OM226142 rev C
39
6 OPTIONS
6.1 INSULATION
TRANSFORMERS
The additional transformers are used in UPS input and/or output, to adapt the voltage, or
to separate the UPS from the mains and/or the load.
They are assembled in external cabinets.
6.1.1 By-pass insulation transformer
It is used when the three phase input mains is without neutral or the galvanic isolation
between the mains and the load is required.
During the UPS normal operating condition the inverter transformer provides for this task,
while during the bypass condition (for example in case of any overload), the mains feeds
the load directly. Generally this configuration is used when the output neutral conductor
must be different from the input, thus discriminating two different grounding systems.
6.1.2 Rectifier insulation transformer
It is used when the galvanic isolation between the three-phase mains and the battery is
required.
6.1.3 Voltage Adaptation Transformers
A voltage adaptation transformer can be connected at the UPS output terminals to adapt
the standard voltage to the value requested by the AC loads. It can also be connected at
the input terminals to adapt the actual mains voltage to the UPS specific value (see
technical specification). In case the galvanic isolation is not required an auto-transformer
can be used.
6.2 12-PULSE
RECTIFIER
The 12 pulses configuration is used to reduce the distortion of the current absorbed from
the mains (THD) to a value lower than 12%.
This ensures that the rectifier does not distort the supply mains, with regard to the other
loads; it also avoids the overheating of the cables due to the harmonics circulation.
This technology uses two 6 pulse rectifier bridges, which operates with an input voltage
having a phase displacement of 30°; the two DC outputs are connected in parallel through
two stabilizing chokes. A DC current transformer provides the feedback signal for the
current sharing control. The galvanic isolation between AC and DC side is provided by the
input transformer, that is designed with two secondary windings displaced by 30°
(delta/delta-star connection). This rectifier can also be designed without galvanic isolation;
in this case the phase-shifted rectifier is provided only on one rectifier bridge, while the
other is supplied directly by the mains through a three-phase filter inductor.
This configuration allows the system to cancel the typical harmonics of the 6 pulses
rectifier, namely the 5
th
and 7
th
. As a consequence only the higher order harmonics remain
(the 11
th
, 13
th
and the multiples). This system is highly recommended for high power
equipment, where the current distortion caused by the rectifier might affect other loads
connected to the mains. It’s also advisable to use 12 pulses rectification in order not to
overload an emergency diesel generator supplying the system in case of mains failure.
The typical harmonic content of the current absorbed by a 12P rectifier is shown below.
Harmonic order
1
5
7
11
13
17
19
THD
Amplitude (I
n
/I
1
)
100 % 1,9 %
1,3 % 7,2 % 5,7 % 0,4 % 0,3 %
9,7 %
