What Are You Looking For?
Dry-type transformers are a class of power transformers that use air or solid insulation instead of liquid. In China, the prevalent models are three-phase solid-cast SC series, such as the SCB9 wound type and SCB10 foil type. These typically operate within 6–35 kV and can achieve capacities up to 25 MVA. Their non-liquid composition lends both safety and eco-friendly appeal. Let’s delve deeper into their design, cooling methods, varieties, advantages versus oil-filled units, installation and diagnostics, acoustic issues, and selection criteria.
Exposed to the atmosphere, this form demands a dry, clean indoor environment—ideally below 85 % humidity at 20 °C. Cooling is achieved via natural convection or forced air. It’s simple and effective but requires strict environmental control.
The transformer’s body is housed within a sealed enclosure, isolating it from ambient air. With somewhat hindered cooling, this explosion-resistant format is well-suited for mining and hazardous environments.
Utilizing epoxy or similar resins for insulation, this compact, streamlined form is apt for smaller capacities. Its monolithic structure provides robustness and high dielectric strength.
Dry-type units utilize:
Natural Air (AN): Permits continuous operation at rated capacity.
Forced Air (AF): Boosts output by up to 50%, ideal for intermittent or emergency overloads.
Note: Overheating and increased impedance during overloading render this economically imprudent for continuous use.
Impregnated Air-Insulated: Rarely used today; wound with wire insulated to Class B, F, or H standards.
Epoxy Cast: Insulation via polyester or epoxy—epoxy prevalent in modern units.
Wound Resin-Insulated: Similar resin-wound construction; now produced by few manufacturers.
Hybrid Insulation:
High-voltage windings cast; low-voltage impregnated.
Or high-voltage cast, low-voltage foil-wound with copper/aluminum.
Inspect packaging integrity, verify nameplate data and documentation, and assess external damage or part displacement.
Ensure leveled foundation with no voids under embedded steel plates. Move transformer with rollers, fine-tune positioning, weld reinforcement steel plates at corners to prevent shifting.
Maintain proper phase-to-phase and phase-to-ground clearance, especially at the cable-to-HV coil interface. For currents >1,000 A, use flexible connectors to accommodate thermal expansion and suppress vibration. Use spring washers or Belleville discs and torque wrenches per manufacturer specs.
Connect grounding at the LV base’s specific terminal, link transformer casing and neutral line reliably to the protective earth system.
Verify all fasteners, electrical connections, and insulation distances. Clean coil surfaces and ensure the surrounding area is free of debris.
Measure winding resistance and tap-changer position; compare to factory data.
Insulation resistance: moisture-indicated if below factory baseline; drying needed when <1,000 Ω/V.
Conduct dielectric withstand tests; remove temperature sensors (e.g., TP100) before testing and reinstall immediately after.
Test any installed fans for proper operation.
Once energized, monitor noise, odors, discoloration, and ventilation. Also ensure:
Relatively low humidity (<70 %) enhances insulation reliability; avoid prolonged shutdowns.
In step-up power stations, never energize under LV open-circuit to prevent overvoltage. Use surge arresters like Y5CS ZnO for protection.
Phase Loss: Silent in one phase until another phase feeds current—indicative of supply or fuse problems.
Tap-Changer Issues: Loud chirps or spark-like hisses point to misalignment or poor contact.
Foreign Objects / Loose Clamps: Metallic knocks or whooshing airflow sounds indicate stray objects or loose iron-core bolts.
Porcelain Bushing Damage: Hissing at night with visible sparks—signs of surface arcing.
Core Ground Break: Tingling intermittent discharge sounds from earth fault.
Internal Discharges: Crisp pops or dull rumbles signal insulation faults—internal or oil-based.
External Line Faults: Froggy croaks from wind-caused arcing; heavy hums for LV short circuits; tiger-like roars for near-source faults.
Overload: Low-frequency drone reminiscent of a heavy aircraft.
Overvoltage: High-pitched whining from excessive magnetic flux.
Winding Short-Circuits: Gurgling, boiling sounds from interlaminar or inter-turn faults.
Fan Noise:
Clang sounds if debris inside.
Continuous grinding indicates worn components—replace promptly.
Loose Enclosure:
Vibrations transmitted through panels.
Use rubber gaskets between casing and base or floor to dampen.
Acoustic Interference:
Resonance between transformer vibrations and electrical room boundaries.
Adjust placement or apply sound-absorbing materials to walls.
Foundation Deficiencies:
Weak flooring can resonate—visible cracks or tremors underfoot.
Reinforce supports and fix the foundation.
Large Tier-1/Tier-2 Loads: Use at least two transformers to maintain continuity during downtime. Keep critical loads consolidated.
Seasonal High Demand: Assign dedicated units for HVAC, heating, or refrigeration.
Concentrated High-Demand Equipment: Examples include arc furnaces, X-ray machines—appoint specialized transformers.
Lighting-Dominant Circuits: Provide dedicated lighting transformers to optimize voltage quality and lamp lifespan.
Normal Conditions: Choose dry-type or oil-filled based on site needs (e.g., industrial, rural substations) with models like S8, S9, S10, SC(B)9/10.
High-Rise or Multi-Story Buildings: Opt for flame-resistant types (SC(B)9, SC(B)10, SCZ variants).
Dusty or Corrosive Atmospheres: Select sealed or explosion-proof variants (e.g., BS9, SH12-M).
Integrated Non-Oil Installations: Ensure IP2X rated enclosure when co-located with other electrical gear.
Dry-type transformers have ascended as indispensable assets in modern power distribution due to their high dielectric resilience, short-circuit robustness, environmental safety, and minimal maintenance. Yet, the market is marked by uneven quality and aggressive price competition—often at the expense of performance. When procuring, technical staff and buyers should employ a comprehensive evaluation—scrutinizing design, build quality, certification, manufacturer reputation, after-sales support, and actual operating parameters—to ensure reliable, safe, and economical transformer deployment.
