Guide for Electrical Testing and Inspection

Electrical Maintenance Program and Principle of Electrical Testing

Electrical Maintenance Activities Fall into three General Categories:

I. ROUTINE MAINTENANCE

• Activities that are conducted while equipment and system are in service. These activities are predictable and can be scheduled, staffed and budgeted. Generally this are schedule on a time –based, run time meter or number of operation schedule.

Examples: 

1. Visual Inspection 
2. Infrared Scans
3. Cleaning
4. Functionality Test
5. Lubrication
6. Oil Test
7. Governor and Excitation System Alignment

II. MAINTENANCE TEST

• Activities that involved the use of electrical test equipment to determine electrical equipment RELIABILITY in an offline state.

Examples: 

1. Insulation Resistance Test
2. Breaker Trip Testing
3. Alternating Current High Potential Test
4. High Voltage Direct Current Ramp Test
5. Battery Load Test

III. DIAGNOSTIC TESTING

• Activities that involve use of test equipment to access condition of equipment after unusual event such as faults, fire or equipment failure/repair/replacement or when equipment deteriorate is suspected.

Examples:

1. Turns Ratio
2. Core Ground Test
3. Insulation Resistance Test
4. Insulation Power Test
5. Winding or Contact Resistance Test

DIFFERENT KINDS OF IDENTIFICATION OF ELECTRICAL HAZARDS

1. Insulation Resistance Test

• Resistance (in megohms) offered by the insulation to an impressed direct voltage. Insulation Resistance is NOT a definite measure of the voltage an insulation will stand, its is an indicator of the suitability of the insulation for continued service.

Application of Insulation Test Method

2. Dielectric Absorption

• The time resistance test is independent of equipment size and temperature. It compares the absorption characteristics of contaminated insulation with the absorption characteristic of a good insulation. The test voltage should be applied over a 10 minute period with the data recorded every 10 seconds for the first minute there after. The interpretation of the of the slope of the plotted graph will determine the condition of the insulation. A continuous increase in resistance in the graph indicates a GOOD Insulation. A flat or downward curve indicates cracked or contaminated insulation. 

3. Polarization Index

• The steepness of the dielectric absorption curve taken at a given temperature indicates the relative dryness of the insulation.

Polarization Index = R10 / R1 = The value must be more than 1.

where:

R10 = MegaOhms at 10 minute reading

R1 = MegaOhms at 1 minute reading

NETA STANDARD / IEEE 43-2000:

Polarization Index should be higher than 1

4. Insulation Power Factor Test

• The purpose of this test is to determine the state of dryness of the winding and insulation system and to determine the power factor for overall insulation, including bushing, oil and winding. It’s a measure of the ratio of power losses to the volt amperes applied during the test. The power factor obtained is measure of watt lost in the transformer insulation system including the bushing. 

The Power Factor should not exceed 0.5% at 20 degrees Celsius

5. Winding Resistance Test or Contact Resistance Test

• This is to check the loose connections and broken strands. The most accurate method is to allow the transformer sit de-energized until temperatures are equalized. Take the reading from the top of each bushing to neutral for wye and across each pair of bushing for delta. Result are compared to other phase in wye connection or between pairs of terminal on delta connected winding to determine if a resistance is too high. Resistance can also be compared to the original factory measurement. Investigate and deviate from adjacent pole or similar equipment / devise by more of 25% of lowest value.

6. Transformer Turn Ratio Testing

• TTR detects shorted turns which indicates insulation failures by determining if the correct turns ratio exist. Shorted turns may result from short circuit or dielectric (insulation ) failure. Measures are made by applying a known low voltage across one winding and measure the induce voltage on the corresponding winding. The voltage ratio obtained by the test is compared to the nameplate voltage ratio. The ratio obtained from the field test should agree with factory data, within 0.5%. New transformer of good quality normally within 0.1%.

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