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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!

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    \$\begingroup\$ See edits to answer. Is your transformer a real, physical transformer, or is this a thought exercise? \$\endgroup\$ – Li-aung Yip Dec 4 '13 at 13:01
  • \$\begingroup\$ It's a real transformer, but it's not yet installed. It's actually quite similar to the one you showed below. Thanks for your answers! \$\endgroup\$ – Stewie Griffin Dec 4 '13 at 13:08
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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.

enter image description here


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.


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".

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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.

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