Dry-type transformers are widely used in power distribution systems due to their high insulation strength, strong short-circuit withstand capability, and advantages such as environmental friendliness, fire resistance, explosion protection, and minimal maintenance. This article provides a comprehensive overview of their construction, cooling methods, installation procedures, commissioning, fault diagnosis, and selection criteria.
The core and windings are directly exposed to the atmosphere.
Suitable for dry, clean indoor environments (ambient temperature 20°C, relative humidity ≤85%).
Cooling methods: natural air cooling (AN) or forced air cooling (AF).
The active part is housed in a sealed enclosure, not in direct contact with the atmosphere.
Due to limited heat dissipation, mainly used in mining (explosion-proof applications).
Epoxy resin or other resins are used as the main insulation.
Simple structure, compact size, suitable for smaller capacity transformers.
Natural Air Cooling (AN): The transformer can operate continuously at rated capacity under normal conditions.
Forced Air Cooling (AF): Output capacity can be increased by 50%. Suitable for intermittent overload or emergency overload operation. However, prolonged continuous overload is not recommended due to increased load loss and impedance voltage, leading to non-economic operation.
Rarely used today. Windings and insulating materials are selected according to different thermal classes (Class B, F, H).
Uses polyester resin or epoxy resin; epoxy resin is the most common choice for cast insulation dry-type power transformers.
A type of resin-insulated transformer, but few manufacturers produce it.
Type A: High-voltage winding cast resin, low-voltage winding impregnated insulation.
Type B: High-voltage winding cast resin, low-voltage winding foil-wound (copper or aluminum foil).
| Feature | Dry-Type | Oil-Immersed |
|---|---|---|
| Fire & explosion risk | Avoided due to non-flammable insulation | Risk of fire/explosion from oil |
| Leakage | No oil leakage issues | Possible oil leakage and aging |
| Maintenance | Minimal or maintenance-free | Regular oil checks and maintenance |
| Installation | Indoor (or outdoor with enclosure); can be placed with switchgear | Requires separate oil containment |
| Accessories | Few (no conservator, safety vent, many valves) | Many accessories, sealing issues |
Check packaging integrity.
Verify nameplate data against design requirements.
Ensure factory documents are complete.
Inspect for external damage, displaced parts, damaged supports or connections.
Check spare parts for damage or shortage.
Check foundation: embedded steel plates must be level with no voids underneath (to avoid increased noise from poor vibration/sound absorption).
Use rollers to move the transformer into position, then remove rollers.
Adjust to exact design position; levelness must meet specifications.
Weld four short channel steels at the four corners of the base onto the embedded plates to prevent movement.
Maintain minimum clearances between live parts and to ground, especially from cables to HV windings.
High-current LV busbars must be independently supported – do not connect directly to transformer terminals to avoid excessive mechanical stress and torque.
For currents >1000A (e.g., 2000A busbars), use a flexible connection between busbar and terminal to accommodate thermal expansion/contraction and isolate vibration.
Ensure proper contact pressure using elastic elements (e.g., disc springs or spring washers). Use a torque wrench for tightening bolts.
The grounding point is on the LV side base with a dedicated grounding bolt marked with the grounding symbol.
Reliably connect this point to the protective grounding system.
If an enclosure is used, it must also be grounded.
For three-phase four-wire LV systems, the neutral line must be connected to the grounding system.
Check all fasteners for looseness.
Verify electrical connections are correct and reliable.
Ensure insulation distances between live parts and to ground meet requirements.
Remove any foreign objects near the transformer; clean coil surfaces.
Ratio and vector group check: Measure DC resistance of HV and LV windings; compare with factory test data.
Insulation resistance: Measure between windings and winding-to-ground. If significantly lower than factory values, the transformer may be damp. If below 1000Ω per volt of operating voltage, drying is required.
Dielectric withstand test: Test voltage must comply with standards. Remove temperature sensors (e.g., PT100) before low-voltage withstand test; reinstall after test.
