Single-phase operation of a three-phase induction generator withthe Smith connection

Single-phase operation of a three-phase induction generator with the Smith connection (SMIG) is analyzed using the method of symmetrical components. It is shown that, despite the asymmetrical nature of the winding connection, balanced currents can be made to flow in the three-phase stator winding. The conditions for achieving perfect phase balance are carefully deduced. With the aid of a phasor diagram, expressions for the line power factor and line current under perfect phase balance are also obtained. The effect of the phase-balancing capacitances on the generator performance is investigated. A simple dual-mode control scheme is also proposed with a view of minimizing the phase imbalance over the practical operating speed range. Experiments conducted on a 2.2 kW induction machine confirm the validity of the theoretical analysis and feasibility of the control method     

A novel excitation scheme for a stand-alone three-phase induction generator supplying single-phase loads

This paper presents the operating principle and steady-state analysis of a novel excitation scheme for a stand-alone three-phase induction generator that supplies single-phase loads. The phase windings and excitation capacitances are arranged in the form of the Smith connection and the excitation scheme is referred to as the SMSEIG. In addition to providing the reactive power for self-excitation, the capacitances also act as phase balancers. With this novel excitation scheme, isolated single-phase loads can be supplied with good phase balance in the induction machine, resulting in high efficiency, large power output, and quiet machine operation. Performance analysis is based on the method of symmetrical components, from which the input impedance of the generator can be determined. Numerical solution of a simplified equivalent circuit for the machine variables, namely the excitation frequency and magnetizing reactance, enables the generator performance to be evaluated for any load and speed. With the aid of a phasor diagram, the conditions for achieving perfect phase balance are deduced and a method to compute the capacitances required is developed. Experimental investigations on a 2.2-kW induction machine confirm the feasibility of the SMSEIG.     

Steady-state analysis and performance of a stand-alone three-phaseinduction generator with asymmetrically connected load impedances andexcitation capacitances

This paper presents a steady-state performance analysis of a stand-alone three-phase induction generator self-excited with unbalanced capacitances and supplying unbalanced loads. Using the method of symmetrical components, the complex three-phase generator-load system is reduced to a simple equivalent passive circuit. A function minimization technique is employed to solve this equivalent circuit in order to determine the excitation frequency and magnetizing reactance. The proposed method enables practically all cases of unbalanced operation of the self-excited induction generator to be analyzed. Emphasis is next focused on single-phase loads and a special phase-balancing scheme, namely the modified Steinmetz connection (MSC), is investigated. It is shown that perfect phase balance of the SEIG can be obtained with an appropriate combination of excitation capacitances and loads. The theoretical analysis is validated by experiments on a 2.2 kW induction machine     

A novel single-phase self-regulated self-excited inductiongenerator using a three-phase machine

This paper presents a steady-state analysis of a novel single-phase self-regulated self-excited induction generator which employs a three-phase machine. Performance equations are derived using the method of symmetrical components, while the pattern search method of Hooke and Jeeves is used for the determination of the machine variables. The advantages of the generator include simple circuit configuration, small voltage regulation, good phase balance, and large power output. With an appropriate choice of the series and shunt capacitances, a nearly balanced operating condition can be obtained for a certain load. The theoretical analysis is validated by experiments performed on a small induction machine     

Performance analysis of a three-phase induction generator connectedto a single-phase power system

This paper analyzes the performance of a three-phase induction generator which is connected to a single-phase power system. Significant improvement in machine performance can be obtained by using a single static phase-converter, provided that the machine is driven in the reverse direction. If two phase-converters, are employed, perfect phase balance can be obtained at any desired value of slip. The theoretical analysis is validated by experiments on a 2-kW test machine     

Finite element analysis of a single-phase grid-connected induction generator with the Steinmetz connection

The Steinmetz connection enables a three-phase induction generator (IG) to operate satisfactorily on a single-phase grid by using only a capacitance phase converter. This paper presents a finite element analysis of this mode of IG operation. A time-stepping, two-dimensional (2-D) finite element method (FEM) is employed in the solver, with the external circuit equations coupled to the set of simultaneous equations formulated with the nodal magnetic vector potentials as the variables. A detailed rotor circuit model that accounts for the current density distribution as well as the end-ring parameters is also presented. Generator performance computed using FEM is compared with that computed using the method of symmetrical components and that obtained experimentally on a small induction machine. The results indicate that FEM gives an accurate prediction of the line current and the output power of the IG.     

