1. BACKGROUND AND PROGRESS REQUIRED BEYOND THE STATE OF THE ART
Current advances in technology offer TSOs many opportunities to implement power system technologies to cope with future network development and operating challenges as renewable energy integration and increased cross-border flows. In particular, connecting North Sea offshore wind farms to the European network requires assessing different novel network architectures (meshed HVDC network, HVDC multi terminal grid, radial DC connections, etc.).
Another challenge is the optimisation of existing network architectures through the integration of power technologies and the revamping of existing lines with, for instance, High Temperature Conductors (HTC), the integration of HVDC lines into the existing AC system.
Other aspects to be considered include the need to deploy ICT, large scale storage technologies and develop expertise in hybrid AC/DC power systems and multi-terminal, vendor independent HVDC Voltage Source Converter (VSC) including HVDC circuit breakers (DCCB).
A. New power technologies
The complexity of the pan-European network requires highly flexible development of transmission capacity and system operation to ensure security of supply. Furthermore, the advent of the Internal Electricity Market has led to increased Advance transmission technologies must be tested and existing lines must be improved. The integration of new technologies into existing infrastructures presents interoperability issues that must be solved.
Regarding passive transmission technologies, offshore wind farms in operation today are connected to the onshore power system with HVAC cables. Due to the high capacitance of shielded power cables, the length of such AC cables, for practical use, is limited by the charge current of the cable. The length of HVAC underwater cables is therefore also limited: this can be overcome by using HVDC cables . The High Voltage Direct Current (HVDC) technology can be used to transport electricity over long distances or to interconnect different power systems whose grid frequencies are not synchronized: DC cable systems become cost effective for transmission distance beyond 100 km. The first DC cable for offshore wind energy was commissioned in 2013 to connect the wind farm Bard Offshore 1 in the German Bight to the German transmission system. Most other wind farms in the German Bight will be clustered at HVDC converter stations at sea and then connected to shore via HVDC Voltage Source Converters (VSC) as well.
Furthermore, an increasing number of high-voltage applications will utilize superconducting technologies like Fault Current Limiters (FCL) , High Temperature Superconductors (HTS), for instance MgB2 cables.
Regarding active transmission technologies , it is crucial to demonstrate that Flexible AC Transmission Systems (FACTS) and Phase-shifting Transformers (PST) installed at a regional scale bring flexibility, enhance security and expand the capability of the network to transport more power coming from renewable energy. In addition, in order to be able to develop meshed HVDC networks, high voltage Direct Current Circuit Breakers (DCCB) still need to be validated at full scale.
Concerning generation technologies , new technologies and new regulatory schemes are required towards renewable energy production aggregation. It is necessary to show, at large scale that aggregated DER as technical VPP combined with load flexibility can lead to a more secure and efficient electricity system having high scalability potential. In particular, there is a need for showing and demonstrating that active and reactive power control can be performed reliably with the help of aggregated wind farms , thus allowing frequency control and voltage control in the system.
Besides, storage technologies such as compressed air energy storage (CAES) require a business case to allow future deployment.
Finally, transmission monitoring technologies as well as transmission network control devices are needed to optimise the capacity of the grid.
Variability of renewable generation can indeed be leveled out provided that sufficient transmission capacity is available. Since constructing additional infrastructure becomes more and more difficult, network flexibility can contribute to the needed capacity. For instance, the capacity of Overhead Transmission Lines (OHL) is limited by sag, which is influenced, through the conductor temperature, by environmental factors (ambient temperature and wind velocity and direction). This is taken into account in European grids by adapting line ratings according to the season of the year, leading to higher security margins than needed in order to take into account the standard case conditions over each season. In this framework, Dynamic Line Rating (DLR) would allow for a more efficient use of the transmission lines, by calculating line ratings based on more accurate weather forecasts and the real conditions of the conductor.
