ABB Relion 620 Series - Page 523

always removed from both sides automatically. The Zro A elimination parameter
cannot change this.
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
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
separately selected.
By using the Zro A elimination parameter, the zero-sequence component of the
phase currents is calculated and reduced for each phase current:
I I x I I I
I I x I I I
I I
L m L L L L
L m L L L L
L m L
1 1 1 2 3
2 2 1 2 3
3 3
1
3
1
3
= − + +
( )
= − + +
( )
= −− + +
( )
1
3
1 2 3
x I I I
L L L
(Equation 73)
In many cases with the earthed neutral of a "wye" winding, it is possible
to make the compensation so that a zero-sequence component of the
phase currents is automatically eliminated. For example, in a case of
a "Ynd" transformer, the compensation is made on the winding 1 side
to automatically eliminate the zero-sequence component of the phase
currents on that side (and the "d" side does not have them). In those
cases, explicit elimination is not needed.
Compensation of tap changer position
The position of the tap changer used for voltage control can be compensated and
the position information is provided for the protection function through the tap
position indication function TPOSYLTC.
Typically, the tap changer is located within the high voltage winding, that is,
winding 1, of the power transformer. The Tapped winding parameter specifies
whether the tap changer is connected to the high voltage side winding or the
low voltage side winding. This parameter is also used to enable and disable the
automatic adaptation to the tap changer position. The possible values are "Not in
use", "Winding 1" or "Winding 2".
The Tap nominal parameter tells the number of the tap, which results in the nominal
voltage (and current). When the current tap position deviates from this value, the
1MRS757644 H
Protection functions
620 series
Technical Manual
523