13,8 kV Bus Ducts Typical failures 2

[ahung]

We have five SW Gas turbines 100 MW each one. This GT operate at base load condition in simple cycle. Recently we do a termographic inspection on 13,8 kV bus ducts (ISO phase BUS)and we found some bolts of bus duct supports with very high temperature according with the termographic report. We think this phenomena its not normal and we need receive some advice to take action before any injuries or consequences due to a failure.

 

[ScottyUK]

Isolated phase bus ducts rely on both the inner busbar AND the outer casing being electrically contiguous. Normally this is achieved using welded joints between sections. The outer casing carries an anti-phase 'image current' of equal magnitude to the main bus current. This is the property which gives IPB a very low external magnetic field because of flux cancellation. Flux cancellation is almost perfect on straight sections of bus, but is not as effective at discontinuities like elbows and tees. At the ends of the bus duct bank - typically at transformer bushings or at generator line connections - the three single-phase ducts are tied together with massive bonding bars. These bars carry significant current, and they are integral to the function of the IPB. Sections of IPB beyond the bonding bars do not benefit from image current cancellation: typically these are bushing cubicles and generator teminal chambers. These structures are isolated from the IPB using rubber bellows. The external flux in these areas can be very large.

IPB is supported on isolating pads which separate the casing from the structual steel supports. If one of these pads is missing or is bridged, image current will flow in the supporting steelwork causing I²R heating in the steel. The bolted joints are the highest resistance part of such structures, so they experience the greatest heat rise.

I suggest you check the isolating pads and the cross bonding is correctly installed and that no weld fractures or loose bolting are causing the return path for the image current to have a higher than normal impedance. It is also important to remember that flux cancellation is not perfect, and that steelwork in the vicinity can get heated by eddy currents induced in it. This is particularly bad in the immediate vicinity of bushings etc outside the cancellation zone. Any loop formed by steelwork in these areas will experience eddy current heating. Distance is the best cure.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[fsmyth]

Whoa.. Sounds like coax with massive currents.
Note that the current will have a null at 775 miles,
so it should be safe to ground it there.
Assuming no reflections.
<als>
P.S. just being silly. No response required.

 

[ScottyUK]

fsmyth,

Just for your interest - continuous currents in excess of 20kA are common, as are voltages up to ~30kV. Prospective fault current between phases in excess of ~300kA are equally common. The virtual elimination of phase-phase faults is a large benefit of IPB: typically generators are impedance earthed with phase-ground fault current limited to 10A or so to protect the generator core, and a phase-ground fault is by far the most probable failure mode for an IPB.


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One day my ship will come in.
But with my luck, I'll be at the airport!