Self-study notes. This page is static reading. It does not ask you to sign in. It does not certify a product, a factory, or a person. Use the edition of each standard named in the contract. Questions: hello@transformerpath.com.
This module is the first of four reading notes. It covers the documents that fix a transformer before any steel is cut: the enquiry, the rating the nameplate will have to carry, the vector group, and the tap changer. Later modules cover factory tests, the checks before the first voltage, and how a dissolved-gas trend is read against a frequency-response baseline. Nothing on these pages is scored, and nothing on them is withheld until another page is finished.
Read the enquiry before you accept the rating
A manufacturer cannot design from a system voltage and a power figure alone. IEC 60076-1 is the general requirement for power transformers. IEEE C57.12.00 is the general requirement used for many liquid-immersed units in IEEE practice. The enquiry should name one of those documents, and the edition, as the base, and then list the departures. A sentence that says the unit shall meet IEC and IEEE names neither document. Temperature rise, test code, and tolerances then stay ambiguous.
Write the rated power of each cooling stage, not one number with the fans assumed. A line that says the transformer is 40 MVA, with no cooling stage beside it, is incomplete. That 40 MVA figure is a power rating. It is not a distribution-transformer class, and IEC 60076-1 does not assign it a standard impedance band. Impedance is a value the enquiry states, at a stated power and a reference temperature, because it sets fault current and voltage regulation.
State the high-voltage, low-voltage, and any tertiary voltages. State the highest voltage for equipment, Um, for each winding. IEC 60076-3 uses Um, not the rated voltage, when insulation levels are selected. State the lightning-impulse withstand, the switching-impulse withstand where that voltage calls for it, and the short-duration power-frequency withstand. If the enquiry wants a standard level, it should say so and name the table. Do not invent a level by scaling from another project.
A 110 kV system is normally assigned a highest voltage for equipment of 123 kV. In IEC 60076-3:2013 Table 2 the minimum row for that Um is a 450 kV lightning impulse, with a 185 kV separate-source AC level beside it. If a chopped wave is specified, the chopped level is the one in the same row, not a second full-wave choice. The purchaser may specify a higher standard level. A 650 kV lightning impulse is not that 123 kV row. It belongs with a higher Um. Use the table in the edition the contract names. IEEE C57.12.00 lists the basic insulation levels for the IEEE voltage class. Do not convert an IEC level into an IEEE level from memory.
State the frequency. A unit designed and tested at 50 Hz is not automatically rated at 60 Hz. Flux is set by voltage, frequency, and turns. Running a 60 Hz design at 50 Hz at the same voltage raises the flux and can saturate the core. No-load loss, noise, and temperature rise all move. That is a design question, not a field adjustment.
State the ambient and the altitude. Normal service conditions in IEC 60076-1 and IEC 60076-2 include a cooling-air temperature not above 40 °C and an altitude not above 1 000 m. Above that altitude, external clearances and temperature rise both need the corrections in IEC 60076-2 and IEC 60076-3. If losses are to be capitalized, state the capitalization rates for no-load loss and load loss and the reference temperature. If they are not capitalized, still ask for guaranteed no-load loss, load loss, and total loss. Those guarantees are what the factory test is judged against.
What the rating plate has to identify
The nameplate is the manufacturer's statement of the rating the transformer was designed and tested to. It is not a statement of remaining life, and it is not evidence that the unit suits every site. IEC 60076-1 sets out the information a power-transformer nameplate carries. IEEE C57.12.00 does the same job for liquid-immersed distribution, power, and regulating transformers built to IEEE practice. Read the plate against the standard named in the contract.
Record the manufacturer, the works serial number, the year, and the number of phases before you record the power. Those items tie a factory test report, a transport record, and a later oil sample to this tank. Rated power is the continuous power at rated voltage and rated frequency under the cooling method printed next to that power. When the plate shows an ONAN value and a higher ONAF value, each value belongs to the cooling stage beside it. The higher number is not available with the fans stopped.
IEC 60076-2 assigns the four-letter cooling code for a liquid-immersed transformer. The first letter is the internal liquid, the second is how that liquid moves, the third is the external medium, and the fourth is how that medium moves. IEEE C57.12.00 uses its own letter code and its own temperature-rise limits. For a conventional mineral-oil unit, buyers commonly specify a 65 K average winding rise and a 60 K top-liquid rise under IEC 60076-2, and a 65 °C average winding rise with a 65 °C top-oil rise under IEEE C57.12.00. Those pairs are not interchangeable. Confirm them in the edition you cite. A plate that only says 65 °C rise does not, by itself, name the document.
Rated voltage is the voltage the winding is designed for at rated power. It is not Um, and it is not the lightning-impulse level. Short-circuit impedance is the voltage, as a percent of rated voltage, that circulates rated current with the other winding shorted. It is stated at a stated power and a reference temperature, usually on the principal tapping. For a liquid-immersed transformer whose average winding temperature-rise limit is 65 K, IEC 60076-1 uses 75 °C as that reference temperature for load loss and for impedance. Confirm the pair in the edition named in the contract. A dry-type transformer follows IEC 60076-11. It does not inherit 75 °C.
