Transformer Nameplate 1

[budhiman]

Hello Everyone,

I came across this transformer nameplate which shows 4 impedance values. My interpretation is a combination of voltage and mVA has a corresponding %Z. Has anyone seen this before and have any idea why four impedances?

 

[prc]

It is a multi LV Transformer 66/34.5-12.47 kV and impedance will change at different voltage ratios. In the impedance table the voltage ratios and MVA shall be shown. In this case it is difficult to guess.

 

[mgtrp]

The four impedances look to be:
10 MVA / 69 kV-34.5 kV: pos = 5.04%, zero = 5.04%
12.5 MVA / 69 kV-34.5 kV: pos = 6.31%, zero = 6.30%
20 MVA / 69 kV-12.47 kV: pos = 9.71%, zero = 9.86%
25 MVA / 69 kV-12.47 kV: pos = 12.28%, zero = 12.32%

The 34.5 kV impedance is almost half that of the 12.47 kV impedance because of the change in base.

The 12.47 kV winding is made up of three equal winding sections that are connected in parallel. These same winding sections are connected in series to give 37.41 kV. A small section of one of the winding sections is left out of the circuit so that you get 34.5 kV rather than 37.41 kV as the nominal voltage. This small missing section is why the 34.5 kV impedance is not quite half of the 12.47 kV impedance.

 

[7anoter4]

I don't think the short-circuit impedance is connected to secondary voltage but only with the rated power. If you'll take a look in IEC 60076-5 Power transformers –Part 5: Ability to withstand short circuit you'll see in Table 1 Recognized minimum values of short-circuit impedance for transformers with two separate windings the only parameter required it is the rated power kVA. So for 20-25 MVA minimum 8-10% is recommended. For 10-12.5 MVA -since these are the same windings connected parallel-and only two windings when for 20-25 MVA 3 series windings are used- it will be one half about, indeed.

 

[prc]

7anonter4,Generally your view is correct. But here, certain sections are kept floating during 12.47 KV connections ie there is no current flow in those sections. This changes the spatial ampere turn distribution and flux fringes out at those gaps, increasing the impedance.

 

[7anoter4]

Thank you, prc.I agree.

 

[Mbrooke]

The IEC usually uses the highest values rather then the base.

 

[bacon4life]

What is the difference between the 10 and the 12.5 rating? In the connections table, they are both listed as OA 55 degrees C rise. However, near the top of the nameplate the TYPE is listed as OA/FA.

 

[prc]

Generally in IEC world, percentage impedance is always mentioned on FA rating while in US it is based on OA rating. In this case NP shows impedances both at OA &FA ratings,but with out clearly indicating the base MVA or voltage rating. Indications given by mgtrp seems correct.