I have a 150kva Y-Y 3phase transformer (12470/7200v - 480/277v) thats currently in service. When looking at the meter logs I see 300-400 amp spikes on the secondary side (This transformer serves irrigationn pumps) for 1-2 hour durations, sometimes even longer. Now my first though is that i need to upsize this transformer to a 300kva or 500kva, but when looking at the peak demand at the meter, it reads around 160kw or so each month. Why dont I see the 300-400amp spikes in the demand calc at the meter? How is this demand calculated (I know its calculated by using kwh, but I thought i would see the 300-400 amp peaks in this calculation?)? Should I upsize this transformer based on the 300-400amps the irrigation pumps pull or am i okay since the peak demand calc at the meter reads 160kw peak. The meter is on a 15min interval. The CT ratios for metering is 200/5.
Transformer Peak Demand Clarification
- Thread starter kel037
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- Wisconsin
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- PE (Retired) - Power Systems
The spike is averaged over the demand interval. A single spike lasting 30secs would hardly show up when averaged over 15 min.
gadfly56
Senior Member
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- New Jersey
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- Professional Engineer, Fire & Life Safety
When he says "1-2 hour durations", I'd think the CT's would catch that.
Yes. This is exactly what I thought as well.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
1-2 hour duration is not a spike.
It is an overload condition if a pump is drawing 300A, on a circuit with 200A CTs.
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Thanks for clarifying that. Why wouldnt this show up in the demand interval if the overload condition lasted at least 1-2 hours?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
It might, assuming the over saturated CT output was in excess of 7.5A.
But morr likely the CTs would fail thermally.
CTs will have a rating factor. If his are 1.5@55 he should be good to go.
I have them now with a 3 rf.
The 160kW demand is what you should base transformer size on.
160kW with a 150 kVA xf. It will hold for a long time if the loads aren't constant and the XF can cool between loadings.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Your are probably utility grade.
For some reason most of the building management systems, I have seen, use cheap CTs particularly rope style for existing equipment.
Those are the ones with about a 3 rating factor at 55c.
no one should use the 30C rating…
Ahh. Rope CTs… I only have those on the Dranetz..Your are probably utility grade.
For some reason most of the building management systems, I have seen, use cheap CTs particularly rope style for existing equipment.
I will double check this.
The transformer was installed in 79 (Wayyy before my time) and has suprisngly held since then.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I’m thinking this is a semantics issue. The irrigation pumps RUN for 1-2 hour periods, during which his logging meter records the spikes, but the spikes do not LAST the entire 1-2 hour period. If his meter is set to Peak Read, then the motor starting current would show the highEST current during that cycle. If it is set up to reset if the current drops to zero, which it would when the pumps are off, then the next pump cycle will again read the peak current in that next cycle.
This is based on the fact that this is a 150kVA transformer, so if feeding pump(s), the pump size is likely not over 125HP (should be even smaller unless there are Soft Starters or something). 300-400A on even 125HP would be causing overload relay trips in a few minutes, not 1-2 hours.
This is based on the fact that this is a 150kVA transformer, so if feeding pump(s), the pump size is likely not over 125HP (should be even smaller unless there are Soft Starters or something). 300-400A on even 125HP would be causing overload relay trips in a few minutes, not 1-2 hours.
Correction: It was installed in 89.
I agree with @Jraef here. It is unlikely that the said "spike" lasted for hours, the circuit breakers I know of will trip in minutes (e.g. 225AT circuit breaker will trip at around 6 minutes with an overcurrent of 400A or 222%.
Even with a load that has a high X/R ratio (say X/R = 30), the worst saturation amperes limit will likely be around 600A, IMO.
Even with a load that has a high X/R ratio (say X/R = 30), the worst saturation amperes limit will likely be around 600A, IMO.
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