Analysing the costs of High Voltage Direct Current (HVDC) transmission

Topics discussed

• Introduction to HVDC
• Cost structure of HVDC
  • Assumptions made in the price calculations
    • Thyristor based HVDC system versus high voltage AC system
    • VSC based HVDC system versus an AC system or a local generation source
  • Comparative cost information

Let’s start with introduction!

It specifies a system used for transmitting or exchanging electrical power by means of direct current.

HVDC is used to transmit electricity over long or very long distance by overhead transmission lines or submarine cables, because it then becomes economically attractive over a conventional AC transmission lines.

In a HVDC system, electric power is taken from a three-phase AC network system converted to DC in a converter station, transmitted to the receiving end by a DC cable or a DC overhead line and then inverted back to AC in another converter station and injected in to receiving end AC network system.

The 12 pulse converter produces odd harmonic currents (h = np ± 1) on the AC side ie 11th, 13th, 23rd, 25th, 35th, 37th and so on… These harmonics are prevented from entering in to the AC network system by providing AC harmonic filters.

I_dc = (V₁– V₂) / R

The 12 pulse converter produces even harmonic currents on the DC side ie 12th, 24th, 36th. These even harmonics are prevented from entering DC overhead line by providing DC filters.

An HVDC link is two rectifier/inverter stations connected by an overhead line or DC cables. Bipolar HVDC line uses only two insulated sets of conductors rather than three. This results in narrower rights of way, smaller transmission towers.

For a given cable conductor area, the line losses with HVDC cables is about 50% of that AC cables. This is due to AC cables requiring more conductors ie three phases, carrying reactive component of current, skin-effect and induce currents in the cable sheath and armour.

Transmitting power over DC lines requires fewer conductors (i.e. 2 conductors; one is positive another is negative).

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Cost structure of HVDC

The cost of an HVDC transmission system depends on many factors, such as:

• Power capacity to be transmitted,
• Type of transmission medium,
• Environmental conditions and
• Other safety, regulatory requirements etc.

Even when these are available, the options available for optimal design (different commutation techniques, variety of filters, transformers etc.) render it is difficult to give a cost figure for an HVDC system.

Nevertheless, a typical cost structure for the converter stations could be as follows:

As a guidance, an example showing the price variation for an AC transmission compared with an HVDC transmission for 2000 MW is presented below.

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Assumptions made in the price calculations

For the AC transmission a double circuit is assumed with a price per km of 250 kUSD/km (each), AC substations and series compensation (above 600 km) are estimated to 80 MUSD.

For the HVDC transmission a bipolar OH line was assumed with a price per km of 250 kUSD/km, converter stations are estimated to 250 MUSD.

The choice of DC transmission voltage level has a direct impact on the total installation cost. At the design stage an optimisation is done finding out the optimum DC voltage from investment and losses point of view.

The costs of losses are also very important – in the evaluation of losses the energy cost and the time horizon for utilisation of the transmission have to be taken into account.

Finally the depreciation period and desired rate of return (or discount rate) should be considered. Therefore, to estimate the costs of an HVDC system, it is recommended that life cycle cost analysis is undertaken.

Two different comparisons are needed to highlight the cost comparison between high voltage AC and HVDC systems:

1. One is between thyristor based HVDC systems and a high voltage AC transmission system; AND
2. The other between a VSC based HVDC system; an AC system and a local generation source.

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Thyristor based HVDC system versus high voltage AC system

The investment costs for HVDC converter stations are higher than for high voltage AC substations. On the other hand, the costs of transmission medium (overhead lines and cables), land acquisition/right-of-way costs are lower in the HVDC case.

The following picture shows the cost breakdown (shown with and without considering losses):

The breakeven distance depends on several factors, as transmission medium (cable or OH line), different local aspects (permits, cost of local labour etc.). When comparing high voltage AC with HVDC transmission, it is important to compare a bipolar HVDC transmission to a double-circuit high voltage AC transmission, especially when availability and reliability is considered.

VSC based HVDC system versus an AC system or a local generation source

VSC based HVDC systems cater to the small power applications (up to 200MW) and relatively shorter distances (hundred of km) segment of the power transmission spectrum.

As a guidance, a price example for a 50 MW VSC transmission with land cable is presented below:

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Comparative cost information

However, the break-even distance and power transfer level criteria and the comparative cost information should be taken in the proper perspective, because of the following reasons:

1. Conserve the environment

In the present (and future) industry environment of liberalised competitive markets and heightened efforts to conserve the environment. In such an environment, the alternative for a transmission system is an in-situ gas-fired combined cycle power plant, not necessarily an option between an AC transmission and a HVDC one.

2. System prices

Second, the system prices for both AC and HVDC have varied widely even for a given level of power transfer. For example, several different levels of project costs have been incurred for a HVDC system with a power transfer capacity of 600 MW.

3. Technological developments

Third, technological developments have tended to push HVDC system costs downward, while the environmental considerations have resulted in pushing up the high voltage AC system costs.

Therefore, for the purposes early stage feasibility analysis of transmission system type, it is perhaps better to consider HVDC and high voltage AC systems as equal cost alternatives.

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References

• High Voltage Direct Current (HVDC) Transmission Systems Technology Review Paper – Roberto Rudervall, J.P. Charpentier and Raghuveer Sharma
• Substation design / Appliation guide – V AYADURAI BSC, C.Eng, FIEE