WHAT THE TWO COLUMNS MEASURE
No-load loss is what the core consumes to stay magnetised. It is drawn from the moment the unit is energised to the moment it is switched off, and the load has nothing to do with it. Load loss is the copper. It rises with the square of the current, so the figure in a table applies only at rated current. At 40 percent load it is around 0.16 of that figure. The split between iron loss and copper loss is old textbook material, and the heat they produce is what the cooling system has to carry away. What matters at the desk is simpler: the two quantities come from two different tests. The open-circuit test gives no-load loss and no-load current, the short-circuit test gives impedance and load loss, and the sequence of transformer testing techniques behind both is routine work. A supplier who sends one column without the other has sent half a data sheet, and the no-load half is usually the one that goes missing.S11-M AND S13-M AT THE SAME RATING
Both tables below come from the XIAOPAI catalogue for the oil-immersed transformer range: fully sealed, 50 Hz, Dyn11, with a tapping range of plus or minus 5 percent in steps of 2.5 percent. Same low-voltage side, same dimensions on the drawing.| Rating (kVA) | S11-M no-load (W) | S13-M no-load (W) | Load loss at 75 C (W), both |
|---|---|---|---|
| 630 | 810 | 570 | 6,200 |
| 800 | 980 | 700 | 7,500 |
| 1000 | 1,150 | 830 | 10,300 |
| 1250 | 1,360 | 970 | 12,000 |
| 1600 | 1,640 | 1,170 | 14,500 |
| 2000 | 2,050 | 1,440 | 17,500 |
THE COLUMN THAT RUNS ALL YEAR
Take that 320 W at 1,000 kVA and multiply by the hours in a year. 2,803 kWh. Three units on one site carry roughly 8,400 kWh of the same difference, with the load profile untouched. This holds while the transformer is energised, which for a distribution unit is most of its life. A plant running one shift and a plant running three pay the same no-load figure. Load loss behaves better: at 40 percent load it drops to about a sixth of the rated value.WHY THE GRADE IS A SITE DECISION
A better core uses more iron and returns the difference through the no-load column, which is why the choice belongs with the load profile rather than with a preference for newer numbers. Most of the engineering that decides this happens before the core is wound, and the levers are well documented in this outline of loss reduction at the design stage. Buyers who run the arithmetic before the enquiry tend to ask for two or three ratings in the same message, with the losses attached, and pick from that short list. There is a practical write-up on taking losses into account when buying that covers the same ground from the purchasing side. One thing the table cannot tell you: what the load actually looks like. Drives, rectifiers and LED lighting draw non-sinusoidal current, and harmonics add losses that no 50 Hz table accounts for. If the plant is full of variable-speed drives, the rated figures are a floor, not a forecast.WHAT TO PUT IN THE SCHEDULE
Six lines, half a page, and all six appear in the XIAOPAI tables for every rating: - no-load loss in W, at the rating offered - load loss in W at 75 C, at the rating offered - no-load current, as a percentage of rated current - short-circuit impedance, as a percentage - oil mass and total mass, in kg - the routine test report for the serial number being shipped The last one cannot be copied from a catalogue, and it is the one that closes the loop on the other five. A routine report normally covers winding resistance, ratio, impedance and load loss, no-load loss and current, plus the applied and induced voltage checks, and the dielectric tests that go with them. If the report arrives with only a ratio check, the loss columns in the offer were never verified on that unit.THE TANK IS NOT JUST A BOX
Losses are the part of the specification that runs, but the fittings decide what you can see and what protects the unit. A sealed distribution transformer carries less of this furniture than a transformer with a conservator, and the difference shows up in maintenance rather than in the table. There is a useful inventory of the common accessories on an oil-filled transformer, from the breather and the oil level gauge to the pressure relief device. On units with a conservator, the Buchholz relay sits in the pipe between the tank and the conservator and reacts to gas or oil surge. It is a mechanical device with a float and a flap, and it is the reason a slow internal fault gets noticed before it becomes a fire. A sudden pressure relay covers the faster case, where the pressure rises before any gas has had time to collect. None of that appears in a loss table, and none of it is optional on the units that carry it.WHAT XIAOPAI SUPPLIES
Wenzhou Xiaopai Electric Power Technology designs and manufactures oil-immersed transformers in 6-10 kV and 35 kV classes: the S9-M, S10-M, S11-M, S13-M, S20-M and S22-M ranges. The distribution tables run from 30 to 2,500 kVA, and the 35 kV power transformer range is published to 31,500 kVA.
The dry-type range is tabulated the same way, so the two technologies can sit on one comparison sheet. Every rating in those tables carries its no-load loss, load loss at 75 C, no-load current, impedance and mass, so the comparison above can be made before the order rather than after commissioning. XIAOPAI publishes the tables for the same reason a buyer should ask for them: a capacity is a request, and a loss column is an answer. One more thing, and it is the easiest question in the whole process. Ask which grade is on the table, S11-M or S13-M, before the technical comparison starts. The two are the same size and the same connection, and they differ in a column that runs for the life of the installation. Asking later usually means asking after the drawings are approved.
