james.wilson Posted April 9, 2009 Share Posted April 9, 2009 star point is the mathamatical centre of the equilateral triangle formed by the phase points. Also called the neutral point. If the load of the 3 phases is balanced (should be...ish) then the star point/neutral point is dead in the middle and is at 0v. However if a phase or phases are not balanced the star point is (for want of a simple explanation) dragged away from centre thus either have more or less of a pd depending upon where it is dragged to. Hence the 'voltage' on the neutral. James securitywarehouse Security Supplies from Security Warehouse Trade Members please contact us for your TSI vetted trade discount. Link to comment Share on other sites More sharing options...
Rulland Posted April 9, 2009 Share Posted April 9, 2009 star point is the mathamatical centre of the equilateral triangle formed by the phase points. Also called the neutral point.If the load of the 3 phases is balanced (should be...ish) then the star point/neutral point is dead in the middle and is at 0v. However if a phase or phases are not balanced the star point is (for want of a simple explanation) dragged away from centre thus either have more or less of a pd depending upon where it is dragged to. Hence the 'voltage' on the neutral. James QFA-couldn't have put it better myself! Link to comment Share on other sites More sharing options...
james.wilson Posted April 9, 2009 Share Posted April 9, 2009 QFA-couldn't have put it better myself! lol i listened when we did ac theory securitywarehouse Security Supplies from Security Warehouse Trade Members please contact us for your TSI vetted trade discount. Link to comment Share on other sites More sharing options...
luggsey Posted April 9, 2009 Share Posted April 9, 2009 star point is the mathamatical centre of the equilateral triangle formed by the phase points. Also called the neutral point.If the load of the 3 phases is balanced (should be...ish) then the star point/neutral point is dead in the middle and is at 0v. However if a phase or phases are not balanced the star point is (for want of a simple explanation) dragged away from centre thus either have more or less of a pd depending upon where it is dragged to. Hence the 'voltage' on the neutral. James Hmmm, the star point on transformers is also earthed to a 'very' good earthing system meaning it's potential is always at earths potential. In other words unless there is a fault it's always 0v. More then that the neutral is bonded to earth at the transformers and at every (x)pole in the supply network. If the suppliers network neutral was to rise in potential by several volts then peoples bonding cables in houses and factories would [potentially] carry huge current and melt. I don't see how phase imbalance can 'drag' this from ground potential? baring a fault it's pretty much impossible unless I'm missing something? Are you talking about phase angles and out of phase current? I'm not taking the proverbial, just trying to understand your point. ----------------------------------------------------------------------------------------------------------------------------------- Life is like a box of chocolates, some bugger always gets the nice ones! My Amateur Radio Forum Link to comment Share on other sites More sharing options...
james.wilson Posted April 9, 2009 Share Posted April 9, 2009 Luggs Yes in uk distribution we do tie the star point to earth and you will usually find a fair bit of current flows through it at the tx due to these imbalaces. Also when a phase disconnects etc. However there will still be a voltage on the neutral at site end on an unbalanced system. My theory point would only be valid on isolated systems but is easier to understand. I also dont know much about 11kv and up as i have never worked on it, if you had or have a HV licence then you would be better at explaining this than me. James securitywarehouse Security Supplies from Security Warehouse Trade Members please contact us for your TSI vetted trade discount. Link to comment Share on other sites More sharing options...
luggsey Posted April 9, 2009 Share Posted April 9, 2009 LuggsYes in uk distribution we do tie the star point to earth and you will usually find a fair bit of current flows through it at the tx due to these imbalaces. Also when a phase disconnects etc. Doesn't 'all' current pass through the neutral? I'm still struggling with the imbalances you mention but now we are on to phase disconnects, I'm no HV engineer so you will have to explain that one to me! However there will still be a voltage on the neutral at site end on an unbalanced system. Unbalanced system, are you sure you don't mean a TT earth (earth stake) where there will always be a difference in potential between earth and neutral? My theory point would only be valid on isolated systems but is easier to understand. I also dont know much about 11kv and up as i have never worked on it, if you had or have a HV licence then you would be better at explaining this than me. The only HV I ever worked on was a test bed for 1KV water boilers for trains, it frightened the c##p out of me then and still does! Isolated system meaning TT earth maybe? James You have lost me James! ----------------------------------------------------------------------------------------------------------------------------------- Life is like a box of chocolates, some bugger always gets the nice ones! My Amateur Radio Forum Link to comment Share on other sites More sharing options...
james.wilson Posted April 9, 2009 Share Posted April 9, 2009 now im getting confused. I didnt mean unbalanced i meant isolated, god knows what i was thinking there. Im not meaning TT system no. As you can see the star point is calculated relative to earth. If you dont have an equal loading then your star point (theory) is nto in the middle. Hence you have the voltage. What effect on this tying the star point to earth has i dont know, but i would assume that the current has to flow somewhere to bring the star point back to centre. securitywarehouse Security Supplies from Security Warehouse Trade Members please contact us for your TSI vetted trade discount. Link to comment Share on other sites More sharing options...
luggsey Posted April 9, 2009 Share Posted April 9, 2009 now im getting confused.I didnt mean unbalanced i meant isolated, god knows what i was thinking there. Im not meaning TT system no. As you can see the star point is calculated relative to earth. If you dont have an equal loading then your star point (theory) is nto in the middle. Hence you have the voltage. What effect on this tying the star point to earth has i dont know, but i would assume that the current has to flow somewhere to bring the star point back to centre. I must have missed something along the line somewhere, the star point is earthed in your diagram so I'm baffled as to how it floats! It 'would' float without the earth connection 'relative' to earth and would then have different potential against a installation with it's own earth, ie. causing a voltage between N-E. Your theory can only work if there is no earthed star point, that's as much as my little brain can handle tonight! ----------------------------------------------------------------------------------------------------------------------------------- Life is like a box of chocolates, some bugger always gets the nice ones! My Amateur Radio Forum Link to comment Share on other sites More sharing options...
james.wilson Posted April 9, 2009 Share Posted April 9, 2009 its a relative earth but forget the earth for a moment. You can see the star point would move IF the currents were not equal. Like i said i dont know what effect have an unbalaced load would have on an earth star point. Maybe someone who knows mre about this could enlighten me? securitywarehouse Security Supplies from Security Warehouse Trade Members please contact us for your TSI vetted trade discount. Link to comment Share on other sites More sharing options...
esp-protocol Posted April 9, 2009 Share Posted April 9, 2009 I think redbull is thinking the starpoint is a calculated figure, that changes relative to the current drawn in each of the three phases. Whereas it really is a physical point of the three sets of windings that is then tied down to earth.... in a simplistic view......!! Link to comment Share on other sites More sharing options...
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