Liebert 30-130kVA Rectifier/Charger, Operation, Input Current Limit, Input Current Inrush

Theory of Operation

2.2.2Rectifier/Charger

The UPS module rectifier/charger consists of input fuses, AC current-limiting circuit, battery equalize charge circuit, DC filter, battery charge-current-limiting circuit, and bridge rectifiers.

Operation

The rectifier/charger converts the AC input power to DC power. This conversion is accomplished by 3- phase bridge rectifiers using SCRs. All phases are individually fused. For all modules, reflected input current THD is less than 30% at full load (which may be reduced to less than 10% with optional fil- ter).

The filtered output of the rectifier/charger provides regulated DC power to drive the inverter and charge the battery.

Input

The input is sized to allow enough current to recharge the battery and supply a full-rated load at the same time.

Input Current Limit

AC input current sensing transformers (CTs) are used to measure current levels. Control circuitry monitors the CTs and restricts the AC current to less than 125% of the full input current rating by reducing the battery charging voltage. This current limit is adjustable from 100 to 125% of the system capacity measured in AMPS, with the default setting at 125% (maximum AMPS). An external dry contact closure (field supplied) activates a reduced second level of the battery charge current limiting circuit for use with a back-up generator.

Input Current Limit, Second Level

A second level of input current limit is initiated by an external contact closure (field supplied for use with back-up generator), and is adjustable from 85 to 100% (factory set at 100%).

During a rectifier re-start following battery discharge, the current slowly ramps up (walks-in) from 20% of the rated input current to 100% over a 15 to 20 second period. The maximum rate of change of the AC input current is 15% per second. The input current walk-in reduces the start-up surge distor- tion effects on all other equipment connected to the same source and prolongs the service life of inter- nal components.

Input Current Inrush

The maximum sub-cycle of inrush current is typically less than 6-8 times normal.

Input Power Factor

The rated input power factor is no less than 0.80 lagging at the nominal input voltage and the full rated UPS load. The optional input filter will improve the power factor to better than 0.92 lagging at full load. Refer to drawings for your specific model.

2.2.3Battery Charging Circuit

The UPS module charging circuit is capable of recharging the battery plant to 95% of full capacity within 10 times the discharge time. Recharging the last 5% takes longer because of characteristics inherent in the battery. DC ripple voltage is limited to 0.5% RMS to preserve battery life during long- term float charging while the UPS system is operating on utility source power.

Operation After Discharge

When commercial power is interrupted, the battery continues to supply DC power to the inverter without interruption to the critical load. If the AC source power is restored before the battery has fully discharged, the rectifier automatically restarts and resumes carrying the inverter and battery recharge load requirements.

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