The successful operation of a transformer is dependent on proper installation as well as on good design and manufacture. An article on how to choose a transformer based on the latter criteria appeared in the May '96 issue of EC&M. The article in this issue covers installation procedures you should consider to help assure your transformer will function properly and safely, and it's focused on installation practices that are common for both dry-type and liquid-filled transformers.
When viewing transformer product literature, you'll probably find that one manufacturer's units will have differences from that of a competing manufacturer. And, each manufacturer will have its own instructions for installing and testing its transformers. Regardless of manufacturer, you should carefully follow these instructions to ensure adequate safety to personnel and equipment. And its important that you follow current NEC practice and applicable local codes.
This material will provide additional general guidelines for installing and testing both dry-type and liquid-filled transformers for placement into service. A note of caution: The information presented here is not meant to supersede any manufacturer's instructions.
Acceptance testing requirements
Before your transformer is scheduled to be shipped to its designated site, it's important that you coordinate with the manufacturer what acceptance tests should be carried out. Each test has a particular objective that helps determine a transformer's suitability for use.
A number of these tests are done on the plant floor, while other tests are conducted usually after delivery is made. Two good references to dry-type transformer requirements are ANSI/IEEE C57.12.01-1989, IEEE Standard General Requirements for Dry-Type Distribution and Power Transformers Including Those With Solid Cast and/or Resin-Encapsulated Windings, which addresses general requirements, and C57.12.91-1995, IEEE Test Code for Dry-Type Distribution and Power Transformers, which addresses testing. (Most of the information in these standards also can be applied to liquid-filled transformers.)
Small transformers, arguable those below 400kVA, usually don't have extensive testing as they are often installed far downstream in a power system; as such, their importance is not as vital as compared with larger units. Also, it's a matter of cost. The expense associated with testing represents a larger portion of a small transformer's cost compared with the testing cost of a large kVA unit.
It's recommended that all conducted tests comply with applicable ANSI/IEEE and NEMA standards. Should a transformer be built to meet special requirements, however, then some additional testing is recommended to ensure that the unit operates as called for. Sometimes, when testing is carried out at the manufacturer's plant, the manufacturer will also allow a purchaser or a representative, such as a consulting engineer, to witness the 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 transformers are sometimes connected in parallel and sometimes in a 3-phase bank);
* Phase relationship for 3-phase units (important when two or more 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 and [I.sup.2]R component of winding losses (usually not required on 600V class units);
* 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. Usually, additional testing means there will be an increase in transformer cost. Before specifying any additional tests, you should contact the manufacturer to find out what data it has accumulated from testing essentially duplicate units. If this data can be used, the extra cost to carry out tests can be avoided without sacrificing the quality of the transformer.
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 (megohmmeter 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, you should select a location 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. Other site considerations require closer analysis.
Foundations. Foundation preparation usually includes an evaluation of soil characteristics and concrete work. For transformers placed outside that are 2000kVA and above, you may wish to have the soil examined. Clay soils are compressible and can cause problems that may require stabilizing back-fill. Most soils are able to withstand a bearing pressure of 2500 lbs/sq ft.
The foundation should be constructed of reinforced, air entrained concrete having at least 3000 psi compressive strength at 28 days after pouring. For pad mount transformers with ratings 75kVA through 500kVA, a typical concrete base would be 5 1/2 by 6 1/2 ft and 10 in. thick with chamfered edges on top of the base and footings extending 20 in. down from each of the ends of the long sides. For units with ratings above 500kVA to 2500kVA, a typical concrete base would be 8 ft by 9 ft and 10 in. thick with chamfered edges on top of the base and footings extending 20 in. down from each of the ends of the long sides. [ILLUSTRATION FOR FIGURE 1 OMITTED]. In addition, to avoid problems, a civil engineer should be consulted for guidance on the above matters.
Structural support. When placed inside or on top of a building, you must consider structural capabilities because a transformer represents a highly concentrated load. For new buildings, you should work with the structural engineers so that the transformer's placement is included in the building plans.
Installing transformers in existing structures may require an analysis of the building for structural support capability since the original structural information may not be available. Structurally speaking, it's generally wiser to place a transformer as close as possible to a column. This may call for compromises regarding the length of the conductors going to and/or from the transformer.
In seismic areas, the stability of the unit with respect to turning over must be evaluated, whether placed outside or in a building, because a transformer usually has a relatively high center of gravity. Usually, lateral bracing and/or an extra solid anchorage will be required. Therefore, it's advisable that you seek guidance on this matter from an engineer knowledgeable in seismic supports and the associated code requirements.
