SecoGear Medium-voltage Switchgear Application and Technical Guide
DET-882
Control Power Equipment
22
©2017 General Electric All Rights Reserved
Generally, the effect of high temperatures for every 15 °F
above 77 °F the lead-acid battery loses 50% of its useful
life. For the same temperature decrease, the nickel-
cadmium loses 20 % of its useful life. The one-minute
rating at -10 °C [15 °F], for instance is half the 25 °C rating.
When calculating battery loads, you must consider three
types of loads:
•
Continuous loads are those that are energized for the
duration of the duty cycle. These have a major effect of
battery capacity.
•
Non-continuous loads are energized for only a portion
of the duty cycle. If the inception of the load is known,
but the end is not or reverse, then you must consider it
as the known portion of the duty cycle.
•
Momentary loads are very short in duration, perhaps a
fraction of a second, but you must treat it as lasting one
full minute.
For more information, refer to the specification sheets or
software programs provided by the battery manufacturer.
For direct calculation, the battery is assumed to have
carried its steady loads for eight hours, and then as the
worst case subject to the maximum load involving the one-
minute rate.
Indoor locations assume that the battery is at 25 °C [77 °F];
outdoor locations at -10 °C [15 °F]. A minimum size limit of
cell is suggested to allow for unknowns: 20 ampere-hours
for lead-acid, or 15 ampere hours for nickel-cadmium.
A small station, for example, with the battery located
indoors, might have three breakers, with closing and
tripping duty, and no steady load except the switchgear
indicating lamps. Two of the breakers have instantaneous
settings on their overcurrent relays, so that per Table 3-3
simultaneous tripping of these two breakers might occur.
Steady lamp load, thus, is 0.035 A x 3 = 0.105 amperes.
Maximum short-time loads, given for both 48 Vdc and
125 Vdc to illustrate procedure, are shown in Table 3-9.
Table 3-7: Battery Sizing, Example A
Current
48 Vdc
125 Vdc
Trip coil
15.84 A
2.78 A
Spring motor
3.2 A
1.2 A
Since two breakers can trip at once in this example,
maximum current from this load is either 15.84 A x 2, or
2.78 A x 2, respectively: 31.68 A or 5.56 A total at 48 Vdc or
125 Vdc. Comparing this with charging motor current, we
see that the trip current is larger, so trip current will be
used in the next step, shown in Table 3-8.
Table 3-8: Battery Sizing, Example B
Current
Lead-acid
Nickel-cadmium
48 Vdc 125 Vdc 48 Vdc 125 Vdc
Max. 1-min.
demand
31.68 A 5.56 A 31.68 A 5.56 A
8-hr. equiv.
(max. 1-min.
demand /
conversion factor
1
)
21.12 AH 3.71 AH 10.92 AH 1.92 AH
Lamp load (0.105 A
x 8 hrs.)
0.84 AH 0.84 AH 0.84 AH 0.84 AH
Total AH (amp-hrs.)
at 8 hr. rate
21.96 AH 4.55 AH 11.76 AH 2.76 AH
1.
Conversion factors to calculate “common rate base” (i.e., from one-
minute rate to eight-hour rate) are: 1.5 for the lead-acid batteries
(pasted plate); 2.9 for the nickel-cadmium batteries (thin plate or high
rate). Please note that conversion factors vary by cell size; therefore,
the factors used in this example are not applicable for batteries of
other sizes.
Analyzing these totals, the lead-acid battery at 48 V with a
nominal ampere-hour rating of 30 AH will be required. As
an alternate at 125 volts, the minimum 20 AH lead-acid
battery will be sufficient. The minimum nickel-cadmium
battery of 15 AH will be sufficient at 48 V and at 125 V.
In addition, since the total ampere-hours required in each
case is less than the ampere-hour capacity of the selected
cell, reserve capacity is available. The matter of reserve
capacity is largely related to how long the charger may be
off. This no-charge condition has been known to last for
several days. Thus, a “DC low-voltage alarm” option in the
charger may be desirable to warn of such conditions.
For the same station, with the battery at outdoor
temperatures, the one-minute demand must be doubled
before converting to ampere-hours. The eight-hour rate
needs a smaller increase of about 30 percent. Note that
these conversion ratios generally decrease as cell size
increases; hence, the approximate size of cell being
considered must be determined before the conversion
factors can be determined.
In arriving at the actual size of the battery bank, care must
be taken to review the calculated amp-hours or cell
requirement and then take into account the recommended
design factor of 10% times the calculated values and then
an aging factor of 25% times the calculated values. The
combined sum of these calculations will provide the actual
size of the battery bank.
Battery Chargers
Battery chargers have been built both as unregulated or
“trickle” chargers, and as voltage-regulated chargers. The
latter type provides longer life for the battery, particularly if
it is a lead-acid battery. Voltage-regulated chargers are
considered standard for switchgear applications.
