Digital Simulation of a High-Performance AC Drive System?Part II

This is the second part of a paper describing the digital simulation of an ac drive system consisting of a three-phase induction motor supplied by a cycloconverter. In Part I, it is assumed that the cycloconverter is connected to a three-phase sinusoidal power source having negligible impedance. In this part, the complete system is simulated, including a three-phase synchronous generator, which in a practical system is driven by a prime mover at constant speed. To simplify the simulation process, it is assumed that the commutation of the generator phase currents takes place instantaneously. The results of the simulation are compared with test results. Both the calculated rms values and waveshapes of voltage and current show good agreement with test data.     

A Cycloconverter?Synchronous Motor Drive for Traction Applications

An electric drive suitable for traction is described consisting of a cycloconverter and a synchronous motor. Essentially, the same system can be used with either single-phase or three-phase input with frequencies around 50-100 Hz. The motor speed range is not limited by the input frequency. The cycloconverter thyristors are commutated by the source voltage at low motor speed and by the motor electromotive force (EMF) at higher speed. The transition between the two regions is smooth and automatic. Only 16 low- switching speed thyristors are required for the single-phase input system. The short time overload capability of this drive at low speed is excellent, and full range regenerative, as well as dynamic braking, is readily accomplished. Compared to dc motors occupying the same space, three-phase ac motors, both synchronous and induction, offer higher output power, higher efficiency, and the elimination of brush and commutator maintenance. Compared to inverters, cycloconverters have higher efficiency and lower cost and weight.     

一种新颖的矩阵式三相/单相变换器的设计与实现

提出了一种新颖的矩阵式三相/单相变换器,变换器由三相整流桥和单相逆变桥组成,其最大特点是开关管只需要采用单极性开关管并且无需直流母线支撑电容。三相整流桥采用基于扇区判断的同步整流控制策略,得到六脉波直流母线电压,并经过单相逆变桥得到频率和幅值可调的单相交流电。设计并制作了一台1.5kVA的样机,PSIM仿真结果和样机实验结果的吻合验证了本文所提出变换器的可行性。     

单极性移相控制电压源高频交流环节AC/AC变换器研究

该文首次提出并深入研究了基于正激Forward变换器的单极性移相控制电压源高频交流环节AC／AC变换器。这类变换器由输入周波变换器、高频变压器、输出周波变换器、以及输入、输出滤波器构成。输入周波变换器将输入不稳定劣质的交流电压调制成双极性三态的高频交流电压，输出周波变换器将此高频交流电压解调成单极性三态SPWM波，经输出滤波后得到稳定优质的正弦交流电压。通过输入周波变换器右桥臂相对左桥臂的移相，让输出周波变换器功率开关在输入周波变换器输出的高频交流电压为零期间进行换流，并借助输出周波变换器换流重叠和输入电压极性选择，从而实现了变压器漏感能量和输出滤波电感电流的自然换流、输出周波变换器的ZVS开关。详细分析了这类变换器在1个高频开关周期内的12个工作模式及其等效电路，获得了变换器外特性曲线与关键电路参数设计准则。原理试验结果证实了这类变换器新概念，为实现新型电子变压器、正弦交流稳压器和交流调压器奠定了基础。     

交-交变频器谐波消除方法的探讨

随着电力电子技术的发展,交-交变频器的应用愈加广泛。在交-交变频器输出的电压波形中含有大量的谐波,为有效地消除谐波的不利影响,采用余弦交点法控制的交-交变频器,通过对输出电压波形的基本结构进行分析,找出谐波的内在规律,提出了谐波同步消除方法,谐波同步消除方法属于有源滤波,利用其特有的谐波分析方法控制谐波电源的输出补偿电压,补偿电压与谐波同时产生,相位相反,从而达到滤波的目的。与现有的其它方法相比,其算法简单、效果明显,更加适用于输出电压频率频繁变化的场合,即使在变频器直流输出时,也可达到明显的滤波效果。     

A variable-voltage current-commutated three-phase inverter

A six-pulse three-phase inverter circuit that provides a wide range of AC output voltages and frequencies is presented. The inverter circuit uses asymmetrical thyristors for all the controllable switches. Through the use of these devices, a small turn-off time, low commutation circuit and switching power losses, and high-surge-current handling capabilities are obtained. The performance of the inverter is evaluated both analytically and experimentally. The inverter no-load power losses are calculated and compared with the experimental values using different supply voltages     

基于双Boost交联拓扑的三相功率因数校正器

研究了基于双Boost交联拓扑的三相功率因数校正器,其主支路三相不控整流器通过外加并联功率因数校正电路来改善网侧电流波形,提高功率因数。首先对主支路的拓扑结构以及工作方式进行分析,总结该拓扑的特点,并针对其特点提出相应的控制策略以及参数设计方案。理论分析和仿真、实验结果表明:这种新型三相功率因数校正器适用于恒功率负载场合,只需通过控制从整流器的选相开关以及双Boost交联变换器即可获得稳定的6脉波直流输出电压、理想的功率因数以及总谐波畸变率,而仅有约1/5的功率经过双Boost交联变换器。     

