There are many problems that can be avoided during transformer installation simply by installing the transformer in the correct environment. Many difficulties and safety hazards can be avoided or minimized by keeping certain factors in mind while positioning a transformer, before it is set up and connected. Likewise, the location can have a bearing on how the transformer should be set up, and what precautions should be taken in the future.
The Standards
Installing transformers in accordance with the ANSI, NEMA, and IEEE standards is critical to ensuring a safe electrical installation as well as a reliable power supply system - especially for those applications where power quality is an issue. Transformer installation is one of the most common-yet-complicated installation practices that are cause for considerable confusion when sizing Over Current Protection Devices (OCPDs) and bonding and grounding conductors.
Many electrical installations can be a challenge in terms of NEC requirements, and transformers can raise that challenge to a new level. A properly designed installation will ensure the conductors and equipment are properly sized, protected and also deal with the overriding issue of grounding. Incorrect installation can lead to fires from improper protection or conductor sizes, as well as electric shock from inadequate grounding.
This article will provide a brief overview of important considerations to keep in mind during installation, of transformers located outdoors and indoors, and for dry-type vs. liquid-filled transformers.
Installation Best Practices Some of the particularly important transformer installation best practices are listed below:
1. Permanent Grounding: Once the transformer is placed permanently - before further inspection is carried out and before assembling the unit - the tank should be permanently grounded with a correctly sized and properly installed permanent ground.
2. Humidity: No access should be permitted to the transformer liquid-filled compartment in conditions of excessive humidity or rain. If humidity exceeds 70% for example, dry air should be continuously pumped into the gas space. Liquid-filled transformers that are shipped with Nitrogen in the gas space must be purged by pumping dry air for at least 30 minutes before service personnel can enter the tank. Oxygen concentrations of 19.5% to 23.5% are advised.
3. Fluid Inspection: If the insulating liquid for inspection needs to be drawn down, make sure you have equipment for clean and dry storage of the liquid during inspection and for filtering the liquid prior to refilling the tank. It is very important that all associated equipment used in the handling of the fluid (hoses, pumps, etc.) are also very clean and dry. If this equipment was used before with a different type of fluid, clean all contaminated items. When you remove the liquid, its level should not go below the top of windings.
4. Pressure Maintenance: Liquid-filled transformers may be stored outdoors upon delivery. Sufficient gas pressure must be maintained to allow a positive pressure of 1 psi to 2 psi at all times, even at low ambient temperature. The pressure-vacuum gauge, if supplied with the transformer, will show pressure variations with ambient temperature. Pressure and ambient temperature readings should be recorded regularly. The manufacturer's instructions must be referred to for storage of accessories.
5. Inspection and filling: You should make a final inspection of the transformer before it is energized, particularly if any work has been done inside the tank. All electrical connections should be checked for tightness. All bushings should be checked for tightness of gaskets, and all draw lead connections should be checked. Electrical clearances inside the tank should be checked. One final check should be made to ensure all tools have been removed.
6. Loading: After applying full voltage, the transformer should be kept under observation during the first few hours of operation under load. After several days, check the oil for oxygen content and dielectric strength. All temperatures and pressures should be checked in the transformer tank during the first week of operation. Except for special designs, transformers may be operated at their rated kVA if the average ambient temperature of the cooling air does not exceed 86'F (30'C) in any 24-hr period, and the altitude does not exceed 3300 ft.
7. Surge arresters: When used, surge arresters must be installed and connected to the transformer bushings/terminals with the shortest possible leads. These arresters may be necessary to protect the equipment from line switching surges and lightning.
When the Transformer is Under Vacuum, Never,Never...
* Apply voltage to the transformer
* Leave it unattended; a positive pressure must be applied
* Stand or walk on the transformer tank
8. Structural Considerations: Multiple transformers can be mounted on a single pole as long as the weight is evenly distributed, and the total weight is well within the safe limits of the pole, any cross-arms or supporting bolts.
9. Mounting: Sub-100 kVA single-phase distribution transformers are typically mounted above the secondary mains. Platform or pad mounting are options for transformers larger than 100 kVA.
10. Protection: Apart from self-protected transformer types, all distribution transformers must have lighting arresters and fused cutouts installed on the primary side.
11. Ground wires: Cover all ground wires with plastic or wood molding to a point 8 feet above the base of the pole.
12. Guying of poles: Correctly installed guy wires can protect the pole line from damage caused by the strain of the line conductors and pole-mounted equipment, and minimize pole line damage caused by severe weather.
Following are a few more generic precautions to observe in the installation of outdoor, indoor, dry or liquid-filled transformers:
Outdoor Transformers
For transformers to be kept outdoors, some of the chief considerations are keeping the public safe - and keeping the transformers safe from outside elements - are:
* Minimizing damage: Outdoor transformers should be located so as to minimize the damage from natural and human elements. For instance, a transformer should be kept out of the way of anything that might fall on it or run into it, such as moving vehicles or cranes.
*Enclosures:.The specific location of an outdoor transformer has some bearing on how the transformer enclosure should be built. It should, for example, have ventilation slits small enough that fingers, sticks and so on cannot be stuck into them.
* Soil Considerations: For particularly heavy transformers, the soil should be a consideration. The soil should be firm enough to support the weight of the transformer without shifting. For instance, large quantities of clay in the soil would not be desirable, since clay compresses under heavy weights.
Indoor Transformers
Transformers kept indoors have further considerations to keep in mind:
* Structural integrity: It can be a good idea to keep particularly heavy transformers as close to columns as possible, since that is where they will be supported best.
* Ventilation: Transformers need to be properly ventilated to reduce the chance of their overheating. Large transformers should not be kept installed against walls, so that as much surface area as possible is exposed to the outside air. Likewise, transformers kept in restricted spaces should be given enough space to ensure that proper ventilation occurs. There should be enough space and ventilation for the transformer so that its surroundings remain at room temperature.
* Noise:.Locations that have sound-absorbent materials on the walls can be the best, or for especially loud transformers they can be kept in areas without many people.
* Water: Excess water is damaging for both dry-type and liquid-filled transformers. If sources of water are unavoidable, then the transformer should be given protection against them such that no water gets inside.
* Contaminants: The presence of air borne contaminants can be a problem for transformers. They should be kept, as much as possible, away from sources of excess dust or chemical pollutants that might damage them.
* Accessibility:Transformers should be located so that they are easily accessible to anyone who may need to maintain, test or repair them.
Liquid-filled Transformers
There are a few requirements that are specific to liquid-filled transformers. These are,
* Since liquid-filled transformers are fire hazards, their location must match up to the relevant fire hazard specifications.
* Keeping safety in mind, they should also be positioned so that the possibility of leakages will not endanger anyone.
Dry-type transformers, on the other hand, overheat more easily than liquid-filled transformers, and can be more susceptible to air contaminants. For these reasons,
* When installing dry-type transformers, their ventilation must be kept a priority. They should have easy access to clean, dry air, and given enough space that heat is able to dissipate easily.
* Likewise, the air should be clean, kept free of dust and other pollutants. A location with filtered air may be optimal.
Although finding the appropriate location for a transformer during installation can seem secondary to installing it correctly, the right location can ultimately reduce costs, and prevent potential safety hazards. Keeping the type of transformer being installed, its purpose and its surroundings in mind during installation is one way to minimize future headaches.
