ABB RELION REX640 Technical Manual page 639

1MRS759142 C
REX640
Technical Manual
fault in the network is seen as differential current unless the zero-sequence component
is correctly eliminated.
The zero-sequence elimination is done with the Zro A elimination setting. The
recommended rules for this setting are:
• For power transformer wye-connected windings with earthed neutral point, Zro
A elimination must be activated.
• For power transformer wye-connected windings with isolated neutral, and for
delta-connected windings, Zro A elimination is not activated unless there is a
separate earthing transformer inside the protected zone (that is, between the
power transformer winding and CTs).
• It does no harm to activate Zro A elimination even if it is not needed.
Compensation of tap changer position
The position of the tap changer used for voltage control can be compensated according
to the actual tap changer position. The position information is provided to the
protection function through the tap changer position indication function TPOSYLTC.
Typically, the tap changer is located within the HV winding, that is, winding 1 of the
power transformer. The Tapped winding setting parameter specifies whether the tap
changer is connected to winding 1, winding 2 or winding 3. The Tapped winding
setting parameter is also used to enable or disable the automatic adaptation to the tap
changer position. The possible values are "Not in use", "Winding 1", "Winding 2",
"Winding 3".
There can be only one tap changer in the transformer, that is, the
function can take into account and compensate only for one tap
changer.
The Tap nominal setting parameter provides the number of the tap which results in the
nominal voltage and current. When the current tap position deviates from this value,
the input current values where the tap changer resides are scaled to match the currents
on the other side.
A correct scaling is determined by the number of steps, the direction of the deviation
from the nominal tap and the percentage change in the voltage resulting from a
deviation of one tap step. The percentage value is set via the Step of tap setting.
The operating range of the tap changer is defined by the Min winding tap and Max
winding tap settings. The Min winding tap setting gives the tap position number
resulting from the minimum effective number of winding turns on the side of the
transformer where the tap changer is connected. Correspondingly, the Max winding
tap setting gives the tap position number resulting from the maximum effective
number of winding turns.
The Min winding tap and Max winding tap settings help the tap position compensation
algorithm find the direction of the compensation. This also ensures that if the current
Section 4
Protection functions
633