Section 6 – Maintenance

DANGER

hazard of electric shock, explosion, or arc flash Only qualified personnel should clean, inspect, maintain and test Unit Substations. Turn off power supplying this equipment before working on it. Disconnect and electrically isolate power to the Unit Substation so that no accidental contact can be made with energized parts. Discharge all static charges held by coils. Failure to follow these instructions will result in death or serious injury.

1. Perform a visual inspection of the Unit Substation and its surrounding environment.

   1. Inspect the enclosure for any physical damage.

   2. Repair any damage, if possible.

   3. Provide guards to prevent future damage.

2. Record operational data while the Unit Substation is still energized and in service.

3. De-energize the Unit Substation prior to performing work on the unit.

   1. Follow all safety procedures to place the unit into an electrically safe condition.

   2. Remove the access covers. Inspect for:

      1. dirt on insulating surfaces and at areas which tend to restrict air flow.

      2. loose connections.

      3. the general condition of the Unit Substation.

4. Inspect the enclosure, Unit Substation, terminals, lead wires, and insulators for signs of overheating from internal or external sources. Check for voltage creepage over insulating surfaces, evident by tracing or carbonization.

5. Check for evidence of rusting, corrosion and paint deterioration. Repair any rust and corrosion where necessary.

The frequency at which the Unit Substation should be inspected depends on operating conditions. For clean, dry locations, annual inspections may be sufficient. However, for other locations, such as where the air is contaminated with dust or chemical fumes, an inspection at three- or six-month intervals may be required. Usually after the first few inspection periods, a definite schedule can be set up based on the Unit Substation’s existing conditions.

• Perform routine inspections, maintenance, and testing after any severe electrical short circuit, ground fault, or environmental event (e.g., flooding) to determine the operational status of the Unit Substation.

• Perform the inspections, maintenance and testing if the Unit Substation has been out of service for an extended period of time. Place strip heaters to maintain the Unit Substation temperature above ambient to prevent condensation from forming in the Unit Substation during extended down time.

DANGER

hazard of electric shock, explosion, or arc flash Only qualified personnel should clean, inspect, maintain and test transformers. Turn off power supplying this equipment before working on it. Disconnect and electrically isolate power to the transformer so that no accidental contact can be made with energized parts. Discharge all static charges held by coils. Failure to follow these instructions will result in death or serious injury.

Perform the following electrical tests during maintenance:

• Check all electrical contacts with a calibrated torque wrench.

• Verify proper spacing.

• Refer to the manufacturer’s specifications for torque values.

• Check all external hardware for tightness.

• Use a low resistance ohmmeter measure contact resistance.

  • Compare values with similar connections looking for unusually high contact resistance points.

  • Investigate any connections that may deviate from common values by more than 50% of the lowest values.

• An infrared scanning device may be used to look for abnormal temperatures at transformer contacts for energized units.

  NOTE: Take care to follow the manufacturer’s procedures, as small errors in scanner operation can cause significant errors in detected temperatures.

Insulation resistance checks may create a capacitive charge on a winding.

1. Ground each winding for at least one minute between tests to drain any static charge created during the test.

2. Perform insulation resistance tests with an appropriate device.

3. Test from winding-to-winding and winding-to-ground.

4. Record all test results for comparison for future readings.

   1. Values are installation site dependent and not pre-set at the factory. All testing is compliant to NEMA ST-20.

   2. Insulation resistance readings should be at least 1 megohm.

   3. Lower readings may indicate the presence of moisture and require a drying procedure to connect.

1. Check for proper values of resistance between the systems ground lug (or point) and the transformer enclosure.

2. Compare with pre-commissioning values, looking for trends that may indicate a compromise in the quality of the ground bond.

1. Perform a turns ratio test on all taps and on the full winding voltages for each phase. Significant changes may indicate a loss of winding continually.

   1. Compare these values with those obtained during the pre-commissioning checks. Voltages between phases should not deviate by more than 0.5% from adjacent coils.

   2. Investigate abnormal readings fround outside of this limit.

2. Verify input and output voltages are correct for the tap and output voltages expected. Investigate abnormal readings.

Ensure terminals are in good working condition for the proper operation of Unit Substations.

• Inspect terminals for alignment, tightness (see torque requirements), pressure, burns or corrosion. Investigate and correct any signs of damage.