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  1. DiscoP

    Explanation for Importance and/or application of the "3*Zero Seq Current" for LG and LLG F

    The main reason to use this is that 3*I0 only appears for ground faults so it can be set lower (and often faster) than an element based on negative or positive phase sequence that need to be set above normal load conditions and co-ordinate with all down-stream protection. As others have said...
  2. DiscoP

    Meter shows current values during CT secondary injection but zero after energization

    Expanding further again on relays, some have true RMS readings but also have raw / Fourier filtered measurements. The true RMS needs a bit more current to display anything compared to the "raw" measurements. Also in addition to different Zero deadbands in different devices and firmware version -...
  3. DiscoP

    Ct saturation

    I'd suggest reading the manuals very carefully, and consulting with the product application support experts of the relay manufacturer. It is true that sometimes a value less than the maximum fault level will be OK and some protection schemes can tolerate some saturation, but there is normally a...
  4. DiscoP

    Reduce the Tripping time - Relay coordination

    There might be quite a lot of things than can be done, depending on what equipment currently exists and how much you are able to spend. Your system needs to be properly studied and designed though.
  5. DiscoP

    CBCT/ZCT on solidly grounded systems:

    My understanding is that the CBCT will normally have an improved accuracy when compared to a residual connection because the fault current has only one iron core to magnetise vs three on the residual connection (assuming the same core materials and size). It's benefits were more noticeable with...
  6. DiscoP

    Protective Relay Trip Circuit Monitoring

    I'd echo cranky108's comments, use a product designed for the function. I've seen people try and save money on this and it come back to bite them. Another example I can remember is back emf from the trip coil damaging the coils they installed for TCS. As thermionic1 said, most of the numerical...
  7. DiscoP

    FIBER OPTIC CABLE SIGNAL TRANSMISSION

    You may find there is "Remote Indication" signalling in the Siprotec relays (depending on the model). If the IED is for feeder differential protection, then the fiber core being used for this protection can also be used to signal CB status etc. There is quite a few additional signals in that...
  8. DiscoP

    CT secondary wye point grounding

    Just to add a comment, the font looks like what GE Multilin uses - so I can give an example from the SR745 from that vendor - it might be relevant here. If you connected early versions of the SR745 using IEEE conventions, it would trip incorrectly for an external fault. The REF input had a...
  9. DiscoP

    Reyrolle type C21 (Duo-bias) Electromechanical relays

    I don't have it, but you could try your local Siemens agent. Siemens bought Reyrolle's relay group out a few years ago. I think the C21 is called a "mono-bias" relay, then they come out wth the 2C21 and 4C21 which were "duo-bias" versions, so might be worth knowing exactly which one you need...
  10. DiscoP

    Why everyvody hates NC contacts?

    Hi David - yeah we do the same where the relay allows. We use a Normally Open supervision contact that will be closed when everything is OK, and then invert the singal in SCADA so that the alarm is raised when the SCADA input is low. It is a much better supervision system, however it creates...
  11. DiscoP

    Why everyvody hates NC contacts?

    I know many people don't like the term "normally" and this can create confusion - sometimes leading to hate. "Normally" is the state of the contact when the coil is de-energised (or reset if latching). However there are many applications where the coil being energised (or set) is normal - for...
  12. DiscoP

    CT Shorted on Primary (secondary Impedance)

    It has been standard practice where I work to disconnect the CTs from the busbar protection for this type of switching set-up, but the main focus is the grounds create a path for fault current and cause the busbar protection to incorrectly trip (which has happened to us). I guess the reasons you...
  13. DiscoP

    CT Testing to Determine Secondary Winding Resistance & Thermal Rating

    I don't think that information relates to the continuous rating of the CT, it is more to do with the insulation rating. The nameplace is showing a short term themral current rating 65kA for 3 sec, but not a continuous rating. If it doesn't say anything on the nameplate, then I think the IEC...
  14. DiscoP

    Frequency relay operation

    Do you know that the UF is based on Voltage or current. I am not familiar with that relay, but some relays can use current instead of voltage (sometimes a setting preference) and unintentionally using current has caused false operations. Another possible false trip is when the AC supply to the...
  15. DiscoP

    Dual ratio CT parameters (Knee point voltage, class, magnetizing current, resistance)

    Yeah - that graph looks about right. You can only say both windings have a 5P rating if stated on the nameplate (or if you can verify it with a test). Note that rating means the error is less than 5% at ALF and connected burden. 0.1% error or 4.9% error can still be called 5P. I would assume...
  16. DiscoP

    Dual ratio CT parameters (Knee point voltage, class, magnetizing current, resistance)

    Rightly or wrongly we re-calculate that the voltage and Rct will rise proportionally to the turns and the exciation current will lower proportionally. If you examine test results you will find that this mostly holds true. So a 5P20 is more like a 10P10 on half the turns, or a 2.5P40 on double...
  17. DiscoP

    Multilin GE SR 760 Relay Protection Element Tolerances

    A relay doesn't measure I2, it calculates it from Ia Ib and Ic (each with their own tolerances) So the I2 "measurement" has three lots of tolerances when compared to each of the phases. This is my best guess as to what that probably means.
  18. DiscoP

    High impedance bus differential - CT Saturation Calculation

    Agree with Stevenal. The CTs will saturate for an internal fault, so you want to make sure the relay operates before this happens. That is why they recommend the CT's Vk be at least twice the setting voltage when using the MFAC Maybe check the NPAG - availble on the GE Grid website (the...
  19. DiscoP

    Frame Leakage protection: 3 phase fault

    10 ohms is the recommendation in the GEC Protective Relays Application Guide. If you have access to this book, it gives an explanation as to why this is normally OK. It is OK protection for switch gear faults, but I wouldn't be relying on it to operate if all three phases are shorted together...
  20. DiscoP

    should I use both element 51 and element 49 for the same motor?

    In addition to JG2828's comments, with the relays I have used the 49 element also takes -ve sequence current into account, and speeds up the operating time when present.

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