reactive positive, negative and zero sequence powers, taking into account higher harmonics. The main power theories, including those embodied in the IEEE 1459 standard, do not allow to evaluate some of power components.Design/methodology/approachA well-known fact is that the three-phase AC system total power with the symmetry of currents and voltages is constant. It corresponds to the electrical energy transfer process in a DC system. In this case, the electrical energy transmission can be taken as high quality. It has been established that the components of active and reactive powers are because of the product of current and voltage of unidirectional sequences. The orthogonal components of the oscillating power are because of the product of the voltage and current components of different sequences, with the exception of the zero sequence.FindingsFor an unbalanced nonsinusoidal mode of a three-phase system, the components of instantaneous power were defined, corresponding to the active and reactive positive and negative and zero sequences powers with the selection of the fundamental and higher harmonics. The active and reactive powers of sequences were divided into two categories – consumed and generated.Originality/valueIt is proposed to use the ratio of “interfere” power RMS value to the total power RMS value to assess the instantaneous power distortion.

Estimation of zero sequence impedance in power transformers using 2D and 3D FEM

This paper describes calculation of zero sequence impedance in three phase, three leg core type power transformers using 2D and 3D finite element method (FEM) calculation on example of 100 MVA power transformer. Paper also describes how winding connection and arrangement affects the size and calculation of zero sequence impedance.

Operation of Current Protection for the Zero Sequence of Overhead Electric Power Transmission Lines Taking the Circuit for Ice Melting on its Lightning-Protection Cables into Account

Operation of Current Protection for the Zero Sequence of Overhead Electric Power Transmission Lines Taking the Circuit for Ice Melting on its Lightning-Protection Cables into Account

60° phase difference current control‐based open‐phase fault‐tolerant operation strategy for open‐end winding permanent synchronous motor

AbstractAs the most favorable candidate of electric vehicle drive system, the fault tolerance of open‐end winding permanent synchronous motor (OEW‐PMSM) fed by dual inverter with common DC bus is of great significance because it is related to the reliability and safety of the system. Aiming at the most common fault of open‐phase, a fault‐tolerant operation strategy based on 60° phase difference current control is proposed by making full use of the characteristics that each phase of OEW‐PMSM can be controlled independently. Through this method, the same stator rotating magnetic field can be generated before and after the open‐phase fault without reconfiguring the controller, so as to reduce the torque ripple after the fault and ensure the continuous and stable fault‐tolerant operation of the motor. In addition, this method can reduce the switching times by 50%. The proposed method can reduce the switching loss by 50% compared with the ordinary modulation method, reduce the switching loss, and will not produce large zero sequence current. Experimental results show that the fault‐tolerant performance of this strategy is significantly improved compared with the traditional methods.

Methodological Bases of the Fire Hazard Reduction in Internal and External 0.38 kV Electrical Networks with Unbalancing Power Consumption

The article shows that the operation modes in three-phase four-wire 0.38 kV electrical networks are objectively associated with unbalancing power consumption. As a result, significant additional electrical energy thermal losses occur in the neutral conductor’s internal electrical networks and external electrical networks l elements, which not only increase the electrical energy consumption, but also create conditions for the fire-hazardous occurrence single-phase short circuits. A mathematical model is proposed, an algorithm and a program for calculating the limit zero sequence values flows in external neutral conductors and internal 0.38 kV electrical networks are developed. Numerical calculations are made and tabular these limit values data for different conductors’ sections made of various conductive materials are presented. The unbalancing operation modes calculations in operating external and internal 0.38 kV electrical networks have been made, on the basis of which the proposed methodology has been tested. The technical means use effectiveness for the operating modes balancing in existing electrical networks with unbalance power consumption is proved.

