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About cos90

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  1. Read through this: pencil and paper section. Don't lose out to a technicality.
  2. Where did you get this Kaplan? Kaplan changed their name to Brightwood and the book I purchased from them looks nothing like this.
  3. You could also see a wye-delta used as a grounding bank, used like a zigzag transformer.
  4. A GSU is a delta-wye. I misspoke. The LV winding is the primary on a GSU transformer.
  5. My apologies for starting this discussion. Here's how you would solve this problem before microprocessor relays came onto the scene: @TNPE you would see a wye-delta transformer applied for Generator Step Up (GSU), the tertiary of a large autotransformer, and at interconnection sites where two systems are operating at a different phase angle. For distribution you typically see a Wye-Wye or Delta-Wye transformer.
  6. Here is the text: In our example, the transformer has the Δ-Y connection. Traditionally, CTs on the Wye connected transformer winding (winding 2) are connected in a delta arrangement, which compensates for the phase angle lag introduced in the Delta connected winding (winding 1), so that line currents from both windings can be compared at the relay. The Delta connection of CTs, however, inherently has the effect of removing the zero sequence components of the phase currents. If there is a grounding bank on the Delta winding of the power transformer within the zone of protection, a ground fault results in differential (zero sequence) current and false trips. In such a case, it is necessary to insert a zero sequence current trap with the Wye connected CTs on the Delta winding of the transformer. This is a very special case. If there weren't so many special cases we wouldn't need engineers.
  7. what if you lose feed from the utility and they close back in before you have someone at the station to turn the differential off? lockout differential on inrush ? genuinely curious
  8. page 372 Phase and zero sequence compensation
  9. cos90


    Just beware it's not perfect and inspired my forum username cos(180) = 0 ??????
  10. cos90


    It's in the code: the nameplate rating is used for overloads only. Everything else is in those motor tables. Also the PPI exams were really tough.
  11. oh i forgot to mention a major source of CT error as mentioned by rg1: variation from the load tap changer!! also if you are not tnpe or rg1 this discussion is way out of scope for the pe exam
  12. @rg1 you have done a good job explaining the details of transformer differential protection @TNPE I am curious what your plan is for the second energization of your transformers if you don't enable differential protection during the first energization?? especially since the next time you energize will be in less than ideal conditions. my comments: every source I can find for power says do not consider "even" harmonics but there is a second harmonic prominent during transformer energization. why does everyone say it then? removing the zero sequence components with delta connected cts is a negative. if you have a ground bank inside of the transformer cts on the delta side and there is an external ground fault you will have a differential trip. im not smart enough to realize this on my own but instead just read it in the ge t60 manual.
  13. Its a really straightforward question: You need to connect 3000kVAR to correct the power factor and you have 100kVAR cans How many cans per phase do you need to connect? 3000/3 = 1000 kVar per phase 1000kVAR/100kVAR = 10 cans per phase Where the questions get strange he draws out networks with series-parallel combinations of capacitor. Only one of these networks has 10 cans, but it was confusing because I thought there had to be other factors which are mentioned by @rg1. Thanks rg1.
  14. I am glad to see you got it. I was worried for you when I saw this question. I was afraid to do anything out of the ordinary for my application.
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