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  1. PowerfulStuff

    Understanding ground fault detection scheme on a Cogen arrangement

    Hi, With respect to the question of a seperate ground fault CT, or summing the phase CT's I had always thought that the main reason for putting in the seperate CT is that is is more accurate and allows for a lower protection setting i.e.- - Under system fault conditions transients may occur...
  2. PowerfulStuff

    Substation Fence Grounding

    esleigh, I went through a similar situation last year. As Kiribanda says the transferred potential risk is as long as the fence. We ended up improving the substation earthing substantially - thus no gravel required and fence was OK as is. regards, PowerfulStuff
  3. PowerfulStuff

    reasons behind this transformer current

    While I have often seen the stray currents (up to 10's of amps) that Rasevskii mentions, with the panel off i.e. no phase current flowing, induced currents should disappear. Another potential issue is that of crossed neutrals where the neutral from another piece of equipment is inadvertently...
  4. PowerfulStuff

    High Resistance Grounding in 15kV Above Application?

    Hi All, One other really good reason to use solid grounding at higher voltages is that transformers can have graded insulation i.e. not much insulation at the neutral end of the winding and progressively increasing towards the line end of the winding. This generates a substantial saving and...
  5. PowerfulStuff

    Overexcited power transformer

    Sounds like a creative thought! If the transformer is operating above rated voltage (ignoring insulation problems) and hence saturated the core of the transformer there should be two consequences- 1/ The Magnetising impedance of the transformer would get much lower, drawing more reactive...
  6. PowerfulStuff

    Tripped Substation

    86ranger, Even with boots and gloves, there's always the potential to trip and get electrocuted. Are you able to supply an above ground earth grid (e.g. cyclone wire tied together with copper bonding conductors) to make it safe? Should the area be fenced off to ensure no-one is walking past when...
  7. PowerfulStuff

    fluctuations in voltage level feeding cement plant

    usamaegypt, I go along with the suggestion of adjusted UV time and propose - * UPS or batteries depending on the control supply. This should be quite inexpensive as you only have short periods of backup required. * Study of VSD parameters. Most newer VSD's have a ridethrough system where...
  8. PowerfulStuff

    fluctuations in voltage level feeding cement plant

    rcwilson, I wholeheartedly agree. I also worked on a cement plant with tripping issues and again a number of large, expensive options were being considered. By actually identifying the problem, it was possible to remove 90% of power related trips. Solutions were as simple as ridethrough...
  9. PowerfulStuff

    Over head line

    86ranger, Have you considered putting phase loss protection in as part of the motor control? Depending on your circumstances, this may be a lot cheaper. Regards, PowerfulStuff
  10. PowerfulStuff

    Failing 2 MW Siemens motor

    Hi, I have a 28 year old Siemens motor that appears to be on the way out. Motor vital statistics are- * 6.6 kV * Wound rotor * rod mill load Over the last few weeks a new rod mill was put in and charged to an aim of 100% motor load. While it is a bit odd to adjust a process based on the...
  11. PowerfulStuff

    Remedy for switching transients- 4160V motors

    Another though is the selection of vacuum contactor. I have read a bit on vacuum circuit breakers (but assume similar things for contactors). The contact faces are a hard material but with particles of soft metal (e.g. copper) embedded and made in a particular shape. The shape and copper mean...
  12. PowerfulStuff

    Oil furan Level standards (test ASTM D5837)

    Gauss2k, Another possible cause for the inconsistency is whether the insulation aging is a bulk or point phenomena. Rapid deterioration in a small section might give the gas ratio without a corresponding change in furans. I've been comparing furan results against paper samples for transformer...
  13. PowerfulStuff

    generator earthing

    keewee, Jumping across the Tasman Sea to Australia, some power distribution companies have it in their contracts that they must approve any standby generator connections. The distributors can be quite helpful is providing information as to what is required to the Australian/NZ standards and...
  14. PowerfulStuff

    Black start follow up

    Thanks for the replies. As a bit more information- #1 - the 32R relay has VT's and CT's between the generator and GSU transformer. It is set very low (0.015) to protect the steam turbine, but with a suprisingly long time (10s). My reason for picking inrush as a possible cause is that it...
  15. PowerfulStuff

    VFD rather than VSD

    Hi, Just one more from a long time ago - many older units were called VVVF drivess - Variable Voltage Variable Frequency. Probably more correct but I haven't seen it used for ages. Regards, Martin
  16. PowerfulStuff

    Black start follow up

    Hi, I've just been through a 'black start' on a 100MW islanded power station after boiler feed water was contaminated with mud! Anyway the power is back on but I had a few quick ones to see of anyone else has seen them before- 1/ During energisation of a new generator step up transformer the...
  17. PowerfulStuff

    Possible Electric Shock?

    Brian, What type of lighting load is it? I came across one where the 'standard' 10A lighting switch is actually rated for only 2A of inductive load (e.g. fluorescent or discharge lighting). The switch construction was only a small spring to open contacts and nothing to stop and arc from...
  18. PowerfulStuff

    Bus Bar Angle Through CT

    Hmm, an interesting one. I'd propose the opposite, but not out of certainty. In my simple thinking it has to work OK as the enclosed current does not change. As far as saturation is concerned, isn't saturation mainly related to the secondary circuit as the primary circuit has a significant...
  19. PowerfulStuff

    Generator Hunting

    Hi, My follow up question is one of terminology - when we talk about the response of generators, isn't the type of generator important? What I mean is, is it a piston engine, turbine or something else? On the question itself, I recently visited a site with multiple Caterpillar Diesels...
  20. PowerfulStuff

    Torque limit and Control

    maypot, I was (un)lucky enough to be run through the maths of vector drive operation. Simply put, it breaks the rotating magnetic field into two components at 90 degrees. One creates the effective flux in the stator and the other induces current in the rotor. The key is knowing where the...

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