Current Sequencing Research Articles

Faulty Feeder Identification Method Considering Inverter Transformer Effects in Converter Dominated Distribution Network

With the increase in inverter interfaced distribution generations (IIDGs), converter-dominated distribution networks (CDDNs) are expected. When a single-phase-to-earth (SPE) fault occurs in a CDDN, since the inverter transformer effects can affect the transient fault characteristics of fault signals, the existing faulty feeder detection techniques may not be applicable. This paper focuses on the inverter transformer effects on transient zero sequence currents (TZSCs). First, the inverter transformer effects on TZSCs are analyzed, and the analytical expressions of TZSCs are derived. It is found that the transient fault characteristics in TZSCs may become weak by considering the inverter transformer effects. As a result, the accuracy and reliability of faulty feeder detection face more challenges. To address this issue, a novel faulty feeder identification method based on the first order accumulated generating operator (AGO) method and improved cosine similarity is proposed. The effectiveness and applicability of the proposed method are validated in the converter-dominated distribution network. Additionally, the field test results also indicate the feasibility of the proposed method.

Multiple 3-Phase PMA-SynRM With Delta Windings for Enhanced Fault Tolerance

In this article, a novel multiple three-phase PM assisted synchronous reluctance machine with delta windings is proposed for enhanced fault tolerance. The delta connected windings offer an additional degree of freedom of the phase currents. Hence, this machine drive could provide higher torque under phase open-circuit condition. More importantly, the delta windings inherently contain zero sequence current path and could help reducing the turn fault current in the event of the most challenging turn short-circuit fault. Owing to the zero sequence current, the residual flux linkage of the fault turn is more effectively nullified leading to lower fault current. The proposed machine performance together with fault tolerant capability under various fault scenarios are investigated and compared with the star connected winding machine by detailed finite element simulation and experimental tests. It shows that the two machines have the same performance under healthy condition whilst the delta winding machine exhibits higher fault tolerance. These advantages are achieved simply by changing the winding connections without any extra cost. Thus, it is more favorable for the fault tolerant applications.

High Impedance Fault Detection Using Multi-Domain Feature with Artificial Neural Network

High Impedance Fault (HIF) poses a threat to human life even though the fault current is low in magnitude. Moreover, the proliferation of power electronic devices give rise to a range of harmonics in the system. Hence it is difficult to identify the HIF by employing classical approach. In this paper an algorithm is developed that uses signatures in both time and frequency domain to identify the HIF. Since the parameters are inconsistent at every situation, it is imperative to use a classifying technique such as artificial neural network to distinguish the fault and normal situation. The algorithm is tested in a IEEE 33 bus system with the presence of nonlinear loads. The harmonic loads are simulated as current source and the harmonic content of the loads are taken with the help of Harmonic analyzer. The relevant features are judiciously selected to improve the accuracy of the algorithm to detect the HIF. The multi-domain features selected are the rate of change of phase current, change in the angle of the sequence currents and relative energy of Digital Wavelet Transform (DWT) coefficients. The proposed algorithm is used to distinguish the condition such as load switching, bolted ground fault, non-linear current harmonics from HIF.

METHOD OF FUZZY PROTECTION AGAINST ASYMMETRIC MODES OF ELECTRIC NETWORKS OF ENERGY SYSTEMS WITH RENEWABLE SOURCES

The paper considers the creation of an intelligent protection system based on fuzzy controllers, which provides detection and effective protection against asymmetric modes, in particular, from open-phase modes in the electrical network of a power system with a complex load. An intelligent system that includes a positive and negative sequence current filter, an analog-to-digital converter, a switch, a calculation unit for the ratio of negative and positive sequence currents, an executive body that turns off the input switch and turns on the sectional switch.

