Safe and Compliant Transformer Commissioning – Essential Requirements
(1) For forced-oil circulation transformers, coolers shall be put into operation one by one, and the number of coolers in service shall be reasonably controlled based on load conditions. For water coolers, the oil pump shall be started first, followed by the water system.
(2) Transformer charging shall be carried out using a circuit breaker on the source side where protective devices are installed. That is, the disconnector shall be closed first, followed by the circuit breaker, to energize the transformer.
(3) According to the national standard GB 50148-2010, new transformers and reactors may be energized by full-voltage impulse closing when first put into service. A total of five impulse closures shall be performed. The duration after the first energization shall not be less than 10 minutes, and the inrush current shall not cause misoperation of protective devices.
For transformers that have undergone winding replacement, when first put into service, three full-voltage impulse closures shall be performed in accordance with DL/T 572-2010.
(4) If no circuit breaker is available on the source side of the transformer, a three-phase gang-operated disconnector equipped with arc-extinguishing horns may be used to switch no-load transformers with a voltage of 110 kV or below and a current not exceeding 2 A. For disconnectors installed indoors, arc-resistant insulating barriers shall be installed between phases.
(5) The use of fuses is permitted for switching no-load distribution transformers and station service transformers of 66 kV and below.
(6) To prevent localized severe overheating of the transformer due to obstructed circulation of insulating oil at low temperatures, for natural oil-cooled or air-cooled transformers, if the oil temperature is below –30 °C, the transformer shall not be immediately loaded to its rated capacity upon commissioning. For forced-oil air-cooled transformers, if the oil temperature is below –25 °C, the transformer shall not be immediately loaded to its rated capacity. In such cases, the transformer shall first be operated under no-load conditions or with a light load not exceeding 40%–50% of the rated load. This is to avoid a large temperature difference between the oil and the windings caused by slow oil flow at low temperatures, which could accelerate insulation aging if the winding temperature becomes too high. However, these requirements may be relaxed during emergency handling.
(7) When ambient temperatures are low, cooling fans may remain out of service. For example, if the top oil temperature of the transformer does not exceed 55 °C, the transformer may operate at rated load even without the fans in operation.
(8) During transformer commissioning, personnel shall not remain in the vicinity to prevent personal injury caused by incidents such as oil spray, fire, or, in the case of dry-type transformers, loud noises or dense smoke resulting from inter-turn short circuits. After the transformer has been energized, if no abnormalities are observed on ammeters, voltmeters, or other monitoring instruments, and no abnormal sounds are heard from a distance, personnel may approach the transformer to listen carefully for any internal abnormal noises. However, care shall be taken not to stand directly in front of the explosion vent to avoid potential accidents.
(9) If the circuit breaker fails to close during transformer commissioning—i.e., it does not stay closed—the cause may be either a protective device operation causing the breaker to trip after closing, or the closing mechanism not being fully latched, resulting in the breaker opening automatically after closing. In such cases, a second closing attempt shall not be made immediately; instead, a waiting period of several minutes shall be observed. If tripping is caused by protective device operation, the cause of the protection operation shall be identified, and the transformer shall be thoroughly inspected for any faults. If the tripping results from the protection failing to withstand the inrush current, consideration shall be given to adjusting the protection settings. If the tripping is due to improper closing operation or failure of the mechanism to latch, an interval shall be allowed before attempting to close again, to avoid transient overvoltages caused by successive impulses that could lead to transformer failure.