The successful operation of a transformer is pendent on proper installation as well as on proper design and manufacture. Part 1 of this article, which addressed installation procedures that are common to both dry- and wet-type units, appeared in the June '96 issue of EC&M. This article covers installation procedures peculiar to dry-type transformers as well as those relating to fully assembled liquid-filled units.
Each transformer manufacturer produces equipment having differences from competing manufacturers. And, each manufacturer has its own instructions for installing and testing their transformers. Such instructions should be carefully followed to assure adequate safety to personnel and equipment. And, it's important that you follow current NEC practice and applicable local codes.
The purpose of this article is to provide general installation guidelines for placing transformers in service. It is not meant to supersede manufacturer's instructions.
Dry-type transformers
Additional site considerations. The location where a transformer is to be placed should not expose the unit to possible damage from moving equipment. Remember, damage to the enclosure of a dry-type transformer may reduce insulation clearances to an unsafe level.
As an added safety precaution, you should keep in mind the possibility of children inserting rods, wire, etc. through an enclosure's ventilation openings of a dry-type transformer, thus coming into contact with live parts.
Transformer ventilation openings are designed in accordance with NEMA and ANSI standards, which require that a 1/2-in. diameter rod cannot be inserted through the openings.
The transformer enclosure is designed to prevent the entrance of most small animals and foreign objects. In some locations, however, you may have to consider additional protection. Transformers installed in public areas must be constructed to be impenetrable to foreign objects or the units must be protected by a fence in a manner that would prevent accessibility by the public and animals.
For dry-type transformers, the core/coil assembly (without enclosure) will usually have mounting and terminal dimensions to suit your enclosure requirements. The enclosure should give protection to the coils and have adequate clearance and sufficient ventilation openings. The manufacturer should always be consulted to determine these requirements.
Top covers may be designed for cable entry or exit with bolt-on cover plates. However, because heat rises, the insulation of cables placed above a transformer may age prematurely. Therefore, side and bottom cable entry and exit are recommended. Note that conduit, bus duct, etc. must be independently supported as the top cover is not designed for these loads.
Minimum electrical clearances regarding the installation of lugs and cables must be observed per NEC Sec. 373-11.
Ventilated dry-type transformers can be designed for installation indoors or outdoors. They will operate successfully where the humidity is high, but under this condition, it may be necessary to take precautions to keep them dry if they are shut down for appreciable periods.
When operating, you should take steps to arrange frequent inspection to ensure that there is no surface contamination on live parts of dry-type transformers. Caution: Make sure you deenergize the unit first. Excess contamination can cause problems and may even result in transformer failure. For locations where severe atmospheric conditions prevail, it may be better to use a liquid-filled transformer. However, if a dry-type is still preferred, you should consider using a totally enclosed, non-ventilated unit; sealed coil unit; or cast coil transformer.
Locations where there is dripping water should always be avoided. If this is not possible, suitable protection should be provided to prevent water from entering the transformer case. Precautions should be taken to guard against accidental entry of water, such as might be obtained from an open window, by a break in a water or steam line, or from use of nearby water.
Adequate ventilation is a must. Adequate ventilation is essential for the proper cooling of dry-type transformers. And the use of clean dry air is desirable for these types of units. It's highly advisable that you take steps to install ventilated dry-type transformers in locations free from unusual dust or chemical fumes. If such conditions exist, a wet-type unit, or another form of dry-type should be employed. Filtered air may reduce maintenance if the location has a contamination problem.
When transformers are installed in vaults or other restricted spaces, you should provide sufficient ventilation to hold the air temperature within established limits when measured near the transformer inlets. For dry-type transformers, this usually will require approximately 100 cu ft of air per minute per kW of transformer loss. The area of ventilation openings required depends on the height of the vault, location of openings, and maximum loads to be carried by the transformers. For self-cooled transformers, the required area of ventilation should be at least one sq ft each of inlet and outlet openings per 100 kVA of rated transformer capacity, after deduction of the area occupied by screens, gratings, or louvers. This is approximately equivalent to the requirements of NEC Sec. 450-45(c). High-efficiency, low-loss designed transformers may lead to reduced ventilation requirements.
