Analysis of three-phase AC-DC converters under unbalanced supplyconditions

A three-phase AC-DC converter is analyzed, and the performance of the converter under balanced conditions is evaluated. It has been observed that the level of unbalance plays a significant part in the converter characteristics, especially at the lower output voltage range     

Analysis of three-phase induction motors utilizing `The IodekiceLocus'-a normalized impedance circle

A method that allows students to analyze the performance of any three-phase induction motor operating at any arbitrary speed, either analytically or graphically, using the same conceptual approach is presented. The method uses a unity-diameter normalized impedance circle with a periphery calibrated in terms of normalized slip, which is defined as the ratio of per unit slip to a readily determined machine constant. Equations that allow the tabulation of the real and imaginery impedance components of the normalized circle as a function of normalized slip are developed. These data may be plotted for use with graphical analysis or used directly in a simplified mathematical analysis. The key factor in the development is shown to be the concept of normalized slip. Using this concept, the specific operating point of a machine can be simply and uniquely determined for any given value of per unit slip     

Power related phenomena in three-phase unbalanced systems

The paper deals with the power properties of three-phase circuits with unbalanced loads supplied from a three-wire line. It discusses various definitions of the apparent power in such circuits and shows that they cannot be interpreted in terms of any physical phenomena. The paper shows that the unbalanced power does not occur as an effect of the power oscillations and explains why this power does occur. The paper presents a decomposition of the supply current into the active, reactive, and unbalanced currents and shows that this decomposition can be considered as a fundamental for the comprehension of the power phenomena in such circuits and for the power factor evaluation     

NEMA motor-generator standards for three-phase induction motors

The National Electrical Manufacturers Association (NEMA) Motor-Generator Standards (MG-1) have gone through numerous revisions and additions. Key information pertaining to motor performance, construction and operating conditions are provided for end users' consideration when specifying electric motors. There is still some confusion regarding how to specify electric motors and how to interpret nameplate data on existing motors. This article provides the user with a condensed version of MG-1 to serve as a quick reference guide     

Watthour meter accuracy under controlled unbalanced harmonicvoltage and current conditions

This work presents the results of phase I of the project on the accuracy of watthour meters when subjected to closely-duplicated real-world unbalanced harmonic voltage and current conditions. Such real-world conditions involved differences in voltage and current magnitudes as well as differences in the voltage and current total harmonic distortion levels of each phase. Tests were performed on a selected sample of nine three-phase and three single-phase watthour meters. Some results showed that under a particular set of unbalanced waveforms with harmonic conditions of I_THD>80% and V_THD>2%, the percentage errors exhibited by these meters ranged from -10.09% to +0.52%     

Applying adjustable-speed drives to three-phase induction NEMAframe motors

This paper addresses why a three-phase induction motor overheats on adjustable-speed drives, under some conditions. It further addresses how the motor may derate when used on a drive and how to determine the amount of derating required     

Modeling and simulation of transmission system shield wires under unbalanced conditions

The purpose of this investigation is to calculate the ground return current under steady-state or transient conditions, given the phase currents in the conductors of two unbalanced three-phase parallel circuits. Thus, the shield wires can be modeled to carry appropriate current depending upon the currents in the three phase conductors. For example, when a fault occurs on a phase conductor, the currents that start flowing in the phase conductors may become unbalanced. Under such conditions, the currents that are coupled into the shield wires will be more important. This investigation can help a transmission system line designer to identify the appropriate shield wires under different transmission system scenarios. A computer package that evaluates the shield wire currents under unbalanced conditions when the transmission lines are randomly spaced is developed.     

