GE 745 Instruction Manual page 283

CHAPTER 7: COMMISSIONING
Note
NOTE
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
 Check that this dropout current level is equal to the
FREQUENCY
CURRENT
If current sensing is disabled in the element, it will remain operated with current reduced to
0.0 A.
 Slowly increase the current and ensure the element operates when the
current reaches a value just above the setting. Set the current to rated CT
secondary.
 Check that the Trip and Pickup LEDs are on and one of the following
trip messages is displayed:
LATCHED: Underfrequency 1 or OPERATED: Underfrequency 1
 Slowly increase the frequency until the Pickup LED and output relays
reset. Note the dropout level, which should be the pickup plus 0.03 Hz.
 Check that the Trip LED is still on.
The trip message will stay on if the
setting is "Latched"; if set to "Self-resetting", the message will reset
when frequency is above the setpoint.
For timing tests, the signal generator must be capable of triggering into step-wise
changing of frequency or ramping down to a pre-selected frequency in only a few
milliseconds.
 Connect the Signal Source and Timer Start triggers as shown in
FIGURE 7–12: Frequency element testing on page 7–44.
 Set the voltage to rated VT secondary value, the current to rated CT
secondary, and the pre-trigger frequency to the nominal frequency (60
or 50 Hz).
If current sensing is not enabled, it is not necessary to connect the
current signal.
 Set the post-trigger to 0.5 Hz below the setting for underfrequency 1.
Reset all targets and relays, if necessary.
 Reset the timer.
 Initiate the frequency step and timer start.
The Interval Timer will record the operating time of element. Compare
this time to the
UNDERFREQUENCY 1 DELAY

Provided that the operate times are not scattered over a wide range, it may be desirable to
repeat this test several times and average the results. If there is a wide scatter, verify the
test setup and ensure the signal source behaves in a consistent manner.
Current input function (voltage input disabled):
 If the frequency elements are using the winding 1 phase A current signal
as a source, verify the operation of the element using the instructions
below.
UNDERFREQUENCY 1
 
setting:
UNDERFREQUENCY 1 TARGET
S4 ELEMENTS FREQUENCY

setpoint value:
PROTECTION SCHEMES
S4 ELEMENTS

MINIMUM OPERATING
UNDERFREQUENCY 1

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