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About TNPE

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  1. 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.
  2. 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?
  3. 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.
  4. 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.
  5. 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).
  6. 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).
  7. 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.
  8. @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.
  9. 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.
  10. It is usually in the form of a toggle switch on the panel in the relay cabinet. You're essentially removing the differential scheme by breaking the differential protection circuit. As for your other post, I don't readily have any documentation that says you should design your scheme with "x" tolerance. But let's be honest, XFMRs (CTs included) are well-designed from all applicable engineering standpoints, and should accomplish and fill the needs for metering, relaying, protection, etc. with precision. I say this with regards to an engineer designing a suitable scheme/configuration. But yes, it is easily doable if done properly. Personally, I'm all about precision. Blame it on the engineer in me. I much rather prefer a well-designed (with all considerations given to CT ratios, saturation, wiring configurations, available fault currents, types of faults available, etc.) scheme as opposed to giving excessive tolerances to avoid unwanted trips/operations. Especially when we're talking about a XFMR, it is the lifeblood of a power system, as well as the most expensive (minus generation), so it is of utmost importance that it is protected properly.
  11. A differential scheme is looking for 0 amps (minus whatever tolerance levels may be in place). Essentially, the burden is comparing input and output current based on the turns ratio (i.e. it should register ~0 amps, if not, it is recognizing an internal fault and opens up). I may be misunderstanding you, but there should not be a tolerance as high as 300 amps, with a 1000 amp system, when utilizing a differential scheme. Depending on XFMR design and voltage ratings, very, very low voltages could be present in an internal fault, where insulation breakdown can (and often) occur in adjacent windings (i.e. consecutive windings on primary or secondary).
  12. To elaborate and expand further on e). above, most all relays should have a differential bypass to address your concerns with inrush current. Generally speaking, when a XFMR is being placed online, it endures an extensive array of tests to validate it's integrity for safe operation (at least it should.. if not, you need to question the engineer overseeing this). Hence, it is practically 100% safe, at this point, to disable the differential protection for energization. Once energized, enable the appropriate protection schemes. I have been around situations where you may isolate a section with limited protection, maybe backing up on the grid to the transmission/generator sections, but that's just the way it is in some scenarios. Safe practices and measures can be put in place to mitigate this, though (i.e. may still be utilizing over-current protection, one-shot enabled, ground trip block, etc.). I'm aware you don't work in the electric utility industry, but you want to eliminate every blink possible. This doesn't mean you put personnel at risk, just that work around these scenarios in the best, safest way possible.
  13. To compensate for the 30 degree phase shift, the CTs on either side should be connected in the opposite configuration of that side of the XFMR. For example, a D-Y XFMR would be CT'd in Y-D for a differential protection scheme. However, relays exist today that can compensate and do the work for you, regardless of winding configuration of XFMR or CTs.
  14. Transformer efficiency

    The "anomaly" with the math is simply that the problem statement explicitly says the XFMR is loaded at 25%. It doesn't say the output is loaded at any percent. Loaded, to me, means input. I get it. I know the math, and I understand it clearly and can see why it's done that way, but the language and math do not jibe. Loads are depleted by series impedance, looking toward the load, not the other way around. Hence, why I adamantly maintain it's not necessarily intuitive. But yes, if you run across this type of problem, solve it by the approach used in academia. I'm not saying academia is wrong. Hell, I've been there and done it and have gone through this whole process. I'm merely saying the verbiage and approach do not mesh. That's all.
  15. Transformer efficiency

    Dont know why this hasn't come to me till now, maybe a cold beer on a Friday evening does the trick, but looking from the output back makes sense. To evaluate any equipment, you need to know how it responds to a certain condition. The only way to do this is to expose it to said condition and record its behavior. I feel like a dipshidiot!!!