GE D90 Plus Instruction Manual page 347

CHAPTER 7: PROTECTION
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
at 2.0 ms in the figure below; as such, the eighth sample validates the change of state
(mark 1 in the figure below). Once validated (de-bounced), the contact input asserts a
corresponding FlexLogic™ operand and logs an event (if event logging is enabled).
A time stamp of the first sample in the sequence that validates the new state is used when
logging the change of the contact input into the event recorder (mark 2 in the figure
below).
Figure 276: Contact input debouncing mechanism and time-stamping
Protection elements, as well as FlexLogic™ equations and timers, are executed sixteen
times in a power system cycle. The protection pass duration is controlled by the frequency
tracking mechanism. The FlexLogic™ operand reflecting the debounced state of the
contact is updated at the protection pass following the validation (mark 3 in the figure
above). The update is performed at the beginning of the protection pass so all protection
functions and protection FlexLogic™ equations, are fed with the updated states of the
contact inputs.
The FlexLogic™ operand response time to the contact input change is equal to the
debounce time setting plus a maximum of one protection pass (depending on system
frequency if frequency tracking is enabled). If the change of state occurs just after a
protection pass, the recognition is delayed until the subsequent protection pass (that is, by
the entire duration of the protection pass). If the change occurs just prior to a protection
pass, the state is recognized immediately. Statistically, a delay of half the protection pass is
expected. Owing to the 0.25 ms scan rate, the time resolution for the input contact is less
than 1 ms.
For example, sixteen protection passes per cycle on a 60 Hz system correspond to a
protection pass every 1.04 ms. With a contact debounce time setting of 3.0 ms, the
FlexLogic™ operand assert time limits are: 3.0 + 0.0 = 3.0 ms and 3.0 + 1.04 = 4.04 ms.
These time limits depend on how soon the protection pass runs after the debouncing time.
Regardless of the contact debounce time setting, the contact input event is time-stamped
with a 1 ms accuracy using the time of the first scan corresponding to the new state.
Therefore, the time stamp reflects a change in the DC voltage across the contact input
terminals that was not accidental as it was subsequently validated using the debounce
timer. Keep in mind that the associated FlexLogic™ operand is asserted/de-asserted later,
after validating the change. The debounce algorithm is symmetrical: the same procedure
and debounce time are used to filter the OFF-ON and ON-OFF transitions.
PROTECTION INPUTS AND OUTPUTS
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