CHAPTER 7: COMMISSIONING PROTECTION SCHEMES
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 7–29
7.5.7 Neutral Time Overcurrent
7.5.7.1 Description
This procedure verifies that the neutral 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 neutral element measures the derived zero-sequence current signal as an
input. Refer to Time Overcurrent Curves on page 5–59 for information on timing curves.
If the relay elements are set for the “Linear” reset characteristic when measuring the
operating times, ensure there is sufficient time between test current injections for the
element to reset fully. Otherwise, erroneous timing measurements will be obtained.
The settings for these elements are found under the
S4 ELEMENTS NEUTRAL OC
setpoints menu. Note that there can only be one or two Neutral Time Overcurrent
elements in service at the same time.
7.5.7.2 Winding 1 Element
To ensure that only the neutral time overcurrent element under test operates the trip
relays (and any other output relays) selected by the logic, disable all protection features
except neutral time overcurrent. Use the general test setup shown in FIGURE 7–9: Ge n e r a l
test setup on page 7–25.
Connect the current supply to terminals X = H1 and Y = G1 to test the winding 1 neutral
element. Monitor the appropriate output relays as per the relay FlexLogic™ settings or
assigned relays from phase IOC2 page.
7.5.7.3 Pickup Level
With the interval timer disabled, apply the current signal and slowly
increase its magnitude until the trip relay (and all the selected auxiliary
relays) operate.
If the relay under test has a very inverse time characteristic, it is easier and more accurate
to increase the current far above the pickup level until the trip relay operates, then reduce
the current to just above the expected operate level.
Slowly reduce the current below the operate level and observe for a
reset action on the trip relay.
This current reset level should be approximately 98% of the pickup
level setting. Once the relay drops out, slowly increase the current until
the trip contact closes. The operate level should correspond to the
S4
ELEMENTS
NEUTRAL OC W1 NTRL TIME OC W1 NEUTRAL
TIME OC PICKUP
setpoint: Since current is being introduced into one
phase only, the input current signal is equal to the 3I
o
signal used by the
element.
When the element operates, check that the Trip, Pickup, and Phase
LEDs are on and one of the following messages is displayed:
LATCHED a: W1 Ntrl Time OC or OPERATED a: W1 Ntrl Time OC
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