Marine Dry-Type Transformer Selection Guide

The dry-type transformers on ships operate in an environment completely different from that on land: salt fog, mold, humidity, vibration, inclination, high temperature and grid fluctuations are all present. They are oil-free, flame-retardant and easy to maintain, making them the mainstream choice for ship power distribution nowadays. However, they are by no means simply land-based dry transformers with a modified enclosure. When selecting models, many buyers only focus on capacity and voltage, while ignoring numerous certifications, protection measures and temperature rise risks. Experienced marine dry-type transformer manufacturers can provide targeted design solutions to avoid such pitfalls.

1. First, confirm the usage scenario and grid conditions

Three key parameters shall be defined prior to implementation: the type of the vessel, the installation location, and the grid configuration of the shipboard power system, which serve as the fundamental basis for formulating the basic scheme.

Varying vessel types entail drastically different load characteristics. Offshore platforms and engineering ships feature a high proportion of impact loads, while ro-pax vessels impose stringent requirements on noise control and fire prevention. The installation location determines the specific operating conditions the equipment will be subjected to: the engine room is characterized by high temperature, intense vibration and oil contamination, deck compartments are constantly exposed to salt spray and condensation, and superstructure compartments feature limited space and strict mandatory noise limits.

Subsequently, the primary and secondary side voltages, 50/60Hz frequency specification, phase number and grounding mode shall be verified thoroughly. Given the severe third harmonic distortion in shipboard power systems, the Dyn11 connection group is selected directly in most scenarios to suppress harmonic distortion and prevent interference with navigation systems and weak-current equipment.

II. Six-step Selection Method

Capacity Calculation: Total Power Is Not the Sole Determinant

The capacity calculation follows the workflow: Load Statistics → Simultaneity Factor → Safety Margin.

Multiply the value by an additional margin factor: 1.2 to 1.3 for conventional power systems, and a value above 1.2 for auxiliary equipment with frequent start-stop cycles.

For example, a medium-sized cargo ship has a total load of 1200kW, a simultaneity factor of 0.8, and a power factor of 0.8, resulting in a calculated apparent power of 1200kVA. With an additional 20% margin reserved, the selected capacity shall be no less than 1440kVA.

2. Voltage, Frequency and Phase Number

Verify the marine power grid system specifications, such as 440V/60Hz, 690V/60Hz, 220V/50Hz, etc. The phase number, connection vector group and neutral point grounding mode shall all be coordinated with system protection. The common connection vector group for marine applications is Dyn11, but the specific configuration shall be determined according to the requirements of short-circuit current and relay protection.

3. Insulation and Temperature Rise: Reserve Sufficient Margin

The minimum insulation class for marine use is Class F (155℃), and Class H (180℃) shall be directly adopted for high-temperature areas such as engine rooms. Under the ambient temperature of 45℃, the temperature rise limit of Class F windings is approximately 95K, and that of Class H is approximately 120K. In practical implementation, the rated limit shall not be fully utilized. The winding temperature rise shall be controlled within 60K and the core temperature rise shall not exceed 50K to ensure reliable service life.

4. Protection and "Three-Protection" Measures: The Environment Determines the Rating

For installation in the engine room, the enclosure shall be at least IP23; for scenarios where non-professional personnel may come into contact with the equipment, the protection rating shall be IP4X or higher; for ocean-going and high salt-fog areas, IP56 or even fully-sealed IP67 is recommended. However, regardless of the IP rating, the three-protection treatments against salt spray, mould and dampness shall be explicitly specified in the requirements.

One point shall be clarified: a higher IP rating is not always better. A fully enclosed structure will lead to heat dissipation problems, so the protection rating shall be balanced with the cooling mode.

5. Cooling Mode: AN or AF

AN (Natural Air Cooling): Featuring a simple structure and almost no maintenance requirements, it is suitable for applications with stable loads, spacious compartments or controllable ambient temperatures.

AF (Forced Air Cooling): Equipped with fans, this mode can deliver approximately 50% more capacity, making it suitable for engine rooms with high ambient temperatures, limited space and large load fluctuations.

If AF mode is selected, the power supply reliability of the fans shall be taken seriously. Once the fan stops, the transformer shall operate derated to the AN capacity. If it is forced to run at full load, the temperature rise will quickly exceed the specified limit.

6. Short-Circuit Impedance and Overload Capacity

Short-circuit impedance affects short-circuit current limitation and voltage regulation rate, and shall be matched with protective electrical appliances.

When multiple units operate in parallel, the short-circuit impedance of each unit shall be identical.

For equipment with high starting current such as large propulsion motors, transformers with high short-circuit impedance and strong overload capacity shall be selected to ensure voltage stability at the moment of startup.

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III. Different Ship Type Selection Focuses

IV. Type Selection Under Special Marine Environmental Conditions

Vibration and Shock Resistance. Vessels are subject to continuous jolting and repeated wave impacts during navigation. Dry-type marine transformer manufacturer shall perform overall structural reinforcement on marine dry-type transformers, with anti-loosening enhancement implemented for all iron cores, windings and fasteners, and be equipped with dedicated marine shock absorbers. Otherwise, long-term sustained vibration will lead to component loosening, abnormal noise and insulation damage. Conventional industrial shock absorbers cannot meet the marine shock and vibration standards.

