The 33rd China International Exhibition on Electric Power Equipment and Technology
Shanghai International Energy Storage Technology Application Expo / Hydrogen Energy Expo
High Voltage Direct Current (HVDC) transmission uses direct current at high voltages (typically ±500 kV to ±1,100 kV) to transmit large amounts of electricity over long distances with lower losses than equivalent AC systems, and to connect asynchronous AC grids. HVDC systems consist of converter stations at each end that convert AC to DC (rectifier) and DC back to AC (inverter), connected by DC transmission lines or cables. The two main converter technologies are Line Commutated Converters (LCC), using thyristor valves — the mature technology used in China's UHV DC projects; and Voltage Source Converters (VSC), using IGBT valves — a newer technology offering black start capability, independent active and reactive power control, and suitability for offshore wind and multi-terminal DC grids. China operates the world's largest HVDC network, with over 30 long-distance DC transmission projects.
5 Key Questions About HVDC
HVDC offers several advantages over HVAC for long-distance transmission: lower transmission losses (no reactive power losses, lower skin effect); no reactive power compensation required along the line; ability to connect asynchronous AC grids (different frequencies or uncontrolled interconnections); better controllability of power flow; no stability limits on transmission distance (AC systems have synchronous stability limits); and lower line cost for very long distances (two conductors versus three for AC, though converter stations are more expensive). HVDC is generally preferred for distances over 600–800 km for overhead lines and 50–80 km for underground/submarine cables.
LCC (Line Commutated Converter) HVDC uses thyristor valves that require an existing AC voltage for commutation, making LCC suitable for bulk power transmission between strong AC grids. LCC systems can handle very high power ratings (up to 12 GW for the Changji–Guquan ±1,100 kV project) but cannot provide reactive power independently and require a minimum short-circuit ratio at the AC connection point. VSC (Voltage Source Converter) HVDC uses IGBT valves that can independently control active and reactive power, provide black start capability, connect to weak or passive AC grids (including offshore wind farms), and operate in multi-terminal configurations. VSC is preferred for offshore wind, urban infeed cables, and multi-terminal DC grids despite higher losses than LCC.
A multi-terminal HVDC (MTDC) grid connects three or more converter stations on a common DC bus, enabling power to be routed between multiple AC systems through the DC network. MTDC grids offer greater flexibility than point-to-point HVDC, enabling power to be redirected between multiple sources and loads in response to grid conditions. China is developing the world's first large-scale MTDC grid in the Zhangbei region, connecting wind, solar, and pumped hydro resources with Beijing's load centre. VSC technology is essential for MTDC grids due to its ability to independently control power flow at each converter station.
An HVDC converter station contains: converter transformer(s) providing the AC voltage for the converter valves; converter valves (thyristor or IGBT) arranged in 6-pulse or 12-pulse bridges; smoothing reactors limiting DC current ripple; DC filters and AC harmonic filters; reactive power compensation equipment (capacitor banks, SVCs, or STATCOMs for LCC; built-in for VSC); surge arresters; DC switchgear; and control and protection systems. The valve hall housing the converter valves is the most distinctive feature of an HVDC station, containing thousands of series-connected thyristors or IGBTs operating at hundreds of kilovolts.
HVDC technology is evolving in several directions: higher voltage ratings (±1,100 kV demonstrated in China, ±1,200 kV under development); higher power ratings per bipole (12 GW demonstrated); VSC technology improvements reducing losses and increasing power ratings; multi-terminal and meshed DC grid development; offshore HVDC for connecting remote wind farms; and hybrid LCC-VSC systems combining the high power capability of LCC with the flexibility of VSC. China's State Grid and Southern Grid are the world's most active HVDC developers, with Chinese manufacturers TBEA, NR Electric, and NARI Technology supplying converter equipment for domestic and international projects.
Key Takeaways
Capacitors improve power quality and system efficiency by correcting power factor and filtering harmonics. As industrial loads become more complex and power electronics proliferate, capacitor applications are expanding. EP Shanghai showcases capacitor technologies for power system and industrial applications.
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