Besides, the coordinated and optimal use of Power Flow Control (PFC) devices, such as Phase-Shifting Transformers (PST), allows the TSOs to optimize the power system in a flexible way by controlling the line flows and to enable enhanced connections between different zones. Combining DLR with the use of PFC devices can push the use of the existing grid beyond its present limits. However, the extended use of the grid can trigger dynamic stability issues. Therefore, a real-time monitoring of the dynamic stability of the grid – for instance, a Wide Area Monitoring System (WAMS) – has to be implemented with for instance Phasor Measurement Units (PMU), to detect possibly dangerous situations. There is indeed a clear need for demonstrating that adequate observation and control schemes (WAMS with PMUs, DLR, PFC, etc.) increase the flexibility and the capacity of the existing power grid within affordable capital and operational costs.
In addition, connecting PFC, like mobile Overload Line Controllers (OLC), which result from an innovative combination of reactor switch control steps with high-end control systems and are able to redirect power flows, would allow to shift the operating point to relief the overload on transmission lines within seconds, allowing more corrective actions rather than costly preventive actions. In addition, this type of device can keep a pre-set target value of a flow in a line during a number of different contingencies with automatic control, which cannot be done easily by changing the taps of a Phase Shifting Transformer (PST), and can be moved within the network, thus increasing the flexibility of the network. The transmission line can therefore be operated closer to its physical limits which results in increased transmission capability for renewable energy integration.
When it comes to massive renewable energy integration into the electricity system (mostly offshore wind energy and photovoltaic power) and effective delivery of electrical power to distant consumption sites, transmission networks will need to evolve throughout the entire European territory. The integration of power technologies will take two forms.
First, optimising existing network architecture will be necessary. For instance, existing AC corridors will need to be repowered to take advantage of existing infrastructures since it will be increasingly difficult to build new overhead lines due to public acceptance. Besides, the possible integration of HVDC lines into existing AC system as well as the influence of hybrid AC/ DC lines on the same tower need to be investigated to facilitate existing infrastructure paths in an optimal manner.
Second, novel network architectures will have to be designed. Offshore, point-to-point DC links, which do bring recognized economic advantages in terms of capital and operational costs, will evolve towards HVDC multi-terminal grids to transport large offshore wind power (like in UK, Germany, Denmark, Netherlands) and to link HVDC and AC networks within flexible operating modes. Dedicated point-to-point DC links will also find opportunities to deliver electric power through densely populated areas to face constraints such as public acceptance, environmental and visual impact, which have gained importance in the last decade.
So far, the impact of security constraints on the investment and operations of meshed HVDC networks have not been assessed. In addition, the absence of naturally occurring current zeros in DC grids makes AC circuit breakers inoperable in DC systems. HVDC Circuit Breakers (DCCB) still need to be validated at full scale to operate an HVDC multi-terminal grid. They are vital in case of faults (e.g. earth faults), since the DCCB must dissipate the energy stored in the transmission medium (cable or line) and withstand the system voltage. Their duty becomes even more demanding if the AC-DC conversion is performed by Voltage Source Converters (VSC). In the event of a fault, some non-controllable power electronic components will let power flow into the DC system even if the active elements of the system are blocked. Therefore, there is a strong need for providing the critical building blocks of DC grid control and protection strategies as well as high voltage DCCB based on large-scale demonstrations, which will allow guaranteeing the security of future HVDC multi terminal grids.
2. OUTCOMES PROVIDED BY THE PROJECTS THAT ADDRESS THE CHALLENGES OF THE CLUSTER
A. The TWENTIES project
The European Union, in its backing for renewable energy integration, especially wind power, launched in April 2010 the TWENTIES project (“Transmission system operation with large penetration of Wind and other renewable Electricity sources in Networks by means of innovative Tools and Integrated Energy Solutions”). Its objective was to significantly advance in developing and demonstrating network architectures with new power technologies and novel network architectures which facilitate the widespread integration of wind power generation into the European electricity system.
By mean of the implementation of six large-scale demonstrations, the project intended to remove the barriers to integrate both onshore and offshore wind power to a greater extent into the European system by 2020.