Transmission technology for the energy transition

While many parts of the world bask in glorious sunshine, other regions experience strong and consistent winds. Wind power turbines successfully exploit these conditions to produce clean and sustainable energy. Given that the higher average offshore wind speeds can result in an energy yield of up to 70% higher than that generated on land, together with the scarcity of available onshore sites, it is no wonder that there is now an increasing number of offshore wind farms being built.

However, as the nearshore sites are being occupied, the further out to sea, the greater the challenge of ensuring the transmission of a stable energy supply to the mainland. This challenge is being met with Hitachi Energy’s voltage sourced converter (VSC)-based high-voltage direct current (HVDC) Light^® technology.

Background

Propelled by political initiatives in different regions of the world, governments have set forth ambitious targets with regard to renewable energy generation. In Europe, the ‘Fit for 55’ legislative package and the Repower EU plan introduced by the EU Commission set targets for all its 27 member states. The revised Renewable Energy Directive EU/2023/2413 entered into force on 20 November 2023 raises the EU’s binding target for the share of renewable energy sources to a minimum of 42.5% for 2030 with an aspiration to reach 45%.

The UK’s Net Zero Strategy aims to decarbonise electricity in the country by 2035 by placing low carbon investment as important to achieving net zero. Driven by the increased global demand for electricity, the need to phase out fossil-based generation, and stay on the 1.5°C pathway, an annual deployment of approximately 1000 GW of renewable power is required between 2023 and 2050.¹ Translated into offshore wind, the global installed capacity would need to reach almost 500 GW by 2030 and 2500 GW by 2050.¹ This target will require the wind industry to massively invest and increase its capacity.

Such radical changes in the amount and type of electricity generation injected in new nodes in the grid will result in changed power flows both in terms of ratings and routing within and between national grids. This would inevitably lead to persistent grid congestion if not properly addressed. Transmission System Operators (TSO) are making significant investments in new transmission assets to reinforce the existing infrastructure, creating strong electricity transmission grids capable of withstanding the future power system’s dynamics. The energy transition required to meet the targets at the needed speed will lead to a complete redesign of the grids. Some of the challenges that will need to be tackled are complex permitting procedures, lack of skilled labour, local opposition against new mega projects, and access to necessary funding.

As a consequence of these demands, the number of HVDC transmission systems is growing at an unprecedented scale. Over the years, HVDC has proven to be a cost-efficient solution for integrating large scale offshore wind power. When wind farm developers are searching for areas with shallow waters together to intercept better wind profiles, the distance from the onshore connection point to the wind farm sites tends to increase. The combination of high power, long distance, and sometimes weak connection points makes HVDC transmission the perfect fit, providing superior grid support and flexibility while at the same time maintaining low operating losses and high availability.

Besides integrating offshore wind power, HVDC is the technology of choice when connecting different markets over long distances, as well as reinforcing existing AC grids. In addition, in several examples, when transmission corridors are being planned to minimise adverse environmental impacts and preserve nature and landscape, HVDC technology plays a pivotal role by allowing the use of underground transmission cables, even for long distances.

The technical development of the VSC technology, as well as the extruded HVDC cable technology, has been intense over the last 25 years. The increase in power and voltage capability has enabled new applications for VSC-based HVDC systems and future scenarios suggest that VSC technology will develop further to meet new and demanding challenges.

HVDC VSC technology development

It is now more than 25 years since the first pioneering VSC-based HVDC system was installed close to Hitachi Energy’s HVDC unit in Sweden. With its capability to transmit 3 MW at ±10 kV DC, it was the beginning of a new power transmission technology era, enabling, among others, the great expansion of offshore wind generation seen today. Until now, the development of the VSC technology has largely been focused on reducing losses, footprint, and increasing reliability, power capacity and voltage levels to create a more sustainable and cost-optimised solution. Many features essential to meeting today’s requirements, such as independent active and reactive power control, black start, etc. were available already from the beginning.

Hitachi Energy has developed today’s multi-modular converter (MMC) topology based on experience from earlier designs. The first generation VSC technology was built around a two-level converter using pulse width modulation (PWM) with high switching frequency. The next step was to increase power and voltage ratings by high-current insulated-gate bipolar transistor (IGBTs) in a three-level converter configuration. With the changed topology, the switching frequency could be reduced while at the same time lowering the harmonic content and, consequently, fewer filters.

The third-generation VSC technology was introduced already in 2005 and went back to a two-level converter topology but with an optimised PWM switching pattern. It retained the losses at the same level but reduced the number of IGBTs while still maintaining low harmonic generation. This resulted in a compact design and reduced footprint, making it especially fit for offshore wind applications.

With a design based on cells (modules), each containing half-bridge two-level converters, the cascaded two-level (CTL) converter was introduced in 2010. To further increase DC voltage, power capacity, reliability, and decrease losses and footprint, the CTL was further developed into an MMC (Figure 2). With a rated semiconductor voltage of approximately 5.2 kV, the MMC has smaller voltage steps than the CTL converter, leading to less distortion of the voltage and current. The improved topology enables a decreased switching frequency and an improved output voltage; thus, the converter losses were decreased even further. It was also possible to reduce the footprint both through optimised mechanical design and by utilising bi-mode insulated gate transistors (BIGT), combining the IGBT and the diode functions in the same chip.

Today’s VSC and cable solutions have taken the next step both in voltage and power. It is possible to design HVDC stations able to transfer about 3 GW in bipole configurations at ± 525 kV, with DC current ratings of about 3 kA. These larger systems can be foreseen mainly together with overhead lines. In links with cables, the current is often limited to about 2 kA depending on the conditions of the soil or seabed. These ratings are approaching levels that may impose a major impact on the transmission grid, and all possible aspects regarding grid resilience must be carefully considered. In many parts of the world, these power levels may set an upper limit due to the potentially severe consequences in the unlikely event of a failure leading to an HVDC trip.

Offshore HVDC development and experiences

For more than 20 years, the offshore wind industry has provided flexible, resilient, and sustainable energy. The global offshore wind outlook estimates that 380 GW of offshore wind in 30+ countries will be installed in the next 10 years.² Driven by political ambitions and commitments, the plans for large scale integration stretch over every continent in the world contributing to the green energy transition.

The world’s first offshore wind connection by HVDC VSC technology was commissioned in 2009 in the German Bight, connecting 400 MW offshore wind to the mainland grid in Germany.³ At that time, the wind farms were connected to the AC collector platform by a 33 kV inter-array cable network. After stepping up the AC voltage to 155kV, the energy was transmitted to an HVDC station on a separate platform that converted the AC voltage into ±150 kV DC for further transmission to the mainland grid.

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