HVDC - High Voltage DC generation
The high voltage in this system is generated from the high voltage secundary winding of the transformer followed by a voltage double circuit.
When the voltage from the transformer goes negative on connection 5 the capacitor C 58 is charged trough D48. When the transformer voltage goes positive on connection 5 the transformer voltage is in series with the capacitor voltage of C 58 and C59 will be charged at a double value trough D47.
The RSS 100NO3 are driving the transformer alternating each one half the square wave comming from the input.That is why the incomming square wave is inverted by the as inverter connected nor gates. 

There are other possibilities to generate HV. but that you have to find yourself