1993/07/12 Caterpillar

All Caterpillar Generators Sets

Reference: SEBD0660, "Operating Environment-And Its Effects On SR4 Generators"; Special Instruction, SEHS9124, "Cleaning and Drying of Electric Set Generators."

There is a potential risk of stator failure for generators stored or operated in contaminated and/or humid environments. Problems can develop in the generator windings when dust, water, salt or grease is present, or anytime the generator windings are below the dewpoint.

Contaminants such as dirt, dust, grease, oil films, salt and water should not be allowed to accumulate on the stator windings. These conditions increase the risk of moisture collecting in the windings. Moisture can provide a conductive pathway between bundles of wire. When this occurs, current will leak from the higher voltage bundle to the lower voltage bundle. The current leak produces heat and causes a breakdown and charring of the winding insulation. If the problem goes undetected, the windings may be severely damaged.

The following preventive maintenance activities will reduce the risk of stator failure for generators stored or operated in contaminated and/or humid environments.

Safety

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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.

Attach a "DO NOT OPERATE" or similar warning tag to start switch or controls before performing any service or repairs.

NOTE: "DO NOT OPERATE" tags are available from Caterpillar. Order Form SEHS7332.

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When power generation equipment must be in operation to make tests and/or adjustments, high voltage and current are present. * Improper test equipment may 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.

New Equipment

Storage

After a new generator arrives on site it should be protected against moisture until it is installed.

When a generator is in storage, moisture condenses in the windings. To minimize condensation, always put the generator in a dry storage area.

When the generator is placed in storage, it is necessary that a megger insulation resistance check be performed and the results recorded. The temperature and level of humidity should also be recorded. This check will provide a baseline for future reference.

NOTE: It is important that this baseline be established when the unit is dry.

To guard against collective contaminants, the generator should be covered with a plastic cover or similar type of protective cloth. The protective cover should extend to the ground, but remain loose around the generator in order to allow it to properly breathe.

Caterpillar recommends maintaining the insulated parts and the air surrounding them at a temperature of at least 5° C (9°F) higher than the room temperature in which the equipment is located. This will prevent condensation. Normal methods of providing the heat required are:

1. Space heaters.

2. Warm air blower. Do Not exceed 206.7 kPa (30 psi).

3. Light bulb (60 Watt minimum) installed inside the generator in proximity to the stator core at the lowest possible location.

It is important that the shaft be rotated 10 revolutions every 60 days throughout the storage period. This process helps to insure that grease continues to cover all bearing surfaces.

Removal From Storage

Operate space heaters for at least 24 hours prior to removing covers.

Before startup of a generator, visually inspect the generator for any foreign material such as rodents, etc. that may have nested in the generator. Use a megohmeter to check insulation resistance for moisture and/or foreign material. Refer to the Service Manual for the procedure.

A resistance reading with a 50% reduction from the reading recorded when the unit was placed in storage (100 megohms for a new unit), is an indication that the winding has absorbed too much moisture. Clean as necessary according to the instructions in Special Instruction, SEHS9124, "Cleaning and Drying of Electric Set Generators".

Visual Equipment Checks

The generator should be checked periodically for any accumulation of dust, water, salt, grease, or oil films. Clean as necessary according to the instructions in Special Instruction, SEHS9124, "Cleaning and Drying of Electric Set Generators".

NOTE: Windings must be kept clean and dry. Any accumulation of dirt will trap moisture. This moisture will reduce the insulation's dielectric strength, causing the insulation to fail.

Recommended Megger Insulation Test Procedure

Main Stator

1. Take generator out of service.

2. Visually inspect for moisture. If moisture exists, do not megger. Dry unit first. Refer to Special Instruction, SEHS9124, "Cleaning and Drying of Electric Set Generators" for drying procedure.

3. Inspect installation to determine what equipment will be tested by the 4C4213 Megger.

4. Discharge the capacitance of the windings.

5. Disconnect T0 from ground.

6. Disconnect regulator sensing lead wires 20, 22, 24.

7. Connect megger tester RED lead to ground (+).

8. Connect megger tester BLACK lead T0 (-).

9. Set voltage for 500 Volts for units that are 600 volts or less.

Set voltage for 1000 Volts for units that are more than 600 volts.

10. Use the 30 - 60 time resistance method.

(a) Apply voltage.

(b) Observe the readings at 30 seconds and again at 60 seconds.

(c) Record the 60 second reading. This reading must be corrected for temperature. Refer to "Approximate Insulation Resistance Variation with Temperature " chart at the end of this article.

(d) Record temperature.

(e) Record humidity.

(f) Remove voltage.

11. Evaluate readings. The actual value of the resistance may vary greatly from one generator to another. For this reason, the insulation condition must be evaluated based on how the 60 second resistance readings compare to the readings taken on previous dates (under similar conditions). A 60 second resistance reading with a 50% reduction from the previous reading indicates that the insulation may have absorbed too much moisture.

The illustration above shows the resistance curve of normal high resistance windings over a period of one minute. The resistance after one minute will be greater than or equal to the resistance after 30 seconds.

The illustration above shows the resistance curve decaying over a period of one minute. If the insulation resistance after one minute is not higher than the resistance after 30 seconds, the windings should be cleaned and thoroughly dried. Refer to Special Instruction, SEHS9124, "Cleaning and Drying of Electric Set Generators" for cleaning and drying information.

12. Discharge leads and windings before disconnecting megger leads by switching megger to "off".

NOTE: Good results from the insulation resistance checks give a good indication of when drying/cleaning and/or repairing is becoming more critical.

Generally insulation resistance will vary greatly with temperature. Tests should be made at the same temperature and humidity (as near as possible) each time.

Evaluating the Test Reading of Operating Generators After Cleaning and Drying

Caterpillar recommends that the minimum acceptable insulation resistance, for generators with less than 1000 volts of operating or rated voltage, is one megohm. Any generator, with more than 1000 volts of operating or rated voltage, should have a minimum insulation resistance of (Rated V/1000) + 1 megohm. (Typical readings will be much higher.).

If the Caterpillar minimum acceptable insulation resistance readings cannot be obtained after cleaning and drying the generator windings, the insulation has probably deteriorated and should be reconditioned by a qualified electrical shop.

Frequency of Insulation Resistance Checks

The environmental and operating conditions in which the generator is operating will determine how often the megger insulation resistance checks should occur. You must be confident the readings are not changing more rapidly than the frequency selected before determining a preventative maintenance schedule. Megger insulation resistance checks must be performed a minimum of once a year.

Generators that work seasonally should be tested and inspected prior to being placed back in service.

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

If the above conditions are not met, the insulation tests should be made every three months.

If the generator is in or near a sea water environment, or in a high humidity (above 75% relative humidity) environment, test the insulation every month.

If the most recent test results were less than 3 megohms, increase the frequency of the insulation test.

Use of Space Heaters

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.

Approximate Insulation Resistance Variation with Temperature Chart.