how to change pole (anode -cathode - anode) in a electrolysis system

If the electrodes are the same material, simply reverse the supply polarity.

Hello hoantran0712

Are you intending to reverse poles of the electrodes under load of 8,000 to 9,000 Amperes? (While the system is actually working)

If so, please advise the total DC Voltage supplied to the process plant electrodes.

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Kind Regards....

You will have large arcing if you try to open the circuits with a contactor while operating. Even if the DC voltage is low during operation, the large di/dt Will produce large over-voltages. It is feasible but the DC contactors needed will be very expensive.

If you have a simple diode rectifier with step-down transformer, I suggest that you turn off the rectifier AC supply and then activate some some DC change over contactors. Since these contactors will not commute the DC current, they simply need to be rated for the load current. You must make sure that they are interlocked properly to prevent opening them under load. This also apply during a power failure. It might take a few seconds for the DC current to decreases to a low level. You might need a mechanically held change over contactor pair.

The other possibility is to have two rectifiers connected in opposite polarity with diodes to prevent back current flow. Then you simply turn the AC on for the required rectifier. This way needs more semiconductors and two AC contactors but you don't need the change over switch. It can be an attractive solution if you already have a spare rectifier.

If you want a rapid change of polarity, we could make you a semiconductor switch rated for that current. Then you would be able to reverse polarity hundreds and even thousands of time per seconds without worrying about over-voltages. Depending on your situation, this could be a simpler solution and maybe less expensive than a pair of DC contactors.