Resurgence of HVDC; why AC is inevitable

Reasons for the resurgence of HVDC

The first power systems (Edison) were DC, but AC won because of the transformer. HVDC has come back strongly since the 1950s because:

1. Mercury-arc valves, then thyristors and IGBTs made reliable high-power AC/DC conversion possible; modular multilevel VSC converters made it more flexible.
2. Long-distance bulk transmission: remote hydro, coal and renewable sources need transfer over 1000+ km, where HVDC is cheaper (beyond the break-even distance) and has no stability limit.
3. Submarine and underground cables: DC cables have no charging current, so long sea links (and offshore wind connections) are possible.
4. Asynchronous interconnection of grids with different frequency or for controlled power trade (back-to-back links).
5. Fast controllability helps damp oscillations and improves stability of AC systems.
6. Lower losses, two conductors, narrow ROW – important for environmental and land issues.
7. No increase of fault level when grids are joined.
8. Renewable energy and multi-terminal DC grid concepts have increased interest further.

Why AC is still inevitable

1. Transformers: AC voltage can be stepped up and down simply and efficiently; DC needs expensive converters.
2. Generation is AC: synchronous generators produce AC directly, and most loads (induction motors, lighting, appliances) use AC.
3. Easy tapping and meshed networks: AC substations can take power out at any point; HVDC tapping needs extra converters, so DC is mostly point-to-point.
4. Circuit breaking: AC current passes through zero twice per cycle, so breakers are simple; DC breakers are difficult and costly.
5. Lower terminal cost for short and medium distances (below break-even distance).
6. No harmonics or reactive demand from converters, and simpler operation and maintenance.
7. Existing infrastructure, standards and experience are all AC.

So HVDC is used for special purposes (long distance, cables, asynchronous links), while AC remains the backbone for generation, distribution and meshed transmission. Modern grids are hybrid AC/DC.