HVDC Classic
General Information
- Description
- Details
- Benefits
- Applications
- References
The High Voltage Direct Current (HVDC) systems are used for energy transmission world-wide. They are a useful supplement or in some cases the only alternative for traditional High Voltage Alternating Current (HVAC) systems.
These HVDC transmission systems are specifically used to:
- economically transmit electrical energy over long distances via overhead lines or cable,
- connect asynchronous grids or grids with different frequencies.
Siemens has been one of the leading companies in the HVDC business for more than 25 years.
HVDC converter stations are complex electrical systems. The main components are shown in the following pictures:
System components: Bipole
HVDC (High Voltage Direct Current) transmission systems connect two separate high voltage AC Systems via a DC link. The basic principle of operation of an HVDC system is based on the conversion of AC to DC and vice-versa by means of converter valves comprising power thyristors, which are the heart of a converter station.
Back To Back HVDC
Two independent neighbouring systems with different and incompatible electrical parameters (Frequency / Voltage Level / Short Circuit Power Level) connected via DC link.
Why Back to Back links?
- different system frequencies
- no additional short circuit power contribution to connected networks
- fully controllable power flow
Long Distance / Sea Cable
Two Remote AC systems, with a distance of normally 300 to 1400km (and more), are coupled together via a DC line. One converter station terminal is usually located close to the power generation and the other station in the load center.
Why Long Distance / Sea Cable HVDC?
- no reactive power loading of the transmission line
- complete control of energy flow
- no contribution to short circuit power in existing AC networks
- reduced losses
The following basic configurations are the most commonly used:
Monopolar HVDC
- One HV line for DC current transmission
- Return path optionally via ground or a LV conductor
- Rating up to 1500 MW
Operating modes of the Bipolar System
If an outage occurs in one pole or one line, there are several operating modes that allow continued operation of the healthy pole. When designed, overload capabilities are used, half of the converter equipment may be shut down is out of order, but less than half of the power rating is lost.
AC technology has proved very effective in the generation and distribution of electrical energy. Nevertheless, there are tasks which cannot be performed economically or with technical perfection by such methods. For instance: economic power transmission over very long distance, power transmission via cables, power transmission between networks operating asynchronously or at different frequencies, input of additional power without increasing the short circuit ratio of the network concerned. For all these tasks HVDC is not only a realistic technical and economic alternative to AC technology, but is also the only possible transmission method.
Economical Considerations
A review of various economic and technical aspects must be considered when examining the viability of an AC or DC transmission system.
The diagram shows that higher terminal cost for an HVDC transmission system have to be compared to the higher line cost for an AC transmission line.
For transmission distances above the “break-even-distance”, an HVDC transmission system represents the more economical solution.
The total investment is broken up into:
- terminal cost
- transmission line cost
- tower cost
- land cost and
- capitalized losses
Overall Conclusion
HVDC transmission results in the better return on investment for transmission applications above the break-even distance.
Only Technical Solution for...
- Asynchronous Interconnections
- Submarine Cable Transmissions
- Tie-Controlled Powe Exchange
- Limitation of Sort-Circuit Currents
- Coupling of two AC systems while maintaining the autonomy of each.
Controllability
The total controllability of HVDC systems is a superior feature / function when compared to an HVAC system.
| DC Connection | AC Connection | |
| load flow | fully controlled by the operator in both directions | uncontrolled, depending on acutal network conditions |
| peak supply | inherent overload capability, e.g. for peak load can be used actively | overload capability not controlled by the operator |
| stabilising connected AC networks | stabilising voltage and frequency by independently controlled active and reactive power supply | no controlled features |
Overload Characteristics
The overload capabilities of an HVDC system depend on system and ambient temperature and on the availability of redundant cooling equipment.
The short time overload capability for a few seconds is normally approx. 50%, the continuous overload capability approx. 10%.
Benefits of overload capability
- improved stability of AC systems
- increased stability limits of parallel AC lines
- shared spinning reserves
- supply of peak loads
- minimized power reduction in case of a pole outage
- economical benefits (at low ambiet temperature)
Applications of HVDC
Today, electric power is normally transmitted as alternating current as this is the easiest and most economic solution when power is consumed near the location it is generated. With the idea of using more renewable energies like wind or solar power, system development strategies go clearly in the direction of hybrid transmissions, consisting of integrated AC/DC interconnections and point-to-point bulk power transmission “highways” (DC backbones).
The most economic solution for long-distance bulk power transmission, due to lower losses, is transmission with High Voltage Direct Current (HVDC). Typically, DC line losses are 30–40% less than with AC lines, at the same voltage levels for long-distance cable transmission as well as to connect two independent AC systems with incompatible electrical parameters DC is the solution, technically and economically.
HVDC has a wide range of applications:
- Back-to-Back Transmission
- Submarine DC cable long-distance transmission
- Long-distance transmission with DC overhead line
Submarine DC Transmission
HVAC cable transmission systems have a limited transmission length, from both technical and economical points of view, whereas the HVDC has practically no restrictions. Therefore, it provides an optimal solution for longer transmission distances. The HVDC transmission is the solution for cable systems longer than 80 - 120 km.
HVDC, with its symmetrical monopole configuration, is a particularly attractive solution for cable applications. A wide range of economic cable designs is available in combination with HVDC. Typical applications include: energy platforms, offshore wind farms, island connections, urban in-feeds etc. which can be connected via submarine cables to the main grids.
Back-to-Back Transmission
DC Back-to-Back is frequently used to couple neighboring AC networks, e. g. two or more national grids, with different frequency or voltage control characteristics. In such cases a synchronous AC interconnection is not possible.
A Back-to-Back connection with HVDC will overcome the above mentioned difficulties: HVDC makes power flow. Furthermore, the interconnection with a Back-to-Back HVDC link will not increase the short-circuit power of the systems. In addition, it prevents the spread of cascading disturbances (firewall for Blackout prevention).
Overhead Line Transmission
The most economical solution for DC transmission is an interconnection via overhead lines (OHL).
DC overhead lines use more narrow transmission corridors for the same power transfer when compared with their AC counterparts and this is often a crucial issue while opting for DC transmission. In the future DC may also be used to increase the power transfer through existing rights-of-way by converting AC lines to DC.
All OHL sections are subjected to a higher risk of fault (from lightning strikes etc.) than the insulated cables and therefore HVDC has the capability to cope with faults on the DC line.
Since 1975, the proven Siemens HVDC technology has provided numerous comprehensive power solutions to customers all over the world. Take a look at our superior experience in the field of HVDC Classic and follow the steady improvements we developed over the years. Whether long distance transmission schemes, sea cable interconnectors or back-to-back stations – we have the adequate solutions for all kind of challenges.
More details about our references
Solution Information
Watch the video: The BritNed Interconnector
