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1、1 1、 外文原文（复印件）外文原文（复印件）A: The Utility Interface with Power Electronic SystemIntroductionWe 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 oper
2、ation 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 harm
3、onic 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
4、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 ar
5、e 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
6、-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 si
7、gnificant 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 i
8、nitial 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 disc
9、ussed here.Generation of Current HarmonicsIn 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 recti
10、fier 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 I
11、d 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
12、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
13、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
14、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 filteri
15、ng 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. Ther
16、efore, 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 FactorAs 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
