| POWER QUALITY ISSUES Ronald F. Schapp P.E. Phone: 513-248-0901 Today, Power Quality issues can be described in numerous ways. Usually reliable electric service is what is desired. What does that mean? It means that today ?s manufacturing plants that use sensitive electronic controls and electronic equipment want to maintain production 100% of the time. This requires continuous, clean power source.Back at the generating station utilities create a perfect sine wave with their generators, free of transients, harmonic distortion and high frequency noise. It is then transmitted at various voltage levels to the end user. If the generator were at the customer site, that would be the ideal solution. Why, because distribution systems are exposed to lightning, accidents, component failures, power factor switching transients, grid switching transients and customers who create harmonics and send them back onto the utility system distorting voltage and adding to reliability problems. There are many electrical conditions that can disrupt a process.
With today’s sensitive equipment such as adjustable speed drives (ASD ?s) voltage sags to 90% of nominal may cause shutdown. Indeed, voltage sags are one of the major causes of production interruptions. This along with single phasing, and reclosure operations constitute 99% of all interruptions of plant operations. These outages are less than 200 milliseconds (ms) or 12 cycles. The remaining 1% of outages are above 200 ms and more commonly extend to one or more hours.Here is a summary of data assembled by EPRI the Electric Power Research Institute. Study of 107,834 incidents of voltage variations: 68% involve 1 phase only Study of 105 sites experiencing power quality problems: 70% involved 1 phase only Q Is there an inexpensive way to solve 99% of the outage problems? Yes and no. Yes, if it is possible to maintain power to control circuits of sensitive equipment such as plastic extruders, compressors, and other loads that would continue to operate for 100 to 1000 ms if their controls did not drop out. Dip Proofing Technologies has an inexpensive solution that connects into the control circuit. Q What if the sensitive drives or other equipment cannot be handled in this way or it is absolutely necessary to maintain power to the motor? Then the solution gets more expensive. Power Systems and Controls makes MG (Motor Generator) sets that can carry through processor loads or entire plants for up to 200 ms. MG sets will carry 67% of their rated capacity indefinitely when subjected to a single phasing condition. This provides protection against 70% of all power outages. Q What about transients from outside the plant? Because there is a mechanical coupling connecting the motor and the generator the plant or process is isolated from the power source through the MG sets. Q What about harmonics? As noted above the MG set isolates the load from source harmonics as well. Q What about internally generated transients and harmonics? The inherent design of a synchronous generator will reduce transients and harmonics generated on the load side to 50%. This may be adequate to eliminate operating problems. Q Aren ?t MG sets expensive?Yes and no. It really depends on the costs that are incurred when a process or plant is shut down. Critical processes may experience extremely high costs associated with an outage making an MG solution very reasonable. Q What if it is necessary to provide continuous uninterruptible power? For industrial applications where there are many motors, rotary UPSs made by Power Systems and Controls are very cost effective because they do not have to be oversized to handle motor inrush current. For data centers we recommend static UPS Q What are TRANSIENTS and HIGH FREQUENCY NOISE? A TRANSIENT is a non-repetitive very high magnitude short time voltage (E) and current(I). It can be many thousand of volts and amps and last for only a microsecond (0.000001 seconds). HIGH FREQUENCE NOISE is a repetitive low magnitude <1000V and very high frequency 100 kilohertz (KHZ) – 100 megahertz (MHZ) occurrence. Q Are there other sources for TRANSIENTS than sensitive electronic equipment? Yes – While 80% of problems come from within a plant from electronic equipment and when motors and electrical equipment start and stop, 20% of the problems come from outside. These are higher magnitudes and are caused by lightning, utility grid switching, capacity switching, accidents, etc. Q What problems do TRANSIENTS cause? TRANSIENTS can damage equipment. High levels may cause immediate failure of P.C.s, printers, fax machines, adjustable speed drives, etc. They may also cause repeated low levels of damage called electronic rust so that when failure occurs it cannot be identified with a specific incident. P.C.s served from the same panelboard can actually cause failure to P.C.s on other circuits. Q What problems do HIGH FREQUENCY NOISE cause? The most frequent result is equipment upset, i.e., the P.C. locks up for no apparent reason and occasional bad data. The major impact is down time and lost productivity. The Solution Q How can these problems be avoided? Electrically connected parallel Transient Voltage Surge Suppression devices can be applied at the service entrance to protect against the high level transients and in panelboard motor control centers (MCC’s), bus duct (B.D.) and at individual equipment. Q Who manufactures this equipment? There are several manufacturers of this type of equipment. However, Current Technology Inc. has been manufacturing this equipment for more than 30 years and is recognized as the leader in their field. Q How does the Current Technology product differ from the others?
Q What is a HARMONIC?
Q What happens in a facility that has a high percentage of single phase non-linear loads such as P.C.s? Following Ohm’s Law (E = IZ), the non-linear load impedance (Z) causes a non-linear current (I) to flow resulting in a distorted current (I) and voltage drop (E). Q So – in simple terms what does all this mean?
The Solution Q What can be done to eliminate this problem? One simple solution is to install a special transformer called a low impedance zero sequence transformer at the panelboard or out on the branch circuits close to the load to eliminate 3rd harmonics. Q If that takes care of the 3rd harmonic what about the 5th and 7th etc.? The normal step down transformers 480:120/208 can be replaced with transformers that phase shift the low voltage from that of standard transformers and eliminate 90% of the 5th and 7th harmonics. Q What can be done to eliminate 5th, 7th etc. HARMONICS caused by three phase non-linear loads? Drive isolation transformers which are recommended by EPRI and drive manufacturers can be furnished with the output voltage phase shifted to eliminated 90% of the 5th and 7th HARMONICS. Q Who manufactures these transformers, where can I get them and who can help me in their application? Power Quality International Inc. has been making these transformers and solving these problems for many years. Just call, fax or email Alexander Equipment Co. We have extensive application experience. |