e-Highway 2050: Methodology for the operational validation of grid reinforcements at 2050

Challenge
What are the challenges to operate the system in the e-Highway2050 scenarios?

Background and assumptions 
The e-Highway2050 project proposed a set of grid reinforcements at the 2050 time horizon in a scenario driven approach. The scenarios for the pan-European transmission system at 2050 have been described here and Europe has been clustered into about 100 different clusters. Then the five energy scenarios at 2050 have been quantified, meaning that demand, storage, exchange and generation have been defined at country and cluster level according to a dedicated approach. Grid architectures and reinforcement options have been detailed as well.

Description of the result
1.      Challenges for system operation of the European electricity system at 2050
System operation will be challenged in the future by the major changes expected in the European electrical system. Three main sources of change are identified, each having a potential impact on the different operating issues:
- The increasing penetration of renewable energy sources: RES behave radically differently than traditional plants (small power electronics device vs. large synchronous generator). In four of the five e-Highway2050 scenarios, during some hours, they are the only generating units connected to the grid, supplying entirely the European load.
- The increasing power exchanges: All the e-Highway2050 scenarios show significant European power flows on the transmission grid
- The increasing number of connections realized with HVDC: HVDC behaves very differently than AC lines. Today, a few DC lines exist in the European system to connect non-synchronous areas and only one DC link is implemented in parallel to AC lines. In e-Highway2050 scenarios, at least 50 GW of more HVDC are foreseen in addition to the 2030 projects of the TYNDP.

2.      Power flows control
In comparison to flows on AC lines, which is determined by Kirchhoff’s laws and the topology of the system, HVDC lines are connected via power electronics (PE) and can be actively set by TSOs. The advantage is a better control and a greater flexibility. The drawback is that HVDC are not responding “naturally” to the variations or contingencies affecting the power system and thus efficient coordinated control rules have to be implemented by TSOs.

Within the project, load flows simulations of the full transmission network of the continental synchronous area were performed for each scenario for two snapshots: winter peak and summer low. The model was built from ENTSO-E 2030 grid with the inclusion of the 2050 inter-cluster reinforcements. For terrestrial reinforcements, two strategies were compared: AC and DC. A security study, between “N-1” and “N” situation, was then performed. In the simulations performed with AC reinforcements, the architectures are robust but with DC reinforcements, overloads appeared when fixed setting points were assumed. It highlights the need for smarter control rules of HVDC.

3.      Voltage control
TSOs have to operate the system within secure voltage limits. The drastic change in the power system expected by 2050 will probably need adaptations in the reactive power compensation means. To assess these issues, AC load flows were tested within the project. For the two most extreme scenarios, Large Scale RES and 100% RES, no convergence could be found. The reason is that existing methodologies and tools are inadequate to study easily such different network configurations. In parallel, in the R&D part of the project [5], an innovative algorithm has been developed to tackle these problems by automatically adapting the reactive power compensations. It could be applied in future studies.

Today, traditional plants are the main contributors to voltage control and their modeling in simulation tools are mature and quite realistic. This is not the case for wind and solar generators which are not systematically taking part to voltage control although it is technically feasible. Without this participation, the situation will no longer be manageable with more RES. This has to be anticipated in network codes and simulation tools should be adapted. HVDC converters also offer new possibilities for voltage control.

4.      Dynamic stability and protection schemes
Traditional plants are synchronous generators; their behavior is well known by TSOs. On the contrary, HVDC, large wind turbines and solar panels are connected to the AC grid via power electronics.  Their dynamic behavior is completely different. Preliminary studies were conducted within the project about short-circuit currents, frequency stability and small signal stability but research is still needed to fully assess the impact of their increasing penetration.

Conclusions
The methodology developed to validate the robustness of the grid architecture proposed by the e-Highway2050 project is based on detailed power flow simulations performed on relevant areas and power flow snapshots of the proposed grid reinforcement solutions. Critical reliability issues in the presence of massive renewables expansion were investigated. These issues include voltage problems at peak and off-peak situations, potential power oscillations in the AC system when a DC link is lost, stability issues of HVDC line in parallel with HVAC (after a fault occurs on the HVAC line), multi-terminal HVDC links solutions operated in parallel with the AC grid.

The studied case studies were designed to address voltage drops, power oscillations and stability issues for certain critical areas and snapshots and were designed on the grid constraints and the resulting likeliness of occurrence of problems.

The simulations allowed the following findings:
- to define credible system weakening factors with help of N-1 to N-k calculations in order to find critical network situations
- to define the network parameters for which one or more of the above reliability loss can be faced
- to propose counter measures for which expected improvements can be quantified via network simulations
- to define curative actions that can restore grid operation under normal condition after disturbances
- to specify changes in performance of the technological countermeasures to be sent to manufacturers for further feasibility validation by 2050
- to abandon some of the retained architectures since leading to potential reliability issues that cannot be properly solved before 2050 within the expected technology performances.
For more details see [1] and [2].

References
[1]    R. Pestana et al, Deliverable D4.1 of eHighway2050 project. Operational validation of the grid reinforcements by 2050. November 2015
[2]    T. Anderski et alia. D2.4 of e-Highway2050 project. Contingency Analyses of Grid Architectures and Corrective Measurements. 2015
[3]    F. Echavarren, L. Rouco, L. Sigrist, Comillas. Deliverable D8.5 of eHighway2050 project. Enhanced methodology to assess robustness of a grid architecture. 2015
[4]    G. Sanchis, RTE et alia, “A methodology for the development of the pan-European Electricity Highways System for 2050”, CIGRE Paris, August 2014 
[5]    B. H. Bakken, M. Paun, R. Pestana, G. Sanchis, “e-Highway2050: A Modular Development Plan on Pan- European Electricity Highways System for 2050”, Cigre Lisbon, April 2013
[6]    e-Highway project http://www.e-highway2050.eu