Are tap changers required?

[Mbrooke]

Are automatic tap changers still recommended for a 10 or 12% impedance, 132 or 66 to 22kv distribution transformer where source voltage does not deviate beyond 1.00pu?

 

[Marmite]

Does the magnitude of the load vary with time? Do you have to maintain the secondary voltage within statutory tolerances? Does the power factor of the load vary?

If so then you need a tap changer.

Regards
Marmite

 

[Mbrooke]

Loading does vary, between 10% to 75%, and up to 105% during a contingency (such as transformer failure in another substation)

My understanding this comes from the naturally high impedance of the transformer itself?

 

[bacon4life]

Where to you find a source voltage with less than 0.5% voltage variation?

We have a number of 115/13.8 kV transformers that do not have LTC's. We usually maintain the transmission voltage within a tight tolerance, but we think of tight as 1 to 2%. Even if your voltage is 1.00 during normal conditions, you should also consider the impact of abnormal conditions. In our area without LTCs, the most limiting factor is low voltage rather than the thermal rating of components.

 

[jghrist]

In my area, line regulators are much more common on distribution than LTC. Regulation of the distribution voltage, by either line regulators or LTC is always done to maintain proper customer service voltages.

 

[Mbrooke]

The tight boundaries in question come by distribution transformers fed directly from or close to sub-transmission transformers, or in some cases being right after line regulators.

For example, on board tap changers maintain voltage close to 1.00pu on all 33kv and most 66 kv sub transmission busses. Anything attached to these busses always sees + or - 1.5% at most. In some 33kv lines pole mounted regulators maintain the voltage close to boundaries because commercial and industrial customers tap directly from the sub transmission system. The pad mounts have no automatic tap changers so its essential the LV output be within +or- 6%. As a result, when distribution substations tap immediately after these regulated lines voltage is considered to be within bounds.

IMO the primary voltage is not a concern in itself, however the naturally high impedance of the transformers (10 to 12% to limit fault current) is raising concerns. My understanding is that voltage regulation is excellent in low impedance transformers, but less so in higher impedance versions.

 

[Mbrooke]

@jghrist: These is exactly one scenario at hand. On your none LTC transformers, assuming a PU voltage of 1.00, how much does the voltage vary between light load all the way up to full load?

 

[crshears]

In my experience customers grouse about the immediacy of low or high voltage far more than they do about the much more intangible concept of their equipment sustaining damage under fault conditions.

Given the relatively wide load range you're suggesting, I'd much prefer to have the regulators in place.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[tinfoil]

1) Have you done the math on the delivered secondary voltage delivered when the load can vary from 10 to 105%? A 12% xmfr will see 'regulation' across this range, even if the source was really 1.00 p.u. all the time. (BTW, although the relationship is not quite linear, higher %Z implies greater internal voltage regulation with varying load)

2) Have you added in the +- 1.5% source variation you mentioned to the worst case of (1)?

3) Is there ANY possibility of a variance from this source of more than 1.5% under THEIR emergency conditions? What is their ALLOWABLE range rather than the range you usually see? Did you allow for this in your answer to (1)?

4) Is there a GUARANTEE that the supplier's regulators will always be present and functional, or is that simply an artifact of their present configuration?

5) Is there any possibility that you could be supplied from a more distant source under planned maintenance or emergency contingency plans? Have you allowed for the worst case from that?

6) Is there any chance that you may want to move these units somewhere else in your system where a tap-changer would be needed?

7) Does your organization have to match specs on their xmfrs for 'spares' purposes? Do these other xmfrs have tap-changers?


If you are satisfied with the answer to all of these and still do not see a need for tap-changers, than maybe you can do without.

 

[jghrist]

If you have distribution feeders with significant voltage drop, most utilities would want to maintain maximum allowable voltage at the beginning of the feeder. This generally means setting the regulated voltage to 1.05 pu (actually less to allow for regulator bandwidth). You need regulators to do this even if the unregulated voltage is 1.0 pu without any variation.

 

[Mbrooke]

1) Have you done the math on the delivered secondary voltage delivered when the load can vary from 10 to 105%? A 12% xmfr will see 'regulation' across this range, even if the source was really 1.00 p.u. all the time. (BTW, although the relationship is not quite linear, higher %Z implies greater internal voltage regulation with varying load)

Hmmm, I never thought like that. Why would higher Z equal more voltage regulation? I was always under the impression that a lower Z would mean less fluctuation... kind of like a pole pig that can take a 300% overload with minimal voltage drop.

Crunching the numbers as long as the source output is within +or- 1%, it will still yield an expectable bandwidth from light to full load.

Normally I wouldn't count on feeders to have voltage regulation within range but because the feeders here are fairly short voltage drop will be limited.

2) Have you added in the +- 1.5% source variation you mentioned to the worst case of (1)?

Not to the primary side no, just the secondary.

3) Is there ANY possibility of a variance from this source of more than 1.5% under THEIR emergency conditions? What is their ALLOWABLE range rather than the range you usually see? Did you allow for this in your answer to (1)?

Possible, yet unlikely due to reliability standards. The source has its own tap changing transformer (the life saver here) which keep a pu close to 1.00, however under serve line outages a voltage drop is not impossible.

4) Is there a GUARANTEE that the supplier's regulators will always be present and functional, or is that simply an artifact of their present configuration?

I will have to think about this, another good point, but if it gets to the point where the supply transformer taps fall out of range it would indicate a very severe contingency. Now that you say it, it might be just an artifact to some degree.

5) Is there any possibility that you could be supplied from a more distant source under planned maintenance or emergency contingency plans? Have you allowed for the worst case from that?

Loss of all sources would cause an outage, but as far as I know under all known contingency (except voltage collapse) pu is maintained from where the transformers will tap from.

6) Is there any chance that you may want to move these units somewhere else in your system where a tap-changer would be needed?

This is a valid point. While there is usually no immediate hurry to move units, in the future there might be.

7) Does your organization have to match specs on their xmfrs for 'spares' purposes? Do these other xmfrs have tap-changers?

Not usually, its generally done case by case, but going back to #6, if a unit is ever moved under emergency situations the one its replacing could end up being a tap changer type.

If you are satisfied with the answer to all of these and still do not see a need for tap-changers, than maybe you can do without.

Most satisfied but you do make points that are well worth re-evaluating in depth.