An efficient diagnostic technique for distribution systems based on under fault voltages and currents

Service continuity is one of the major aspects in the definition of the quality of the electrical energy, for this reason the research in the field of faults diagnostic for distribution systems is spreading ever more. Moreover the increasing interest around modern distribution systems automation for management purposes gives faults diagnostics more tools to detect outages precisely and in short times. In this paper, the applicability of an efficient fault location and characterization methodology within a centralized monitoring system is discussed. The methodology, appropriate for any kind of fault, is based on the use of the analytical model of the network lines and uses the fundamental components rms values taken from the transient measures of line currents and voltages at the MV/LV substations. The fault location and identification algorithm, proposed by the authors and suitably restated, has been implemented on a microprocessor-based device that can be installed at each MV/LV substation. The speed and precision of the algorithm have been tested against the errors deriving from the fundamental extraction within the prescribed fault clearing times and against the inherent precision of the electronic device used for computation. The tests have been carried out using Matlab Simulink for simulating the faulted system.     

Electrostatic and environmental analyses of high phase order transmission lines

This paper presents an electrostatic modeling and environmental effect analysis of the high phase order transmission lines (HPOTL) using the charge simulation method. Both the conversion of the existing EHV and UHV double circuit transmission lines to six-phase lines, and the use of new, compact circular configuration HPOTL are introduced and discussed in the light of electric field computation at both the subconductor/conductor surface and the ground level. A new technique is also presented to decrease drastically the electric field at the ground level with a negligible increase in the conductor surface gradient and the charging current of the bottom phases. In this technique, a very few number of extra ground wires under each circuit are added, where they can also be utilized as optical ground wires for telecommunication purposes. Different phase arrangements, voltage ratings and conductor coating of the above-mentioned two cases are studied to investigate the possibilities of increasing the power handling capabilities without augmenting the environmental effects. It seems that HPOTL give a promising cost-effective means for increasing the power handling capability either for the existing double circuit transmission lines by 73% via increasing the operating voltage, or for the compact HPOTL via using six or more phases.     

Magnetic field reduction using high phase order lines

Ground level electric, and more recently magnetic, fields from overhead power transmission lines are increasingly important considerations in right-of-way specification, with states setting or planning to set edge-of-right-of-way limits. Research has been conducted on high-phase-order power transmission wherein six or twelve phases are used to transmit power in less physical space and with reduced electrical environmental effects compared with conventional designs. The first magnetic field testing verifies predictive methods for determination of magnetic fields from high-phase-order lines. Based on these analytical methods, field profiles are determined for lines of different phase order but comparable power capacity. Potential advantages of high phase order as a means of field mitigation are discussed     

小电流接地电网单相接地故障的小波选线方法

中性点非有效接地配电网发生单相接地故障时,尽快选出故障线路对电网运行有重要意义,但选线问题迄今尚未能很好解决。在分析了小电流接地电网发生单相接地时暂态特征的基础上,提出应用小波变换提取故障暂态信息,实现故障选线的方法。由于利用暂态量,该方法适应性、灵敏性均优于现有的基于稳态量方法。     

Distortion-free measurement of high impulse voltages

It is shown that every high voltage divider has a limited bandwidth of some MHz, resulting in a limitation for the measurement of front oscillations or front-chopped lightning impulse voltages. It is pointed out that a voltage divider can be easily calibrated and can be constructed to prevent EMC-problems during impulse voltage measurement. Using a spherical electric field sensor with optoelectronic data transmission, a voltage measurement system with an upper bandwidth of 25 MHz has been constructed. The advantage of this system is the distortion-free measurement of all impulse test voltages in any circuit. After a short review of the principle and characteristics of the sensor, some practical applications are highlighted, showing that it can improve high-voltage measurement     

Surge voltages and currents into a customer due to nearby lightning

This paper presents experimental results of lightning surges incoming into a customer due to lightning to an antenna of the customer, a pole and a ground nearby the customer, and briefly discusses lightning current distribution in the customer, a distribution line and a telephone line. Based on experimental results, modeling of each component is explained, and EMTP simulations are carried out. The ground voltage rise is represented by a mutual resistance between grounding electrodes. EMTP simulation results have been observed to agree qualitatively with the measured results, and it becomes possible to investigate lightning surges and current distribution in a customer by an EMTP simulation.     

