Sequence Voltage Research Articles

Study on Selective Insulation On-line Monitoring Technology for Mine Power Network

In this paper, by studying the change rule of zero sequence voltage when single-phase leakage occurs, the detection method of insulation decline of mine cables is studied, especially the additional low-frequency signal sampling technology of power cables. Combined with the conventional leakage protection technology, the online monitoring equipment and supporting software for the insulation parameters of power cables are developed to realize the online monitoring of the insulation of each branch cable of the mine low-voltage power supply system and achieve the purpose of selective leakage protection.

Design of the inverter in high accuracy and development of work therein by using compound ligation

A ligation (CL) system with a very high frequency in an inverter was used, it is a novelty in this study. We also employed a three-phase dynamic load (positive sequence voltage) to correct the power factor and account for active and reactive power in the output system this work is a novelty. The technique through compound ligation (CL) can improve inverter output waveform by reducing losses caused by ingrained agents in voltage source inverters (VSI), such as dead time caused by overload or voltage drop in an inverter's output, or abnormal load current conditions such as a short circuit current occur in the output stage of the inverter, and so on. The invention relates to the conversion of D.C. to A.C. power inverters, it is preferable to use compound ligation to achieve high efficiency, the device is lightweight, and has low losses and good precision. In addition, the present invention relates to improving an inverter load request sensation circuit, smoothing operational current, inverter response, and inverter spontaneous power factor improvement, as well as correction of the reactive power of passive components. In addition, an inverter with high active power (P) equal to (2.6×106 Watt), reactive power (Q) (5.4×107 VAR), was designed, and used positive sequence voltage (1.6×108 Watt ), as well as the switching period (10µs), The system's total harmonic distortion (THD) in voltage and current was 0.11 percent, while the system's accuracy was 99 percent. This is developed by using FPGA and oscillator circuit and programmable peripheral interface 8255 A as well as, ultrasonic PWM with a high-frequency range of (20–500 kHz) as demonstrated evidenced by the results obtained

Weak grid integrated solar photovoltaic‐based distributed generation system proficient in supplying VAR requirement with current restriction

AbstractThis study presents a grid interactive solar photovoltaic (PV) system proficient with low voltage ride through capability. When the supply voltage drops more than 10%, the solar PV system remains connected and continues to supply the active power to the loads as well as to the grid. Along with the active power supply, the solar PV array system supplies the reactive power for stabilising the system. Due to the supply of both the active and reactive powers, there is a chance for a grid fed inverter (GFI) current to exceed the maximum current limit. It may result in the tripping of inverter protection. Therefore, a current limiting logic is incorporated to keep the inverter current within the safe limit. The unbalanced voltages are responsible for circulation of negative sequence currents in the system. As the prime aim of the present work deals with the supply of positive sequence currents to the grid, it is required to estimate the positive and negative sequence voltages separately. Intending the estimation of sequence voltages from unbalanced supply voltages, a frequency adaptive higher order complex filter (FAHOCF) is used. A MATLAB model of grid interactive solar PV array is developed and simulated results are verified with test results.

Series Capacitive Compensation to Increase the Railway Traction Power Supply System Transmission Capacity

The article addresses the problem of improving the numerical analysis and operation of series capacitive compensation (SCC) units in the return wire of 25 kV power supply system traction substations to increase the railway capacity. Numerical analyses of the SCC impedance for two versions are considered: according to the condition of voltage equalization in the traction substations power supply arms and according to the condition of minimizing the negative sequence voltage. It is shown that the existing regulatory documents correctly place focus on the SCC impedances determined from the condition of ensuring equal power supply arm voltages. It is also shown that for numerical analyses of SCC impedance, the connection of the 110(220) kV power supply line impedances should be taken into account. To this end, it is proposed to connect, in the equivalent analysis circuit diagrams, two adjacent substations to the external power supply system through an equivalent delta of the impedances of 110(220) kV power lines. To effectively increase the voltage in the traction network while maintaining the minimum equalizing current values, it is advisable to connect the SCCs at both adjacent substations. In so doing, the no-load voltage values should be kept at the same level to reduce the equalizing current first component, which is determined by the difference of the transformers’ no-load voltages. It is shown that the connection of the SCC does not entail a growth in the equalizing current second component, which is determined by the difference of voltage losses at adjacent substations, including the voltages across the SCCs. If the SCC is connected at only one substation of the inter-substation zone, it is advisable to introduce automated control of the transformer OLTC at the substation without an SCC with a limited number of switching operations (up to 10 per day) with increased loads in the traction network.

