Winding Insulation Maintenance and Cleaning and Drying of the Generators {4450} Caterpillar

Generator Set

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Introduction

Winding insulation breakdown in generators causes current leakage and lower high voltage resistance, which allows short circuit condition to occur. Generators stored in or operated in contaminated environments are especially at risk. Environment contaminants include excess dust, water, salt, grease, and humidity and should not be allowed to accumulate on the generator windings. This Special Instruction provides information for testing and maintenance procedures.

Reference: Refer to the following documents for more information:

• "IEEE STD 43-2000 Recommended Practice for Insulation Testing of Rotating AC Machinery"

• Engine News, SEBD0660, "Operating Environment and Effects on SR4 Generators"

• Application and Installation Guide, LEBW4993, "Generator Systems"

• Application and Installation Guide, LEBE5294, "EP Applications, Engine & Generator Sizing"

Important Safety Information

Work safely. Most accidents that involve product operation, maintenance, and repair are caused by failure to observe basic safety rules or precautions. An accident can often be avoided by recognizing potentially hazardous situations before an accident occurs. A person must be alert to potential hazards. This person should also have the necessary training, skills, and tools in order to perform these functions properly. Safety precautions and warnings are provided in this instruction and on the product. If these hazard warnings are not heeded, bodily injury or death could occur to you or to other persons. Caterpillar cannot anticipate every possible circumstance that might involve a potential hazard. Therefore, the warnings in this publication and the warnings that are on the product are not all inclusive. If a tool, procedure, work method, or operating technique not recommended by Caterpillar is used: Ensure that such tool, procedure, work method, or operating technique are safe for you and for other people to use. Ensure that the product will not be damaged, or be made unsafe, by the operation, lubrication, maintenance, or repair procedures that are used.

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Personal injury or death can result from high voltage. When power generation equipment must be in operation to make tests and/or adjustments, high voltage and current are present. Improper test equipment can fail and present a high voltage shock hazard to its user. Make sure the testing equipment is designed for and correctly operated for high voltage and current tests being made. When servicing or repairing electric power generation equipment: Make sure the unit is off-line (disconnected from utility and/or other generators power service) , and either locked out or tagged DO NOT OPERATE. Remove all fuses. Make sure the generator engine is stopped. Make sure all batteries are disconnected. Make sure all capacitors are discharged. Failure to do so could result in personal injury or death. Make sure residual voltage in the rotor, stator and the generator is discharged.

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Personal injury or death can result from a spark. An incompletely discharged capacitor can cause a spark. Do not use the insulation tester in an explosive atmosphere.

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Personal injury or death can result from electrocution. The megohmmeter is applying a high voltage to the circuit. To avoid electrocution, do not touch the instrument leads without first discharging them. When finished testing also discharge the generator windings.

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Personal injury or death can result from the engine starting. Regardless of the cleaning method used; to avoid serious injury or death, make sure there is no voltage applied to the windings. Make sure the engine can not be started.

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Personal injury can result from working with cleaning solvent. Because of the volatile nature of many cleaning solvents, extreme caution must be exercised when using them. If unsure about a particular cleaning fluid, refer to the manufacturer's instructions and directions. Always wear protective clothing and eye protection when working with cleaning solvents.

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Personal injury can result from air pressure. Personal injury can result without following proper procedure. When using pressure air, wear a protective face shield and protective clothing. Maximum air pressure at the nozzle must be less than 205 kPa (30 psi) for cleaning purposes.

Checking Equipment

The generator should be checked periodically for any accumulation of dust, water, salt, grease, or oil films. If cleaning is necessary, refer to the methods that are covered in "Cleaning"and "Drying" for more information.

Note: Windings must be kept clean and dry. Any accumulation of dirt will trap moisture. This moisture will reduce the dielectric strength of the insulation.

Note: The use of a space heater is recommended whenever the unit is not running. Even generators that cycle on and off within a 24 hour period should use space heaters during the off hours.

The frequency of the insulation resistance checks will be determined by the environmental conditions and the operating conditions during operation of the generator. At the minimum, an insulation resistance test must be performed annually.

For determining a preventive maintenance schedule, use the following guidelines:

• Generators that have a seasonal operation should be tested and inspected prior to being put back in service.

• If the generator is installed in an enclosed building with low humidity and minimal temperature variations, test the insulation annually.

• If the generator is not in a controlled environment, the insulation tests should be performed every 3 months.

• If the generator is in, or near, a salt laden water environment, or in a high humidity environment (above 75% relative humidity): Test the insulation every 3 months.

Required Tooling

Caterpillar recommends the use of an industry standard calibrated insulation tester for checking the insulation resistance. Insulation testers apply a nominal voltage to test insulation resistance of the generator windings.

Note: Before performing an insulation resistance test, disconnect any of the following items: grounds, loads, meters, and controls. See tool test voltages listed below per generator rated voltages.

Note: Certain conditions will give acceptable or even high insulation resistance values but the unit may still fail. However, these tests are used to determine when cleaning, drying, and repair is necessary.