A FinFET SRAM cell design with BTI robustness at high supply voltages and high yield at low supply voltages

In this paper, a SRAM cell structure which uses pMOS access transistors and predischarged bitlines is presented. By using the strained pMOS transistor technology, the degradation of the read static noise margin (SNM) at high supply voltages due to the aging, especially in the presence of symmetric stress, is suppressed. In contrast to conventional cell, the write margin of the proposed cell does not degrade considerably at low supply voltages. To assess the efficacy, the proposed cell is compared with conventional cell for two cases of unstrained and strained pMOS. A comparative study is performed using mixed mode device/circuit simulations for a gate length of 22 nm. The results show that the read SNM degradation due to the symmetric aging at the supply voltage of 1 V is about 6% after three years for the proposed strained structure, while degradations are 14%, 12%, and 11% for the unstrained proposed structure, unstrained, and strained conventional structures, respectively. In addition, the proposed cell has both read and write cell sigma yields higher than six for supply voltages ranging from 1 V down to 0.5 V while the other structures have read or write yields less than six at the minimum supply voltage. Through some work function tuning, the cell sigma yields of the other structures reach above six for both read and write while being still lower than those of the proposed structure. Copyright © 2015 John Wiley & Sons, Ltd.     

Frequency dependence of phase shift between exciting and induction pulse currents in inductor-detail electromagnetic systems

The mutual phase shift between an exciting (primary) pulse current in a disk inductor and an induction (secondary) pulse current in a flat metallic detail depending on the frequency and degree of display of the superficial effect in the detail’s wall was investigated based on the classical electricity theory. It was shown that the phase shift can vary in the range of 0.5π-π at the electrotechnological application of this system to treat metals using the pressure of a strong pulse magnetic field.     

Computer analysis of transient voltages in large grounding systems

A computer model for transient analysis of a network of buried and above ground conductors is presented. The model is based on the electromagnetic field theory approach and the modified image theory. Validation of the model is achieved by comparison with field measurements. The model is applied for computation of transient voltages to remote ground of large grounding grid conductors. Also computation of longitudinal and leakage currents, transient impedance, electromagnetic fields, and transient induced voltages is possible. This model is aimed to help in EMC and lightning protection studies that involve electrical and electronic systems connected to grounding systems     

Bearing currents and shaft voltages of an induction motor underhard- and soft-switching inverter excitation

Bearing currents and shaft voltages of an induction motor are measured under hard- and soft-switching inverter excitation. The objective is to investigate whether the soft-switching technologies can provide solutions for reducing the bearing currents and shaft voltages. Two of the prevailing soft-switching inverters, the resonant DC-link inverter and the quasi-resonant DC-link inverter, are tested. The results are compared with those obtained using the conventional hard-switching inverter. To ensure objective comparisons between the soft- and hard-switching inverters, all inverters were configured identically and drove the same induction motor under the same operating conditions when the test data were collected. An insightful explanation of the experimental results is also provided to help understand the mechanisms of bearing currents and shaft voltages produced in the inverter drives. Consistency between the bearing current theory and the experimental results has been demonstrated. Conclusions are then drawn regarding the effectiveness of the soft-switching technologies as a solution to the bearing current and shaft voltage problems     

大容量PWM逆变器对交流电机的轴电压和轴承电流的影响

本文就在大容量ＰＷＭ逆变器供电情况下,逆变器对交流电机的轴承造成损坏的起因,影响和补救措施作了综述,着重介绍了所产生的轴电压和轴承电流对轴承的损坏作用。     

高压大功率测量中正确判断相序的研究

在高压大功率测量中,由于电压互感器与电流互感器对应相序错误,使功率测量结果不正确。本文介绍用Ｓ２－ＷＤ三相功率变送器正确判断了并进行功率测量的方法。     

电力系统故障分析中故障端口开路电压的计算

在不修改故障前原网节点阻抗参数矩阵的条件下,研究了电力系统故障分析中横向故障和纵向故障端口开路电压的计算方法。根据横向故障端口开路电压的物理意义和叠加原理,在已知电力系统内各发电机电势幅值和相位角或已知各发电机电势幅值相等、相位角相同的条件下,推导出计算横向故障端口开路电压的方法。根据纵向故障端口开路电压的物理意义、网络等值方法和叠加原理,在已知电力系统内各发电机电势幅值和相位角或已知各发电机电势幅值和断相支路两侧等效电源相位差的条件下,推导出计算纵向故障端口开路电压的方法。文中的计算方法可满足各种情况下电力系统故障分析计算的需要。     

