1 Introduction

Ever since the first power transformers were introduced in the 1880s, they have been transported from the manufacturing plant to the installation site. Ever since the first power transformers were introduced in the 1880s, delays in transportation and installation have resulted in a need to put transformers into short-term storage. The growing importance of a reliable electricity supply to the economy and modern society led to a need for spare transformers and reactors, which sometimes needed to be stored for long periods before use.

Despite its importance in the procurement process, until recently, very little has been published on transformer or reactor storage, and standards included very few requirements related to transformer storage. IEC standard 60076-1 (2011) does not mention storage, but IEC standard 60076-11 (2018) does include some requirements for storage of dry-type transformers and reactors. CIGRE working group A2.34 gave some guidance on storage of spare components and materials, but not complete transformers (CIGRE brochure 445 2011). They also recommended that the operation and maintenance manual for transformers should include storage instructions. CIGRE working group A2.36 gave some guidance on storage of complete transformers (CIGRE brochures 528 and 530, 2013). It was apparent that further work was needed on the subject of transformer transportation, and so CIGRE Working Group A2.42 was established to continue the work of A2.36 (CIGRE brochure 673 2016). It includes some additional material on transformer storage.

As was mentioned in passing above, CIGRE Working Group A2.34 considered storage of transformer components, especially bushings. CIGRE Working Group A2.43 on bushing reliability considered bushing storage in more detail and included a chapter on bushing storage in CIGRE Brochure 755 (2019).

There is now increased interest in new solutions to overcoming some of the new challenges presented by transformer and reactor installation. CIGRE Study Committee A2 established Working Group A2.58 to investigate the subject of transformer installation, pre-commissioning, and trial operation. They have discussed storage in more detail.

2 Storage for Different Lengths of Time

Power transformers and reactors may need to be stored to accommodate constraints in manufacturing, transport, or installation. During manufacturing, the user or purchaser should be in communication with the manufacturer about any potential delays to the project. Typically transport plans and permits are completed several weeks in advance, therefore it is advisable to notify relevant parties immediately if there is a change in the project and requirement to store the transformer. This storage may take place at the factory, the installation site, or alternative storage facilities. In some cases, transformers or reactors are purchased for use as spares and may need to be stored for indefinite periods.

There are essentially three options for storing liquid-immersed transformers and reactors:

  • Drained of liquid and filled with dry gas

  • Partly filled with liquid, usually to cover the top of the core and winding assembly

  • Substantially or completely assembled, and ready for service

The first option is suitable for short-term storage to accommodate constraints in manufacturing, transport, or installation. This option can typically be used for up to 6 months following dismantling for transport after testing. Note that no liquid containment is required, as the transformer is not liquid filled.

For long-term storage, the core and winding assembly must be immersed in liquid. This prevents loss of liquid impregnation and moisture ingress. It thus preserves the dielectric strength of the insulation and prevents corrosion of the core and other parts. Some means of accommodating changes in liquid volume with temperature is required, e.g., a pressurized gas cushion in the top of the tank or a conservator. The second and third options are thus suitable for long-term storage.

The second option is applicable for medium power transformers and reactors, which are transported only partly drained and with dry gas in the upper part of the tank. Note that transporting transformers and reactors partly drained of liquid is not always considered to be good practice due to the possibility of movement of the liquid during transport making the transformer or reactor unstable, and also complications when topping up during installation. It may also be applicable where it is necessary to extend short-term storage of transformers or reactors, which are transported, drained of liquid, and filled with dry gas. The transformer or reactor must be stored in a suitable liquid containment area.

For long-term storage, according to the second option, it is also necessary to consider how the components will be stored. This is especially important for components that may become contaminated or deteriorate if not stored correctly, e.g., bushings and coolers. Note that pilfering loose items for use as spares with other transformers or reactors is a widespread practice.

The third option is often considered to be preferable for spare transformers and reactors. Note that this means they must be dismantled on deployment and may not be the best choice in all cases.

The third option is also applicable to distribution and other small power transformers, which are transported fully assembled and liquid filled. As with the second option, the transformer must be stored in a suitable liquid containment area.

2.1 Dry-Type Transformers

IEC standard 60076-11 (2018) includes some requirements for storage of dry-type transformers and reactors.

Unless otherwise specified, all dry-type transformers and reactors according to IEC standard 60076-11 (2018) should be suitable for transport and storage down to minimum temperatures lower than the required minimum operating temperature. The default requirement is for a minimum transport and storage temperature of −25 °C, and a minimum operating temperature of −5 °C.

