Operation of power transformers
When your transformer arrives on site, various procedures should be carried out to assure successful operation (installation, testing and various checkings). The successful operation of a transformer is dependent on proper installation as well as on good design and manufacture.
The instructions mentioned in the manufacturer manual or in Standards MUST be followed to ensure adequate safety to personnel and equipment.
This technical article will provide 10 general guidelines for installing and testing both dry-type and liquid-filled power transformers for placement into service //
- Standard and special transformer tests
- Site considerations
- Preliminary inspection upon receipt of transformer
- Plan for the prevention of contaminants
- Making connections that work
- Controlling sound level
- Make sure the transformer is grounded
- Final inspection and testing
- Applying the load
- Adjustment for correct tap setting
1. Standard and special transformer tests
Standard transformer tests performed for each unit include the following //
- Ratio, for voltage relationship;
- Polarity for single- and 3-phase units (because single-phase power transformers are sometimes connected in parallel and sometimes in a 3-phase bank);
- Phase relationship for 3-phase units (important when two or more power transformers are operated in parallel);
- Excitation current, which relates to efficiency and verifies that core design is correct;
- No-load core loss, which also relates to efficiency and correct core design;
- Resistance, for calculating winding temperature
- Impedance (via short circuit testing), which provides information needed for breaker and/or fuse sizing and interrupting rating and for coordinating relaying schemes;
- Load loss, which again directly relates to the transformer’s efficiency;
- Regulation, which determines voltage drop when load is applied; and
- Applied and induced potentials, which verify dielectric strength.
There are additional tests that may be applicable, depending upon how and where the transformer will be used. The additional tests that can be conducted include the following //
- Impulse (where lightning and switching surges are prevalent);
- Sound (important for applications in residential and office areas and that can be used as comparison with future sound tests to reveal any core problems);
- Temperature rise of the coils, which helps ensure that design limits will not be exceeded;
- Corona for medium voltage (MV) and high-voltage (HV) units, which helps determine if the insulation system is functioning properly;
- Insulation resistance (meg-ohmmeter testing), which determines dryness of insulation and is often done after delivery to serve as a benchmark for comparison against future readings; and
- Insulation power factor, which is done at initial installation and every few years thereafter to help determine the aging process of the insulation.
2. Site considerations
When planning the installation, the location is selected, that complies with all safety codesyet does not interfere with the normal movement of personnel, equipment, and material. The location should not expose the transformer to possible damage from cranes, trucks, or moving equipment.
3. Preliminary inspection upon receipt of transformer
When received, a transformer should be inspected for damage during shipment. Examination should be made before removing it from the railroad car or truck, and, if any damage is evident or any indication of rough handling is visible, a claim should be filed with the carrier at once and the manufacturer notified.
4. Plan for the prevention of contaminants
Develop a procedure for inventory of all tools, hardware, and any other objects used in the inspection, assembly, and testing of the transformer. A check sheet should be used to record all items, and verification should be made that these items have been properly accounted for upon completion of work.. Making connections that work
The connections shall be made, between the transformer’s terminals and the incoming and outgoing conductors, carefully following the instructions given on the nameplate or on the connection diagram. Check all of the tap jumpers for proper location and for tightness.
Re-tighten all cable retaining bolts after the first 30 days of service.
Before working on the connections make sure all safety precautions have been taken. Arrangements shall be made to adequately support the incoming/outgoing connecting cables, so that there is no mechanical stress imposed on transformer bushings and connections. Such stress could cause a bushing to crack or a connection to fail.
6. Controlling sound level
All power transformers, when energized, produce an audible noise. Although there are no moving parts in a transformer, the core does generate sound. In the presence of a magnetic field, the core laminations elongate and contract. These periodic mechanical movements create sound vibrations with a fundamental frequency of 120 Hz and harmonics derivatives of this fundamental.
For example, if the transformer is installed in a quiet hallway, a definite hum will be noticed. If the unit is installed in a location it shares with other equipment such as motors, pumps, or compressors, the transformer hum will go unnoticed. Some applications require a reduced sound level, such as a large unit in a commercial building with people working close to it.
Occasionally, the installation of some method of sound abatement will be called for.
8. Final inspection and testing
Once the transformer has been located on its permanent site, a thorough final inspectionshould be made before any assembly is accomplished and the unit is energized. Before energizing the unit, it’s very important that all personnel installing the transformer are alerted, that lethal voltages will be present inside the transformer enclosure as well as at all connection points.
The installation of conductors should be performed only by personnel qualified and experienced in high-voltage equipment. Personnel should be instructed that should any service work be required to the unit, the lines that power the transformer must be opened and appropriate safety locks and tags applied.
A careful examination should be made to ensure that all electrical connections have been properly carried out and that the correct ratio exists between the low and high-voltage windings. For this test, apply a low-voltage (240V or 480V) to the high-voltage winding and measure the output at the low-voltage winding.
8. Final inspection and testing
Once the transformer has been located on its permanent site, a thorough final inspectionshould be made before any assembly is accomplished and the unit is energized. Before energizing the unit, it’s very important that all personnel installing the transformer are alerted, that lethal voltages will be present inside the transformer enclosure as well as at all connection points.
The installation of conductors should be performed only by personnel qualified and experienced in high-voltage equipment. Personnel should be instructed that should any service work be required to the unit, the lines that power the transformer must be opened and appropriate safety locks and tags applied.
A careful examination should be made to ensure that all electrical connections have been properly carried out and that the correct ratio exists between the low and high-voltage windings. For this test, apply a low-voltage (240V or 480V) to the high-voltage winding and measure the output at the low-voltage winding.
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