Transformers should be located at least 12 in. to 18 in. away from walls and other obstructions that might prevent free circulation of air through and around each unit, unless the unit is designed for wall mounting and installed per the manufacturer.
Units rated for 600V operation and used for lighting are labeled for minimum distance, usually between 3 in. and 12 in. (6 in. for most units). This varies among manufacturers, and it's recommended that you follow such instructions. Also, accessibility for maintenance should be taken into account before locating the transformer. If it's to be located near combustible materials, the minimum separations established by the NEC, and local fire ordinances must be maintained.
Handling and lifting. Because of a dry-type transformer's high center of gravity, the unit is subject to tipping over during handling. It's wiser to apply force for moving such units at the bottom rather than near or at the top of the transformer.
When lifting the unit by the core/coil assembly, only use the lifting devices/holes provided on the core clamps. Make sure you take care to prevent damage to bus work, wiring, and termination assemblies during lifting. When lifting, you should increase tension gradually; do not jerk, jar, or move the transformer abruptly.
When moving transformers, avoid contamination by foreign objects. This is especially true for dry-type units as tools or other objects may be dropped into the air passages of the coils. It's important to instruct your work crews to take steps to prevent any foreign material from falling into or onto the coils, terminals, and insulators. Hardware, connecting parts, tools, or any foreign material should not be allowed on top of the core and coil assembly. Foreign material lodged in a coil duct can cause electrical failure or overheating.
Remove shipping supports. For dry-type transformers, after the transformer has been placed in its permanent location, you should arrange to have the bold-down bolts securing the core and coil assembly to the base or enclosure loosened but left in the holes to act as horizontal restraints. This loosening releases the sound isolation pads for maximum effectiveness. Also remove any shipping braces on the core and coil busses or the enclosure. For easy identification, these braces are usually painted a different color from the remaining assembly parts, made of wood, or left unpainted. Should it be necessary to move the transformer, replace the hold-down bolts and braces for the moving operation. Because each manufacturer uses different shipping procedures, you should carefully follow the specific instructions provided by the firm.
Extra grounding provisions. Make sure that the flexible grounding jumper between the core-and-coil assembly and the enclosure is intact, or that the core-and-coil assembly is directly grounded from the core clamp by a flexible means.
Insulation resistance testing. Variable factors affecting the construction and use of dry-type transformers make it difficult to set limits for this test. Experience indicates that for some transformers (manufacturers' guidelines may differ, and they should be followed), a minimum of 2 megohms (1 min reading at approximately 25 degrees C) per 1000V of nameplate voltage rating, but in no case, less than 2 megohms total, may be a satisfactory value of insulation resistance (IR) for the application of the high-potential test. If the IR is less than the above minimum values, do not energize the transformer. When this condition exists, the next step is to dry out the transformer and then do another IR test. Section 6.4.1 of the ANSI/IEEE Standard C57.94-1982, IEEE Recommended Practice for Installation, Application, Operation, and Maintenance of Dry-Type General-Purpose Distribution and Power Transformers, states that should the manufacturer's recommended values not be available, the readings as shown in the Table (above) may be used. If the IR is less than the minimum values stated in the above paragraph or in Table 1, do not energize the transformer. When the latter condition exists, the next step is to dry out the transformer and then do another IR test.
Other recommended on-site tests for dry-type units include the following:
* Winding resistance measurements,
* Polarity verification,
* Insulation power factor, and
* High potential (hi-pot) test to 75 % of factory test level, per ANSI C57.12.911995, IEEE Test Code for Dry-Type Distribution and Power Transformers, providing the transformer was not subjected to severe dielectric tests before this.
It's preferable that the dielectric tests (i.e., power factor and hi-pot) be done last. Note that the value of the power factor tests on dry-type transformers will be higher than on liquid units because the air is measured as a dielectric.
Drying varnish-insulated transformers. If the megohmmeter readings are low, it's an indication that the transformer needs to be dried. In fact, megohmmeter readings are of value in determining the status of drying. Measurements should be taken before starting the drying process and at 2-hr intervals during drying. The initial value, if taken at ordinary temperatures, may be high even though the insulation may not be dry. Because IR varies inversely with temperature, the transformer temperature should be kept approximately constant during the drying period to obtain comparative readings.
