Unbalance Factor Research Articles

Performance analysis of future PEA distribution networks under high penetration of PEV home charging using the Monte Carlo method

This article presents the performance analysis of future Provincial Electricity Authority of Thailand (PEA) distribution networks under high penetration of plug‐in electric vehicle (PEV) home charging using the Monte Carlo method. Network performance indices considered in this study are the voltage profile, power losses, load factor, and voltage unbalance. The voltage profile and power losses are evaluated based on the PEA criteria. In addition, the voltage unbalance factors (%VUF) are evaluated according to the IEC 61000‐2‐4: 2000‐06 standards. A selected PEA distribution network based on the existing data is used for the simulation study with the DIgSILENT PowerFactory. The penetration of PEV home charging at each node and phase is assigned using the Monte Carlo method, which in particular is used to randomly generate patterns of PEV charging behaviors. The simulation study presents five cases; the average points of charge, max‐to‐min charge, min‐to‐max charge, central feeder charge, and random point of charge. All cases are compared with a base case (without PEVs). Charging scenarios at phases ‘a’, ‘b’, and ‘c’ are assumed having overlapped time duration. The PEV charging duration per phase is assumed 6 h. Simulation results indicate that PEV charging time, charging point, and penetration levels at each phase and node are significant factors impacting the network performance. Voltage profiles slightly drop in all cases, meanwhile the average voltage drop increases by 7% compared with the base case; case 3 has the maximum voltage drop, which is deeper than PEA criteria. The power losses increase in all cases. The average power losses have increased by 103.52%. The %VUF increases gradually with the length of the feeder. The maximum %VUF occurs at the end of the feeder for all cases and is higher than 2.0% of the IEC standard. On the other hand, PEV charging improves the load factor. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

LIM Control Strategy Supported by Genetic Algorithm with Unbalanced AC Source

AbstractThis work presents the improvement of an open loop control strategy for linear induction motors operating at low speeds. The improvement is provided through the application of genetic algorithms in determining unbalance factors of the supply voltages of the linear motor. For this, a computational model of the linear motor was used as the evaluation function. The computational model was developed based on the equations of the linearized induction motor. The proposed methodology is validated through the comparison between computational results and experimental data performed in a linear motor prototype. This methodology allows to evaluate the influence of the unbalance of the supply voltages for linear motors working at low speeds.

Design of a direct control strategy for a static shunt compensator to improve power quality in polluted and unbalanced grids

Many control techniques for three-phase active filters with the aim of improving the power factor and reducing the current harmonic distortion at the source level exist today and are widely reported in the scientific literature. Almost all of these methods are designed for a perfect sine-wave source and do not perform sinusoidal current when voltages are distorted or unbalanced. The work developed in this paper concerns the active filtering on a perturbed and unbalanced network. First, the proposed control method is described and validated by simulations and experimental results, and then the proposed method is compared to the classical control laws such as the original p-q theory, the modified p-q theory and the p−q−r method. Experimental results are conducted on a three-wire low power distribution network under perturbed voltage and non-linear load using a DSPACE control system. The assessment is performed on the source currents after filtering, and a comparative evaluation in terms of unbalance factor and current Total Harmonic Distortion (THDi) is carried out for all methods.

Analysis of the power quality impact in power supply system of Urban railway passenger transportation – the city of Ulaanbaatar

The urgency and expediency of additional passenger transportation by urban railway as an extension of the transport infrastructure of megacities has been proved. The fact that the urban railway uses AC 2x25 kV, which is uses to out of the three phases of city’s power supply system negatively impacts. Thus, it creates a significant asymmetry. In this paper, a comparative analysis on the negative sequence component of the voltage unbalance, which occurs in the nodes of connection of traction substations through both a three-phase transformer or a Scott transformer are been carried out. A mathematical method was created for determining the negative sequence voltage unbalance factor is based on the phase coordinates and symmetrical components methods. The results show that the Scott transformer provides a more symmetrical load distribution to the three-phase electric grid, when compared to the three-phase transformer and was verified using a simulated model in the Matlab program. Consequently, the use of a Scott transformer has better symmetrising abilities than a three-phase transformer, which gives it a significant advantage in the traction power systems.

Power Quality Analysis for Ship-Photovoltaic Power System: A Case Study

Due to the seriousness of oceanic environmental pollution and the strict restrictions imposed by marine pollution protocol, the photovoltaic (PV) system has been tried to be implemented in traditional ships that use fossil fuel for power generation. This paper investigates the impacts of a large PV system on the ship power system. The installation of PV system of various operation modes on ship power system will significantly affect the power quality of ship grid. In this paper, one practical ship-PV power system of the COSCO TENGFEI vehicle carrier is selected for study. Various power quality issues like the voltage unbalance factor, voltage deviation, frequency deviation, harmonic ratio as well as total harmonic distortion (THD) are investigated and calculated by the calculation methods suggested by the International Electrotechnical Commission (IEC) and ship classification societies. The results will be compared with the relative standards to demonstrate the feasibility of the PV system to be installed in the ship power system.

