Published by Dranetz – BMI Case Studies

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Despite their differences, continuous-process industries share underlying characteristics: they maintain continuous operations in facilities that represent substantial start-up costs and time but can be interrupted or disrupted by seemingly minor fluctuations in power quality. If the product stream is disrupted, lost productivity and lost product can create a large financial burden. For example, a voltage sag in a paper mill can waste a whole day of production and inflict a $250,000 loss, while a 5-cycle interruption at a glass manufacturing facility can cost a minimum of $200,000.

Duke Energy is one of the most proactive utilities in providing power quality solutions for its customers. The company’s Power Quality Group goes beyond the electric meter, right into its customers’ plants, to help eliminate power quality problems.

To document the interaction of power quality and process quality for continuous-process industries, Duke Energy and Clemson University developed a Webstand simulator in the Power Quality Lab of Clemson University in South Carolina. The Webstand, a portable, process-level power quality research tool, models the winding/unwinding (web) processes typical of the textile industry. The Webstand incorporates the Signature System to monitor and model the interactions between power quality and process quality, enabling improvements to the system’s robustness without impacting the facility’s production. The Webstand was subjected to numerous dips of various magnitudes, durations, and combination of phases. The resulting data provided the information necessary to:

• Determine sources of potential process interruptions
• Validate control system models
• Anticipate the correct output for a given duration based on the magnitude of a sag
• Anticipate the speed of degradation for system components
• Establish individual quality of process and quality of power supply requirements to maintain consistent product quality performance

Following the proof-of-concept, the Signature System configuration used at the Clemson Power Quality Lab was expanded for installation at a thin-film extrusion plant that operates with an allowable downtime of 4 hours per year. The resulting system is capable of monitoring critical points throughout the system, including the various winders/unwinders, laminators, infeed and accumulator load cells, metering roll, chill roll, fire alarm system and electric supply distribution system. Monitoring data can quickly identify whether a fault occurred on the distribution system or within the facility and the possible cause of the disturbance, along with correlating the effect in the production line.

Using the Signature System for quality of process monitoring helps ensure that critical process equipment is functioning properly, that industrial processes are not interrupted, and most important, that the quality of the final product consistently meets its specifications.

Published by Dranetz – BMI Case Studies

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Elevator control panels for the lower zone mid-rise elevators were on the nineteenth floor of a forty-story office building in a large midwestern city. It was fed with 480V transformer on the same floor, with the feeders to the controls rated at 800A.

Excessive noise and vibration of the feeder wires and conduit were noticed on a number of upper floors.

Monitoring was done on at the service panels on the 19^th, 25^th and 37^th floors. Different combinations of the three voltages and three currents were monitored for only a short duration before the source of the problem became evident.  Figure 1 and 2 below show the time plot of voltage and current, respectively, on one phase, which were representative of all the phases.

As the current values increase, a corresponding decrease in the RMS level of the voltage is observed. The maximum current is over 1200A on the 800A service.

The voltage transients during this 40 second window were nearly 300V. Closer examination of these transients can be seen in Figure 3 and 4. Notice how the voltage notches appear in the same position on the wave of each of the phases.

The overall RMS voltage values were observed to be fluctuating. Such fluctuations often result in light flicker, though none was reported at this location. Figures 5 and 6 shows the voltage fluctuations and the current during the startup of an elevator.

 

Analysis

During one 20 second measuring period, there were 77 transients recorded.

• Amplitude: 236.5 volts.
• Absolute amplitude (from zero crossing): -684.
• Rise time: 1.0851 microseconds
• Frequency (1/4*rise): 230.4 kilohertz.

Though there was no corresponding transient in the phase current to suggest the origin of the transient, the phase current was observed to be switching at the same time. The transient frequency was greater than 100 khz suggesting the origin of the transient was local to the monitoring location.

There were also a number of zero crossing errors observed on the voltage channels.
The worst zero crossing width was 277.6 microseconds and the worst zero crossing delta voltage was 28.5 volts.

Voltage Notch Analysis

Voltage notches were the most prevalent type of event observed. As noted, these occurred at the same phase position on all phases and corresponding to the current waveform. A one second monitoring period noted 3026 such notches, with a worst notch area of 0.42159 volt-seconds. The depth of the notch was most severe on the voltage phase corresponding to the current phase that had the step change in current.

