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Line impedance query

me1899
Hi 


I am looking at a job where a supply to a replacement CT scanner has to be calculated.


The current scanner is fed by a cable with line impedance 0.06 at source end of the sub panel that feeds it.


The new scanner must have a line impedance no more than 0.06 so basically I can't use the way on the current panel.


The client engineer has said to run from the intake and provide a cable calc to prove the line impedance is OK for the new scanner feed (this is D and B).


I am using AMTECH and was wondering if there was a way to output line impedance or if I need to show a calculation another way?


The client engaged a firm to do line impedance tests, and their report shows values measured between L1-L2, L2-L3 and L1-L3, but looks like they did testing at the sub panel and not at the intake.


This is basically like PSCC right? So I can get PSCC from AMTECH, divide by root 3 and work out impedance from there?


Am I on the right track? Assistance would be much appreciated.


Regards


Anisur
  • 0
    Hi All


    As an update, looking at the line impedance readings between phases, we instead took measure L - N voltage and divided by corresponding PSCC to get impedance of each phase. That gave us enough play to propose a 95mm 5 core of sub 30m length.


    The manufacturer and main consultant have been advised and give a copy of cable calcs. I note the manufacturer asked for CPC to at least match phase conductor size, so we put in a 5 core and that gets less than 0.06 for Zs.


    From site we also confirmed that the feeding board is genny backed so we have asked the question of the main consultant seeing as the unit is not due to be supplied with a UPS - just waiting an answer from them.


    Sorry for slow update, but been bogged down.


    Hope this will be OK now and all relevant parties will be satisfied.


    I really appreciate and hugely grateful for all the valuable contributions that have been given to this question.
  • Former Community Member
    0 Former Community Member
    Couple of points:


    1 - do you need generator back up to the scanner - they are an aggressive load and you need to be careful the generator can actually close onto the load step. Some are supported (as front line diagnostic tools) some are unsupported as they aren't used in emergency response


    2 - In terms of the system controls, they won't survive the blackout between LoM and Generator start - up to 15 seconds is permitted. So, you may need a small UPS to keep the "brains" functioning before the generator supplies bulk power. This UPS is usually part of the equipment supplier package


    Regards


    OMS
  • 0
    Thanks OMS, some responses below:


    1 - We have asked question about the generator of the main consultant - it was never mentioned in the tender and at the minute I have no idea of the genny size or anything like that, The main consultant has to guide us as to whether we look to stop the scanner coming on genny or if we go for it. I'll post updates as I get them.


    2 - Sorry, should have said, the control panel has a little UPS provided by the manufacturer, but not the scanner itself.
  • Former Community Member
    0 Former Community Member
    OK - sounds like you have this in hand via the design consultant


    Good luck - and make sure the services penetrations don't prejudice the shielding of the scanner room


    Regards


    OMS
  • 0
    Hi all, just an update FYI. The main consultant came back after speaking himself with the manufacturer and said:

    "Yes I spoke to them and they confirmed that they require 0.06 of a ohm on each conductor. i.e no one conductor can have an impedance of greater than 0.06 of an ohm, from transformer to there unit.

    In relation to the generator they are more concerned about how quick it turns on as the capacitors have to discharge and re energise, I would ask XXX to formally advise you on what the time is from mains failure to generator taking up load and of course is the existing CT on the generator."


    From that I can work out line impedance for a conductor at the source by dividing L-N voltage by L-N fault current and then add on the submain conductor impedance.


    On Genny, a bit more digging required.
  • Former Community Member
    0 Former Community Member
    If you look at HTM 06-01, the generator should be on line within 15 seconds of a LoOP event (Loss of Offsite Power) ie both primary and secondary HV have failed (or you want the set idling if primary has failed just in case secondary follows it


    It is entirely feasible to delay the circuit breaker feeding the CT for longer  - eg, you open the CB on loss of supply with a spring recharge on the breaker, and then when it detects generator volts, it counts for a pre set time before closing - you may need this anyway to manage the load step


    Sounds like said consultant doesn't really know what he wants and is making this up on the hoof


    Regards


    OMS
  • 0
    Thanks OMS, all fun and games


    Don't think the guy wants to be involved - sounds like his fee is very small.


    At least the line impedance thing is sorted - I will raise an RFI / proposal to them on the genny bits collating all the bits, as it's getting fragmented

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