Liebert Series 600T manual Inverter, Output Regulation and Overload Performance

2.2.4Inverter

The inverter is a solid state device that converts the DC output of the rectifier/charger or the battery to AC power.

Operation

The inverter converts DC power—from either the battery or the rectifier/charger—into three pulse-width-modu- lated/six-step waveforms. These waveforms are filtered into low-distortion sine wave power. The inverter is con- trolled by an Application-Specific Integrated Circuit (ASIC). This integrated circuit controls the precise synchronization, amplitude, and frequency of the output voltage.

In addition to the inverter efficiently supplying a regulated AC output from a DC source, the output isolation transformer acts as a second stage of isolation between the critical load bus and the commercial source power. The inverter is configured to handle most critical load inrush surges. It maintains output voltage Total Harmonic Distortion (THD) within specifications even when handling non-linear computer loads. Refer to non-linear load characteristics.

Output Regulation and Overload Performance

The inverter is capable of sustaining full output voltage ±2% of the nominal voltage) for up to 150% overload at the output for as long as 30 seconds without reducing the output voltage. It can also handle at least 125% of the rated current for up to 10 minutes and 104% of the rated current continuously. The System Control Cabinet (SCC) will transfer the critical load to the bypass source if the overload exceeds the system capacity or the capac- ity of the modules on-line. The multi-module overload capacity (in terms of percent of rated full load current of modules on-line) is 140% for 20 seconds, 108% for 10 minutes, and 104% continuously.

Note that in normal operation the SCC will transfer the load to bypass before a UPS module exceeds its overload capacity. If a module overload condition is sustained (beyond inverter capacity) the UPS module is automatically taken off-line (disconnected from the critical bus) and is shut down. If the UPS system is operating in the redun- dant mode, the load may remain on the UPS system. Refer to 3.4 - Automatic Operations.

Non-Linear Load Characteristics

Computers and computer equipment with switching power supplies generate non-linear currents rich in fifth and seventh harmonics.

The inverter pulse-width-modulated/six-step waveform, coupled with the output filter, provides a natural path for reducing the fifth and seventh harmonic currents produced by the load. The inverter/filter limits the output voltage THD to less than 5% with up to 100% typical electronic data processing (EDP) loads. EDP equipment characteristically includes both non-linear and linear load components.

Unbalanced Load Characteristics

Unbalanced loads are actively regulated. The phase-to-phase voltage balance is maintained to within 2%, even with a 50% load imbalance.

The three-phase, root mean square (RMS—approximately 70% of the peak) average voltage is also regulated through a separate control circuit (phase-to-phase sensing).

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