GE 745 Instruction Manual page 290

PROTECTION SCHEMES
7.5.18 Overexcitation
7.5.18.1 Volts per Hertz
7–52
is to operate.
Note that operation occurs if the rate criterion is satisfied and the
frequency is below the threshold.
 Initiate ramping action and verify that the element operates once the
frequency drops below the threshold.
 Check that the Trip and Pickup LEDs are on and one of the following
trip messages is displayed:
LATCHED: Freq Decay Rate 1 or OPERATED: Freq Decay Rate 1
If the target was selected as Latched, the Trip LED and the
message remain on.
 Set the current level to a value below the
FREQUENCY DECAY

 Repeat ramping action and verify that element does not operate.
If current sensing has been disabled for this element, operation will
continue down to a current level of approximately 2% of nominal.
 For timing tests, an approximate operate time is obtained if a timer is
triggered at the same time as the ramping action and the time interval
between the trigger point and the relay operation measured.
 From that measured time, subtract the time required for the frequency to
reach the threshold value.
The expected time must be computed using the rate of change and the
effect of the
FREQUENCY DECAY DELAY

The following procedure applies to both volts-per-hertz elements; make the necessary
changes where appropriate. The volts-per-hertz operating levels are set in terms of the
relay-input voltage divided by the frequency of that voltage.
 Disable all elements except volts-per-hertz 1.
 Monitor the appropriate contact.
 Use the test setup in FIGURE 7–12: Frequency element testing on page
7–44 with variable-frequency voltage source.
The Volts-per-hertz settings are found in the
setpoints menu.
PER-HERTZ 1
 Apply a voltage starting at 60 Hz and increase the magnitude until the
element operates.
 Reduce the frequency in steps of 5 Hz and repeat the measurement.
The element should operate at a consistent value of volts/hertz equal to
the setting of the element.
 Check that the Trip and Pickup LEDs are on and one of the following
trip messages is displayed:
CHAPTER 7: COMMISSIONING
MINIMUM OPERATING CURRENT

S4 ELEMENTS FREQUENCY

time delay.
S4 ELEMENTS

745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
S4 ELEMENTS FREQUENCY

value.
FREQUENCY DECAY

OVEREXCITATION VOLTS-