A Hybrid Carrier-Based DPWM With Controllable NP Voltage for Three-Phase Vienna Rectifiers

In this article, a hybrid carrier-based discontinuous pulsewidth modulation (HCB-DPWM) scheme is proposed for Vienna-type rectifiers. By injecting modified zero sequence components (ZSCs), the proposed HCB-DPWM-I scheme solves the issue of alternating input current distortion under unbalanced neutral point (NP) voltage conditions. The proposed HCB-DPWM-II scheme can also active NP voltage balancing control in case of extreme imbalance NP voltages. The flexible transition between the HCB-DPWM-I and the HCB-DPWM-II is also analyzed. Finally, a 10-kW prototype was built to conduct experiments. Experimental results demonstrate that the HCB-DPWM-I can be preferred to run Vienna-type rectifiers without regulating the NP voltage, and the HCB-DPWM-II can balance the NP voltage under severe NP voltage imbalance conditions. Compared with existing DPWM schemes, both the current THD performance and efficiencies can be significantly improved by the proposed HCB-DPWM scheme, especially under unbalanced NP voltage conditions.

Image encryption algorithm based on digital filters controlled by 2D robust chaotic map

This paper introduces a new cryptosystem algorithm based on digital filters; as an encryption operation, second-order and third-order IIR digital filters are used, and as a decryption operation, second-order and third-order FIR digital filters are used: Chaos in digital filters is caused by the overflow non-linearity function. Both IIR and FIR digital filters are controlled by the first random sequence of zeros and ones produced by the 2D robust chaotic map, while the filters coefficients are generated by the second sequence, so that each pixel is encrypted and decrypted with different coefficients. Fisher-Yates shuffle has been used to obtain a random permutation of the position of the pixels of the original image, so that, the rows are shuffled first, then the columns, based on random numbers obtained by a hyperchaotic map 2D-LICM and 2D-SLIM respectively. Another algorithm based only on a single digital IIR/FIR filter is also presented in this work to allow us to demonstrate the development and high security of the proposed approach. Simulation and experimental results with performance analysis are carried out to demonstrate that our algorithm has high security and can resist various attacks such as differential attacks and statistical attacks.

Continuous comprehensive line selection technology based on D-S evidence theory

Aiming at the single-phase grounding fault in 380 V three-phase AC power system with ungrounded neutral, a fault line selection method considering zero sequence current ratio method, zero sequence current phase ratio method and dual frequency method is proposed in this paper, which makes full use of the fault information such as zero sequence current and insulation resistance to ground of each branch when single-phase grounding fault occurs, The fault degree function and reliability function corresponding to each line selection method are constructed to form a comprehensive line selection strategy. After continuous comprehensive line selection, the multiple comprehensive line selection results are fused by using D-S evidence theory to obtain the final line selection results. The simulation calculation on MATLAB/ Simulink platform shows that this method is feasible and improves the accuracy of line selection to a great extent.

An Analytical Zero Sequence Method to Locate Fault in Distribution Systems Rich in DG

Locating high impedance faults (HIF) in a distribution system increases the system’s reliability and improves the service’s quality for customers. However, this type of fault produces currents of low magnitude, making it difficult to locate using conventional techniques. Therefore, this paper proposes a method for the location of single-phase high impedance faults in distribution systems with photovoltaic cells in low voltage. The method is based on the use of the sequence component of the current circulating in the medium voltage feeders of the distribution system, which are obtained from synchronized phasorial meters positioned along the feeder. The method considers nonsymmetrical lines, unbalanced loads, capacitors banks, photovoltaic DGs, and load variation. The performance of the proposed method was evaluated and tested on several distribution systems using ATP or PSCAD® and the results show that the method can locate HIFs with high precision. Also, a comparison was made with recently published methods to show the efficiency of the proposed method.

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.

