HVDC Classic – powerful and economical — High-performance power transmission

HVDC – the silver bullet? Major challenges for grid operators

The brochure opens by framing HVDC as the answer to optimized grid operation in the face of technical constraints and market development: an ever-increasing share of volatile renewable infeed calls for future-proof, flexible solutions that meet regional regulations and standards. In more than 50 projects worldwide, Siemens Energy has proven its high-voltage direct-current (HVDC) technology to be the best solution for long-distance transmission, grid access, and grid stability.

High power transmission capability: the energy transition from fossil to renewable resources is dramatically changing load flows and requiring improvements to the existing power transmission infrastructure. Increasing distances between power generation and load centers mean higher transmission capabilities are essential, and emerging international electricity markets call for improved transmission capacities and new corridors for power transfer and cross-border interconnectors.

Optimal efficiency: power losses must be kept to an absolute minimum. The ability to flexibly increase current or even temporarily overload power lines in an emergency enhances power grid efficiency, while the minimized right of way required for overhead lines and cables compared to AC systems reduces costs.

Flexibility for future challenges: very fast and accurate power flow control becomes essential as infeed from intermittent renewable sources increases — and fluctuates with the weather. Power also needs to be transmitted in diverse directions and into different regions or even countries, depending on market requirements; other challenges include more flexible grid configurations, redundancies, and more grid-stabilizing functionalities.

Keeping costs low: the lowest achievable CAPEX and OPEX are indispensable, possibly over the entire lifecycle of the investment — supported during operation by high availability and a low-loss solution with minimized operating and maintenance costs. Safety and security for a reliable power supply round out the challenge list: the impact of failures on security of supply must be limited, the highest safety standards maintained in maintenance and operation, and the grid must have optimal resilience against natural disasters, terrorist attacks, and cyber attacks.

The 'Major challenges for grid operators' box summarizes: low investment and operation costs; highest efficiency with minimum losses; maximum operational availability and reliability and the best possible resiliency requirements; compact, adaptable, and economical solution; power exchange between interconnected systems and asynchronous grids; maintenance-friendly, safe, and reliable design with comprehensive lifetime services; future-oriented, flexible solutions for varying power market requirements.

HVDC Classic: Proven technology for sustainable performance

Siemens Energy's HVDC Classic (with line-commutated converter) technology helps grid operators solve diverse technical and economic challenges — while improving grid performance and stability and providing an outstanding control of power flows.

Lowest transmission losses: while HVDC Classic features the lowest losses of all HVDC technologies, it is especially efficient in long-distance transmission over 600 km and more. In this case, HVDC transmission typically features 30 to 50 percent lower transmission losses than comparable HVAC (high-voltage alternating-current) overhead lines. It can also carry 30 to 40 percent more power given the same right of way, and the HVDC transmission link offers an overload functionality that helps supply sufficient power in emergencies and improves grid resilience without requiring more infrastructure investments.

Sustainable savings: HVDC Classic offers the lowest CAPEX and OPEX and has set the efficiency benchmark in long-distance bulk power transmission. With a power rating of up to 6 GW at a voltage level of ±600 kV and up to 10 GW at ±800 kV, HVDC Classic solutions offer very high power transmission capabilities that boost performance and provide a firewall against blackouts in existing overloaded AC grids.

Enhanced grid stability: any HVDC Classic system can improve grid stability, and in special cases the addition of FACTS devices can enhance voltage stability even further — optimizing grid stability such that it achieves the performance of Siemens Energy's voltage-sourced converter technology (HVDC PLUS). Increased security of supply can be achieved by arrangements of series and parallel connected converters in each pole, and multi-terminal setups take this a step further by connecting several stations, for example across several countries.

Ease of maintenance and safety: the converter modules have been redesigned to facilitate easier, faster, and much safer installation, service, and maintenance activities. Thanks to the C-shaped design of these next-generation valve modules, all components can be accessed without having to leave the lifting platform.

Operational advantages (as boxed in the brochure): high power and current transmission capability; optimized grid resilience thanks to sufficient transmission capacity to stabilize AC networks; a very high level of system reliability and redundancy of all key components of the converter control; state-of-the-art control and protection system with hardware and software in hot standby and proven in practice; all current HVDC Classic systems in line with latest cyber security standards (e.g. NERC CIP ready); minimized maintenance and service requirements and the highest health and safety standards.

