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High Voltage
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You may already be using predictive maintenance (PdM) techniques on your motors and drives. But how often do you inspect the power to your equipment? By adding basic power quality measurements to production equipment maintenance procedures you can head off unexpected failures in both production equipment and your power system.
Cost savings
Insurance claims data in the NFPA 70B maintenance standard shows that roughly half of the cost associated with electrical failures could be prevented by regular maintenance. A study published in IEEE 493-1997 says that a poorly maintained system can attribute 49 % of its failures to lack of maintenance.
To determine the cost of a failure, it helps to consider three key categories:
• Lost income (gross margin) due to downtime
• Cost of labor to troubleshoot, patch, clean up, repair and restart
• Cost of damaged equipment and materials, including repairs, replacements and scrapped material
Integrating power quality into PdM
Unlike a comprehensive electrical system survey, predictive maintenance power quality focuses on a small set of measurements that can predict power distribution or critical load failures. By checking the power quality at critical loads, you see the effect of the electrical system up to the load. Your predictive maintenance inspection route probably already includes any motors, generators, pumps, A/C units, fans, gearboxes, or chillers on site.
Voltage stability, harmonic distortion, and unbalance are good indicators of load and distribution system health and can be taken and recorded quickly with little incremental labor. Current measurements can identify changes in the way the load is drawing. All of these measurements can be taken without halting operations and generate numbers that can easily be entered into maintenance software and plotted over time.
For each measurement point or piece of equipment, determine what limit should trigger corrective action. Limits should be set well below the point of failure, and as time goes on limits may be "tightened" or "loosened" by analyzing historical data. The appropriate limits depend somewhat on the ability of your loads to deal with power variation. But for most equipment, your maintenance team can devise a set of default, "house limits" based on industry standards and experience.
The cost of three-phase power analyzers is lower now than ever and it should only take roughly 15 minutes to take the readings discussed in this article. (Storing voltage sag data will add more time, since it requires picking up the data after a day or so.)
Voltage
Good voltage level and stability are fundamental requirements for reliable equipment operation.
• Running loads at overly high or low voltages causes reliability problems and failures. Verify that line voltage is within 10 % of the nameplate rating.
• As connections in your system deteriorate, the rising impedance will cause "insulation resistance drops" in voltage. Added loads, especially those with high inrush, will also cause voltage decline over time. The loads farthest from the service entrance or transformer will show the lowest voltage.
• Neutral to ground voltage tells you how heavily your system is loaded and helps you track harmonic current. Neutral to ground voltage higher than 3% should trigger further investigation.
Voltage sag count
Taking a single voltage reading tells only part of the story. How is the voltage changing during an hour? During a day? Sags, swells and transients are short-term variations in voltage. The voltage sag (or dip) is the most common and troublesome variety.
Sags indicate that a system is having trouble responding to load requirements and significant sags can interrupt production. Voltage sags can cause spurious resets on electronic equipment such as computers or controllers, and a sag on one phase can cause the other two to overcompensate, potentially tripping the circuit.
Sags have several dimensions: depth, duration, and time of day. Utilities use a special index to track the number of sags that occur over a period of time. To gauge the depth of the sags, they count how often voltage drops below various thresholds.
The longer and larger the voltage variations, the more likely equipment is to malfunction. For example, the Information Technology Industry Council (ITIC) curve specifies that 120 V computer equipment should be able to run as long as voltage does not drop below 96 V for more than 10 seconds or below 84 V for more than 0.5 seconds.
Current
Current measurements that trend upward are a key indicator of a problem or degradation in your load. While equipment is running, monitor phase, neutral and ground current over time. Make sure none of the currents are increasing significantly, verify that they're less than the nameplate rating, and keep an eye out for high neutral current, which can indicate harmonics and unbalance.
Voltage unbalance
In a three-phase system, significant differences in phase voltage indicate a problem with the system or a defect in a load
• High voltage unbalance causes three-phase loads to draw excessive current and causes motors to deliver lower torque.
• Unbalance is tracked in percentages. The negative sequence voltage (Vneg) and zero sequence voltage (Vzero) together identify any voltage asymmetry between phases.
• Using a power quality analyzer to do the math, high percentages indicate high unbalance. EN 50160 requires Vneg to be less than two percent.
Voltage harmonic distortion
Harmonic distortion is a normal consequence of a power system supplying electronic loads such as computers, business machines, electronic lighting ballasts, and control systems. Adding or removing loads from the system changes the amount of distortion, so it's a good idea to regularly check harmonics.
Harmonics cause heating and reduced life in motor windings and transformers, excessive neutral current, increased susceptibility to voltage sags, and reduced transformer efficiency.
