Download AN ELECTRONICALLY ISOLATED 12

AN ELECTRONICALLY ISOLATED 12-PULSE AUTOTRANSFORMER
RECTIFICATION SCHEME TO IMPROVE INPUT POWER FACTOR AND LOWER
HARMONIC DISTORTION IN VARIABLE-FREQUENCY DRIVES
ABSTRACT:
Diode rectifier front-ends with large dc bus capacitors used in variable-frequency drives
draw discontinuous current from the power system, resulting in a poor input power factor and
high input current distortion. The 12-pulse and 18-pulse techniques used to reduce input current
harmonics are popular because of their simplicity but are bulky and expensive. Autotransformer
techniques require additional external magnetic components that are bulky and add to the cost of
the system. A new electronically isolated 12-pulse autotransformer topology that does not need
bulky external magnetic components is proposed in this paper. The proposed topology employs
active switches to achieve electronic isolation between the two rectifiers, resulting in continuous
input current with reduced harmonics. The proposed non regenerative active scheme achieves
less than 5% total harmonic distortion at a much lower cost compared with other passive or
active topologies currently in the market.
To reduce the influence of wiring inductance between switch–diode pairs and the main dc
bus capacitance of the inverter section, a snubber capacitor is employed on the inverter module.
A laminated bus structure can be used between the switches (S1∼S4) and the inverter section to
reduce inductance. Switches S1-S2 and S3-S4 are operated at a high switching frequency with an
equal duty ratio of 0.5 to optimize the rating of the switches. The criterion for establishing an
optimal switching frequency will be discussed later in this paper. The equal duty cycle allows
balanced current flow from each three phase set of the autotransformer, thereby reducing the
input current THD. The voltage across switches S1∼S4 is the instantaneous difference between
the voltages across either CDC1 or CDC2 and CDC as the switches that are OFF have the
voltage of either CDC1 or CDC2 at one side of the switch and CDC on the other side of the
switch. Hence, the voltage rating of the switches need not be that of the full dc bus voltage of the
VFD.