Aboutcleggsan Expertise All technical areas of Electronics Engineering.
Experience BSEE, MBA, Design, R&D, University Research.
Senior Life Member of IEEE. Life Fellow of AES.
Organizations IEEE, Consumer Electronics Society, Audio Engineering Society.
Broad teaching experience; work experience mostly in consumer electronics and conversion from analog to digital technologies. Pioneer in digital audio at all levels.
Question I have a customer with a strange problem.
In a high-rise hotel he has a 480v buss riser. On every sixth floor he has a 150-kva 3 phase transformer serving the 120v 3 phase loads in each room and the hallways. Each transformer has 2 disconnects fused at 300 amps. Each disconnect serves 3 panels. 3 on the west and 3 on the east.
Each panel serves 2 floors.
The transformers are overheating.
I took an amp reading with a true RMS meter. The readings were taken in each disconnect switch. A Phase is 15 amps on average. B phase is 18 amps on average. C Phase is 199 amps on average.
The 480 primary readings were, A phase 100A. B phase 100A. C phase 15A.
The 480V buss riser continues to the pent house and feeds chillers and mechanical equipment.
This is happening at 3 locations on 3 different transformers, on the same 480 V buss riser. The loads are all incandescent lighting and receptacles.
The hotel has a 30% occupancy so the loads on A and B phase seem normal.
What is happening with the C phase high currents?
Rick Munch
Frischhertz Electric Co.
New Orleans, LA
Answer Well, you may have confused the readings OR the lines are mismarked and the A phase and C phase are really the same, in which case there is an excessive load on the C Phase that is pulling 199 amps (average, as you reported).
In any event, there is a heavily imbalanced loading condition which needs to be looked at right away - which you are doing. And it is no wonder they are overheating with loads like you reported.
I have no clue what to tell you, but I would suspect shorted turns within one of the transformer windings; shorts on the line somewhere; or some equipment that is drawing the currents and the readings are accurately reporting the heavy draw. It may be worth going around to the possible load lines and checking every piece of equipment hanging on the lines. You mentioned refrigeration equipment - they are notorious for failures of this kind.
Wish I could give you some better ideas.
Good Luck.
I am very curious. Could you let me know the outcome when you find the solution?
