Prototyping an advanced protection scheme for enabling a low voltage DC last mile distribution network with renewables
Growing international awareness of the environment has increased the urgent need to reduce carbon emissions within the electricity, transport, and building sectors. Traditional low voltage alternating current (LVAC) public distribution systems are now facing pressure to connect large numbers of small-scale renewables such as micro-wind and solar panels. This is in addition to the need for supplying the increased low carbon heat demand such as heat pumps and the increased transport demand such as electric vehicles.
However, existing electricity distribution systems were not originally designed for accommodating such changes. As a result, significantly increased power flow capacity and reduced energy waste will be required over and above the provision of smart controls. Such challenges increase the need for a rethink of the new standard designs to be adopted in the last mile of public electricity distribution networks. Low voltage direct current (LVDC) distribution systems, combined with smart controls, have the potential to facilitate this transformation with advantages over the corresponding LVAC systems.
The use of LVDC systems is not new. The first public electricity supply introduced in 1882 (Edison’s Pearl Street Station) was an LVDC. Since that time AC has dominated due to the capability of AC for transmitting power over long distances. In today’s digital era and low carbon economy, the use of electronic devices run on DC and microgenerators that generate DC has been rapidly growing. This, in addition to advanced power electronics, has motivated the return of LVDC distribution systems after more than 120 years. They can be used to power different sized data centers, facilitate the connection of end-users’ electronic loads and sources, and potentially to be used in last mile cables to reduce energy losses and increase power capacity.
LVDC systems do, however, present significant safety and protection challenges. DC faults are more difficult to detect and clear. Their associated arcs are more aggressive than in AC, and they require longer time to be cleared. This makes the risk of fire in DC systems higher than in AC. In addition, the residual current devices (RCDs) which are commonly used in AC systems to protect against electric shocks and fire are not commercially available for DC systems. DC systems will also require AC-DC power electronics converters for providing DC supply. Such interface devices have poor short circuit fault capability and can trip for remote faults. This can lead to substandard protection selectivity and unnecessary disconnection of larger part of the system. Such issues increase the need for fast and reliable DC protection solutions that reduce the risk and the cost of operating such systems.
This impact acceleration account (IAA) project has prototyped a novel DC protection scheme that addresses the outstanding challenges for protecting an LVDC last mile distribution network. The scheme concept which provides fast speed and good selectivity level for clearing DC faults on LVDC systems were initially developed during the EPSRC Transformation of the Top and Tail of Energy Networks project.
Proof of the concept
The new protection algorithm has been successfully tested on a low power demonstrator, representing an LVDC last mile network with distributed generation that was built for the purpose.
The algorithm has been implemented in the Laboratory Virtual Instrument Engineering Workbench (LabVIEW) system design platform, and its performance has been tested on the demonstrator. All the measurements of the currents and voltages from the demonstrator hardware are interfaced to the simulated protection scheme through a National Instrument (NI) CRIO-based FPGA interface.
The experimental results have been widely disseminated, and exhibited at the Low Carbon Networks and Innovation (LCNI2014) conference in Aberdeen, at the PAC world conference in Glasgow, and at the HubNet Smart Grid Symposium 2015 in Birmingham.
Supporting the IEC system evaluation group on LVDC systems
The impact champion is now a technical member of the IEC SEG4 committee which evaluates the standards of LVDC systems and products. The IEC SEG4 work covers the application of LVDC systems in developed and developing countries by which the DC systems can be utilised to deliver energy efficiency and power remote areas. This evaluation group is actively engaged with relevant LVDC systems stakeholders and will deliver the recommendations on LVDC standards to the standardisation Management Board of the IEC.
The University of Strathclyde LVDC work has now been internationally recognised, and the impact champion was invited by the IEC and the Bureau of Indian Standards (BIS) to present the research results at the 1st International LVDC “Redefining Electricity” conference in New Delhi in India in October 2015.
Supporting the new IET code of practice on LVDC systems
The research results have further significantly supported the writing of two chapters of the new IET LVDC code of practice. This code of practice provides the technical guidance, and “sets out requirements for the growing demand for LVDC power systems, covering specification, design, selection, installation, commissioning, operation and maintenance”.
As stated on the IET website, the standard is useful to “designers, installers and operators of LVDC power distribution in buildings – including electrical, IT and telecommunications professionals – and of interest to customers, owners and insurers of LVDC power applications”.
Based on the experience gained from the LVDC project, the Impact Champion has significantly contributed to forming an LVDC European consortium (led by Strathclyde) of research institutes and industrial partners across 7 EU countries, and coordinated the consortium during developing a H2020-MSCA Innovative Training Network bid which was submitted on 12 January 2016. The bid is also supported by the IET, IEC, and EMerge Alliance organisations.