Unbalanced Voltages Impacts on the Energy Performance of Induction Motors

<p class="Abstract">This paper presents the results of a study about the effects of unbalanced voltages on the energy performance of three-phase induction motors. The principal contribution of this paper is that presents a detailed analysis of the influence of positive and negative sequence voltage components and the angle between them on several characteristics such as: line currents, losses, efficiency and power factor under different voltage unbalanced conditions. A three-phase induction motor of 3 HP was used as a case study. The results of the investigation show that the positive sequence voltage must be considered together with the voltage unbalance factor (VUF) or percent voltage unbalance (PVU) index to evaluate the performance of the induction motor. It is also shown that the behavior of the motor load influences on the positive sequence parameters next to the voltage, while in the case of negative sequence only influences the negative sequence voltage.</p>

Development and Analysis of a Sensitivity Matrix of a Three-Phase Voltage Unbalance Factor

Unbalanced phase voltages in electricity networks prevent efficient and stable delivery of power and energy services from utility companies to end-users, and vice versa. While variations in voltage magnitude and power loss have been studied extensively, little attention has been paid to the development of linearized expressions to account for the variation in voltage unbalance. Using a standard definition of the voltage unbalance factor (VUF), this study is the first, to the best knowledge of the authors, in which a VUF sensitivity matrix is mathematically developed and analyzed with respect to variations in three-phase (3- ph ) voltages and, consequently, 3- ph unbalanced load demand. To demonstrate its utility, we performed simulation case studies and compared the estimates made using the matrix to those from a power flow calculation algorithm. The VUF sensitivity matrix is expected to have a variety of practical applications for power systems, such as optimal scheduling and real-time control, which enables improvement in network power quality.

Determination of inductance for suppressing current unbalance of 3 phase inverter under input voltage unbalance condition using MATLAB Simulink

The installation of the alternating current reactor at the input side of an inverter is 1 of the effective methods available to protect the diode of the rectifier and to minimize the harmonic distortion of the input current in the inverter under the unbalanced input voltage conditions. In this paper, the unbalance behavior of the currents flowing through the diodes of the converters, the total current harmonic distortion (THDi), and power factor of the power supply systems have been simulated by means of the software MATLAB Simulink, and the simulation results have been confirmed to be available from the practical field test. Using the simulation results dealt with in this paper allows the designers of inverter to conveniently determine the reasonable values of the inductance for alternating current reactors according to the desired permissible current unbalance factor and values of THDi of the converter, and the power factor of the power supply system in the case that the capacity of an inverter is given.

Fractional-order modeling and dynamic analyses of a bending-torsional coupling generator rotor shaft system with multiple faults

ABSTRACT Unexpected vibrations induced by the crack fault and other unbalance factors in rotor system seriously affect the health and reliability of the generator. Here, to explore the vibration performances, a bending-torsional coupling model of the generator rotor shaft system is established, in which electromagnetic malfunction (unbalanced magnetic pull) and mechanical failures (fractional-order damping, crack and contact-rubbing) are considered. Then, the simulation is done by a modified Adams-Bashforth-Moulton algorithm. Based on the simulation, the correctness of the new coupling model is verified by comparing with previous model and experimental data. At the same time, the new coupling model is analyzed to obtain the dynamic evolutions of the generator rotor shaft system with the changes of crack depth ratio, the fractional order of damping, rotational speed ratio and mass eccentricity of rotor. In addition to this, some critical values and ranges are proposed. Finally, these results can efficiently provide a theoretical reference for the design of generator rotor system and be applied to forecasting and diagnosing vibration faults in generator rotor shaft system.

