Page 159 - Power Quality in Electrical Systems

Uninterruptible Power Supplies 141

CNV. Leav Linv INV.
0.4 mH 0.4 mH
Lr
´
IN Cdc (6 w) OUT
Cr
25 µF 25 µF
Zσc
10 µF

Cdc´ = 100 µF 4Ω
Cr = 100 µF
Lr = 2.8 mH

Power rating- IM FW
200 V, 5 kVA
Power maintaining time- FW INV./CNV.
1 min
Figure 9.11 Electrical diagram of double-conversion UPS with flywheel energy stor-
age, rating 5 kVA, 1 min [9.16].

■ Minimum end-of-life disposal problem [9.16 and 9.19].
■ Ambient temperature (0 to 40°C) compared to batteries (20 to 25°C)
[9.19].
■ Measure available energy accurately by calculating speed and energy.
■ Can provide AC generator or DC converter output.

Con factors include the following:
■ Installed cost 1 to 1.4 times that of batteries [9.19]
■ Storage expansion not easy, requires adding units of comparable size

Other pro and con factors include availability and operator’s experi-
ence with flywheels.

Fuel cells
Fuel cells, using hydrogen as a fuel, have become a possibility to
replace lead-acid batteries in standby applications [9.20]. Figure 9.12
shows a comparison of the acquisition cost of lead-acid batteries and
fuel cells for a 10-year period in a standby power application. The
rising cost of batteries is based upon an assumption of replacement at
36- to 60-month intervals [9.21].