Unless otherwise specified, all dry-type transformers and reactors according to IEC standard 60076-11 (2018) should be protected against moisture and other contamination during transport and storage. Note that transformers and reactors according to IEC standard 60076-11 (2018) are, unless otherwise specified, suitable for operation in an environment where no condensation can occur on transformers and there is negligible pollution. This is typical of clean, dry, indoor installations. It would follow that such transformers are only suitable for storage in clean, dry, indoor locations.

3 Preparation for Storage

3.1 Storage Without Liquid

Large power transformers and reactors are generally transported drained of liquid, and filled with dry gas. Medium power transformers and reactors may also be transported drained of liquid, and filled with dry gas. The dry gas may be either air or nitrogen. Use of air is generally preferable, especially in the case of indoor or underground installations. Nitrogen may be required if the transformer is filled with natural ester or may be requested by certain customers. Where there is a requirement for nitrogen, then great care is necessary to ensure that the transformer or reactor is safely vented before access during installation on-site or where access to internal parts is needed. There are credible reports of several fatal accidents caused by lack of care with transformers filled with nitrogen for transport.

When it is necessary to put medium or large power transformers or reactors into short-term storage to accommodate constraints in manufacturing, transport, or installation it is usually preferable for this to be done in the transport configuration. For medium to large power transformers or reactors this would be drained of liquid and filled with dry gas.

Medium and large power transformers and reactors can typically be stored for up to 6 months following dismantling for transport after test. For longer storage periods, there is a risk of loss of liquid impregnation from the solid insulation which may adversely affect its dielectric strength. There is also a risk of moisture ingress which may also adversely affect its dielectric strength. Ingress of moisture may also result in corrosion of the core and other parts. If the solid insulation is not adequately dried on installation, it may also affect the life of the transformer. Moisture acts as an accelerant in the solid insulation aging process. For further information, see CIGRE Brochure 738 (2018).

When transporting and storing without oil, CIGRE brochure 673 (2016) recommends filling the transformer with dry gas (air or nitrogen) at a pressure of 200 mbar gauge at 20 °C while the liquid is drained. The dew point for the dry gas would typically be in the range −50 °C and −60 °C. After draining the liquid, CIGRE brochure 673 (2016) recommends connecting a gas bottle or cylinder via a pressure-reducing regulating valve. CIGRE brochure 673 (2016) also recommends fitting a pressure gauge, at a place where it can easily be checked during transport. The location of the gas bottle or cylinder and gauge should be shown on the transport drawing. The pressure in the gas bottle or cylinder may be as high as 200 bar. The pressure inside the tank should be regulated to 350 mbar gauge. The dew point should be rechecked before transport. CIGRE brochure 673 (2016) recommend that it should be less than −40 °C at 20 °C ambient.

Normal procedures for liquid filling and circulation should be followed at the end of storage, unless there is an indication of moisture ingress where additional measures may be needed to dry the solid insulation.

3.2 Storage Partly Assembled, with Liquid

Medium power transformers and reactors may be transported only partly drained of liquid, with dry gas in the upper part of the tank. As with transformers and reactors which are drained of liquid for transport, the dry gas may be either air or nitrogen. Although the volume of dry gas is less than for transformers which are drained of liquid, use of dry air remains preferable for safety reasons. Nitrogen is required if the transformer is filled with natural ester, and may be required by certain customers.

When it is necessary to put medium or large power transformers or reactors into short-term storage to accommodate constraints in manufacturing, transport, or installation it is usually preferable for this to be done in the transport configuration. For many medium power transformers and reactors, this would be partly drained of liquid, with dry gas in the upper part of the tank.

There are limits to how long it is possible to store medium and large power transformers and reactors without liquid. If these are exceeded, then there is a risk of loss of liquid impregnation and also of moisture ingress. This may adversely affect the dielectric strength of the insulation and also cause corrosion of the core and other parts. Therefore, it is recommended to fill with liquid for periods greater than 6 months. If the storage is temporary, and especially if the transformer or reactor is not at its final location, there are advantages to filling the transformer to cover the core and winding assembly with dry gas in the upper part of the tank. Alternatively, if this is not feasible, then it is possible to assemble the conservator and associated pipework, but not the other components, and fill the transformer completely. The breather must be fitted and, if necessary, connected to an auxiliary supply. If the control cabinet is assembled onto the transformer or reactor during storage, then the anticondensation heater should also be connected to an auxiliary supply.