Fan test: If fans are installed, energize them and verify normal operation.
After thorough checks, energize the transformer. During trial operation, pay special attention to:
Abnormal sounds, noise, vibration.
Unusual odors (e.g., burning, pungent smell).
Discoloration due to local overheating.
Proper ventilation.
Important notes:
Although dry-type transformers have good moisture resistance, they are open-type and can still absorb moisture. For high reliability, operate at relative humidity below 70%. Avoid long-term idle periods. If insulation resistance drops below 1000Ω/V, stop trial operation and dry the transformer.
For step-up transformers in power stations (unlike oil-immersed types), do not operate with the LV side open-circuited, as overvoltages from the grid or lightning can cause insulation breakdown. Install overvoltage arresters (e.g., Y5CS zinc oxide arresters) on the HV busbar side for protection.
| Sound Type | Possible Fault |
|---|---|
| No sound on one phase, normal on others | Phase loss (power supply, HV fuse blown, or broken HV lead wire due to vibration) |
| Loud “chirping” or “sizzling” sparking | Tap changer not fully engaged or poor contact |
| “Ding-dong” knocking, “whoosh” blowing, or “zira” magnetic attraction sounds | Loose core bolt, small metal objects left inside, or foreign matter – generally not critical, can be fixed at next outage |
| “Sizzling” or “hissing” with visible sparks at night | Dirty or cracked HV bushing – surface flashover |
| “Bibob” light discharge sound | Core grounding broken |
| Sharp “crackling” (air discharge) or dull “crackling” (through oil-like medium) | Internal discharge – insulation distance insufficient; requires inspection and reinforcement |
| “Jee-wah” intermittent sound (like a frog) during windy conditions; “humming” or “roaring” for low-voltage short circuit | External line break or short circuit |
| Low “humming” like a heavy aircraft | Severe overload |
| Louder and sharper “humming” | Overvoltage causing over-excitation |
| “Gurgling” like boiling water | Inter-turn or layer short circuit in windings |
Metallic rattling (“crackling”) at start: Foreign object inside fan – clean it.
Continuous friction noise after start: Fan quality issue – replace fan.
Vibration from the transformer can cause the enclosure to rattle. Solution: install rubber pads between enclosure and floor, and between enclosure and transformer base.
Sometimes “humming” varies in intensity due to sound wave reflection from walls. Adjust transformer position or add sound-absorbing materials on walls.
Poor foundation amplifies vibration and noise. Reinforce the mounting surface if cracks or noticeable vibration are present.
Large amount of Level 1 or Level 2 loads: Install two or more transformers so that remaining capacity can serve critical loads if one fails.
Large seasonal loads (e.g., HVAC chillers, electric heating): Use dedicated transformers.
Large concentrated loads (e.g., large X-ray machines, arc furnaces): Use dedicated transformers.
Large lighting loads or when sharing with power loads degrades lighting quality: Install dedicated lighting transformer.
Normal conditions: Oil-immersed or dry-type (e.g., S8, S9, S10, SC(B)9, SC(B)10) for industrial/commercial substations.
Multi-story or high-rise buildings: Use non-flammable or flame-retardant types (e.g., SC(B)9, SC(B)10, SCZ(B)9, SCZ(B)10).
Dusty or corrosive environments: Use enclosed or sealed types (e.g., BS9, S9-, S10-, SH12-M).
Non-flammable oil-free switchgear and transformers in same room: Transformer must have IP2X protective enclosure for safety.
Dry-type transformers are increasingly favored for their high insulation strength, strong short-circuit withstand capability, and benefits such as fire resistance, explosion protection, and low maintenance. Proper installation, commissioning, and fault diagnosis are essential for safe and reliable operation. However, the market varies widely in quality and price – some manufacturers cut corners to compete. When purchasing, consider all factors and choose a product that meets your specific needs.
For more information, please contact: xinhong electrical.
