barcode generator in vb.net free download Transient Overvoltages Transient Overvoltages 161 in Software

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Transient Overvoltages Transient Overvoltages 161
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471 Nuisance tripping of ASDs
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Most adjustable-speed drives typically use a voltage source inverter (VSI) design with a capacitor in the dc link The controls are sensitive to dc overvoltages and may trip the drive at a level as low as 117 percent Since transient voltages due to utility capacitor switching typically exceed 130 percent, the probability of nuisance tripping of the drive is high One set of typical waveforms for this phenomenon is shown in Fig 440 The most effective way to eliminate nuisance tripping of small drives is to isolate them from the power system with ac line chokes The additional series inductance of the choke will reduce the transient voltage magnitude that appears at the input to the adjustable-speed drive Determining the precise inductor size required for a particular application (based on utility capacitor size, transformer size, etc) requires a fairly detailed transient simulation A series choke size of 3 percent based on the drive kVA rating is usually sufficient
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472 Transients from load switching
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Deenergizing inductive circuits with air-gap switches, such as relays and contactors, can generate bursts of high-frequency impulses Figure 441 shows an example ANSI/IEEE C6241-1991, Recommended Practice for Surge Voltages in Low-Voltage AC Power Circuits, cites a representative 15-ms burst composed of impulses having 5-ns rise
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480-V Bus Voltage (phase-to-phase) 1500 1000 500 Voltage (V) 0 500 1000 1500 333
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667 Time (ms)
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Figure 440 Effect of capacitor switching on adjustable-speed-drive ac current and dc
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Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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Transient Overvoltages 162 Four
ac Drive Current during Capacitor Switching 300 200 100 Current (A) 0
100 200 300 333
667 Time (ms)
dc Link Voltage during Capacitor Switching 750
Current (A)
500 333
667 Time (ms)
(Continued)
times and 50-ns durations There is very little energy in these types of transient due to the short duration, but they can interfere with the operation of electronic loads Such electrical fast transient (EFT) activity, producing spikes up to 1 kV, is frequently due to cycling motors, such as air conditioners and elevators Transients as high as 3 kV can be caused by operation of arc welders and motor starters The duration of each impulse is short compared to the travel time of building wiring, thus the propagation of these impulses through the
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
Transient Overvoltages Transient Overvoltages 163
1000V
1000V
5000 V/div VERTICAL 50 ms/div HORIZ
400V
400V
200 V/div VERTICAL 1024 s/div HORIZ
Fast transients caused by deenergizing an inductive load
wiring can be analyzed with traveling wave theory The impulses attenuate very quickly as they propagate through a building Therefore, in most cases, the only protection needed is electrical separation Physical separation is also required because the high rate of rise allows these transients to couple into nearby sensitive equipment EFT suppression may be required with extremely sensitive equipment in close proximity to a disturbing load, such as a computer room High-frequency filters and isolation transformers can be used to protect against conduction of EFTs on power cables Shielding is required to prevent coupling into equipment and data lines
473 Transformer energizing
Energizing a transformer produces inrush currents that are rich in harmonic components for a period lasting up to 1 s If the system has a parallel resonance near one of the harmonic frequencies, a dynamic overvoltage condition results that can cause failure of arresters and problems with sensitive equipment This problem can occur when large transformers are energized simultaneously with large power factor correction capacitor banks in industrial facilities The equivalent circuit is shown in Fig 442 A dynamic overvoltage waveform caused by a thirdharmonic resonance in the circuit is shown in Fig 443 After the expected initial transient, the voltage again swells to nearly 150 percent for many cycles until the losses and load damp out the oscillations This can place severe stress on some arresters and has been known to significantly shorten the life of capacitors
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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