Additional Techniques and Troubleshooting

There are numerous electrical tests that, while not forecast orientated, provide indications of the system condition.

4.6.2.1 Time Domain Reflectometry

A voltage spike is sent through a conductor. Each discontinuity in the conductor path generates a reflected pulse. The time difference between initial pulse and reception of the reflected pulse

indicate the location of the discontinuity. This technology is used for power cables to help locate faults within a cable run. The test is performed with the cable de-energized.

4.6.2.2 Power Factor and Harmonic Distortion

Maintaining optimum power factor maximizes the efficient use of electrical power. Power factor is the ratio of real power to reactive power usage. Dual channel data-loggers are used to

determine the phase relationship between voltage and current, then calculate the power factor.

Addition of power factor enhancing capacitors is then evaluated as a means of improving power system power factor.

Harmonic distortion is a result of having non-linear loads on the power system. These loads include laser printers, desktop computers, and SCRs found in variable speed motor controls.

High levels of harmonic current cause excessive heating in transformers and cables, which reduce service life and cause spurious tripping of circuit breakers, which can be a major inconvenience. A harmonic analyzer is used to measure the harmonic current and identify the source. Filters can then be placed on the circuit to minimize the impact. This test is performed with the system energized.

4.6.2.3 Motor Starting Current and Time

Starting current in electric motors can routinely exceed five times full load running current. The amount of starting current combined with the duration of the starting surge can indicate the condition of electrically driven equipment. Higher starting current and longer duration of the surge can indicate mechanical problems such as increased friction due to misalignment of the mechanical components of the equipment. Alternatively, coastdown tests using timing devices and vibration monitoring equipment can verify the presence of magnetically induced vibrations or mechanical friction.

4.6.2.4 Transformer Turns Ratio (TTR)

This test measures the turns-ratio of a transformer and is mainly used as an acceptance test. It can also be used as a trouble shooting tool when other electrical tests reveal a possible problem.

During routine maintenance tests a TTR can be performed to identify short circuited turns, incorrect tap settings, mislabeled terminals, and failure in tap changers.

The test is performed by applying a voltage across the primary windings and measuring the resulting voltage across the secondary winding. The ratio of active windings can be calculated.

This measurement can be used to determine the condition of the transformers inductive capability. The turns ratio measurement can show that a fault exists but can not determine the reason or location of the fault. This test is done with the transformer de-energized.

4.6.3 Applications

4.6.3.1 Equipment to be Monitored

Specific equipment that can be monitored by electrical condition monitoring techniques are listed below:

a. Electrical Distribution Cabling— Megohmmeter, VLF Testing, Time Domain Reflectometry, HiPot, Infrared Thermography (if visible) and Airborne Ultrasonics.

b. Electrical Distribution Switchgear and Controllers— Breaker Timing, Insulation Power Factor Testing, Visual Inspection, Infrared Thermography and Airborne Ultrasonics.

c. Electrical Distribution Transformers—Oil Analysis, Turns Ratio, Power Factor and Harmonic Distortion.

d. Electrical Motors—Motor Current Signature Analysis, Motor Circuit Analysis, Megohmmeter, HiPot, Surge Test, , Starting Current and Coast Down Time, Infrared Thermography, Airborne Ultrasonics.

e. Generators—Megohmmeter, VLF Testing, and Coast Down Time.

f. Distribution System—Infrared Thermography, HiPot, Airborne Ultrasonics, Power Factor and Harmonic Distortion.

4.6.3.2 Conditions Monitored

Temperature, voltage, current, resistance, complex impedance, capacitance, insulation integrity, phase imbalance, mechanical binding, and presence of arcing.

4.6.3.3 Detection Interval

Monitoring intervals of several weeks to several years for various technologies will provide sufficient condition information to warn of degrading equipment condition. Specific

expectations of the length of warning provided should be factored into developing monitoring intervals for specific technologies. Additionally, some monitoring intervals will depend on outage cycles. Some of the electrical condition monitoring can be done with the system energized. Several of the technologies outlined are also effective when used for acceptance testing and certification.

.4.6.3.4 Accuracy

Accuracy is dependent on the testing technique applied and rating of the instrument.

4.6.3.5 Limitations

The technologies presented can be divided into two categories:

a. Energized— Those technologies that can safely provide information on energized systems and require the system be energized and operational. These technologies include Infrared Thermography, Airborne Ultrasonics, Motor Current Readings including Starting Current, Motor Current Spectrum Analysis, VLF Testing, Power Factor and Harmonic Distortion, Battery Impedance Testing, Insulation Oil Analysis (Including Gas-in-Oil).

b. De-Energized— Technologies that require the circuit to be de-energized include Surge Testing, HiPot Testing, Time Domain Reflectometry (TDR), Megohmmeter, Motor Circuit Analysis, , Circuit Breaker Timing, Transformer Turns Ratio, and Insulation Power Factor Testing.

Each technology will require specific initial conditions to be set prior to conducting the test. For instance, prior to an Infrared Thermography survey, typical equipment powered through the switchboard should be running to bring the distribution equipment to normal operating temperatures. Higher load accentuates problem areas. Conducting the survey at low load conditions may allow a problem to remain undetected.

4.6.4 Logistics

4.6.4.1 Equipment Required

A comprehensive electrical testing program includes: Infrared Camera, ultrasonic noise detector, multi-meters/volt-ohmmeters, clamp on current transformers, Insulation Power Factor test set, Time Domain Reflectormery test set, Breaker Timing test set, Contact Resistance (micro-ohmmeter) test set, Battery Impedance test set, VLF test set, motor current signature analysis soft-ware, and integrated motor circuit analysis testers.

4.6.4.2 Operations

Electricians, electrical technicians, and electrical engineers should be trained in electrical PT&I techniques such as motor current signature analysis, motor circuit analysis including complex phase impedance and insulation resistance readings and analysis.

4.6.4.3 Training Available

Equipment vendors and independent companies provide training. See Appendix C for training sources.

4.6.3.4 Cost

a. Equipment—Equipment costs vary from $20 for a simple multi-meter to

approximately $40,000 for integrated motor-current analysis (MCA) testers. A full inventory of electrical testing equipment can cost in excess of $100,000.

b. Training—Training averages between $750 and $1,000 per week per person trained for each technology.