In frequency converters, there are two rectifier alternatives, and, as everything, both have advantages in relation to each other
Diode Front End
The most common, and simplest, rectifier designs use diodes, and has the following benefits:
- It is the most price-competitive solution, since diodes are less costly than using power electronics, as the active front end alternative, not only in the inverter but also in the rectifier
- In terms of reliability, it should also be a better option, since it implies having less active parts that are more given to fail than passive elements as in the case of diodes
- Power density is higher, this is, cabinets containing diode rectifiers usually will lead to less length, thus being capable of fitting more power conversion in less space
- As the number of pulses increases (6-pulse, 12-pulse, 18-pulse, 24-pulse, 36-pulse), the harmonic content dropped to the grid is lower, with 24-pulse diode front end rectifiers usually being able to loosely cover IEC and IEEE requirements
Active Front End
Active rectification is a technique for improving efficiency of rectification by replacing diodes with actively controlled switches, with some of the advantages listed below
- Allows a regenerative operation, this is, to revert back the energy to the grid, and although it is not currently a mode of operation expected in a Shore connection application, there could be some cases were it could be useful
- Whenever a shipyard is testing the generators in the motors of their vessels, they might need to drop the energy back to the system, otherwise they would need to burn it
- With Floating Production System (FPSO/FSO) or similar vessels, there’s the possibility that, if they are producing energy in the dock, they can source it back to the grid, I would say this is not something realistic at the moment, but I know there are some proposals or concepts that value it
- The harmonics content dropped to the grid is lower than the diode front end solutions, in our case, with distortions with a THD lower than 1%
- The active filtering part is implemented by controlling the current harmonics such that these cancelling harmonics (such 5th, 7th, 11th,..., and zero sequence harmonics) effectively eliminate those that would be created in a standard variable frequency drive and motor/generator system.
- The power factor at that point is corrected up to 1, between the load capacity and the power capacity of the rectifier
- It has a better power stability, with a better quality unaffected from mains power fluctuations, that can react injecting power in the cases that there are variations in the grid, with a higher flexibility and possibilities of slightly stepping up the dc-bus voltage if its needed