Evaluation of voltage imbalance on low‐voltage distribution networks considering delta‐connected distribution transformers with a symmetrical NGS

The main purpose of this study is to evaluate the effect of distribution transformers with a delta-connected neutral-grounded structure (NGS) on the voltage imbalance of low-voltage distribution networks. The proposed NGS for delta-connected distribution transformers has a symmetrical structure. That is, the stability and reliability of the operating characteristic of the proposed delta-connected NGS are better than other grounding methods such as corner-grounding, mid-tapped grounding, a zig–zag grounding transformer, and an asymmetrical NGS. In this study, the coupling-free equivalent circuit mathematical models for various methods of grounding a delta-connected structure are first developed. Furthermore, these coupling-free equivalent circuits can be simply implemented in the MATLAB/Simulink platform. Next, the IEEE test feeder system is used as a benchmark for verification. To determine the feasibility of the proposed NGS and evaluate its operational performance, a Monte Carlo simulation-based three-phase power-flow method was proposed to compare the operational characteristics of various grounding structures (i.e. system imbalance resulting from asymmetric load conditions or asymmetric transformer connections) such as phase-to-ground voltages, line currents, power consumption, and voltage unbalance factor. The results provide a valuable reference for distribution engineers and building technologies seeking a suitable method of grounding delta-connected power service systems.

Control the Three-level Rectifier Under Unbalanced Grid Conditions

This paper proposes a novel control method for Three-level rectifier under unbalanced grid conditions. An average model is established to analyze the influence of the unbalanced factor. It can be seen that the unbalanced factor will generate the negative-sequence current. The negative-sequence current can be suppressed by a proportional integral resonant (PIR) controller. Additionally, the unbalance of neutral-point (NP) voltage will generate in the three-level rectifier. Therefore, this paper proposes a space vector modulation for balancing the NP voltage. The simulation results confirm the performance and effectiveness.

Voltage Regulation Method for Voltage Drop Compensation and Unbalance Reduction in Bipolar Low-Voltage DC Distribution System

Owing to the development of highly efficient electric power converters, the number of studies on the construction of low-voltage dc (LVDC) systems is gradually increasing. To use LVDC distribution systems, voltage regulation methods to compensate for the voltage drop and to limit the voltage unbalance are essential. In a bipolar LVDC distribution system, a voltage drop and voltage unbalance could occur because of the load current and the variation in the amount of power supplied to the poles. However, not enough research has been conducted on voltage regulation methods for LVDC distribution systems. To reduce the voltage drop and the voltage unbalance, this paper proposes a voltage regulation method based on the neutral to line drop compensation (NLDC) method, which employs a modified LDC to calculate the sending-end reference voltage. In the NLDC method, the neutral current and neutral line impedance are taken into consideration to compensate for the neutral line potential fluctuation and voltage drop on the pole. The next sending-end voltage is determined by checking the voltage unbalance factor, and it is adjusted to maintain it within the allowable voltage unbalance factor. A voltage regulation algorithm and a bipolar LVDC system model are implemented using the Electromagnetic Transients Program.

A voltage unbalance factor coding technique for three-phase induction motors

A voltage unbalance factor coding technique for three-phase induction motors

Development of hardware and software for calculation and monitoring of the unbalance factor in three­phase voltage system

Hardware and software for measuring and monitoring the unbalance factor of the three-phase voltage system is developed. The shortcomings of the existing technical and software solutions for calculating the unbalance factor, as well as problems of applying the methods used for analog control systems to digital systems are described. The block diagram of the microprocessor system for measuring the voltage unbalance factor in the ship three-phase network with the effective values of phase voltages is used. The analysis of the analytical expressions to calculate the unbalance factor allowed minimizing the utilization of system hardware necessary for the formation of all necessary signals. The optimized structure of the digital filter to calculate the effective value of the deterministic periodic signal at the optimum speed of a discrete system is proposed. An increase in the sampling rate in the optimized filter structure does not lead to an increase in the number of arithmetic operations of addition and multiplication, which is an advantage of the proposed system in comparison with similar ones. This allows monitoring of instantaneous changes in the unbalance factor of three-phase voltage and enables the system to operate in real time. The developed hardware and software can be integrated into the automated control system of the power plant and used in voltage control systems, as well as for the timely detection of voltage unbalance to protect equipment and prevent violation of normal operation.

Unbalance Level Regulating Algorithm in Power Distribution Networks

The paper dwells on the unified power quality indexes characterizing the phenomenon of voltage unbalance in three-phase systems. Voltage unbalance is one of the commonest occurrences in the town mains of 0.38 kV voltage. The phenomenon describes as inequality of vector magnitude of phase voltage and shearing angle between them. Causes and consequences of the voltage unbalance in distribution networks have been considered. The algorithm, which allows switching one-phase load, has been developed as one of the methods of reducing the unbalance level. The algorithm is written in the function block diagram programming language. For determining the duration and magnitude of the unbalance level it is proposed to introduce the forecasting algorithm. The necessary data for forecasting are accumulated in the course of the algorithm based on the Function Block Diagram. The algorithm example is given for transforming substation of the urban electrical power supply system. The results of the economic efficiency assessment of the algorithm implementation are shown in conclusion. The use of automatic switching of the one-phase load for explored substation allows reducing energy losses (active electric energy by 7.63%; reactive energy by 8.37%). It also allows improving supply quality to a consumer. For explored substation the average zero-sequence unbalance factor has dropped from 3.59% to 2.13%, and the negative-sequence unbalance factor has dropped from 0.61% to 0.36%.

