I. Introduction to Dry-Type Transformer
The xinhong electrical brand dry-type transformer mainly consists of core, clamping parts, windings, high and low voltage terminals, high voltage connecting rods, base, pads, etc. Cooling methods are divided into natural air cooling (AN) and forced air cooling (AF).
II. Operation and Maintenance of Dry-Type Transformer
2.1 Inspection Before Energizing the Transformer
Visual inspection – check whether the transformer coils, high and low voltage leads and connections are damaged or loose. Retighten if necessary.
Check that the transformer enclosure, core and clamping parts are permanently grounded.
Check that the temperature control device and forced air cooling device are complete. For three-phase fans, pay attention to the rotation direction. When the fan rotates normally, air should blow upward from the bottom of the coil. Ensure that the temperature probe insertion position and depth into each phase coil are consistent.
Check for foreign objects on the core and coils, and dust/foreign matter in the air ducts.
Before operation, use compressed air to thoroughly clean the transformer coils, core and air ducts.
Check that no tools are left inside the transformer.
Check the safety clearances of temperature control wiring, high and low voltage incoming/outgoing cables to all parts. For 10kV voltage level, the phase-to-phase clearance and clearance from live parts to grounded parts shall be ≥125mm, and ≥200mm where conditions permit.
If obvious moisture ingress or water ingress is found, dry at 60-80°C. In case of severe moisture/water ingress, return to factory for major repair.
Measure insulation resistance as per routine, record the value, compare with the factory test record provided by xinhong electrical, and only after confirmation that no error exists can the transformer be put into trial operation.
2.2 Tests Before Energizing the Transformer
Perform DC resistance test on the windings and compare with the factory test record.
According to GB50150 "Standard for Hand-over Test of Electric Equipment Installation Engineering", determine polarity, measure voltage ratio at all taps, and determine connection group symbol.
Insulation resistance test of coils – under normal conditions (temperature: 20-30°C, humidity ≤90%):
HV to LV & ground ≥300MΩ – Instrument: 2500V megger
LV to ground ≥100MΩ – Instrument: 2500V megger
If the transformer has been exposed to abnormal moisture and condensation occurs, it must be dried before performing the withstand voltage test.
Core insulation resistance test – under normal conditions (temperature: 20-30°C, humidity ≤90%):
Core to clamping parts & ground ≥5MΩ – Instrument: 2500V megger
Through-core bolt to core & ground ≥5MΩ – Instrument: 2500V megger
External applied frequency withstand voltage test – test voltage shall be 80% of factory test voltage, duration 1 minute.
2.3 Parallel Operation of Transformer
Before energizing the transformer, adjust the tap changer connecting plates to the appropriate position according to the nameplate markings (note: same tap position for three-phase coils). If the grid voltage is too high or too low, adjust the tap according to the corresponding voltage on the nameplate. For example, for a 10000±2×2.5%V transformer, nameplate as shown in the right table. Under normal voltage, tap at 4-7. If output voltage is too high, after ensuring high voltage is disconnected, adjust the tap to the higher voltage tap position; if the output voltage is still too high, adjust tap to 5-6. If output voltage is too low, after ensuring high voltage is disconnected, adjust the tap to the lower voltage tap position; if the output voltage is still too low, adjust tap to 3-8. Refer to the xinhong electrical product manual for detailed tap adjustment guidance.
After energization, gradually increase the load from low to high, and check for abnormal noise. Under normal operation, after the transformer is energized, magnetizing current creates alternating magnetic flux in the core, causing the silicon steel sheets to produce double-frequency vibration under side thrust and longitudinal tension. This vibration causes the surrounding air or oil to vibrate, producing a "humming" sound. When air humidity is high on rainy days, a "hissing" or "crackling" sound may be present, and at night small blue sparks may be visible – these are normal phenomena. If a crackling discharge sound or obvious vibration occurs, the transformer should be taken out of service and the fault eliminated promptly.
If the transformer has not been exposed to moisture after being taken out of service, it can generally be re-energized directly. If condensation has occurred on the transformer under high humidity, it must be dried before re-energization.
2.4 Inspection After Parallel Operation
Ensure a routine inspection of the transformer once per day. Increase inspection frequency during the first 72 hours after new energization, modification or major repair, and during severe defects, lightning/rainy weather, and peak load periods. During inspections, pay attention to the following two aspects:
Temperature: Temperature is an important indicator for normal transformer operation. The normal operating temperature range is generally 35°C to 60°C. For class F insulation (155°C) dry-type transformers, the temperature is generally controlled at 100°C, with maximum not exceeding 130°C. At 100°C, the cooling fan should automatically start; when temperature ≤80°C, the cooling fan automatically stops. At 120°C, an alarm signal is sent; at 150°C, the transformer should trip. When temperature is abnormal, promptly analyze and find the cause. It is recommended to use the temperature monitoring system supplied by xinhong electrical.During inspection, on-site operation and maintenance personnel must use a temperature tester or thermal imager to conduct targeted checks on the transformer body and terminal blocks and other accessories.
