Protection Schemes
745
Transformer Management Relay
Commission
ing
http://www.GEindustrial.com/multilin
7–21
GE Multilin
Neutral Time
Overcurrent
a) 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
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.
b) 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: General Test Setup on page 7–18.
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.
c) Pickup Level
1. 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.
2. 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 3
I
o
signal used by the element.
3. 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
4. Reduce the current until the element resets. The reset level should be 97% of
the operate level. When the element resets, the Trip and Message LEDs should
remain on if the
W1 NEUTRAL TIME OC TARGET
was selected as “Latched”.
Otherwise only the Trip LED remains on.
5. Reset indicators and clear messages.
d) Operating Time
Using a table like the one shown below, select three (3) or four (4) values of current
multiples at which timing is to be measured. Enter the expected operating times
from the timing curve applied in the settings. Using the setup in FIGURE 7–9:
General Test Setup on page 7–18 and the Interval Timer enabled, set the current
level to the desired value and apply suddenly by closing the double-pole switch.
Record the operate time and compare to the expected value. Repeat for all desired
values of current.
Summary of Contents for Multilin 745
Page 2: ......
