GE 745 Instruction Manual page 247

CHAPTER 7: COMMISSIONING
7.4 Metering
7.4.1 Description
7.4.2 Current Inputs
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Accuracy of readings taken in this section should be compared with the specified
accuracies in Specifications on page 2–5. If the measurements obtained during this
commissioning procedure are 'out-of-specification' verify your instrumentation accuracy.
If the errors are truly in the relay, advise the company representative.
The general approach used to verify the AC current inputs is to supply rated currents in all
the input CTs. Displayed readings will then confirm that the relay is correctly measuring all
the inputs and performing the correct calculations to derive sequence components,
loading values, etc. Since the displayed values are high-side values, you can use this test to
verify that the CT ratios have been correctly entered.
 If you are using a single phase current supply, connect this current signal
to all the input CTs in series, winding 1, 2 and 3, if using a three-
winding configuration, and the ground CT input(s). Adjust the current
level to 1 A for 1-amp-rated relays and to 5 A for 5-amp-rated relays.
Some elements may operate under these conditions unless all
elements have been disabled!
 With the above current signals ON, read the actual values displayed
under
The actual values can be quickly read using the EnerVista 745 Setup
software.
 Read the RMS magnitude and the phase of the current signal in each
phase of each winding.
Note that the Winding 1 Phase A current is used as the reference for
all angle measurements.
I = I
phase rms displayed phase input
The phase angle will be 0° for all phase currents if the same current is injected in all phase
input CTs. Sequence components will be:
I aI
+
a
I CT Ratio × --------------------------------- -
=
1
2
I a
+
a
--------------------------------- -
I CT Ratio ×
=
2
I CT Ratio input current
=
zero-sequence
I 3 × CT Ratio input current
=
neutral
I Ground CT Ratio input current into Ground CT
=
ground
A2 METERING CURRENT.

x CT ratio for that winding
2
a I
+
b c
0, all 3 currents in phase, and a
=
3
I aI
+
b c
0, the 3 currents are in phase
=
3
×
×
×
METERING
∠ 120°
=
(EQ 7.1)
7–9