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Projects: Projects for Investigator
Reference Number NIA_SPEN_0028
Title Transition to low voltage DC distribution networks – Phase 1
Status Completed
Energy Categories Energy Efficiency(Transport) 10%;
Other Power and Storage Technologies(Electric power conversion) 20%;
Other Power and Storage Technologies(Electricity transmission and distribution) 70%;
Research Types Applied Research and Development 100%
Science and Technology Fields ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering) 100%
UKERC Cross Cutting Characterisation Not Cross-cutting 100%
Principal Investigator Project Contact
No email address given
SP Energy Networks
Award Type Network Innovation Allowance
Funding Source Ofgem
Start Date 01 March 2018
End Date 01 December 2018
Duration ENA months
Total Grant Value £142,000
Industrial Sectors Power
Region Scotland
Programme Network Innovation Allowance
Investigators Principal Investigator Project Contact , SP Energy Networks (100.000%)
  Industrial Collaborator Project Contact , SP Energy Networks (0.000%)
Web Site https://smarter.energynetworks.org/projects/NIA_SPEN_0028
Objectives To address the problem listed above this project will carry out the following activities across two distinct Phases. Phase 1 will directly inform the requirements and refinement to the scope for Phase 2 which will be registered separately depending on the results and outcome of Phase 1. Phase 1 will carry out a detailed review of LV DC network requirements from a UK DNO point of view and determine the optimal scope for Phase 2 as below based upon the latest learnings that are available from across the world. This includes: A literature review on latest technical and commercial learnings available for LV DC distribution networks from around the world. This will include protection requirements on both network and customer sides and the impact on existing network earthing. Desktop modelling of the expected capacity release and potential losses improvements that could be achieved by operating the existing LV AC circuits at DC under different applications using case studies of real network within SPD. Modelling of the transient and steady state performance of LV DC networks under fault conditions to inform protection requirements at different voltages and fed by different converter topologies. Developing technical specification requirements for LVDC DNO networks Developing technical specifications and laboratory test schedule for the cable testing planned within Phase 2. Market review of the available cable testing facilities that could be used within Phase 2 based upon requirements set out by the technical specification for LV cable testing at DC. Phase 2 will carry out physical testing of existing LV AC cables of different types and sizes to determine the impact of converting circuits to DC for the purposes of increasing transfer capacity and reducing consumer losses. The results of the laboratory test will be compared with modelling activities conducted within Phase 1 and required adjustments to the model will be considered. This will inform DNOs on the practicalities of running a DC distribution network for applications such as rapid Electric Vehicle charging or supplying DC customers: Laboratory testing of typical 4-core and 3-core cables (new and existing) for operation under DC, using unipolar and bipolar cable configurations. This includes monitoring of partial discharge and cable temperature at different DC voltage levels and loading to determine: o The impact on cable health and rate of ageing when converted to DC operation. o The impact of a DC fault on the cable and surrounding area (informing operation and maintenance costs and damage level). Development of a CBA methodology for comparing the deployment of LV DC network vs traditional AC network design and reinforcement. Run the developed model and the CBA methodology on example networks (case studies) under several distinct applications to demonstrate the potential customer and network benefits. Phase 1: 9 months and to include the following activities: Literature review of existing and previous academic and practical projects in LV DC distribution from around the world: The resulting document will contain current best practices and standards of LVDC networks that provide UK DNOs with the necessary background knowledge to develop LV DC networks. This document will also highlights any gaps in knowledge and testing that require further investigation. This will inform both the desktop modelling and technical specifications for testing of LV cables at DC. As a minimum the following content will be covered: o Protection requirements (relay, earthing etc.), o Potential network capacity increase over distance (assumptions and requirements) o Recommendations on DC voltage levels and associated operational risks and H&S issues o Customer requirements (voltage, equipment specifications, Health and Safety recommendations and risks) o LV DC networks configurations, comparison between bi-polar and uni-polar technologies, recommendations on suitability for UK distribution network application o Recommendations on LV DC metering technologies and potential commercial arrangements Desktop modelling to demonstrate the performance of LV DC networks and their ability to improve network capacity and reduce losses: o Evaluation of transfer capacity over distance for different cable types used by UK DNOs. o Fault transient behavior under DC operation considering different converter types and voltage levels and different earthing arrangements: DC faults profiles are significantly different from AC, and they can have different characteristics based on the following factors: the types of the converters providing DC and the local devices interfacing with the DC cables (e.g. LCTs); operating voltage levels; DC cable configurations and earthing arrangement; and used protection schemes and protection devices. With an LV DC network high transient discharging current with high rate of change can be experienced due the capacitive nature of LVDC followed by a steady state fault current without natural zero crossings. Such a profile will generate large thermal energy (larger Joules compared to AC) in the LV DC network which has to be absorbed and dissipated in the right time. In general this has to be done faster than in AC, to due to the aggression of DC arcs especially when a higher DC voltage is used compared to equivalent RMS AC. However, the level of such phenomena will depend on how the LVDC is designed and the type of protection is used. This task will investigate fault characteristics and potential