Robust multimachine power systems control via high order sliding modes

This article presents a formal study of the transient stabilization problem, including a stability proof. A decentralized nonlinear robust control scheme for multimachine electrical power systems is presented. The mathematical model takes into account the interconnections between electrical elements of the power system such as generators, electric networks, loads and exciters. Each generator is considered as a seventh order system that includes the mechanical and rotor electrical dynamics with static exciter dynamics. The proposed control scheme is based on the Block Control methodology and Second Order Sliding Modes technique using the Super Twisting algorithm. A nonlinear observer is designed to estimate the rotor fluxes of the synchronous machines. The designed decentralized control scheme requires the only local information for each local controller. This control scheme was tested through simulation on the well known reduced 9 buses equivalent model of the WSCC system.     

A combined directional and faulted phase selector element based onincremental quantities

In this paper, differential incremental or superimposed voltage and current quantities are used in order to determine directionality of a fault in a network. It is shown that this can be accomplished by performing three scalar products between three voltage and current phasor pairs. The relative levels of these same three scalar products can be furthermore used to identify the faulted phases. While, in ultra-high-speed directional comparison relays, superimposed voltage and current had to be treated in the time domain in order to get speed. In this paper, all computations are done in the frequency domain     

A simple approach to analysis of faulted power systems

In this paper the common approach for analysis of symmetrical faults and the linear circuit theory are used to analyse the unbalanced disturbances in a power network. The positive, negative and zero sequence impedance data of the faulted power system are fed separately to the program developed for analysis of symmetrical (three-phase) faults and the node voltages corresponding to each sequence impedance data are obtained. These sequence node voltages are then corrected to yield the correct sequence node voltages by multiplying them by the actual ‘impedance ratios’ obtained from the orientation of the sequence network for the particular type of fault considered. Numerical examples are presented to support the developed theory.     

Decreasing the influence of arc feed currents and transient recovery voltages in single-phase conditions in EHV lines

One of the most efficient ways to solve the problem of reactive power compensation connected with restriction of unacceptable levels of voltage on extrahigh voltage (EHV) power-transmission lines is installation of additional shunting reactors (SR) in the EHV networks. Moreover, installation of shunting reactors leads to the occurrence of resonance overvoltages under single-phase operation conditions of power transmission (for example within the one-phase automatic reclosing time), as well as to the occurrence of deenergized current phase of the arc feed in the arc channel, which prevents fast deionization of another channel and increasing transient recovery voltage. In this paper, measures for decreasing the influence of arc feed currents and transient recovery voltages in single-phase conditions of EHV lines are considered.     

A study of the effect of earthwire materials on lightning induced voltages and currents in transmission lines

Two materials commonly used for overhead static conductors are compared for their response to voltages and currents induced in a transmission line by a lightning stroke in the proximity. Mean characteristics of initial and subsequent strokes form the basis of the necessary field calculations. Frequency domain solution techniques are used to compute the total field, considering both the lightning source field and the reaction field of the induced currents.The results indicate only slight differences in the voltages induced in lines using either aluminum or steel overhead earthwires. However, the magnitudes of induced currents are very sensitive to the type of earthwire material.     

Motor shaft voltages and bearing currents and their reduction inmultilevel medium-voltage PWM voltage-source-inverter drive applications

This paper presents test results on problems and solutions of motor shaft voltages and bearing currents in medium-voltage pulsewidth modulated (PWM) drive systems. Tests show that multilevel medium-voltage PWM voltage-source inverter drives can cause motor bearing currents, similar to a low-voltage PWM drives, even with one motor bearing insulated. Common-mode voltages generated as a result of PWM switching are observed on motor windings and capacitively coupled to the motor shaft, leading to bearing currents. Potential solutions, including altering common-mode circuitry, changing the grounding scheme, providing common-mode filtering, and grounding the motor shaft, are investigated. Test results on their effectiveness are presented