电气类外文翻译.doc

1、 外文原文（复印件）A: The Utility Interface with Power Electronic SystemIntroduction We discussed various powerline disturbances and how power electronic converters can perform as power conditioners and uninterruptible power supplies to prevent these poweline disturbances from disrupting the operation of critical loads such as computers used for controlling important processes, medical equipment, and the like. However, all power electronic converters (including those used to protect critical loads) can add to the inherent powerline disturbances by distorting the utility waveform due to harmonic currents injected into the utility grid and by producing electromagnetic interference, To illustrate the problems due to current harmonics ih in the input current is of a power electronic load, consider the simple block diagram of Fig. 1-6A-1. Due to the finite (non-zero) internal impedance of the utility source which is simply represented by Ls in Fig. l-6A-1, the voltage waveform at the point of common coupling to the other loads will become distorted, which may cause them to malfunction. In addition to the voltage waveform distortion, some other problems due to the harmonic currents are as follows: additional heating and possibly overvoltages (due to resonance conditions) in the utilitys distribution and transmission equipment, errors in metering and malfunction of utility relays, interference with communication and control signals, and so on. In addition to these problems, phase-controlled converters cause notches in the utility voltage waveform and many draw power at a very low displacement power factor which results in a very poor power factor of operation. The foregoing discussion shows that the proliferation of power electronic systems and loads has the potential for significant negative impact on the utilities themselves, as well as on their customers. One approach to minimize this impact is to filter the harmonic currents and the electromagnetic interference (EMI) produced by the power electronic loads. A better alternative, in spite of a small increase in the initial cost, may be to design the power electronic equipment such that the harmonic currents and the EMI are prevented or minimized from being generated in the first place. Both, the concerns about the utility interface and the design of power electronic equipment to minimize these concerns are discussed here.Generation of Current Harmonics In most power electronic equipment, such as switch-mode dc power supplies, uninterruptible power supplies (UPS), and ac and dc motor drives, ac-to-dc converters are used as the interface with the utility voltage source. Commonly, a line-frequency diode rectifier bridge as shown in Fig.1-6A-2 is used to convert line frequency ac into dc. The rectifier output is a dc voltage whose average magnitude Ud is uncontrolled. A large filter capacitor is used at the rectifier output to reduce the ripple in the dc voltage Ud. The dc voltage Ud and the dc current Id are unipolar and unidirectional, respectively. Therefore, the power flow is always from the utility ac input to the dc side. These line-frequency rectifiers with a falter capacitor at the dc side were discussed in detail in other section. A class of power electronic systems utilizes line-frequency thyristor-controlled ac-to-dc converters as the utility interface. In these converters, which were discussed in detail, the average dc output voltage Ud is controllable in magnitude and polarity, but the dc current Id remains unidirectional. Because of the reversible polarity of the dc voltage, the power flow through these converters is reversible. As was pointed out, the trend is to use these converters only at very high power levels, such as in high-voltage dc transmission systems. Because of the very high power levels, the techniques to ffdter the current harmonics and to improve the power factor of operation are quite different in these converters, as discussed in other section, than those for the line-frequency diode rectifiers. The diode rectifiers are used to interface with both the single-phase and the three-phase utility voltages. Typical ac current waveforms with minimal filtering were shown in other section. Typical harmonics in a single-phase input current waveform are listed in Table 1-6A-1, where the harmonic currents Ih are expressed as a ratio of the fundamental current Il. As is shown by Table 1-6A-l, such current waveforms consist of large harmonic magnitudes. Therefore, for a finite internal per-phase source impedance Ls, the voltage distortion at the point of common coupling in Fig. 1-6A-1 can be substantial. The higher the internal source inductance Ls, the greater would be the voltage distortion.Current Harmonics and Power Factor As we discussed