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The wide-band frequency electro-magnetic (EM) noise is generated during turn-on and turn-off processes of thyristor valves in converter stations. It will be conducted along the primary circuitry, and may interfere with secondary systems by the way of conducted and radiated coupling. The plusmn800-kV UHVDC transmission project is one of newly developed technique and used barely in the world. The electro-magnetic interference (EMI) problem is one of most important issue that should be studied deeply in the design stage. Transients during turn-on and turn-off processes are introduced. Characteristics of electromagnetic disturbance (EMD) caused by valves are analyzed. Main influence factors on the characteristics of the EMD source such as voltages, currents, operation modes, are described based on calculation results.