The liquid may also represent an environmental hazard or a fire safety hazard, especially in the case of mineral oil and similar liquids. Environmental and fire safety hazards are reduced by the use of natural or synthetic esters, although the use of nitrogen in the case of natural esters also represents a safety hazard as mentioned above. In any case, the transformer or reactor must be stored in a suitable liquid containment area. An emergency plan must be available in case of an oil spill, and suitably qualified workers must also be available to put it into action if required.

Normal procedures for liquid filling and circulation should be followed during preparation for storage, unless there is an indication of moisture ingress where additional measures may be needed to dry the solid insulation. If the conservator is assembled, then the transformer or reactor should be completely filled with a suitable reserve in the conservator to allow for changes in oil volume with temperature and minor oil leaks. If the conservator is not assembled, then the transformer or reactor should be filled with oil to cover the core and winding assembly as completely as possible. In practice this normally means filling to approximately 100 mm below the lid. This will likely be sufficient to cover the windings and associated insulation, the core, frame, and associated insulation, although perhaps not all of the leads or the insulation blocking between the core and winding assembly and the tank. The upper part of the tank is then filled with dry gas, in much the same way the complete transformer or reactor was filled with gas for transport. The dry gas pressure then needs to be maintained in much the same way the complete transformer or reactor, i.e., by connection to a gas bottle or cylinder via a pressure-reducing regulating valve, with a pressure gauge for monitoring.

Consideration should be given to storage of any components which are not assembled onto the transformer or reactor, to avoid pilfering and preserve their condition. Special precautions may be necessary for components which might otherwise become contaminated or deteriorate, e.g., bushings and coolers.

To preserve the paint finish and to avoid excessive corrosion during storage, it is important that transformers or reactors are not stored for a prolonged period in a more corrosive atmosphere than that specified. Any damage to the paintwork during transport should be made good before storage. Note in particular that dock and harbor areas typically have high salinity and often high pollution owing to other causes, and may therefore not be suitable for long-term storage of transformers or reactors which have not been specified for such an environment.

Where the transformer or reactor is stored at a substation or other electrical installation, the tank should be earthed according to normal procedures. The core and frame should also be earthed, via the tank. Any bushings which are fitted, e.g., neutral bushings, should also be earthed.

3.3 Storage Fully Assembled, with Liquid

Distribution and other small power transformers are usually transported fully assembled and liquid filled. As with the second option, the transformer must be stored in a suitable liquid containment area.

When it is necessary to put distribution and other small power transformers into short-term storage to accommodate constraints in manufacturing, transport, or installation it is usually preferable for this to be done in the transport configuration. For distribution and other small power transformers, this would usually be fully assembled and liquid filled.

There are limits to how long it is possible to store medium and large power transformers and reactors without liquid. If these are exceeded, then there is a risk of loss of liquid impregnation and moisture ingress. This may adversely affect the dielectric strength of the insulation, and cause corrosion of the core and other parts. Therefore, it is recommended to fill with liquid for periods greater than 6 months. There are advantages to fully assembling the transformer or reactor during storage for a spare in long-term storage. Firstly, it preserves the condition of components which might otherwise become contaminated or deteriorate if stored separately, e.g., bushings and coolers. Secondly, it discourages the widespread practice of pilfering loose items for use as spares with other transformers or reactors.

The breather must be fitted and, if necessary, connected to an auxiliary supply. The control cabinet would typically also be assembled during storage, and the anticondensation heater should also be connected to an auxiliary supply. The coolers would typically also be assembled during storage and, if possible, they should also be connected to an auxiliary supply, preferably via the control cabinet. This will allow checking of cooler function, and operation for short periods to prevent moving parts from seizing.

The liquid may also represent an environmental hazard or a fire safety hazard, especially if using mineral oil and similar liquids. Environmental and fire safety hazards are reduced by use of natural or synthetic esters. In any case, the transformer or reactor must be stored in a suitable liquid containment area. An emergency plan must be available if there is a liquid spill, and suitably qualified workers must also be available to put it into action if required.

Normal procedures for assembly during installation should be followed during preparation for storage. After assembly, normal procedures for liquid filling and circulation should be followed during preparation for storage, unless there is an indication of moisture ingress where additional measures may be needed to dry the solid insulation.

