DC link voltage definition for high power/high voltage inverters
The blog aims professional converter designers
We deal with inverters and active rectifier units. We discuss two level hard switched topology (the details are identical for 3 and more levels).
There are few factors determining the rated dc link voltage of the power converter:
the device 100 FIT voltage DC value
the device maximum voltage blocking (forward) capability- peak value
the required rated ac motor voltage- rms line to line value
the modulation method
the system dynamics
For a 4500 Vpeak device (IGCT and IGBT) the 100 FIT value is 2800 VDC.
With a 4500 Vpeak device, the rated dc link voltage is approx. 80-85% of 2800 VDC. The value depends on input rectifier type, active or passive (diodes). In view of lower rated dc voltage the FIT value will be lower (better MTBF).
In view of the rated dc link voltage the motor voltage will be under 3300 Vrms- the standard voltage level. it is worth noting that most of the motors are special motors for converter applications, so design of a motor for a lower input voltage, say 3150 V, has no impact on price or performances. In case of retrofit projects special attention is needed.
There are modulation techniques that can produce more ac side voltage. When deciding on the modulation index at rated condition one should consider:
available DC link voltage
permitted common mode maximum value
As rated modulation index we propose 0.92 to 0.95.
The system dynamics depends on type of application. Metal applications require high dynamic and Marine application low dynamic, so for metal application we propose the 0.92 value and for marine 0.95 (even more- less phase current at maximum load) values.