CHAPTER 8: COMMISSIONING DIFFERENTIAL CHARACTERISTIC TEST EXAMPLES
T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 8-3
8
The two tests can be repeated for Phases B and C.
Figure 8-1: Differential restraint characteristic
8.2 Differential characteristic test examples
8.2.1 Overview
The T35 commissioning tests are based on secondary current injections, where two or three individually adjustable
currents are required. The differential protection compares the magnitudes of the varying HV and LV currents in real time.
Therefore, the test set currents and their angles must be an exact replica of the HV and LV currents and angles shown in
the diagrams, along with the correct CT polarity and orientation.
Ensure that the thermal rating of the relay current inputs is not exceeded. Stopping the injection of the currents to the relay
by using contact outputs triggered by protection operation can prevent this from occurring.
Due to the complexity of the mathematics defining the operating characteristic of the region between Breakpoint 1 and 2,
the use of a factory-supplied Microsoft Excel simulation utility is highly recommended. This utility indicates graphically
whether the relay should operate, based on the settings and winding current injection. This allows the tester to define and
confirm various points on the operating characteristic. The spreadsheet can be found at GE Grid Solutions website (look for
the T35/T60 Percent Differential Element Simulator in the support documents for the product).
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