faulty meter in Sub
faulty meter in Sub
(OP)
The volt meters in a lot of our subs meter only the red and blue phases and calculate the white phase. These are old electro-mechanical meters not electronic or transducer types. We lost white phase but the meter continued to give a soild (although not quite normal) reading. On paper we have looked at the possiblity of the red and blue phases being wired incorrectly but it does not appear to work. How would the meter appear to work correctly when everything is ok and then show volts on white phase when it was dead?






RE: faulty meter in Sub
RE: faulty meter in Sub
RE: faulty meter in Sub
RE: faulty meter in Sub
RE: faulty meter in Sub
Aaron connection will show:
kWhr=kWhr1 + kWhr2=Ea x Ia + Eb x Ib + Ec x Ic=Reading1 + Reading2=kWhra + kWhrb + kWhrc + kWhrcoil1 + kWhrcoil2
where
kWhrcoil1 is an energy consumption of kWhr1 potential coil
and
kWhrcoil2 is an energy consumption of kWhr2 potential coil
RE: faulty meter in Sub
It’s not entirely clear from the description, but comments here are based on a scenario of a single ammeter and a three- (or four-)position ammeter switch.
See page 28 in http://www.geindustrial.com/products/brochures/sbcat.pdf Figures 11 and 12 deal with an ammeter switch with overlapping contacts to permit two CTs to monitor each phase in a 3ø 3-wire circuit, depending on switch position. In the problem case, it may be that the states represented in the contact logic has changed yielding the false reading. [The two-CT and ammeter-switch application will not work correctly in a 4-wire circuit with any neutral current.]
RE: faulty meter in Sub
RE: faulty meter in Sub
RE: faulty meter in Sub
As far as voltages are concerned, the same principle applies for the open delta PT connection, where the third phase-phase voltage is not measured directly but is inferred from the vector sum of the two measured phases.
RE: faulty meter in Sub
I am a trainee protection tester and have been asked to do some research on this. Ther powers that be basically want to know if there is a fundamental error in metering the current this way or if there is a problem at this particular substation.
Zero sequence current should not be caused by an open circuit on the white phase and when i draw a circuit diagram and try to follow it, it appears the white phase should read zero but i can't figure out why it did not.
RE: faulty meter in Sub
Smeaton’s 1977 Switchgear and Control Handbook §14 13-14 contains basic information similar to the GE reference. For a 3-wire circuit, the middle phase current is measured by connecting the ammeter to the polarity terminals of CT secondaries on the two outer phases, with the non-polarity terminals of the CT secondaries connected together.
Common sense and the usual CT-handling procedures apply.
RE: faulty meter in Sub
RE: faulty meter in Sub
RE: faulty meter in Sub
RE: faulty meter in Sub
As you pointed out when added using complex numbers or vectors the white phase current should show zero but it did not and it was not significantly off normal opperating load. We already calculated the 1.73 factor if the red and blue phase are added, this was ruled out by one of the senior testers.
as i said as soon as i can get the relevant information i will post it. your help has been much appreciated.
RE: faulty meter in Sub
I apologize for the mistake, but I thought the installation contained one ammeter, an ammeter switch and two CTs on the outer phases. Are there 3 CTs involved?