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Introducing HVDC What Is HVDC? Brochure Introducing HVDC What is HVDC? High-voltage direct current (HVDC) transmission is an efficient technology designed to deliver large amounts of electricity over long distances. The technology is a key component in the future energy system based on renewable energy sources. 3 2 4 1 3 1 HVDC Converter station Rectifier | 2 HVDC converter station Inverter | 3 AC | 4 DC What are the benefits of HVDC? centers where it is needed, hundreds or even thousands of HVDC systems can transmit more electrical power over long- kilometers away. Once installed, HVDC transmission systems er distances than a similar AC transmission system, which become an integral part of the electrical power system, means fewer transmission lines are needed, saving both improving the overall stability and reliability. money and land. In addition to significantly lowering electrical losses over long distances, HVDC transmissionis also very The HVDC systems core component is the power converter, stable and easily controlled, and can stabilize and intercon- which serves as the interface with the AC transmission sys- nect AC power networks that are otherwise incompatible. tem. The conversion from AC to DC, and vice versa, is achieved by controllable electronic switches, called valves. The HVDC market is growing rapidly and has become an important part of many transmission grids; not least because it can connect remote sources of electrical generation – often emissions-free renewable sources like hydro or wind – to load 2 Introducing HVDC Why use HVDC? There are a number of criteria which help to decide if AC or DC is best suited for a particular customer application, including: investment costs, system losses, system availability, power and voltage levels, means of transmission, availability of land and AC network support. Each transmission link has its own set of requirements justi- Electricity is usually transmitted using three-phase AC systems. fying the choice of HVDC, but the most common points in In DC systems, only two conductors are necessary to trans- favor include: mit electricity, and with lower losses than AC systems of − Lower investment cost similar scope. DC long distance transmissions require only a narrow power corridor. − Long distance water crossing − Lower losses − Asynchronous interconnections − Controllability − Limit short-circuit currents 1 − Environmental concerns Why use high voltage instead of low voltage? To reduce electrical losses. When electric power is transmit- 2 ted over long distances, electrical losses are lower if the cur- rent is low. This is obtained by using high voltage. 3 Why DC instead of AC? 1 AC overhead line | 2 DC overhead line | 3 DC land cables To reduce electrical losses and costs – and sometimes because it is the only possible solution. AC cable transmission links have What is an HVDC transmission link? a maximum distance of 50 to 100 km. For longer distances, An HVDC transmission link consists of three main com- HVDC is the only solution. Furthermore, with HVDC it is also ponents: a station to convert the alternating current of possible to connect grids that could not otherwise be con- the grid to direct current, the transmission equipment nected, e.g. with different frequencies – 50 Hertz and 60 Hertz. itself in the form of cables and overhead lines, and another station that converts DC back into AC so that it can be used by consumers. Investment Cost Critical Distance Total AC Costs Total DC Costs DC Cable Terminal AC Line Costs Converter Converter Costs station station DC Line Costs AC DC AC AC Terminal Costs Distance HVDC has a higher initial cost – the converter stations – but because An HVDC transmission link consists of a station for converting the grid’s the means of transmission (the overhead lines and the cables) are less alternating current to direct current, an overhead line or cable, and the expensive per kilometer with DC, there is a break-even distance. station that converts the directs current into alternating current Introducing HVDC 3 Can HVDC support renewable energy systems? 1 Wind power | 2 Solar power | 3 Hydro Alternatives to burning fossil fuels for electricity, includ- With an increasing number of installed HVDC links, in com- ing hydro, wind and solar generation, are often located bination with a rising need of back-up power for the often in remote locations. Hence the need for robust electrical volatile power supply from renewable energy generation, the transmission systems to ensure high availability, minimal concept of continental HVDC grids are now being discussed maintenance and of course, low losses. around the world. HVDC transmission systems offer the best technical and In order to meet the market demands, ABB has a grid simu- economical long distance transmission solutions, integrating lation center, located in Sweden, where faults in the surround- renewable generation, and stabilizing power networks. With ing AC grids are simulated and the advanced control system its inherent technical properties and superior ability of control, of an HVDC grid is verified. HVDC is a key component in the future energy system based on renewable energy sources. 