Application of High-Voltage Direct Current (HVDC) Transmission for Offshore Wind

As the demand for power is increasing and greater emphasis is being placed on offshore wind, government policies are providing incentives for utilities to adopt renewable energy sources. Utilities and developers are investing time and money to improve the transmission of renewable energy from offshore wind.?

?Most power grids today use high-voltage alternating current (HVAC) for transporting energy over long distances, but that technology is susceptible to losses during transmission, has limits on power transfer over long distances, and has limited power control capability. A high-voltage direct current (HVDC) system converts the power from alternating current (AC) to direct current (DC) at the sending end, transmits the power using DC, converts the power back to AC at the receiving end, and delivers the power to the receiving end AC grid.??

?HVDC technology is being applied for large bulk power transfer over long distances, as well as in the interconnection of renewable energy sources. Most offshore wind farms are being connected via HVDC transmission systems, which offer some noteworthy benefits.?

?HVDC Converter Technologies?

HVDC converter technologies are well established and include two categories: line-commutated converters (LCCs) and voltage-sourced converters (VSCs). With LCCs, electronic switches can only be turned on, whereas VSCs can be switched on and off. Both technologies offer advantages and disadvantages for utility applications.?

Line-commutated converters (LCC), the more traditional systems used in HVDC systems, offer much more power and the following characteristics:?

• Multiterminal applications can be complicated.?

• Excellent for clearing DC line faults quickly with fast, automatic restart capabilities.?

• Current ratings of thyristors are much higher (6 kA), allowing for higher-capacity converter valves.?

• 3,000-MW bipole is no problem.?

• No black start capability.?

• Requires AC filtering.?

• Consumes reactive power at converter stations.?

• Larger footprint than VSC.?

Voltage-sourced converters (VSC), a newer and more compact technology, have lower power capabilities and the following characteristics:?

• Preferred for multiterminal applications.?

• Challenges with clearing DC line faults (requires tripping of the entire system, full bridge converters or HVDC breakers).?

• Current ratings of insulated-gate bipolar transistors (IGBTs) of approximately 3 kA limit converter valve capacity.?

• 3,000-MW bipole is at the upper limit of today’s technology.?

• Black start capabilities.?

• Good for integrating renewables.?

• Requires no AC filtering.?

• Capable of providing independent reactive power control at converter stations.?

• Smaller footprint than LCC.?

Bulk Power Transmission Efficiencies?

Transmission cost depends on numerous factors, including the size and quantity of conductors, equipment needed at the terminal stations, and transmission tower size.?

?HVDC is particularly well suited for bulk power transmission over long distances for several reasons:?

• A bipolar HVDC system and symmetrical monopole HVDC system offers reliability comparable to a double-circuit HVAC line, significantly reducing the transmission line costs and right-of-way requirements.?

• Losses in a transmission line depend on the resistance of the line. One factor that impacts the resistance is skin effect, which causes the effective resistance to increase with increasing AC frequency. Use of DC eliminates the skin effect, reducing overall transmission losses.?

HVDC transmission systems require converter stations at each end of the line to convert the AC to DC and back. The cost of HVDC converter stations can be substantially more than a conventional AC substation with similar power throughput. For offshore wind HVDC systems, the offshore platform to support one end of the HVDC system is another significant cost to consider. Those expenses may be counterbalanced by reduced transmission line costs and reduced losses. This becomes more evident as the distance and/or power transfer level increases.?

?Cable Length Advantage?

HVAC transmission cable length is limited because as the length of cable increases, the capacitive charging current increases. It can reach a point that the capacitive charging current approaches the total current-carrying capacity of the cable. HVDC has no capacitive charging current, and higher levels of power can be delivered over longer distances. HVDC cable length is theoretically only limited by capital cost.??

Enabled applications include connecting offshore wind farms. As the development of offshore wind generation assets increases, the location of those generators is moving farther from the shore. The increasing distances between the generators and the onshore point of interconnection means the necessary cable length is increasing. HVDC can be a viable transmission option, unlocking the true potential of renewable energy.?

?Power Controllability?

Within an HVAC system, the ability to control power flows in any given parallel path is limited. Power flows are dictated by the relative impedance of various parallel paths from a given source of generation to a given load. HVDC, on the other hand, offers very fast and accurate control of the power flowing within its system. The operator can select the amount of power to be transmitted over the link. If that power is available at the sending end, it is converted to DC, transmitted to the receiving end, converted back to AC and injected into the receiving AC system. Auxiliary control functions can further enhance the AC system’s stability by providing frequency control and damping of power swings within the AC grid.?

?Your Project?

Before sinking a lot of money — potentially $2 billion to $6 billion — into a project of that expense and complexity, it makes good sense to conduct an initial desktop study and sweep for anything that would be likely to stop the project dead in its tracks.?

Burns & McDonnell can draw on deep experience with both transmission infrastructure and environmental permitting — as well as a broad understanding of how to work with relevant regulatory bodies effectively — to investigate a project on your behalf.??

?We perform consulting and owner’s engineer services for clients throughout North America and the United Kingdom. We support projects from concept through commissioning and even support owners through the operational periods of the facilities. We can help troubleshoot operational issues and can support the upgrades to be expected through the life of the asset, of which there could be many.?

As clients consider an HVDC transmission solution, they should be aware that the market may no longer support a traditional RFP process. The demand for HVDC converter systems and cable systems has driven the procurement process to become simpler for suppliers, who may request a preferred supplier agreement from the first inquiry about scope and costs.???

Burns & McDonnell helps clients through all modern project phases, many of which may not have experienced by those considering these facilities. The definition of the project, including the development of level of specifications and commercial terms, is different today than 5 years ago. It is critical to understand what requirements can be influenced by purchasers.?

Our process includes the following as a relationship is being developed with an HVDC converter OEM:?

• Assembling basic information and general data (site conditions, codes and standards, etc.).?

• Identifying non-equipment requirements (training, spares, documentation).?

• Identifying performance and equipment requirements.?

To support the overall project, our owner’s engineer services typically support and provide oversight during the following phases:?

• Studies??

• Engineering?

• Procurement (factory acceptance testing)?

• Construction?

• Commissioning/Trial Operation?

• Spares, reports, manuals and as-builts?

• Startup?

• Commercial operations?