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
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.
When planning the installation, the location is selected, that complies with all safety codes yet 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.
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.
Important note // If the transformer is moved or if it is stored before installation, this inspection should be repeated before placing the transformer in service!
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.
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.
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.
Grounding is necessary to remove static charges that may accumulate and also is needed as a protection should the transformer windings accidentally come in contact with the core or enclosure (or tank for wet types). Note that for MV power transformers, the secondary neutral is sometimes grounded through an impedance.
Ensure that all grounding or bonding systems meet NEC and local codes.
Once the transformer has been located on its permanent site, a thorough final inspection should 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.
However, for low-voltage (600V and below) transformers, this is not practical! Here, a transformer turns ratio indicator should be used to measure the ratio. Any control circuits, if any, should be checked to make sure they function correctly. These include the operation of fans, motors, thermal relays, and other auxiliary devices.
Correct fan rotation should be visually verified as well as by checking indicator lights if they are installed. Also, a one-minute, 1200V insulation resistance test of the control circuits shall be done. (If the power transformer has CT circuits, they should be closed.)
All windings should be checked for continuity. An insulation resistance test shall be carried out to make certain that no windings are grounded.
Before energizing a 3-phase transformer, arrangement for monitoring the voltages and currents on the low-voltage side shall be done. Then, without connecting the load, energize the transformer. The magnitude of the voltages shown (line-to-ground and line-to-line) should be very similar. If this is not the case, de-energize the transformer and contact the manufacturer before proceeding further.
Next, connect the load and energize the transformer. While monitoring the voltages and currents, gradually increase the load in a stepped or gradual application until full load is reached. Both the voltages and currents should change in a similar fashion. If this does not happen, de-energize the transformer and contact the manufacturer.
The maximum continuous load a transformer can handle is indicated on its nameplate.
After installation, check the output voltage of the transformer. This should be done at some safe access point near or at the load.
When changing taps, the same changes must be made for all phases. Consult the transformer diagrammatic nameplate for information on what tap must be used to correct for extra high or extra low incoming line voltage. The same adjustment should be made to compensate for voltage drop in the output due to long cable runs.
When the load-side voltage is low, tap connections below 100% of line voltage must be used to raise the load voltage.
If the load-side voltage is high, tap connections above 100% of line voltage must be used to lower the load voltage.
Reference // Best practice manual for power transformers