Preparations Before the Distribution Transformer is Put Into Operation: 4 Key Points

Before being put into operation, distribution transformers must undergo a series of strict on-site tests. These tests are the key steps to ensure that the transformers can be safely and reliably put into operation. They can be divided into 4 parts: visual inspection, electrical tests, protection verification, and impact closing test.

1. Appearance and Installation Inspection
Before powering on, a detailed visual inspection of the power transformer and its accessories is necessary.
- Label verification: Check whether the model, rated capacity, voltage level, connection group, etc. on the transformer's label are consistent with the design requirements.
- Visual inspection: Check if the transformer's appearance is damaged, and whether the connections (such as covers, bushings, valves) are well sealed and if there is any oil leakage.
- Oil level inspection: For oil-immersed transformers, check if the oil level in the oil reservoir is clear and matches the oil level line corresponding to the ambient temperature.
- Key accessory inspection:
- Check if the explosion-proof membrane of the explosion-proof tube is intact.
- Check if the desiccant (silica gel) in the breather has changed color and become ineffective.
- Check if the grounding of the shell is firm and reliable.
- Check if the primary and secondary outlet bushings and wire connections are in good condition, and if the phase colors are correct.
- Installation quality inspection: Verify if the installation position, center line, and elevation of the transformer meet the design requirements, and check if the fasteners are complete and tightened.

II. Electrical Handover Test
This is the core step for evaluating the insulation condition and electrical performance of the transformer. It must be carried out in accordance with the national standard "Electrical Installation Engineering - Electrical Equipment Commissioning Test Standard" (GB 50150). Only after all the tests are passed can it be put into operation.
1. Insulation resistance test: Use a 1000-2500V megohmmeter to measure the insulation resistance between the windings and the ground, as well as between the windings themselves. The test results should not be lower than 70% of the factory value, or there should be no significant decrease when compared with historical data.
2. DC resistance measurement: Measure the DC resistance of the windings together with the bushings. For distribution transformers, the difference between the DC resistances of each phase should be less than 4% of the average value, and the difference between phases should be less than 2% of the average value. This test can effectively check the contact of the tap changer, the welding of the wires, and whether there are broken strands or short circuits inside the windings.
3. Voltage ratio and connection group verification: Verify the voltage transformation effect of the transformer and whether the connection group is consistent with the nameplate marking.
4. No-load and load characteristic tests:
- No-load current and no-load loss (iron loss) measurement: Verify the material and assembly quality of the core.
- Short-circuit impedance and load loss (copper loss) measurement: Verify the condition of the winding structure and the conductive circuit.
5. Power frequency withstand voltage test: Test the main insulation strength of the transformer under the specified power frequency voltage.
6. Transformer oil test (applicable to oil-filled transformers): Conduct tests such as voltage withstand, impurities, and moisture on the transformer oil to check if the oil quality is qualified.

III. Protection and Grounding Inspection
The correctness of the protection device and grounding system is the guarantee for the safe operation of the transformer.
- Fuse inspection: For distribution transformers protected by fuses, it is necessary to verify whether the fuse specifications are correct. Generally, the primary-side fuse is selected at 1.5 to 2 times the rated current, and the secondary-side fuse is selected based on the rated current. If the fuse is too large, it will lose its protective function; if it is too small, it may cause false melting during normal operation or when an overload is allowed.
- Ground resistance test: Using a ground resistance tester, confirm that the grounding connection method and ground resistance value of the transformer's neutral point comply with the design requirements. This is an important measure to ensure the safety of personnel and equipment.

IV. Impact Closing and No-load Test Run
After completing the above static checks and tests, the power-on trial operation phase will commence.
- No-load impulse closing: Conduct an impulse closing test (typically 5 times) on the transformer without load. The purpose is to verify the correctness of the transformer and its secondary phase alignment, and to test the insulation and mechanical strength of the transformer using the excitation surge current. During the closing process, listen for any abnormal sounds inside the transformer and check if the instrument indications are normal.
- Secondary phase alignment and voltage check: After the transformer is running without load, check if the voltage on the low-voltage side is balanced and complete the secondary phase alignment to ensure the phase is correct.

In summary, the on-site inspection before commissioning is a rigorous closed-loop process: from the exterior to the interior, from insulation to protection, from static to dynamic, each step is indispensable and together ensures that the distribution transformer can be safely and reliably put into operation.