Long-distance transmission: Focus on resilient grids
HVDC has for years been supporting grid resilience. Innovations offer a similar performance for HVDC PLUS with its additional stabilizing functions. Future HVDC developments will serve the fast and flexible expansion of DC grids for sustainable energy systems.
by Rhea Wessel
In 2010, HVDC technology had a major breakthrough with the first ever HVDC PLUS system installed in San Francisco, the so-called Trans Bay Cable Link. Now, six years later, one focus is increasingly on standardizing HVDC design and operating principles. “We want to help customers reduce lead times and be more responsive to meet the new requirements of a more sustainable energy system, such as those from the changing energy mix,” says Frank Schettler, Principal Key Expert and Product Life Cycle Manager of Siemens’ HVDC PLUS solutions. “Our goal is for customers to have the most competitive grids possible through technology that makes their grids more flexible, such as multi- terminal HVDC systems.”
At the same time, R&D and design improvements to HVDC also focus on innovation and resilience. The technology is critical as the stabilizing backbone of the network: It is HVDC that helps the network avoid an interruption of power supply or blackouts.
Some HVDC systems can be complemented by flexible AC transmission systems (FACTS), which are power electronic solutions that stabilize AC grids and reduce power delivery costs by supplying inductive or capacitive power to the grid. “The HVDC and FACTS systems that we deliver are not allowed to be disconnected if a fault occurs in an AC system. They are designed to help the system exactly when needed most – when it breaks down. This creates special design requirements compared to other components,” Schettler explains.
Resilience in the network is indeed the Holy Grail for operators, and in general, it is achieved in three phases, says Schettler. “First, operators must prepare for potential disruption scenarios; second, the network itself must be built to withstand such stresses; and finally, reliable protection and control functions are needed.”
From the outset, HVDC technology from Siemens is built with robustness in mind, for instance to minimize the impact on the system when a fault does occur. “If there’s a fault, maybe only part of the link will fail briefly instead of the complete link,” says Schettler. “By far, most faults in transmission systems are temporary: Power flow should commence immediately after the fault has been cleared.”
In addition, when HVDC PLUS is used with full-bridge topology, for example, it keeps interruptions of power at a minimum with extremely fast interruption of the fault current. It also supports the AC system with AC voltage control during the entire fault clearing process.
For the third phase of ensuring resilience, the recovery phase, Siemens researchers have focused on developing control mechanisms that allow an operator to restart the system very quickly – and, if the problem is with the network overall, to use HVDC technology to help the network recover. As Schettler points out, “We can actually start up the network from our HVDC links.”
Moreover, high mechanical requirements have to be fulfilled. In New Zealand and in San Francisco, for instance, the network was designed to very high seismic standards due to the high risk of earthquakes in both areas.
Operators have to be competitive. They need a technology that can grow with demand.Mirko Düsel, CEO of Transmission Solutions, Siemens
HVDC – a foundation for renewable energy
It is no exaggeration to say that HVDC has the potential to change power distribution around the world because the technology allows renewable energy that is generated far from load centers to be transmitted efficiently. At the same time, the fast and robust control capabilities of HVDC systems from Siemens help stabilize the AC grids. In the case of the Ultranet project being implemented in Germany, for the first time, HVDC will bring these benefits without changing the face of the landscape using existing corridors combining AC and DC lines on the same tower.
As changes in technology and business models take hold, Mirko Düsel, the CEO of Transmission Solutions at Siemens, expects the future will bring a move to smaller DC grids that require control mechanisms and reliable transmission between AC and DC: “I think the technology is moving in he direction of multi-terminal instead of point-to-point because it is far more flexible in system planning. Our customers, the operators, ultimately have to be competitive. They need a technology that can grow with demand. What if they need to integrate another wind park to be more competitive?”
The key word here is “multi-terminal”. It not only makes individual operators more flexible, but allows for the development of a pan-European HVDC grid. Düsel sees the grid evolution as beginning with HVDC grids connecting a small number of converter stations. Then, a full-bridge modular multi-level converter (MMC) in combination with fast disconnectors would provide reliable and cost-efficient fault clearing and fast recovery of energy transmission. In a second stage of grid development, small HVDC grids would become interconnected, and the full-bridge MMC would provide an increased DC voltage control range as needed for longer distances.