To preserve the paint finish and to avoid excessive corrosion during storage, it is important that transformers or reactors are not stored for a prolonged period in a more corrosive atmosphere than that specified. Any damage to the paintwork during transport should be made good before storage. Special care is necessary for distribution transformers with cooling radiators or fins integral with the tank (corrugated tanks). For such transformers, their life may be limited by the life of the tank as it is challenging to refurbish or repair the tank in the field.

Where the transformer or reactor is stored at a substation or other electrical installation, the tank should be earthed according to normal procedures. Likewise, any free-standing coolers or other components should also be earthed according to normal procedures. The core and frame should also be earthed, via the tank. All bushings fitted should also be earthed, if necessary, using temporary connections.

4 Maintenance During Storage

CIGRE Working Group A2.34 made a survey on maintenance practices and used the results to make recommendations concerning what maintenance tasks should be performed at what frequency (CIGRE Brochure 445 2011). Their recommendations are strictly only applicable to transformers in service. However, they can be adapted for transformers in storage.

A maintenance task list, adapted from CIGRE brochure 445 (2011), may be found in Tables 16.1, 16.2, and 16.3. There different levels of maintenance tasks are defined – light, regular, and intensive. These are appropriate in different circumstances as described below.

Table 16.1 Typical maintenance task intervals storage without liquid (Adapted from CIGRE Brochure 445 (2011))
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Table 16.2 Typical maintenance task intervals storage partly assembled, with liquid (Adapted from CIGRE Brochure 445 (2011))
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Table 16.3 Typical maintenance task intervals storage fully assembled. (Adapted from CIGRE Brochure 445 (2011))
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The levels of maintenance tasks are appropriate in different circumstances as follows:

Light

  • Transformer or reactor is stored at a secure location

  • Transformer or reactor is not stored in a harsh environment

  • No major environmental or health and safety concerns

  • Equipped with components known to be suitable for long-term storage

  • Low probability transformer or reactor will urgently need to be deployed

  • Low consequences in the case of a delay in deployment

Intensive

  • Transformer or reactor is not stored at a secure location

  • Transformer or reactor is stored in a harsh environment

  • Major environmental or health and safety concerns

  • Doubts over the suitability of components for long-term storage

  • High probability transformer or reactor will urgently need to be deployed

  • High consequences in the case of a delay in deployment

Regular

  • A situation between the two previous ones

Note that light maintenance is not appropriate for transformers or reactors, filled or partly filled with dry gas due to the need to make regular checks on gas pressure.

4.1 Acceptance Criteria for Checks

Recommendations concerning gas pressure during transport and storage are made in CIGRE Brochure 673 (2016). A typical requirement would be for a positive pressure of 350 mbar, and for the dew point of the gas to be below −40 °C at 20 °C ambient.

Recommendations concerns liquid quality are given in the relevant maintenance guide, viz. IEC standard 60422 (2013) for oil, IEC standard 60944 (1988) for silicone, and IEC standard 61203 (1992) for synthetic esters. The scope of checks is normally limited to moisture content and breakdown voltage, as a wide range of checks to monitor liquid aging is not normally necessary.

5 Storage of Components and Materials

5.1 Bushings

CIGRE Working Group A2.43 on bushing reliability considered bushing storage in more detail. A chapter on bushing storage was included in their final report (CIGRE brochure 755 2019).

Storage recommendations for bushings vary depending on the type of construction (both internal and external) and the storage environment. In general, bushings require protection from moisture ingress, ultraviolet (UV) radiation, ozone, corrosion, and damage from wildlife and mechanical damage. CIGRE brochure 755 (2019) makes recommendations concerning bushing storage, with reference to previous work by CIGRE National Committees in the ASEAN countries (Transformer Bushing Guide 2014). CIGRE brochure 755 (2019) also recommends that bushings should be handled and stored by trained personnel and in accordance with the manufacturer’s recommendations.

Note that trapped charge in capacitance-graded bushings in storage may represent a safety hazard. CIGRE brochure 755 (2019) recommends that bushings should only be handled with the internal insulation short circuited. This is most easily done by connecting the flange to the top or bottom connection as appropriate.

Bushings can typically be stored in the transport configuration for up to 12 months. The oil end of the bushings should be securely wrapped with plastic film, with a bag of desiccant material (e.g., silica gel) if possible. This is especially important for resin-bonded paper (RBP) and resin-impregnated paper (RIP) bushings, which may absorb moisture at the liquid end. The bushing should then be placed in a suitable packing case, preferably the bushing manufacturer’s original packing case. The packing case should preferably be stored in a secure warehouse, and protected from direct sunlight, moisture, and wildlife.

