GE UR Series L90, Instruction Manual

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10-24 L90 Line Current Differential System GE Multilin
10.7 LINES WITH TAPPED TRANSFORMERS 10 APPLICATION OF SETTINGS
10
10.7LINES WITH TAPPED TRANSFORMERS 10.7.1 DESCRIPTION
The L90 protection system could be applied to lines with tapped transformer(s) even if the latter has its windings connected
in a grounded wye on the line side and the transformer(s) currents are not measured by the L90 protection system. The fol-
lowing approach is recommended.
If the setting
SYSTEM SETUP

L90 POWER SYSTEM

ZERO-SEQ CURRENT REMOVAL
is “Enabled”, all relays at the line ter-
minals are calculating zero-sequence for both local and remote currents and are removing this current from the phase cur-
rents. This ensures the differential current is immune to the zero-sequence current outfeed caused by the in-zone
transformer with a primary wye-connected winding solidly grounded neutral.
At all terminals the following is being performed:
I_L_0 = (I_L_A + I_L_B + I_L_C) / 3) : local zero-sequence current
I_R_0 = (I_R_A + I_R_B + I_R_C) / 3 : remote zero-sequence current
Now, the I_PHASE – I_0 values (for Local and Remote) are being used instead of pure phase currents for differential and
restraint current calculations. See the
Theory of Operation chapter for additional details.
For example, the operating current in phase A is determined as:
I
2
op_A = |(I_L_A – I_L_0) + (I_R_A – I_R_0) |
2
: squared operating current, phase A
where: I_L_A = “local” current phase A
I_R_A = “remote” current phase A
I_L_0 = local zero-sequence current
I_R_0 = remote zero-sequence current
I
2
op_A = operating (differential) squared current phase A
The restraint current is calculated in a similar way.
When the
ZERO-SEQ CURRENT REMOVAL
feature is enabled, the modified (I_0 removed) differential current in all
three phases is shown in the
ACTUAL VALUES

METERING

87L DIFFERENTIAL CURRENT
menu. Local and remote
currents values are not changed.
10.7.2 TRANSFORMER LOAD CURRENTS
As the tapped line may be energized from one terminal only, or there may be a low current flowing through the line, the
slope setting of the differential characteristic would not guarantee stability of the relay on transformer load currents. Conse-
quently, a pickup setting must be risen accordingly in order to prevent maloperation. The L90 forms its restraint current in a
unique way as explained in Chapter 8. Unlike traditional approaches, the effects of slope and pickup settings are combined:
the higher the slope, the lower the pickup setting required for the same restraining effect.
Assuming the line energized from one terminal and the current is below the lower break-point of the characteristic one
should consider the following stability conditions in order to select the pickup (P) and slope (
S
1
) settings ( I
LOAD
is a maxi-
mum total load current of the tapped transformer(s)).
The above calculations should take into account the requirement for the pickup setting resulting from line charging currents.
Certainly, a security factor must be applied to the above stability conditions. Alternatively, distance supervision can be con-
sidered to prevent maloperation due to transformer load currents.
• Two-terminal applications: • Three-terminal applications:
NOTE
I
op
2
I
LOAD
2
I
REST
2
2 S
1
2
I
LOAD
2
2 P
2
Stability condition: 2 S
1
2
I
LOAD
2
2 P
2
I
LOAD
2

I
op
2
I
LOAD
2
I
REST
2
4
3
---
S
1
2
I
LOAD
2
2 P
2
Stability condition:
4
3
---
S
1
2
I
LOAD
2
2 P
2
I
LOAD
2
