Two enquiries can carry the same first line — 2,000 kVA, 10 kV primary, rectifier duty — and not be the same machine. One feeds an electrochemical line through a twelve-pulse connection at 720 V DC and 8,000 A. The other feeds a plating rectifier at 60 V DC and 12,000 A. The first wants two phase-shifted secondaries and a current ordinary bushings handle. The second wants a secondary that behaves like a busbar, with ampere-turns balanced against forces the first design never sees. Same kVA, opposite designs.
That is why the data sheet matters more than the rating. A rectifier transformer has no output voltage of its own in the way a distribution unit does: what the process needs is DC volts and DC amps at the busbar, and everything else — kVA, secondary voltage, vector group, number of windings, cooling — follows from those two numbers plus the connection. Send a kVA figure on its own and the first task is to ask for the DC side, which is a round trip before the drawing office can start.
Start on the DC side: three numbers that set the rest
Give us DC voltage, DC current and the rectifier connection, and the rest is arithmetic you can check. DC power is simply Pdc = Vdc × Idc, so 720 V at 8,000 A is 5,760 kW. For a six-pulse bridge the ideal no-load DC voltage is Vdc0 = 1.35 × VLL (that constant is 3√2/π = 1.3505), and the secondary line current is I2 = √(2/3) × Idc = 0.8165 × Idc. Apparent power follows: S = √3 × VLL × I2 = (√3 / 1.35) × 0.8165 × Pdc = 1.05 × Pdc. Our 5,760 kW job therefore lands near 6,050 kVA, on a secondary of 720 / 1.35 = 533 V carrying 0.8165 × 8,000 = 6,532 A.
Reverse the habit and the same arithmetic has to be guessed. Assume a connection you did not ask for and the error surfaces later, either as a secondary voltage that puts the semiconductors in the wrong class or as a busbar section that cannot carry the current. The mechanical side of carrying six thousand amps is treated separately in our article on designing a transformer for high current applications.
Connection and pulse number decide the secondary, not the tank
Pulse number is six times the number of six-pulse groups: twelve pulse is two groups 30° apart, twenty-four pulse is four groups 15° apart. Twelve pulse is cheap to build on one tank because a star secondary and a delta secondary of the same turns ratio are naturally half an hour apart on the clock. Twenty-four pulse is not, and the displacement has to be wound in.
Then the bridges are either in series or in parallel, and that halves or holds the current per winding. In series, each bridge carries the full 8,000 A and each secondary sees 6,532 A. Paralleled through an interphase transformer, each bridge carries 4,000 A and each secondary sees 3,266 A. Both sit inside a secondary design rated up to 10,000 A, but the lead arrangement, the terminal count and the internal bracing are not interchangeable between them. Say where the pulse number came from — a utility harmonic limit, your own study, or the rectifier supplier’s standard — because it decides how many windings the tank has. What each pulse number cancels on the AC side is set out in our piece on rectifier transformer harmonics.
Duty class and cooling: what actually sizes the copper
IEC 60146 defines load duty classes for semiconductor converters, and each class carries a defined short-time overload above 100 per cent continuous current. Ask the rectifier supplier which class the process falls into, because the class sets the conductor section more than the kVA does. A unit that must ride through a repeated overload needs copper and cooling a steady-duty unit of the same rating does not.
Cooling is the other half of the same decision. Dry-type designs run AN or AF, oil-filled designs ONAN or ONAF, and each needs something from your plant. Ratings quoted from IEC 60076 assume a 40 °C ambient and installation below 1,000 m; above either, the rating has to be corrected. Ambient and altitude are the fields most often left blank, and the two that return later as a change order.
The grid-side and site lines that quietly move the date
On the primary side we need the system voltage and the frequency. Our rectifier units are built for 6–35 kV and ratings up to 20 MVA, and 60 Hz is a normal build, not an adaptation. Short-circuit impedance is the third grid-side question. If your protection study has fixed a value, give it; if not, ask us to propose one, because it interacts with commutation on the rectifier and with the fault level your switchgear must break.
On the site side: indoor or outdoor, the IP rating you expect, whether entry is by cable or busbar and from which face, the ambient range, the altitude, whether a harmonic filter shares the bus, and how you want voltage regulated — on-load or off-circuit tap changing, with the range and the step size. None of these are hard questions. Each one that goes unanswered gets answered by assumption in the drawing office, and assumptions are corrected after contract, which is the expensive place to correct them.
What a complete data sheet lets us commit to
With the DC figures, the connection, the duty class and the site conditions on the page, a quotation can carry a drawing and a date instead of a range. Delivery is 30 days from approved drawings and the warranty runs 2 years. Type-test and certification requirements are handled per project rather than assumed from a fixed package; send the standards and tests the project calls for and we will confirm what can be arranged.
If all you have today is a rating and a voltage, send it anyway — enough to start the conversation, not enough to price the job. The same questions in another form are covered in our custom transformer checklist, and the usual arrangements are shown on the rectifier transformer range.
Frequently Asked Questions
Can you quote a rectifier transformer from a kVA rating and a primary voltage?
We can give a budget range, not a firm price. Without the DC voltage and current and the connection, the secondary arrangement is a guess, and the guess changes copper quantity, tank size and whether a tap changer is needed. A budget figure is fine for screening; a quotation needs the DC side.
Do you build 60 Hz rectifier transformers?
Yes. State the frequency, because 60 Hz is not a detail: at a given voltage it changes the core cross-section and flux density. A 60 Hz unit is designed for that frequency rather than adapted from a 50 Hz design.
Are your rectifier transformers certified?
Certification and type-test requirements are supported per project rather than claimed from a fixed package. Send the standards and tests your project requires and we will confirm what can be arranged before any order is placed.
Send Us the Data Sheet
DC voltage and current, connection and pulse number, duty class, primary voltage and frequency, and the site conditions. Email info@xsdfftransformer.com, call +8615867897761, or message WhatsApp +852 5416 2620. The same list can be sent through our enquiry page.