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Power System – Causes for Over Voltage and Prevention

Power System – Causes for Over Voltage and Prevention. Power System – Causes for Over Voltage and Prevention. Terminology. Normal. Power System – Abnormalities.

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Power System – Causes for Over Voltage and Prevention

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  1. Power System – Causes for Over Voltage and Prevention

  2. Power System – Causes for Over Voltage and Prevention Terminology Normal

  3. Power System – Abnormalities 1. Voltage Drop/ Shot interruptions 2.Harmonic waves due to voltage changes 3.Temporary voltage increases 4.Switching surges 5. Lightning surges

  4. Power System – Abnormalities 1.The voltage falls below normal: Sag 2.The reverse of a sag: Swell 3.Increase in voltage above 110% of nominal for more than one minute - Over-voltage.

  5. Power System – Abnormalities 4.Noise - used to describe very small and persistent disturbances. These do not have damaging effects but can be a nuisance 5.Harmonics are a recurring distortion of the waveform that can be caused by various devices including variable frequency drives, non-linear power supplies and electronic ballasts. 

  6. Power System – Abnormalities 6. When the voltage drops below 10% of its nominal value it is called an interruption or a blackout.  7. Transients are very short duration (sub-cycle) events of varying amplitude.  Often referred to as "surges", transients are probably most dangerous power abnormally

  7. Power System – Causes for Transients External Causes: Lightning - Direct hit Distant Lightning - Transients on Utility lines Brownouts/Blackouts - Sags & Recovery Grid Switching - Utility switching Other Users - Adding/Removing loads line

  8. Power System – Causes for Transients Internal Causes: Switching of: Electrical motors Elevator Motors Compressors Welding or heavy machinery Power Overloads

  9. Power System – Lightning types Stepped Leader Streamer

  10. Power System-Facts -Lightning • A strike can average 100 million volts of electricity • Current of up to 100,000 amperes • Can generate 54,000 oF • Lightning strikes somewhere on the Earth every second • Kills 100 US residents per year

  11. Power System-Lightning Effects Side Flash Direct strike

  12. Power System-Lightning Effects Once the building is struck, lightning current can cause damage either by spark over or intrusion through service lines

  13. Power System-Lightning Effects Lightning generates strong electromagnetic radiation in a broad band of frequencies

  14. Power System-Lightning Effects Once entered into nearby structures, these radiation may damage sophisticated electronics irrespective of whether they are in operation or not

  15. Power System-Lightning Effects A service line may directly encountered with lightning or a nearby hit may induced large voltage pulses in the service line

  16. Power System-Transient Effects Catastrophe Failure Safe Zone Degradation

  17. Power System-Transient Effects Random faults (Annoying) Memory Loss Data Errors Random System Halts Degradation (Un-noticed Damage) Break-Down of ICs Loss of Circuit Traces, or Insulation Shortening of Component Thermal Runaway of Semi-Conductors Burn-Out (Catastrophic Failure) Board Damage & Failure System Down System Failure

  18. Power System-Transient Effects Damage to a modem card The surge has come through the communication line. In this case the damage to the modem has prevented further damage to the computer

  19. Power System-Lightning types • Two types of Lightning – Direct & Indirect • Direct – Stroke A-Cloud to tall object – protection is possible, Stroke B-Between clouds-protection not possible

  20. Power System-Lightning types • Indirect – Line directly below the cloud gets electrostatically charged. When the cloud discharges to earth thro lighhtning, -ve charge on the line gets isolated and it travels fast on the line in both directions in the form of waves. Indirect lightning is the major cause for transients in elec lines

  21. Power System-Lightning Protection • OH grounding wire • Lightning arrester • Earthing screen

  22. Power System-Lightning Protection • OH grounding wire

  23. Power System-Lightning Protection • Lightning arrester

  24. Power System-Lightning Protection • Earthing screen – A grid of earthed copper conductors criss crossing the Substation provide a low resistance path for lightnings. They do not guard against travelling waves

  25. Power System-Lightning Arresters • Rod gap • Rod gap • Horn gap • Multigap • Expulsion type • Valve type

  26. Power System-Lightning Protection • Horn gap • Multigap

  27. Power System-Lightning Protection • Expulsion Type

  28. Power System-Lightning Protection • Valve Type

  29. No damage Possible damage Destruction Motors & Transform. Relays Valves Passive components Diodes Transistors (power) Integrated circuit ICs Semi-conductor diodes 8 -8 -6 -4 -2 0 2 4 6 J 10 10 10 10 10 10 10 10 10 Susceptibility of components to damage.

  30. Protecting Equipment Against Transients

  31. Lightning Current: Peak Value • Typical value for the first stroke is 30 kA • Typical value for the subsequent stroke is 15 kA • Values over 250 kA has been recorded

  32. Power System-Peterson Coil

  33. Power System-Peterson Coil

  34. Power System-Peterson Coil - sample

  35. Principles of Surge Protection Devices • Clamping output voltage to a safe level Diverting surge current to earth • Does NOT prevent lightning but protects against effects

  36. Principle of Power Protection A protector performs like a switch controlled by voltage. If the voltage is higher than the rated voltage of the electrical line to be protected, then the protector changes its state to low impedance and derives current to earth. The usual state of the protector is being in high impedance, so that the protector is transparent for the installation.

  37. Examples for typical Up values

  38. Surge protection devices • Spark gaps (air gaps) • Gas discharge tubes (GDTs) • Zener diodes (avalanche diodes) • Metal oxide varistors (MOVs) • Transobers • Relays • Fuses • PTCR (Positive Temperature Coefficient Resistor) • TBU (Transient Blocking Unit)

  39. MOV/GDT/SAD MOV GDT SILICON DIODE

  40. Earthing for lightning protection - principles Divert current as soon as possible Use dedicated low impedance connection Make sure other systems are bonded to it, once!

  41. Clean and unclean lines should not be placed together

  42. Bundled wires introduce much less voltage drop than unbundled wires

  43. Details of SPDs

  44. Gas Discharge Arresters • Two electrodes, close together, enclosed in a tube filled with gas • When the voltage rises, a low impedance arc is formed between the two electrodes • Symbol: • Also called gas-filled surge arrester

  45. Gas Discharge Arresters

  46. Metal Oxide Varistor • Variable resistor – resistance depends on voltage • Symbol: • The most common type of varistor is the Metal Oxide Varistor, or MOV

  47. MOVs

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