5. Project‐specific considerations regarding the choice of transmission line
5.2.4 Summary of project‐specific use of UGC technology
Apart from the obvious visual advantages and maintaining property values, underground cable transmission systems have one primary advantage over overhead lines:
Reduced maintenance costs as components of underground cable installations require less maintenance as they are not as exposed. However, external equipment like shunt reactors and switchgears will add to the maintenance costs of a 400 kV cable installation.
The application of 400 kV underground cables is relevant for short distances, assuming the technical issues listed in Section 5.2.2 can be mitigated efficiently without setting precedents and limitations for the future development of the Danish transmission grid.
It must be emphasized that the share of underground cables of future 400 kV grid expansion projects must be seen in the context of the accumulated amount of installed cable circuits in the surrounding transmission grid, as this total amount of cables dictates the applicability of EHV cables in a given transmission grid. For the same reason, it is important that any choice between 400 kV underground cables and overhead lines includes a long‐term system design perspective.
An increased share of 400 kV underground cables in the transmission grid introduces a series of unknown factors, and thereby considerable risks. Due to limited operational experience of transmission systems with a significant share of underground cables, relevant technical aspects are examined further in Chapter 6.
Attempts to overcome the technical challenges related to underground cables, all originating from the laws of physics, may delay further expansion of renewable generation in Denmark, as there will be a limit to the total amount of 400 kV underground cables which can be established in a transmission grid.
5.3 400 kV HVAC gas‐insulated transmission lines (GILs) 5.3.1 Usability
Installation of GIL systems over long distances (>1 km) has not been done anywhere in the world. However, GIL systems buried directly in the ground rather than installed in tunnels are commercially available, which add potential to the use of GIL systems over long distances in the future.
The installation of long GIL systems introduces a number of unknown factors, and thereby considerable risks, due to limited operational experience. Identified risks mainly relate to construction schedules, installation and long‐term reliability issues.
For tunnel installations in urban areas, GILs are considered a competitive solution instead of UGCs, especially for high power transmission capacity applications. GIL systems in tunnels add the advantage of personnel safety, as an internal arc (short circuit) between the conductor and the metallic pipe will not cause a pressure rise in the tunnel. In addition, GIL systems are considered fireproof due to the absence of flammable
materials.
Figure 29: GILs installed in tunnels (SIEMENS).
5.3.2 Technical considerations
The following sections describe well‐known technical issues that must be taken into account when considering the application of GIL systems.
5.3.2.1 Transmission capacity
GIL systems can be designed for a transmission capacity equal to an overhead line, reducing the space needed for the installation as the required transmission capacity can be achieved with a single circuit whereas two circuits are normally required for undergrounded cable systems.
Figure 30: Double circuit GIL installation (Foto: SIEMENS).
5.3.2.2 Reactive power compensation
GIL systems have the advantage of having electrical characteristics similar to those of overhead lines, which is important to system operation. The capacitance of GIL systems is low, allowing long lines to be installed without the substantial need for reactive compensation of an underground cable circuit.
Reactive power compensation of GIL systems does not pose any technical challenges from an operational point of view. Detailed studies will determine the design and optimum location of the required reactive compensation.
5.3.2.3 Load balancing of the system
GIL systems have lower positive sequence impedances as compared to similar overhead lines. As such, they will tend to carry a greater share of the transmitted power when operating in parallel with overhead lines.
Detailed analysis must be conducted to establish whether this could effectively be counteracted by inserting reactors in series, thereby increasing apparent reactance of the GIL system.
5.3.2.4 Short‐circuit level
A GIL system may result in an increase of the short‐circuit level of the transmission grid and thereby exceed the current design limit of 40 kA. If this limit is exceeded, existing components in parts of the transmission grid and at lower voltage levels must be replaced to withstand the elevated short‐circuit level. Detailed analyses must be conducted to establish the extent of the required replacement of grid components. In addition, it must be investigated if short‐circuit levels can be kept within limits by inserting reactors in series, thereby increasing apparent reactance of the GIL‐system
5.3.2.5 Temporary overvoltage
The risk of temporary overvoltages (TOV) related to GIL systems is considered low; however, detailed analyses must be conducted to establish whether special countermeasures are required for each individual case.
5.3.2.6 Amplification of background harmonics
The risk of amplification of background harmonics related to GIL systems is considered low; however, detailed analyses must be conducted to establish whether unacceptable amplification might occur.
5.3.2.7 Effect on existing HVDC links control systems
The application of long GIL systems is not expected to introduce any risk towards HVDC controller stability (wide band stability).
5.3.3 Construction schedule
A detailed construction schedule of a very long GIL system has not been prepared, but a time schedule for the installation of a 5 km GIL system comprising two circuits (a total of 30 km single phase tubes) has been developed.
The expectation is that the discussed length of 5 km can be installed and commissioned within 3 years.
Installation and commissioning of the entire length of the proposed 400 kV transmission lines with GIL technology within the timeframe available seems extremely unlikely.