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Recent content by davidbeach

  1. davidbeach

    Primary and Backup Relay Timing

    It depends on what you mean by primary and backup. If you're doing two of the same things, more Main A and Main B, then set them the same. But if you're meaning what we used to do, one good relay and one so-so relay then you may want to consider a different approach. What we do in that case...
  2. davidbeach

    Ceiling fan issue

    Yea, off as in not moving or off as in no applied voltage? They may look very similar...
  3. davidbeach

    Phase-earth fault in YNd11 step-up transformer while operating at no load

    An overcurrent element just above pickup can take an excruciatingly long time to trip.
  4. davidbeach

    Transmission Line Geometry and Impedance/Losses

    Oh, so spend over 80% of the cost of a two circuit line to end up with a single circuit line? Where NERC's rules around being prepared for the next contingency apply, there's and advantage to having more lines rather than fewer. We're in the process of splitting some superbundles into separate...
  5. davidbeach

    Transmission Line Geometry and Impedance/Losses

    Looks like 5 single conductors, not bundles. What's the cost of the transformation between the special 5 phase line and the surrounding 3-phase system? That's got to eat up most of the "savings", the rest of them likely consumed by 5 pole breakers (although that might be an advantage of live...
  6. davidbeach

    Disconnect sizing question

    Not so fast down playing the fault current level. Go with what the utility says. If that 500kVA is comprised of three 167kVA cans they might have impedances closer to 2%.
  7. davidbeach

    UPS Load Test

    Once did a comprehensive commissioning test of a data center that included a 300kW UPS. In order to test out everything, UPS, HVAC, etc. the contractor had to go purchase a pallet load (200) of 1.5kW heaters. All plugged in and all on high. 300kW of heat being dumped into the room. HVAC...
  8. davidbeach

    Spain and Portugal power grid collapse | Report Released

    I like rotating mass, but batteries may actually allow a system to remain functional with little inertia. One of the reasons is that batteries can “spin up” much faster than any conventional rotating generation and available wind and solar probably has no headroom. There have been events where...
  9. davidbeach

    FIU Bridge Update - New Bridge Has Broken Ground

    Lifts = Elevators 🙄
  10. davidbeach

    Spain and Portugal power grid collapse | Report Released

    Probably all going to turn out that all the resources should have been in Volt var control rather than in power factor control.
  11. davidbeach

    Directly from 30 kV to 0.4 kV? Or step down the voltage in stages, typically from 30 kV to 6 kV, then from 6 kV to 0.4 kV.

    Lots and lots of installations with 34.5kV to 690V or 480V transformers, so "most cases" is probably not accurate. Probably depends on many factors including local custom.
  12. davidbeach

    Transformer operations

    How much does top oil temperature vary with that cycle?
  13. davidbeach

    Transformer test report - zero sequence values

    A bit closer than Dave's 85% is R0 = R1 and X0 = 0.85 * X1. But that's for generic core form 3-phase transformer with three legs. More legs (but unusual on a DY), shell form, or a bank of three single phase units result in X0 = X1. Some designs and material selections can also push that 0.85...
  14. davidbeach

    Inrush current of transformer

    Did you ever look at a raw event capture of that? You don't see the load initially, all you see is all the transformers being energized (aka inrush). Yeah, there's a bunch more load to carry for the first bit than there was before the circuit tripped, but most of what a cold load setting...
  15. davidbeach

    L-G Fault Zero Sequence Current in un-faulted phases

    The zero sequence current in each phase is identical to the zero sequence current in the other two phases. The positive sequence current in each phase has the same magnitude as that in each of the other two phases with an angular difference such that A leads B leads C by 120 degrees each...
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