To simplify installation, you should request from the manufacturer a simplified outline drawing of the transformer. By studying the overall mounting and terminal dimensions, it's possible to plan the installation with an orderly arrangement of connections. Also, with this information, it will be easier to plan site arrangements.
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. Subsequently, covers or panels should be removed and an internal inspection should be made for damage or displacement of parts, loose or broken connections, dirt or foreign material, and for the presence of water or moisture. If the transformer is moved or if it is stored before installation, this inspection should be repeated before placing the transformer in service.
Handle and lift with care
Transformers are designed with provisions for lifting, jacking, and/or rolling. These provisions will vary depending upon the weight, size, and mechanical configuration of the unit. The weight distribution should be studied by examining the inside of the transformer enclosure for dry-type units. If appropriate, supports should be used so that the transformer enclosure is not crushed when the unit is lifted.
You may lift transformers with enclosures having lifting lugs by using appropriate slings or chains. Larger units will have provisions for lifting from the base frame or from clamps at the top of the core. Make sure the rigging crew is experienced in lifting and moving heavy delicate equipment. Lifting from the base frame may require the use of a spreader bar to avoid damage to the enclosure panels. [ILLUSTRATION FOR FIGURE 2 OMITTED]. Units lifted from the top core clamps will sometimes require that the top cover or part of the cover be removed.
Transformers should be maintained in an upright position when being moved. There should be no attempt to handle a transformer in any other position. If this isn't possible, you first should contact the manufacturer to explore other options. Exercise care during handling to prevent equipment damage and/or personnel injury.
If the transformer can't be lifted by a crane, it can be skidded or moved on rollers, as shown in Fig. 3, on page 42. Take care not to damage the base or tip it over. When rollers are used on transformers without a structural base, you should use skids to distribute the stress over the base. Large enclosed units with base frame type enclosures may be jacked using the base frame angles. The transformer should be jacked evenly on all four corners to prevent warping or tipping over.
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
When you start making the connections between the transformer's terminals and the incoming and outgoing conductors, carefully follow 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. As appropriate, you should make arrangements to adequately support the incoming/outgoing connecting cables to ensure 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.
Transformers are usually designed and built to provide good electrical connections using either copper or aluminum cable. A protective plating or compound that prevents surface oxidation on the aluminum terminals is usually applied at the factory. You should not remove this coating from tap and line terminals. Also, when aluminum conductors are used, give them a protective compound treatment at the terminal as specified by the cable manufacturer.
Representative torque requirements for making connections using steel nuts/bolts are shown in Table 1, on page 42.
Some equipment could have a different torque requirement than shown. This is especially true if bronze or other type material is used for the nuts/bolts. To avoid problems, you should follow the instructions provided by the transformer manufacturer. Torque specifications are sometimes listed on the hardware. After applying proper torque, you should wait a minute or so, and then re-tighten all bolts to the specified torque.
You should use commercially available, properly sized, UL-listed mechanical- or compression-type lugs. These terminations should be attached to the cables as specified by the termination or cable manufacturer. Such terminations are available from electrical distributors. Do not install washers between the terminal lugs and the termination bus bar as this will introduce an added impedance and will cause heating and possible connection failure.
Some transformer manufacturers recommend that the cable size be based on an ampacity level of 125% of nameplate rating. When speaking to consulting engineers on this topic, we've found that they recommend the cable be sized for the transformer's nameplate rating. You take your choice; extra safety and extra cost or regular-sized cables. Whatever the choice, the cable insulation rating must be adequate for the installation. The cables you install must be kept as far away as possible from coils and top blades. If in doubt about clearances, do not hesitate to call the transformer manufacturer. Information on minimum wire bending space clearances at terminals for conductors is found in NEC Sec. 373-6, Deflection of Conductors, and referenced in Sec. 450-12 on Terminal Wiring Space.
Controlling sound level
When testing a transformer for sound level, you should recognize that all 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.
The location of a transformer relates directly to how noticeable its sound level appears. 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. You should consider this when planning the unit's installation.
