The voltage dependence of HVDC (High Voltage Direct Current) converters as a load in ac systems is studied in this paper. Analytical studies are conducted for various HVDC control modes. It is shown that HVDC converter behaves between constant power and constant current. Its reactive power is more close to constant current type load. It is also demonstrated that the unfavorable load voltage relationship was mainly caused by the large amount of capacitive compensation at the converter bus. In fact, those capacitors play more role than the converters themselves in driving the connected ac system to be vulnerable to voltage instability.