in other section, the power factor PF at which an equipment operates is the product of the current ratio Il / Is and the displacement power factor DPF: In Eq. (1-6A-I), the displacement power factor equals the cosine of the angle 1. The current ratio Il / Is in Eq. (1-6A-l) is the ratio of the rms value of the fundamental frequency current component to the rms value of the total current. The power factor indicates how effectively the equipment draws power from the utility; at a low power factor of operation for a given voltage and power level, the current drawn by the equipment will be large, thus requiting increased volt-ampere ratings of the utility equipment such as transformers, transmission lines, and generators. The importance of the high power factor has been recognized by residential and office equipment manufacturers for their own benefit to maximize the power available from a wall outlet. For example from a 120V, 15A electrical circuit in a building, the maximum power available is 1.8 kW, provided the power factor is unity. The maximum power that can be drawn without exceeding the 15A limit decreases with decreasing power factor. The foregoing arguments indicate the responsibility and desirability on the part of the equipment manufacturers and users to design power electronic equipment with a high power factor of operation. This requires that the displacement power factor DPF should be high in Eq. (1-6A-I). Moreover, the current harmonics should be low to yield a high current ratio I1 / Is in Eq. (1-6A- 1).B: A Three-phase Pre-converter for Induction Heating MOSFETBridge InvertersIntroduction High frequency power supplies, based on MOSFET bridge inverters, are already widely used for induction heating applications. These units require dc input voltages of about 400V to allow efficient operation of the MOSFETs employed. This supply voltage is usually obtained by using a three-phase rectifier stage, appropriate smoothing components or by employing thyristor phase- angle control to the mains supply. This kind of mains frequency power supply allows output power control of the induction heater, but it suffers from highly distorted input current waveforms with a low power factor. New legislation has been proposed to limit the maximum magnitude of harmonics drawn from the mains supply and different strategies have been suggested to reduce mains pollution. Investigations have been made to replace mains frequency power supplies by switched mode pre-converters. Switched mode converters can be designed to draw sinusoidal input currents thus avoiding the need for large and expensive mains frequency filters. At the same time these converters provide output power control and implementation of a small size high frequency isolation transformer. Power factor corrected three-phase ac-dc switched mode converter systems have usually been obtained using three identical single-phase converters with a common output filter. These systems overcome problems of mains pollution, but suffer from the disadvantage of a relatively large number of components and the need for complicated control and synchronization circuits. To reduce component costs, a structure based on a boost converter with three-phase input diode rectifier has been suggested. However, when operated direct-off-line from a three-phase 415V mains supply, this structure leads to high output voltages above lkV. In this paper, a novel method to achieve power factor correction for three-phase ac to dc power converters is described. The proposed topology is based on the buck converter and allows therefore output voltages to be below the maximum input voltage. The proposed topology utilizes a three- phase diode rectifier at the mains input and a single active switching device. The active switching device operates under zero-current switching conditions, resulting in very high converter efficiencies and low RFI emissions.Zero-current switching technique allows semiconductor devices to be operated at much higher switching frequencies and with reduced drive requirements compared with conventional switched mode operation. The proposed single-ended resonant converter with three-phase diode rectifier offers good opportunities for medium power, ac to dc applications. It combines simplicity and ease of control with high converter efficiency and high output power capabilities. It will be shown in the paper, that these characteristics make the converter very suitable as a direct replacement for the conventional mains frequency power supply used to supply induction heating MOSFET bridge inverters.General Description A block diagram of the proposed induction heating system is shown in Fig. 1-6B-1. Block 1 represents the pre-converter that produces the dc supply voltage to feed to