Inspection, Acceptance, Energization, and Maintenance of Dry Type Transformers

Inspection Before Operation - Some Preparatory Work Before Energization

1.1. Verify if all the accessories removed from the transformer have been properly reinstalled.

1.2. Check if there are any unrelated items around the transformer, especially carefully inspecting the transformer body and air ducts for any foreign objects. Recheck all fasteners with a wrench.

1.3. Check if the transformer core and clamping parts are reliably grounded. Pay special attention to whether the grounding piece removed during the inspection of the core insulation has been restored.

1.4. Check if the casing is reliably grounded.

1.5. Ensure that the temperature measuring device's probe is properly installed and fixed, and check and adjust it according to the instructions of the temperature measuring device.

1.6. Check if the blower cooling system is reliably grounded, verify if the fan blades rotate in the correct direction, and ensure that the airflow is directed from the bottom of the transformer to the coil. If the direction is reversed, the three-phase power supply phase sequence should be adjusted (if the fan is a three-phase motor). Ensure there are no foreign objects on the fan or in the air ducts.

1.7. Finally, use dry, clean compressed air to blow off dust on the transformer, and clean any stains with anhydrous alcohol.

Acceptance Inspection On Site - Tests Required for Field Reception

Tests to be conducted after the transformer installation is completed and before operation:

1. Measure the DC resistance of the windings at all tap positions.

2. Perform polarity checks, measure the voltage ratio of the windings at all taps, and determine the connection group designation.

3. Check whether the transformer casing is securely grounded, remove the core grounding piece, and use a 2500V megohmmeter to check the insulation condition of the core. After meeting the requirements, reinstall the grounding piece and check whether the core grounding is good (the core should only be grounded at one point).

4. Test the insulation resistance of the coils. Under normal conditions (temperature: 20–30°C, humidity ≤90%):

- High voltage coil to low voltage coil and ground ≥ 300MΩ, Instrument: 2500V megohmmeter

- Low voltage coil to high voltage coil and ground ≥ 100MΩ, Instrument: 2500V megohmmeter

In a relatively humid environment, the insulation resistance of the transformer may decrease. Generally, for each 1000V rated voltage, the insulation resistance value should not be less than 2MΩ (measured at 25°C for one minute) to meet operational requirements. However, if the transformer is abnormally humid and condensation occurs, no matter the insulation resistance, it must be dried before a withstand voltage test or operation (the drying method depends on site conditions; the simplest method is to use hot air drying, infrared lamps, or a combination of both. The heating process should be gradual, with the ambient temperature around the transformer maintained at 60°C–80°C. The transformer can only be put into operation after drying to meet the required standards).

5. Test the insulation resistance of the core. Under normal conditions (temperature: 20–30°C, humidity ≤90%):

- Core to clamps and ground ≥ 2MΩ, Instrument: 2500V megohmmeter

Similarly, in a more humid environment, this value may decrease, but as long as the resistance value is ≥ 0.1MΩ, operation is possible. Drying treatment can generally be used to meet the required standards.

6. For on-load tap-changing transformers, necessary inspections and tests should be performed according to the on-load tap-changer operating manual before commissioning.

- External power frequency withstand voltage test: the acceptance test voltage should be 85% of the factory test voltage (during the test, the temperature controller and sensor plug must be disconnected).

Precautions During Energization

1. Before the transformer is put into operation, the tap changer should be adjusted to the appropriate position according to the transformer nameplate and tap indicator.

2. For off-load voltage regulation, the connection plate of the tap changer should be connected to the corresponding position according to the grid voltage, nameplate, and tap indicator.

3. For on-load voltage regulation, please refer to the operating manual of the on-load tap changer. The tap switch can only be put into operation after testing under power-off conditions.

4. After confirming that the protection devices are activated, the transformer should undergo three full-voltage no-load switch-ons to test the transformer's ability to withstand operational overvoltages and inrush currents. The interval between two voltage surges should be more than 5 minutes. If no abnormalities occur, the transformer can run under no-load for 24 hours.

5. If the transformer is equipped with a temperature controller and temperature display instrument, please refer to the respective manuals for wiring the instrument power supply and other secondary circuits. Once the temperature control and display systems are properly tested, first put the transformer into operation, and then activate the temperature control and display systems.

6. The transformer should be energized in a no-load condition. The peak value of inrush current can reach 8 to 10 times the rated current. The current quick-action protection setting value for the transformer should be greater than the inrush current peak.