Generator Components Insulation Resistance Test Voltage

For the main stator, the recommended test voltage will vary depending on the generator AC output voltage, see Table 1.

Insulation Resistance Tests

A permanent record of the resistance readings of the insulation and the values of the temperature readings for the windings should be kept. All readings must be corrected to winding temperature for trending or comparison purposes.

The minimum insulation resistance value for the main rotor and the exciter components is 5 megaohms, but higher values should be expected.

Note: The Polarization Index test (PI test) is an additional insulation test, described in a later section, that is effective at identifying how moisture is affecting the health of insulation of winding. Since random wound windings tend to be more susceptible to retaining moisture, the PI test may be a more effective measure of moisture content in random wound windings than form wound windings. If insulation is identified to be low with the insulation resistance test, performing the PI test on a random wound, as well as a form wound, winding is still recommended to determine if the presence of moisture is cause for low insulation value.

If the recommended values cannot be obtained after cleaning and drying the generator windings, the insulation has deteriorated. All such insulation should be reconditioned by a qualified electrical shop.

Illustration 1	g03346059

For example, 100 megaohms at 50 degree C are converted to 200 (2 x 100) megaohms at 40 degree C.

Trending insulation resistance over time will help prevent sudden failure and therefore unplanned generator down time.

Note: Results from the insulation resistance test give a good indication of the time that cleaning and repairing is becoming more critical. The insulation resistance will vary greatly with temperature. The test should be performed as near as possible to the same temperature and humidity.

Periodic readings of the insulation resistance should establish a trend in deterioration of the insulation. Resistance readings with a 50% reduction or more from the previous reading may indicate an abnormal condition.

Polarization Index

Polarization Index (PI) test is an indicator of moisture and the cleanliness of all the generator windings. Polarization Index is defined as the ratio of the insulation resistance measured after 10 minutes to the insulation resistance value measured after 1 minute

Polarization Index = Insulation Resistance after 10 minutes / Insulation Resistance after 1 min.

If the minimum acceptable PI values cannot be obtained after cleaning and drying the generator windings, the insulation has deteriorated. All such insulation should be reconditioned by a qualified electrical shop.

Note: If the 1 min insulation resistance is above 5000 megaohms, the calculated PI may not be meaningful. In such cases, the PI may be disregarded as a measure of winding condition.

PI Criteria:

PI>=2

• Normal range

PI<2

• significant moisture in the windings

• Recomended to clean and dry immediately

Reference Generator Test Report Data Sheet

Illustration 2	g03443658

Cleaning

The generator should be checked periodically for any accumulation of dust, water, salt, grease, or oil films. These contaminants should not be allowed to accumulate on the windings.

Generators that have been submerged, flooded, or filled with dirt may need extreme cleaning. Refer to "Extreme Cleaning" for more information.

Generators may be routinely cleaned by the following methods:

Brushing

Brush using only fine soft bristled NON ABRASIVE brushes to loosen and remove debris.

Vacuuming

Dry contaminants may be removed by vacuuming. A soft bristle NON ABRASIVE brush may be used in order to loosen the dirt before vacuuming or during vacuuming. Also note that "vacuuming" and using "pressurized air" can be performed on an assembled unit. Washing and steam cleaning must be performed on a disassembled unit.

Note: In order to dry the insulation completely, refer to the procedures in "Drying" for more information.

Washing

If the equipment for drying is available, washing may be performed in a dealer shop. Refer to the procedures in "Drying" in order to determine the equipment that may be needed.

A liquid cleaner designed to remove grease, oil, dirt, and other contaminants from electric motors, parts, and equipment can be used. Heavy-duty cleaners at the recommended concentration levels can also be used. Refer to the following procedure in order to clean the generators:

1. Create the cleaning solution as per the manufacturer recommendations. If a pressure washer is used do not exceed 205 kPa (30 psi).

2. Clean the insulation and rinse the insulation with fresh warm water.

   Note: Cleaning should be done as rapidly as possible in order to avoid long periods of exposure of the insulation to the solvent.

3. Wipe off excess moisture with a clean dry cloth.

4. Bake dry in an oven before placing back into service. Refer to the "Ovens" in "Drying" for more information.

Steam Cleaning

Steam cleaning should only be performed if there is a great amount of grease or dirt present. Use a qualified electrical rebuild shop to steam clean the generator.

Pressure Air

The generators may be cleaned with filtered pressurized air.

Note: This method is the least desired method of cleaning the generators. Pressurized air can relocate the dirt instead of removing the dirt.

Carefully blow the dirt out of the windings by using 135 kPa (20 psi) to 205 kPa (30 psi) filtered dry air.

Once the dirt is out of the windings, refer to the procedures in "Drying" for more information in order to dry the unit.

Extreme Cleaning

This method is for electrical equipment that has been submerged completely, flooded, or filled with dirt.