fault levels of LV DC under different short circuit faulted conditions. This includes the simulation testing of faulted LV DC with different cable configurations, different converters interface (including converters without fault tolerance such as two-level VSC and converters with fault level capabilities such as full bridge MMC), and different earthing arrangements. The outcomes of the task will include: Understanding the behavior of LV DC networks under different fault conditions Identification of prospective fault levels under different LV DC and converter configurations, and this will help to identify the potential fault level management and protection requirements Identification of the thermal energy generated during transient and steady state DC fault phases. This will feed to the Phase II testing for understanding how much energy is expected and passed in LVDC cables under different DC faulted conditions. o Time-domain analysis for different load profiles (24hrs) will be considered to demonstrate how losses/voltage profile/loading can be improved at DC when compared to existing AC operation. Development of technical specifications for the testing of LV cables at DC. This will include a review of the LV cable types that are prevalent across the UK distribution network so that the testing within Phase 2 is optimized. This includes developing plans for the type of testing required, test schedules, and testing facility technical capability requirements. A market review of the available cable testing facilities based upon the technical specifications for testing to identify the potential testing facilities which meet the testing capabilities requirement and can be invited for the procurement of the activities within Phase 2. Phase 2 – Cable Testing and BaU Integration (to be registered separately) The following describes the initial scope for Phase 2 prior to the refinements that will be made based on the learnings gathered from Phase 1. Work Package 1: Cable Laboratory Testing (3 months) Laboratory testing of both 3-core and 4-core cable at increasing DC voltage levels and cable configurations (unipolar vs bipolar). The impact of DC on cable rate of ageing will be determined using partial discharge and cable temperature monitoring. The impact of cable failure will be tested to understand the HSE & O&M requirements for converted DC circuits. The energy released from DC fault is normally higher than in AC due to the high transient discharge currents and steady state fault currents without zero crossings. The test will provide understanding the impact of such phenomena which will give design engineers and district staff the confidence that circuits can be operated at DC without additional unmanageable risk. Work Package 2: Reporting and BaU documentation (6 months) The development of case studies for network reinforcement and including a detailed CBA to demonstrate the financial benefit that would be realised using a DC reinforcement/design approach. Recommendations for network applications which would benefit the most from the deployment of LV DC distribution. Provide DNOs with a proven business case to demonstrate whether low voltage DC networks can be a financially and technically competitive alternative to costly conventional reinforcement and network design. Evaluate and understand the performance of existing low voltage cables under DC operation to provide adequate learnings for the technical and operational requirements of converting LV AC networks to DC operation. Provide a road map of potential applications that could benefit from low voltage DC supplies (rapid EV charging, street lighting) along with a detailed cost benefit analysis using relevant case studies.
Abstract It is well documented that the shift towards a low carbon society will result in a step change in how electricity is generated and consumed across distribution networks. In particular, the uptake of Low Carbon Technologies (LCTs) such as Electric Vehicles, Photovoltaics and Heat Pumps is reducing the available capacity within LV networks and creating a requirement for costly and time consuming network reinforcement. In fact, the potential UK network reinforcement caused by the uptake of Electric Vehicles alone has been estimated to reach £34bn - £48bn by 2040. Furthermore, these LCTs are connected to an AC distribution network despite the fact that most consume and generate DC power. This creates the requirement for often low efficiency three stage converters between DC devices and the AC network, increasing both network demand and customer losses and equipment cost. To overcome the challenges discussed above existing LV AC circuits could be converted to DC networks operation to release additional capacity within existing network infrastructure whilst significantly reducing customer losses. In theory, an LV DC distribution network could support longer LV feeders and reduce the number of secondary substations required to supply an area. This is due to improvements in transfer capacity caused by improved circuit voltage drop and increased cable thermal capacity.  In addition to this, if the losses associated with EV charging were reduced by only 4% using a DC supply UK consumers could save up to £52m annually by 2040. For targeted applications LV DC network operations could deliver significant value to UK electricity consumers in the near future if innovation funding is invested into its development. However, there are number of technical and commercial challenges which need to be addressed before LVDC can be fully adopted by UK DNOs as a business as usual (BaU) approach to facilitate the uptake of LCTs. Some of these challenges include: The impact on cable ageing when converting existing low voltage AC cables to LV DC at different voltage levels and cable types. The optimal LV DC voltage for distribution networks when considering both customers (reduced losses) and network requirements (increased transfer capacity). How an increased transfer capacity over distance could benefit and change network design practices i.e. longer LV feeders, less secondary substations, smaller cable etc. The H&S requirement for LV DC adoption both within the network and at the point of customer connection. The number of opportunities for UK DNOs to convert existing LV AC circuits to DC including a proven Cost Benefit Analysis (CBA) for different applications demonstrating the financial benefit. The operational and maintenance requirements for existing LV AC circuits which have been converted to DC i.e. is the impact of a cable fault at DC any more or less severe?
Publications (none)
Final Report (none)
Added to Database 02/12/22