Load loss is the loss measured in the short-circuit test, at rated current, and then corrected to that reference temperature. No-load loss is measured at rated voltage and rated frequency with the other winding open. Both can be guaranteed. IEC 60076-1 tolerances, unless the contract replaces them, include these for a tested unit: total losses +10 percent; no-load loss and load loss each +15 percent, provided the total losses stay within +10 percent; no-load current +30 percent. The tolerance on voltage ratio at the principal tapping is the lower of ±0.5 percent of the declared ratio and one tenth of the actual percentage impedance. For a two-winding transformer, the principal-tapping impedance tolerance is ±10 percent when the guaranteed impedance is below 10 percent, and ±7.5 percent when it is 10 percent or greater. Other tappings, and transformers with more than two windings, have their own rows in the same table. IEEE tolerances are the IEEE table. Do not mix the two.
Walk a vector group back to the windings
An IEC vector-group symbol has three parts. The capital letter is the high-voltage connection. The lowercase letter is the lower-voltage connection. The clock number is the phase displacement. D or d is delta. Y or y is star. Z or z is zigzag. N or n means the neutral of that winding is brought out. A is used for an auto-connected winding. Dyn11 is a delta high-voltage winding, a star low-voltage winding with the neutral brought out, and clock number 11. IEC 60076-1 defines the notation.
The clock number multiplied by 30 degrees is the angle by which the lower-voltage voltage lags the high-voltage voltage. Clock 0 is in phase. Clock 1 is 30 degrees of lag. Clock 11 is 330 degrees of lag, which is the same as 30 degrees of lead. It is not 30 degrees of lag, and it is not "11 times 30 degrees of lead." Dyn11 and YNd1 are not the same displacement. They cannot be substituted because both symbols contain the digit 1.
Dyn11 is the usual distribution symbol because the delta high-voltage winding gives triplen currents a path inside the transformer, and the star low-voltage winding with a brought-out neutral can supply phase-to-neutral loads. A Yyn connection can also present a low-voltage neutral, but without a delta path the neutral current has to be limited or a stabilizing winding has to be added. The enquiry has to say which behaviour is required. IEEE nameplates often show a phasor diagram and an angular displacement in degrees instead of a clock number. Convert one into the other only when you can point to the same two phasors on both drawings.
The displacement comes from how the windings are seated on the core and how the terminals are labelled. Changing it after the windings are built means reterminating, a new ratio test, and a new diagram. Off-circuit links that choose between two groups exist only when the manufacturer provided them and the plate lists both. Two units can share a bus only when the frequency matches, the tap positions make the voltage ratio match, the phase displacement matches on the terminals that will actually be joined, and the percent impedances are close enough for the load share you will accept. The IEC impedance tolerance of a single unit is not automatically tight enough for paralleling. Close the parallel breaker on the test sheet for this serial number, not on the symbol in the enquiry.
Match the tap changer to a real winding
Tappings change the turns ratio. A de-energized tap changer is used only with the transformer isolated. An on-load tap changer moves between positions in service, within the range and the step on the plate. IEC 60214-1 is the tap-changer standard. The enquiry is incomplete until it says which winding is tapped, whether the changer operates on load or only off circuit, the number of steps, the step size, and whether rated power is held across the whole range.
IEC 60076-1 distinguishes constant-flux, variable-flux, and combined regulation. In constant-flux regulation the voltage per turn stays constant over the range, so the tapped winding's voltage varies and the current limit can change. In variable-flux regulation the flux varies. If the plate does not say which regime applies, the tapping diagram and the test report must. An on-load changer on a star winding is commonly placed at the neutral end, where the voltage to earth is lower than at the line end. A delta winding has no neutral, so the changer is a line-end or a bridging arrangement. A specification that says only "on-load taps, plus or minus 10 percent on HV" does not yet identify the winding end, the number of positions, or the flux regime. Those three facts decide the insulation of the changer and the current it switches.
The tapping diagram, the nameplate range, and the winding the changer is built into have to describe the same thing. A changer specified for the high-voltage neutral, built instead at a line end, is a different insulation and a different current. Find that mismatch on the diagram, before the active part is finished.
Check your reading
The answers follow the questions. This is a reading check, not a gate.
- In Dyn11, what does 11 mean?
- What lightning-impulse level is the IEC 60076-3:2013 Table 2 minimum for Um = 123 kV, and is 650 kV that row?
- Is a 40 MVA rating, written with no cooling stage, a complete distribution specification?
- Which loss is measured with one winding shorted, at rated current?
- Where is an on-load tap changer on a star winding commonly connected, and why does a delta winding differ?
Answers. Clock 11 means the lower-voltage voltage lags the high-voltage voltage by 330 degrees, which is 30 degrees of lead. For Um = 123 kV the IEC 60076-3:2013 Table 2 minimum lightning impulse is 450 kV, with 185 kV as the separate-source AC level in that row. 650 kV is not that row. A bare 40 MVA line is not a complete specification and it is not a distribution class; the cooling stage and the impedance still have to be stated. Load loss is the short-circuit measurement, corrected to the reference temperature, which is 75 °C for the common 65 K liquid-immersed case and not for every insulation class. An on-load changer on a star winding is commonly at the neutral end. A delta winding has no neutral, so the changer cannot be specified as a neutral-end device.
Continue
- Module 2 — Factory acceptance testing
- What belongs in a transformer inquiry
- How to read a power transformer nameplate
- Vector group, clock number, and paralleling
- Cooling codes and temperature rise
- Engineer Track
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