Power quality


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Power quality

  2. 2. INTRODUCTION Power Quality means quality of the normal voltage supplied to your facility. The growing use of microprocessors and electronic equipments has made us to focus on power quality . Equipment and machinery can be damaged or even fail when subjected to power anomalies . Voltage provided should be as close as possible to nominal voltage and waveform must be pure sine wave free from any harmonics and other disturbances.
  3. 3. POWER QUALITY PROBLEMS: Power Quality problems can be broadly classified into following categories.• Voltage sags• Micro-interruptions• Long interruptions• Voltage spikes• Voltage swells• Harmonic distortion
  4. 4. Voltage SagsA decrease of the normal voltage level between 10 and 90% of the nominalrms voltage at the power frequency, for durations of 0,5 cycle to 1 minute.Causes:• Faults on the transmission or distributionnetwork.• Faults in consumer’s installation.• Connection of heavy loads and start-up oflarge motors.Consequences:• Malfunction of microprocessor-basedcontrol systems that may lead to a processstoppage.•Disconnection and loss of efficiency inelectric rotating machines.
  5. 5. Micro-InterruptionsTotal interruption of electrical supply for duration from few millisecondsto one or two seconds.Causes:• Opening and automatic reclosure of protection devices.• Insulation failure, lightning and insulator flashover.Consequences: •Tripping of protection devices. • Loss of information and malfunction of data processing equipment. • Stoppage of sensitive equipment (such as ASDs, PCs, PLCs).
  6. 6. Long Interruptions Total interruption of electrical supply for duration greater than 1 to 2 seconds. Causes: • Equipment failure in the power system network. • Storms and objects (trees, cars, etc) striking lines or poles, fire. • Human error, bad coordination or failure of protection devices. Consequences: • Stoppage of all equipment.
  7. 7. Voltage SpikesVery fast variation of the voltage value for durations from a several microseconds to few milliseconds. Causes: • Lightning. • Switching of lines or power factor correction capacitors. • Disconnection of heavy loads. Consequences: • Destruction of components and of insulation materials. • Data processing errors or data loss. • Electromagnetic interference.
  8. 8. Voltage SwellsMomentary increase of the voltage, at the power frequency, outside thenormal tolerances, with duration of more than one cycle and typically lessthan a few seconds.Causes: • Start/stop of heavy loads. • Poorly dimensioned power sources. • Poorly regulated transformers.Consequences: • Flickering of lighting and screens. • Damage or stoppage or damage of sensitive equipment.
  9. 9. Harmonic DistortionVoltage or current waveforms assume non-sinusoidal shape. Thewaveform corresponds to the sum of different sine-waves withdifferent magnitude and phase, having frequencies that are multiplesof power-system frequency.Causes:Classic sources: electric machines working above the knee of themagnetization curve (magnetic saturation), arcfurnaces, welding machines, rectifiers, and DC brushmotors.Modern sources: all non-linear loads, such as power electronicsequipment including ASDs, switched mode powersupplies, data processing equipment, high efficiencylighting.
  10. 10. Consequences of harmonic distortion• Increased probability of occurrence of resonance.• Nuisance tripping of thermal protections.• Electromagnetic interference.• Increase in the losses.• Loss of efficiency in electric machines (e.g. 5th harmonic).
  11. 11. SolutionS for PQ ProblemS
  12. 12. Grid AdequacyMany PQ problems have origin in T& D network.A proper planned and maintained grid will avoidmany PQ problems.- High level of redundancy;- Cleaning of insulators;- Trimming of trees nearby power lines…
  13. 13. Distributed Resources1.Distributed Generation (DG)2.Energy Storage (restoring technologies)• Flywheels• Supercapacitors• SMES
  14. 14. Distributed Generation•Used to provide “clean power” to critical loads, isolating them fromdisturbances with origin in the grid.• Backup generators to assure energy supply to critical loads during sustainedoutages.•The most common solution is the combination of electrochemical batteries UPSand a diesel genset. At present, the integration of a flywheel and a diesel genset in asingle unit is also becoming a popular solution, offered by many manufacturers.
  15. 15. Distributed Resources – Energy Storage SystemsRestoring technologies. Energy storage systems, also known as restoringtechnologies, are used to provide the electric loads withride-through capability in poor PQ environment.
  16. 16. FlywheelsElectromechanical device that couples a rotating electric machine(motor/generator) with a rotating mass to store energy for shortdurations.
  17. 17. Super capacitorsNew technology applied to capacitors • High power density • Long life and non-toxic
  18. 18. Superconducting Magnetic Energy Storage (SMES)Energy is stored in the magnetic field of a coil made of superconductormaterial. • High power density • Very fast response • Very expensive (on development)
  19. 19. Codes and StandardsNeed to regulate: • the minimum PQ level that utilities have to provide to consumers, and • the immunity level that equipment should have.Most relevant standards: • CBEMA curve • ITIC curve • IEC 61000 • EN 50160:2001 • IEEE standards - 519-1992 →Harmonics - 1100-1992 →Powering and grounding sensitive equipment - 1159-1992 →Monitoring power quality - 1250-1995 →Service of sensitive equipment
  20. 20. CBEMA curve. ITIC curve
  21. 21. Enhanced Interface Devices•Using proper interface devices, one can isolate the loads from disturbances deriving fromthe grid. Some of the enhanced interface devices are: A. Dynamic Voltage RestorerA dynamic voltage restorer (DVR) acts like a voltage source connected in series with theload. The output voltage of the DVR is kept approximately constant voltage at the loadterminals. B. Transient Voltage Surge suppressors (TVSS)Transient voltage surge suppressors are used as interface between the power source andsensitive loads, so that the transient voltage is clamped by the TVSS before it reaches theload. C. Noise Filters Noise filters are used to avoid unwanted frequency current or voltage signals (noise) from reaching sensitive equipment. D. Static VAR Compensators Static VAR compensators (SVR) use a combination of capacitors and reactors toregulate the voltage quickly.
  22. 22. E. Harmonic FiltersHarmonic filters are used to reduce undesirable harmonics. They can be divided intwo groups:• passive filters• active filters.Passive filters (left) consist in a low impedance path to the frequencies of theharmonics to be attenuated using passive components (inductors, capacitors andresistors).Active filters (right) analyse the current consumed by the load and create acurrent that cancel the harmonic current generated by the loads.
  23. 23. Make End-use Devices Less Sensitive• In most cases, making the end-use devices less sensitive to PQ disturbances is more cost effective than buying equipment to mitigate these problems.• Some measures to increase equipment immunity: – Add a capacitor with larger capacity to power supplies; – Use cables with larger neutral conductors; – Derate transformers;
  24. 24. ConclusionsThe availability of electric power with high quality is crucial for the running of the modern society. If some sectors are satisfied with the quality of the power provided by utilities, some others are more demanding.To avoid the huge losses related to PQ problems, the most demanding consumers must take action to prevent the problems. Among the various measures, selection of less sensitive equipment can play an important role. When even the most robust equipment is affected, then other measures must be taken, such as installation of restoring technologies, distributed generation or an interface device to avoid PQ problems.