GE D90 Plus Instruction Manual page 371

CHAPTER 7: PROTECTION
NOTE:
Six VT breaker flashover application
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
input indicating the breaker status is off), and no flashover current is flowing. A contact
showing the breaker status must be provided to the relay. The voltage difference will not
be considered as a condition for open breaker in this part of the logic.
Voltages must be present prior to flashover conditions. If the three VTs are placed after the
breaker on the line (or feeder), and the downstream breaker is open, the measured voltage
would be zero and the flashover element will not be initiated.
The flashover detection will reset if the current drops back to zero, the breaker closes, or
the selected FlexLogic™ operand for supervision changes to high. Using supervision
through the Supervision A
trip operand that will not allow the flashover element to pickup prior to the trip. The
flashover detection can be used for external alarm, re-tripping the breaker, or energizing
the lockout relay.
Consider the configuration below.
Figure 312: Breaker flashover application example
The source 1 (SRC1) phase currents are feeder CTs and phase voltages are bus VTs, and
contact input 1 is set as breaker 52a contact. The conditions prior to flashover detection
are:
1. 52a status = 0.
2. VAg, VBg, or VCg is greater than the pickup setting.
3. IA, IB, IC = 0; no current flows through the breaker.
4. ∆VA is greater than pickup (not applicable in this scheme).
The conditions at flashover detection are:
1. 52a status = 0.
2. VAg, VBg, or VCg is lower than the pickup setting.
3. IA, IB, or IC is greater than the pickup current flowing through the breaker.
4. ∆VA is greater than pickup (not applicable in this scheme).
The per-phase voltage difference approaches zero when the breaker is closed. The is well
below any typical minimum pickup voltage. Select the level of the
setting to be less than the voltage difference measured across the breaker when the close
or open breaker resistors are left in service. Prior to flashover, the voltage difference is
larger than the
Voltage Difference Pickup
between two live voltages per phase or when the voltage from one side of the breaker has
dropped to zero (line de-energized), at least one per-phase voltage is larger than the
Voltage Pickup setting, and no current flows through the breaker poles. During breaker
flashover, the per-phase voltages from both sides of the breaker drops below the pickup
value defined by the
Voltage Pickup
setting, and flashover current is detected. These flashover conditions initiate FlexLogic™
pickup operands and start the
This application do not require detection of breaker status via a 52a contact, as it uses a
voltage difference larger than the
the breaker contact will ensure scheme stability.
through Supervision C settings is recommended by selecting a
setting. This applies to either the difference
setting, the voltage difference drops below the pickup
timer.
Pickup Delay
Voltage Difference Pickup
CONTROL ELEMENTS
Voltage Difference Pickup
setting. However, monitoring
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