As a transformer is heated, the presence of moisture will be evident by the rapid drop in resistance measurement. Following this period, the IR will generally increase gradually until near the end of the drying period, when it will increase more rapidly. Sometimes, it will rise and fall through a short range before reaching a steady state because moisture in the interior of the insulation is working its way out through the initially dried coils. A curve, with time plotted on the X-axis (horizontal axis) and resistance on the Y-axis (vertical axis), should be plotted, and the run should be continued until the resistance levels off and remains relatively constant for a period extending 3 to 4 hrs.
Use caution when testing the insulation. The IR readings should be taken from each winding to ground, with all windings grounded except the one being tested. Before taking IR measurements, the winding should be grounded for at least 1 min to drain off any static charge. All readings should be for the same time of application of the test voltage, preferably 1 min.
Drying of the core/coil assembly. When it's necessary to dry out a transformer before installation (or after an extended shutdown under relatively high humidity conditions), one of the following methods may be used. (Before applying any of these methods, free moisture should be blown or wiped off of the windings to reduce the time of the drying period.)
When providing external heat, it's important that most of the heated air passes through the winding ducts, not around the sides. Three options are available here.
* Directing heated air into the bottom air inlets of the transformer enclosure.
* Placing the core and coil assembly in a nonflammable box with openings at the top and bottom through which heated air can be circulated. (By not using the transformer's enclosure, you avoid possible damage to gauges, gaskets, etc.)
* Placing the core and coil assembly in a suitable ventilated oven.
With either of the first two external heating methods, you can use resistance grids or space heaters. These may be located inside the case or box, or placed outside and the heat blown into the bottom of the case or box. A couple notes of caution here: The core and coil assembly should be carefully protected against direct radiation from the heaters, and the air temperature should not exceed 110 [degrees] C.
Drying by internal heat is relatively slow and should be used only when the other two methods are unavailable. The transformer should be located to allow free circulation of air through the coils from the bottom to the top of the enclosure. One winding should be short-circuited, and sufficient voltage at normal frequency should be applied to the other winding to circulate approximately 75 % of normal current. The winding temperature should not exceed 100 [degrees] C as measured by resistance or by thermometers placed in the ducts between the windings. The thermometers used should be of the alcohol type or thermocouples. Take care to protect the operator from dangerous voltage.
Drying by external and internal heat is a combination of the two methods previously described and is, by far, the quickest method. The transformer core and coil assembly should be placed in a nonflammable box, or kept in its own case when suitable, and external heat applied (as described in the first method) as current is circulated through the windings (as described in the second method). The current required will be considerably less than when no external heating is used, but should be sufficient to produce the desired temperature in the windings. However, the temperature attained should not exceed that stated in the foregoing paragraphs.
Applying load. Before applying load to a dry-type unit, you may wish to review ANSI/IEEE Standard C57.96-1989, Guide for Loading Dry-Type Distribution and Power Transformers.
You will observe vapor or smoke when initially applying a load to a varnish-insulated transformer. Do not be concerned. As the unit is brought up to full load, some temporary vapor or smoke may be given off from the unit's coil/core assembly. This is not an uncommon occurrence. It's due to the heating of residual varnish in the coils. This condition will disappear in a few hours after stabilizing at normal operating temperature.
Special procedures for tap selling. For dry-type transformers, after the correct tap connection has been determined from the nameplate, use the following procedure to change taps.
* Deenergize the transformer. Safety is of the utmost importance here. Make sure there is no back feed from a low-voltage tie breaker. Verify that the transformer is de-energized by testing.
* Remove front access panels from the transformer enclosure.
* When the output voltage requires adjustment, either up or down, you should change the percentage tap jumpers found on the front surface of the coils.
* Change the tap jumper on each phase to the correct tap connection. The tap jumper must be on the same tap position on all phases.
* The tap jumper must be installed on the upper side of the coil tap, with lugs on the ends of the cable tap jumpers positioned for maximum electrical clearances from the ground and other live parts. Be sure all bolts are tightened as described elsewhere.
* Replace the front access panel.
* Energize the transformer and recheck the output voltage.
