Q4Power System - I

(a) Voltage Source Converters (VSC) in HVDC

A Voltage Source Converter (VSC) used in modern HVDC schemes employs self-commutating semiconductor switches — typically Insulated Gate Bipolar Transistors (IGBTs) — arranged in a bridge configuration (commonly a Modular Multilevel Converter, MMC, in current-generation VSC-HVDC schemes), which can synthesize an AC voltage waveform of controllable magnitude, frequency and phase angle from a constant-polarity DC voltage bus, using high-frequency pulse-width modulation (PWM) or multilevel switching techniques, entirely independent of the connected AC system's own voltage waveform (unlike LCC, which requires the AC voltage itself for commutation).

Because the VSC can independently set both the magnitude and phase angle of its synthesized AC voltage relative to the AC system voltage, it can independently and continuously control both active power flow (via the phase angle difference between converter and system voltage) and reactive power exchange (via the magnitude difference), operating in all four quadrants of the P-Q plane, without needing the AC system to supply a commutation voltage — allowing VSC-HVDC to connect to weak AC grids or fully passive networks with no local generation (such as offshore wind farm platforms), to black-start a de-energized AC network, and to eliminate the commutation-failure risk inherent to LCC converters during nearby AC system faults.

(b) Comparison of AC and DC Transmission Systems

• Losses: DC has no skin effect and no reactive power flow, giving lower losses over long distances; AC suffers additional skin-effect and reactive-current-related losses that grow with distance and line reactance.
• Stability: DC transmission has no power-angle stability limit since converter control directly sets power flow; AC long-distance transmission is constrained by the steady-state and transient stability limits related to line reactance and the sine of the power angle.
• Charging current: AC lines/cables draw continuous capacitive charging current, requiring reactive compensation and severely limiting achievable transmission distance for submarine AC cables; DC has no charging-current limitation at all.
• Cost: DC needs costly converter stations at both ends but cheaper line/cable construction (fewer conductors, narrower corridor); AC needs no converter stations but more expensive line/tower construction for equivalent power — DC becomes economical only beyond a certain break-even distance.
• Interconnection: AC permits easy multi-point tapping and requires synchronism between interconnected systems; DC (particularly two-terminal schemes) does not easily support intermediate tapping, but uniquely allows interconnecting asynchronous (different-frequency or unsynchronized) AC systems.
• Fault interruption: AC benefits from mature, well-established circuit-breaker technology exploiting the natural AC current zero; DC circuit interruption is inherently more difficult (no natural current zero), historically limiting multi-terminal DC grid development, though this is improving with modern hybrid DC breakers.
• Control flexibility: DC (especially VSC-based) offers fast, precise, independent control of active and reactive power flow, valuable for damping oscillations and supporting weak AC grids; AC power flow is governed by the network's inherent impedance and cannot be as flexibly and independently controlled without additional FACTS devices.

In summary, DC transmission (whether conventional LCC or modern VSC-based) offers superior efficiency, controllability and stability characteristics for very long-distance or submarine bulk power transmission and for asynchronous interconnections, at the cost of substantially higher converter-station capital investment, while AC remains the more economical, operationally simpler choice for shorter-distance, meshed, multiply-interconnected transmission and distribution networks.