A new dimension in power rating: parallel, series, and multi-terminal converters

Siemens Energy has developed a variety of technologies to meet the need for ever-higher power transmission capacities. One of them is the new six-inch thyristor with a rated current up to 6.25 kA: it has a high blocking voltage and increased power density, allowing a robust design with a minimum number of components. This development enables up to nearly 14 GW of power transmission in bipolar HVDC Classic systems.

Siemens Energy is delivering the world's most powerful converter transformers to China to create the world's first 1,100 kV HVDC transmission link. This component features 19-meter-long valve bushings that enable the insulation clearance required in air. The Changji–Guquan link is 3,284 kilometers long and has a transmission capacity of 12 GW; its special converter transformers can be directly connected to China's 1,050 kV AC grid, another world's first.

Parallel converters: one answer to the increasing demand for large power transfers, offering very high bulk power transmission, availability, and reliability due to the redundant design. It is also very flexible in operation, with an option to increase current ratings; thanks to its very high currents and minimized height of the transmission towers and valve halls, these installations also enjoy improved public acceptance.

Series converters: this design features improved redundancy and availability during converter failures. It enables grid operators to realize very high transmission voltages and power transfer, yet it is constructed using standardized components and designed to facilitate low investment and high cost advantages during operation, achieved by reducing losses and a simplified operation.

Multi-terminal installation: a system of three or more converter stations that can be built in different locations, offering highly flexible operation and adaptation to changing power flow needs — the perfect solution for connecting AC grids because it offers fast control and support for AC network stability and increased efficiency. In addition, a project can be developed in stages, allowing an early start of power transmission and revenues.

A station-layout photo on this spread labels the main elements of an HVDC Classic converter station: valve hall, DC hall, control building, converter transformers, AC switchgear, and AC filters.

Technology that explores new frontiers: four decades of thyristor development

Siemens Energy is at the forefront of HVDC development and has set many milestones over more than four decades of research and practical implementation. Its overload capability, the advantages of Siemens Energy light-triggered thyristors, and the option to choose between voltage-sourced (HVDC PLUS) and line-commutated converters (HVDC Classic) are part of the company's HVDC success story. (The printed section heading reads 'Technology that explore new frontiers', as published.)

A development chart (1970–2021) shows the continuous improvement of thyristor technology for maximum power density and compact design: from 1.5-inch thyristors with 1.65 kV blocking voltage around 1970 — when roughly 28,000 thyristors per 1,000 MW were needed for both valve halls — to today's six-inch light-triggered thyristors with 8.5 kV blocking voltage, so that with the latest generation only 600 thyristors are required to transmit 1,000 MW of power (the chart's annotation reads '< 600 thyristors/1,000 MW').

An economical solution: depending on system and ambient temperature and on the availability of redundant cooling equipment, the overload capabilities of thyristor-based HVDC Classic systems are an extremely economical asset. The cost benefits are amplified by the rugged system design, which allows for both short-term and long-term overloads if the appropriate cooling is installed. For grid operators this means improved stability of the AC systems, shared spinning reserves, and reliable supply for peak loads; even in the event of a pole outage, power reduction can be minimized.

Light instead of electronics: the thyristor valves convert AC into DC — but while it is common to use electronics to trigger the thyristors, Siemens Energy has developed a more reliable trigger based on fiber optics and a light impulse. The light-triggered thyristor (LTT) uses far fewer electronic components and is therefore more reliable. Fire-retardant and self-extinguishing materials make the thyristors robust and safer in terms of fire prevention, and parallel cooling of the valve levels with de-ionized water helps support maximum utilization of the thyristors.

Large range of high-power applications: the current-carrying capacity of the thyristors, up to 6.25 kA, makes it possible to transmit power at high voltages and currents over very large distances, which cannot be achieved by any other AC or DC transmission technology. The HVDC Yunnan–Guangdong link in China was the first 800 kV project ever realized with overhead lines, and the Western HVDC link in the UK set a world record for 600 kV of power transmitted via subsea cable.

