ABB Relion REG670, Technical Manual

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corresponding value for Ur_W1 will be calculated and used in the above 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). 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
IdMin , will be increased by the set
range of the load tap changer. Alternatively the 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
IDiffAlarm a delay on pickup
timer is started. When the pre-set time, defined by setting parameter tAlarmDelay , has
expired the differential current alarm 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
1 and equation 2 . 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
1MRK502052-UEN B
Section 6
Differential protection
121
Technical manual