Harmonic Analysis

The voltage channels’ total harmonic distortion was typically under 4%, varying under different loading. The fifth harmonic was the predominate one, at 2.4%. This corresponded to the current harmonics, as shown in Figure 8 for the current waveform in Figure 7.

Probable Cause

The voltage transients, whether appearing as notches, zero crossing errors, or just unipolar transients, were probably the resulting of the electronic switching load, such as in elevator controllers. While these were quite excessive, the amplitude and width did not seem to be severe enough to cause malfunction of the elevator controls. Continued operation in this manner may eventually lead to such failures. Notching can cause timing problems with phase controlled loads, trip protective relaying, stress power electronics, and cause excessive heat in motors and transformers. It often takes power conditioning devices , chokes or special filters to “fill in” the notches and smooth out the waveform

The voltage transients and the voltage fluctuations were caused by excessive current draw through the inadequately sized and supported service. This also produced the “thumping” sound, as the magnetic fields generated by the three phases tend to push the cables apart. A general rule of thumb is that the conductors will jump when the current is 120% of maximum rating. In this case, the current levels peaked at over 50% of rating, but were within the steady-state rating of the system.

The elevator manufacture recommended that the size of the feeders from the transformer should be increased by at least 50% for both of the elevator controllers. Caution should be used that the conduit has adequate size to accommodate such, in accordance with NEC. The utility engineer noted more correctly that the problem was the result of the conduit being inadequately supported to the walls.

Published by Dranetz – BMI Case Studies

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Executive Summary

Defiance Metal Products Company contracted with Electrotek Concepts to perform field measurements of their welding operations, and to make recommendations for remediation of Power Quality problems at the facility.

The customer has been experiencing both major and minor electrical problems. The minor problems include occasional observable flicker in the overhead HID lighting system. The major problems include welder trip during operation and damage to protective MOVs.

Electrotek Concepts engineer installed a Dranetz 8020 PQ Node at the 2000 amps main service disconnect to monitor overall plant electrical power quality during the survey. Measurements were taken at individual spot welders using a Dranetz Power Platform PP1. The service voltage is 277/480 grounded wye.

The customer has upgraded the electrical service to the welding building from a 300 kVA transformer to a 1500 kVA unit. This action has achieved a very stable voltage waveform as seen by the various welders. Even with heavy current draws, the voltage waveform showed very little notching and no significant transients were detected. The only drawback to such a “stiff” service supply is the availability of greater fault current flow in case of short circuits or heavy welder loading. This high available fault current should be taken into consideration in the sizing and rating of fuses and circuit breakers.

The proper operation of the welding equipment is dependent not only upon the quality of the electrical power at the main service but also the quality of the electrical power in the main welder circuits. The high currents involved can cause premature deterioration and failure of any non-bolted electrical connections in the overall power circuit, such as the contacts in circuit breakers and electrical contactors and power relays. The electronic firing circuits will attempt to balance-out the problem of phase imbalance during the weld cycle, but extreme voltage excursions (as can happen with damaged electrical contacts) cannot be fully compensated for. The results will be occasional bad welds and system faults.

Some of the readings taken during welder operation did indicate the possibility of poor electrical conductivity in the welder circuit. This was later confirmed when the suspect circuit breaker was inspected and found to have severely burned contacts.

The report is organized with the following further observations:

Measurements – 8020 PQ Node.  These readings were taken at the main service entrance and profile the voltage and current for the entire welding operation. Charts are drawn to illustrate these profiles as well as noteworthy electrical disturbances that were triggered during the site visit.

Measurements – Power Platform PP1. These readings were taken at individual welder locations and the charts show the voltage and current variations during typical weld cycles.

Of special interest is Figure 32, which shows the voltage change as the circuit breaker on Welder 121-R is closed. Notice the voltage does not change smoothly, which is indicative of poor conductivity of the circuit breaker contacts. Also note Figures 36 and 38, which show voltage transients on Phase “B” during a welder fault. These voltage profiles also suggest a problem with electrical continuity on the center phase of the circuit breaker. Subsequent inspection of the circuit breaker proved this to be true.

