Directional Overcurrent (67) Protection; Polarization Methods - Basler BE1-11g Instruction Manual

9424200994 Rev U

Directional Overcurrent (67) Protection

The 67 element provides directional supervision for the overcurrent tripping elements. Two reference
quantities for each polarizing method are compared to establish directional signals for controlling
operation of the phase, ground, and negative-sequence overcurrent elements. Directionality is derived
from a comparison between internally calculated sequence voltages V1, V2, V0 (magnitude and angle)
and calculated values of I1, I2, 3I0, I0, (magnitude and angle) and measured IG (magnitude and angle).
Regardless of fault direction, the angle of the sequence voltages and the ground current source will
always be the same while the angle of the currents (I1, I2, 3I0/IN, I0, IG operate) will change based on
the direction of fault current flow.
On protection systems equipped with two sets of CTs, the directional element can monitor CT circuit 1 or
CT circuit 2. The CT source is selected on the Directional settings screen (Figure 70) in BESTCOMSPlus.

Polarization Methods

The polarization methods are as follows:
●
Positive-Sequence Polarization – Forward direction is detected is when the apparent Z
) is equal to the positive-sequence maximum torque angle (MTA), ±90°.
of V /I
1 1
●
Negative-Sequence Polarization – Forward direction is detected is when the apparent Z
is equal to the negative-sequence maximum torque angle (MTA), ±90°. (See Note 1.)
-V /I )
of
2 2
●
Zero-Sequence Voltage Polarization – Forward direction is detected is when the apparent Z
) is equal to the zero-sequence maximum torque angle (MTA), ±90°. (See Note 1.)
(angle of V /I
0 0
However, the BE1-11g has two forms of zero-sequence voltage available to it (calculated V
phase voltages or V
(calculated I from the phase currents or I
0
results in four options for zero-sequence voltage polarization:
○
Calculated V
○
Calculated V
○
V verses calculated I
x
○
V verses I
x G
○
All four forms of zero-sequence voltage polarizations use the same MTA value.
●
Zero-Sequence Current Polarization – Forward direction is detected is when the phase angle of
current in the ground CT input (IG) is in phase with the calculated I
Each of the four internal polarization methods has designated internal bits that are used in the BE1-11g
for direction identification, one for forward direction and one for reverse direction. Combined, these eight
bits are referred to as the directional status byte and are used to control the various overcurrent elements.
Note 1: The negative and zero-sequence angle of maximum torque has a built in 180-degree phase shift
that arises out of the calculation methods described at the end of this chapter.
Positive-Sequence Polarization is used to determine direction for three-phase faults. Under these
conditions, very little negative or zero-sequence quantities are present, making the other polarization
methods unreliable for this fault condition. For close-in faults, the BE1-11g will also need to depend on
memory voltage to determine direction (see below). Positive-sequence bits are used to supervise the
elements in single or 3 phase mode.
To provide memory, the positive-sequence voltage is stored continuously until a fault occurs. Memory
voltage is used when the positive-sequence voltage falls below the minimum acceptable level of 12 volts.
The BE1-11g maintains memory voltage for 20 cycles to allow tripping for close in faults. When using
memory voltage polarization, the BE1-11g assumes nominal system frequency.
BE1-11g
from a broken delta VT) and two forms of zero-sequence current available to it
X
from the protection system's IG1 or IG2 input). This
G
verses calculated I
0 0
verses I
0 G
0
1
, ±90°.
0
Directional Overcurrent (67) Protection
113
angle (angle
angle (angle
2
angle
0
from the
0