Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 2 replies
  • Subscribers 27 subscribers
  • Views 163 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

How can you determine by measurement if you have two separate sources if supply?

Former Community Member
Former Community Member
Hi everyone, I am looking for some answers for the following situation:


There are two identical three phase step-down transformers 11kV to 0.4kV, connected to a 11kV ring main, each have an UPS system connected downstream and both are connected to the same Automatic Transfer Switch:

TX1 → UPS1 → ATS SOURCE 1

TX2 → UPS2 → ATS SOURCE 2


I want to plug in a parallel supply ciruit in the ATS for source 1 (considering the new and existing circuit would be live) and I want to double check by measurement that the new circuit is not connected to another source by mistake.


I have measured at the ATS the voltage and frequency on the same source, U=400V / Uo=230V / 50Hz and I wasn't expecting any voltage between the two separate sources but this is what i found:

TX1/ L1 - TX2/ L1 = 2V / 700-800Hz

TX1/ L1 - TX2/ L2 = 400V / 50Hz

TX1/ L1 - TX2/ L3 = 400V / 50Hz


TX1/ L2 - TX2/ L1 = 400V / 50Hz

TX1/ L2 - TX2/ L2 = 2V / 700-800Hz

TX1/ L2 - TX2/ L3 = 400V / 50Hz


TX1/ L3 - TX2/ L1 = 400V / 50Hz

TX1/ L3 - TX2/ L2 = 400V / 50Hz

TX1/ L3 - TX2/ L3 = 2V / 700-800Hz


From the readings above I can see that the different frequency between TX1/ L1 and TX2/ L1 etc is an indicator of a different source of supply.


Is my assumption correct? 

Why is there a voltage between the two sources of supply if there is no physical connection due to the internal windings in the transformer?

Why is there a highly fluctuating frequency between the same lines of the two sources?


Thank you in advance!


Andrei Ionescu
Parents
  • 0
    Of course you will get a potential difference between the two sources as the neutrals will be linked which, by the way, must be kept intact with UPS systems to prevent floating neutrals - 3 pole switching only.

    The 700-800Hz seems like the PCM frequency of the UPS invertors and is being imposed on the line albeit at a very low level, I think that is quite normal.

    Not sure about the rationale of paralleling another source alongside the existing source to one leg of the ATS, sounds a bit dodgy to me.
Reply
  • 0
    Of course you will get a potential difference between the two sources as the neutrals will be linked which, by the way, must be kept intact with UPS systems to prevent floating neutrals - 3 pole switching only.

    The 700-800Hz seems like the PCM frequency of the UPS invertors and is being imposed on the line albeit at a very low level, I think that is quite normal.

    Not sure about the rationale of paralleling another source alongside the existing source to one leg of the ATS, sounds a bit dodgy to me.
Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines