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Grades 9–12 · Systems

How does an HVDC link connect two electricity grids?

A high-voltage direct-current link can transfer electrical power between alternating-current networks, even when the networks do not share the same frequency or phase. At each end, power-electronic converter equipment connects the AC system to a DC circuit. One terminal supplies DC power while the other converts received power back to AC; suitable links can reverse the transfer direction.

S50 visual explanationThe sending converter receives 202 MW and loses a supplied 2 MW. The DC line receives 200 MW at 400 kV with 10 ohms total circuit resistance. Current is P/V; line loss is I²R. The receiving converter loses another supplied 2 MW. Converter behavior is otherwise idealized.Two converters join AC networks through DCDC sending end: 200 MW at 400 kVAC → DCDC → ACConverterConverterComplete DC circuitTotal resistance 10 ΩAC grid AAC grid BIn: 202 MWOut: 195.5 MW500 ALine loss 2.5 MWConverter losses: 2 MW at each end

In the diagramTwo converters join AC networks through DCDC sending end: 200 MW at 400 kVAC → DCDC → ACConverterComplete DC circuitTotal resistance 10 ΩAC grid AAC grid BIn: 202 MWOut: 195.5 MW500 ALine loss 2.5 MWConverter losses: 2 MW at each end

An explicit outgoing-and-return DC circuit joins two converters. At 400 kV and 200 MW entering the line, 500 A causes 2.5 MW line loss. Including both converter losses, 202 MW supplied becomes 195.5 MW delivered.

The transmission circuit needs a complete current path. Depending on the design, this may involve two conductors or a specified return arrangement. The diagram uses an explicit two-conductor circuit, not electricity disappearing into one cable. Switching devices in the converters control electrical waveforms. Ordinary transformers may serve the AC side, but a transformer alone cannot continuously change a steady DC voltage.

High voltage reduces current for a stated transmitted power because P = VI. In the idealized DC sending-end example, 200 megawatts at 400 kilovolts requires 500 amperes. A total circuit resistance of 10 ohms dissipates I²R = 2.5 megawatts. Holding DC sending power and resistance fixed while halving voltage doubles current and increases resistive loss fourfold. These values are illustrative, with voltage measured across the complete DC circuit.

Line resistance is only part of the budget. The example assigns 2 megawatts of loss to each converter. Supplying 202 megawatts to the sending converter leaves 200 megawatts entering the DC line and 195.5 megawatts delivered after both line and receiving-converter losses. Real converter losses vary with operating conditions. Cable insulation, capacitance, distance, terminal cost and control requirements also affect technology choice. This explains a useful transmission architecture rather than proving that DC is always preferable to AC.

About this explanation

Starting knowledge: Use P=VI, I²R and conservation of energy; distinguish AC and DC.

Sources

  • U.S. Department of Energy — Connecting the Country with HVDC
  • OpenStax — University Physics Volume 2: Transformers
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