A dry-type transformer is often the most important and most expensive electrical equipment in a power substation or an industrial plant. Because it uses air or cast resin instead of oil for cooling and insulation, its accessories play a decisive role in temperature control, safety and operational flexibility. Understanding what each accessory does, and checking it regularly, is the fastest way to achieve optimum performance and extend the product life. This guide covers the standard accessory set of a cast resin dry-type transformer and the maintenance points that go with it.
Why Accessories Matter for Dry-Type Transformers
Dry-type transformers are designed for indoor use, fire-safe installations and locations where oil-filled units are not permitted. Because there is no oil to absorb heat or indicate faults, the operator relies on instruments and auxiliary devices to monitor temperature, protect personnel and keep the unit within its rated envelope. Regular maintenance checks on these accessories are not optional: a thermometer that is not calibrated, a cooling fan that fails, or a loose grounding connection can turn a minor overload into a costly failure. The design and test requirements for dry-type transformers are defined in IEC 60076-11, with national equivalents such as GB 1094.11.
Temperature Monitoring Accessories
Digital type thermometer
The digital thermometer provides a continuous readout of the winding temperature, usually with alarm and trip contacts that protect the transformer against overheating. Modern units can log temperature history and communicate with the control system.
Dial type thermometer
The dial thermometer is a simple mechanical instrument located on the top of the body. It gives a local, battery-free temperature reading and is useful for routine visual inspection and as a redundant check alongside the digital unit.
Electrical and Safety Accessories
HV and LV terminals
The high-voltage terminals are arranged on top of the coil, and the low-voltage terminals on top of the body. Both must be checked before an operation: terminal bolts loosen with thermal cycling, and a poor connection causes hot spots that damage the coil and the cable lugs.
Grounding terminal
The grounding terminal is arranged on the bottom of the frame and connects the transformer enclosure and core to the station earth grid. It should be inspected and torque-checked before commissioning and at every maintenance interval, because a broken ground leaves the enclosure live under fault conditions.
Label and rating plate
The label carries the safety instructions and operating restrictions, including the notice that contact is restricted during operation. The rating plate states the rated power, voltages, impedance and standard, and should always be legible and matched to the design documentation.
Mechanical and Installation Accessories
Lifting lug
Lifting lugs are arranged on top of the frame for crane handling. Use all lugs with a properly rated spreader beam and keep the lifting angle small, because a one-point lift can distort the frame and crack the cast resin coils.
Bi-directional wheel
Bi-directional wheels allow the transformer to be travelled lengthways or sideways inside the room, which simplifies positioning during installation and future maintenance access. Wheels must be locked after the unit is in its final position.
Anti-vibration pad
The anti-vibration pad is placed between the frame and the floor to reduce noise and vibration transmitted to the building. Check the pads for compression and deterioration, and replace them if the unit shows abnormal noise or movement.
Cooling and Protection Accessories
Cooling fan
Cast resin transformers can be equipped with auxiliary ventilation that permits a power increase of 30 to 40 percent above the natural-air-cooled rating. Fans are controlled by the winding temperature: they start at a set point and stop below it. Keep the air intake and outlet clear, and test the fan direction and contactor during maintenance.
Enclosure
The enclosure protects personnel against accidental contact with live parts and gives the transformer a defined degree of protection against dust and objects. Enclosures are available in different protection ratings, and ventilation openings must never be blocked, because the enclosure directly influences the cooling performance and therefore the allowable load.
| Accessory | Main function | Key pre-operation check |
|---|---|---|
| Digital thermometer | Winding temperature readout, alarm and trip | Calibration and alarm set points |
| Dial thermometer | Local mechanical temperature reading | Glass and pointer condition |
| HV / LV terminals | Electrical connection of windings | Bolt torque and insulation distance |
| Grounding terminal | Safety earthing of frame and core | Continuity and torque |
| Lifting lug | Crane handling of the unit | Weld and lug condition before lift |
| Bi-directional wheel | Movement in the room | Locking after positioning |
| Anti-vibration pad | Noise and vibration damping | Compression and wear |
| Cooling fan | Forced cooling, 30 to 40% extra power | Rotation direction, airflow, contacts |
| Enclosure | Personnel protection, degree of protection | Ventilation openings clear |
Tap Terminal Link and Voltage Adjustment
The tap terminal link on the low-voltage side adjusts the transformer output to suit the system conditions, and it may only be operated in the no-load state. Changing the tap position while the transformer is energised causes arcing and can destroy the connection. Record the tap position, and after any change verify the output voltage with the unit energised.
Maintenance Routine That Keeps Accessories Reliable
Schedule a periodic maintenance routine per the manufacturer instructions and IEC 60076-11. Before each operation, check the grounding, terminals, labels and thermometer readings. At annual intervals, test the cooling fan operation, inspect the enclosure seals and ventilation, check the anti-vibration pads, and verify the alarm and trip circuits of the digital thermometer with a simulated signal. Keep the dust off the coil surfaces, because dust layers reduce heat dissipation and accelerate insulation ageing.
Frequently Asked Questions
Q1. Why does a dry-type transformer need so many accessories?
Because there is no oil to cool the windings or indicate faults, the transformer relies on instruments, fans and safety devices to stay within its rating. Each accessory covers one part of that protection chain.
Q2. Can the cooling fan really increase the transformer power?
Yes. With auxiliary forced-air cooling, a cast resin transformer can typically carry 30 to 40 percent more load than its natural-air-cooled rating, which is why the fan condition directly affects the usable capacity.
Q3. How often should the thermometer be checked?
The digital thermometer and its alarm and trip circuits should be checked at every maintenance interval, typically once a year, and calibrated according to the manufacturer instructions. The dial thermometer should be compared against a master instrument during the same visit.
Q4. What happens if the grounding terminal is loose?
The enclosure and core may stay at a dangerous potential during a fault, and circulating currents can damage bearings and insulation. Loose grounding is one of the first items to check when unexplained noise or vibration appears.
Q5. Is it safe to change the tap position while the transformer is running?
No. The tap terminal link may only be operated in the no-load state. Operating it under load creates arcing, burns the contacts and can damage the winding.
Q6. Which standard applies to dry-type transformer accessories?
The transformer itself and its accessories are designed and tested to IEC 60076-11, with national equivalents such as GB 1094.11. Accessory ratings such as the protection degree of enclosures follow IEC 60529.