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Theory of Operation
11
2.2.2 Rectifier/Charger
The UPS module rectifier/charger consists of an input circuit breaker, AC current limiting circuit,
battery equalize charge circuit, DC filter, battery charge current limiting circuit and bridge rectifiers.
Optional items are an isolation transformer and a 12-pulse rectifier (these are standard on 1000kVA
units).
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 1000kVA models and
modules with the 12-pulse rectifier option, input current reflected THD is less than 9% at full load
(which may be reduced to less than 4% with optional filter). For all modules with 6-pulse rectifiers,
input current reflected THD is less than 30% at full load (which may be reduced less than 7% with
optional filter).
The filtered output of the rectifier/charger provides regulated DC power to drive the inverter and
charge the battery.
Input Circuit Breaker
The input circuit breaker (CB1) is sized to allow enough current to recharge the battery and supply a
full rated load at the same time. The circuit breaker contains a thermal magnetic trip mechanism and
an undervoltage release that interrupts power, preventing damage to the system, if there is an inter-
nal AC overcurrent condition or a short circuit. This circuit breaker must be closed manually.
Isolation Transformer
The optional input isolation transformer—standard with 1000kVA models and with the 12-pulse rec-
tifier option—has a dry type core and copper windings with Class H insulation. The transformer on
modules with standard 6-pulse rectifier has wye primary and delta secondary windings. For models
with the optional 12-pulse rectifier, the transformer has a delta primary, with delta and wye second-
ary windings. The isolation transformer provides the initial step of critical bus isolation. This reduces
the AC shock hazard at the battery and at other DC components and prevents a DC fault from dis-
rupting upstream AC circuitry.
The transformer has a nominal tap and one tap 6% below the rated nominal input voltage (normally
used for 460 VAC input).
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% and is fac-
tory-set at 115%.
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%). This second level of
input current limit may be used to set the maximum amount of input current permitted under all
operating conditions of connected load and battery recharge.
During a rectifier restart following battery discharge, the current slowly ramps up (walks-in) from
20% of the rated input current to 100% over 15 to 20 seconds. The maximum rate of change of the AC
input current is 15% per second. The input current walk-in reduces the start-up surge effects on all
other equipment connected to the same source and prolongs the service life of internal components.
Input Current Inrush
The maximum sub-cycle of inrush current due to the optional input isolation transformer is typically
less than five times the rated input current for the first 1/2 cycle with the optional input filter. With-
out this transformer, inrush current is typically less than three times nominal.
Input Power Factor
The rated input power factor is not less than 0.85 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 your submittal package or installation drawings in the installation manual for your
specific model.