GE D30 Instruction Manual page 446

DISTANCE ELEMENTS (STEPPED DISTANCE SCHEME)
Given the equivalent systems shown in the following figure, the angular difference between the zero-sequence or
negative-sequence currents at the relay, and the fault current can be calculated as follows:
where:
A is the local equivalent system
B is the remote equivalent system
L is the line
d is the per-unit (pu) intended reach of the zone (typically 0.7 to 0.9)
If abs Θ ( ) abs Θ < (
0
the fault current), and zero-sequence polarizing should be selected. Otherwise, negative-sequence is a better polarizing
signal.
The calculated Θ angles reflect errors between the ideal and applied polarizing angles. Therefore, after selecting the
polarizing current, the corresponding angle is chosen as the non-homogeneity correction angle. For example, calculations
yielding Θ = 5° and Θ
0
8
homogeneity correction.
Often the non-homogeneity correction angle is used to apply extra security rather than correct the angle between the
relay and the fault currents. For extra security, set the angle to negative values.
Approach with care the ability to select an optimum polarizing signal and to correct for non-homogeneity. The best
polarizing signal and the correcting angle both depend on system parameters and intended reach of the zone. If the
system configuration is static, the selection is straightforward.
If the system configuration can change significantly, consider all the relevant system topologies for calculations. Either a
compromise choice is made, or different settings are applied via switchable setting groups depending on system
conditions. In highly non-homogenous and dynamic systems where the correction for non-homogeneity is not feasible,
consider conservative reach (reduced) and/or correction angle (negative) settings.
8-4

Θ = angle ---------------- -

0
I
0_RELAY

---------------- -
Θ = angle

2
I
2_RELAY
Figure 8-1: Understanding system homogeneity
I
F
Z dZ
0A 0L
I
0
Z dZ
1A 1L
I
2
I
F
) , the zero-sequence network is more homogenous (the zero-sequence current better approximates
2
= –1° result in selecting the negative-sequence for polarization, and the value of –1° for non-
2
I Z + Z + Z
 
F 0A 0L
= angle ------------------------------------ -
 
( )Z
Z + 1 d
–
0B
I Z + Z + Z
 
F 1A 1L
------------------------------------ -
= angle
 
Z + ( 1 d )Z
–
1B
(1 – d)Z Z
0L 0B
(1 – d)Z Z
1L 1B
D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: APPLICATION OF SETTINGS

0B

0L

1B

1L
I
0
I
F
I
2
DEPENDING ON SYSTEM
PARAMETERS, I0 OR I2 MAYBE A
BETTER APPROXIMATION OF THE
FAULT CURRENT ANGLE
842742A1.CDR
Eq. 8-2
Eq. 8-3