Modeling and enhanced control of DFIG under unbalanced grid voltage conditions

This paper presents a mathematical model of a doubly fed induction generator (DFIG) based on stator voltage orientation (SVO) in the positive and negative synchronous reference frames under unbalanced grid voltage conditions. The oscillations of the DFIG electromagnetic torque and the stator active and reactive powers are fully described during grid voltage unbalance. A new rotor current controller implemented in the positive synchronous reference frame is proposed. The controller consists of a proportional integral (PI) regulator and a harmonic resonant (R) compensator tuned at twice the grid frequency. Thus, the positive and negative sequence components of DFIG rotor currents are directly regulated by the PI + R controller without the need of involving positive and negative sequence decomposition, which indeed improves the dynamic performance of DFIG-based wind power generation system during small steady-state and relatively larger transient network unbalances. The theoretical analysis and the feasibility of the proposed unbalanced control scheme are validated by simulation studies on a 1.5-MW wind-turbine driven DFIG system. Compared with conventional single PI current control design, the proposed control scheme results in significant elimination of either DFIG power or torque oscillation under unbalanced grid voltage conditions.     

电压偏差条件下笼型感应电机的损耗特性

为了研究电压偏差条件下笼型感应电机的损耗特性,以一台Y225M-8型22 kW电机为例,利用场-路耦合时步有限元法,在额定电压±10%范围内,定量分析电机带轻载及额定负载时各项损耗的变化情况,并进一步针对Y系列3种不同容量电机,得出其在电压偏差条件下带不同负载时总损耗变化的一般规律.经损耗分析发现,无论在欠电压还是过电压下,满载总铁耗均明显高于轻载总铁耗,而铁耗中的附加铁耗通常要高于基本铁耗.对5.5 kW和22 kW电机进行实验研究,通过对比其在额定电压下满载运行时实测和计算损耗值,验证了所提出的损耗计算方法的正确性.     

Early detection of stator voltage imbalance in three-phase induction motors

Online health monitoring of electric motors is of paramount interest to various applications. As the operation of industrial processes becomes more complex, the cost of health monitoring increases dramatically. To this end, much efforts have been directed towards enhancement of fault diagnostics and prognostics in electric motors, largely based on conventional signal processing and pattern classification. This paper formulates and experimentally validates a recently reported technique, called Symbolic Dynamic Filtering (SDF), for early detection of stator voltage imbalance in three-phase induction motors. The SDF-based imbalance detection algorithm is built upon the principles of wavelet transforms and symbolic time series analysis.     

Demagnetization diagnosis in permanent magnet synchronous motors under non-stationary speed conditions

Permanent magnet synchronous motors (PMSMs) are applied in high performance positioning and variable speed applications because of their enhanced features with respect to other AC motor types. Fault detection and diagnosis of electrical motors for critical applications is an active field of research. However, much research remains to be done in the field of PMSM demagnetization faults, especially when running under non-stationary conditions. This paper presents a time–frequency method specifically focused to detect and diagnose demagnetization faults in PMSMs running under non-stationary speed conditions, based on the Hilbert Huang transform. The effectiveness of the proposed method is proven by means of experimental results.     

电网故障时功率变换器并网同步策略

针对传统同步参考系锁相环(synchronous reference frame phase-locked loop,SRF-PLL)在电网故障时检测出的电压幅值与相位等存在低频干扰量,无法保证并网同步要求的问题,提出了一种功率变换器并网同步策略.该并网同步策略是对传统SRF-PLL的拓展,由设计的正序分量提取算法和传统的SRF-PLL组合而成,详细阐述了该并网同步策略的理论依据和实现方法.理论分析表明:在电网故障时,该并网同步策略具有动态响应时间快,结构简单,谐波分量抑制能力强的优点.并且对电网基频在允许范围内(50±0.5Hz)偏移时不敏感,能够有效解决传统SRF-PLL在电网故障时无法保证并网同步要求的缺点.仿真与实验结果验证了理论分析的正确性和可行性以及该并网同步策略应用的有效性.     

低压配电网三相不平衡运行的影响及治理措施

我国低压配电系统大都采用三相四线制Y/Yno(Y/Y)接线方式,由于单相负载的不均衡性等原因,造成配电变压器处于三相不平衡运行状态.从电能质量的角度分析不平衡运行所造成的三相电压不平衡及其危害,详细分析和推导三相不平衡所带来的配变损耗和线路损耗,并针对低压配网的三相不平衡问题提出了管理和技术上的治理措施.