The recent trend for wind farms to be constructed offshore has led to the development of a new HVDC technology. HVDCplus allows power to be transmitted reliably over distances of more than 100 km, and provides several advantages over conventional HVDC technology. Offshore wind parks and oil platforms will be linked to mainland power systems in future. This is not only a logistical challenge but it also requires new transmission concepts, such as the HVDCplus (High Voltage Direct Current power link universal system), a new technology developed by Siemens Power Transmission and Distribution (PTD).
This type of HVDC power link presents a new generation of high voltage direct current (DC) transmission systems based on voltage-sourced converter technology. It is suitable for a wide range of applications, and combines a compact design with high dynamic control response. It promises users many technical and economic advantages compared to conventional line-commutated converter systems.
Demand for DC power
The share of electricity generated from renewable sources is fast increasing. Besides solar, wind energy is becoming more popular. With mainland wind parks now a proven success, offshore wind parks with a large installed generating capacity of more than 1 GW are planned particularly for North Sea and Baltic locations.
The reasons for this development are:
- More powerful wind energy resources offshore (e.g. more wind, more uniform wind velocities)
- Reduced public acceptance of large wind parks on mainland sites (environmental impact)
- Availability of advanced large-capacity wind generator technology
- Government legislation promoting wind energy.
Many wind parks are situated at least 100 km from the grid terminal on the coast. The economic and technical limits of alternating current (AC) power
transmission systems are physically exceeded over these distances. New transmission concepts are therefore required.
There are approximately 70 oil production platforms in the North Sea with a total power demand exceeding 2 GW. Like the offshore wind parks, most of these platforms are located at distances of more than 100 km from the coast. Every one of these platforms has to be supplied with power by its own generating plant.
In the future, off-shore power generation will be replaced by a supply of electricity from the mainland. With such power supply, production platforms will have more electricity at their disposal; costly maintenance and servicing procedures on the offshore power plants will be significantly reduced, and CO2 and NOx emissions will also be considerably cut. HVDCplus can also be used for coupling of asynchronous networks (back-to-back), series compensation (SSSC), shunt compensation (SVC), and combined series and shunt compensation (UPFC).
If modern combined cycle power plants on the mainland are used to generate energy for oil platforms, it will be possible to achieve major reductions in CO2 and NOx emissions in comparison with existing offshore generation systems. Supplying the production platforms with power from the mainland will make a contribution to meeting Kyoto obligations.
Long-distance power transmission across the sea is only possible by submarine cable. Due to the growing unpopularity of overhead power lines and time-consuming licensing procedures, cable transmission systems are likely to become more important for mainland power transmission systems than in the past. Technical and economic parameters limit the cable-based transmission of AC to distances of 80 to 100 km. DC links offer stable and reliable power transmission over longer distances.
The key elements of the DC transmission links are line-commutated converters or self-commutated converters which convert DC into AC and vice versa. Line-commutated converters with thyristor technology have previously dominated the high voltage sector. But costly additional equipment is needed before line-commutated HVDC links can be used to supply island networks such as oil platforms, or to connect with wind parks, for example. However, it is the large amount of space this technology needs that precludes its use in offshore installations.
Due to its operational performance and optimized design, the HVDCplus voltage sourced converter provides a number of advantages over the line-commutated converters equipped with thyristors which have previously featured in high-voltage applications:
- No additional equipment is needed for connection to isolated AC voltage systems (e.g. wind parks and production platforms)
- High dynamic control response and performance
- Structure of a high voltage DC network with several converter stations (multi-terminal configuration) will be simplified
- Much smaller space requirement and lower weight than line-commutated HVDC links
- Separate control of active and reactive power is possible, enabling active voltage support of the AC network
- Reliable system operation is assured even in no-load operation, giving uninterrupted power supply.
The voltage sourced converter (VSC) has already been established in the industrial field. Insulated gate bipolar transistors (IBGT) are mainly used as switching elements in these converters. With the latest developments in power electronics, IGBT elements now allow cost-effective use of voltage sourced converters for high voltage DC transmission links. HVDCplus is designed as a monopolar system in the power range up to 125 MW and as a bipolar system up to 250 MW. Several 250 MW units can be connected in parallel for ratings higher than 250 MW.
A typical HVDCplus 250 MW long-distance transmission system results in a bipolar design and comprises:
- High voltage DC circuit
- Voltage sourced converters
DC power is transmitted at a voltage of à‚±150 kV and a current of 850 A. Transmission is based on the difference of voltages UDC between stations A and B. If the voltage at A is higher than at B, power is transmitted from A to B and vice versa.
In comparison to conventional HVDC links it is not necessary to change the polarity of the conductors for reverse power flow. If cable systems are used for long-distance power transmission, significant cost savings will be made on the cables because they no longer have to be dimensioned for high internal over voltages resulting from voltage polarity reversals.
Main and return conductors are used to prevent DC current flowing into the ground. DC is converted into AC and vice versa in the voltage sourced converter. IGBT elements with a reverse voltage of approximately 4.5 kV and a current carrying capacity of approximately 1200 A are used as switches. Several IGBT elements are connected in series to generate the DC transmission voltage of à‚±150 kV. The direct voltage generated by the converter is stored and smoothed in a DC link capacitor CL.
The IGBT elements can be switched at frequencies up into the kHz range. Switching/firing of the valves is based on the optimized pulse pattern (OPM) technique which is used to prevent harmonic interference modulating the IGBT valves, which results in lower filter costs.
The converter, which is a rugged two-level design, is connected to the transformer via a Radio Interference (RI) filter on the AC side. When switching the IGBT element, the converter generates relatively high rates of voltage rise, which the RI filter attenuates so that they are no longer critical for transformer insulation.
The transformer has one primary and two secondary windings. One of the secondary windings is star-connected and the other is delta-connected. This special three winding transformer design is beneficial as it eliminates specific harmonic currents.
Filters are installed on the primary side of the transformer to minimize the converter’s harmonic interaction with the AC grid. These filters are considerably smaller compared to a conventional HVDC system and are not breaker-switched. Above all, these much smaller filters provide significant space savings.
According to Dr. Ralf Bergmann, project manager for Siemens PTD, the company developed the technology in 1999 after frequent requests from clients. At this time, switching technology had been developed and was readily available, enabling Siemens to offer a new supplement to complement its existing HVDC technology.
Unlike traditional transmission systems, HVDCplus can provide power for distances over 100km.
Although HVDCplus is not currently installed in a commercial application, Siemens PTD is in negotiations to install the technology in a project in the Atlantic ocean. The project will include 120 km of cabling transmitting 250 MW of energy.
The cost and time of installing HVDCplus will depend on the distance and the demands of each individual project. However, Bergmann estimates that HVDCplus each project will take between 26 and 28 months to install.
The HVDCplus is guaranteed for three years after installation but it has an expected lifespan of 30 years. Annual maintenance checks will be carried out throughout its life which will result in a one week shutdown each time.
The HVDC technology has been developed to include tools for modern digital simulation (e.g. EMTDC) and real-time simulations using the Transient Network Analyzer. It also features state-of-the-art control and protection systems based on proven hard and software technology.
This new technology provides numerous technical and economic advantages compared to conventional HVDC links. Particular features are:
- Rugged two-level design
- Compact dimensions
- No ground return system
- System is easy to expand by parallel connections for multi-terminal applications
- More potential applications for high voltage direct current transmission.
As well as being an economic asset, the HVDCplus technology will contribute significantly to environmental protection by connecting offshore wind parks and oil platforms to mainland grids.