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  • Users: mgtrp
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  1. mgtrp

    Low insulation resistance - Utility scale PV plant

    I think there might be a decimal point in the wrong place with that calculation. 1000V / 1000kOhm = 1mA, not 1A. Which is a lot less heating in any problematic connector, and unlikely to give the type of unambiguous evidence of damage that 1-2 kW might.
  2. mgtrp

    Load shedding

    This is a little misleading. If the generators are connected to a common busbar, the frequency measured by VTs on the busbar or the on the feeder will for all practical purposes be identical. Generators might have different speed setpoints as part of a droop load-sharing scheme, but this does...
  3. mgtrp

    ROCOF settings used for islanding

    More information about what the OP is actually trying to achieve might be useful. A ROCOF scheme would typically be for detecting when the generation and some of the nearby distribution network have already become islanded from the wider grid. They're only useful/effective if there is a...
  4. mgtrp

    How do you determine X"d, X'd, Xo on a small generator when the info is not given on the namepl

    Which version of the standard did you get that from? It doesn't match the my copy of the red book, which is the 1993 version (and I believe the still-current version), which show Xd" and Xd' headings on the opposite columns.
  5. mgtrp

    Transformers at low load and impact on power factor

    The charging current for 3km of 132kV cable should swamp the magnetizing current of the 56 MVA transformer. It should ruin your site power factor even when operating under load. Therefore, I'd be fairly sceptical of your load flow results showing the same power factor at the DNO and at the...
  6. mgtrp

    Transmission system overvoltages during major load throw-offs

    As far as I know, tripping of lines for voltage control is done by operator action, not automatically.
  7. mgtrp

    Transmission system overvoltages during major load throw-offs

    Taking lines out of service is still done in the UK--the retirement of synchronous generation and the addition of large amounts of inverter-based embedded generation has made high voltages an issue. Certain circuits are designated for voltage control. Another utility that I previously worked...
  8. mgtrp

    Can cut-out fuses have a current rating of 250 Amp at 23 kV?

    Cut-out based reclosers are making an entry though... (for example, here) With regards to the original question, I haven't come across cutout fuses above 200A. IEEE C37.42 only seems to have standard ratings up to 200A, so I suspect that this is what the holders are designed for. Cheers, mgtrp
  9. mgtrp

    Transformer 30/0.4kV unbalanced currents

    What ratio are the primary CTs? If they're fairly high ratio (e.g., 200-1 or 200-5 are quite common) then a few amps difference between phases probably doesn't mean too much. Cheers, mgtrp
  10. mgtrp

    Grounding of primary neutral of a step up Transformer in Solar Generaton Sytem

    Be careful comparing the descriptions in the linked eng-tips post (explanations from waross) and the IEEE standard--I believe that waross's explanations largely relate to transformers with a wye-connected HV and delta LV (i.e., Ynd), while the IEEE standard is referring to transformers with a...
  11. mgtrp

    TCC curve in excel

    It's not perfect, but the nice folks at Basler have made an attempt at fitting the recloser curves already. If you hunt around on their website near the bottom of the 'Technical Resources' section of any of their protection relays, you should find it. Cheers, mgtrp
  12. mgtrp

    430 MVA GSU Tripping on 51G1N (Neutral Ground Overcurrent)

    Do the protection relays provide any oscillography? That'd be the first place that I'd be looking for clues. Neutral CTs don't usually produce false currents, even if misconnected, so I'd be wondering about the possibility of there being an open circuit on the HV side of the transformer...
  13. mgtrp

    Star Star Auto-transformer with Delta tertiary

    Is the tertiary grounded via a reactor, or is that a shunt reactor connected in the tertiary (the '4' next to it makes me think three phases plus a fourth on the star point)? Cheers, mgtrp
  14. mgtrp

    Differential Trip on Transformer

    Assuming that there's no parallel feed transformers or zero-sequence sources on the LV distribution network, the LV CTs should only see fault current for a faults that are downstream of them (not on the transformer). But, if the fault is downstream of the CTs, then the differential should not...
  15. mgtrp

    ANSI equivalent to IEC 62271?

    Further to Scotty's answer, see here.
  16. mgtrp

    Zero Sequence Network

    It does include the zero-sequence impedance of the delta-star transformer, it is the j0.1pu on the left.
  17. mgtrp

    GIS high speed earthing switch

    At my previous utility, there were several high-speed ground switches used to trigger the remote protection. This was in cases where there were no communications between the substations, so the local protection would close the ground switch for certain fault types. I suspect that if these...
  18. mgtrp

    Autotransformer teritary

    No, the opposite--it is very lightly loaded. VArs are being controlled to lower the voltage.
  19. mgtrp

    Autotransformer teritary

    My former utility had them for transmission intertie to control VAR flows between the transmission systems (different voltages). We would have struggled to operate the system without OLTCs!
  20. mgtrp

    High Resistance grounding on Medium Voltage System

    Squaring the ratio is right; squaring the resistance is not. It comes about because R=V/I, so when you convert to primary values by the turns ratio you end up with R = (V*n)/(I/n) - the turns ratio is in both the V and the I terms. Expanding becomes R = V/I * n^2. Actual reflected resistance...

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