The abbreviated notation of IEC 60076-1, such as (160 ± 10 × 1.5 %)/66 kV, gives the tapped winding, the number of steps and the size of each step. The principal tapping is at the centre of the range unless stated otherwise. The notation describes how the tappings are distributed. It does not say what voltage, current or power applies at each tapping.
Short answer
The tapping range notation of IEC 60076-1 is written with three pieces of data: the voltage of the tapped winding, the number of steps and the size of each step. With them you know how many tappings there are and how much the transformation ratio changes. You still do not know what voltage, current or power applies at each tapping. More on how to specify the rated and tapping quantities in rated and tapping quantities of a transformer.
Terms used by the standard
These terms are defined in clause 3.5.
| Term | Meaning |
|---|---|
| Tapping | A specific connection of the tapped winding, with a defined number of effective turns |
| Principal tapping | The tapping to which the rated quantities refer |
| Tapping factor | The ratio of the tapping voltage Ud to the rated voltage Ur. Given as a ratio or as a percentage |
| Plus tapping | A tapping with a tapping factor greater than 1 |
| Minus tapping | A tapping with a tapping factor lower than 1 |
| Tapping step | Difference, in percent, between the tapping factors of two adjacent tappings |
| Tapping range | Variation of the tapping factor, in percent, relative to the value 100 |
If the factor varies from 100 + a to 100 - b, the range is written +a %, -b %, or ±a % when a = b.
The tapping factor depends on Ud. Ud is the voltage that would be obtained at no load across the terminals of the winding, at that tapping, when rated voltage is applied to an untapped winding.
How it is written
Clause 5.1 of IEC 60076-1 gives this example: a transformer with a 160 kV tapped winding, with 21 tappings in total distributed symmetrically. It is written:
(160 ± 10 × 1.5 %)/66 kV
It reads in parts:
- 160: rated voltage of the tapped winding, in kV.
- ± 10: ten steps above and ten below the principal tapping.
- × 1.5 %: each step changes the tapping factor by 1.5 %.
- / 66 kV: rated voltage of the other winding.
The example gives these figures:
| Item | Value |
|---|---|
| Number of tappings | 21 (the principal, 10 plus and 10 minus) |
| Tapping range | ±15 % (10 × 1.5 %) |
| Minimum tapping factor | 85 % |
| Maximum tapping factor | 115 % |
| Rated ratio | 160/66 kV |
The transformation ratio at each tapping comes from clause 3.5.8. If the tapped winding is the high-voltage one, the tapping ratio is the rated ratio multiplied by the tapping factor. If it is the low-voltage one, you divide by the factor.
The principal tapping is at the centre of the range unless otherwise specified. The steps of the tapped winding are equal unless the design says otherwise. If the steps are not equal, the offer must say so (clause 5.4).
Asymmetric range
The standard also provides for a range that is not distributed symmetrically around the rated voltage. In that case the notation gives separately how many steps there are above and how many below. The example in clause 5.1 uses the same 1.5 % step and a different number of steps on each side.
What the notation does not say
Clause 5.1 makes this clear with a note. The abbreviated notation is only a description of the arrangement of the tapped winding. It does not imply actual variations of the voltage applied to that winding in service.
To complete the information, one of two things is needed (clause 5.2):
- A table with the tapping power, voltage and current for each tapping.
- A text that states the voltage variation category and the limits of the full-power tappings.
Those categories (constant and variable flux) are explained in the guide on CFVV and VFVV. The power side is in full-power and reduced-power tappings. To dig deeper into where to place the taps of an autotransformer, read autotransformer with taps at the neutral or the X terminal.
The standard also gives the rule for the nameplate: when the range exceeds ±5 %, the nameplate carries the full table (clause 7.2). See the power transformer nameplate.
Special cases in clause 5.1
- If the transformer has several windings and only one has tappings, what the clause says applies to the combination of the tapped winding with any of the untapped windings.
- In an autotransformer with tappings in the neutral, the number of effective turns changes in both windings at once. In that case the tapping data are a matter of agreement. The requirements of the clause are used as far as they apply.
What data to include when ordering a transformer with tappings
Clause 5.4 lists what must be in the enquiry and in the order:
- Which winding carries the tappings.
- Number of steps and tapping step (or range and number of steps).
- Voltage variation category and, if combined, the changeover point.
- Whether maximum current limitation applies (reduced-power tappings) and for which tappings.
It also allows two routes. The purchaser may specify all the data from the start. Or the purchaser may supply load cases with active and reactive power, with the direction of flow, and the corresponding voltages. The manufacturer then chooses the tapped winding and sets the quantities.
What not to do
- Do not read the notation as a list of service voltages.
- Do not assume the principal tapping is the maximum-voltage tapping. It is the reference tapping for the rated quantities.
- Do not ask for an asymmetric range without saying how many steps go on each side.
- Do not mix up the tap-changer (the device) with the range (the arrangement of the winding). The equipment is described in on-load tap-changer.
When to call us
Defining the tapping range and its category is part of the specification and of the voltage regulation study. At Tiza Group we do it with you. See our engineering design and studies service. To check the tappings of the equipment, we carry out power transformer testing and commissioning in Colombia, or elsewhere on request. Write to us and we will send you a quote.
Related guides
- On-load tap-changer
- Transformer turns ratio test
- Tap-changer dynamic resistance test
- Parallel operation of transformers with tap-changer
- Power transformer
- Autotransformer Taps at the Neutral or the X Terminal
- Rated and Tapping Quantities: How to Specify Them
References
- IEC 60076-1, Power transformers - Part 1: General, clauses 3.5, 5.1, 5.2, 5.4 and 7.2. Clause numbering of edition 2.1 (1993, with amendment 1 of 1999).
- IEC 60606, Application guide for power transformers.
Frequently asked questions
What does (160 ± 10 × 1.5 %)/66 kV mean?
The 160 kV winding has tappings, with 10 steps above and 10 below the principal tapping, of 1.5 % each. That makes 21 tappings in total. The rated voltage of the other winding is 66 kV. It is the example IEC 60076-1 gives in clause 5.1.
Where is the principal tapping?
At the centre of the tapping range, unless otherwise specified. It is the tapping to which the rated quantities refer. The other tappings are identified by their tapping factor.
Does the range notation tell me the actual voltage at each tapping?
No. The standard makes clear that the abbreviated notation only describes the arrangement of the tapped winding. It does not indicate the actual variations of the voltage applied in service. That is defined by the tapping quantities (clauses 5.2 and 5.3).