Stationary-Frame Grid-Forming Inverter Control Architectures for Unbalanced Fault-Current Limiting

Grid-forming (GFM) control offers promising performance features for inverter-based resources (IBRs) across scales. However, design, analysis, and benchmarking of GFM IBRs during unbalanced faults remains largely unexplored. In this article, we outline a stationary-reference-frame nested-loop control architecture for GFM IBRs and integrate the same with novel current-limiting strategies. The architecture improves on virtual-impedance and current-reference-saturation limiting as well as state-of-the-art methods for control of voltage-source inverters. Electromagnetic-transient simulations for a modified IEEE 14-bus network validate salient features of the proposed control architectures. The proposed virtual-impedance limiter is shown to provide better voltage support during faults than the current-reference-saturation limiter (quantified via sequence voltages). On the other hand, the current-reference-saturation limiter offers better (and more accurate) fault-current contribution.

Two novel current-based methods for locating earth faults in unearthed ring operating MV networks

Automated fault location plays a key role in improving the reliability of modern power grids—it helps to reduce the power outage time and the number of affected customers. Earth faults in isolated systems are among the fault types that are difficult to detect and locate with a reasonable accuracy, and there has therefore not been a well-established method for detecting and locating such faults in isolated systems. This paper presents two novel current-based methods for locating earth faults in unearthed closed-ring MV networks. The methods are based on the utilisation of sequence components of standard current and voltage measurement at a primary substation and they do not require new devices or measurements in the field. Both methods are tested with a simulation of a network based on data provided by a Norwegian network operator and their prediction performances were investigated considering different scenarios on fault resistance, load asymmetry and measurement errors. Both methods performed well and showed good accuracy. The simulation results are promising and strengthens the prospect of further testing of the methods in real networks and adoption of the methods for a real-world implementation. • Novel earth fault location method for ungrounded ring operating MV networks. • Earth fault location method utilising the ratio of change in negative sequence current. • Earth fault location by utilising the ratio of change in zero sequence current & voltage. • Equations for sequence currents & division of zero sequence current in ring network. • Robust fault location against fault resistance, load asymmetry and measurement errors.

A Coordination Control With Extra Active Power Exchange Way to Extend Negative Sequence Current Compensation Range for STATCOM Based on Hybrid Cascaded Converter

The star-connected Cascaded H-Bridge (CHB) converter is one of the most attractive multilevel topologies for medium/high-voltage STATCOM. However, due to the fact that CHB has separated dc-links, three-phase unbalanced active powers cannot exchange automatically, which, thus, requires extra zero sequence voltage (ZSV) to eliminate them. Especially when negative sequence current is compensated, the negative sequence current compensation range (NCCR) is seriously limited. To extend the NCCR, this article combines the CHB and the converter with a common dc-link, which is utilized to exchange and eliminate the unbalanced active power generated from CHB. Based on this new active power exchange way and conventional ZSV injection, a novel cluster voltage balancing control method is proposed to eliminate the unbalanced cluster active powers. Moreover, based on the proposed control, the NCCR is identified, which is larger than that based on the conventional control. Finally, the proposed control and the derived NCCR are verified by simulation and experimental results on a 400 V/7.5 kvar Hybrid Cascaded Converter STATCOM.