Zero sequence method for energy recovery from a variable-speed windturbine generator

An innovative power conversion system to convert energy from a variable-frequency wind-powered induction generator to a fixed frequency output is presented. A standard six-switch DC link current regulated pulse width modulated (CRPWM) inverter is simultaneously modulated with two current components. A three-phase balanced current component at the induction generator's optimum operating frequency transfers energy from the generator to the converter. A single phase zero sequence current component at a fixed 60 Hz frequency transfers energy from the converter through a zero sequence filter to the load. Unity power factor output is shown both in simulation and experiment, though any arbitrary power factor output may be readily commanded. Maximum power capture for a variable-speed wind turbine is achieved using proven control techniques. This method uses only half of the active power switching devices of conventional conversion methods. Simulation and experimental verification are shown     

一种综合使用可再生能源系统的设计与研究

提出一种新的能量转化装置——内馈式感应发电机,叙述了其运行方式;并在内馈式感应发电机为主要动力装置的基础上建立可再生能源综合系统,给出了系统结构图,阐述各个系统元件之间的相互关系。     

Improvements in RF monitoring system on generators

An improved RF monitoring system for generators is described. A capacitive coupling is shown to be much more sensitive than the currently used magnetic coupling through a current transformer. The increase in sensitivity results in an improved coupling between the generator's neutral grounding lead and the monitor. An RC high-pass filter is shown to be an effective coupler, because it does not attenuate the RF noise or create saturation problems as in the case of magnetic coupling. By selecting an RC filter with higher capacitance than the stray capacitances created by the typical generator grounding transformer, the coupler can detect arcing signals in any generator, regardless of the type of neutral grounding transformer used. Even though there is an ohmic contact to the neutral lead, the protections to the added RF instrumentation assure that the protective relaying scheme is not degraded     

Measured efficiency improvements of induction motors with thyristor/triac controllers

The deployment of inexpensive thyristor/triac controllers improves the efficiency and does not increase the power factor of induction motors (as measured at the input of pole transformer) operating under variable torque and constant speed constraints. Computer-aided testing yields detailed input/output powers for the drive. The improvement of the power factor does not increase the real-power capability (RPC) of a transformer supplying power to such drives due to current harmonics. Although the total harmonic current distortion (THD/sub i/) may exceed IEEE 519 and IEC 555 recommendations, publications show that the impact on the apparent power derating of a 25 kVA pole transformer is small if THD/sub i/<50%.     

基于光学电流互感器的发电厂保护优化研究

本文总结了采用柔性光学电流互感器的发电厂继电保护针对不同场景的优化研究和探索,介绍了发电机主保护方案优化、转子绕组匝间保护、选择性定子接地保护、核电辅助变断相监测保护、抽水蓄能等变频启动机组保护优化等保护优化方案和效果,利用柔性光学TA良好的传变特性和灵活的安装方式,较好地解决了现有发电厂保护存在的问题,可为现场类似场景的问题解决提供参考思路。     

Control of a switched reluctance generator for variable-speed wind energy applications

This paper presents a novel control system for the operation of a switched reluctance generator (SRG) driven by a variable speed wind turbine. The SRG is controlled to drive a wind energy conversion system (WECS) to the point of maximum aerodynamic efficiency using closed loop control of the power output. In the medium and low speed range, the SRG phase current is regulated using pulsewidth-modulation (PWM) control of the magnetizing voltage. For high speeds the generator is controlled using a single pulse mode. In order to interface the SRG to the grid (or ac load) a voltage-source PWM inverter is used. A 2.5-kW experimental prototype has been constructed. Wind turbine characteristics are emulated using a cage induction machine drive. The performance of the system has been tested over the whole speed range using wind profiles and power impacts. Experimental results are presented confirming the system performance.     

