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  1. electrolitic

    4 or 2 diodes

    It is an old question, still unclear: Why some designers apply 4 blocking diodes, instead of 2, when connecting two DC power sources in parallel. There is a risk of, under certain circumstances, the output voltage be the sum of the two sources. Is there a reason that justifies this risk?
  2. electrolitic

    Capacitors discharge

    High voltage capacitors connected in parallel with induction motor needs a dedicated discharge circuit before next motor starting? Thanks.
  3. electrolitic

    Brushless excitation

    Even new synchronous machines, specially Rotary Compensators, still use brushes/rings for field winding excitation current instead of brushless system. Probably the possibility of field polarity reversion is the reason. When/why is this reversion necessary? Thanks.
  4. electrolitic

    Blocking diodes

    It is an old question : some important switchgear designers apply 4 blocking diodes when connecting two control DC power supplies in parallel. Others, 2 just at positive leads. It is well known the risk of serious continuous overvoltage ( 2 times Vnom) at load, under certain fault conditions...
  5. electrolitic

    Metering CT for protection

    I think that a metering CT can be connected to a protection ground relay if installed at HRG circuits of transformers and generators. I can see some pros and no cons. Am I right? Is this feasible/usual? Thanks.
  6. electrolitic

    Rotary frequency converters

    Recently I took notice of an old (50's) but still running electric equipment. Two MG sets, 20 MW each, (60 Hz Synchronous Motor driving a 50Hz Synchronous Generator). One of the sets has an special sliding cradle that permits the turning (+/- 20 degrees) of the complete SM stator (hidraulic...
  7. electrolitic

    Residual VT

    Condition: single phase fault on an unearthed neutral 3-phase system Question: Can I obtain 59N protection using just 2 single phase VTs connected in open delta configuration? Thanks
  8. electrolitic

    Redundant power supply

    Two DC PS units connected in parallel through blocking diodes. Two diodes or four diodes? Thanks
  9. electrolitic

    Cardew

    On LV isolated network Schneider recommends a surge limiter (Cardew C) to discharge overvoltages to earth, normally connected between neutral and earth. In case of unavailable neutral the catalog shows just one surge limiter connected between one of the three phases and earth. Is this enough or...
  10. electrolitic

    Short circuit asymmetrical factor

    Do I need to consider the calculated short circuit asymmetrical factor in case of arc fault in a MV system? For bolted faults I already need to provide a zero crossing delay for safe breaker opening. Thanks.
  11. electrolitic

    CT 50/60Hz

    Recently I had started a thread: If a CT originally designed to be applied at 50 Hz circuit is installed at 60Hz which would be the influence over the secondary saturation voltage? The answers were perfect but one doubt remained if related to the accuracy threshold at low current settings...
  12. electrolitic

    CT saturation voltage

    If a CT originally designed to be applied at 50 Hz circuit is installed at 60Hz which would be the influence over the secondary saturation voltage? Thanks
  13. electrolitic

    Unbalance currents

    Can I consider Pt100 sensors at induction motor stator windings a good protection against overheating caused by unbalance currents (max unbalance voltage=5%).RTD has a relatively slow reaction time. Can it respond adequately to the heating process? Thanks
  14. electrolitic

    Generator stator ground fault protection

    All papers that I could find describes gen stator ground fault (over 80-90% of stator winding) protection for unit-connected generator (connected to a step transformer). Is it possible to protect generators connected in parallel? Thanks.
  15. electrolitic

    motor inrush reduction

    I connect a 3,5MVA transformer, Z=7%, 11.0kV/3.3kV to a 2,5 MW induction motor, Is=6 In. Circuit breakers at primary and secondary. Can I suppose around 30% of reduction voltage at starting, Is reduction to aprox. 4,0 In as it was a reator starting (70%) without additional breakers and bump...
  16. electrolitic

    CT accuracy

    10P20 CT it means a protection CT with composite error of +/- 10% and that error will be maintained up to 20 times the rated primary current of the CT. Is it correct? Is it constant? More and more current measurements/monitoring using the modern protection relays instead of dedicated...
  17. electrolitic

    VFD AC/DC supplied

    Sirs Can I connect two 5,0kW VFD (diode front end) to a selectable (open transition) AC 3ph or a DC from battery bank? Fuse protection and blocking diodes at DC entry. If yes, which should be the DC voltage compared to AC? Could the built-in precharge be used for both supplies? Control power to...
  18. electrolitic

    VFD and HRG system

    If I have a VFD in a HRG system the ground fault is difficult to be detected. It depends where the fault occurs. Can be at AC entry side, DC bus or drive output, sometimes running at very low output frequency. Expensive microprocessor based relay reading a CT at neutral resistor circuit can...
  19. electrolitic

    Electric symbols

    I received an electric single line diagram drawn in USA where it appears two small circles close to a low voltage circuit breaker. Inside one circle is written EO. Inside the other, DO. What do they mean? Thanks.
  20. electrolitic

    Peak Torque

    Rotary blade saw, 125kgm2,running at 1200rpm, driven by AC induction motor 610kW, 1780rpm. Motor starting torque 1.8 times Tn, motor max torque 2,7 times Tn. VFD for no-load starting up to nominal speed. During open transition (applied VFD cannot run in parallel with the grid) a high peak...

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