GE L90 Instruction Manual page 539

CHAPTER 5: SETTINGS
5.7.15.2 Disturbance detector
SETTINGS  GROUPED ELEMENTS  SETTING GROUP 1(6)  SUPERVISING ELEMENTS  DISTURBANCE
DETECTOR
 DISTURBANCE
 DETECTOR
The disturbance detector (50DD) element is an 87L-dedicated sensitive current disturbance detector that is used to detect
any disturbance on the protected system. It is intended for such functions as trip output supervision and starting
oscillography The disturbance detector also signals the 87L function that a disturbance (possible fault) occurred and to
resize the operating window to remove the pre-fault current. It is essential to have the disturbance detector enabled for
applications where the 87L operating time is critical.
If the disturbance detector is used to supervise the operation of the 87LP or 87LG function, it is recommended that the 87L
trip element be used. The 50DD SV disturbance detector FlexLogic operand must then be assigned to an
setting, and the 50DD SV 87LG must be assigned to an
enabled.
The disturbance detector function measures the magnitude of the negative-sequence current (I_2), the magnitude of the
zero-sequence current (I_0), the change in negative-sequence current (DI_2), the change in zero-sequence current (DI_0),
and the change in positive-sequence current (DI_1). The disturbance detector element uses net local current, computed as
a sum of all sources configured in the current differential element, to detect system disturbances. If the weak condition
exists at one terminal, the voltage-based disturbance detector measures the changes in three-phase voltages (DVA, DVB,
DVC).
The adaptive level detector operates as follows:
• When the absolute level increases above 0.12 pu for I_0 or I_2, the adaptive level detector output is active and the
next highest threshold level is increased eight cycles later from 0.12 to 0.24 pu in steps of 0.02 pu. If the level exceeds
0.24 pu, the current adaptive level detector setting remains at 0.24 pu and the output remains active (as well as the
disturbance detector output) when the measured value remains above the current setting.
• When the absolute level is decreasing from 0.24 to 0.12 pu, the lower level is set every eight cycles without the
adaptive level detector active. Note that the 50DD output remains inactive during this change as long as the delta
change is less than 0.04 pu.
The current delta level detectors (DI) detectors are designed to pick up for the 0.04 pu change in I_1, I_2, and I_0 currents.
The DI value is measured by comparing the present value to the value calculated four cycles earlier.
The voltage delta level detectors (DV) are designed to pickup for the 0.1 pu change in any of three-phase voltages and the
old voltage is greater than 0.8 pu. The DV value is measured by comparing the present value to the value calculated four
cycles earlier.
The settings are explained as follows.
— This setting is used to enable/disable the operation of the disturbance detector.
DD FUNCTION
— Selects a FlexLogic operand that activates the output of the disturbance detector upon events
DD NON-CURRENT SUPV
(such as frequency or voltage change) not accompanied by a current change.
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
DD

FUNCTION: Disabled
DD NON-CURRENT SUPV:

Off
DD CONTROL LOGIC:

Off
DD LOGIC SEAL-IN:

Off
DD SEAL-IN BY 87LG:

Disabled
DD VOLTAGE SUPV:

Disabled
DD

EVENTS: Disabled
87LG TRIP SUPV
Range: Disabled, Enabled
Range: FlexLogic operand
Range: FlexLogic operand
Range: FlexLogic operand
Range: Disabled, Enabled
Range: Disabled, Enabled
Range: Disabled, Enabled
setting as well if the ground differential function is
GROUPED ELEMENTS
5
87LP TRIP SUPV
5-315