Effect of Voltage Unbalances on the Performance of a Three-phase Transformer

Mitigation and adaptation to climate change have been major driving forces in modernizing electric power infrastructure. Low power quality has a significant impact on electrical equipment efficiency. Voltage unbalances one of the issues with low power quality. This study used a simulation method for a power supply with a voltage unbalance to evaluate the effect of voltage unbalance on the performance of 3-phase transformers. The research results show that when voltage unbalance occurs, the working point with the maximum efficiency shifts and transformer efficiency is reduced in all load cases. When there is an increase in VUF, the efficiency of the transformer is reduced. With a small VUF, the drop is insignificant, but when the VUF is greater than 3%, the drop starts to increase. In the case of voltage unbalance, when the positive sequence voltage is greater than the phase voltage, both the no-load loss and the load loss increase significantly. Both these losses are reduced when the positive sequence voltage is less than the phase voltage. Due to phase shift, both these losses are almost unchanged. The research results are the starting point for general studies on voltage unbalance and power quality.

Effects of Negative Sequence Voltage Subharmonics on Cage Induction Motors

In some power systems, voltage waveforms contain, apart from harmonics, interharmonics and subharmonics that are components of frequency less than or not an integer multiple of the fundamental frequency. Voltage subharmonics and interharmonics may be of both a positive and negative sequence, independently of their frequency. Previous papers on induction motors under subharmonics have been generally limited to the components of a positive sequence. This study deals with the effect of negative sequence subharmonics on the work of induction motors. Investigations were performed using the 2D finite element method and an experimental method. Differences between the impact of positive and negative sequence subharmonics are discussed. It was found that negative sequence voltage subharmonics can result in significant current subharmonics, torque pulsations and vibration. Further, because of possible resonance, motors that are comparatively resistant to positive sequence subharmonics might be especially sensitive to negative sequence subharmonics of the same frequency and vice versa.

A New Unbalanced Voltage Compensation Method Based on HOPF Oscillator for Three-Phase DC/AC Inverters With Unbalanced Loads

In islanded microgrids, distributed generators (DGs) are employed as distributed compensators to improve the power quality on the load side. Due to the access of unbalanced loads, the low-voltage microgrid will naturally exhibit three-phase unbalance, which may cause additional power losses and deteriorate power quality. Therefore, this paper proposes a novel unbalanced-voltage compensation control method based on the Hopf oscillator, which introduces the negative-sequence voltage and current into the traditional Hopf oscillator to achieve the negative sequence voltage droop characteristics. It can not only realize the negative sequence droop control of a single inverter, but it is also capable to support the reactive power automatic distribution between multi-parallel grid-forming inverters. Compared with the traditional unbalanced voltage compensation method based on filtered negative sequence reactive power, the proposed unbalanced voltage compensation method can slightly improve the dynamic response. Experimental results verify the effectiveness of the proposed control strategy.

Overcurrent Suppression Method for Multiple Wind Farms Connected to MMC-HVDC

To achieve asymmetric fault ride-through (FRT), it is crucial to limit the overcurrent for both the grid-forming modular multilevel converter (MMC) and the grid side converters (GSCs) of wind turbines. Compared with the MMC, GSCs enable better system stability when providing negative sequence paths, while the overcurrent can occur due to the negative sequence current. To address this issue, a negative sequence overcurrent suppression method is proposed, which introduces a negative sequence voltage proportional current control in each GSC. The redistribution of negative sequence currents is achieved in the manner that each GSC (filter included) is equivalent to a resistor with resistance determined by the proportional coefficient. Through this control, the negative sequence current can be evenly distributed so that the overcurrent can be effectively suppressed. Finally, simulations are performed in MATLAB/Simulink to verify the validity of the proposed negative sequence overcurrent suppression method.

A Mathematical Approach of Voltage Sag Analysis Incorporating Bivariate Probability Distribution in a Meshed System

In this paper, different mathematical expressions are derived to compute the residual magnitude of voltage caused by faults along the line and on the bus. Symmetrical and unsymmetrical faults are taken into consideration, and the consequences of the various fault distributions are considered. A new way of assessing a sag is proposed that incorporates the method of fault position and mathematical expression based on sequence currents and voltages. The fault impedance is introduced to obtain a better result. A fast and efficient load flow analysis technique produces quick computational results. In addition, the sag analysis is performed using the bivariate joint discrete probability distribution method that gives a clear idea about the probability of occurrence of sag in a meshed network. The suggested approach is applied in the IEEE 39-bus system and with an existing real-time electrical power distribution system in India.

