Schweitzer Engineering Laboratories SEL-311B, Instruction Manual

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Date Code 20011205 Testing and Troubleshooting 13-21
SEL-311B Instruction Manual
When Z2F
£
0:
m 2 Z • 0.25 - F 2 Z • 0.75 = FT 2 Z Equation 13.2
When Z2F > 0:
m 2 Z • 0.25 - F 2 Z • 1.25 = FT 2 Z Equation 13.3
Where:
I
V
= m 2 Z
2
2
The reverse directional threshold is calculated using Equation 13.4 when Z2R is positive or
Equation 13.5 when Z2R is negative.
When Z2R
³
0:
m 2 Z • 0.25 + R 2 Z • 0.75 = RT 2 Z Equation 13.4
When Z2R < 0:
m 2 Z • 0.25 + R 2 Z • 1.25 = RT 2 Z
Equation 13.5
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 F32Q, F32QG, R32Q, or R32QG elements.
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.
Negative-Sequence Directional Element Supervisory Conditions
There are a number of supervisory conditions that must be fulfilled before the relay asserts the
negative-sequence directional elements. These supervisory conditions are described below:
Magnitude of 3I
2
The SEL-311B 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 50QFP setting, the F32Q and F32QG elements do not assert to indicate direction. If the
magnitude of 3I
2
applied is not greater than the 50QRP setting, the R32Q and R32QG elements
do 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
elements (F32Q, F32QG, R32Q or R32QG) to operate.