ABB RELION REX640, Technical Manual, Page: 660 / 1960

If
Clock number is “Clk Num 1”, “Clk Num 5”, “Clk Num 7” or “Clk Num 11”, the vector
group matching is done on one side only. A possible zero-sequence component
of the phase currents at earth faults occurring outside the protection area is
automatically eliminated in the numerically implemented delta connection before
the differential current and the biasing current are calculated. This is why the vector
group matching is almost always made on the star connected side of the “Ynd” and
“Dyn” connected transformers.
If
Clock number is “Clk Num 0” or “Clk Num 6”, the zero-sequence component of
the phase currents is not eliminated automatically on either side. Therefore, the
zero-sequence component on the star connected side that is earthed at its star
point has to be eliminated by using the Zro A elimination parameter.
The same parameter has to be used to eliminate the zero-sequence component
if there is, for example, an earthing transformer on the delta-connected side of
the “Ynd” power transformer in the area to be protected. In this case, the vector
group matching is normally made on the side of the star connection. On the side
of the delta connection, the elimination of the zero-sequence component has to be
eliminated by using the Zro A elimination parameter.
By using the Zro A elimination parameter, the zero-sequence component of the local
phase currents is calculated and reduced for each phase current:
I I I I I
LOC A LOC A LOC A LOC B LOC C
− − − − −
= − × + +






_ _ _ _ _
1
3
(Equation 152)
I I I I I
LOC B LOC B LOC A LOC B LOC C
− − − − −
= − × + +






_ _ _ _ _
1
3
(Equation 153)
I I I I I
LOC C LOC C LOC A LOC B LOC C
− − − − −
= − × + +






_ _ _ _ _
1
3
Fail safe function
To prevent malfunction during communication interference, the operation of
LNPLDF is blocked when the protection communication supervision detects severe
interference in the communication channel. The timer reset stage is activated in
case the stabilized stage is started during a communication interruption. The
protection communication supervison is connected internally from PCSITPC to
LNPLDF (dotted OK line).
Protection functions 1MRS759142 F
660 REX640
Technical Manual