4 Introducing HVDC DC Grids The evolution of the HVDC Grid in Europe and elsewhere will be in steps. The customers planning a grid will start by ordering point-to-point systems that should be grid-enabled for a future extension to a three- or multi-terminal system. In parallel there will be a few first regional multi-terminal projects. Over time, the point-to-point connections and the regional multi-terminal projects will be connected with each other to more extensive grids. The first small regional systems can be operated without DC breakers, but as the size and complexity increase, breakers will be gradually introduced. What is an HVDC grid? An HVDC electricity grid that can operate: − Independently of one or several disturbances (isolate a failure) 2 − In different operation modes in the connected AC and DC systems Technology gaps for the full realization include: 1 − DC breakers − Power flow control − Automatic network restoration − High voltage DC/DC converters Global rules and regulations for operation are required for market acceptance. Why DC Grids and not DC single links or AC? − It is the only relevant offshore solution − Loss reduction and increased power capacity versus AC − Less visual impact Why now? − Drivers are offshore wind, remote solar and grid constraints − HVDC Light systems and components are mature 3 1 Offshore wind | 2 Hydro | 3 Solar Introducing HVDC 5 What are the applications? 1 1 2 3 4 1 Connecting offshore wind | 2 City-center infeed | 3 Power from shore | 4 Connecting remote loads Connecting remote generation and for the daily need of often hundreds of persons working Some energy sources, such as hydro and solar power, are on the platform. If the power is instead supplied from shore, often located hundreds or thousands kilometers away from via an HVDC link, costs go down, emissions are lower and the load centers. HVDC will reliably deliver electricity gener- the working conditions on the platform are improved. ated from mountain tops, deserts and seas across vast dis- tances with low losses. DC links in AC grids HVDC links within an AC grid can be successfully utilized Interconnections to strengthen the entire transmission grid, especially under Connecting AC grids is done for stabilization purposes and demanding load conditions and during system disturbances. to allow energy trading. During some specific circumstances, Transmission capacity will improve and bottlenecks be dis- the connection has to be done using HVDC, for example solved. when the grids have different frequencies or when the con- nection has to go long distances over water and AC cables City-center infeed cannot be used because of the high losses. HVDC systems are ideal for feeding electricity into densely populated urban centers. Because it is possible to use land Connecting offshore wind cables, the transmission is invisible, thus avoiding the opposi- Wind parks are often placed far out at sea, because the wind tion and uncertain approval of overhead lines. conditions are more advantageous there. If the distance to the grid on land exceeds a certain stretch, the only possible Connecting remote loads solution is HVDC - due to the technology’s low losses. Islands and remotely located mines often have the disadvan- tage of a weak surrounding AC grid. Feeding power into the Power from shore grid with an HVDC link, improves the stability and even pre- Traditionally, oil and gas platforms use local generation to vents black-outs. supply the electricity needed to run the drilling equipment 6 Introducing HVDC Is this technology new? ABB has participated in the delivery of more than half of the world’s around 150 HVDC links. The world’s first commercial HVDC link, situated between the Swedish mainland and the island Gotland, was delivered by ABB already in the 1950’s. HVDC is an established technology that has been in use for long distances. The technology can use overhead lines or almost sixty years. During the first thirty years, it was a niche environmentally sound oil-free underground and underwater technology, with a limited number of installations per year. cables. With the changes in demands due to evolving environmental needs, HVDC has become a common tool in the design of ABB’s most recent development is ultrahigh-voltage direct future global transmission grids. current (UHVDC) with a rated voltage of up to 800 kilovolts (kV). Transmission at this voltage represents the biggest leap There are two types of HVDC, the classic technology using in transmission capacity and efficiency in more than two dec- thyristors for conversion, and VSC (voltage source converters) ades. technology, by ABB called HVDC Light, which uses transistors (IGBT’s) for conversion. A classic HVDC transmission typically Upgrades has a power rating of several hundreds of megawatts (MW) As a general rule, it is assumed that the major electrical and many are in the 1,000 – 3,000 MW range.
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