An application-range chart (DC voltage vs. DC current) places HVDC Classic with cable around the 600 kV / 2 kA region (Western Link, UK, 2,200 MW), HVDC Classic with overhead lines from roughly 800 kV / 3 kA (Yunnan–Guangdong, CN, 5,000 MW; Jinping–Sunan, CN, 7,200 MW) up to 1,000 kV / 5 kA (Changji–Guquan, CN, 12,000 MW), and HVDC Classic back-to-back in the low-voltage region (Black Sea, GEO, 2 x 350 MW).

HVDC Classic Services: Partners for decades

Investments in the transmission network are based on long-term calculations of power demand, mirrored in the life expectancy of the transmission equipment. Even high-quality installations require regular maintenance and other services to keep them perfectly efficient. Siemens Energy's dedicated after-sales services start with on-site condition assessments of all assets, complemented by continuous monitoring of critical systems, which minimizes unplanned downtime through preventive maintenance. A lifecycle wheel on this spread runs from engineering, manufacturing, delivery, commissioning, and training through operation, maintenance, repair, upgrade, refurbishment, and replacement to disposal (printed 'Diposal', as published).

The service offering is grouped into four pillars. 'We make your assets more transparent': on-site condition assessments, condition monitoring and diagnostics, remote services, asset management and advisory services. 'We ensure high asset availability': preventive maintenance, field service and repair, spare parts, 24/7 expert hotline and technical support, obsolescence management. 'We optimize asset performance': refurbishment, upgrade and uprate, patch management. 'We support you in operation management': asset operation, spare-part management, customer qualification and training, cyber security services.

A 'Typical life time of systems' chart gives lifetime bands per system layer over a 5–40-year axis, with a general recommendation per layer (reproduced in the table below). For HMI and COM, industrial IT lasts ~5–8 years and standard IT ~2–4 years; control and protection (C&P) lasts ~15–20 years; main components and sub-systems last ~30–40 years.

Case study — HVDC Inter-Island link: the backbone of New Zealand's power grid

Since 1965, the power grids on New Zealand's North and South Islands have been connected across the Cook Strait by the HVDC Inter-Island link. The comprehensive upgrade project, finalized 2013, included the replacement of Pole 1 by a new Pole 3, the replacement of the control system for the existing Pole 2 (a third-party system), and a new system for reactive power control. All installations fulfill the strictest seismic requirements ever implemented in an HVDC installation.

Safeguarding power in the 'Shaky Isles': due to its location on several seismic fault lines and the high number of resulting earthquakes, New Zealand is sometimes referred to as the 'Shaky Isles'. Haywards Substation, a key asset of New Zealand's transmission grid, is located directly on one of these fault lines, so the national grid operator demanded the strictest seismic requirements ever implemented anywhere in the world. Over a period of four years, Siemens Energy designed, built, tested, installed, and commissioned a state-of-the-art thyristor-based HVDC converter and interconnector system at the Haywards site 25 miles north of Wellington and at Benmore, the hydro power plant's substation in the far South Island. Both systems are capable of withstanding a one-in-2,500-years earthquake event; seismic measures include rubber compensators for damping of vertical and horizontal oscillation of the complete valve hall/transformer foundation.

Prepared for the future: the upgraded interconnector has a designed capacity of 1,400 MW at 350 kV, of which currently only 1,200 MW are being used due to the limited capacity of the submarine cables. The project also included a new reactive power controller, which controls reactive power flow and voltage in major parts of the 220 kV system of the North Island by direct control of existing and new reactive power sources. The new Pole 3 has a continuous rating of 700 MW in both directions and, like Pole 2, is capable of operating in bipolar and monopolar configurations; further advantages are increased reliability and increased flexibility due to the new controls.

Designed and tested in every respect: the complexity of the AC system interfaces and the staged replacement of the existing third-party control systems resulted in an unusually high number of different operational scenarios for which the system had to be configured and tested. A very comprehensive on- and off-site test program was implemented to ensure the link will be a robust and reliable backbone for New Zealand's power grid. It will provide the required capacity to fulfill New Zealand's plan to achieve 90 percent renewable power by 2025.

Selected HVDC references (1975–2021)

Siemens Energy positions itself as a reliable and experienced partner in the development, installation, commissioning, and operation of HVDC Classic solutions. Numerous references around the world — plotted on a world map and listed in the table below — demonstrate its role as a technology leader offering highly efficient solutions for economical long-distance power transmission and interconnecting grids operating asynchronously or at different frequencies.

Figures & drawings

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