Transformer Tap setting
Transformer Tap setting
(OP)
Hi,Everybody,
I am running ETAP motor starting study. I set the fixed tap at -10% on Primary side,in the mean time, I set up Auto LTC at 100%. In this case, I got the minimum voltage dip compared to other cases. Does the fixed tap setting affect the voltage dip while the Auto LTC is also in service? According ETAP, it does. But I am not sure it is a real situation. I don't have much experience in transformer setting. Can anybody here explain this to me better?
Thanks a lot in advance.
I am running ETAP motor starting study. I set the fixed tap at -10% on Primary side,in the mean time, I set up Auto LTC at 100%. In this case, I got the minimum voltage dip compared to other cases. Does the fixed tap setting affect the voltage dip while the Auto LTC is also in service? According ETAP, it does. But I am not sure it is a real situation. I don't have much experience in transformer setting. Can anybody here explain this to me better?
Thanks a lot in advance.






RE: Transformer Tap setting
RE: Transformer Tap setting
RE: Transformer Tap setting
RE: Transformer Tap setting
RE: Transformer Tap setting
RE: Transformer Tap setting
If you set the LTC to a high voltage, you can keep the value of the voltage during the dip higher. The extent of the sag is the same, but by starting at a higher value you end up at a higher value as well. This might be helpful if you're trying to help voltage sensitive equipment ride through the event. We've done this to help a voltage sensitive customer ride through fault induced voltage sags. They see fewer shutdowns with LTCs set to deliver voltage at 105% of nominal than they did at 100%.
Load and no-load taps do affect the impedance, but the effect is small and usually neglected.
RE: Transformer Tap setting
RE: Transformer Tap setting
Thank you for the response.What I am trying to do is to reduce the possible voltage dip when starting a motor in the event that the system is weak.I did some simulation tests by ETAP software. With Manual LTC and Auto LTC working simutaneously, I can achieve the lowest voltage dip. But I am not sure about the time delays of LTC (initial and operating).What would the real picture look like?
RE: Transformer Tap setting
RE: Transformer Tap setting
I do have a softstarter-VFD for 2 motors(5000 HP), but I am thinking about arcoss- line starting under emergency situations in the event of VFD failures.
RE: Transformer Tap setting
Autotransformers typically have less impedance than 2 winding transformers.
RE: Transformer Tap setting
Tap changers are electro-mechanical devices operated by a hand wheel or by a motor – both inherently slow operations. An automatic tap changer controller has a dead band and a time delay that would prevent it from automatically compensating for the motor voltage drop. The controller could not sense the dip and get the transformer to react in time. You would need a response time of milliseconds, but have a system that responds in minutes.
If you are worried about starting without the VFD/Soft Start, buy two of them so you have a spare, or install two motors with two VFD's. Otherwise, just live with the voltage dip on across the line starting. Use the methods suggested above to minimize the dip. I once wrapped rope around a large fan’s shaft and had a tractor to pull the rope to get the fan spinning before we hit the motor with full voltage. That didn't reduce the initial voltage dip, but it shortened the acceleration time and length of the dip to within the motor’s limits.
Good lcuk with your design.
RE: Transformer Tap setting
Did you by any chance video this episode? I would love to see it: my safety department would be in apoplexy!
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Experience is what allows us make a completely new mistake every day!
RE: Transformer Tap setting
Problem was, they wrapped the rope in the wrong direction, spun the fan backwards and tripped on inrush. Second attempt worked better. The rope trick reduced the acceleration time about 5 seconds, enough to keep below the rotor or transformer damage curves.
Would have made a good “Things Not To Do” video. Plant went out of business a few years ago. (Thank goodness!)
RE: Transformer Tap setting
RE: Transformer Tap setting