the RF MOSFET bridge inverter. Its output voltage should be controllable over a wide range to control the output power of the inverter and it must be able to operate with a wide range of load resistance to compensate load changes of the induction heating inverter stage. The pre-converter should operate direct-off-line from a three-phase 415V mains supply, drawing sinusoidal input current waveforms with a power factor approaching unity. Block 2 shows the RF MOSFET bridge inverter.The required maximum supply voltage of the MOSFET bridge lies between 300V and 400V. Block 3 represents the control and protection circuit used to stabilise the output power and to allow reliable operation of the induction heater in an industrial environment.Principle of Converter Operation A circuit diagram of the proposed three-phase ac to dc converter topology is shown in Fig. 1- 6B-2. The converter input currents are filtered through the input inductors L1, L2, L3. These inductors are designed so that the converter input currents are approximately constant over a whole switching cycle. During the OFF time of switch S, all three capacitors are charged by the input currents I1, I2,I3. Consequently the three capacitor voltages Uc1, Uc1, Uc1 begin simultaneously to increase at a rate proportional to their respective input currents. If discontinuous operation is assumed the initial voltages of all capacitors C1, C2, C3 are zero when the switch ceases conducting. Hence, the peak voltage across each capacitor at the end of the OFF interval is proportional to their respective phase input current during the same OFF interval. Since capacitor voltages always begin at zero, it means that their average values during OFF time are linearly dependent on the phase input currents. During the ON time of switch S the energy stored in the three input capacitors C1, C2 and C3 is discharged through the six rectifier diodes VD1 VD6, the switch S and the resonant inductor Lr. The rate of current decrease is dependent on the phase currents I1, I2, I3 and the switch current I0. The average value of the capacitor voltages Uc1, Uc2, Uc3 during the ON time are not linearly dependant on their phase input currents. To draw sinusoidal input currents from the mains supply the converter must draw input currents averaged over each switching cycle which are proportional to the phase voltages. Assuming steady state converter operation, the average phase input voltages over each switching cycle must be equal to the appropriate average input capacitor voltages during the switch OFF time plus the average input capacitor voltages during the switch ON time. Average input capacitor voltages during the switch OFF time have been shown to be proportional to the phase input currents, but during the switch ON time this is not true. However, if the switch ON time of the converter is mucteshorter than the switch OFF time, then the shape of the phase input currents will approach a sinusoidal waveform with unity power factor.2、外文资料翻译译文A:效用界面与电力电子系统介绍 我们之前介绍了许多种电力线的干扰情况和电力系统转换器是如何在作为电力调节器和电力电子变换器时,用来防止那些电力线扰动干扰操作的临界荷载,例如电脑用于控制重要步骤,医疗设备,以及类似其他情况。但是,所有的电力电子变换器(包括那些用来保护临界荷载)都能增加固有的扰动,歪曲效用波形。由于谐波电流流入到公用电网和产生电磁干扰的问题,我们阐述了由于谐波电流ih在输入电流的电力电子负载,得出简单的框图。1-6A-1图。由于以有限的(非零效用)内部阻抗为代表的资源,呈现在图. l-6A-1中，电压波形的点将成为到另一个常见的耦合载荷,这可能导致他们图像扭曲甚至失真。谐波电流除了电压波形畸变小的问题,还会产生一些其他问题:额外的加热和可能会导致超电压(由于共振条件)的情况，在效用的分布和传输设备、错误的计量和故障效用的继电器、干扰通讯与控制信号这些方面，都会出现问题。除了这些问题,转换器引起电压波形和许多功率位移功率因数变化,并使这些因数降低。结果导致定向控制转换器在一个非常低的功率因数下的运作。上述的讨论表明,电力电子系统的扩散和载荷对公用电网,以及对他们的客户都有显著的潜在负面影响。减小这种影响的一个办法是过滤由电力电子负载产生的谐波电流和电磁干扰(EMI)。尽管其初始投资成本很小,但它是设计电力电子装置和防止或减少对这些谐波电流所产生的电磁干扰的首要原因。关于对效用接口和电力电子设备的设计从而来尽量减少这些影响，我们都在此进行讨论。谐波电流的换代在大多数电力电子设备中,如很多的开关型直流电源、不间断电源系统(UPS)和交直流电机驱动,交直流转换器，都被作为界面与效用的电压源。通常一个线频二极管整流桥（如图Fig.1-6A-2）是用来将直流转换成交流。整流器输出直流电压平均大小Ud是不可控的。一个大的滤波电容器用于整流器输出来减少直流电压Ud。直流电压Ud和直流电流Id分别是单极和单向的。因此,功率总是从交流侧输入直流侧。这些线频整流器与直流侧电容将在其他部分进行详细讨论。一个电力电子类系统利用线频交直流转换器作为效用界面。在这些转换器中详细讨论了直流输出电压,可控平均电压Ud,但是电流Id的大小和极性仍然是直流和单向的。由于可逆直流电压的极性,以及功率流通过这些转换器是可逆的。作为以上所指出,这一趋势只有与这些转换高的使用功率水平有关,如在高压直流输电系统。由于极高的功率水平，用来过滤谐波电流的技术和提高功率因数的运作有很大的不同,如前所述,这些转换器比那些在其他部分的线频二极管整流器更好。二极管整流器既具有单相、三相效用的电压是是用来接口的。典型的交流电流波形与最小的过滤波形在其他部分显示。单相输入典型谐波电流波形的特点列于表1-6A-1,那里的谐波电流Ih表示为基本电流的比值Il。如表中所示，电流波形表,包括大型谐波的大小。因此,对于一个有限的独立的内部各相源阻抗Ls,电压畸变的结果如表所示。内部源电感越高, Ls越容易产生电压畸变。谐波电流和功率因数我们讨论过功率因数的其他部分 ,在其中一个设备操作的积是流动比率Il /Is及其位移功率因数DPF:在例(1-6A-I)中，位移相等时的功率因数等于角1余弦值。流动比率Il /Is在例(1-6A-l)是均方根值之比的基本频率，电流分量的均方根值,总电流。如何有效地的显示设备功率因数的效用;当前制定的设备在低功率因数的运转下，对于给定电压和功率电平中将增大,这样如变压器、输电线路、发电机这些设备的二次利用率将大大提高。住宅以及办公室开发商已经意识到了提高功率因数的重要性并将其应用到电源的插座上以最大可能得提高获得的利益。例如,从120伏15A电流在一幢房子里,提供统一的最大可用功率是180千瓦的功率因数。最大功率抽取的电流不超过15A来限制减少功率因数。上述论据表明部分设备制造商和用户的职责和义务是设计电力电子装置使其在高功率因数的条件下运作。这要求在例子中位移功率因数应当较高。此外,在例子中谐波电流应该低于高电流比率。B：三相变频器对感应加热MOSFET桥逆变器介绍高频电源、基于MOSFET桥逆变器,已经广泛应用于感应加热领域。MOSFET的这些单元需要在直流输入电压400V情况下的才可以高效运行。得到的电源电压通常是通过使用一