Dear Group,
I've been asked to measure sequence
susceptances (B1, B0) on an overhead line. Basically there are only 2
measurements that I can in practice perform:
1) measure capacitance from each phase to ground
and
2) measure capacitance between phases.
So now I can convert from the capacitance to a susceptance at 50Hz and this
gives me 6 values of B.
B(a-e), B(b-e), B(c-e) and B(a-b), B(a-c), B)b-c).
How can I convert these into the sequence susceptances B1 and B0?
also how does disparity in the first group (to earth) or the second group
(between phases) effect the results.
Many thx
Danny
--
Daniel Indyk C.Eng C.Phys

Harder way (theoretically better)
You will have a Baa which will be the sum of the Bae,Bab,Bac values (sum of
admittances connected to the a line (earth as reference). Ditto for Bbb,
Bcc.
the resulting matrix will be
Baa -Bab -Bac
-Bba Bbb -Bbc Bab=Bba etc.
-Bca -Bcb Bcc
Chances are that the Bab is not Bac and Baa is not Bbb etc (too much to hope
for)
This is an abc matrix which can be transformed to a zpn matrix . The
result will not likely be diagonal and that is a lot of work. This is
generally true for both impedances and admittances and is a fact that is
generally ignored.
Easier way:
If the equivalence of full transposition is assumed, then simply take B1 as
the average of Baa ,Bbb Bcc values and a zero sequence value that is simply
the average of your Bae,Bbe and Bce values. The results will be good
enough- particularly for susceptances in a typical situation .
If the to earth results were the same and the between phases group were the
same, the matrix would be symmetrical and converting to sequence components
would result in a diagonal matrix - no coupling between sequences. This is
what is generally done in the usual calculation of sequence impedances. In
the case where the groups values differ, there will be coupling between
sequences - this is generally ignorable as indicated above. Where it is not-
then the advantage of sequence components is lost.

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