overheating bus-duct

[Boof87]

I have a 1750A bus-duct 4-wire WYE 240/415 50hz with a run of approx 70 meters supplying a mssb supplying 3 chillers in a high rise, which is overheating and melting the insulation the neutral is only rated at 100%. The data logger i have had on the ACB in MSB at last download reads 481.2 A @ .85 P.F, 522.3 A @ .85 P.F, 519.32 @ .87 P.F with a neutral current of 0. I've drawn the vectors and should have a neutral current of approx 84A on the neutral, the C.T on the neutral is rated at 3000A which normally doesnt register anything over 30 A. I have a N-E voltage at MSSB of 4.5 V and a current draw of 8.5A on the earth. The problem only occurs with all 3 chillers running which phase currents dont exceed 1000A. All chillers have VSD. Have had to tighten the pin connections which join the bus-duct. Could this be a problem with the neutral or should i be looking at high frequency harmonics from the VSD? am i anywhere near where i should be looking? Thanks in advance

 

[waross]

Loose connections may have led to heat corrosion damage in the neutral connections. If so, problems may persist even after tightening. Harmonics will be summing on the neutral.
Will your instruments indicate the high frequency currents and voltages accurately?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Boof87]

yer the data logger will, in the last week or two have had readings of 2.5% voltage distortion on the phases and between 150 and 175 % THD on the neutral at times, the C.T is accurate its just with low currents it seems to be a bit ordinary dont wanna put a smaller one on which is 1000A just incase current exceeds. the meter is a gridsense PM40 series. What about the high N-E voltage considering it is 0.334 at MSB? sorry im only an electrical technician and dont have a wealth of knowledge on these matters.

 

[rbulsara]

Perform a thermo-scan (IR scan) of the bus duct to find loose connections. Neutral to voltage does not overheat the bus. not the harmonics or loading appears to be the culprit.

Rafiq Bulsara

http://www.srengineersct.com

 

[Boof87]

thermal imaging was done on the entire bus, temperature was around 100'C this was how we discorvered the issue. problem is is that it gets that hot on the imager the whole bus glows gets very hard to determine a hot joint.

 

[rbulsara]

100 deg C is rather high, although a Class B insulation is rated for 130 deg C. If the temperature is uniform along the length of the bus and no individual hot spot is found, this may be a lack of ventilation issue. Check with the manufacturer as to what is their take on this.

Who performed the IR scan and what did their report say?

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Your neutral current may be returning on the bus enclosure rather than the neutral bus.

Bill
--------------------
"Why not the best?"
Jimmy Carter