Evaluation of Pattern Recognition Algorithms for Applications on Power Factor Compensation

This paper assesses different applied pattern recognition algorithms to decide the most appropriate power factor compensator for a particular point of common coupling. Power factor, current unbalance factor, total demand distortion, voltage harmonic distortion and reactive power daily variation, as well as human expertise, are the key parameters used to set each recognition algorithm. These algorithms are then trained with a series of both simulation and experimental data. Numerical results consistently indicate the decision-tree algorithm with depth 20 as the best classifier for power factor improvement in terms of all metrics considered in this work.

Individual control method for hybrid voltage regulator

In recent years, photovoltaic power generation systems (PVs) have been installed in distribution systems after feed-in tariff operation. Voltage unbalance due to the difference in interconnection rates of PVs at each phase reduces power quality. To reduce the voltage unbalance, the hybrid voltage regulator (HVR), which can control each phase voltage, was developed. Here, the authors propose an HVR individual control method to control the unbalance factor and phase voltage within a specified range. Numerical simulations were performed to validate the proposed method. The results showed that the proposed HVR method reduced the unbalance factor from 3.0 to 1.6% while maintaining the specified phase voltage range.

Classification of voltage sag disturbance sources using fuzzy comprehensive evaluation method

The classification and recognition of voltage sag disturbance sources is the foundation of mitigating voltage sags. This study analysed characteristics of various kinds of voltage sags, including sags caused by short-circuit faults, energising of transformers and starting of large induction motors. According to the features of different sources, an evaluation index set was formed by three-phase voltage unbalance factor, duration ratio and increments of second harmonic. After that, the weight of each element was calculated by analytic hierarchy process. A novel kind of estimation criterion for fuzzy membership function parameters was proposed, inspired from 95% probability value, based on the method of undetermined coefficient to establish the functions. Consequently, according to it, fuzzy membership functions were created for each index. Further, comprehensive index membership degrees were calculated to judge the disturbance source according to the maximum of the degrees. The presented estimation criterion can efficiently reduce the influence of sample errors and thus improve the accuracy of the whole process, which was verified with the use of data obtained from both simulations and measurements.

Probabilistic Modeling of Electric Vehicle Charging Pattern Associated with Residential Load for Voltage Unbalance Assessment

It has been recognized that an increased penetration of electric vehicles (EVs) may potentially alter load profile in a distribution network. As EVs are regarded as a diversely distributed load so a deterministic method, to predict EV charging load, may not account for all possible factors that could affect the power system. Thus, a stochastic approach is applied that takes into account various realistic factors such as EV battery capacity, state of charge (SOC), driving habit/need, i.e., involving type and purpose of trip, plug-in time, mileage, recharging frequency per day, charging power rate and dynamic EV charging price under controlled and uncontrolled charging schemes. A probabilistic model of EVs charging pattern associated with residential load profile is developed. The probabilistic model gives an activity based residential load profile and EV charging pattern over a period of 24 h. Then, the model output is used to assess the power quality index such as voltage unbalance factor under different electric vehicle penetration levels at different nodes of the system. An uneven EV charging scenario is identified that could cause the voltage unbalance to exceed its permissible limit.

Algorithm for estimating derating of induction motors supplied with under/over unbalanced voltages using response surface methodology

One of the adverse effects of unbalanced three-phase voltages on the induction motors (IMs) is overheating of the windings. The IMs should be loaded at less than their rated power in case of unbalanced supply voltages to prevent this overheating. Recent studies have pointed out that the maximum allowable loading ratio or derating factor (DF) of the IMs have various values for several combinations of the magnitude and angle of the complex voltage unbalance factor (CVUF) operating with a combination of unbalanced over- and under-voltage cases. This means that determination of DF requires plenty of experimental efforts for all possible unbalanced voltage conditions. In this study, the effective root-mean-square voltage definition which is defined in IEEE Standard 1459 is combined with the CVUF for proper identification of over and under unbalanced voltage conditions. An algorithm based on response surface methodology is proposed to estimate a precise DF for a broad range of unbalanced supply voltages. The simulation results are presented to validate the effectiveness of the proposed algorithm. It is apparently figured out that the proposed algorithm has better accuracy compared with the conventional approach reported in National Equipment Manufacturer's Association Standard MG1.