Schweitzer Engineering Laboratories SEL-321 - Magnitude of 3 I

Date Code 20011026 Maintenance and Testing 7-23
SEL-321/321-1 Instruction Manual
When Z2F > 0:
m2 Z• 0.25 - F2 Z• 1.25=FT2Z Equation 7.4
Where:
I
V
=m2Z
2
2
The reverse directional threshold is calculated using Equation 7.5 when Z2R is positive or
Equation 7.6 when Z2R is negative.
When Z2R ≥ 0:
m2 Z• 0.25 + R2 Z• 0.75 = RT2Z Equation 7.5
When Z2R < 0:
m2 Z• 0.25 + R2 Z• 1.25 = RT2Z Equation 7.6
The threshold equations have a unique property. When Z2m = │Z2R│, Z2RT = Z2R and when
Z2m = │Z2F│, Z2FT = Z2F. This property is used to simplify the test procedures outlined on the
following pages.
It is possible to create test conditions for a Z2c that lies between the forward and reverse
thresholds. In this case the relay does not set the 32QR or the 32QF element. While these
conditions can be contrived in the lab, correct Z2F and Z2R settings prevent this from occurring
for real power system faults.
Settings for both Z2F and Z2R may be either positive or negative, depending upon the constraints
of the relay application. Section 5: Applications of this instruction manual provides guidelines
for the selection of Z2F and Z2R settings.
Negative-Sequence Directional Element Supervisory Conditions
There are a number of supervisory conditions that must be fulfilled before the relay asserts the
32QF or 32QR elements. These supervisory conditions are described below:
Magnitude of 3I
2
The SEL-321 Relay uses the 50QF and 50QR negative-sequence overcurrent elements to
supervise operation of the directional element. If the magnitude of applied 3I
2
is not greater than
the 50QF setting, the 32QF element does not assert to indicate direction. If the magnitude of 3I
2
applied is not greater than the 50QR setting, the 32QR element does not assert.
Magnitude Comparison of Positive-Sequence Current to Negative-Sequence Current
The relay multiplies the measured positive-sequence current (I
1
) magnitude by the a2 setting,
then compares the result to the measured magnitude of negative-sequence current (I
2
). The
magnitude of I
2
must be greater than the magnitude of I
1
multiplied by a2 for the directional
element (32QF or 32QR) to operate.