Test Example 1 - GE T60 Instruction Manual

DIFFERENTIAL CHARACTERISTIC TEST EXAMPLES
Consider the above system, which illustrates the importance of CT orientation, polarity, and relay connection. These
factors also apply when performing the tests outlined in the next examples.
The transformer high voltage (HV) and low voltage (LV) side fault currents, and angles are all related. More specifically, the
HV and LV primary fault currents are displaced by 180°. The CT polarity marks point away from the protected zone and are
connected to the ~a terminals of the relay. The displayed current is what is reported by the relay.
The ~a and ~b terminal identifications are illustrative only. See the CT/VT Modules section in Chapter 3 for specific
terminal identification.

8.2.2 Test example 1

8.2.2.1 Overview
Transformer data — 20 MVA, 115/12.47 kV, CT (HV) = 200:1, CT (LV) = 1000:1, Y/y0° with a grounded LV neutral
Test set configuration
8
The fault current distribution for an external b-c fault is identical for the HV and LV transformer sides and can be simulated
easily with two current sources. Connect the first current source to the relay Phase "B" and "C" terminals, corresponding to
the HV winding CTs in series, and the second source to the Phase "b" and "c" relay terminals, corresponding to the LV CTs.
Ensure that the polarity is correct and the relative phase angles are similar to that shown in the figure, that is, 180°
between IB and IC, 180° between Ib and Ic, 180° between IB and Ib, and 180° between IC and Ic. Follow the magnitudes
and angles of the injected currents from the following tables to ensure that the test performs correctly
Operating criteria
The differential element operates if the differential current (I
restraint current magnitude (I
largest compensated current. Compensation refers to vector and magnitude corrections applied to the currents from the
HV and LV transformer sides.
8-4
Figure 8-2: Current distribution on a Y/YG0° transformer with b-c fault on LV side
). The differential current I
r
) exceeds the characteristic defined by the relay settings for
d
is the vector sum of the compensated currents, and I
d
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: COMMISSIONING
is the
r