As current harmonics interact with impedance, they're converted into voltage harmonics. Total Harmonic Distortion (THD) is a sum of the contributions of all harmonics. By tracking Voltage THD over time you can determine if distortion is changing. For voltage harmonics, IEEE 519 recommends less than 5 % THD.
Fluke Corporation offers an extensive range of power quality test tools for troubleshooting, preventive maintenance, and long-term recording and analysis in industrial applications and utilities. For more information on Fluke Predictive Maintenance Products and Services go to http://www.fluke.com/pdm.
Steve Glad is a writer with Structured Information (Arlington, MA). He frequently writes about industrial technology topics. Web site: http://www.strucinfo.com/
Partial Discharge Analysis Prevents High Voltage Equipment Failure
The partial discharge process occurs in electrical equipment submitted to high voltage stress. It represents the dielectric breakdown of an electrical insulation and usually has a sporadic character. Because partial discharge occurs randomly and unexpectedly, regular partial discharge analysis is highly recommended with any piece of high voltage equipment. The effects of the partial discharge phenomenon can be very grave and even lead to a complete breakdown. Timely detection can help you avoid very unpleasant situations. Find out more about avoiding the extremely hazardous effects of partial discharge, with the aid of partial discharge switchgear and other devices, at elec-di.com
The partial discharge process can be very hazardous to high voltage equipment. It usually starts within cracks, voids, bubbles in liquid dielectrics or enclosures within solid dielectrics. Partial discharge within insulating materials commences in the dielectric, within gas-filled voids. In this particular situation, the partial discharge phenomenon is the result of high voltage stress across the void. Partial discharge can sometimes be relatively harmless, but in most cases its effects are devastating. For instance, discharges which take place within polymeric cables are extremely hazardous. Once partial discharge begins, it progressively deteriorates the insulating materials up to the point where it causes a complete breakdown of the equipment. Partial discharge switchgear can help you escape the devastating consequences this process may have.
Once the partial discharge process commences there is a high probability that the high voltage equipment will end up failing entirely. If you want to avoid the destruction of your electrical equipment you should use partial discharge analysis and monitoring devices. Partial discharge can also be prevented through thorough design and careful material selection. Manufacturers should use partial discharge detectors in all of the production stages. Periodical check ups are also important when trying to avoid any unpleasant surprises. If you want to make sure your high voltage equipment will function perfectly on the long run, thorough partial discharge analysis is the only reliable solution.
There are several commonly used methods of accurately detecting partial discharge. For instance, a very widespread detection method is by using a calibrator. Partial discharge current flows are usually very brief and discharges take the form of random pulses. With the help of a calibrator, the intensity of the pulses is measured against the voltages obtained from a calibration unit and is then discharged onto a measuring device. Another frequent detection method is the so-called wideband detection. In this situation, the pairing impedance consists of a small Q parallel-resonant RLC circuit which leads to a decrease in the existing voltage and an amplification of the discharge generated voltage. Other partial discharge analysis and detection methods include field testing, laboratory testing, narrow band detection or differential discharge bridge methods.
The importance of preventing complete system failure as a consequence of partial discharge occurrences has been fully acknowledged at the beginning of the past century. Electrical discharges usually occur across a portion of insulation between two conducting electrodes and lead to the so-called partial discharge phenomenon. Once the process is triggered, it can have extremely destructive consequences if left undetected. The high voltage equipment may even completely fail after such an occurrence. Using partial discharge analysis methods and devices, such as partial discharge switchgear, can help you save a lot of time and money along the way. At elec-di.com you will find a full range of products such as partial discharge switchgear to help you avoid problematic situations.
About the Author
When discussing high voltage equipment, partial discharge is an extremely common problem. When the process occurs, it may soon lead to complete failure of the equipment. Partial discharge analysis helps avoid unpleasant situations. Find out more about partial discharge switchgear and other devices by visiting our website.
Electric power is transferred over large distances at very high voltages. Explain how the high voltage?
Electric power is transferred over large distances at very high voltages. Explain how the high voltage reduces power losses in the transmission lines. (25/12)
the amount of power lost as heat during transmission using line wires is given by P=i^2 R
Where I is the rms value of ac current in the wires and R is the resistance related with the wires.
When voltage is stepped up using transformer, Voltage is increased by reducing the current in the circuit. therefore at high voltage only less amount of current flows through the wires, hence transmission loss becomes less (i becomes less hence P becomes less)
TenneT Completes Acquisition Of E.ON Extra-High Voltage Transmission Network
TenneT Holding and E.ON AG have recently completed the sale of E.ON's extra-high voltage transmission network. All conditions of the sale have been fulfilled. In the takeover, TenneT has procured all shares in E.ON subsidiary, Transpower Stromübertragungs GmBH (transpower), with effect from 31 December 2009
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