GE 745 - 7.5.15 Negative-sequence Time Overcurrent; 7.5.15.1 Description; 7.5.15.2 Winding 1 Element

7–40 745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION SCHEMES CHAPTER 7: COMMISSIONING
phase input CTs. Refer to Time Overcurrent Curves on page 5–59 for additional information
on timing curves.
If the relay elements are set for “Linear” reset characteristic when measuring the operating
times, ensure that there is sufficient time between test current injections for the element to
reset fully. Otherwise, erroneous timing measurements will be obtained. Settings for these
elements are found in the
S4 ELEMENTS  NEG SEQ OC  W1 NEG SEQ TIME OC settings
menu.
7.5.15.2 Winding 1 Element
To ensure that only the negative-sequence time overcurrent element operates the trip
relays (and any other output relays selected by the logic),
 Disable all protection features except negative-sequence time
overcurrent.
 Use the general test setup shown in FIGURE 7–9: General test setup on
page 7–25.
 Connect the current supply to terminals X = H1 and Y = G1 to test the
winding 1 negative-sequence element.
 Monitor the appropriate output relays as per the relay FlexLogic™
settings or assigned relay settings.
7.5.15.3 Pickup Level
 With the interval timer disabled, apply the current signal and slowly
increase its magnitude until the trip relay and all selected auxiliary
relays operate.
If the relay 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 operate level.
 Slowly reduce the current below the operate level and observe for a
reset action on the trip relay.
This reset level for the current should be approximately 98% of the
pickup level.
 Once the relay drops out, slowly increase the current until the trip
contact closes. The operate level should correspond to the
S4 ELEMENTS
 NEG SEQ OC  W1 NEG SEQ TIME OC  W1 NEG SEQ TIME OC
PICKUP
setting.
NOTE
Note
With current applied to a single phase, the negative sequence current component is
calculated from:
(EQ 7.12)
Hence, the phase current will be three times the pickup setting.
 Check that, when the element operates, the Trip and Pickup LEDs are
on, and one of the following messages is displayed:
I
neg seq
1
3
---
I
phase
×=