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  1. JJHorak

    SF6 Switch/breaker at very low gas pressure

    Did you ever hear a discussion on whether there a point of low SF6 where internal flashover becomes a concern? I suppose in a well monitored breaker it is not realistic to go to 0 pressure without first getting an alarm and having time to isolate the breaker, but that is not the case here...
  2. JJHorak

    SF6 Switch/breaker at very low gas pressure

    I know you do not operate an SF6 breaker if gas pressure is low, but assume dead tank SF switch where there is at least a little positive pressure: a) Is there a gas pressure level where there a risk of an internal line to ground fault? b) If the breaker is open, is there a gas pressure where...
  3. JJHorak

    Phase distance relay overreach

    My only notable experience with impedance relays seeing through a xfmr are in large auto-xfmrs where there are line relays on the remote side of the xfmr that can be relied upon to trip in less than a second (likely instantaneously in the reach of the xfmr relay). I seem to recall a case where a...
  4. JJHorak

    E Rated Fuse

    Overload protection is best done by design of the attached load, not by tripping fuses and relays. If you rely on fuses and relays to keep you from overloading the system, you have an unreliable system and a notable design problem. Fuse amp rating is the current they can carry without damage...
  5. JJHorak

    Mutual coupling of different voltages

    With complex enough software models of the system, ground overcurrent is still used. I concur it would be nice to bypass the tedious checking of their coordination for a zillion different outages. I guess till all the old EM relays go away, they remain even in new uP relays, at least in the...
  6. JJHorak

    Reactors on Substation Feeders

    Maybe they are related to closed transition of two sources feeding an important load. J. Horak, P-R Engineering, Colorado
  7. JJHorak

    Unbalanced Delta Delta Transformer Load Calculation

    I am not going to put my brain through the strain of figuring out everyone's logic, but I see a basic problem statement error that makes this difficult for me: In a delta system, loads are phase to phase, yet the problem starts my mentioning KVA on a single phase basis (phase A = ##kVA, phase B...
  8. JJHorak

    reactive power in transmission underground cables

    On David's comment of moving cables farther apart: I d not think that will decrease phase to ground or phase to phase capacitance of sheilded cables, but I suppose it will increase inductance a tad. On bacon4life comments: Nice set of options. Increasing voltage rating of cable will force...
  9. JJHorak

    PhD in Electrical Power Engineering

    "Oh come on John". Hmmmm, Oh well, I will move on past what sounds like a bad attitude or an anger of some sort. My suggestion was related to the fun of a young and bright mind fully understanding the physics of magnetics fields with earth and ground wire return, well enough to finding a set of...
  10. JJHorak

    PhD in Electrical Power Engineering

    Zero sequence of transmission lines. I tried to figure it out once, and was overcome by the complexity. Most of what people use is referred to as Carson's equations, but if you pull up a copy of Carson's paper, it provids un-derived equations that jump to results that I have never seen proven...
  11. JJHorak

    Mutual coupling of different voltages

    The voltage difference makes me wonder how much a fault at the end of say the 345kV line creates a current in the 115kV line, and vice versa. If there is low Io in the parallel line for a fault on the parallel line, then the parallel line would have low effect on the reach of the relays on the...
  12. JJHorak

    Phase distance relay overreach

    The way impedance relays looking into xfmrs are are set, load current is not really a problem. Assume xfmr Z = 9% per unit, and system impedance 1% on the xfmr base, for arguement and also a reasonable set of values. Through fault current is 10 x the load (self cooled MVA). An impedance relay...
  13. JJHorak

    Delaying bus differential

    Reliance on lockout relays to prevent damage to high impedance bus diff relay from damage has been an industry standard since the invention of the high impedance bus relay (1930's?) and I am unaware of a concern about their failure and subsequent damage to the relay. Or maybe I am just unaware...
  14. JJHorak

    Why are transformers connected additive or subtractive?

    Can some of you more experienced people comment on whether I have mixed up additive and subtractive in the two bullets? The two versions of the xfmr arise out of different approaches to making the winding polarity orientiation recognizable without having to look at the busihg polarity marks. -...
  15. JJHorak

    How does the positive-sequence voltage relay measure?

    I stand corrected again. One sample is not used in relays to trip. It takes a cycle's worth of data to accurately calculate RMS and fundamental frequency magnitude, though you can calculate either RMS or fundamental frequency data off of a quarter or half cycle of data if you assume that quarter...
  16. JJHorak

    How does the positive-sequence voltage relay measure?

    In most digital relays that I am aware of protective elements pickup at only one sample below the setting; there is no inherent delay. If there is no time delay, you will have a trip with only one sample is out of range. Programmable delay settings will allow you to specify that the trip...
  17. JJHorak

    CT star point

    You are right David. I can't say the wiring reversal will cause a 180deg shift unless I also looked at how the relay or meter was wired.
  18. JJHorak

    Transformer Inrush and Frequency of Breaker Closures

    I have not heard of a rating/standard on frequent inrush limitations, but a couple tidbits that come to mind: --Maybe you could reason through heating effects using some guesstimated average I^2R, keeping in mind that inrush I^2R is only in the primary. Then compare to rated continuous I^2R. --I...
  19. JJHorak

    CT star point

    All your currents will be off by 180degrees, unless there is some other question or point you are asking.
  20. JJHorak

    CT Saturation

    No, I said you could drive it up to 281 A if your burden is negligible. If you have 0.1 ohm burden, the calc becomes 18/(0.064+0.100), or ~110A secondary, which gives some agreement with the C class rating. Yes, the statement that "if burden is less than X you will get at least Y current before...

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