A Hybrid Compensator for Unbalanced AC Distribution System With Renewable Power

A hybrid compensator with a new coordinated control is proposed for mitigating power quality aspects and integration of renewable power in an unbalanced three-phase four-wire distribution system. The proposed compensator comprises of thyristor switched capacitor (TSC) banks, delta connected thyristor controlled reactor (TCR), a 3rd and a 5th harmonic tuned passive LC filter & a low rating three-phase four-wire D-STATCOM. The TSC-TCR operates asymmetrically for the base var compensation and also eliminates negative sequence current during unbalanced loading at the point of common coupling (PCC). The D-STATCOM works in coordination with the TSC-TCR and the passive filters to compensate fundamental var, zero sequence current, voltage flicker & 7th harmonic currents at the PCC. The D-STATCOM thus serves as a supportive var compensator as well as an active filter in selective harmonic compensation mode, thereby improving the var compensation and harmonic current filtering at the PCC. A new coordination control system is proposed to control the operation of the D-STATCOM which is also employed to integrate a PV source & a doubly fed induction generator (DFIG) based wind energy conversion system (WECS) to the PCC. This scheme with grid connected and islanded mode of operation using D-STATCOM exhibits low-voltage ride-through capabilities. Further, the chance of resonance of the passive filters used with source inductance is eliminated in the proposed scheme. Simulation results show that the proposed hybrid system effectively compensates var, negative & zero sequence current, reduces voltage flicker and harmonic currents at PCC.

ДВУХСТОРОННИЙ МЕТОД ОПРЕДЕЛЕНИЯ МЕСТА ОДНОФАЗНОГО ЗАМЫКАНИЯ НА ЗЕМЛЮ В СЕТЯХ С ИЗОЛИРОВАННОЙ НЕЙТРАЛЬЮ

Relevance Currently, on the part of enterprises operating 6–35 kV distribution networks, there is a request for a deep modernization of networks and the transformation of existing, outdated networks into digital ones. A separate aspect of modernization is the introduction of high-precision measuring devices and new principles of data transmission. At the same time, the probability of damage for various reasons remains. Single-phase earth faults in networks with isolated neutral are still the most common problem in distribution networks that do not have universal solutions. The problem is particularly acute with determining the location of single-phase earth faults. The opportunities provided by the introduction of digital networks can become the basis for the implementation of new algorithms for relay protection and determining the location of damage. Aim of research The article considers, developed by the authors, a two-way method for determining the location of damage in single-phase earth faults according to the parameters of the emergency mode when installing current transformers in all three phases. The two-way method of determining the location of damage is based on the use of zero-sequence currents and voltages, which are determined by calculation for phase current and voltage differences at the beginning and end of the line. The algorithm is focused on digital electrical networks equipped with high-precision measuring transformers and a communication channel. To calculate the parameters of the zero sequence, the differences of the phase emergency parameters at the beginning and at the end of the line are used. Research methods To develop and study a two-way method for determining the location of damage in single-phase earth faults, the principle of superposition in combination with the method of symmetrical components was used. Decomposition of three-phase currents and voltages into symmetrical components is a linear procedure. Therefore, the zero-sequence current in a three-phase line, in accordance with the superposition principle, can be represented as the sum of three zero-sequence currents, each of which is determined by one of the phase currents at zero values of phase currents in the other two phases. This application of the method of symmetric components is valid only for purely emergency values of phase currents. Therefore, in advance, the load current must be excluded from the phase currents of the emergency mode. Results It is shown that the use of the method of symmetric components for single-phase earth faults separately for the damaged phase and undamaged phases makes it possible to determine the zero-sequence current at the site of damage and the intrinsic zero sequence current of the damaged line. It is shown that this separation of the zero-sequence current makes it possible to distinguish two components of the zero sequence in the voltage difference at the ends of the line. Based on the allocation of two components in the current and voltage of the zero sequence, an algorithm for two-way determination of the damage location has been developed, which allows determining the distance to a single-phase earth fault in networks with an isolated neutral. To study the two-way method of determining the location of damage, a network model and a device for determining the location of damage were implemented in the MATLAB Simulink software package. A functional diagram of a two - way damage location detection device is presented. The performed studies on the model of the electrical network have shown the high accuracy of the proposed algorithm for determining the location of damage.