A Static Variable-Frequency Three-Phase Source Using the Cycloconverter Principle for the Speed Control of an Induction Motor

This paper describes a variable-frequency source employing the cycloconverter principle. SCR's have been used as the principal switching and voltage-regulating devices. The logic of the firing circuit has been discussed. The factor affecting output frequency stability has also been presented. A three-phase variable low-frequency oscillator has been used as the reference to achieve the variation of frequency. Voltage variation has been affected by phase delaying of the firing pulses in a manner similar to that in a phase-controlled rectifier. Provision has been made for changing the phase sequence at the output in order to facilitate reversal of the direction of rotation of the motor. Experimental results include output waveforms of various frequencies (0-30 Hz), and speed variations of the experimental motor (1 hp, 100 V, 1700 r/min) in either direction of rotation thereby providing the practical feasibility of the SCR cycloconverter for speed control of induction motors.     

Analysis and Performance Assessment of Six-Pulse Inverter-Fed Three-Phase and Six-Phase Induction Machines

This paper describes a model for a six-phase induction motor driven by an inverter operating in a six-pulse (square wave) mode. The model is implemented, and performance, in terms of torque, current, and efficiency, is compared to the performance of a three-phase motor (supplied with either a sine and six-pulse voltage). The models are verified experimentally to a degree of accuracy, and it is illustrated that the improvement in inverter efficiency when in six-pulse operating mode may improve the performance of the overall system, although the improvement is marginal in the example used. The potential of this drive configuration is demonstrated     

A Three-Phase Soft-Switched Isolated AC/DC Converter Without Auxiliary Circuit

This paper describes an ac/dc converter system consisting of a voltage-source converter (VSC) with purely capacitive snubbers and a two-phase by three-phase cycloconverter, connected via a medium-frequency (MF) transformer. By alternately commutating the two converters, it is possible to achieve beneficial switching conditions for all semiconductor devices. A commutation and modulation algorithm is described, which allows for pulsewidth-modulation control of the output voltage while maintaining soft switching. Low-load operation of the converter is a potential difficulty because the load current may be insufficient for recharging the snubber capacitors of the VSC. However, if the cycloconverter is used to momentarily short circuit the transformer, a quasi-resonant commutation mode of the VSC can be achieved, making a fast and soft commutation of the VSC down to zero load possible, without an auxiliary circuit. Furthermore, the design and operation of a 40-kVA prototype converter system are described. The experimental results from the prototype clearly show the practical feasibility of the studied concept.     

基于Matlab的交-交变频同步机系统变频器性能分析

讨论基于Matlab的交-交变频器带不同负载的建模与仿真。对交-交变频器输出电压及输出电流进行了谐波特性分析,指出输出频率、输出电压比、“死区”对交-交变频器的影响。并且对该系统在不同负载情况下的运行特性进行简要分析。     

Impact of cycloconverter harmonics

Findings from a study on the harmonics generated by a 15 MVA three-phase 13.2 kV 60 Hz to single-phase 24/12 kV 25 Hz static frequency converter (SFC) or cycloconverter are summarized. At light loads, some of the characteristic harmonics (e.g. 10 Hz, 110 Hz) have magnitudes that exceed the fundamental frequency. Since the three-phase components at these frequencies do not necessarily nullify each other, many frequency components, in addition to zero-sequence components, can be measured in the neutral (residual) current. Because of low system impedance at harmonic frequencies, the voltage at the 13.2 kV substation supplying the SFC is not severely affected. A worst-case scenario for the voltage spectrum is presented. The corresponding distortion factor is calculated to be 4.12%. The power factor of an assumed parallel load is calculated using this scenario, and the effect of the SFC harmonics on the power factor is found to be insignificant. However, depending on the system impedances, the shunt capacitors for power factor compensation at the assumed load may be ineffective due to harmonic resonance     

电流源高频交流环节AC/AC变换器研究

提出了基于反激F1yback变换器的电流源高频交流环节AC/AC变换器电路结构与拓扑族。该电路结构由输入周波变换器、高频储能式变压器、输出周波变换器以及输入、输出周波变换器构成，能够将一种不稳定畸变的交流电变换成问频率稳定的正弦交流电压；该电路拓扑族包括单四象限功率开关式、推挽式、半桥式、全桥式等四种电路。以单四象限功率开关式电路拓扑为例，分析研究了这类变换器工作模式、稳态原理与电压瞬时值反馈控制策略，给出了变换器的外特性曲线、关键电路参数设计准则。原理试验结果证实了这类变换器新概念的正确性与先进性。     

Digital Simulation of a High-Performance AC Drive System?Part I

With the advent of solid-state devices, various methods have been devised to use induction motors for variable-speed drives. Most of these schemes use a variable-frequency source; that is, the power is supplied through a cycloconverter or inverter to the induction motor. However, the voltage obtained by such methods does not have a sinusoidal waveshape. To predict the performance, therefore, we have to simulate the system using either an analog or digital computer. A method to simulate digitally a drive system consisting of a cycloconverter connected to a constant three-phase sinusoidal voltage source and an induction motor is given.     