Measurements – 8020 PQ Node

 

 

Measurements – Power Platform PP1

 

 

Conclusions and Recommendations

The electrical measurements on the 3-phase welder 121-R were made on the load side of the molded-case circuit breaker in the welder panel. A voltage transient was noted on the “B” phase that could not be fully explained unless one considered a break in the supply voltage to that phase. Examination of the circuit breaker revealed heavily burned contacts and, in essence, zero-conductivity in the center phase. These “cold” characteristics could easily change when the breaker is energized, and it seems it has been delivering at least partial power in the period before its failure. However, the damage to the circuit breaker contacts could easily explain why this particular welder experienced random failures.

This same logic could be used to explain problems occurring with other welders. At the time of the survey, it was not conclusive that all important electrical contacts in the other welders (especially circuit breakers and contactors) were in good shape. All should be considered under suspicion, and further considerations as to what could still be causing problems should be suspended until an analysis is made of existing circuit conditions. At a minimum, this would consist of cold-load ohmmeter measurements of conductivity and full-load tests of voltage drop across circuit breaker and contactor line and load terminals.

Published by Dranetz – BMI Case Studies

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Engineering laboratory adjacent to offices. A computer system used by laboratory personnel exhibited intermittent failures and data errors. Typically, these would start around 10 a.m. On some days there were no failures.

A power monitor was installed at the panelboard serving the computer system. The monitor was connected, line-to-neutral (Channel A) and neutral-to-ground (Channel B). The recorded line-to-neutral waveforms were relatively undistorted sine waves, which gave no clues as to the source of the line disturbances.

An event summary of the RMS voltage, Figure 1, revealed a pattern of repetitive sags in the RMS voltage, beginning just after 10 a.m. The regular nature of these sags, as well as corresponding swells on the neutral, were made even clearer by increasing the resolution to a few minutes, Figure 2.

The regular repetition of the voltage sags was a powerful clue, indicating automatic switching of another load on the circuit. The neutral-to-ground waveform, Figure 3, indicated the load was linear.

A little more detective work soon located a laser printer in one of the nearby offices whose print fusing heater switched on every minute or so. The high current demand and resulting potential developed on the neutral was causing the computer errors. The printer was usually powered up around 10 a.m., after the user had created text to be printed. Moving the printer to another branch circuit removed the source of computer interference.

Published by Dranetz – BMI Case Studies

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This medium-sized manufacturing facility, located in an industrial park that experienced an unexplained shutdown of several adjustable speed drives (ASDs) , wreaking havoc on key process equipment. Each day, at approximately 6 am, the utility-owned PF capacitor kicks on to improve the voltage of inductive loads prevalent in many of the park’s manufacturing facilities. The ASDs are conditioned to anticipate this expected power quality event and are typically able to ride through the problem. So when one of the ASDs closed down and interrupted the continuous stream manufacturing process, the facility manager needed to learn why, correct the problem, and prevent it from happening again.

As you can see from the attached screen capture (Figure 1), a second, unanticipated capacitor switching event occurred shortly after the first. This event was categorized by the Signature System Capacitor Switching Answer Module, enabling the facility manager to pinpoint the exact source of the problem. Further analysis showed that the ASD shutdown was the result of an overcurrent trip, which was quickly remedied preventing hours of downtime, at a loss of $10,000/hour.

Published by Dranetz – BMI Case Studies

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A medical clinic in Las Vegas, Nevada, employing sensitive computed tomography (CT Scan) system. Power was brought to the system from a 480 volt service feeding a 480-to-208 volt isolation transformer.

CT Scan system was experiencing repeated computer lockups and component failures.

The medical equipment service company installed a power monitor on site to analyze the power to the system. A single day of monitoring was enough to identify the case of the failures. Figure 1 shows the disturbance on the line caused by a utility power factor correction capacitor bank located just one block away.

Figure 2 shows the attenuation effect of the isolation transformer, still not enough to protect the CT Scan system from such a severe transient.

Expansion of the waveform data, Figure 3, revealed the ringing frequency of the system, when energized by the capacitor bank, was between 1 kHz and 1.5 kHz. This allowed the specification of a treatment device to mitigate the problem.

The medical equipment company recommended the use of an active-tracking filter specifically designed for this type of disturbance around 1 kHz. The filter was installed on the 480-volt line to protect all of the downstream equipment. Figure 4 shows the difference between the input (Channel C) and output (Channel B) of the filter during a subsequent capacitor bank switching operation.