Power Sharing in Three-Level NPC Inverter Based Three-Phase Four-Wire Islanding Microgrids With Unbalanced Loads

The droop-based control schemes are widely used for power-sharing and control of the isolated microgrids. The conventional droop control scheme with inner current and voltage controllers can work effectively under ideal voltage. However, the existence of unbalance tends its performance to worsen. This paper aims to propose an enhanced droop control method to improve the power-sharing, and maintain the distributed generators’ (DGs) terminal voltages and the system frequency under the balanced as well as unbalanced loads. The enhancements of the proposed control scheme include the droop controller, inner current and voltage controller, and virtual impedance loop. The proposed virtual impedance deals with the accurate sharing of powers in the case of unequal line impedance and the control of zero sequence current. Moreover, to enable the proposed control to compensate for the unbalance, three-level neutral point clamped (NPC) inverters are used to form a three-phase four-wire microgrid. With this control scheme, the voltage unbalance factors (VUFs) of the DGs’ terminal voltages are reduced from 4% to less than 1%, negative sequence reactive powers, which are also an indication of unbalance, have been reduced significantly, and the system frequency is maintained within the standard permissible limit of 2.5%. Furthermore, a local load controller is also proposed to control the reactive loading of the DGs when they have local loads supplied from their terminals. The investigations have been carried out under simulation as well as lab-based environments to validate the efficacy of the proposed control method.

Fault transient feature extraction method of 66kV transmission line based on improved wavelet threshold and CEEMDAN-FME

In the 66kV transmission line, the compensation effect of the arc suppression coil, so the amplitude of fault zero sequence current is weak after the occurrence of single-phase grounding responsibility. The fault characteristics are not obvious, and there is environmental noise interference during fault signal acquisition, which further increases the difficulty of ground fault feature extraction. To solve the above problems, a fault transient feature extraction method of 66kV transmission line is proposed, based on improved wavelet threshold and Complete Ensemble Empirical Mode Decomposition with adaptive noise (CEEMDAN) - Fuzzy measure entropy (FME). The characteristics of single-phase grounding transient fault in small current grounding system are analyzed. The transient fault signal was preprocessed by an improved wavelet threshold method, and the transient fault characteristics were extracted accurately by CEEMDAN-FME processing. The proposed method has good anti-noise performance through the simulation comparison and verification of denoising processing and transient feature extraction by MATLAB. It can effectively extract fault features when the interference degree is significant.

Assessment of the Performance of Active Energy Meters Under Unbalanced Conditions

Over recent decades, electrical power concepts under nonsinusoidal and unbalanced conditions have been continually researched by several academics around the world. The results from these studies present a direct relationship between the performance of electric energy meters and the physical reality of the power balance between loads and sources. As such, this study presents a wide-ranging subject analysis that contemplates analytical, computer, and laboratory developments. This, in turn, opens the possibility of the physical representation of active power under unbalanced conditions, directly impacting the amount of active energy measured by the polyphase meters for billing purposes (from different manufacturers and models) and the composition of technical losses on distribution systems. The results showed that when using the fundamental active power of positive sequence as a reference, as suggested by IEEE Std. 1459/2010, none of the meters available worldwide correctly accounts for active energy. In this context, the study also presents the results from a power measurement campaign performed with 162 different low-voltage three-phase residential consumers located on five different electric energy utilities to understand the actual levels of current unbalance on this specific type of load. In this context, the main contributions of this work are related to (i) the quantification of measurement differences between active energy meters, highlighting the lack of equity in the measurement process of active energy under unbalanced conditions and (ii) the demonstration that the components of zero and negative sequence currents, produced by unbalanced loads, increase the technical losses of distribution systems. Furthermore, the results indicate the urgency of reviewing active energy measurement protocols for billing electricity consumers.