For long-term storage, bushings should be stored in the vertical position. This is especially important for bushings with liquid-impregnated insulation, e.g., oil-impregnated paper (OIP) bushings. It prevents loss of liquid impregnation from the internal insulation and preserves the dielectric condition of the bushing. Loss of dielectric condition can result in failure of the bushing on deployment, with potentially very serious consequences for the environment and for health and safety.

For longer-terms storage of RBP and RIP bushings, the transformer end of the bushing should be immersed in liquid as in service. The top and, if necessary, bottom connections and also the underside of the bushing flange should be protected from corrosion by applying a protective lubricant or equivalent. CIGRE brochure 673 (2016) also recommends protecting the upper insulator and the sight glass of the bushing during storage to prevent accidental damage.

Experience with resin-impregnated synthetic (RIS) bushings is more limited, as these have only recently become available. Moisture absorption during storage is less likely with RIS bushings than with some alternatives, e.g., RBP and RIP. The liquid end of the bushing needs to be protected against UV radiation.

Any bushing with a silicone rubber external insulator needs to be protected from wildlife during storage. Although the silicone rubber has no nutritional value, there are numerous reports of bushings in storage being damaged by wildlife.

5.2 Coolers

Coolers represent a particular challenge during storage, as they are particularly prone to accidental damage, contamination, and corrosion. For long-term storage, it is therefore preferable for the cooler to be assembled and filled with liquid.

Radiators and similar cooling equipment are manufactured from thin metal (typically 1 mm). They are prone to accidental damage, which may compromise corrosion protection. They are also challenging to refurbish or repair in the field. Separable radiators can be replaced during corrective maintenance, and the life of the transformer is not affected. However, for distribution transformers with cooling radiators or fins integral with the tank (corrugated tanks) the life of the transformer may be limited by damage during transport and storage.

Best practice is to protect radiators, whether separable or integral with the tank, against accidental damage during transport and storage using suitable packing material. Care should also be taken with cranes and vehicles around where loose radiators are stored, and especially where transformers with assembled or integral radiators are stored.

Coolers contain lots of locations where contamination may become trapped during transport and storage. Internal corrosion may also produce contamination during transport and storage.

When filled with liquid, internal surfaces of coolers are protected against corrosion. Therefore, internal surfaces of coolers usually have little or no protection against corrosion and may be vulnerable during transport, especially during storage without liquid. The condition of the coolers can thus best be preserved by assembly onto the transformer and then filling with liquid. If this is not possible, then all cooler components should be filled with dry gas and securely blanked during transport and storage. As with complete transformers or reactors, the dry gas may be either air or nitrogen. Although the volume of dry gas is less than for transformers which are drained of liquid, use of dry air remains preferable for safety reasons. Nitrogen is required if the transformer is filled with natural ester, and may be required by certain customers. It is preferable to use steel blanking plates of a suitable thickness together with gaskets or O-ring seals on each cooler component. The use of polymer bungs or plugs is not recommended, especially for long-term storage.

As with complete transformers or reactors, to preserve the paint finish and to avoid excessive corrosion during storage, it is important that coolers are not stored for a prolonged period in a more corrosive atmosphere than that specified. Any damage to the paintwork during transport should be made good before storage. Note that coastal locations such as docks and harbor areas typically have high salinity and often high pollution due to other causes. Therefore these storage locations may not be suitable for long-term storage of transformers that have not been specified for such an environment.

Fans and pumps may seize during long-term storage and should therefore be exercised at suitable intervals. Fans and pumps to be connected to a suitable auxiliary supply, preferably via the control cabinet. Note also that pumps should not be exercised unless immersed in liquid.

5.3 Other Components

CIGRE brochure 445 (2011) notes that gaskets and seals are prone to deterioration during storage. These may be needed on installation, especially if transformers or reactors are stored without liquid or stored partly assembled with liquid. Deteriorated gaskets and seals may cause problems during liquid filling or result in liquid leaks after filling.

CIGRE brochure 445 (2011) recommends that gasket and seal material should be sealed during storage, and protected from light. Gasket and seal material should be stored at a temperature between 5 °C and 30 °C, and at a relative humidity between 45% and 70%.