What is the purpose of these didoes? I couldn't find it because I don't know it's name.
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On the left is a full-bridge fully-controlled thyristor rectifier. With that circuit it's possible to turn DC into AC if the outer DC voltage is ever higher than the rectified AC voltage. The current flow is reversed then.
D1 stops this reversed current from the DC connector.
In addition, D2 and D1 function as a crowbar for reversed polarity on the DC connector.
There is no real reason to have D1, though, because the ability to turn DC into AC relies on firing the thyristors in the correct pattern. One could simply forbid the control logic to use that pattern.