7 1/2% vs. 10%, +/- LTC, pros and cons?

[EddyPach]

Can somebody tell me why a +/- 7 1/2 % on-load tap changer should be prefered versus a +/- 10% one? For some reason the last one seems more reasonable. It is required in the LV side of a 230 -115 kV Autotransformer.
Thanks for the advise.

 

[stevenal]

+/- 10 is ANSI standard. See C57.12.10.

 

[dpc]

The only advantage to the smaller range would be lower cost, I think. But that is likely to be a small amount.

As mentioned by stevenal, the +/- 10% is standard for a load-tap changer. If you want to limit the tap range for some other reason, your LTC controller should be able to do that.

I don't see much downside to the standard +/- 10%

 

[electricpete]

Stevenal is right... 2 2.5% taps above and 2 2.5% taps below is standard.

When you ask why 7.5% might be preferable in some applications or more expensive, I can offer two ideas:

1 - If you still have 5 taps total spanning 7.5%, then each tap is roughly 7.5%/4 =1.875% gives a little finer adjust capability (in case where it is known that extreme variations of 10% won't be encountered?).

2 - The 7.5% transformer might be more expensive precisely because it is non-standard... special order?

just thinking out loud.

 

[gordonl]

I think there is a little confusion here. Is the standard +- 10% or +-5%. My experience is with off load tap changers, but we purchase +- 5% as described by electricpete and haven't ever needed a larger range. But for your case the range you require is going to depend on the level of swings present in your system, buying the reduced range could save a small amount in the short run, but could be a large problem if you require the tap in the future for regulation.

Adding the taps which increase the secondary voltage are more more expensive because they add turns to your secondary winding for the secondary tap changer, so you could possibly use +7.5% -10% to save some cost. (Reduced MVA at lower taps)

 

[electricpete]

gordon - you're right, I was very confused. Two 2.5% taps above and below is 5% above and below (as my seven-year old would say... duhh!)... which is the standard for off-load tapchangers.

For LTC's it is 16 taps*5/8% per tap in each direciton gives 10%.

 

[dpc]

The original question was about **load** tap changers. The ANSI standard is +/- 10%, but you can get whatever you want, at a price. I don't know what the IEC standard range is, or if there is an IEC standard for this.

For the no-load (or off-load) tap changers, I think we all agree the ANSI standard is +/- 5% with 2 taps above and 2 taps below. But other arrangements are often used. It's not unusual to specify more taps below the nominal rating than above since most transformers end up set below nominal to boost secondary voltage anyway.

These are really different beasts. The no-load tap changer is actually changing the number of turns in the primary winding. The on-load tap changer is generally a separate auto-transformer (essentially) connected to the LV winding output.

 

[EddyPach]

Thank you all.

 

[prc]

My comments are limited to EddyPach'squestion:
Yes +/-10%tappings are more sensible for225-110 KV autotrf and it will not affect the cost .Depending on system requirements there can be OLTC for +25/-5% tappings too!