At present, only a few multi-terminal applications exist worldwide with line-commutated converter technology. A range of technologies from Siemens are available for multi-terminal configurations, including DC compact switchgear (DCCS) and a fast disconnector switch. As multi-terminal applications grow in number and network providers and operators gain experience with the configuration, HVDC PLUS provides critical stability to the network.
Siemens has handed over to the customer most of its HVDC PLUS projects in operation today, for instance connecting offshore wind farms and the converter stations for the largest transmission system of this kind between France and Spain, called Inelfe.
Making HVDC smarter: control mechanisms for DC?
Looking forward, engineers at Siemens are developing control mechanisms for DC similar to those used for AC grids. AC grid controllers monitor where power flows, measure power in the network, and predict how large loads will be at a certain time in the future. Using the example of load prediction, Markus Engel, Product Life Cycle Manager of Control & Protection, notes: “A DC controller could also say, ‘What would be my wind infeed at a certain time?’ The controller would estimate how much power will flow in different directions and can link that to business information about long-term contracts.”
Schettler adds that when a flexible DC transmission system is available, advanced IT capability and “intelligence” are needed to make the best use of this system. For multi-terminal HVDC systems, the HVDC grid controller is a critical component. “The more converter stations we connect to the HVDC grid, the more important it is to control the power flows and develop an HVDC grid controller. We already have good solutions, and they will be part of upcoming multi-terminal systems,” says Schettler. Siemens is taking a leading role worldwide in developing commonly agreed functional requirements for HVDC grids. This is a prerequisite for the interoperability of systems sold by multiple vendors.
Indeed, control and protection functions are a hotbed of innovation, and changes in the technology apply to convertors, too. As Engel explains, “If you talk about multi-level convertors, and thousands and thousands of individual capacitors voltages that need to be controlled inside one converter, then we need a really powerful control system behind that.”
These powerful control systems can be modeled through network simulation tools that help researchers understand the impact of hypothetical configurations and develop the right control functions from the comfort of their desktops. The tools help operators simulate normal operations and model possible faults to anticipate problems early on and adapt design plans accordingly. “Advanced simulation technology is the basis for optimized use of HVDC and FACTS systems for a reliable supply of power that is both safe and efficient,” says Engel.
That means that the new innovations expected in HVDC and FACTS systems are also the basis for economic growth, since economic growth depends on a secure, efficient and sustainable power supply. Düsel believes that “demand for energy is on the rise around the world. HVDC has served us well for decades, and I expect the latest evolutions of the technology to do the same for decades to come. I am really curious how HVDC grids will be configured in 20 years, with the onset of multi-terminal systems and the sharing of overhead lines for AC and DC.”
Rhea Wessel, journalist based in Frankfurt, Germany.
Picture credits: Guy Frederick, Siemens AG.
A scenario for a step-by-step evolution
HVDC multi-terminals connecting a small number of converter stations are established.
Full-bridge MMC in combination with fast disconnectors provides reliable and cost-efficient fault clearing and fast recovery.
The small HVDC grids become interconnected. Full-bridge MMC provides increased DC voltage control range.
Selectivity between sub grids may need additional fast devices, like fault current limiters or DC breaker applications.
The expected step-by-step growth of HVDC grids requires standardization of HVDC grid design and operating principles.
HVDC makes an essential contribution to grid resilience
Redundant systems and components can fail without having impact on the power supply capability of the transmission system.
Responsive infrastructure systems incorporate automated monitoring, short feedback loops, and controls at multiple points, enabling transparency of performance data and rapid adjustment to maintain functionality.
Diversity and flexibility
Redundancy and flexibility in infrastructure systems mean that services may be supplied via a number of pathways, using distributed resources and multifunctional equipment. If one pathway fails, another can be used to achieve the same service.
Robust infrastructure is able to withstand the impacts of hazard events without significant damage or loss of function.
Coordination between systems means that knowledge is shared, planning is collaborative and strategic, and responses are integrated for mutual benefit.
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