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TNPE

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About TNPE

  • Rank
    Principal in Charge

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  • Gender
    Male

Previous Fields

  • Engineering Field
    Electrical (Power)
  • License
    PE
  • Calculator
    Casio
  • Discipline
    Electrical

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604 profile views
  1. Turns ratio confusion

    I used the last two weeks to tidy up any loose ends on topics I was weaker on. I didn't use a cheat sheet, rather I learned my materials while studying (developed comfortability) and let that be my "cheat sheet." There's not a template that is all-inclusive such that any person could follow it and pass. If there were, I'd gladly give it to you. You know your strengths and weaknesses. Bolster your strengths and mitigate your weaknesses. I cant stress enough how important it is to know your materials. Really, just as important if not more so than being a "student" of the subject matter. There are dozens (maybe hundreds) on this very forum who may even admit that. Some fail the exam due to underestimating what's required to pass and they sleepwalk during preparation, others miss the mark by not being proficient with their resources. Good luck! If you have any other questions during the countdown, please post them.
  2. Turns ratio confusion

    Hard to have a defined system if everyone knew the problems, therefore, either diminishing the value of being a PE or being impossible to pass. Best I can give you is this, the test is almost identical in difficulty and style to the published practice exam.
  3. Turns ratio confusion

    I have worked a lot of them at some point in my past, but I was speaking to presentation and overall layout of the book. I takes a fundamental, foundational approach.
  4. Turns ratio confusion

    I, too, am 32. Took the exam at 31, though. I don't necessarily agree with rg1 with regards to Chapman. I think you can almost pass the test exclusively with his book, Graffeo, NEC and NESC. Now, I wouldn't show up with only those materials, but it is easily passable with them alone. I do not own a copy of Wildi, but I have made a cursory scan of it. IMHO, it is more of an "academic" book, but Chapman is better suited for this type of scenario (Chapman was actually a college text of mine). This exam is not about "academics" per se, rather it is more about fundamentals and knowing how to work problems that take an average of 6 minutes to complete. The student in me loves academia, but the PE in me loves preparing to pass the test the first time around. By no means, I'm not telling you not to use Wildi, but don't get bogged down in the academics. Learn the approaches and techniques and move on.
  5. Turns ratio confusion

    If you're talking about Stephen Chapman, then yes, it is an essential. I used it almost exclusively when I took the PE (also used Graffeo, NEC and NESC, where practical). I also used a few other college texts for specific problems, but used Chapman and Graffeo for at least 75-80% of the exam.
  6. Phase angle shift

    No, it's correct. You're dealing with voltages here, not currents. Now, had it been worded in terms of asking for proving the currents, then it would be lagging. Draw it out and it'll make sense. Use this to get started: VAB=VA-VB Plug and chug along and make substitutions and simplifications where needed (ideally, convert VB angle to (-1/2 + j(sqrt3/2))and you will find it to be: (sqrt 3)V/_+30
  7. Dodge the problem entirely by doing what I mentioned in other posts. No need to complicate it and make it more expensive at the same time. I have yet to see a scenario where this configuration would be needed, at least with regards to HV, MV and distribution utility applications.
  8. That's my point, it is almost impractical to use this winding configuration on the utility scale when other more practical, existential options are available. Most transmission step-downs (e.g. 161-69kV or similar) are wye-wye grounded with a delta tertiary. The tertiary serves two purposes, trapping zero sequence and providing station service to a 13.2 kV bus (typically). Generally speaking, to pick up ground faults, you want your down line sections to be looking back at a grounded secondary. Without this, you have to use a more expensive alternate approach to detect "ground" faults. Whether you accomplish this by beefing up the relays or adding a zigzag. Why even go that route when it's not necessary?
  9. So, this sounds like a zigzag has been installed near the delta so that a ground reference can be made for the secondary side. In this case, yes, I can see where you would have to protect in a seemingly unconventional manner to offset the introduction of zero sequence from the zigzag. That said, can anyone explain to me where a XFMR arrangement like the one presented in the OP would be used, or even practical, in a utility environment? Personally, from my experience and the applications I'm accustomed to, I would never use that arrangement at the sub-transmission or distribution levels.
  10. Per the original drawing, no zero sequence current flows to the relay coils. Zero sequence circulates in delta CTs and no zero sequence present on wye CTs due to the absence of a neutral. X2 bushings are bonded together and floating. Am I missing something you're trying to say? The primary of the XFMR winding is wye grounded, thus zero sequence currents will flow between that point and it's upline source. This is a rather odd connection. I'm used to the whole configuration being flipped.
  11. Turns ratio confusion

    No problem. The confusion with most of these scenarios is attributed directly to language. However, if you think of it intuitively, it makes perfect sense. As I've said before, the PE exam will ask you EXACTLY what they're looking for. There should be no confusion if you're well-versed (and from what I've seen, you are, and you should have no trouble). As far as it being legislated, I don't know that I would define it that way. Just know that a voltage ratio and XFMR winding ratio can be different, depending on winding configuration. Just remember, you will always be relating the phase voltages to one another when working with winding ratios (Always, Always, Always).
  12. Turns ratio confusion

    The voltage ratio may be given in line quantities, but the turns ratio of the XFMR is always with regards to phase (e.g. phase for delta is LL and phase for wye is LN).
  13. That's a good question. If you did lose feed from the G&T, or whoever your up line supplier is, it's customary that they be in contact with their downline utilities to coordinate events such as this and for safety measures (i.e. all clear). With today's technology, you should also be able to handle this remotely via SCADA.
  14. @cos90 Yes, you're correct in saying the level of this conversation is safely out of range for what the PE will test for. Second energization? Hopefully you don't lose your XFMR often. I'm speaking to initial energization. That said, should you lose a XFMR, and you have no indication of internal damage, I would disable differential protection and let the OC carry it, just as you would during initial energization. But be aware of XFMR behavior and noises. If it starts emulating a washing machine, kill it. Without an advanced scheme or alternate profile, energization could be extremely difficult, even impossible (the inrush would trigger the differential and prevent energization). On the other side, if you suspect internal damage or have shrapnel and oil spewed everywhere, it's safe to say energizing ain't happening soon. At this point, I hope you have means to backfeed or access to a spare XFMR or a mobile sub.
  15. Also, differential schemes are EXTREMELY fast!!!! Much faster than a generic over-current scheme. But it should be. We're talking about protecting a piece of equipment that could be valued in a range from ~$500K to several million. Differential schemes are akin to a hot line tag for over-current devices. Operation can occur within the first half cycle to 3 cycles.
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