Basler Electric BE1-11t - Manual (Page 97)

9424200995 Rev H 85
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.
The directional element can monitor CT circuit 1 or CT circuit 2. The CT source is selected on the
Sensing Transformers settings screen in BESTCOMSPlus
®.
Polarization Methods
The polarization methods are as follows:
● Positive-Sequence Polarization – Forward direction is detected is when the apparent Z
1
angle (angle
of V
1
/I
1
) is equal to the positive-sequence maximum torque angle (MTA),
±90°.
● Negative-Sequence Polarization – Forward direction is detected is when the apparent Z
2
angle (angle
of -V
2
/I
2
) is equal to the negative-sequence maximum torque angle (MTA), ±90°. (See Note 1.)
● Zero-Sequence Voltage Polarization – Forward direction is detected is when the apparent Z
0
angle
(angle of V
0
/I
0
) is equal to the zero-sequence maximum torque angle (MTA),
±90°. (See Note 1.)
However, the BE1-11t has two forms of zero-sequence voltage available to it (calculated V
0
from the
phase voltages or V
X
from a broken delta VT) and two forms of zero-sequence current available to it
(calculated I
0
from the phase currents or I
G
from the protection system's IG1 or IG2 input). This results
in four options for zero-sequence voltage polarization:
○ Calculated V
0
verses calculated I
0
○ Calculated V
0
verses I
G
○ V
x
verses calculated I
0
○ V
x
verses I
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
0
,
±90°.
Each of the four internal polarization methods has designated internal bits that are used in the BE1-11t 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 BE1-11t 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 BE1-11t maintains memory voltage for 20 cycles to allow tripping for close in faults. When using
memory voltage polarization, the BE1-11t assumes nominal system frequency.
BE1-11t Directional Overcurrent (67) Protection