DC overlay systems are quite radical evolutions of the legacy of AC interconnected systems developed over the past decades. Due to a strong shift in type and location of power generation as well as a shift towards a digitalized energy sector, continuing incremental AC interconnection upgrades will not necessarily lead to the most cost-effective and timely solutions to cope with the energy transition. DC overlay systems can be designed as radial multi-terminal systems or in a meshed way, providing the characteristics of a grid. Two-terminal long-distance DC corridors emerged in the 1960s and with the rapid advancements in power electronics and control systems the first multi-terminal, non-meshed, HVDC system was commissioned in the 1990s. Meshed Multi-Terminal DC grids (MTDC), in which more than one power-flow path between two grid terminals will exist, is still being stretched at the research level to solve the challenges of integration with the AC meshed grid. Once that stage is passed, this will create a so-called DC overlay grid. The concept of DC overlay grids may one day also allow interconnecting the various large electricity networks on a global level. Furthermore, a DC overlay grid system is able to enhance flexibility of the entire transmission grid being able to cope with the characteristics of renewable power infeed in a more effective way.
Radial DC overlay systems:
Interconnection of N terminals with one power-flow path between two terminals of the DC grid.
Meshed DC overlay systems:
Interconnection of N terminals with more than one power-flow path between two terminals of the DC grid.
Components & enablers
- Transmission corridor technologies (Current Source Converters, LCC) with capacities of around 4-8 GW per circuit, and continuing developments towards 2050
- Voltage Source Converters with ratings in the range of 1-3 GW per circuit, and continuing developments towards 2050
- Advanced operational coordination
- Advanced grid planning technique
- Tailored fault clearing strategies to the specific HVDC/HVAC grid characteristics
- HVDC Circuit breakers
- HVDC Gas insulated switchgears
- Flexible DC Transmission devices
Advantages & field of application
The choice between an extension of a grid in AC or DC depends on a variety of technical, economic, environmental and technical factors, the profitability threshold between the two types of current systems varied overtime depending on the use cases. The first resurgence of a DC system was registered in 1954 in Sweden when the island of Gotland was connected back to the mainland.
With the increasing need to integrate remote large-scale renewables and the growing share of distributed DC connected energy resources, DC transmission will become more relevant and its integration within the current AC system will contribute in several ways to achieve a cost-efficient energy transition.
Major advantages of the integration of DC systems will be:
- Increase of transmission capacity by leveraging existing AC corridors to create new higher capacity DC corridors, boosting transmission capacity with limited additional environmental and social impact.
- Enhancement of power flow control which enables better utilization of the lines closer to thermal limits.
- Increase in ancillary services provision such as voltage/frequency regulation or power oscillations damping.
- Enhancement of flexibility in the overall transmission grid being able to cope with the characteristics of renewable power infeed. To date, there exist more than 180 HVDC operational projects all over the world. A few non-meshed multi-terminal systems are in operation in Europe, North America and Asia. In the next ten years, over 25,000 km of HVDC transmission lines will be built and operated in parallel to over 300,000 km HVAC transmission lines according to the TYNDP estimates, yet most of these are case-by-case point-to-point connections.
Research and development is being accelerated in this field to overcome the technical and regulatory barriers to operate and control MTDC system and integrate them in meshed AC systems. Such integration will combine the benefits of AC and DC technologies and open the door to new devices and systems, such as HVDC circuit breakers, HVDC gas insulated switchgears and Flexible DC Transmission System devices that can bring benefits to the security, reliability, performance and economics of a DC overlay grid.
Concepts like the North Sea Wind Power Hub already show advanced DC grid layouts complementing the AC onshore system. The Med Grid idea is already linking European, North African and Middle Eastern areas around the Mediterranean area.
More visionary approaches such as a global grid based on DC backbones may allow to exchange high levels of renewable energy supply in a cost-effective and secure way.
Technology Readiness Level
For the estimation of the TRL of DC grid systems, it has to be distinguished between
• TRL 9 for radial multi-terminal systems;
• TRL 4 for meshed multi-terminal systems.
Research & Development
Current fields of research: DC Voltage control in a multi terminal set up; protection types for VSC HVDC, Static and Dynamic stability of hybrid AC/DC systems, sizing and location of converters, protections and HVDC breakers, Flexible DC transmission systems (FDCTS); Probabilistic grid planning.
Innovation priority: Currently point-to-point systems have one converter controlling the DC voltage at its bus and the other converter controlling the active power. In a multi terminal set up this should be ideally spread out over multiple converters, allowing them to collectively stabilize the voltage after a fault or converter outage, e.g. through droop control. With multiple converters connected to the same synchronized system, it is evident to find appropriate power set points depending on the goal of optimization of the whole system.
Other: The development of grid codes for Multi-Terminal DC (MTDC) systems and standardization protocols are essential to inspire confidence to different grid operators for interconnecting each other through MTDC grids.
Best practice performance
Best practice application
The point-to-point 200 MW, 200 kV DC interconnection between Italy and Sardinia was extended in 1988 with an MTDC station of 50 MW at in Corsica.
Using thyristor-based LCC converters, the station was equipped with high changeover switches to enable bidirectional flow. The two older existing mercury arc valves based LCC stations were replaced in 1992 with two new thyristor-based LCC stations. The three MTDC stations form together the SACOI interconnection which operates as an MTDC system. The transmission system has three overhead line segments and two subsea cable sections. The line segments are 22km long on Italy, 156km on Corsica and 86 km over Sardinia whereas the undersea cable is 105 km between Italy and Corsica and 16 km between Corsica and Sardinia.
Bidirectional flow between Corsica, Sardinia and Italy’s mainland was facilitated by the construction of the MTDC stations.
Zhoushan 5-Terminal project
World’s first five terminal DC project at high voltage level. The five-terminal system connects five islands with the main power grid providing 400 MW at a voltage of ±200 kV DC in order to stabilize the weak power grids on the islands.
The system is designed as radial multi-terminal system in a non-fault selective way, which results in a disconnection of the five terminals in case of a DC fault. A refurbishment with DC circuit breakers is intended in order to provide selectivity.
The project will enhance security of supply for the islands power grids.
North Sea Wind Power Hub
Authorities, TSOs, wind industry and other stakeholders of countries around the North Sea have reviewed the potential large-scale coordinated infrastructure over the past decade. A recent example is the large-scale North Sea Wind Power Hub proposed by among others TenneT NL, TenneT DE and Energinet.
The whole system is intended to function as a hub for win energy transport and as an interconnection hub to connected countries and a location for possible Power to Gas solutions.
The project will enable Europeans to extract the wind energy from the North Sea up to levels of 180 GW and transmit DC power far inland directly to commercial centres. Synergies with gas/H2 are being pursued.
The concept of a global grid still speaks to the imagination of many. The CIGRE WG C1.35 performed a recent feasibility study on this.
The interconnection of 15 large electricity systems using DC corridors.
Interconnecting 15 large electricity systems worldwide, mainly by DC corridors, would allow to integrate higher levels of RES at lower cost than compared with ‘isolated’ areas. The interconnections give high flexibility benefits to accommodate flows from PV energy at any time of the day as well as wind energy from the globes most resourceful areas.
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