Reactor power requirments in HVDC line commutated converter

The remote generation and system interconnections lead to a search for efficient power transmission at increasing power levels. The problems of AC transmission particularly in long transmission have laid to development of DC transmission. However, as generation and utilization of power remain at alternating current, the DC transmission requires conversion at two ends i.e. from AC to DC at sending end and back to AC at receiving end. It is done at converter stations i.e. rectifier station at sending end and inverter station at receiving end. The physical process of conversion is such that the same station can switch from rectifier to inverter by simple control action, thus facilitating power reversal. The converters in HVDC station are line commutated type. They require that the current initiation in the valve can only be delayed with respect to the zero crossing of converter bus AC voltage. These results in lagging power factor operation of the converter. It needs requirement of reactive power sources to be connected at converter bus for better voltage control. The reactive sources are required at both the rectifier and inverter stations. While rectifier station appears as load in system, the inverter station can be viewed as generator consuming reactive power. This characteristic of inverter is not desirable and need suitable modification by providing adequate Var compensation.