Schweitzer Engineering Laboratories SEL-421-4 - Page 1028

7.4
SEL-400 Series Relays Instruction Manual Date Code 20171006
Metering
Instantaneous Metering
Table 7.3 Instantaneous Metering Accuracy—Power
Quantity Description Power Factor Accuracy (%)
a
a
Power accuracy is valid for applied currents in the range (0.1–1.2) • I
NOM
, and applied voltages
from 33.5–75 V.
At 0.1 • I
NOM
3P Three-phase rms real power Unity ±0.40
–0.5 or +0.5 ±0.70
3Q
1
Reactive power –0.5 or +0.5 ±0.50
At 1.0 • I
NOM
3P Three-phase fundamental real power Unity ±0.40
–0.5 or +0.5 ±0.40
3Q
1
Reactive power –0.5 or +0.5 ±0.40
Example 7.1 Calculating Exact Error Coefficients
Consider the case of a 5 A relay during normal operating conditions. The
secondary current in the CT is 1.0 A for nominal system operation. Noting
that this current is greater than 10 percent of I
NOM
(1 A > 0.5 A), calculate
the error coefficient:
error = ±(0.2% • 1.0 A) ± (0.8 mA • I
NOM
)
= ±(0.002 • 1.0 A) ± (0.0008 A • 5)
= ±(0.002A ± 0.004A)
= +0.002 A to +0.006 A
and
= –0.006 A to –0.002 A
Equation 7.1
Figure 7.2 represents the calculated accuracy range. The error is very small,
indicating that the relay measures normal operating currents accurately.
Figure 7.2 Typical Current Measuring Accuracy
Measured Current (A)
0.994
0.998
1.006
1.002
01.0
1.0
I
NOM
= 5 A
Actual Value