CHAPTER 7: COMMISSIONING PROTECTION SCHEMES
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 7–39
Inject the I
1
current such that the ground differential pickup value
divided by the I
1
current is less than the slope setting. Set I
2
= 0 A.
The element will operate since the current appears as ground
differential.
The slope is calculated from the values of I
ground differential
and I
max
as
shown below:
(EQ 7.11)
where I
max
represents the maximum phase current for the
winding being measured.
As I
2
is increased, the element will reset when the percentage of
slope drops below the slope setting.
Slowly increase I
2
until the element operates again.
Calculate the slope at this point.
Decrease the slope setting to 0% then continue to increase the I
2
current
until the element resets.
Slowly increase I
2
until the element operates again.
The reset level should be 97% of operate level. When the element resets, the Trip and
Message LEDs should remain on if the
W1 RESTD GND FAULT TARGET was selected as
“Latched”. Otherwise only the Trip LED should remain on.
7.5.14.6 Winding 2 or 3 Elements
Since the second restricted ground fault element can be set with completely different
parameters than the first element winding, it is necessary to repeat the full set of tests
described in this section for each winding.
To test the second element,
Disable all protection elements except for the winding 2 (or winding 3
as appropriate) restricted ground fault element.
Connect the ground current signal to terminals F12 and E12.
Repeat all the tests described for the winding 1 element in this section.
Note
The blocking from logic input, if enabled, can be tested as described in earlier tests for
other elements.
7.5.15 Negative-sequence Time Overcurrent
7.5.15.1 Description
This procedure verifies that the negative-sequence time overcurrent performance
matches the in-service settings. Since these elements can have any one of a multitude of
timing curves, a table of expected operating times versus applied current should be
prepared prior to testing the elements. The negative-sequence element measures the
derived negative-sequence component of the phase current signals connected to the
%slope
I
ground differential
I
max
------------------------------------------
100%×=
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