CHAPTER 7: COMMISSIONING PROTECTION SCHEMES
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 7–45
Check that this dropout current level is equal to the S4 ELEMENTS
FREQUENCY
UNDERFREQUENCY 1 MINIMUM OPERATING
CURRENT
setting:
Note
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
UNDERFREQUENCY 1 TARGET
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
S4 ELEMENTS FREQUENCY UNDERFREQUENCY 1
UNDERFREQUENCY 1 DELAY setpoint value:
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.
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