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Widow · (This post was last modified: 09-28-2020, 06:38 AM by Widow.)

6: MAINTENANCE REQUIREMENTS - ELECTRICAL MONITORING

6.1: Electrical Condition Monitoring

A major portion of components onboard Chesterfield Station rely on electrical equipment to operate. This includes everything from the power distribution system to electric motors, the electrical systems efficient operation is crucial to maintaining operational capability. Electrical condition monitoring encompasses several technologies and techniques that provide critical information so a comprehensive system evaluation can be performed. Monitoring key electrical parameters provides the information to detect and correct electrical faults such as high resistance connections, phase imbalance and insulation breakdown. Since faults in electrical systems are seldom visible, these faults are costly (increased electrical usage), present safety concerns (fires) and life cycle cost issues (premature replacement of equipment). Voltage imbalances of as little as 5% in motor power circuits result in a 50% reduction in motor life expectancy and efficiency in 3 phase AC motors. A 25% increase in motor temperatures can be generated by the same 5% voltage imbalance accelerating insulation degradation.

6.2: Standard Tests

6.2.1: Insulation Power Factor

Insulation Power Factor, sometimes referred to as dissipation factor, is the measure of the power loss through the insulation system to ground. It is a dimensionless ratio, which is expressed in percent of the resistive current flowing through an insulation to the total current flowing. To measure this value a known voltage is applied to the insulation and the resulting current and current/voltage phase angle is measured. IR is the resistive current, IC is the capacitive current, IT is the resultant, or total current, and V is the applied voltage. Usually, IR is very small compared to IT because most insulation is capacitive in nature. As a comparison, look at the similarities between a capacitor and a piece of electrical insulation. A capacitor is two current carrying plates separated by a dielectric material. An electrical coil, such as would be found in a transformer or motor, is a current carrying conductor, with an insulation material protecting the conductor from shorting to ground. The conductor of the coil and ground are similar to the two conducting plates in the capacitor, and the insulation of the coil is like the dielectric material of the capacitor. The dielectric material prevents the charge on each plate from “bleeding through” until such a time that the voltage level of the two plates exceeds the voltage capacity of the dielectric. The coil insulation prevents the current from flowing to ground, also until such a time that the voltage level exceeds the voltage capacity of the insulation. As the impedance of the insulation changes due to aging, moisture, contamination, insulation shorts, or physical damage the ratio between IC and IR will become less. The resulting phase angle between the applied voltage and resultant current then becomes less, and the power factor will rise. Consequently, the power factor test is primarily used for making routine comparisons of the condition of an insulation system. The test is non-destructive, and regular maintenance testing will not deteriorate or damage insulation.

6.2.2: Megohmmeter Testing

A hand-held generator is used to measure the insulation resistance phase-to-phase or phase-to-ground of an electric circuit. Readings must be temperature-corrected to trend the information. Winding temperatures affect test results. An enhanced technique compares the ratio of the Megohmmeter readings after one minute and ten minutes. This ratio is referred to as the polarization index.

6.2.3: High Potential Testing

High potential testing applies a voltage equal to twice the operating voltage plus 1000 volts to cables and motor winding testing the insulation system. This is typically a go/no-go test. Industry practice calls for high potential testing test on new and rewound motors and on new cables. This test stresses the insulation systems and can induce premature failures in marginal insulation systems. Due to this possibility, High potential testing is not recommended as a routinely repeated condition monitoring technique, but as an acceptance test. An alternative use of the equipment is to start with a lower voltage and increase the applied voltage in steps and measure the change in insulation resistance readings. In repaired equipment, if the leakage current continues to increase at a constant test voltage this indicates the repair is not to the proper standard and will probably fail soon. In new equipment, if the equipment will not withstand the appropriate test voltage it indicates the insulation system or construction method is inadequate for long term service reliability.

6.2.4: Battery Impedance Testing

Batteries are DC energy storage devices with many shapes, sizes, capacities, and types. All batteries have a storage capacity which is dependent on the terminal voltage and internal impedance. A battery impedance test set places an AC signal between the terminals of the battery. The resulting voltage is measured and the impedance then calculated. This measurement can be accomplished without removing the battery from service since the AC signal is low level and "rides" on top of the DC voltage of the battery. Two comparisons are then made: first, the impedance is compared with the last reading for that battery: and, second, the reading is compared with other batteries in the same bank. Each battery should be within 10% of the others and 5% of its last reading. A reading outside of these values indicates a cell problem or capacity loss. Additionally, if the battery has an internal short the impedance tends to go to zero. If there is an open the impedance will try to go to infinity, and premature aging due to excessive heat or discharges will cause the impedance to rise quickly. There are no set guidelines and limits for this test. Each type, style, and configuration of battery will have its own impedance so it is important to take these measurements early in a battery's life, preferably at installation. It should take less than an hour to perform this test on a battery bank of 60 cells.

6.2.5: Surge Testing

Surge Testing utilizes equipment based on two capacitors and an oscilloscope to determine the condition of motor windings. This is a comparative test evaluating the difference in readings of identical voltage pulses applied to two windings simultaneously. Like high potential testing, the applied voltage equals two times operating voltage plus 1000 volts. This test also is primarily an acceptance, go/no-go test. Data is provided as a comparison of wave-forms between two phases indicating the relative condition of the two phases with regard to the insulation system (short circuits). Because of the repeated stress of the insulation system, Surge testing is not recommended for routine condition monitoring.

6.2.6: 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.

6.3: Circuit Breakers

6.3.1: Timing Tests

For in-service circuit breakers, a digital contact and breaker analyzer can be used to measure the contact velocity, travel, over travel, bounce back, and acceleration to indicate the condition of the breaker operating mechanism. A voltage is applied to the breaker contacts and a motion transducer is attached to the operating mechanism. The breaker is then closed and opened. The test set measures the time-frame of voltage changes, and plots the voltage changes over the motion waveform produced by the motion transducer. The numbers are normally printed out from the test set, and the chart is stored in memory for downloading into a computer. Analyzing and trending this information allows for adjustments to the breaker operating mechanism when necessary. This test is not applicable to molded case breakers or low voltage

6.3.2: Contact Resistance

This test is used to determine the contact condition on a breaker or switch without visual inspection. The results of this test can be trended over time to help in scheduling maintenance activities before the contacts degrade significantly. Most manufacturers of high and medium voltage circuit breakers will specify a maximum contact resistance for both new contacts and in-service contacts. The contact resistance is dependent on two things, the quality of contact area and the contact pressure. The contact quality can degrade if the breaker is called upon to open under fault conditions. The contact pressure can lessen as the breaker springs fatigue due to age or a large number of operations. To measure the contact resistance a DC current, usually 10 or 100 amps, is applied through the contacts. The voltage across the contacts is measured and the resistance is calculated using Ohms law (V=IR). This value can be trended and compared with maximum limits issued by the breaker or switch manufacturer.

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