Tony Walker, technical support group
At the annual UK cable test seminar held at the end of last year, papers were presented by some of the most experienced engineers working in the cable and cable test technology fields. This article is based on the paper that was presented by Chris Jones of Sinclair Knight Merz, with additional material from the paper presented by Peter Herpertz of Megger.
Today, the need for dependable and efficient methods of transmitting large amounts of power over long distances is more pressing than ever before. In many cases, the optimum solution is to transmit the power via Extra High Voltage (EHV) cables but, as we shall see, there are many challenges that must be overcome if solutions of this type are to be implemented successfully.
As would be expected, market forces are behind the growth in cable-connected projects in general and in High Voltage Direct Current (HVDC) transmission in particular. The expansion of renewable generation, particularly offshore wind, means that new connections and system reinforcement are essential. There is also an increased interest in interconnectors between regions and countries, and in the bulk transfer of power over long distances.
Other factors that tend to favour cable-based solutions are environmental concerns and stakeholder opposition. These have to be taken into account, of course, in every project, but they are usually bigger issues with overhead line connections than with cable connections.
When implementing cable-connected projects to satisfy these market needs, one of the first and most fundamental decisions that has to be made is whether to use High Voltage Alternating Current (HVAC) or HVDC transmission. In some cases, this decision is straightforward, as it is in the case of international links, for example, where synchronising the power grids of the countries involved to allow AC transmission would be problematic. In other cases, however, the decision is much more complex.
There are pros and cons for both transmission technologies. HVAC cable links generally operate at between 132 kV and 550 kV, whereas present-day HVDC cable links can operate at up to 600 kV. HVAC cables suffer from charging current effects that limit the amount of power that can be transferred over long distances, whereas HVDC cables only suffer I2R losses. HVDC systems, however, need expensive AC/DC and DC/AC converters to allow them to interface with the AC distribution network.
When all of these factors are taken into account, a general rule of thumb emerges: for transmission over distances of less than 60 km, HVAC is usually preferable, whereas for distances over 120 km, HVDC is likely to be the better option. For intermediate distances between 60 and 120 km, the optimum solution can usually only be determined by carrying out a detailed and careful assessment of the project.
This will need to take into account capital cost, operating cost, the cost of electrical losses, and the potential cost of lost generation due to non-availability of the connection should faults or failures occur. It is interesting to note that the decision has come down in favour of HVAC technology for almost all offshore wind farms to date.
For land-based projects, there is still significant global discussion on the relative merits of overhead lines and cables. Cables are however being adopted on many projects as they are seen as a “pragmatic solution”, and it is reasonable to expect that the use of cables on transmission projects will continue to increase in the future.
Considerations for offshore transmission are, as would be expected, rather different. Here the use of overhead lines, except for very short distances, is not feasible. There are also numerous technical, environmental and cost factors that are unique to offshore work. These include complicated logistics, which are much influenced by weather and the availability of vessels; costs, which can be five to six times higher than onshore costs; and the large amount of personnel training that is needed for all offshore activities.
Transmission distances offshore can be up to 200 km, and the investment needed for many of the schemes that are currently being proposed is on a scale that has not been seen in our industry since the 1960s. In addition, today’s planning and consenting requirements are very different from those that applied in the past and, adding another layer of complexity, the regulatory regime chosen for the UK is very different from the regimes in many other parts of the world.
In this connection, it may be of interest to note that a recent UK government document available on the National Archives website gives an example of the development timeline and key decision gates for an offshore wind project. This shows the shortest possible timescale for project completion to be six years and, in practice, many projects can be expected to take much longer.
Another key challenge that designers of cable systems face is choosing the most suitable cable technology. HVDC XLPE (Cross Linked Polyethylene) cables are, at present, limited to operation at voltages up to 320 kV, which means, in practice, that they can transmit up to around 500 MW per cable. There is a need for HVDC XLPE cables that will operate at 400/500 kV and these are, at the present time, under development but are not yet available.
HVCD MIND (mass impregnated non-draining) cables are usually considered as having an operating voltage limit of 550 kV, although 600 kV cables are currently being delivered for use on the Western Link project, which will transfer power between Scotland and Wales.
In the HVAC arena, three-core submarine cables typically operate at up to 245 kV and have a power transmission limit of around 330 MW, but very recently a 420 kV three-core submarine cable with a capacity of 550 MW has been delivered. For underground applications, 550 kV single-core XLPE cables are available, and could potentially be used to transmit up to 2,000 MW per circuit, but to date the demand for these has been small.
After the type of cable has been decided, the next set of challenges relate to installation. With onshore installations, the first of these is securing cable easements, which can be a complicated and time-consuming process. Once this process is complete and cable installation is underway, the issues become ensuring that the cable is handled carefully and that joints are correctly made.
As might be expected, there are even more challenges associated with subsea cable installation, not the least of which is chartering suitable cable-laying vessels with the capability, in larger projects, of handling sections of up to 150 km of cable. The handling and laying risks are, for the most part, comparable with those encountered in onshore installations, but there is a vital additional requirement to provide protection for the cable after installation.
Options for this include ploughing or jetting to bury the cable with the use of additional protection in the form of rocks, concrete mattresses, gabion bags and even concrete bridges to protect the cable against damage by anchors and similar hazards at particular locations along the cable route.
One final challenge is deciding whether to test the cable as part of the commissioning process and, if so, what test methods to use. While it might seem self-evident that cables should be comprehensively tested before being put into service, finding test methods that will prove adequate for this task, especially in the case of subsea cables, is difficult. On some projects this can leave no option but to energise the newly installed cable and monitor its performance while it is in service.
The key challenges associated with cable installations include evaluation of costs and project viability, addressing procurement issues such as finding cable suppliers with sufficient manufacturing capacity and the ability to meet delivery requirements, and availability of suitable installation vessels. Once the cable is in service, issues such as cable failure, fault location and repair times come to the fore.
If we are to continue to meet the growing need for cable-connected projects, there are a number of future requirements that are highly desirable if not essential. Among these are the needs to reduce costs and project risks thereby making projects more financeable.
It will also be necessary to speed up the project delivery process, particularly in the areas of planning and consent, and to achieve better co-ordination to avoid route sterilisation where an existing cable blocks the desired route for a new cable. More certainty will be needed on projects and also more supplier capability and capacity.
In conclusion, the demand for cable projects is increasing and will continue to increase for the foreseeable future with offshore wind and interconnectors being key areas of growth. Both HVAC and HVDC technologies will have a role to play in meeting this demand. Growing route lengths, tighter project programmes and the need for risk mitigation will undoubtedly lead to new approaches to cable testing, fault location and repair. In short, market demand will drive supply chain development and innovation.