Double-conversions systems are characterized by their topology. In these systems, the incoming line is first converted to dc. The dc then provides input power to a dc-to-ac converter (i.e., an inverter). The inverter output is ac, which is used to power the critical load.

Many different types of inverters are used, each employing a variant of available technology. (Note that the recently revised NEMA PE 1-1993 [B23], identifies the double-conversion system as a “rectifier inverter.”)

Historically, the double-conversion UPS has found the most prominence in the industry. The double conversion UPS system has been available for many years and has proven to be reliable when operated within its design limits.

This type of system is the static electrical equivalent to the motor-generator set. The battery is connected in parallel with the dc input to the inverter, and provides continuous power to it any time the incoming line is outside of its specification or fails.

Switching to the battery is automatic, with no break in either the input to the inverter or the output from it.

The double-conversion system has several advantages:

— It provides excellent frequency stability.
— There is a high degree of isolation from variations in incoming line voltage and frequency.
— A zero transfer time is possible.
— Operation is relatively quiet.
— Some systems can provide a sinusoidal output waveform with low distortion.

In the lower power UPS applications (0.1–20 kW), the double-conversion UPS has the following disadvantages. (Many of these disadvantages can be minimized if the system is carefully specified to use the latest topologies.)

— There is lower overall efficiency.
— A large dc power supply is required (typically, 1.5 times the full rated load rating of the UPS).
— Noise isolation line to load can be poor.
— There is greater heat dissipation, which may affect the service life of the UPS.

In addition, if the inverter is the pulse width modulated type, the high-frequency circuitry may produce electromagnetic interference (EMI). This may require special filtering and shielding to protect sensitive equipment from radiated and conducted interference.

The double-conversion UPS may also produce excessive battery ripple current, possibly resulting in reduced battery life (see IEEE Std 1184-1994).

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