Q3Power System - I

Advantages of HVDC transmission: (1) Lower transmission losses over long distances, since DC transmission has no reactive power flow and no skin effect, allowing full conductor cross-section utilization; (2) Asynchronous interconnection — HVDC links can interconnect two AC grids that are not in synchronism (different frequency or unsynchronized phase) without requiring them to operate in lockstep; (3) No stability limit on transmittable power due to line reactance/angle, since DC has no concept of power-angle stability limit that constrains long AC lines; (4) No charging current/reactive power compensation required along the DC line, since there is no line capacitance charging current at DC (though AC-side reactive compensation is still needed at the converter stations); (5) Reduced right-of-way and tower cost for a given power level, since DC transmission requires fewer conductors (typically a bipolar 2-conductor scheme) for the same power compared to a 3-phase AC line; (6) Precise, fast controllability of power flow through converter firing angle control, useful for damping oscillations and providing frequency support between interconnected systems; (7) Economical for very long-distance bulk transmission and submarine cable crossings, where AC cable charging current would otherwise be prohibitive.

Disadvantages of HVDC transmission: (1) High cost of converter stations (rectifier and inverter equipment, transformers, filters) at each terminal, making HVDC economical only beyond a certain 'break-even distance' compared to AC; (2) Converters generate significant harmonics on both AC and DC sides, requiring extensive and costly filtering equipment; (3) Converters (particularly LCC-based) consume reactive power that must be locally supplied via capacitor banks or synchronous condensers; (4) Circuit breaking on the DC side is more difficult and costly than AC, since DC has no natural current zero crossing to aid arc interruption; (5) Lack of simple, economical multi-terminal tapping — unlike AC lines, where additional substations can be tapped relatively easily along the route, extending an HVDC line to serve multiple intermediate load points (multi-terminal HVDC) is technically more complex and less common; (6) Converter station control and protection systems are considerably more complex than conventional AC switchgear, requiring specialized maintenance expertise.