A Novel Microgrid Islanding Detection Algorithm Based on a Multi-Feature Improved LSTM

Islanding detection is one of the conditions necessary for the safe operation of the microgrid. The detection technology should provide the ability to differentiate islanded operations from power grid disturbances effectively. Given that it is difficult to set the fault threshold using the passive detection method, and because the traditional active detection method affects the output power quality, a microgrid islanding detection method based on the Sliding Window Discrete Fourier Transform (SDFT)-Empirical Mode Decomposition (EMD) and Long Short-Term Memory (LSTM) network optimized by an attention mechanism is proposed. In this paper, the inverter output current and voltage at the point of common coupling (PCC) are transformed by the SDFT. The positive sequence, zero sequence, and negative sequence components of voltage and current harmonics are calculated and reconstructed by adopting the symmetrical component method (SCM). Meanwhile, the current and voltage are decomposed into a mono intrinsic mode function (IMF). The symmetric components of voltage, current, and IMFs are used as inputs to the deep learning algorithm. An LSTM with the features extracted to classify islanding and grid disturbance is proposed. By using the attention mechanism to set the weight values of the features of hidden states obtained by the LSTM network, the proportion of important features increases, which improves the classification effect. MATLAB/Simulink simulation results indicate that the proposed method can effectively classify the islanding state under different working conditions with an accuracy level of 98.4% and a loss value of 0.0725 with a maximal detection time of 66.94 ms. It can also reduce the non-detection zone (NDZ) and detection time and has a certain level of noise resistance. Meanwhile, the problem whereby the active method affects the microgrid power quality is avoided without disturbing the current or power of the microgrid.

Grounding fault test technology of distribution network based on zero sequence power direction judgment

Distribution network grounding fault testing technology relies on a single criterion, which has a positioning blind area, resulting in large ranging error. In this paper, a grounding fault test technology for distribution network based on zero sequence power direction determination is proposed. The direction from the power supply side to the load terminal is specified as the positive direction of the network, the fault equivalent zero sequence current is modeled, and the high-frequency component of transient zero sequence current containing main fault characteristics is extracted. The fault line is determined based on the zero sequence power direction. For the fault line, the zero sequence power is less than zero, and the absolute value is much greater than the normal line, and the line selection device can select the line normally. The grounding fault test model of distribution network is established, and the fault current difference of each measuring point is calculated. When the difference is the smallest, it is identified as the fault location. The average ranging error of the test technology based on zero sequence power direction determination is 50.68m, which is 20.24m and 23.55m lower than that based on wavelet analysis and EMD. Therefore, it can accurately determine the fault line and has strong positioning advantages.

New Insights Into Calculations of AC Interference at Fundamental and Harmonic Frequencies Taking Account of the Phase Relationships of the Currents

Electromagnetic interference to pipelines and railways from AC sources has long been a cause for concern. Methods and standards have evolved to enable calculations of the voltages produced under different conditions. These take into account the AC frequency, the soil resistivity and the pipeline characteristics. However, the approximations presented in some standards fail to take into account the phase relationship of the currents in the AC conductors and how that affects the induced currents in the earth. This paper re-examines these issues by introducing a revised complex image method for 3-phase systems that provides a simple and accurate way to include the influence of induced currents in calculations of the induced emf in nearby conductors. Normal positive sequence 3-phase currents produce fields in neighbouring conductors that tend to cancel. For the associated image currents in the earth, the cancellation is so complete that they have no influence on the induced emf. A different situation occurs with the zero sequence currents that occur during fault conditions and triplen harmonics. These currents are in phase, so there is none of the cancelling effect and the contributions from each conductor add to give a larger induced emf in a neighbouring conductor.

Analysis of Fault Characteristics of Distribution Network with PST Loop Closing Device under Small Current Grounding System

With the rapid development of cities and the increasing complexity of distribution network systems, the cableization of transmission lines and the diversification of power users have brought new challenges to the supply reliability of distribution networks. Short-time outages caused by power outages and maintenance are one of the factors that affect the reliability of the power supply in the distribution network. The non-stop load transfer through a phase-shifting transformer (PST) operation can effectively improve the reliability of power supply to complex distribution systems. Considering the operation mode and structure form and fault type of the urban distribution network in China, comparing and analyzing the applicable scenarios of different neutral grounding methods, and based on the structure and zero sequence path characteristics of PST, an improved PST-based phase shifting and grounding transformer loop closing device with low power consumption is proposed. The fault characteristics of the PST-based loop closing device under the small current grounding system are also analyzed by the sequence component method, and, finally, the effectiveness of the phase-shifting and grounding transformer device is verified by simulation under PSCAD/EMTDC for fault routing and protection configuration of the urban distribution network in China.

