To integrate both onshore and offshore wind power to a greater extent into the European system by 2020, DC grids (DCGs) play an important role. In fact, in addition to optimization of AC and DC transmission infrastructures, and potential improvement of reliability and security of supply, DCGs are expected to provide additional functionalities and meet some requirements: wind power transfer (including their capability for smoothing of wind power fluctuations); wind power interconnection (i.e. use of the DCG to exchange power between AC zones);ancillary services (e.g. voltage support , frequency support to onshore AC grids, etc.).
In spite of those potential benefits, no DCG currently exists, as major barriers still remain. The TWENTIES project aims at developing and demonstrating network architectures with power system technologies and novel grid architectures which facilitate the widespread integration of wind power generation into the European electricity system.
One of the objectives of the project, as pursued in one of its large-scale demonstrations (“DEMO 3: DC Grid”), is to clarify and overcome some significant ones, either technological or economic. To this aim a preliminary analysis to implement different possible DC Power Flow Control (PFC) strategies has been carried out, i.e.:
- Autonomous PFC (wind power transfer and AC interconnection)
- Partial PFC and wind spillage
- DC PFC for AC network security
For the first case, it was proved that DC grid topologies (such as the “DC backbone grid” shown in Figure 1) make it possible to design dedicated autonomous controls for onshore converters, such as Voltage Source Converters (VSC) , to transfer power according to a predefined behaviour (wind power mitigation or not, with possible AC inter-area power exchange), thus accommodating for wind variability in a communication-free system.
Fig. 1 – Tree-like (plain lines) and meshed (plain and dashed lines) DC backbone grid examples (VSC are Voltage Source Converters; GS “Grid Side”; WS “Wind Side”)
By using this control strategy it was also verified that flexible PFC in normal and disturbed conditions, ancillary services to the AC mainland network (voltage support, frequency control , Power System Stabilizer), and Fault Ride-Through capability (i.e., to maintain the DC voltage during an AC contingency or the loss of an onshore converter) can be provided using local measurements only. Fault clearance would then involve de-energizing the complete network from the onshore AC grid.
Some complex DC grid structures (possibly resulting from existing grid extensions) may result in partial PFC. The principles of a simple PFC device were analyzed, resulting in a gain in network flexibility and significant savings on wind spillage. Such devices can limit wind spillage in case the grid becomes under-rated compared to the offshore wind generation (for example, following the connection of supplementary wind farms to it).
Lastly, PFC provided by the DCG was also considered as a mean to alleviate the AC network it connects to, by using preventive strategies to reduce the high current risk on the mainland network. The principle is to shift DC power injections thanks to a risk-based control strategy to minimize the overall high current risk over a certain time interval (relevant for the evaluation of contingency probability; e.g. 15 minutes), while minimizing either the redispatching costs or the redispatched energy of conventional generation and power injections from the DC grid.
The simulations carried out demonstrate that DC grid injections can help to reduce congestions on the onshore AC network it connects to; naturally, a higher number of DC injections will provide more flexibility to solve AC congestions, while ensuring AC network security at lower redispatching costs. Additionally, risk-based approaches can ensure AC system security at lower costs with respect to conventional deterministic preventive controls.