ASSESSMENT OF TECHNICAL ALTERNATIVES TO STRENGTHEN THE 400 KV TRANSMISSION GRID WSP
Project No.: 70051622 | Our Ref No.: December 2018
Independent Report Page 7 of 51
2 TECHNICAL REVIEW
2.1 INTRODUCTION
In Chapters 4 and 5 of the Report, Energinet has given an overview of the characteristics, merits, and disadvantages of four different technical alternatives that have been considered for the required West Jutland 400 kV transmission grid reinforcement.
In Chapter 4, Energinet describes the four main transmission line technologies that they have considered for the transmission grid reinforcement project. The technologies considered in this high-level assessment were as follows:
• 400 kV HVAC overhead lines (OHL)
• 400 kV HVAC underground cables (UGC)
• 400 kV HVAC gas-insulated transmission lines (GIL)
• High voltage direct current (HVDC)
Chapter 5 of the Report comprises a high-level technical assessment of the four transmission technologies for the proposed project. The technologies were evaluated in terms of:
• Usability
• Technical considerations
• Construction schedule
• Environmental impact
• Cost
It should be noted that WSP has not been asked to comment on the need for the reinforcement.
However, the new transmission capacity required was also not clear from the report. Generally, when new reinforcements are being considered, it would be expected that the reinforcement capacity required would be clearly identified using generation scenarios and carrying out
contingency analysis of the network. A discussion on this point was undertaken with Energinet in which Energinet stated that whilst the exact location of the offshore wind farms is not currently known, the worst case capacity assumptions and taking into account N-1 requirements the calculations show that around 2030 the maximum capacity provided by a 400 kV overhead line is starting to be exceeded. Energinet is therefore looking into whether utilisation of a second 400 kV circuit of the Northern reinforcement (Endrup-Idomlund) will be needed by 2030.
The following Subsections of this report present WSP’s appraisal of Energinet’s evaluations of the four different technical alternatives.
2.2 ALTERNATIVES CONSIDERED
In Chapter 5 of the Report, Energinet has presented an assessment of the relative merits of each of the four transmission technologies and provided reasons why some technologies and reinforcement options should be discounted from further consideration.
The results of that assessment are presented in Table 2-1 and the scoring system used by Energinet is presented in Table 2-2. Energinet’s conclusion was that the HVAC OHL solution achieves the highest score in all categories apart from Environmental Impact. The Report notes
WSP ASSESSMENT OF TECHNICAL ALTERNATIVES TO STRENGTHEN THE 400 KV TRANSMISSION GRID
December 2018 Project No.: 70051622 | Our Ref No.:
Page 8 of 51 Independent Report
however that the evaluation was of a qualitative nature and that more detailed development and design is needed to fully investigate the impact.
Table 2-1 - Relative merits of different transmission technologies (reproduced)
Technology HVAC OHL HVAC UGC HVAC GIL HVDC
Table 2-2 – Energinet’s scoring system for transmission technology assessment (reproduced)
Rating Description
1 Least preferred, high difficulty, unacceptable 2 Major technical challenges, difficult, poor
acceptability and very risky
3 Known technical challenges, difficult, limited acceptability and high risk
4 Known technical challenges, acceptable and some risk
5 Preferred, no technical challenges, fully acceptable and low risk
Energinet report Section 5.6
The Report notes that GIL has some technical advantages compared with underground cables. The GIL solution is however rejected in the Report due to:
• Lack of operational experience for the type of application, i.e. directly buried in open landscape and areas of special environmental interest
• Lack of experience of long horizontal drilling techniques for GIL purposes or the establishment of tunnels for GILs under these areas
• Timescales – not considered feasible to construct by 2023
• Costs – estimated at 2-3 times the cost of the underground cable solution.
The Report also rejects the HVDC solution on the basis of:
• Lack of robustness compared with HVAC solutions
ASSESSMENT OF TECHNICAL ALTERNATIVES TO STRENGTHEN THE 400 KV TRANSMISSION GRID WSP
Project No.: 70051622 | Our Ref No.: December 2018
Independent Report Page 9 of 51
• Limitations for future expansion and integration of renewables
• Increased cost – roughly 5 times the cost of the “equivalent” HVAC solution.
It is noted that it is not clear from the report in what respect the HVDC solution is equivalent, as the total capacity provided by HVDC is less than the maximum HVAC capacity. This is all subject to N-1 transmission system studies which are not included within the report.
Although not covered in the table above, the Report also considers an alternative based on 150 kV and 220 kV UGCs with full undergrounding. The Report rejects this solution as it would require numerous circuits in order to achieve the required transmission capacity and would require major restructuring of the transmission grid in Jutland.
Having rejected the GIL, HVDC, and 150/220 kV based solutions, the Report then considers four alternatives based on combined use of OHL and UGC with a share of UGC ranging from 6% to 100%. The Report notes that the locations and exact lengths of the individual cable sections are to be defined during the environmental impact assessment.
The share of UGC and OHL for each of the four alternatives is given in Section 5.7 of Energinet’s Report. Note that the distance from Endrup to the border with Germany is approximately 75 km and therefore the part of the line within Germany is approximately 16 km, which will all be OHL.
Alternative A is the reference solution. It is understood that this was the solution originally put forward by Energinet. As indicated below, it includes approximately 6% UGC on the Endrup-Idomlund section and 11% of the Endrup-Klixbüll section.
Alternative B has an UGC share of 12-15% of the length.
In the answers to clarification questions from WSP, Energinet has stated that Alternatives A and B would require reactive compensation stations at Endrup and Idomlund substations and at a new substation at Stovstrup between Endrup and Idomlund. It is understood that in Energinet’s view, Alternative B represents the solution with the maximum possible share of UGC without requiring additional compensation stations (beyond those required for Alternative A).
Energinet has stated that Alternatives C and D with 50% and 100% UGC would require considerably more compensation as follows:
• Alternative C:
o Three minor compensation stations between Endrup and the Danish-German border, including expansion of Endrup substation.
o Five (including two minor) compensation stations between Endrup and Idomlund including expansion of Endrup, Idomlund and Støvstrup substations.
• Alternative D:
o Four compensation stations between Endrup and the Danish-German border, including expansion of Endrup substation and a new substation at the border.
o Five compensation stations between Endrup and Idomlund including expansion of Endrup, Idomlund and Støvstrup substations.
Energinet has not considered an UGC share of between 15% and 50% because the technical challenges and risks associated with a length of 15% are already becoming significant. It was therefore concluded that there would be limited value in understanding these intermediate cable shares.
WSP ASSESSMENT OF TECHNICAL ALTERNATIVES TO STRENGTHEN THE 400 KV TRANSMISSION GRID
December 2018 Project No.: 70051622 | Our Ref No.:
Page 10 of 51 Independent Report