Fault Line Selection with Correlation Analysis of Zero Sequence Current and Voltage under High-Low Frequency

When a single-phase ground fault occurs near the zero or maximum point, the fault line selection of resonance grounding system is not sensitive enough. After analyzing the transient current and line impedance characteristics of zero sequence network, and making full use of the decaying DC component, power frequency component and high frequency component of the transient zero sequence current of the feeder, a fault line selection method based on correlation analysis of zero sequence current and bus zero sequence voltage derivative in high and low frequency band is proposed. The effectiveness of the method is verified by the examples from the aspects of fault closing angle, transition resistance.

Grounding fault location of large turbo generator based on stator winding distribution

Stator grounding is the most common fault of a large generator. If the accurate coil of the fault can be located quickly and effectively, the maintenance progress of the generator can be greatly sped up, and the fault generator can be put back into use as soon as possible. For this purpose, a stator grounding fault location method for a large steam generator is proposed, which takes the distribution of winding potential into account. Firstly, the fault characteristics and the influence of winding potential distribution are analyzed when a single-phase ground fault occurs in a large generator, and the amplitude equation and phase equation of zero sequence voltage are constructed. On this basis, the potential distribution of stator winding can be analytically expressed, so the fault location can be directly calculated by the numerical solution method. Simulation analysis verifies the effectiveness of the proposed method.

Research on STATCOM control strategy of Cascaded H-bridge considering unbalanced conditions

STATCOM, a star cascade H bridge, usually uses zero-sequence voltage injection to achieve inter-phase DC voltage balance. Different from negative sequence current injection, the phase-to-phase DC voltage balance control based on zero sequence voltage injection does not generate zero sequence current in three-phase and three-wire systems, which is beneficial to the power quality of the system. In the traditional control algorithm, the zero sequence voltage is calculated by the unbalanced active power and current. The control algorithm contains a large number of complex operations and matrix operations, which need to consume a lot of controller resources and is not conducive to system design and expansion. Moreover, in order to improve the dynamic performance under voltage imbalance, the positive and negative sequence separation is introduced into the feedforward control to increase the complexity of the control algorithm. In order to simplify the control algorithm, a PI regulator is proposed to linearly adjust the DC component of zero sequence voltage in DQ coordinate system. At the same time, through theoretical analysis, it is found that the zero-sequence voltage in feedforward control is equal to the zero-sequence voltage in power grid, so the positive and negative sequence separation can be removed, further simplifying the algorithm.

Integration of PV Panels and EV Chargers on the Modular Multilevel Converter Based SST

This article develops a solid-state transformer (SST), which integrates PV panels and EV chargers together through the open-dc modular multilevel converter (MMC). Different from the well-known MMC, which has a common dc link, the discussed MMC is open-circuited at the dc link so that it should be analyzed from the three-phase viewpoint. The developed SST performs like two parallel-connected cascaded converters, and they are operated with opposite power flow directions. Compared with the conventional SST solution using two separate cascaded H-bridge (CHB) converters, the developed SST has relatively low component count so that it is expected to be of superiority in cost, footprint, and even power density. In addition, the usage of zero sequence voltage in the CHB converter can be replaced by circulating currents in the open-dc MMC so as to greatly reduce the risk of overmodulation. After the thorough power flow analysis on the open-dc MMC and the corresponding controller design, the developed SST is capable of freely working at any operating points with PV panels and EV chargers. Moreover, the developed SST takes the unbalanced grid conditions into account, which is another key contribution of this article. Finally, the balanced and unbalanced operations of the developed SST are both confirmed by the laboratory test results, and furthermore, the dynamic performance of the developed SST considering the time-variant characteristics of PV panels and EV chargers is also demonstrated through the PLECS simulation platform.

