Chemical Processes often have large power rectifiers as their primary source of DC Power. If these rectifiers are turned off, either for maintenance or a malfunction, there is a danger to the hardware of the chemical process.
To solve this problem, standby rectifiers are installed in parallel with the main rectifiers. These rectifiers, known as Polarisation Rectifiers, pass electric current through the process and prevent damage. As these rectifiers are used as stand by, they have to be extremely reliable.
During start up of the electrolyser, as the main rectifier is off, the PR supplies current to the electrolyser, and continues to supply the current as long as the output voltage of the main rectifier is less than the voltage of PR. The moment the main rectifier voltage is equal to the PR, the PR supplies current equal to or less than the set point current.
During normal operation this unit operates in parallel with the main rectifier supplying rated current each polarisation rectifier consists of 2 rectifiers which are designed to operate in parallel redundant mode, A third rectifier known as the Booster rectifier may also be used.
Normally both paralleled rectifiers supply 50% of the output current. In case of failure of one of the rectifiers, the healthy unit supplies 100% current to the Electrolyser without any interruption.
The Booster polarisation rectifier is connected between the positive and negative end of predetermined set of elements.
The PR is designed to operate is constant current mode and the current is adjustable manually.
The PR has various protections like surge protection, fast acting semiconductor fuses, DC overload, DC over voltage, short circuit protection and AC under voltage and also fast acting current limiting which protects the PR from supplying current higher than the set value.
With built in power factor correction circuitry, the PF is better than 0.95 Lag. Efficiency is greater than 85-90%. Output voltage setting is possible from 0 to 100%, regulation is better than 1% and ripple <1%.
Different configurations like 6 pulse or 12 pulse thyristor bridge switching results in reduced harmonics and ripple.
Masstech also offers SMPS based modular configuration where n+1 redundancy makes the system more economical. Also Power factor > 0.93 is achieved without power factor correction circuitry and efficiency is better than 90%.