Fully assembled liquid-filled transformers External inspection. This inspection calls for different criteria than for dry-type units. One of the first things you should check upon receipt and placement of the transformer is its fluid level. This is done by inspecting the liquid level gauge. If such a gauge is not used, check the level by opening the fill plug. Be careful when doing this. You should take steps to prevent the entrance of moisture or foreign objects. Moisture, dirt, and foreign objects can weaken the insulation level of the transformer fluid and greatly shorten the transformer's life.
Any unit that doesn't have the proper fluid level should be checked for leaks and topped off. When adding fluid to the transformer, use only quality oil, or if applicable, the appropriate fire-resistant fluid.
Transformers with liquid level gauges will have an indicator for the minimum allowed fluid level as well as a minimum 25[degrees]C level. Transformers without such a gauge must have the liquid level checked by removing the 25 [degrees] C level plug. If the liquid level is more than 1/2 in. below the 25 [degrees] C line, consult the manufacturer before energizing the transformer.
Presently, all transformers are filled or processed at the factory with dielectric fluid containing no detectable PCBs, in accordance with Federal Polychlorinated Bi-phenyl (PCB) Regulations. The presently accepted limit for PCBs in dielectric fluid is 1 ppm as based upon the latest detection capability. You should take precautions so that PCB contamination is not introduced during field filling or maintenance of a transformer. The use of old pumping equipment and old hoses that were at one time used for servicing transformers that contained PCB dielectric fluid is one of the main culprits for introducing PCBs into new transformers. And when storing dielectric fluid, you should make sure that the container being used is extremely clean and dry.
Testing the dielectric fluid. Upon receipt of the transformer, you should carry out certain tests using a sample of the dielectric fluid. These tests include dielectric withstand (per ASTM D1816 or D877), and dissipation (power) factor (per ASTM D924). The primary purpose of the tests is to confirm that the properties of the dielectric fluid meet the conditions established by the appropriate standards. But, the tests also set benchmark values for comparison with periodic testing done while the transformer is in use.
Standard dielectric fluid test procedures are found in ASTM D3487 for conventional mineral oil, ASTM D5222 for fire-resistant hydrocarbon fluids, and ASTM D4559 and D4652 for fire-resistant silicone liquids.
If analysis of dissolved gases in the dielectric fluid is to be performed as part of a preventive maintenance program, it's also advisable to set benchmark data for the new fluid. Fire-resistant (less-flammable) fluids also should be tested per ASTM D92 to confirm a minimum 300 [degrees] C fire point, which is required by the NEC for this class of fluid.
ANSI/IEEE is the best source for required acceptance tests for various fluid properties. Such applicable standards are C57.106-1991, IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment, for conventional mineral oil; C57.121-1988, IEEE Guide for Acceptance and Maintenance of Less Flammable Hydrocarbon Fluid in Transformers, for fire-resistant hydrocarbon fluid; and C57.111-1989, IEEE Guide for Acceptance of Silicone Insulating Fluid and Its Maintenance in Transformers, for fire-resistant silicone fluids.
Internal tank inspection
Fully assembled liquid-filled transformers are usually shipped ready for installation and do not require internal inspection. However, if the transformer's tank must be opened, you should prevent the entrance of moisture, dirt, and foreign objects.
Mounting. It's important that you mount a liquid-filled transformer on a flat level pad strong enough to support the weight of the transformer (Part 1 of this article, in the June '96 issue, contained information on concrete bases for transformers). When supplied, hold-down cleats or brackets, as shown in the Figure (above), should be used to bolt the transformer securely to the pad. The unit should not be tilted in any direction greater than 1.5 degrees, as a greater tilt will cause deviations in the liquid level near fuses, pressure relief devices, and/or other accessories specifically located at or near the 25 [degrees] C liquid level. The result of a deviation in the liquid level can increase the possibility of a disruptive failure.
Locating the transformer. Since a conventional oil-filled transformer contains a combustible insulating fluid, transformer failure can cause fire and/or explosion. Alternatives, such as NEC compliant fire-resistant fluids (which have a reduced fire and/or explosion potential) should be considered when locating this type of transformer in buildings, or in close proximity to buildings or public thoroughfares. You should refer to the latest edition of the NEC for guidance in locating liquid-filled transformers and be guided by local codes. (Refer to the article "Knowing Liquid-Filled Transformer Installation Requirements" in the June '96 issue.)