A New Guidance for Circuit Breaker Rate Selection Including the Impact of Zero-Sequence X/R Ratio

The multiplying factor (MF) is critical for the correct design, sizing, and selection of protection devices in a power system. This factor depends on the X/R ratio at the point of failure. The standards establish an equivalent MF for the X/R ratios for both three-phase and single-phase fault accordingly for engineering practices. However, X/R ratios for single-phase faults ignore how much from the total X/R ratio comes from the positive-sequence or zero-sequence X/R ratio. This research demonstrates that the total X/R ratio at the point of failure is not the only variable that affects how the multiplying factor changes. The results demonstrate with dynamic short-circuit simulations using EMTP-ATPDraw, which it is also necessary to establish the MF considering the differences of the X/R ratios of each sequence, respectively. To support the engineering designer in the correct selection of the protection device, this article proposes a methodology complementary to the standards, which allows establishing the multiplying factor using the X/R ratios of positive and zero sequence separately. The results show that both oversized and undersized design situations can occur when ignoring the zero sequence in particular scenarios.

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.

Research on Low-Resistance Grounding Fault Line Selection Based on VMD with PE and K-Means Clustering Algorithm

Aiming at the fault line selection problem in the single-phase grounding system of the distribution network, a new fault line selection method based on VMD and permutation entropy feature extraction combined with K-means clustering algorithm is proposed. This method is a hybrid algorithm that can effectively identify fault line selection. Firstly, a simulation model is built and its zero sequence current is collected. The variational modal decomposition method is used to decompose the collected zero-sequence current into multiple intrinsic modal functions, which can not only effectively reduce the influence of harmonic components and noise in the characteristic signal but also facilitate the calculation. The extracted intrinsic mode function is calculated by permutation entropy (PE), and the calculated entropy value is constructed into a matrix to highlight the fault characteristics of the line; then, the matrix is subjected to K-means cluster analysis through the preprocessing algorithm and the faulty line is correctly distinguished. Then, regression verification is performed. Finally, it is verified by the recorded wave data of the real test site and then analyzed and compared with other algorithms. The proposed method shows that when a single-phase ground fault occurs, the ground fault line selection can be effectively identified under different transition resistances, grounding resistances, and fault distances. Therefore, this method can accurately identify the fault line selection, and the accuracy rate is 100%, which has a certain use value.

High sensitivity fault location technology for distribution networks considering measurement error

Accurately identifying weak fault characteristics is a critical issue for locating high impedance faults (HIFs) in distribution networks. Due to the measurement error of current sensors, the weak fault signals are easily submerged by the measurement noise interference. Nonetheless, the impact of measurement error is rarely considered explicitly. The present work addresses this issue by proposing a weak fault characteristics extraction algorithm based on an adaptive Fourier decomposition method (FDM) that can effectively suppress noise interference, and accurately locate faults with high sensitivity. First, the generalized Fourier expansion method is applied to obtain a series of independent and uncorrelated Fourier intrinsic band functions (FIBFs) adaptively by decomposing the noisy zero sequence current (ZSC) signal sampled from current sensors. Then, the FIBFs are employed to construct a set of Fourier-Hilbert spectra, which are utilized to map the noisy ZSC into a series of frequency distributions that includes the weak fault characteristic information of the HIF. The proposed density-distance search scheme can quickly identify the frequency center of gravity on both sides of the fault point to achieve reliable convergence of the clustering algorithm. Numerical simulations and field testing demonstrate that the proposed method can accurately locate HIFs with high sensitivity. Moreover, the observability and reliability of the distribution networks are improved.

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.