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

Analysing the costs of HVDC High Voltage Direct Current (on photo: A transmission line of constant voltage of 600 kV DC, at 2400kilometers built in Brazil in 2012; by IVOLINES via Flickr)

Topics discussedIntroduction to HVDCCost structure of HVDC

Assumptions made in the price calculations

Thyristor based HVDC system versus high voltage AC systemVSC based HVDC system versus an AC system or a local generation source

Comparative cost information

Lets start with introduction!It specifies a system used for transmitting or exchanging electrical power by means of direct current.

HVDC transmission is widely recognised as being advantageous for long distance bulk power

delivery, asynchronous interconnections and long submarine cable crossings.

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

Comparative HVDC & AC Transmission Costs

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

HVDC converter station uses thyristor valves to perform the conversion from AC to DC and vice versa. Thevalves are normally arranged as a 12-pulse converter. The valves are connected to the AC system by means of aconverter transformers.

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 providingAC harmonic filters.

12 pulse converter

Idc = (V1- V2) / R

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

A large smoothing reactor is always installed on the DC side to reduce ripple in the DC current. This largesmoothing reactor also filters these harmonics. However for a submarine cable application instead of DCoverhead line DC filters are not required.

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

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

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

Comparing HVDC and HVAC

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Cost structure of HVDCThe cost of an HVDC transmission system depends on many factors, such as:

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

Even when these are available, the options available for optimal design (different commutation techniques, varietyof 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:

Cost structure of HVDC

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

Price variation of HVDC and HVAC

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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, converterstations are estimated to 250 MUSD.

It is strongly recommended to take contact with a manufacturer in order to get a first idea of costs and alternatives.The manufacturers should be able to give a budgetary price based on few data, as rated power, transmissiondistance, type of transmission, voltage level in the AC networks where the converters are going to be connected.

The choice of DC transmission voltage level has a direct impact on the total installation cost. At the design stagean 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 thetime 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, toestimate 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 ACand HVDC systems:

1. One is between thyristor based HVDC systems and a high voltage AC transmission system; AND2. 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 otherhand, the costs of transmission medium (overhead lines and cables), land acquisition/right-of-way costs are lowerin the HVDC case.

Moreover, the operation and maintenance costs are lower in the HVDC case. Initial loss levels are higher in theHVDC system, but they do not vary with distance. In contrast, loss levels increase with distance in a high voltageAC system.

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 localaspects (permits, cost of local labour etc.). When comparing high voltage AC with HVDC transmission, it isimportant 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.

The graph below shows that, the VSC based HVDC system is the better alternative economically when

HVDC cost breakdown

compared to either an high voltage AC system or a generation source local to the load centre (e.g., dieselgenerator).

VSC based HVDC system is the better alternative economically

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

Price example for a 50 MW VSC transmission

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Comparative cost informationHowever, the break-even distance and power transfer level criteria and the comparative cost information shouldbe 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 toconserve 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 transfercapacity of 600 MW.

What this shows therefore is that, in addition to the criteria mentioned above (power levels, distance, transmissionmedium, environmental conditions etc.), the market conditions at the time of the project is a critical factor, perhapsmore so than the numerical comparisons between the costs of an AC or DC system.

3. Technological developments

Third, technological developments have tended to push HVDC system costs downward, while the environmentalconsiderations 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 toconsider 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 RobertoRudervall, J.P. Charpentier and Raghuveer SharmaSubstation design / Appliation guide V AYADURAI BSC, C.Eng, FIEE

Analysing the costs of High Voltage Direct Current (HVDC) transmissionTopics discussedLets start with introduction!Cost structure of HVDCAssumptions made in the price calculationsThyristor based HVDC system versus high voltage AC systemVSC based HVDC system versus an AC system or a local generation source

Comparative cost information1.Conserve the environment2. System prices3.Technological developments