Capacitors Required for Maximum Power of a Self-Excited Single-Phase Induction Generator Using a Three-Phase Machine

This paper aims to determine the optimal capacitors required for maximum output power of a single-phase self-excited induction generator (SEIG), using a three-phase machine feeding inductive as well as capacitive loads. The generator consists of a three-phase star-connected induction machine with three capacitors and a single-phase load. The algorithm, which gives directly the values of the optimal capacitors for the maximum power output and the maximum power available, has been developed using the steady-state model of the SEIG and sequential unconstrained minimization technique (SUMT). The variations of the maximum power output with power factor (pf) of loads (both inductive and capacitive) and speed of the SEIG have been presented. The voltage regulation of the generator is small due to the effect of the two series capacitors. Experimental results have shown the effectiveness and accuracy of the developed algorithm.     

A Brushless, Self-Excited Single-Phase Synchronous Generator Operating with Load and Exciting Currents Flowing in Armature

This paper describes principles, analyses and experimental studies of a brushless and self-excited single-phase synchronous generator which was devised by one of the authors. The analyses and test results of performances of the generator system of which the armature winding is supplied with an exciting power from a transformer which is connected with the generator armature winding through a reactor or a condenser are presented. It can be found from the test results that the necessary exciting power is less in case of the armature winding supplied with DC exciting current from the transformer through the condenser.     

功率因数及三相不平衡对变压器振动特性的影响

针对现有基于振动信号的变压器故障检测技术往往忽视功率因数及三相不平衡影响的现象,分析了功率因数及三相不平衡对于变压器振动特性的影响,并搭建试验平台分别测试了不同功率因数、不平衡度下的变压器振动信号。结果表明,振动信号的振动幅值随功率因数增大而增大,但频谱分布受影响较小;三相不平衡对振动幅值与频谱分布均有较大影响,在进行基于振动信号的变压器故障检测时,必须考虑其三相不平衡情况,否则将影响检测结果的准确性。     

Effect of excitation capacitors on transient performance ofreluctance generators

The transient responses of a reluctance generator connected to an infinite power system excited by a bank of terminal capacitances are compared to those when load excitation is used. A mathematical model is developed to simulate the machine with its terminal capacitor. With the aid of a least-square-error method, this model is used to optimize the machine parameters. The capacitance excitation requirements for different load conditions are then computed using a steady-state model. The comparison of the transient responses shows that the terminal-capacitor excitation method has several advantages over the load excitation method. It reduces the first rotor swing and gives more damping to the subsequent rotor oscillations. It also increases the critical fault-clearing time and hence the transient stability limits. In addition, it suppresses all power frequency torque oscillations, which are quite pronounced when load excitation is used     

基于双馈发电机的太阳能热动力发电系统并网研究

对双馈发电机的空载并网控制技术进行了研究,通过调节转子励磁电流控制双馈发电机定子电压,在双馈发电机定子电压和电网电压的幅值、频率和相位完全一致时进行并网,并网完成后将其从并网控制切换到发电控制,根据实际吸收太阳能功率进行功率实时调整。     

Interrelation between configuration of electrodes for induction electric charging and collector current wind EHD power generator

This work aims to realize a wind electrohydro- dynamical (EHD) power generator using induction electric charging with a water jet. The electrodes for induction electric charging are composed of an orifice of which inner radius is 1.5mm and a ring made of a copper wire of which diameter is 1.5mm. It is important to design the electrodes that maximize induction electric charge. In this paper, it is experimentaly clarified that the ring electrode of inner diameter of 3.7cm gives the collector short current of the wind EHD power generator more than that of 4.7cm but has dispersion of the collector short current more than that of 4.7 cm.     

Short-Circuit Current of Wind Turbines With Doubly Fed Induction Generator

The short-circuit current contribution of wind turbines has not received much attention so far. This paper considers the short-circuit behavior, especially the short-circuit current of wind turbines with a doubly fed induction generator. Mostly, these wind turbines have a crowbar to protect the power electronic converter that is connected to the rotor windings of the induction generator. First, the maximum value of the short-circuit current of a conventional induction machine is determined. The differences between a crowbar-protected doubly fed induction generator and a conventional induction generator are highlighted and approximate equations for the maximum short-circuit current of a doubly fed induction generator are determined. The values obtained in this way are compared to the values obtained from time domain simulations. The differences are less then 15%     

变压器侧出线的短网参数

通过对变压器采用顶出线和侧出线两种结构的短网参数计算比较,得到变压器侧出线结构可使短网电抗减小,电抗不平衡系数降低,从而降低电耗,提高三相电弧功率均衡和功率因数。