7 CHECKING THE DIFFERENTIAL ELEMENT
Testing of the differential element during commissioning is not necessary unless explicitly requested.
T
o avoid spurious operation of any other protection elements, all protection elements except the transformer
differential protection should be disabled for the duration of the differential element tests. This is done in the
product’s CONFIGURATION column. Make a note of which elements need to be re-enabled after testing.
The following tests demonstrate correct operation of the differential protection. Testing should be performed on
individual current inputs of each phase and on each winding, but note that the timing test should only be
performed on the high voltage winding. Testing the low-set element is sufficient to test the correct operation of the
differential protection.
Using a suitable current injection method, slowly increase the current from 0 Amps and note the pick-up value at
which the element operates. Reduce the current slowly and note the drop-off value at which it resets. Check that
the pick-up and drop-off are within the range shown in the table below.
Current level
Pick-up 0.90 x In to 1.1 x In
Drop-off 0.90 x pick-up to 1 x pick-up
where In = Is1/(amplitude matching factor)
and Is1 is the low set setting which can be found in the Is1 cell in the DIFF PR
OTECTION column.
The amplitude matching factor is used to compensate for a mismatch in currents due to the line side current
transformer ratios. There is one amplitude matching factor for the high-voltage side, one for the low-voltage side
and one for the tertiary-voltage side. You will find these in the SYSTEM CONFIG column. Use the appropriate
amplitude matching factor to calculate the required injection current. This depends on whether it is being injected
into the HV, LV, or TV current transformer inputs.
1. Whilst connected to the HV windings, connect the output contacts for the low set differential protection
function to trip the test set and also to stop a timer.
2. Configure the test set so that when the current is applied, the timer starts.
3. Inject 5x In into the HV CT input.
4. Check that the element operates within the range of 30 ms to 35 ms and record the time.
Note:
If this test has been successfully performed there is no need to carry out the tests described in the protection timing checks
section.
7.1 USING THE OMICRON ADVANCED MODULE
In software versions 05 and onwards, transient bias can be disabled or enabled. In software version 06 onwards,
the CT Saturation f
unction was separated from the No gap function.
Testing Differential Operating Characteristics
When testing the differential operating characteristic, it is important to first disable transient bias. The CT
saturation and No Gap settings may be either disabled or enabled. An example is shown below:
Chapter 19 - Commissioning Instructions P64x
428 P64x-TM-EN-1.3
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