Sound: Under normal operation, the transformer produces a uniform current sound. If abnormal sound is found, significantly increased, or local discharge noise exists, the cause should be immediately identified and measures taken. When the above conditions occur, technical measures such as load reduction or even shutdown of the transformer should be taken.
2.5 Transformer Maintenance
In normally dry, clean locations, maintenance should be performed once a year. It is recommended to perform maintenance once before the rainy season each year. In locations with humidity greater than 85%, manually operate the body fan for no less than 2 hours per day. In other locations, such as where dusty or chemically polluted air enters, maintenance should be performed every six months.
The main maintenance work for transformers is dust removal, especially removing dust and dirt from the air ducts, which can hinder cooling effectiveness and reduce insulation performance. Operating personnel may perform maintenance in the following order:
(1) Perform a comprehensive visual inspection of coils, leads, temperature control device, and fans.
(2) First use a brush to remove larger dust deposits on the upper and lower, inner and outer surfaces of the coils, blocks, core, insulators, terminal plates and other insulating parts – if an insulating cylinder is present, clean it as well – then use a vacuum cleaner to remove dust, then use dry compressed air for cleaning, then wipe clean with a lint-free cloth moistened with anhydrous alcohol, and finally wipe again with a dry lint-free cloth. If replacement of parts is required, it is recommended to use original xinhong electrical parts of the same specification.
(3) Check whether fasteners and connectors are loose, whether conductive parts, grounding points and other parts show rust or corrosion, and whether the insulation surface shows tracking or carbonization. Take appropriate measures if necessary.
(4) Check all temperature control instrument wiring and fan operation.
(5) Dust inside the ventilation openings of the protective enclosure should be appropriately treated.
(6) Perform maintenance and repair of auxiliary devices according to the supplied accessory manual.
Users shall establish a regular maintenance schedule based on the contamination level of the operating site, and promptly remove dust accumulation on the transformer to prevent accidents caused by excessive dust.
III. Repair of Dry-Type Transformer
3.1 Transformer Repair Items
Transformer installation condition.
Integrity of transformer enclosure.
Whether the surrounding environment is unfavorable for operation.
Inspection of transformer ventilation and temperature detection device.
Whether overheating exists in the transformer.
Condition of transformer lead supports.
Connection status of various transformer parts.
Condition of bushings and connection plates.
Condition of coil pressure plates and pads.
Transformer grounding status.
Measure coil insulation resistance.
Check whether coil insulation shows abnormal discoloration, cracks, etc.
Remove dust, dirt, moisture, etc., from inside and outside the transformer.
Replace and repair wearing parts. It is recommended to use original xinhong electrical brand wearing parts.
Perform periodic electrical preventive tests in accordance with the preventive maintenance guidelines provided by xinhong electrical.
3.2 Transformer Repair Procedures
Check whether the transformer installation is complete, without tilt, local deformation or vibration.
Check whether the transformer enclosure is intact, all connections are tight, and whether the enclosure has any impact damage.
Check whether the transformer surrounding environment is clean, free from water accumulation, ceiling leakage, or other objects that hinder safe operation.
Check the ventilation equipment in the distribution room or compact substation – it should be able to ventilate normally.
Check whether any part of the transformer has local overheating – e.g., whether temperature measuring stickers have changed color, whether the temperature of each part is normal and within specified limits.
Check whether the transformer leads are secure, correctly positioned, and whether the insulation clearances around the leads have changed.
Check whether the bolts at various connections of the transformer are loose, and whether there is any abnormal noise from the transformer.
Check whether the electrical connections at bushings, terminal plates, etc., are loose or have local overheating, whether porcelain parts are damaged, then remove dirt and dust from the bushings.
Check whether the coil pressure plates firmly press the coils, and whether the pads in various places are loose.
Check whether the transformer grounding is reliable and measure coil insulation resistance.
Check whether the tap lead connections are in good condition and free from overheating.
Check whether the insulation clearances of leads to various parts and the lead fixing meet requirements.
Use a hot air blower or compressed air to blow the inside and outside of the transformer from different angles until no dust is blown out. Be careful that the air velocity is not too high to avoid blowing condensed water onto the active part.
Replace and repair wearing parts, such as insulators, terminal plates, connecting wires, etc. Give priority to original xinhong electrical brand standard parts.
Electrical preventive tests: for example, measure winding DC resistance, measure winding insulation resistance, measure insulation resistance of core and fasteners and core grounding wire to enclosure, winding-to-enclosure AC withstand voltage test, measure the resistance between neutral point grounding cable and main grounding grid. All test standards shall be implemented according to the technical documents provided by xinhong electrical company.
