A marine transformer is not a land unit in a painted box. It runs at 45 °C ambient with 95 % relative humidity, breathes air laced with salt, rolls 22.5° and trims 15°, and does all of that without a maintenance crew available for a week at a time. The electrical duty is usually modest — 50 kVA to 3 MVA on most commercial vessels — but the environmental duty is severe enough that the design, the enclosure and the approval paperwork all diverge from an ordinary IEC 60076 unit.
The governing document is IEC 60092-303, applied together with the rules of the classification society that will survey the ship: CCS, DNV, ABS, Lloyd’s Register, BV, NK or KR. A transformer that satisfies IEC 60076-11 but has no class certificate is not installable on a vessel, and that distinction — certification, not specification — is where most first-time marine purchases go wrong.
Four conditions separate the marine duty from an industrial one, and each drives a specific design decision.
The standard sets the electrical requirements; the class rules add the approval route, the enclosure minima and the alarm philosophy. The temperature-rise limits are the numbers that most often decide whether a proposed design is acceptable.
| Parameter | Limit | Consequence for the design |
|---|---|---|
| Winding temperature rise, dry type, Class F | 95 K | Class H (120 K) is available but buys margin by spending insulation life; Class F with a H-class insulation system is the usual marine compromise |
| Winding temperature rise, dry type, Class H | 120 K | Allows a smaller core and a higher losses unit; check the space and cooling first |
| Liquid-filled, ON | 65 K | Thermometer method; the oil temperature gauge is a surveyed instrument |
| Liquid-filled, OD | 70 K | Forced cooling; the pump becomes a single point of failure |
| Short-circuit withstand | 2 s at winding terminals | Bracing and clamping must be demonstrated by test, not assumed from a land design |
| Enclosure, above 1 kV | IP23 minimum | IP4X where unqualified persons have access; IP44 to IP56 for deck and weather deck locations |
| Ambient design basis | 45 °C, 95 % RH, 1,000 m | Above 45 °C or 1,000 m the unit must be derated, not merely relabelled |
The dielectric test levels are lower than a land transmission unit of the same voltage class, because marine systems are impedance-earthed or isolated and the surge environment is bounded by the vessel’s own distribution. A 12 kV winding is tested at 28 kV AC for one minute; the pattern follows 3.6 kV at 1.1 kV, 20 kV at 7.2 kV, 38 kV at 17.5 kV, 50 kV at 24 kV and 70 kV at 36 kV.
Most class rules state a preference for air-cooled dry-type transformers. That preference is well founded, but the choice is not automatic.
| Criterion | Cast-resin or VPI dry type | Liquid filled |
|---|---|---|
| Fire and smoke | No liquid fuel; F1 class available | Requires oil containment, alarms and a surveyed tank |
| Inclination behaviour | Insensitive to list and trim | Oil level and Buchholz behaviour must be assessed for 22.5° roll |
| Salt and humidity | Resin-encapsulated, tracking-resistant; enclosure and heaters still required | Tank and bushings need C5-M finish and sealed breathers |
| Size at 1 MVA | Larger footprint, higher no-load loss | Compact, lower loss, cheaper per kVA above roughly 2 MVA |
| Maintenance at sea | Inspection and cleaning only | Oil sampling, breather, gaskets, level alarms |
| Best fit | Auxiliary, lighting, deck machinery, shore connection step-down | Large propulsion and bow-thruster supply where space dominates |
For a 6.6 kV or 11 kV shore connection stepping down to 440 V, a cast-resin or VPI winding inside an IP44 steel enclosure is the standard answer. For a 3 MVA propulsion supply, the oil-filled unit is still usually lighter and smaller, and the class society will accept it once the alarms and the tank are right.
Two enclosure families cover most of the fleet. A drip-proof steel enclosure at IP22 or IP23 ventilates freely and is appropriate inside a machinery space or a covered deck. A sealed epoxy-encapsulated or resin-cast unit at IP55 or IP56 suits a weather deck or an open location, where driven spray reaches the equipment.
The finish specification matters as much as the IP number. Offshore and deck-mounted units are normally specified to corrosivity category C5-M under ISO 12944: hot-dip galvanised steel, a zinc-rich primer, and a multi-coat epoxy system with a total dry film thickness in the 240–320 µm range, verified by salt-spray testing to ISO 9227. A standard workshop paint system will look sound for two years and then fail at the weld seams and the enclosure base, which is exactly where water sits.
