Page 146 - Uninterruptible Power Supplies

Static Uninterruptible Power Supplies
144 Chapter Five

Battery characteristics impose certain design conditions on the rec-
tifier charger, assuming that a valve regulated lead acid (VRLA) bat-
tery is used then the normal float charge level is 2.27 volt per cell
(VPC), and at the end of discharge the voltage level may be as low as
1.6 VPC. The end of discharge voltage is dependent on the autonomy
period and design of the battery, guidance from the battery manufac-
turer is required. There is the possibility that the system may require
to be fitted with a boost charge facility, this may be required as an ini-
tial charge when commissioning the module, this voltage will be 2.3 to
2.4 VPC. Such a requirement will be switched on manually and also
switched off either manually or shut down by a timer. Boost charging
may be required for flooded-type lead acid and nickel-cadmium cells.
The rectifier/charger will also need to ensure that the ripple on dc out-
put is within specified limits. Battery manufacturers limit the ripple to
7 percent of the C3 (3-h) capacity or 5 percent of the C10 rating under
these conditions temperature rise within the cell will not exceed 5°C. In
practice a 1-percent voltage ripple is normally accepted. Ripple does
affect battery life, optimum ripple damage occurs between 100 to 250 Hz
which normally with modern design UPS modules does not cause a
problem.
The rate of charge is also important and in practice a current limit is
set, thus the dc voltage gradually increases to the normal float charge
level. The accepted standard for recharging a battery is to 95 percent
capacity within 10 times the discharge period. If this requires shorten-
ing, the solution may be to increase the ampere hour rating of the cell
thus avoiding exceeding the current charging limitation.
Other types of battery may require variations to the above charac-
teristics. Details are given in Chap. 7.

Harmonics and Effect on Design
Clearly rectifiers will induce harmonic distortion onto the incoming sup-
ply. Such harmonics are the cause of concern to power networks and lim-
itations are imposed to attenuate such problems. Electricity Council
G5/4 gives recommendations designed to limit the harmonics in a dis-
tribution network to below those thought to cause EMC problems. In
effect it recommends 5 percent total harmonic distortion (THD) as a
maximum. There are some exceptions to the rule: For example, excur-
sions outside the limit for limited time periods may be allowed, and the
recommendation is not retrspective. It should be noted that this recom-
mendation supersedes an earlier paper G5/3 which allowed a 10 percent
THD for ratings above approximately 100 kVA. In the United States the

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