Note: Even with the best treatment and careful testing, the insulation of a generator that has been submerged may be damaged beyond repair.

Flush the unit with fresh warm water in order to remove all traces of salt and silt. Washing should continue until tests of the fresh water show the insulation to be free of salt.

Note: Complete disassembly of the generator may be necessary, in order to accomplish a proper cleaning.

Bake and dry the unit completely with externally applied heat before placing back into service. Refer to the procedure in "Drying" for more information.

Note: If the generator is used in or near salty air and high humidity environments, special electric component cleaners and protectors are needed to prevent corrosion.

Drying

Drying may be accomplished by external heat, internal heat, a combination of internal heat and external heat, or circulating current.

External heat is the most preferable method. Insulation drying time can vary from a few hours to several days, depending on moisture content and the process used for drying.

Note: Drying sometimes does not produce the required results. The use of a qualified rebuild shop for drying and redipping of the generator may be necessary.

Illustration 3	g01697133
Typical curves of the resistance in the insulation that is drying (3) Curve of the temperature in the windings (4) Heat turned off. (5) Drying time in hours (6) Temperature in degrees Celsius (7) Curve of insulation resistance during drying process (8) Insulation resistance in megaohms

Note: When new insulation or damp insulation is being dried, the resistance will probably fall rapidly as the temperature is raised to a value for drying. After reaching a minimum for a given temperature, the resistance will again rise as moisture is driven out of the insulation. The actual values will vary with each situation. Refer to Illustration 3.

If the windings or insulation become wet during the resistance measurement: Use a five megaohms resistor for protection, in series with the red positive lead, to limit the voltage across the circuit under test. Use this method until drying is well in progress, and the resistance has reached an acceptable level.

During the drying period, check the windings every 4 hours, or every hour, for the method of circulating current with the insulation resistance tests. Refer to "Insulation Resistance Tests" for more information.

Drying is complete when the testing demonstrates that PI > = 2. Record these readings in the Generator Test Data Sheet and compare to previous readings. Keep these records for future reference.

External Heat

Ovens

The best oven is a forced air drying oven for drying electrical equipment. Radiant ovens sometimes cause localized overheating.

Heat the generator to a maximum temperature of 85 °C (185 °F) for a minimum of 4 hours.

Alternate Method

A tent formed by a tarp or a canvas with heating lamps or a portable space heater may be used as an alternate method.

A hole should be left in the top of the tarp for ensuring proper circulation through the generator and for permitting the moisture to exhaust. Heat the generator to a maximum temperature of 85 °C (185 °F) for a minimum of 4 hours.

Internal Heating

If a generator operates under one of the following conditions, electric space heaters should be installed as part of the generator:

• Generators are in damp environments and go for long periods of time without operation.

• Generators operate regularly in an environment with moisture laden air.

Circulating Current

Drying can be accomplished by circulating low voltage current through the windings. Since the voltage is low, breakdown of the insulation will not occur. More likely, the breakdown might have occurred with normal operation voltage and wet insulation.

Illustration 4	g01753557
Schematic for the external power source (9) Battery (12 VDC) (10) Switch (11) F1 (12) L (exciter field that is being dried) (13) F2 (14) 0 to 10 Ohm Rheostat

Refer to the following procedure in order to dry the windings with circulating current:

1. Follow all safety procedures before continuing.

2. Disconnect the generator load and open the breaker.

3. Disconnect the leads of the exciter field from the voltage regulator with the engine stopped. Typically, the leads have the following labels: F1 (11) and F2 (13).

4. Connect the output leads for the generator together. Typically, the leads have the following labels: T1, T2 and T3.

5. Install an AC current probe (clamp-on probe) to generator output lead T1.

6. Adjust rheostat (14) in order to give maximum resistance.

7. Connect the external power source to wires F1 (11) and F2 (13). Switch (10) remains open.
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   NOTICE
   Do not exceed the rated phase current that is listed on the generator nameplate. Exceeding the rated phase current will damage the generator windings.

8. Run the generator set at idle speed. Residual voltage will cause current to circulate within the generator windings.

9. While line current is being monitored, slowly increase rpm until rated line current is obtained, or until full generator set speed is obtained.

   Note: When line current is being measured on multiple lead units, measure the current in each conductor per phase, and add the currents together.

10. If the generator reaches full speed and more line current is still necessary, repeat the following procedure until the rated phase current is obtained:

    1. Run the generator at idle speed.

    2. Close switch (10).

    3. Slowly turn rheostat (14) to the rated phase current.

11. Continue running the current at these settings for 1 hour.

    1. Stop the drying procedure.

    2. Verify the insulation resistance with the insulation tester. If minimum PI value cannot be obtained, continue to step 11.c.

    3. Continue the Circulating Current drying procedure, checking insulation resistance values every hour until the insulation resistance test is acceptable.

    4. After cleaning and drying the generator windings, if the minimum acceptable PI values cannot be obtained, the insulation has deteriorated. All such insulation should be reconditioned by a qualified electrical shop.