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  1. Oblsss

    25 kV Disconnector Switch Control Circuit Questions

    From the answers provided, it seems that it is acceptable to lose control of a MV disconnector over an electrical fault in the heater or the light of a control box. This does not make a lot of sense to me! Any tips or suggestions from the more experienced? [bigears]
  2. Oblsss

    25 kV Disconnector Switch Control Circuit Questions

    @crshears Actually I think there is no big difference in terms of continuity of service, since the 230 V AC are provided from the output of a UPS system installed in the nearest station ...
  3. Oblsss

    25 kV Disconnector Switch Control Circuit Questions

    Dear all, I'm checking a quote for a 25 kV Disconnector Switch and I noticed that in the attached circuit diagram the manufacturer uses 230 V AC (Europe) for all circuits, namely for the Control box auxiliaries (heating, light bulb, socket), the control circuit, the motor circuit and the...
  4. Oblsss

    X/R Ratio for CT Saturation/Check

    Hello, Could you please provide a link to the IEEE CT saturation tool? Usually, the protection relay manufacturer provides some application examples which give some standard values for the X/R ratio. BR,
  5. Oblsss

    Short circuit analysis of DC Traction Power System - Running rails and Conductor/3rd Rail impedances

    cranky 108 thank you for your answer! Unfortunately it is not easy for me to communicate with your above references as I live and work in Europe! The system I am talking about does not have a catenary feeding line nor a copper neutral (you can click on the link to see a layout of the system)...
  6. Oblsss

    Short circuit analysis of DC Traction Power System - Running rails and Conductor/3rd Rail impedances

    Dear Sirs, I am trying to build a model of a DC Traction Power System, composed of 10 Traction Substations (TSS) feeding a third rail, also known as conductor rail, system (750 V DC). The main objective is to build a representative model that will confirm the points where the max/min di/dt...
  7. Oblsss

    MV substation auxiliaries T/F (50 kVA) trips when main T/F (3.3 MVA) is energized

    @Kiribanda Main transformer is T1 due to higher power (3.3 MVA) compared to the Aux. T2 (50 kVA). T1 is a 3 winding Transformer, D0doy11 connected.
  8. Oblsss

    MV substation auxiliaries T/F (50 kVA) trips when main T/F (3.3 MVA) is energized

    @Kiribanda Please notice that: [highlight yellow]We are experiencing several nuisance trips of the auxiliaries T/F CB when the main transformer is energized.[/highlight] If I disconnect the residual CT connection there will be no tripping because this CT is the input for the 50N/51N...
  9. Oblsss

    MV substation auxiliaries T/F (50 kVA) trips when main T/F (3.3 MVA) is energized

    @davidbeach The installation was performed as described in your post: Point one was connected through an insulator to the earthing bar of the cable compartment (no electrical connection), passed through the core balance CT and then terminated at the earthing bar of the cable compartment...
  10. Oblsss

    MV substation auxiliaries T/F (50 kVA) trips when main T/F (3.3 MVA) is energized

    Thank you for your answers, @RRaghunath Relay auxiliary voltage comes from a different power supply! @davidbeach I have 4 similar substations, the problem occurs consistently at 2 of them since day one. (This may relate to the relation between inrush current and external grid impedance?) For...
  11. Oblsss

    MV substation auxiliaries T/F (50 kVA) trips when main T/F (3.3 MVA) is energized

    Dear Sirs, We have a MV switchboard consisting of three CB's: 1) one connected to the 20 kV grid, 2) the second feeding a 3.3 MVA 3-winding transformer and 3) the third feeding the substation auxiliaries 50 kVA 2-winding transformer. We are experiencing several nuisance trips of the...
  12. Oblsss

    Hermetically sealed Oil Immersed Transformer trip due to low oil level (vacuum loss???)

    Thanks for the answer prc! "Approx 0.8 % volume increase for every 10 C temperature rise." Do you mind sharing a source for this information? Does the same apply for a 10 C temperature decrease? According to your experience, 0.8 % volume increase for mineral or synthetic oil? For this...
  13. Oblsss

    Hermetically sealed Oil Immersed Transformer trip due to low oil level (vacuum loss???)

    Hello everybody, I had to work on an incident in a Traction Substation where a Hermetically sealed Oil Immersed Transformer, aka corrugated tank completely filled without conservator, tripped due to low oil level after shut down (in reality it did not trip, it just could not be brought back in...
  14. Oblsss

    Non-shielded/Unscreened Power cables - Voltage rating 1.8/3.0 (3.6) kV - Safety issues

    @ Kiribanda Thank you for your answer. Nevertheless, I will have to disagree with you. [highlight #FCE94F]The metallic screen is making an equipotential surface for the cable so that the insulation is not subjected to unbalanced electromagnetic stresses[/highlight] and contain the electric...
  15. Oblsss

    Non-shielded/Unscreened Power cables - Voltage rating 1.8/3.0 (3.6) kV - Safety issues

    First of all, thank you both for your answers! @ Kiribanda Nevertheless the problem remains, ie the separate ground return cable cannot contain the electric field within the cable core and the cable sheath shall not be safe to touch with the cable energized. Please recall that I am talking...
  16. Oblsss

    Non-shielded/Unscreened Power cables - Voltage rating 1.8/3.0 (3.6) kV - Safety issues

    @ 7anoter4 When voltage values like "2001-5000 V" or "2.4 kV" are mentioned in the NEC, should I consider them as phase-phase or phase to earth voltage values? I did a quick search and found that, according to NEC, [highlight #FCE94F]"110.4 Voltages. Throughout this Code, the voltage...
  17. Oblsss

    Non-shielded/Unscreened Power cables - Voltage rating 1.8/3.0 (3.6) kV - Safety issues

    Hello 7anoter4, Thank you for the reply! Regarding type MC cables, in the IEC world, these would probably be considered screened/shielded cables according to par. 9.1 of IEC 60502-1: I have in mind that there are differences between USA & IEC specifications and that is why my main question...
  18. Oblsss

    Non-shielded/Unscreened Power cables - Voltage rating 1.8/3.0 (3.6) kV - Safety issues

    Dear all, In the IEC world, every cable designed for a voltage level greater than 1 kV shall have a metallic layer surrounding the core(s), par. 7.3 of IEC 60502-1. With regard to safety of personnel, the above requirement ensures that the outer sheath of the cable will be safe to touch...
  19. Oblsss

    Thermoset vs Thermoplastic insulation stripping tools

    @waross The first possibility may be the answer i was looking for since in europe we are not used to awg/kcmil units for conductor csa ... The tools are not of high quality but they must have performed well in the past since we didn't have any complaints! Regards, George
  20. Oblsss

    Thermoset vs Thermoplastic insulation stripping tools

    Dear all, In an attempt to install some THHN cables we damaged two of our cable stripping tools. These tools were previously succesfully used for similar works on building cables with thermoplastic PVC insulation rated for 70 deg Celsius. Αs the root cause of the tools damage, apart from bad...

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