GE b30, Instruction Manual

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CHAPTER 5: SETTINGS CONTROL ELEMENTS
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 5-237
5
There are 48 identical digital elements available, numbered 1 to 48. A digital element can monitor any FlexLogic operand
and present a target message and/or enable events recording depending on the output operand state. The digital element
settings include a name to be referenced in any target message, a blocking input from any selected FlexLogic operand,
and a timer for pickup and reset delays for the output operand.
DIGITAL ELEMENT 1 INPUT
— Selects a FlexLogic operand to be monitored by the digital element.
DIGITAL ELEMENT 1 PICKUP DELAY
— Sets the required time delay from element pickup to element operation. If a pickup delay
is not required, set to "0," To avoid nuisance alarms, set the delay greater than the operating time of the breaker.
DIGITAL ELEMENT 1 RESET DELAY
— Sets the time delay to reset. If a reset delay is not required, set to “0.”
DIGITAL ELEMENT 1 PICKUP LED
— This setting enables or disabled the digital element pickup LED. When set to “Disabled,” the
operation of the pickup LED is blocked.
Figure 5-126: Digital element logic
Circuit monitoring applications
Some versions of the digital input modules include an active voltage monitor circuit connected across form-A contacts.
The voltage monitor circuit limits the trickle current through the output circuit (see technical specifications for form-A).
As long as the current through the voltage monitor is above a threshold (see technical specifications for form-A), the
Cont
Op 1 VOn
FlexLogic operand is set (for contact input 1—corresponding operands exist for each contact output). If the output
circuit has a high resistance or the DC current is interrupted, the trickle current drops below the threshold and the
Cont Op 1
VOff
FlexLogic operand is set. Consequently, the state of these operands can be used as indicators of the integrity of the
circuits in which form-A contacts are inserted.
Example 1: Breaker trip circuit integrity monitoring
In many applications it is desired to monitor the breaker trip circuit integrity so that problems can be detected before a trip
operation is required. The circuit is considered to be healthy when the voltage monitor connected across the trip output
contact detects a low level of current, well below the operating current of the breaker trip coil. If the circuit presents a high
resistance, the trickle current falls below the monitor threshold, and an alarm is declared.
In most breaker control circuits, the trip coil is connected in series with a breaker auxiliary contact that is open when the
breaker is open (see figure). To prevent unwanted alarms in this situation, the trip circuit monitoring logic must include the
breaker position.
Digital elements run once per power system cycle.
As such they can easily fail to react to an input signal or a block signal with a duration less than one power system
cycle. This also means that digital element output can react up to one power system cycle later than the pickup
and reset delay settings indicate.
Do not use digital elements with transient signals, such as communications commands. Do not use digital
elements where random delays of up to one cycle cannot be tolerated, such as in high speed protection.