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  1. HenryOhm

    Probability Distribution of Transformer Inrush

    First, I really appreciate the replies, and from some very knowledgeable posters that I've learned from just reading threads I'm not even in. FWIW, we have no fuses involved and the transformers will always be unloaded during transformer inrush. Yes, I don't discount the 0.5% probability case...
  2. HenryOhm

    Probability Distribution of Transformer Inrush

    Hi All, I was just curious if anyone has any better or contradictory information on transformer inrush probability than this excerpt I found (out of a PhD thesis by Jinsheng Peng, "Assessment of Transformer Energisation; Transients and Their Impacts on Power Systems", Pg. 174): "Furthermore...
  3. HenryOhm

    690V Motor Cable Insulation off Microgrid

    Hi All, I have worked with two large system integrators on 4-5 different projects the last few years. They have competing views of cable insulation for 690V motors operating off VFD. One recommends a specific VFD cable with 2000V rating, three symmetrical ground wires, shield, etc. The other...
  4. HenryOhm

    Good Book on AFE PWM Drives?

    All, My textbook "Power Electronics: Converters, Applications and Design" by Mohan/Undeland/Robbins has served me well for over 20 years. I've used it for DC drives, LCIs and through to PWMs. It does have fairly good coverage of an AFE's use to interface to a grid. The DC bus voltage of Vd...
  5. HenryOhm

    T'do Under Short Circuit with AVR Trip of Generator

    Thanks for everyone's helpful discussion! I should clarify, I am only concerned about arc flash energies inside the switchboards that these generators are feeding into. There is a pair of switchboards with bus-tie interconnection and six generators each about 1MW in size. At least one would...
  6. HenryOhm

    T'do Under Short Circuit with AVR Trip of Generator

    RRaghunath, thanks. But, with the short circuit of the field, it will still be flowing. Won't that mean it will continue generating voltage and feeding the fault? Wouldn't opening a generator main breaker be the fastest way to interrupt that fault?
  7. HenryOhm

    T'do Under Short Circuit with AVR Trip of Generator

    All, I am on a project where the systems integrator is proposing an AVR trip for generator short circuit rather than generator OCPD trip. I am concerned about the implications for arc flash incident energies and the time it would take for: 1) the control system to recognize the fault...
  8. HenryOhm

    Arc Flash, IEC 61641 "Arc Free" and NFPA 70E / IEEE 1584

    OK, I've posted there as well. Thanks for the suggestion. Of course, if anyone here has any thoughts, would certainly appreciate it.
  9. HenryOhm

    Arc Flash, IEC 61641 "Arc Free" and NFPA 70E / IEEE 1584

    All, To the best of my knowledge, North American standards like NFPA 70E and IEEE 1584 regarding arc flash have no equivalent to IEC 61641 for "arc ignition protected zones". I understand the IEC used to refer to this as "arc free". In Section 1 (Scope) of the IEC 61641, it ends with the...
  10. HenryOhm

    Inrush on Primary of Three-Winding Transformer When Other Two are In Use?

    I am working with an energy storage application. A three-winding transformer is in use. The utility feeds the primary winding medium voltage. One low-voltage secondary feeds an energy storage inverter with lithium-ion behind it. The third winding steps down the utility power and feeds a...
  11. HenryOhm

    Motor List Price Multipliers and $/kW at 1000kW

    I am making a speadsheet/chart of different motor costs to the end user (industrial). I used catalog list prices from ABB/Baldor for primarily 6-8 pole machines, TWMC for larger medium voltage and Lafert and Toshiba for PM motors. I also added a curve from USDOE for a more common type, not...
  12. HenryOhm

    Typical Subtransient Reactance for Smaller Synchronous Generators?

    All, I know this has been discussed before in more general terms. Referring to the IEEE Std 141 typical reactance values, they list xd" as 0.16 per unit for 12 poles or less (salient-pole with damper windings). In my experience with low-voltage diesel-driven 4-pole gensets in the 100kW -...
  13. HenryOhm

    Voltage Suppression for 19H 50A DC Motor Field

    ScottyUK, That's a pretty fascinating (and large!) application. Thanks for sharing that. I wish I could implement something like that at this juncture. But, the selenium suppressors are needing replaced fairly quickly so I obviously was hoping for a less involved solution. Still, it might...
  14. HenryOhm

    Voltage Suppression for 19H 50A DC Motor Field

    Gunnar, much appreciated. Unfortunately, the exciter does reverse its voltage output when online to collapse the field quickly and regenerate back. My initial thought along these lines was to just use two avalanche/zener diodes (a typical cathode connected power diode module with three...
  15. HenryOhm

    Voltage Suppression for 19H 50A DC Motor Field

    I have a project involving large 7000Hp DC motors with 19H fields with a resistance of about 5ohm. They normally need around 20-50A of field current for most all operating scenarios. Currently, the system has selenium suppressors installed for overvoltage suppression if the exciter suddenly...
  16. HenryOhm

    Wye-Delta Transformer Feeding 6-pulse SCR Exciter

    waross, thanks for the help. I had to spend some time digesting what you said. At first I thought that the voltage across the disconnected primary would go to zero based on Figure 5.33 of this link...
  17. HenryOhm

    Wye-Delta Transformer Feeding 6-pulse SCR Exciter

    I am investigating possible arcing of connections on the primary side of a wye-delta transformer feeding a 6-pulse, three-phase SCR-based exciter. The arcing led to a small electrical fire so there isn't a lot of evidence left of where things might have started. The transformer is 18kVA and...
  18. HenryOhm

    Transformer Percent Impedance with 120 vs 240V Dual-Voltage Outputs

    magoo2, Thanks, I think you've finally set me straight and untied the knot I was in!! Since the test will try to get the variac on the primary to output rated secondary current, I was probably missing that the doubling of output current with the parallel 120V output is still running the same...
  19. HenryOhm

    Ground fault on delta-delta system (ungrounded)

    FWIW and to follow-up Cranky108's post for a ground resistor that gets added in for tracing.... I've seen an entire system with low resistance grounded delta-wye transformers where the grounding resistor had another resistor, I think of equal size, in parallel, and with a contactor. At the...
  20. HenryOhm

    Transformer Percent Impedance with 120 vs 240V Dual-Voltage Outputs

    prc/waross, Thanks for the help, unfortunately, the transformer is installed and back at the client site, in operation, and a plane flight away for me. Sorry, but I was hoping that the nameplate verbiage might have been obvious to someone more experienced with these dual winding 120/240...

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