【導讀】無功功率補償畢業(yè)論文中英文資料外文翻譯文獻

　　

【正文】 d increase in voltage distortion. Harmonics and Power Factor – Displacement and Total As discussed, harmonic distortion and power factor correction are seldom considered as separate topics. This is due to the dramatic effect on system impedance at harmonic frequencies that can result from the addition of conventional power factor correction capacitors. The relationship, unfortunately, does not end there. Due to their nonlinear nature, the presence of harmonic loads can sometimes fool the power engineer into considering unnecessary power factor correction in the first place! Power Factor of a PWM Drive – An Extreme Example? The pulsewidthmodulated (PWM) variablefrequency drive (VFD) produces a characteristic current waveform when energized from a sinusoidal voltage source. Thisthreephase device produces a voltage and current waveform for one phase that resembles the following graphic: If the power parameters (real, reactive, and apparent) associated with this PWM device are measured with a truerms meter, the typical values would show a relationship of real (kW) to apparent power (kVA) of approximately . The engineer might conclude from this knowledge that the power factor of the device is poor, and that a circuit containing many of these PWM drives (not unmon) would require power factor correction capacitors. Unfortunately, this line of reasoning is incorrect and can lead to disastrous results. While the kW/kVA relationship indicated above is accurate, is not the power factor of the device. At least, it is not the plete picture of the power factor. Further measurements would reveal that the displacement angle between voltage and current for this device is 0. That is, the current and voltage are in phase with each other. Or, more accurately, the fundamental (60Hz) ponent of voltage and the fundamental (60Hz) ponent of current are in phase, as shown below. Since harmonic loads like PWM drives are able to consume power in a nonlinear fashion。 that is, by turning on and off in a manner not proportional to the applied instantaneous voltage, their kW/kVA relationship is not equal to the phase angle between fundamental voltage and current. This peculiarity, in fact, has required the establishment of two power factor definitions. These two power factors are equal for undistorted (sinusoidal) voltages and currents Displacement Power Factor (dPF) – Cosine of the phase angle between fundamental voltage and fundamental current. Total (sometimes referred to as “True”) Power Factor (tPF) – Real power (kW) divided by apparent power (kVA). Power factor correction capacitors primarily affect the displacement power factor for a circuit. If PFC’s are applied on a circuit that already has a high dPF, then the fundamental current ponent could be shifted into a leading relationship to fundamental voltage. This situation can result in voltage regulation and distortion problems for the circuit. In addition, the addition of large PFC’s on a PWM circuit can also increase the likelihood of harmonic resonance problems, and the resulting excessive voltage distortion issues introduced earlier. Harmonic Mitigation – Two Passive Techniques Harmonics Attenuation The earlier voltage and current distortion discussion, and voltage distortion estimates assume that the current distortion remains unchanged regardless of the circuit impedance, but this is not entirely true. Harmonic current distortion is affected by the amount of circuit impedance. In fact, an engineer will discover that placing the same harmonic producing load at two different nodes in a power system will result in two different levels of load current distortion. Power system designers can utilize this effect, called attenuation, as one method of passive harmonic mitigation. The current waveforms below show the effects of introducing a series line reactor (“choke”) at the terminals of a 100 hp pulsewidth This attenuation effect is often employed to reduce the harmonic distortion associated with threephase ASD’s. The ASD operation is not adversely affected, provided the line reactor chosen for the application does not exceed about 5% impedance (relative to the drive base). Harmonics Cancellation In addition to attenuation, harmonic current distortion can be reduced by cancellation. Cancellation occurs because individual harmonic ponents of a distorted current are affected differently when passing through normal power system transformers. The magnitude of harmonic currents, like the 60Hz ponent, increases or decreases consistent with the transformer turns ratio. The phase angle of harmonic ponents, however, is influenced by the type of connection of the three phase transformer. The 5th and 7th ponents, for example, experience a 30176。 phase angle shift through a power system transformer connected deltawye, as pared with the same current ponents transmitted through a wyewye or deltadelta connected transformer. This phaseangle effect can be used with multiple ASD’s to reduce the current distortion on the circuit feeding the drives. As demonstrated in the diagram below, the alternating bination of deltawye and wyewye connections can produce much lower harmonic distortion for similarlysized and similarlyloaded drives. The bination of line reactors and deltawye transformers produces a similar cancellation effect. Common Harmonic Current Signatures IEEE 519a (Draft) Table Despite the preponderance of electronic loads, there are surprisingly few categories required to characterize the major harmonicproducing devices in industrial and mercial facilities. Electronic machines that share similar rectifier configurations create similar characteristic harmonic current signatures, as the table below demonstrates.