Stainless steel 316L is the alternative where the budget allows, particularly for the enclosure base, the lifting eyes and the terminal box. Mixing stainless fixings with a painted carbon-steel frame is the classic galvanic couple that quietly destroys both. Where the same vessel also carries process plant, the deck machinery duty is closer to our oil and gas transformer range than to a utility distribution unit, and the enclosure logic transfers.
A transformer rated at 40 °C ambient and installed in a 45 °C space is running five kelvin closer to its limit than the design assumed. For liquid-filled designs the practical derating is roughly 1 % of kVA per kelvin above the rated ambient, so 5 K costs about 5 % of rating. Dry-type units have a smaller winding margin and derate a little faster.
Altitude is the second correction. Above 1,000 m the reduced air density weakens both cooling and external insulation, and a unit specified for a mountain site needs a documented correction rather than a verbal assurance.
Frequency is the third. A 50 Hz design operated at 60 Hz at the same voltage is safe on flux — the volts-per-hertz ratio falls — but its reactance rises by 20 %, which reduces short-circuit current and changes the protection settings that were calculated for 50 Hz. A 60 Hz design operated at 50 Hz at the same voltage saturates the core, overheats the laminations and will fail. Dual-frequency nameplates exist, but they are a design feature and must be requested, not assumed.
Approval is a documentation exercise as much as a technical one. The submission normally consists of the type-test report, the routine-test report for the individual serial number, the winding-material declarations, the enclosure IP certificate and the finish system data. Class rules also control the supplier of the electrical steel and the winding conductor: changing either without approval invalidates the certificate.
Routine tests are unremarkable — winding resistance, ratio, impedance, insulation resistance, applied and induced voltage, and a temperature-rise check where the rating is large enough to warrant it. The type test is where the marine content lives: the vibration sweep, the inclination demonstration and, for liquid-filled units, the alarm and trip functions under simulated low level and high temperature.
Practical advice from the procurement side: confirm the class society and the notation before the order, not during the survey. A unit built to CCS rules cannot simply be re-certificated by DNV afterwards, and re-testing on the quay is the most expensive way to discover that. It also helps to agree the winding system early — the difference between cast resin, VPI and open-wound construction is what decides how the unit behaves in salt air.
IEC 60092-303, “Electrical installations in ships — Equipment — Transformers for power and lighting”, applied together with the rules of the relevant classification society. IEC 60076-11 supplies the dry-type design and test requirements that IEC 60092-303 references. A certificate to IEC 60076 alone does not make a unit marine-approved.
Normally not without modification. A land unit will be missing the vibration and inclination evidence, may have no anti-condensation heaters, and its finish will not meet C5-M. The electrical design might be adequate; the approval evidence will not be, and the class surveyor is entitled to reject it on that basis.
Inside a machinery space, IP23 satisfies the class minimum for equipment above 1 kV. On a weather deck, driven spray and salt require IP55 or IP56, and often a canopy as well. IP23 in an exposed location fills with salt-laden moisture and the winding fails within a few years.
Plan on roughly 5 % for a 5 K rise above a 40 °C rating for liquid-filled units, and slightly more for dry type. The correct approach is to state the ambient in the enquiry and let the manufacturer issue the rating at that ambient — derating a nameplate after the fact is how margins disappear.
Because a 60 Hz design run at 50 Hz at the same voltage saturates. The reverse direction is safe on flux but raises reactance and changes protection settings. Ships are built to both frequencies and shore supplies vary, so the frequency range belongs on the data sheet.
C5-M, per ISO 12944, on a hot-dip galvanised substrate, with a documented dry film thickness and salt-spray test evidence. Offshore atmospheric corrosivity is the highest category normally specified, and the enclosure base, welds and fixings are where the coating fails first.
A marine transformer is a certification project wrapped around a fairly ordinary transformer. Get the class society and notation fixed first, state the real ambient, humidity, inclination and enclosure location, and specify the finish by category rather than by colour. The failures that recur at sea are almost never electrical design errors — they are an IP rating chosen for a sheltered location, a paint system chosen by price, and a heater circuit left out because nobody asked for it.
If you are specifying a unit for a vessel or an offshore platform, send us the class society, the system voltage and frequency, the location and the ambient conditions. We will return a proposed rating with the derating, the IP and finish specification, and the test evidence the surveyor will ask for.
