Page 160 - Uninterruptible Power Supplies

Static Uninterruptible Power Supplies
158 Chapter Five

Voltage 100% 100% 100%
Current 100% 110% 100%
Power 100% 110% 100%

Mains
0% 10%

~ =
= ~
10%
1 Load
2

(d)
Figure 5.14

to the load and 10 percent for battery recharging). Under this condition the
battery recharging is a feedback of power from inverter 2 to the battery.
It should be explained that inverter 2 is synchronized to the mains
supply and acts as the voltage regulator to the load. Inverter 1 com-
pensates for variations in power factor and, although the power for bat-
tery charging is fed from Inverter 2, the control of this function is
governed by inverter 1. Additionally, inverter 1 compensates for any
waveform variation between mains and the load.
Claimed advantages for this system are low harmonics induced onto
the mains supply and high efficiency. The high efficiency at full load is
certainly superior to a normal double conversion system but, with most
UPS systems where a load factor of 70 to 80 percent is expected, there
is little difference in overall system efficiency.
An example of a passive standby UPS module, wherein the load is
normally fed via a switch/filter/conditioner, is shown in Fig. 5.15.
Simultaneously, the battery is in charging state. On loss of mains, the
load is fed from battery/inverter/filter.
Such units are normally to be seen where ratings are low, in other
words, 2 kVA and below. Advantages of the module are low cost and
compact, lightweight. Care should be taken to establish the qualities of
the subassembly containing the switch/filter/conditioner. The switch
may be a mechanical device which may not be sensitive enough for
computer loads. Clearly the quality of filtering and conditioning of raw
mains should also be reviewed. In addition, the system depends on
mains frequency and there is no true isolation.
Line interactive UPS is shown in Fig. 5.16. Under normal operation
the load is fed directly from mains with some voltage conditioning

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