An Introduction to Electric Power Quality
The Definition of Electric Power Quality
Electric power quality is judged on the reliability and the quality of delivery. The reliability is measured in up-time. Reliability is considered low when there are frequent outages of any duration.
Power system reliability can also be judged by the mean time between failures or MTBF. When there is a long time between failures, the MTBF is greater. The quality of the power delivery is measured the variability of the voltage and current. Electric power quality is sometimes abbreviated as p.q. or PQ in technical documents.
Judging Power Quality
Electric power quality is poor when there are long power outages or dangerous power surges that can hurt employees or start fires. Power quality is considered intermediate when the power outages are very brief.
Power quality is rated as better if there are only voltage sags instead of power outages. The highest power quality occurs when the power is delivered in a steady stream at the desired voltage without any interruptions at all with little variation in the current frequency.
The Impact of Poor Power Quality
Lower power quality decreases the reliability of equipment. Voltage fluctuations cause sudden accelerations and decelerations of motors, wearing them out faster. Current fluctuations may cause programmable logic controllers to malfunction, slowing production or causing it to work incorrectly. Power surges may speed up a motor, accelerating an assembly line that feeds too much product to a processing center.
Power system improvements can help organizations save money if the designers balance the cost of the project against the cost savings or benefit of the project. Variations in the frequency of alternating currencies can disrupt the operation of sensitive electronics such as computers and big screen TVs. Voltage surges may burn out resistors and capacitors on circuit cards until the electronic device can no longer function.
IEEE Standard 1159
- IEEE Standard 1159-2009
IEEE 1159 - IEEE Recommended Practice for Monitoring Electric Power Quality
Methods to Improve Power Quality
Alternating current or AC electrical power quality can be improved by adding harmonic filters and capacitor banks. AC power quality is also improved when AC controllers are installed. Shunt active power filters dampen alternating current harmonics by injecting a harmonic that is phase shifted 180° from the operating current.
Uninterruptable power supplies or UPS improve power quality because they provide power for a short term. Uninterruptable power supplies (UPS) offer their greatest benefit when a power grid has periodic but brief power outages. UPS devices also improve system performance by letting devices finish their current cycle before being shut down properly. Installing surge protectors prevents a large percentage of overloads.
Batteries or super-flywheel devices as long term power storage that can supply electricity when demand spikes or the grid goes down for hours. Setting up production lines and offices on micro-grids, their own separate miniature power grids, instead of sharing a single power grid, increases the entire power system’s reliability. When one system is overloaded, the other mini-grids continue operating since they are unaffected.
Another method of improving electric power quality is building a smart grid into your facility. Or the smart grid could automatically shut down non-critical equipment to lighten the load. It could also turn on alternative power sources automatically to meet sudden increases in demand.
Direct current or DC systems can be improved by adding protective devices to the power system. You may add better circuit breakers and relays to the power system. You could install automatic or manual isolating switches or double circuits so that the production line isn't shut down when the primary one is burned out.
Power quality monitoring devices and software to report this information to a central location has the potential to improve power quality, since maintenance staff gain real time information of irregularities and act before a serious electrical problem occurs. When power demand is close to exceeding the power grid’s capacity, send notices to employees to turn off unneeded equipment.
IEEE Standards on Power Quality
IEEE standard 1159 describes the recommended practices for monitoring power quality. IEEE standard 493 outlines the methodology the IEEE recommends for determining the cost-benefit analysis of projects intended to improve power quality.
IEEE 2030 is the first IEEE standard for smart grids, outlining the terminology used in smart grid standards and communication protocols smart grid components will use to talk to each other. Implementing smart grids in an industrial or commercial facility improves overall power quality by balancing supply to demand and shifting power flow when circuits go down.
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