Another consideration is that all indoor liquid-filled transformers require containment to control possible leaks of the fluid. Outdoor liquid-filled transformers containing more than 660 gal of fluid must meet the EPA's requirement for use of some type of containment to control possible leaks. Local codes may also have containment requirements.
Venting. Before accessing the inside of a transformer's tank, you must vent the unit by manually operating the pressure relief device normally provided, or by removing the vent plug. The transformer should be vented before it's energized if it has been pressurized for a leak test, since pressure will increase as the transformer reaches its operating temperature.
Treatment of bushings. Remove dirt and foreign material from all bushings. Carefully inspect the bushings for cracks or other damage before placing the transformer in service. If there are problems, contact the manufacturer and do not energize the transformer. Also, do not energize the transformer with the shipping caps on the bushings or inserts.
High-voltage porcelain bushings (when provided) are externally clamped gasketed bushings with eyebolt-type terminals. The primary cables enter the compartment from below and attach to the bushing terminals. The terminals will accommodate No. 8 through 250 kcmil cable.
Separable insulated connectors. Separable insulated connectors may be universal bushing wells, integral bushings, or bushing wells with inserts installed. They may be either loadbreak or nonload break. All connectors must be dry and clear of any contamination before installation. Unused terminals should be properly terminated to prevent possible contamination. Follow the manufacturer's instructions and warnings relating to the use of these terminations.
During installation, the recommended sequence of connections is to first make all ground connections, then the low-voltage (secondary) connections, and finally the high-voltage (primary) connections. Caution: Before making any connections, verify that all safety precautions have been taken in assuring that all conductors (both high-and low-voltage, the latter possible having back feed) not be energized. A transformer should be removed from service by reversing the above sequence of connections.
Carefully check the transformer nameplate for its rating and the connections that can be made to it. Avoid excessive strain on the bushing terminals or insulators; this could loosen the contact joints or damage the insulators. Remote energizing of a transformer, at a minimum using a hot stick, is recommended as a routine procedure.
Adjustments to the top changer. The tap changer provides a means of changing the voltage ratio of a transformer. These units serve a useful role to helping assure that the load receives the called-for voltage. Many transformers are supplied with an externally operated high-voltage tap changer, located near the high-voltage bushing. To change tap connections, use the instructions provided by the manufacturer.
Dual-voltage transformers, according to ANSI recommendations, should not have tap changes. For dual-voltage transformers that have tap changes, consult the name-plate to determine the proper tap changer setting for each voltage option.
Cabinet security. Before leaving the site of an energized transformer, you should make sure that all protective or insulating barriers are in place, the cabinet is completely closed, and all locking provisions are properly installed.
Accessories and their function. It's important that you're aware of the following accessories and their function.
Pressure relief device. A standard pressure relief device, located on the tank above the liquid level, relieves excessive internal tank pressure automatically and reseals at a lower positive pressure. It can be manually operated by grasping the end-cap (or ring, if provided) and slowly pulling it away from the tank until pressure is relieved.
Thermometer. When supplied, a thermometer indicates the liquid temperature near the top of the tank. The temperature-sensitive element is mounted in a leakproof well, permitting removal of the device without lowering the liquid level. These devices are usually furnished with an additional pointer, red in color, to show the highest temperature attained since last reset.
Liquid level gauge. When supplied, this gauge is located in the low-voltage compartment and indicates liquid level variation from the liquid level at a temperature of 25[degrees]C. This component is a readily available accessory and allows the fluid level to be easily checked without exposing the dielectric fluid to the atmosphere. You should make sure your transformer has one.
Pressure-vacuum gauge. When supplied, a pressure gauge is located in the low-voltage compartment above the bushings in the air space. The gauge indicates whether the gas space in the tank is under a positive or negative pressure.
Nameplate. A nameplate is supplied on each transformer according to Section 5.12 of ANSI Standard C57.12.00-1993, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. You should refer to the nameplate for transformer ratings and for proper connections of the transformer to the system. No internal connections should be made inside the transformer other than those shown.
