GE 745 Instruction Manual page 264

PROTECTION SCHEMES
7.5.4.4 Operating Time
7.5.4.5 Reset Time
7–26
If the element 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. Then, current can be slowly reduced below the
operate level and observed 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 pickup setting.
 Check that one of the following messages is displayed:
LATCHED a: W1 Phase Time OC or OPERATED a: W1 Phase
Time OC
The message will indicate
Using a table like the one shown below, select three (3) or four (4) values of current
multiples at which the timing is to be measured.
 Enter the expected operating times from the timing curve applied in the
settings.
 Using the setup shown in FIGURE 7–9: General test setup on page 7–
25 and the Interval Timer enabled, set the current level to the desired
value.
 Apply suddenly by closing the double-pole switch.
 Record the operating time.
 Compare this to the expected value.
 Repeat for all desired values of current.
Current multiple Nominal time
1.5
3
5
A precise measurement of the reset time requires a relay test set capable of dynamic
operation, with three sequenced stages, each with programmable current levels and time
duration external contact, and flexible triggering. To perform such a test, please contact
GE Multilin for detailed test instructions.
A simple verification the selected reset mode can be obtained using FIGURE 7–9: General
test setup on page 7–25. The procedure consists of performing repetitive operating time
measurements in quick succession. If the reset is selected for instantaneous, the operating
time will always be equal to the nominal time derived from the selected curve. If the reset is
selected as linear, the operating time will vary as a function of the time between successive
application of the current signal. If performed at current multiples of 2 to 3 times the
pickup level, the variations in operating time will be easier to detect.
or , depending on the setting for the target.
LATCHED OPERATED
Measured time
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: COMMISSIONING