Schweitzer Engineering Laboratories SEL-421-4 - Page 1062

8.24
SEL-400 Series Relays Instruction Manual Date Code 20171006
Monitoring
Station DC Battery System Monitor
trol Voltage Supply” (Section 6.5) as an alternating component (ripple) of 5 per-
cent peak or less. (This definition is valid if the minimum instantaneous voltage
is not less than 80 percent of the rated voltage.) The relay measures ac ripple as a
peak-to-peak waveform, consequently, DC1RP and DC2RP should be set at or
greater than 10 percent (2 • 5% peak) of the equalizing voltage. Table 8 .10 shows
the ac ripple threshold settings for this example.
DC Ground
If a battery system is centered around chassis ground, then the magnitude of the
voltage measured from the positive terminal to ground and from the negative ter-
minal of the battery to ground should be approximately one-half of the nominal
battery system voltage. The ratio of the positive-to-ground battery voltage to the
negative-to-ground battery voltage is 1 to 1, or 1.00. Equation 8.5 is the balanced
(no grounding) ratio for a 125-Vdc battery system.
Equation 8.5
If either terminal is partially or completely shorted to chassis ground, then the
terminal voltage will be less than the nominal terminal-to-ground voltage. This
causes the ratio of positive voltage to negative voltage to differ from 1.00.
Equation 8.6 is an example of the unbalanced (grounding) ratio for a partial short
circuit to ground on the negative side of a 125-Vdc battery system.
Equation 8.6
The relay uses this voltage ratio to calculate a ground detection factor.
Figure 8.13 shows a graphical representation of the ground detection factor set-
ting and battery system performance.
Table 8.10 Example DC Battery Monitor Settings—AC Ripple Voltages
Setting Description Indication Value (Vac)
DC1RP AC ripple threshold, Mon. 1 Charger malfunction 14
DC2RP AC ripple threshold, Mon. 2 Charger malfunction 6
k
Vdc1
pos
Vdc1
neg
---------------------
62.50 V
62.50 V
------------------- 1.00===
k
Vdc1
pos
Vdc1
neg
---------------------
62.50 V
59.10 V
------------------- 1.06===