7PG21 Solkor Rf Description of Operation
©2011 Siemens Protection Devices Limited Chapter 1 Page 13 of 13
4 Theory of Summation Transformer
The main purpose of the summation transformer is to enable either balanced or unbalanced three phase currents
to be re-produced as a single phase quantity. This makes it possible in a feeder protection to compare the various
fault currents on a single phase basis over only two pilot cores. As this device is essentially a transformer it can
also be used to reduce the burden imposed by the pilot circuit on the current transformers by changing the
impedance levels. In addition, it provides isolation between the current transformers and the pilot circuit and
makes it possible to have the current transformers earthed and the pilots unearthed.
R
Y
B
N
Primary
nx
turns
x
x
SUMMATION TRANSFORMER
Fault Type Effective Primary Ampere-turns Relative Output
R-E I(nx + x + x) = Ix. (n+2) n+2
Y-E I(nx + x) = Ix. (n+1) n+1
B-E I(nx) = Ix. (n) n
R-Y I(x) = Ix. (1) 1
Y-B I(x) = Ix. (1) 1
B-R I(2x) = Ix. (2) 2
3P I(√3x) = Ix. (√3) √3
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