GE D30 Instruction Manual page 471

CHAPTER 9: THEORY OF OPERATION
Section (a) of the following figure shows the effect of adaptive reach control for low-current external fault. The reach is
reduced sufficiently to cope with both transient and steady-state overreach. Section (b) shows a high-current external
fault. The air gaps or MOVs conduct the majority of the fault current and neither steady-state nor transient overreach
takes place. The relay does not reduce its reach as it is not necessary. Section (c) shows a high-current internal fault.
Because of the large current, the reach is not reduced and the element responds to this internal fault. Traditional approach
leaves this fault out of the relay reach.
The neutral and negative-sequence directional protection functions of the relay cope with the voltage and/or current
inversions by adding appropriate offset to their polarizing signals as explained in the Ground Directional Overcurrent
section. The offset impedance can always be selected successfully to guarantee correct fault direction discrimination
regardless of the degree of compensation and location of the series capacitors and the potential source.
See the Application of Settings chapter for recommendations on settings for series compensation applications.
D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 9-7: Dynamic reach control
X
THE REACH IS
DYNAMICALLY
REDUCED BY
V /abs(I)
L
SERIES COMPENSATED LINES
SET REACH (Z )
R
ACTUAL REACH
FOR VERY HIGH
CURRENTS
ACTUAL REACH IS
A FUNCTION OF
CURRENT
MAGNITUDE
ACTUAL REACH
FOR VERY SMALL
CURRENTS
R
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