A 73 - Storage of Transformers in the Field

Power Transformers may have to be stored in the field due to various reasons. It may?be due to the delay in civil works or delay in the mobilization of other equipment for substations or power stations. The storage of the transformer at the site may be in different conditions – as in transported condition with gas sealing; partially assembled with oil filling, or in fully assembled condition as a spare unit. Such spare units may be assembled and connected to the grid in such a way that spare units can be switched into service with minimum delay-eg. one phase of an auto-transformer bank that can be switched into operation as a replacement for transformer in any phase. In other situations (like one phase of an HV DC converter transformer bank or generator transformer) the failed unit has to be pulled out from the foundation and the fully assembled spare unit has to be moved from its storage place and pushed to the position of the failed unit.

??????The transformer should be stored upright on a solid foundation suitable for the?period?of storage. If timbers or cribbing are used, the timbers should be closely and evenly spaced. Transformers should not be stored on?blocks or jacks. In high seismic areas, consideration should be given to anchoring or tying units down during storage. The preferred method of storage is to store the transformer under insulating liquid with all the accessories fully installed.

1.?Transformers with Dry gas filling

Large transformers are sent from the factory to the installation site with nitrogen or dry air filling to reduce the transport weight. It may be necessary to store the transformer under gas filling till the commencement of installation work. Dry nitrogen, being an inert gas, is more compatible with cellulose paper. But generally dry air is preferred for filling during transport and storage. Dry air storage has the advantage of reduced exposure of transformer insulation during erection work and internal inspection because purging of the gas is not required before a person can enter the unit.

If it is not possible to store the transformer in the completely assembled condition with oil filling, the transformer is to be filled with dry gas under positive pressure, to prevent the ingress of moisture during the storage period. Storage under gas filling shall normally be limited to a maximum period of six months. This is recommended as chances of moisture entry through oil seals are more when stored with gas filling. ?For natural ester-filled units, usage of dry air for storage is generally limited to a one-week period, if no additional procedures are taken. After the transformer is filled with dry gas/ air, one or several gas cylinders may be attached to the tank to replenish gas when required. To provide a constant positive pressure in the transformer tank, the gas cylinders are connected via a pressure reducing and pressure relief valve. The valves are adjusted to maintain a positive pressure in the tank of approx. 0.25 bar. The pressure is to be checked and recorded every two weeks. If the storage period exceeds six months, contact the manufacturer for guidance.

Caution:

a) The pressure must not fall below 0.05 bar. In case of leakages from the transformer tank or in case of pressure loss from the gas cylinders, the leakages are to be sealed.

b) If there are heating elements provided in the motor drive cabinet and control cabinets, they are to be connected to the power supply, to prevent water condensation.

??Long time storage & change of cylinders:

When the transformer is under storage for longer than one month, the filling pressure shall be checked and recorded once a week.

If the unit is opened and pressurized again the pressure must be rechecked after 24 hours. The pressure in the storage bottle may decrease with time.

Note:

1)???Nitrogen gas is sometimes used instead of dry air. In that case, a visible caution plate is affixed to the tank giving this information.

2)???Depending on the period of dry air storage, it will be necessary to adopt more than the normal time of hot oil circulation and settling time before energization. “NOTE 3—Allow the transformer to stand the prescribed time (or longer) before energizing. This permits the moisture in the liquid and insulation to approach equilibrium. When a unit has been stored gas filled for 6 months or longer a stand time of a minimum of 96 h is required” Annexure D of IEEE C.57.93

2.????A transformer in Disassembled Condition, but with Oil Filling (under temporary storage)

?In case it is not possible to install /commission the transformer completely with oil filling, the transformer tank shall be filled with oil to a level such that at least the entire insulation of the core and coil assembly is completely immersed in oil.

?a. Such transformers shall be provided with a dehydrating breather during storage to avoid the ingress of humid air into the transformer tank. Check the silica-gel content of the dehydrating breathers every two weeks. When more than 1/3 of the silica-gel filling has changed color, the filling shall be replaced or reactivated by heating at 120 °C (24 hrs.). At the same time look for any oil traces on the tank. Should any leakage be detected, they are to be sealed immediately.

?b. If the transformer is kept in a sealed condition without a breather, the gap to the tank cover above the oil level shall be sufficient to take care of the oil expansion from ambient temperature variation (air gap at tank top for oil expansion shall be at least equal to approximately 8% of the height of tank).

?c. In case a heater is installed in the motor drive cabinet, and control cabinet, they should be connected to a power supply to prevent condensation of water. Check their heating elements every month. The motor drive cabinet and control cabinet are to be checked for water condensation.

?3.???Storage of transformer?accessories and components

??Transformer accessories and components, such as radiators, coolers, bushings,?pumps, motors, control cabinets, gaskets, connectors, gauges, bushing turrets, conservators, interconnecting pipes, etc are stored before installation, such that they are available when needed and not damaged or deteriorated due to open atmosphere. Certain accessories like fans, motors, control cabinets, gauges, relays, etc shall be stored in a clean and dry environment. Bushings shall?be stored indoors in their shipping crates with their upper part in a raised position. If stored for longer periods, the bottom tail portion of RIP bushings shall be covered with metal covers with gaskets to prevent moisture ingress into the bushing condenser core. If storage is for more than three months, coolers, radiators, and conservators shall be kept with gas or oil filling. Transformer oil when stored in drums shall preferably be kept indoors in a horizontal position, with their caps in a 3-9 o’clock position to prevent moisture entry through the screwed caps.

??4. ??Transformer in fully Assembled, Oil-Filled Condition-Kept as Spare

When storing a fully assembled, oil-filled unit,?the following points may be taken care of. Such units are kept as spare and are expected to be energized at short notice.

a. Make sure that the air cell is in working condition and the breather is not bypassed by leaks in the oil conservator. This can be ensured by making sure that the oil sight window (prismatic oil level gauge) provided on the conservator side is full always. If an air cell leakage detector relay is provided, monitor the same for alarm.

?b. Change /re-activate the silica gel in the breather as soon as the color of the gel changes.

?c. Monitor the BDV and water content of oil (in the main tank, oil-filled cable box, and tap-changer compartment) once in six months.

?d. Check the capacitance and tan delta of windings and bushings once a year. Check insulation resistance of windings also once a year.

?e. Keep running the fans and pumps (where applicable) of the cooler bank once a month for one hour.

?f. Oil leakage at the normal head is to be checked before long-term storage.

?g. Just before putting on to service, check the above parameters after 24-48 hours of hot oil circulation of the transformer.

??h. All pre-commissioning tests/checks shall be carried out as per the manufacturer’s guidelines before taking the unit into service

5. ???Transformer in fully Assembled Condition, with RIP bushings in Cable Box (Oil to Oil /Oil-SF-6 Bushings)

?During the installation and storage of transformers, make sure that the RIP bushings in cable boxes are not exposed to the open atmosphere as they will absorb moisture. When the bushings are mounted in the cable box and waiting for the cable pothead, keep the cable box filled with oil/dry air. In the case of transformers with GIS directly connected to them, there may be a time delay in the installation of GIS getting connected to the transformer. On such occasions, RIP bushings in the SF6 side of the transformer shall be covered by a leak-proof metal canopy, with silica gel packets inside, to avoid the bushings absorbing moisture from the atmosphere.

?6.??References:

1)???IEEE Standard C57.93-2019 Oil-filled Transformer Maintenance -Clause 5.13

2)???CIGRE TB 445-2011 -Transformer Maintenance