GE 845 Instruction Manual page 231

CHAPTER 4: SETPOINTS
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845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
DEBOUNCE TIME
Range: 0.0 to 16.0 ms in steps of 0.5 ms
Default: 10.0 ms
The Debounce Time defines the time required for the contact to overcome 'contact
bouncing' conditions. As this time differs for different contact types and manufacturers,
set it as a maximum contact debounce time (per manufacturer specifications) plus some
margin to ensure proper operation.
EVENTS
Range: Enabled, Disabled
Default: Enabled
For example, to use Contact Input '1' as a status input from the breaker 52b contact, to
seal-in the trip relay and record it in the Event Records menu, make the following settings
changes:
CONTACT INPUT 1 NAME: "52b"
CONTACT INPUT 1 EVENTS: "Enabled"
The 52b contact is closed when the breaker is open and open when the breaker is closed.
A raw status is scanned for all Contact Inputs synchronously at the constant rate of one
protection pass (1/8 cycle) as shown in the figure below. The DC input voltage is compared
to a user-settable threshold. A new Contact Input state must be maintained for a user
settable debounce time in order for the relay to validate the new contact state. In the
figure below, the debounce time is set at 2.5 ms; thus the 3rd sample in a row validates the
change of state (mark no. 2 in the diagram). Once validated (debounced), the new state will
be declared and a FlexLogic™ operand will be asserted at the time of a new protection
pass. 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 no. 1 in the
diagram).
Protection and control elements, as well as FlexLogic™ equations and timers, are executed
eight 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 debounce (marked no. 2 on the
figure below). The update is performed at the beginning of the protection pass so all
protection and control functions, as well as FlexLogic™ equations, are fed with the
updated states of the Contact Inputs.
The FlexLogic™ operand response time to the Contact Input change is related to the
debounce time setting plus up to one protection pass (variable and depending on system
frequency if frequency tracking enabled). For example, 8 protection passes per cycle on a
60 Hz system correspond to a protection pass every 2.1 ms. With a contact debounce time
setting of 3.0 ms, the FlexLogic™ operand-assert time limits are: 4.2 + 0.0 = 4.2 ms and 4.2
+ 2.1 = 6.3 ms. The 4.2 ms is the minimum protection pass period that contains a debounce
time, 3.0 ms.
Regardless of the contact debounce time setting, the Contact Input event is time-stamped
with 1 protection pass accuracy using the time of the first scan corresponding to the new
state (mark no. 1 below). 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. The debounce algorithm is symmetrical: the same
procedure and debounce time are used to filter the LOW-HIGH (marks no.1 and 2 in the
figure below) and HIGH-LOW (marks no. 3 and 4 below) transitions.
INPUTS
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