7. After the transformer is put into operation, the load should be increased gradually from light to heavy, and any abnormal sounds from the product should be checked. Avoid connecting a large load blindly all at once.

8. If the transformer is equipped with an on-load tap changer, after powering on, the on-load tap switch should be operated through one full cycle to check whether the transformer output voltage meets the requirements and if the switch operation is normal (under no-load conditions).

9. After the transformer is taken out of operation, it can generally be put back into operation without any special measures. However, if the transformer has been exposed to high humidity and condensation has occurred, drying treatment must be carried out before it can be re-energized.

10. All tests and energizing processes of the transformer should be recorded and filed for reference.

Maintenance

1. Daily maintenance of transformer

To ensure the transformer operates properly, regular inspection and maintenance are necessary.

1.1 Generally, in dry and clean environments, an inspection should be carried out once a year or longer. In other cases, such as where dust or chemical fumes may enter the air, an inspection should be done every three to six months.

1.2 During inspections, if excessive dust accumulation is found, it must be removed to ensure proper air circulation and prevent insulation breakdown. Special attention should be given to cleaning the transformer’s insulators and the raised pads at the base, using dry compressed air (2–5 atmospheres) to blow out dust from the ventilation ducts.

1.3 Check if fasteners and connectors are loose, and if any conductive parts show signs of rust or corrosion. Also, examine the insulation surfaces for tracking or carbonization. If necessary, take appropriate measures to address these issues.

1.4 After several years of operation (recommended after five years), it is advised to perform tests on insulation resistance, DC resistance, and power frequency withstand voltage every one or two years to determine whether the transformer can continue operating.

2. Notices for Maintenance

2.1 All the spare parts can be used after the inspection of spare parts is good.

2.2 After equipment arrived at the site, first thing is to check the product item no.;

2.3 Before work you must disconnect the main switch of low voltage, if there is contact cabinet, and it should be disconnected.

2.4 Disconnect the high voltage switch and it ground, hang "do not switch" sign;

2.5 High voltage terminals must be grounded with wire firmly

2.6 Check the control power of over temperature alarming and over temperature trip

2.7 Repeat the examination above step from 3 to 6, and then we can replace components after safety confirmed and wiring diagram read carefully.

2.8 After the replacement of components, count materials and tools; the sensor of transformer temperature controller should be fixed by plug; Check and tighten transformer fasteners. Check whether there is any other fault or trouble hidden on transformer; close enclosure door and temperature control door after the inspection of transformer;

2.9 Demolition of the ground wire of transformer HV terminals, remove "do not switch" sign;

2.10 Restore power to transformer, after running for a few minutes normally, close low voltage switch; set up temperature control instrument power, after observation for a few minutes, do manual or simulation run; power can be restored after everything is normal.

Transformer safety precautions

1. The power supply for the temperature controller (and fan) should be obtained from a switchboard and not directly connected to the transformer.

2. Before the transformer is put into operation, a thorough inspection of the grounding system in the transformer room must be conducted.

3. The door of the transformer enclosure should be securely closed to ensure electrical safety.

4. Measures must be taken to prevent small animals from entering the transformer room to avoid accidents.

6. When entering the transformer room, staff must wear insulated shoes and maintain a safe distance from live parts. Do not touch the transformer.

7. If the transformer noise suddenly increases, immediately check the transformer load and grid voltage, closely monitor the temperature changes, and promptly consult with relevant personnel.

8. The transformer should be inspected every 3–5 years, during which preventive tests can also be conducted.

9. The installation, testing, operation, and maintenance of the transformer must be carried out by qualified professionals.

9. The electrical safety clearance of the transformer should not only consider electrical safety but also ensure proper ventilation, cooling, and ease of operation for staff. Refer to the table below for the recommended clearances:

10. In general, to facilitate the adjustment of taps, the high-voltage side should be kept at a suitable distance from obstacles. If you do not request a transformer enclosure at the time of purchase but need to install a metal fence during installation, the metal fence must be reliably grounded. Refer to the table below for the safe distance between the metal fence and the live parts of the transformer:

At this time, compliant warning signs should be posted on the metal fence. Before entering the fenced area, the transformer must be de-energized.

11. After the transformer is put into operation, it is strictly prohibited to touch the transformer body to prevent accidents.

12. The testing, installation, and maintenance of the transformer must be carried out by qualified professionals.