INST-®®1
INSTALLATION AND MAINTENANCE
OF
DRY TYPE TRANSFORMERS
- GENERAL
This outline serves as general instructions for the operation and maintenance of dry type transformers. The transformer and related equipment covered by these instructions is intended to be installed, operated and serviced by qualified technicians familiar with electrical codes and practices peculiar to devices of this type.
In addition to the use of this guide, the manufacturer should be consulted for recommendations on special conditions and applications.
Reference IEEE C57.94 and NEMA ST-20 for additional installation and maintenance guidance.
- INSPECTION
Upon receipt of equipment, thorough inspection must be given to locate and remove any special packaging or blocking that may be installed by the manufacturer for protection during transit. The unit should be checked for loose connections or bolts if rough handling is evident.
Identify any initial electrical tests performed once the transformer is received in order to identify shipping damages. See section IV for field testing.
In the event of damage or if any indication of rough handling is visible a claim must be promptly filed with the transport company and the manufacturer immediately notified of this damage. If no physical damage is present, tighten any parts that may have worked loose during transit.
Should a transformer be moved, or if it is stored before installation, the inspection should be repeated before placing the transformer in service.
The fans, thermal relays, metering and other devices should be checked.
- INSTALLATION
Location: Some of the factors, which should be kept clearly in mind in locating dry-type transformers, are ventilation, accessibility and atmospheric conditions.
Double VPI Ventilated dry-type transformers may be installed indoors in drylocations or outdoors in specifically built NEMA 3R enclosures. These enclosures are weatherproof for outdoor applications.
Adequate ventilation is essential for proper cooling of transformers. Clean dry air is desirable. When transformers are installed in restricted spaces, sufficient ventilation must be provided to hold the air temperature within limits as stated on the nameplate when measured near the transformer inlets. This usually will require approximately 100 cubic feet of air/minute/50 KVA of transformer rating.
Installation must not be made in areas with abnormally high ambient. Care must be used to avoid areas where corrosive chemical vapors or fumes may contact the unit for continuous periods of time.
Normally transformers are designed for suitable operation at their rated KVA up to an altitude of 3300 feet (1000M) at ambient temperature not exceeding 40°C and an average ambient of 30°C for any 24-hour period (See NEMA TR27-4.07). For derating factors, which apply to installations at high altitudes, due to decreased air density, see 1200.520 of American National Standard C57.12-1973.
Grounding: The case and core assembly of these transformers should be permanently grounded with a properly sized strap or conductor per the National Electrical Code.
Storage: Ventilated dry-type transformers should be stored in a warm, dry location with uniform temperature. Ventilating openings should be covered to keep out dust. If it is necessary to leave a transformer outdoors it should be thoroughly protectedto prevent moisture and foreign material from entering. Condensation and absorption of moisture can be prevented or minimized by the addition of space heaters or small portable heaters. Units with enclosures design for outdoor operations should follow the same procedures while not energized.
Energization: before placing in service, check operation of fan motors, metering equipment, temperature monitor and any other peripheral devices. Verify selection of and connection of voltage taps: check tightness of all connections.
For units bearing a UL label please see VII. UL instructions and restrictions.IV. RECOMMENDED FIELD TESTING
Pre-service tests should be made after installation and prior to placing a new transformer into service to determine serviceability and to record data for future comparison.
1. Insulation Resistance Test
The test should be made in accordance with IEEE Standard 057.12.91-2001 and should be performed before the applied voltage test. For future comparison the Temperature and Humidity should also be recorded.
Winding KV Class / Minimum Insulation Resistance (Ma)1.2 / 600
2.5 / 1000
5.0 / 1500
8.7 / 2000
15.0 / 3000
25.0 / 5000
34.5 / 7000
Normally dried transformers may have readings 5 to 10 times greater than the above minimum values.
- Applied Voltage Test
Both dc and ac sources are acceptable for applied voltage testing. Initial installation tests using ac test equipment should be limited to 75% of the factory test values. If dc do not exceed the factory test values listed below from Table 5 in IEEE Standard 057.12.01
1998.Winding BIL / Factory Value (kV) / 75% Factory Value (kV)
10 / 4 / 3
20 / 10 / 7.5
30 / 12 / 9
45 / 19 / 14.25
60 / 31 / 23.25
95 / 34 / 25.5
110 / 37 / 27.75
125 / 40 / 30
150 / 50 / 37.5
170 / 55 / 41.25
200 / 70 / 52.5
- Ratio Test Perform test in accordance with IEEE Standard 057.12.91-2001.
