As power demand grows, transmission capacity must increase. The electric power transmission market is scaling HVDC to higher voltages and higher capacities, using larger converters and thicker cables.

The Voltage Level (kV)

HVDC systems are classified by voltage (e.g., ±500 kV, ±800 kV, ±1100 kV). The long distance power transmission market explains that higher voltage reduces current for the same power, which reduces losses (I²R) and allows for longer distances. The voltage is bipolar (± means two conductors: +500 kV and -500 kV). The voltage level is limited by insulation (air, solid, liquid). The highest voltage today is ±1100 kV (China).

The Power Capacity (GW)

The power capacity of an HVDC link is determined by voltage and current. The hvdc transmission market has links up to many GW (e.g., 12 GW). A higher voltage also allows for higher capacity (for the same current). The capacity is also limited by the converter valves (thyristors or IGBTs). To increase capacity, the valves are connected in series and parallel. The cooling system must handle the losses.

The Ultra-High Voltage (UHV) DC (China)

China is the leader in UHV DC (voltages above ±800 kV). The high voltage direct current market notes that China uses UHV DC to transmit power from its remote coal and hydro plants in the west to the coastal cities. The distances are many km. The power is many GW. The system uses LCC technology (thyristors). The towers are very tall. The insulation is large.

The ±1100 kV Changji-Guquan Project (China)

The Changji-Guquan project is the world's highest voltage HVDC link. The electric power transmission market notes it operates at ±1100 kV and can transmit many GW over many km. The link connects Xinjiang (coal) to Anhui (load center). The converter stations are huge. The towers are massive. The project demonstrates the feasibility of UHV DC.

The Limitations of UHV DC

UHV DC requires large right-of-way (the towers are tall and wide). The hvdc transmission market notes that the insulation (air) is the limiting factor. The cost of the converter stations increases with voltage. The losses are not proportionally lower because corona discharge (ionization of air) increases at high voltage. The economic optimum is found.

The Voltage Sourced Converter (VSC) Voltage Limit

VSC (using IGBTs) is currently limited to lower voltages (e.g., ±500 kV). The smart grid transmission market notes that the IGBTs have a limited blocking voltage (few kV). To reach high voltage, many IGBTs must be connected in series (which is difficult). The IGBT modules must be balanced. The losses increase. VSC voltage levels are increasing (e.g., ±800 kV prototypes). VSC may eventually catch up.

The Submarine Cable Voltage Limit

Submarine HVDC cables are limited by the insulation (extruded XLPE). The long distance power transmission market uses XLPE for voltages up to ±525 kV. Higher voltages require mass-impregnated (MI) cables (which are bulkier). The trend is toward higher voltage XLPE. The cable thickness increases with voltage.

The Overhead Line Voltage Limit (Corona)

For overhead lines, corona discharge (ionization of air around the conductor) limits the voltage. The hvdc transmission market uses bundled conductors (two or more sub-conductors) to reduce the electric field. The bundle spacing is optimized. The corona creates losses (and radio interference). For very high voltage, the bundle size increases.

The Converter Station Footprint (Space)

High voltage converter stations require large space for: (1) Valve halls (containing thyristors/IGBTs), (2) Transformers, (3) AC filters (for LCC), (4) Reactive compensation (for LCC), (5) DC switchyard. The electric power transmission market notes that the footprint increases with voltage. For UHV DC, the station may occupy many hectares. The cost of land is significant.

The Cooling System (Losses)

HVDC converters have losses (heat). The high voltage direct current market uses: (1) Water cooling (most efficient), (2) Air cooling (for smaller systems). The cooling system includes pumps, heat exchangers, and fans. The heat is dissipated to the atmosphere (or to a river). The cooling system is a significant part of the converter station.

The Economic Optimum Voltage

There is an economic optimum voltage for each distance and power. The hvdc transmission market uses an optimization model that considers: (1) Converter cost (increases with voltage), (2) Line cost (decreases with voltage, because less current), (3) Losses (decrease with voltage). The optimum voltage increases with distance and power. For very long distance (many km), UHV DC is optimal.

The Future: ±1500 kV DC

Research is underway for ±1500 kV DC. The electric power transmission market is developing: (1) New insulating materials (gas, solid, composite), (2) New tower designs (to reduce corona), (3) New converter valves (SiC devices). The goal is to transmit many GW over many km with lower losses. The electric power transmission market is scaling up. And the long distance power transmission market continues to push the limits of voltage and capacity, enabling the transmission of ever-larger amounts of power over ever-longer distances.

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