GE D90 Plus Instruction Manual page 386

CONTROL ELEMENTS
NOTE:
Six VT breaker flashover application
376
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 is not 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
is zero and the flashover element is not initiated.
The flashover detection resets 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 through Supervision C
that does not allow the flashover element to pick up 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 shown.
Figure 315: 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. This 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 does not require detection of breaker status via a 52a contact, as it uses a
voltage difference larger than the
breaker contact to ensure scheme stability.
settings is recommended by selecting a trip operand
setting. This applies to either the difference
setting, the voltage difference drops below the pickup
timer.
Pickup Delay
Voltage Difference Pickup
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Voltage Difference Pickup
setting. However, monitor the