Analysis and treatment of current unbalance abnormal situation of 750 kV double-circuit parallel transmission line

At present, the biggest problem faced by short-distance double-circuit parallel lines is the three-phase parameter asymmetry problem caused by non-transposition. This leads to three-phase current asymmetry, especially during heavy load performance is more prominent. Take a 750 kV double circuit parallel erection line in actual operation as an example, use EMTPE electromagnetic transient simulation software to simulate and analyze the current imbalance phenomenon. This paper studies the influence of phase sequence arrangement, distance between loops, distance between wires, line length, equivalent impedance, ground wire conditions and line power flow on current imbalance, and puts forward reasonable restraining measures. The simulation results show that when the line power flow is heavy, the double circuit lines are arranged in the same phase sequence and the distance between circuits is relatively short, there will be a large amplitude of zero sequence current on the line, mainly circulation current. If it is not suitable to adjust the protection measures, it can be considered to adjust the double-circuit lines to reverse phase sequence operation mode. The research results can provide reference for the engineering design of double-circuit parallel transmission lines.

STUDY OF RESONANCE PHENOMENA WHEN CONNECTING 0.38 KV BATTERIES OF TRANSVERSE COMPENSATION CAPACITORS TO THE NETWORK

The article deals with the operation of capacitor banks as part of regulated capacitor units connected to the 0.38 kV electrical network of power transformers of 10/0.4 kV consumer substations. It is known that capacitor banks are used to compensate for the reactive power of electrical receivers and equalize the load of phases in three-phase four-wire networks of 0.30 kV by creating reverse sequence currents equal in magnitude, but rotated by 1800 relative to the load currents due to the inclusion of a different number of capacitors between the network phases. The wide presence of non-linear electrical receivers in electric networks causes the occurrence of higher harmonic components of voltage and current, which cause an increase in the current in capacitors, since the reactive resistance of capacitors decreases with increasing frequency. At the same time as the current in the capacitors increases, which can be regulated with the help of constructive measures, in unfavorable cases resonance phenomena may occur in the networks. Widespread use of capacitor banks creates risks of resonance of voltages and currents both at the main frequency of three-phase alternating current of 50 Hz and at frequencies of 150, 250, 350 and 550 Hz due to higher harmonic components of voltage and current. The obtained results indicate that for the resonance of currents to occur at the main frequency for power transformers of different capacities, the power of the capacitor banks should be from 0.02 kV×Ar per 1 kV×A of power (for a 630 kV×A transformer) to 0.034 kV×Ar (for a 25 kV×A transformer), and for the occurrence of voltage resonance, respectively, from 18.7 kV×Ar/kV×A to 26 kV×Ar/kV×A. In addition, calculations show that to increase the power factor even by a small amount (from 0.9 to 0.95), it is necessary to install capacitor banks of at least 0.14 kV×Ar/kV×A, and to increase the power factor from 0, 7 to 0.95 - only 0.66 kV×Ar/kV×A.

Fault Ranging Method for Overhead-Cable Hybrid Distribution Lines Based on CEEDMAN Energy Relative Entropy

According to the parameter characteristics of cable-overhead line and the folding and reflection process of traveling waves when the wave impedance is discontinuous, a traveling wave abrupt change extraction method based on the empirical mode decomposition of fully adaptive noise ensemble (CEEDMAN) energy relative entropy is proposed for the first time. Through the combination of CEEDMAN and energy relative entropy, the transient traveling wave signals are generated when reflection and refraction occur, and the energy value of traveling waves will change abruptly, and the reflected and refracted wave heads will be captured by the change of relative energy. Combined with the magnitude difference of zero sequence current, the fault area can be determined. Traveling waves have different wave speeds in overhead lines and cable lines, and fault ranging is performed by determining the corresponding propagation speed. Finally, simulation verifies that the method can accurately determine the fault zone with high ranging accuracy under the conditions of different fault locations, different transition resistances and different initial phase angles.