A New Method of Fault Direction Identification for Different Types of Renewable Energy Source Integrations

The characteristics of the short circuit current of renewable energy sources are quite different from those of traditional generators. When applied to tie lines and collector lines of renewable energy sources, the traditional directional elements may not operate correctly. A new method of fault direction identification suitable for inverter power sources and doubly fed power sources based on the fault features of renewable energy sources is proposed. The fault type is judged by sequence voltage. The fault direction is identified by comparing the amplitude between the sequence currents and the phase angle between the sequence voltage and the sequence current, without distinguishing whether the crowbar of the DFIG is triggered. The results of digital simulations and experiments show that this method is not affected by the types of renewable energy sources connected to the power grid and can correctly judge the fault direction in different fault types, which has good engineering application value.

A Novel Islanding Detection Technique Based on Positive-Feedback Negative Sequence Current Injection

This article proposes a new active anti-islanding technique to detect unintentional islanding with high speed and reliability. In grid-connected mode, the controller has no influence on the power system. When the grid is disconnected, the inverter injects negative sequence current in a positive feedback loop, and causes the output negative sequence voltage to increase to a threshold value that causes the inverter to stop operating. The proposed method is nonintrusive and does not cause disturbance to the system in grid-connected mode. The method is tested using PSCAD simulation software, and using controller hardware-in-the-loop (C-HIL) using the physical control board of the inverter. The inverter considered in this article is a photovoltaic inverter with 840-kW rated power. Optimization of the parameters of the proposed control, as well as small signal stability analysis in grid-connected mode are presented. Verification results show that the controller can detect unintentional islands and stop the inverter within 500 ms.

A comprehensive review of fault diagnosis and fault-tolerant control techniques for modular multi-level converters.

Many faults occur in the modular multi-level converters (MMCs), including unbalancing capacitor voltage, lower and upper arm unbalancing, the line to line voltage unbalancing, sensors and actuators fault, system fault, and sub-modules fault in high as well as medium voltage applications. Several fault-tolerant approaches are presented to overcome these problems, such as active fault-tolerant control system (AFTCS), passive fault-tolerant control system (PFTCS), hybrid fault-tolerant control system (HFTCS), redundant system technique, special power circuit with the controller, and zero sequence voltage methods, which we will explain extensively in this article. This review emphasizes the types of faults in the MMCs and discusses the protection methods under failure conditions. The MMC is more popular in high voltage applications because it not only improves the quality of the grid but also has good harmonic performance in high power transmission. There is no need for any isolated dc sources to operate it. When faults are removed, the efficiency and reliability of the system will be increased. This extensive explanation of the current literature on MMC fault diagnosis and control techniques will conclude which methods provide a more valuable solution. Finally, this paper discusses the best approach to reduce MMC faults and provides a future research direction to the readers.

Hybrid voltage and current control strategy for virtual synchronous generator under unbalanced voltage conditions

AbstractThe traditional VSG strategy does not consider the negative sequence component, which will lead to current imbalance and power oscillation under unbalanced voltage. Several improved control strategies of VSG are usable for operating under unbalanced voltage nowadays. These control strategies usually require a complex PLL (phase‐locked loop) to detect sequence components, and a current reference generator to convert voltage references to current references. This paper proposes an improved control strategy for VSG. The main contributions are in threefold: (1) A hybrid voltage and current control strategy by simultaneously controlling the positive sequence output voltage and negative sequence output current is proposed, which can achieve different operation demands under unbalanced voltage conditions without changing the original characteristics of VSG. (2) A positive and negative sequence decoupling method is designed to acquire the synchronization phase information directly from the positive sequence voltage control branch, thereby avoiding the use of complex PLL (phase locked loop). (3) A general formulation for the negative sequence current references is presented to control the power oscillations or the current balance flexibly. The correctness and effectiveness of the control strategy are verified by simulation and experimental results.