ABB ret650, Technical Manual

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proportional bias, which makes the protection operate for a certain percentage
differential current related to the current through the transformer. This stabilizes the
protection under through fault conditions while still permitting the system to have
good basic sensitivity. The following chapters explain how these quantities are
derived.
6.1.4.2 Fundamental frequency differential currents
The fundamental frequency differential current is a vectorial sum (sum of
fundamental frequency phasors) of the individual phase currents from the different
sides of the protected power transformer.
Before any differential current can be calculated, the power transformer phase shift,
and its transformation ratio, must be accounted for. Conversion of all currents to a
common reference is performed in two steps:
• all current phasors are phase-shifted to (referred to) the phase-reference side,
(whenever possible the first winding with star connection)
• all currents magnitudes are always referred to the first winding of the power
transformer (typically transformer high-voltage side)
The two steps of conversion are made simultaneously on-line by the pre-programmed
coefficient matrices, as shown in equation
1 for a two-winding power transformer, and
in equation 2 for a three-winding power transformer.
These are the internal compensation algorithms within the differential
function. The protected power transformer data is always entered per
its nameplate. The Differential function will adapt nameplate data and
select proper reference windings.
1 1_ 1 1_ 2
_ 2
2 2 _ 1 2 _ 2
_ 1
3 3 _ 1 3 _ 2
IDL IL W IL W
Un W
IDL A IL W B IL W
Un W
IDL IL W IL W
é ù é ù é ù
ê ú ê ú ê ú
= × + × ×
ê ú ê ú ê ú
ê ú ê ú ê ú
ë û ë û ë û
1 2 3
EQUATION1880 V1 EN
(Equation 1)
where:
1.
is the resulting Differential Currents
2. is the current contribution from the W1 side
3. is the current contribution from the W2 side
Section 6
1MRK 504 135-UEN A
Differential protection
88
Technical manual