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POWER QUALITY phần 9

BẢNG 7,1 Tần số phân loại Tần số phân loại ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF Dải tần số 3-30 Hz 30-300 Hz 300-3000 Hz 30-30 kHz 30-300 kHz 300-3000 kHz 3-30 MHz 30-300 MHz 300-3000 MHz 3-30 GHz 30-300 GHz | TABLE 7.1 Frequency Classification Frequency Classification Frequency Range Application ELF 3-30 Hz Detection of buried objects SLF 30-300 Hz Communication with submarines electrical power ULF 300-3000 Hz Telephone audio range VLF 3-30 kHz Navigation sonar LF 30-300 kHz Navigation radio beacon MF 300-3000 kHz AM maritime radio HF 3-30 MHz Shortwave radio citizen s band VHF 30-300 MHz Television FM police mobile UHF 300-3000 MHz Radar television navigation SHF 3-30 GHz Radar satellite EHF 30-300 GHz Radar space exploration Source Cheng D. K. Fundamentals of Engineering Electromagnetics 1st ed. Prentice Hall Upper Saddle River N.J. 1993. With permission. 7.5.9 Bandwidth Bandwidth commonly refers to a range of frequencies. For example in Table 7.1 a bandwidth of 300 kHz to 300 MHz is assigned to radio broadcast and marine communication. Any filter intended to filter out the noise due to these sources must be designed for this particular bandwidth. 7.5.10 Filter A filter consists of passive components such as R L and C to divert noise away from susceptible equipment. Filters may be applied at the source of the noise to prevent noise propagation to other loads present in the system. Filters may also be applied at the load to protect a specific piece of equipment. The choice of the type of filter would depend on the location of the noise source the susceptibility of the equipment and the presence of more than one noise source. 7.5.11 Shielding A metal enclosure or surface intended to prevent noise from interacting with a susceptible piece of equipment. Shielding may be applied at the source if the source is known or at the susceptible equipment. Figure 7.5 illustrates the two modes of shielding. 7.6 POWER FREQUENCY FIELDS Power frequency fields fall in the category of super low frequency SLF fields and are generated by the fundamental power frequency voltage and currents and their harmonics. Because of the low frequency content these fields do not easily interact with .