Derating for high ambient temperature

IEC 60076-2 defines the reference conditions against which temperature rise is measured and declared: a maximum ambient temperature of 40 degrees Celsius, an annual average of 20 degrees and a monthly average of 30 degrees. The temperature-rise limits themselves are set at those conditions. For oil-immersed units, they include a 65 K average winding rise, a 60 K top-liquid rise and a hot-spot rise of about 78 K at rated current. Add the 40 degree reference ambient to a 78 K rise and the reference hot-spot lands at about 118 degrees.

Now put the same transformer on a site whose design maximum is 50 degrees. The rise is a property of the design, not of the weather, so the hot-spot moves to about 128 degrees. Nothing inside the transformer has changed. What changed is the ambient the standard assumed. Above the reference ambient, IEC 60076-2 expects the output to be reduced, and that reduction is what the site specification has to state.

The reduction is not a fixed percentage, because it depends on the design. A unit with a lower winding gradient and more cooling surface loses less output per kelvin than a compact unit already close to its thermal limit. A published inverter step-up transformer datasheet gives a worked example: a unit rated 3425 kVA at 45 degrees ambient delivers about 3083 kVA at 50 degrees. That is a 10 percent reduction over 5 K, or roughly 2 percent per kelvin for that design.

Two cautions follow. Do not extrapolate that 2 percent per kelvin to other designs or other temperature ranges; the relationship is not linear, and it steepens as the unit approaches its limit. And do not apply the design maximum as if it were continuous. A site that reaches 50 degrees for four hours on twenty days a year is not the same thermal duty as a site that sits at 50 degrees for three months. IEC 60076-7 provides the framework for calculating hot-spot temperature over a real ambient and load profile rather than a single worst case.

Transformer design for 50 degree ambient is not a matter of adding a margin to the nameplate. Several things change together.

Cooling surface increases, either through more radiator area or by moving from ONAN to ONAF, and the fans and pumps themselves must be rated for the higher ambient: a fan selected at 40 degrees may not deliver its curve at 55 degrees. Conductor cross-section increases to hold loss density, and therefore the winding gradient, inside limits. Insulation and structural materials are selected for the higher continuous temperature, including thermally upgraded paper, gaskets and seals that do not take a permanent set, and a paint system that survives a solar-exposed surface temperature. The oil expansion range widens, which affects conservator volume and the oil preservation system. Tap changer drives, control cabinets and terminal boxes each need their own thermal check.

The consequence for procurement is that a 50 degree design cannot be evaluated by comparing nameplate kVA. Two offers of the same rating can differ by several percent in usable output at the design maximum, and the difference will not appear on the datasheet unless the derating curve is requested.

High ambient is rarely the only site condition. On desert sites, airborne dust settles in radiator fins and reduces the effective cooling surface, and it does so gradually, so the loss of capacity appears months after commissioning. Forced cooling with dusty air needs filtration that itself restricts flow. Radiator designs with wider fin spacing and accessible cleaning paths cost less to maintain over 25 years than the output they preserve.

Diurnal swing is the second factor. A site that runs at 48 degrees by day and 5 degrees at night drives oil expansion and contraction, breathing and moisture ingress on a daily cycle, and thermally cycles the winding. That is a different duty from a site with a stable 35 degrees, even where the annual average is similar. The low end matters as well: the same specification may require operation at minus 5 degrees.

Solar gain is the third. A dark tank in full sun runs well above air temperature, which is why some specifications quote a surface temperature as well as an air temperature. Saudi Electricity Company connection standards for large-scale solar, for example, specify outdoor design conditions of minus 5 to plus 55 degrees, a hottest-month average of 40 degrees, an annual average of 30 degrees, a solar-exposed surface temperature of 75 degrees, altitude up to 1000 m and an equivalent salt deposit density of 0.3 to 0.5 mg/cm2. The salt deposit figure matters because it sets the pollution class, and therefore the creepage distance, on a site that is also hot.

The practical difference between a correctly derated transformer and a nominal one is decided by which temperature is used as the basis. Three figures are needed: the sustained summer maximum, the design extreme with its return period, and the duration and frequency of the peaks. A specification that gives only a design maximum will be over-derated for most of the year and may still be wrong at the peak.

For a containerised unit there is a fourth: the air temperature inside the enclosure. A transformer in an ISO container or a walk-in enclosure sits in air that is hotter than the site ambient, because the enclosure traps heat and adds solar gain of its own. Sizing against the site air temperature rather than the internal air temperature is a common and expensive error, and it compounds with harmonic derating. Ask for the internal temperature record, or for the design internal temperature if the container is not yet built.

The specification is where the derating becomes auditable. State the conditions, state the standard basis for the temperature rise, and state the conditions at which the rated output applies. Without the third item, a quoted rating has no defined meaning.