Often the location and the method in which a transformer is placed have much to do with the perceived sound as does the actual decibels generated. Locating a unit at the end of a long, narrow room, or in the corner of a room can cause a megaphone effect and amplify the transformer's sound. Mounting the unit on a platform that has less mass than the transformer will make the platform serve as a sounding board, just like the body of a violin. Even mounting the unit a distance that is an exact multiple of the 120 Hz wavelength from a solid reflective surface may reinforce the sound waves, causing the transformer to seem louder than it actually is. These considerations should be taken into account, as well as the use of sound absorbing materials on walls (for low frequency sound) and vibration isolation pads under the unit.
A transformer is designed to produce a minimum sound level when the connections to primary and secondary terminals are made with flexible connectors, when all transit bolts and shipping braces are loosened so the unit will float on rubber isolation pads, as shown in Fig. 4, on page 44, and when all enclosure hardware is tightened so panels do not vibrate.
Some manufacturers in the industry have extensive data on sounds produced by their transformers, and they usually can determine the sound level for a particular design quite accurately. You should note, though, that transformers serving large harmonic loads can produce a higher audible noise.
There are NEMA standards for transformer sound and depending upon the kVA rating of a unit, the sound it produces must be under a certain decibel level. The usual sound levels for liquid-filled transformers range from 40 dB to 60 dB for units below 500kVA, about 65 dB for units between 4000kVA to 5000kVA, 73 dB for transformers with ratings between 6000kVA to 7500kVA, and 76 dB for units between 8000kVA and 10,000kVA.
Dry-type transformers have sound levels that are somewhat higher. Sound levels associated with certain kVA ratings will vary depending upon the type of transformer and manufacturer.
Make sure the transformer is grounded
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).
Before applying any voltage to the transformer, you should make sure that the tank for wet-types, or the enclosure and core assembly for dry-types, is permanently and adequately grounded. You should ground the transformer as per NEC Sec. 450-10 and check the grounding of the neutral as applicable per NEC.
Note that for MV transformers, the secondary neutral is sometimes grounded through an impedance.
Ensure that all grounding or bonding systems meet NEC and local codes.
Final inspection and testing
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 you alert all personnel installing the transformer 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, you should arrange for a one-minute, 1200V insulation resistance test of the control circuits. (If the power transformer has CT circuits, they should be closed.) But be careful here: Before applying this voltage, check with the manufacturer's manuals. Some microprocessor-based electronic devices may not be able to withstand the voltage.
As prescribed by NEMA standards, transformers are shipped with both high and low-voltage windings connected to their highest rated voltage (except transformers that have taps above the rated voltage, in which case they will be shipped connected for rated voltage). You should check the internal connections with the diagram on the nameplate to make sure they are correct for the application. The tap setting should also be verified for the proper voltage.
All windings should be checked for continuity. You should arrange for an insulation resistance test to be carried out to make certain that no windings are grounded.
You will find it beneficial to carry out this testing for future comparative purposes, and also for determining the suitability of the transformer for energizing or application of a high potential test.
It's important that you have an understanding of the manufacturer's warranty. A number of manufacturers require that insulation resistance testing be successfully completed prior to the transformer being placed in service for the warranty to be valid. Some manufacturers require that the megohmmeter readings and date of energizing be sent to them within a specified time after the transformer is placed in service for the warranty to be valid. The insulation resistance test should be conducted immediately prior to energizing the transformer or the beginning of the dielectric test.
Caution is required when operating in parallel
When transformers are installed for parallel operation, their rated voltages, impedances, and turn ratios ideally should be the same and their phasor relationships identical. If these parameters are different, circulation current will exist in the circuit loop between these units. The difference in impedance should not exceed 7.5%. The greater the differences in these parameters, the greater the magnitude of the circulating current. When specifying a transformer to be operated in parallel with existing units, all these parameters should be discussed with the transformer manufacturer.
Applying the load
Before energizing a 3-phase transformer, you should arrange to monitor the voltages and currents on the low-voltage side. 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, deenergize 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. If you cannot gradually increase the load, then full load may be applied. 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. However, a specially designed unit may have specific load capabilities not indicated on the nameplate. If you have some doubt as to the load capability of the unit, contact the manufacturer.
Adjustment for correct tap setting
After installation, you should check the output voltage of the transformer. This should be done at some safe access point near or at the load. Never attempt to check the output voltage at the transformer. Dangerous high voltage will be present within the transformer enclosure.
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.
This article will continue in subsequent issues of EC&M. Part 2 will address installation procedures for dry- and wet-type transformers, the latter being fully assembled. Part 3 will cover installation procedures for liquid-filled transformers that are shipped unassembled.