9328900990 Rev L BE1-951 Protection and Control 4-23
46 Curve
The 46 curve is a special curve designed to emulate the I
2
t withstand ratings of generators using what is
frequently referred to as the generator’s K factor. Do not confuse the 46 curve with the 46 element. The
46 curve was designed for use with the 46 function. But, in actuality, the 46 curve may be selected for use
with the 51P, 51N, 151N, and 51Q protection functions as well (though in actual practice, it is doubted
that this will be done very often).
To use the 46 curve, the user should determine the K factor of the generator and the continuous (I
2
)
2
t
rating of the generator (supplied by the manufacturer) and use this to set the time dial and pickup for the
46 curve by the process described in Appendix A, Time Overcurrent Characteristic Curves. The K factor
is the time the generator can withstand 1 per unit I
2
where 1 pu is the relay setting for nominal current.
67 - Directional Overcurrent 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.
Excluding the angle associated with line impedances Z1, Z2, and Z0, consider the following: Standing at
the bus and looking at a typical 4 wire power system, the angle difference between the positive-sequence
voltage and positive-sequence current for 3-phase faults is 0 degrees while an angle difference of 180
degrees exist between negative-sequence voltage and current (for all fault types except 3-phase) and
zero-sequence voltage and current (for ground faults). To establish a simple, consistent methodology for
providing a forward or reverse directional decision in the BE1-951, an internal “angle adjustment factor” of
180 degrees was included in the V2 and V0 voltage calculations. As a result, an in-phase condition (0
degrees) between the positive, negative, and zero-sequence component voltages and currents will result
in “forward fault detection” and out-of-phase conditions (180 degrees) result in “reverse fault detection.”
Of course, the actual angle is that of the Z1/Z2 and Z0 impedance angle which is covered later. For zero-
sequence current polarizing, a true angle difference of 0 degrees exists between the measured zero-
sequence current source (IG) and the internally calculated 3IO reference value IN. Forward or reverse
declarations are relative to the polarity connections of the relay’s current and potential circuits. Typical AC
connections are provided in Section 12, Installation.
Internal Polarization Methods
Each of the four internal polarization methods has designated bits: 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. The polarization methods are as follows:
• Positive-Sequence Polarization – Compares the angle and minimum magnitudes of V1 and I1.
• Negative-Sequence Polarization – Compares the angle and minimum magnitudes of V2 and I2.
• Zero-Sequence Voltage Polarization – Compares the angle and minimum magnitude of the
internally calculated V0 to 3I0/IN or IG, or compares an external source of 3V0 to 3I0/IN or IG.
• Zero-Sequence Current Polarization – Compares optional ground CT input (IG) to 3I0/IN.
Positive-Sequence Polarization. This 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 relay will also need to depend on
memory voltage to determine direction (see below). Positive-sequence bits are used to supervise only the
phase over current elements.
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.
Due to minute errors in the sample rate and variations in the power system, the memory voltage becomes
less accurate over time. Conservatively, the BE1-951 can maintain memory voltage accuracy to less than
5° error for approximately one second. This should be adequate, as close in faults are expected to trip in
very short time intervals.
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