Inverter, Non-Linear Load Characteristics | Liebert 1000kVA instruction

Theory of Operation

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-modulated/six-step waveforms. These waveforms are filtered into low-distortion sine wave power. The inverter is controlled by digitally controlled logic. This logic controls the precise synchro- nization, 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 commer- cial 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 han- dle at least 125% of the rated current for up to 10 minutes and 104% of the rated current continu- ously. The System Control Cabinet (SCC) will transfer the critical load to the bypass source if the overload exceeds the system capacity or the capacity of the modules on-line. The multi-module over- load 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 an individual module overload condition is sustained (beyond inverter capac- ity) 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 redundant mode, the load may remain on the UPS sys- tem. Refer to 3.5 - 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 nat- ural path for reducing the fifth and seventh harmonic currents produced by the load. The inverter/fil- ter 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).