Change taps on primary or secondary side of transformer?

Question

I have a 5500/380V Dyn11 transformer, with possible tap settings of 0.95, 0.975, 1.00, 1.025 and 1.05 on both primary and secondary side.

Due to high loading, I want to change the tap settings to either 0.975/1.00, or 1.00/1.025. However, I'm not sure on what side of the transformer I should change the setting.

My intuition tells me that it makes more sense to select the primary side, but I'm not certain.

1. One reason for changing the setting on the primary side is: The current is much higher on the secondary side, thus increasing the number of windings there will cause higher losses.
2. Another one: More windings might lead to saturation (I would however think that the transformer was designed so that this won't be a problem).

Does the transformer type play a role? Does it matter if the transformer is Dy, Yd, Dd, Yy etc? How will it affect the impedance?

Will there be any side-effects of selecting one or the other? Are there any reasons for choosing to change the settings on the secondary side?

Thanks!

Answer 1

In Australia, at least, the tap-changer is always on the HV winding. I don't recall ever seeing a transformer with the tap-changer on the LV winding.

I believe this is for economic reasons (it's cheaper or easier to build it this way). However I haven't looked this up so treat the previous statement with a grain of salt. The J&P Transformer Book, originally my Martin Heathcote, is all about the details of design, construction, and maintenance of power transformers and could probably tell you more.

To give a more concrete example, here is an example of a transformer nameplate showing the possible tap positions. (Serial numbers have been obscured to protect the innocent.)

Note that the nominal voltage ratio is 11,000 / 440 V and five taps of 2.5% are provided, two taps up and two taps down.

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You say your transformer has both HV tapchanger and LV tapchanger - is this a real, physical transformer, or a theoretical transformer? Having both HV and LV tap changer would be an extra expense and I am not sure if there would be any advantage to providing both.

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A quick skim of J&P Transformer Book (12e) §2.4 mentions that HV tap-changers imply operation at "constant flux density", while LV tap-changers imply operation at "variable flux density".

Answer 2

Your intuition is correct i.e. the tappings are provided on the primary side chiefly for these two reasons: (1) Current is higher on the secondary side and hence sudden changing of the tappings would lead to arcing problems. (2) The HV winding is wound over (outside) the LV winding to minimize the cost of insulation. This implies that the terminals on the primary (HV) are easily accessible compared to secondary.

Answer 3

Losses would be about the same on both sides, because the secondary has fewer turns of thicker wire. I would think arcing, if anything would be worse on the HV side if changing taps live unless the switch if oil filled. As far as DY YD DD YY goes, it makes no difference. The impedance line to line would be slightly higher for Y, as the voltage is made up of two vectors that are 120 deg. apart instead of 180, but I don't know what that has to do with the switching. In most countries (except Canada & Alaska where they run HV 3 phases & a neutral) all those big 3 phase transformers would be DY because the HV supply has no neutral. I would suggest changing primary taps to compensate for supply voltage variation, & secondary to compensate for loading of the transformer. If it's one or the other, do it on the primary, as the load causes resistance voltage losses in both the primary & secondary.