Inclination Resistance. Vessels often operate in an inclined state during navigation and operation. The equipment shall meet the requirements of transverse inclination ≤15° and longitudinal inclination ≤7.5°, to ensure the normal operation of the temperature control system, cooling fans and internal structures under inclined conditions, and avoid heat dissipation failure and fan stalling caused by inclination.

Three-Protection Performance Against Salt Spray, High Humidity and Condensation. High-concentration salt spray and high humidity are most likely to cause metal corrosion and insulation performance degradation. The overall metal structure shall be coated with dedicated anti-corrosion coating for salt spray resistance, and built-in moisture-proof heaters that automatically switch on and off during shutdown shall be installed to prevent condensation from creeping along the windings and avoid insulation damp aging. The protection level shall be selected as required: IP20 is sufficient for installation in the engine room, while the protection level can be upgraded to IP30 for dust and splash prevention in high-humidity areas on the deck, and ventilation and heat dissipation shall be taken into account to avoid sacrificing heat dissipation performance for higher protection level.

Capacity Derating Verification Under High Temperature Conditions. Due to the relatively high ambient temperature in the cabin, the rated capacity must be corrected according to the maximum ambient temperature inside the cabin. Classification societies generally conduct assessment at an ambient temperature of 45℃, and the 40℃ temperature rise standard for land-use transformers shall not be directly applied. Long-term operation under high temperature will lead to excessive temperature rise and frequent overload alarms.

V. Classification Society Certification and Factory Tests (Essential for Procurement) Certification: Select the corresponding classification society certificate in accordance with the vessel's flag state, including CCS (China Classification Society), LR (Lloyd's Register), DNV (Norway), etc. Products without valid certificates cannot pass the acceptance check for on-board installation. It is also necessary to verify that the capacity and voltage specifications covered by the certificate are consistent with the project requirements, as a certificate with mismatched specifications is invalid even if it is authentic. When sourcing, confirm that your dry-type marine transformer manufacturer holds valid class certificates matching your technical scope.

Factory Tests: The full set of tests including transformation ratio test, connection group test, DC resistance test, power frequency withstand voltage test, induced withstand voltage test, partial discharge test, temperature rise test, vibration test and inclination test shall be completed to confirm that the product meets the qualification requirements. A qualified dry-type marine transformer manufacturer shall complete all above test items before delivery.

VI. Considerations for Fire Prevention, Installation, Operation and Maintenance Dry-type transformers are inherently flame-retardant, suitable for use in narrow and enclosed cabins, and eliminate the risk of fuel fire compared with oil-immersed transformers. However, they shall still be kept at a safe distance from flammable pipelines. As the cabin space is extremely valuable, the overall dimension, lifting space and weight of the equipment shall be verified in advance, and the load-bearing capacity of the hull shall also be calculated. In terms of operation and maintenance, sufficient access space for maintenance shall be reserved to facilitate dust cleaning of windings and on-site replacement of temperature control modules. A reliable dry-type marine transformer manufacturer will provide certified drawings, weight data and professional maintenance guidance in advance.

VII. Common Misconceptions

Calculating only the total power without reserving a sufficient margin. Marine loads feature large fluctuation and strong motor starting impact, and insufficient margin will lead to tripping and even cause full-ship voltage drop.

Directly applying land-use dry-type transformers on vessels. Such transformers are not equipped with anti-vibration reinforcement, have not passed inclination verification and are not covered by classification society certification, and will be scrapped quickly under the combined effect of salt spray and mold, which will definitely fail the acceptance check. Many purchasers make this mistake by choosing general suppliers rather than a professional dry-type marine transformer manufacturer.

Neglecting three-protection and moisture-proof measures. Even with a high IP rating, the equipment will still suffer rapid corrosion without proper three-protection treatment; without moisture-proof heaters, the windings will get damp after shutdown, leading to rapid insulation degradation.

Blindly pursuing excessively high protection levels. A higher IP rating will result in poorer heat dissipation performance, which shall be coordinated with the cooling mode.

Unreliable power supply for AF cooling fans. If the transformer continues to operate at full load after the fans stop, the temperature rise will directly exceed the allowable limit.

Incorrect selection of connection group. On vessels with severe harmonic interference, an incorrect connection group will amplify harmonics and interfere with navigation weak-current systems, so the Dyn11 connection group is preferred.

Inconsistent impedance of parallel transformers. This will lead to uneven load distribution, and in severe cases, circulating current will be generated.

Non-compliant shock absorber selection. Conventional industrial shock absorbers cannot meet the marine shock and vibration standards.

Mismatched certificate scope. A certificate is still invalid if the covered capacity and voltage specifications do not match the project requirements, even if the certificate itself is authentic.

VIII. Type Selection Checklist

Vessel type, installation location

Rated capacity (kVA)

Primary voltage / secondary voltage, frequency

Connection group, impedance voltage

Insulation class, cooling mode

Protection level, enclosure material

Requirements for fans, temperature control units and

moisture-proof heaters Transverse inclination / longitudinal inclination requirements Classification society certification requirements

Noise limit, weight, overall dimension