Control of ILC in an Autonomous AC–DC Hybrid Microgrid With Unbalanced Nonlinear AC Loads

This article presents a multiobjective control scheme for a bidirectional interlinking converter (ILC) of renewable energy based ac–dc hybrid microgrid. The prime focus of the power management algorithm is to feed the ac loads, and dc loads from ac source, and photovoltaic battery energy storage (BES), respectively. The second objective is to utilize the solar power to maximum capacity, by feeding it to dc load, BES, and ac load, in the given order of priority. The ac and dc subgrids operate independently, while supporting each other in case of overloading/underloading, thus reducing the amount of power reserve required in each, and eliminating the need of dump loads. The ANFIS-MN-based power quality control is integrated in order to ensure the total harmonics distortion of ac source current remains below 5% as per IEEE 519 standard, irrespective of the nature of load connected. In addition, the ILC control prevents any negative sequence currents from entering the ac source, even if there is any open circuit fault leading to unbalanced loading on the three phases. The performance of ANFIS-MN-based ILC control is demonstrated to be superior when compared with the conventional and contemporary techniques under different scenarios.

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.

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.

PLL Synchronization Stability of Grid-Connected VSCs Under Asymmetric AC Faults

This paper investigates the phase-locked loop (PLL) synchronization stability of grid-connected voltage source converters (VSCs) under asymmetric ac faults. Negative sequence current control has been widely adopted to enhance the stability under asymmetric ac faults and the PLL plays a vital role in the maintenance of synchronization in VSCs. However, the positive and negative sequence current both have an important effect on the synchronization stability of PLL under asymmetric ac faults. Furthermore, the influence of negative sequence current and circuit parameters on PLL synchronization stability is different under different asymmetric ac fault types, which further increases the complexity of analysis. Hence, the unified synchronization stability coefficient (SSC) is proposed to evaluate synchronization stability of the VSC in this work. The parametric impacts of the PLL, the influence of positive/negative sequence current, and the circuit parameters on PLL synchronization stability are revealed analytically during asymmetric ac faults. Time-domain simulations are performed in Matlab/Simulink, and the obtained results validate the theoretical findings.

Single-phase Grounding Fault Location Technology Based on Edge Calculation of Fault Components of the Positive-sequence Current

In the one-phase-to-ground fault of the distribution network, due to the difference in the distribution of the positive sequence current constituents of each line, the information contained in the fault path and the non-fault path also has a certain difference. On this basis, the edge of the fault constituent in the positive sequence current is used to locate the one-phase grounding. The method first uses phase current subtraction to multiply the period difference before and after the fault, and then uses the inverse transformation before and after the fault to determine the positive and null sequence constituents of the fault. Symmetrical composition. By analysing the amplitudes of the positive sequence and null sequence currents of the three-phase fault constituents, and according to the corresponding relationship between the amplitudes and the adaptive threshold, the location of the fault section is realized. A specific fault point location criterion and steps are given in this paper, and it is verified by ATP simulation software.

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.

A proposed method for reduction of induced zero-sequence current in cable system

<span>Zero-sequence circulating currents are particularly problematic because they can occasionally lead to protective relay failure. However, it is not usually understood that systems with numerous cables strung along the same path will produce zero-sequence currents. In this paper, the operation of parallel connections of single core underground cables is examined. The examination is based on power cables arrangement operating in parallel two-circuit with and without the transmission grid. It was found that symmetrical cable arrangement might significantly lower zero-sequence currents in a twin-circuit. On the other hand, the largest value of the zero sequence currents at asymmetrical configuration. To provide an effective method about reduction of zero sequence current induced in a cable system, several connection scenarios such as the transposition of the cable and sheath and the application of sheath cross bonding are examined. Therefore, in this paper, this phenomenon will be analyzed and discussed. ATPDraw is used to simulate and analysis this kind of study. Also, the effect of sheath transposition is explained and analyzed.</span>

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.