V. MAINTENANCE: MUST BE DONE WITH TRANSFORMER IN A DE-ENERGIZED CONDITION
When the transformer is operating under ideal atmospheric conditions, the maintenance required is at its very minimum. For clean, dry locations, an inspection annually may be sufficient. For locations where the air is contaminated with dust or corrosive chemical fumes, an inspection at three or six month intervals may be required. After the first few inspection periods a definite schedule can be set up based on the existing conditions.
With the transformer deenergized, inspections should be made for dirt, especially accumulations on insulation surfaces or for those, which tend to restrict airflow. It is necessary to check electrical connections to insure mechanical tightness and the terminal boards for signs of overheating and voltage creepage over insulating surfaces as evidences by tracking or carbonization.
Cleaning: In the case of accumulation of dirt on the transformer windings or insulators, the dirt should be removed to permit free circulation of air and to guard against the possibility of insulation breakdowns. Particular attention should be given to cleaning top and bottom ends of winding assemblies and to cleaning out ventilating ducts.
With the transformer deenergized, the windings may be cleaned with a vacuum cleaner, a blower, or compressed air. The use of a vacuum cleaner is preferred as the first step in cleaning followed by the use of compressed air of Nitrogen. The compressed air should be applied at a relatively low pressure (not over 100 pounds per square inch). Tap changers, terminal boards, bushings and other major insulating surfaces should be brushed or wiped with a dry cloth. The use of liquid cleaners is undesirable because of the deteriorating effect on insulating materials.
DANGER-HAZARDOUS VOLTAGE WILL CAUSE SEVERE INJURY OR DEATH
VI. DRYING OF CORE AND COIL ASSEMBLY
Moisture is not usually a problem in dry-type transformers that are continually in service. Heat from the unit evaporates any moisture, which might collect.
In the event that it is necessary to dry out a transformer before installation or after an extended shut down under relatively high humidity conditions, one of the following methods should be used.
(1)External Heat
(2)Internal Heat
(3)External and Internal Heat
Free moisture should be blown or wiped off the windings to reduce the time required for drying.
Drying by External Heat: External heat may be applied to the transformer by one of the following methods:
(1)By directing heated air into the bottom air inlets of the transformer.
(2)By placing the core and coil assembly in a nonflammable box with openings at the bottom and top through which heated air can be circulated.
(3)By placing the core and coil assembly in a suitably ventilated oven.
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INST-OO!
When using either of the first two external heating methods, heat may be obtained by the use of space heaters or resistance coils. These may be either located inside the enclosure or may be placed outside and heat blown into the bottom through the winding ducts. The core and coil assembly should be carefully protected against direct radiation from the heaters. It is recommended that the air temperature should not exceed 110°C.
Drying by Internal Heat: One winding should be short-circuited, and sufficient voltage at normal frequency should be applied to the other winding to circulate approximately 90% full load current as stated by the nameplate. The transformer should be located to allow circulation of air through the coils from the bottom to the top of the case. It is recommended that the winding temperature does not exceed 100°C as measured by resistance or by thermometers placed in the ducts between the layers of windings.
This method is relatively slow and should not be used if one of the other two methods is available.
Drying by External Heat and Internal Heat: This is a combination of the two methods previously described and is also the quickest method. External heat is applied as described in the first method and current is circulated through the windings as described in the second method.
Cautions: Constant attendance during the drying process is desirable and in event of an emergency, a suitable fire extinguisher should be conveniently available. All work must be done by personnel familiar with these methods.
VII. UL INSTRUCTION AND RESTRICTIONS.
When a unit is bearing a UL or CSA label you must:
- Do not install on or over combustible surfaces.
- Do not install in areas accessible to the public
- Warning: More than one lie circuit
- Do not install closer than 12 inches to surrounding surfaces
- Use 90°C cable sized to 75°C ampacity.
- De-energize transformer before changing taps.
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