全桥Boost型高频环节DC-AC变换器

在全桥Boost型高频环节AC-AC变换器基础上,提出将输入周波变换器变为高频逆变器,并且在输出负载与输入电源之间联接高频电气隔离反激式变换器能量回馈电路,实现全桥Boost型高频环节DC-AC变换.该变换器是由输入滤波电路、储能电感、高频逆变器、高频变压器、周波变换器、输出滤波电路依序级联构成,深入分析了其控制策略与稳态特性,并给出了关键电路参数与仿真结果.这种变换器适用于要求输入电流纹波小、负载短路时可靠性高、输出容量大、体积和重量小的逆变场合.     

Frequency-response characteristics and error estimation in RMS measurement

Root-mean-square (rms) calculation is a popular method adopted in power system parameter classification. Typical examples are its application in voltage sag classification and relay protection. Starting from the viewpoint of frequency response, this paper seeks to study the characteristics of the rms method, for both the single-frequency and mixed-frequency signals. Analysis is made on its dependence on sampling rate, sampling window size, as well as point-on-wave through strict mathematical deductions. Interesting discoveries include the locations of the "pitfall" frequencies, the minimum sampling points for correct rms output as well as the fluctuation of rms magnitude. Estimation is made on the maximum rms output errors due to the presence of harmonics as well as frequency deviation. Reasonable approximation is adopted in such estimation. Such error estimations are useful in measurement of power system voltages, or any other parameters with relatively higher fundamental but lower harmonics levels.     

A SIMULATION STUDY OF AN OPTIMAL NUMERICAL CONTROL OF A THREE-PHASE CYCLOCONVERTER

Abstract

The paper presents a study of the problem of quantization which is linked with the numerical control voltage of a three-phase cycloconverter controlled by a microprocessor. The study makes a compromise between either using a large number of quantizing steps of the desired control voltage, which requires an Increase in microprocessor memory size, or using small number of steps, which leads to a high degree of harmonic presence in load current. In order to carry out such a study, a complete digital simulation of the whole system (source, cycloconverter, load and control devices) is used. Two types of numerical control-voltage are compared subject to the harmonic spectrum of the load current. Calculation is given for the optimum number of steps and the type of the numerical control-voltage     

一种新颖的高频环节DC/AC变换器的控制方法研究

在简要论述了具有高频环节的DC/AC变换器特点基础上 ,提出了一种基于移相控制思想的控制方法 ,该方法可以使交 -交变频器的所有开关都在零电压条件下开通和关断 ,而且可以安全地重叠换相 ,从而有利于高频变压器漏感中储能放电。同时也提出将用于通用型直流电压源逆变器的PWM波生成方法 ,经过同步处理后 ,可以应用到该交 -交变频器中。本文以空间电压矢量生成软化PWM波为例 ,进行了输出电压的谐波分析 ,并且针对三相感应电动机负载 ,进行了实验研究 ,实验结果表明上述控制方法是正确的     

A Fast-Response DC Motor Speed Control System

This paper describes the design, construction, and testing of a thyristorized speed control unit for a separately excited dc motor. The motor is fed from a three-phase six-pulse fully controlled thyristor bridge. A speed loop with a proportional plus integral controller maintains the desired speed irrespective of the load variations on the motor. An inner current control loop protects the thyristors from overcurrents. This loop also provides fast response overcoming the effect of disturbances such as variations in supply voltage. The design aspects of the control loops are discussed, and experimental results are given.     

A Study of Asymmetrical Gating for Phase-Controlled Converters

The low power factor of phase-controlled converters when the output voltage is less than the maximum is of-concern in high-power equipment. In a converter consisting of two commutating groups in cascade, the usually low power factor can be improved by firing or gating the thyristors in the two groups asymmetrically. One of the groups is fully advanced (or retarded) to minimize its reactive power, while the other group is controlled to give the desired dc Output. The technique is not recommended for single three-phase bridge converters because of third-hannonic output ripple, second-harmonic line current distortion, and danger of commutation failure. These objections are overcome by combining pairs of three-phase bridges. When regeneration is not required, half of the thyristors can be replaced by diodes, reducing the cost. The method should offer substantial improvement in the power factor of cycloconverter ac motor drives as well as dc motor drives.