ABB RELION 670 Series Technical Manual page 131

1MRK 502 066-UUS B
mentioned equations. By doing this, complete on-line compensation for load tap changer
movement is achieved. Differential protection will be ideally balanced for every load tap changer
position and no false differential current will appear irrespective of actual load tap changer
position.
Typically the minimum differential protection pickup for power transformer with load tap changer
is set between 30% to 40%. However with this load tap changer compensation feature it is
possible to set the differential protection in the IED more sensitive with a pickup value of 15% to
20%.
Load tap changer position is measured within the IED by Tap changer control and supervision,
(TCLYLTC ,84). Within this function block, the load tap changer position value is continuously
monitored to insure its integrity.
When any error in the load tap changer position is detected an alarm is given. This signal shall be
connected to the OLTCxAL input of the differential function block. While OLTCxAL input has a
logical value of one the differential protection minimum pickup, originally defined by setting
parameter
differential current alarm feature can be used to alarm for any problems in the whole load tap
changer compensation chain.
It shall be noted that:
• two-winding differential protection in the IED can on-line compensate for one load tap
changer within the protected power transformer
• three-winding differential protection in the IED can on-line compensate for up to two load tap
changers within the protected power transformer
Differential current alarm
Fundamental frequency differential current level is monitored all the time within the differential
function. As soon as all three fundamental frequency differential currents are above the set
threshold defined by setting parameter
pre-set time, defined by setting parameter
is generated and output signal IDALARM is set to logical value one. This feature can be effectively
used to provide alarm when load tap changer position compensation is used and something in the
whole compensation chain goes wrong. This alarm can be as well used with some additional IED
configuration logic to desensitize the differential function.
Bias current
The bias current is calculated as the highest current amongst all individual winding current
contributions to the total fundamental frequency differential currents, as shown in equation and
equation . All individual winding current contributions are already referred to the power
transformer winding one side (power transformer HV winding) and therefore they can be
compared regarding their magnitudes. There are six (or nine in the case of a three-winding
transformer) contributions to the total fundamental differential currents, which are the
candidates for the common bias current. The highest individual current contribution is taken as a
common bias (restrain) current for all three phases. This "maximum principle" makes the
differential protection more secure, with less risk to operate for external faults and in the same
time brings more meaning to the breakpoint settings of the operate - restrain characteristic.
It shall be noted that if the zero-sequence currents are subtracted from the separate contributions
to the total differential current, then the zero-sequence component is automatically eliminated
from the bias current as well. This ensures that for secondary injection from just one power
transformer side the bias current is always equal to the highest differential current regardless of
Technical manual
IdMin , will be increased by the set range of the load tap changer. Alternatively the
IDiffAlarm a delay on pickup timer is started. When the
tAlarmDelay , has expired the differential current alarm
Section 6
Differential protection
M13039-436 v6
M13039-179 v5
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