DC technologies for widespread renewable deployment and efficient use of energy

This speech discussed the background, research experience, research activities and future opportunities for widespread renewable deployment and efficient use of energy

Experience from research into low voltage DC distribution system protection : recommendations for protecting hybrid HV DC-AC grids

This paper presents experience and outcomes of a research project concerned with protecting an LVDC “last mile” distribution network. The paper introduces the following contributions that reduces the risks associated with shifting from AC to DC for LV distribution purposes: understanding of how an LVDC system behaves during fault conditions through presentation and analysis of simulation results; outlining the issues associated with using traditional LV overcurrent protection for protecting future LVDC networks; and simulation of a new DC protection scheme that provides fast DC fault detection and location with a good level of selectivity. In addition, the paper presents a discussion of the lessons learned from the LVDC protection research project and how they can be utilised to understand and address the protection challenges in a higher voltage hybrid DC-AC grid

Protection analysis for plant rating and power quality issues in LVDC distribution power systems

Low Voltage DC (LVDC) distribution systems have the potential to be considered as an efficient platform for facilitating the connection of more distributed energy resources. The applications of LVDC are still at an early stage due to the lack of mature experience and standards. Over and above, the protection challenges that are presented by integrating DC installations in existing AC systems are one of the key issues that are delaying the wide uptake of LVDC technologies. In response to these issues, this paper discusses the international installation progress of LVDC systems and their relevant standards in different sectors. This includes data centres, buildings, and utility last mile distribution systems. The paper also investigates the impact of using traditional LV protection methods on the performance of a faulted LVDC network, and on the associated post-fault power quality performance. A typical UK LV network is energised using DC and modelled in PSCAD, and used for the protection studies under different DC fault conditions

Fault analysis of an active LVDC distribution network for utility applications

Low Voltage DC (LVDC) distribution systems are new promising technologies which can potentially improve the efficiency and controllability of existing LV distribution networks. However, they do introduce new challenges under different fault conditions. This paper investigates the performances of an active LVDC distribution network with local solar photovoltaics (PVs) and energy storages under different short-circuit faulted conditions. A typical UK LV distribution network energized by DC is used as a test network, and modeled using PSCAD/EMTDC. The LVDC is interfaced to the main AC grid using fully controlled two-level voltage source converter (VSC), and supplies DC and AC loads through DC/DC converter and DC/AC converter respectively. The response of an LVDC with such converters combination with different topologies and fault management capabilities are investigated through the simulation analysis

A novel protection scheme for an LVDC distribution network with reduced fault levels

Low Voltage Direct Current (LVDC) distribution is one of the new promising technologies that have the potential to accelerate the wider integration of distributed renewables. However, adding new power electronics to convert AC to DC will introduce new forms of faults with different characteristics. Converters with inherent fault current limiting and blocking capabilities will significantly limit the fault currents, resulting in significant impacts on the performance of existing LV overcurrent protection schemes. New protection methods based on the change in the DC voltages have been proposed recently by different researches. The issue with these methods is that the protection relays of the un-faulted feeders will also see the same change in the voltage for certain faults, leading to substandard selectivity and unnecessary tripping. This paper investigates these challenges, and presents a novel DC protection method which is based on the use of the combination of two components: the voltage change (dv/dt) and the change of current (di/dt). The new method is mainly developed to detect and locate DC faults with reduced fault current levels within DC distribution networks. The introduced protection concept is tested on an LVDC distribution network example using PSCAD/EMTDC simulation tool

Electro-thermal analysis of power converter components in low-voltage DC microgrids for optimal protection system design

Bidirectional power converters are considered to be key elements in interfacing the low voltage dc microgrid with an ac grid. However to date there has been no clear procedure to determine the maximum permissible fault isolation periods of the power converter components against the dc faults. To tackle this problem, this paper presents an electro-thermal analysis of the main elements of a converter: ac inductors, dc capacitors and semiconductors. In doing this, the paper provides a methodology for quantifying fault protection requirements for power converter components in future dc microgrids. The analysis is performed through simulations during normal and fault conditions of a low voltage dc microgrid. The paper develops dynamic electro-thermal models of components based on the design and detailed specification from manufacturer datasheets. The simulations show the impact of different protection system operating speeds on the required converter rating for the studied conditions. This is then translated into actual cost of converter equipment. In this manner, the results can be used to determine the required fault protection operating requirements, coordinated with cost penalties for uprating the converter components

Multi-zone LVDC distribution systems architecture for facilitating low carbon technologies uptake

Low voltage direct current (LVDC) distribution systems have recently been considered as an alternative approach to provide flexible infrastructure with enhanced controllability to facilitate the integration of low-carbon technologies (LCTs). To date, there is no business-as-usual example of LVDC for utility applications and only few trials have been developed so far. The deployment of LVDC in general will present revolutionary changes in LV distribution networks. This will require are thinking of network design principles and the enablement of integrated solutions. This discussion paper reviews the current practice in utility-scale LVDC distribution networks worldwide. The paper also presents a new multi-zone architecture approach which can be used to better understand future of LVDC systems, and exploit their inherent flexibility to allow synergistic integration of multiple energy technologies

Multi-sample differential protection scheme in DC microgrids

This paper proposes a novel solution to the issue of protection instability caused by time synchronization error in high-speed differential protection schemes for DC microgrids. DC microgrids provide a more efficient platform to integrate fast-growing renewable energy sources, energy storage systems, and electronic loads. However, the integration of distributed generators (DG) may result in variable fault current magnitude and direction during fault conditions, potentially causing mis-coordination of conventional time graded overcurrent relays. One identified solution to this issue utilizes high-speed differential protection schemes to maintain effective selectivity in DG-dominated DC microgrids. However, as DC short-circuit fault currents are highly transient, microseconds of synchronization error in the measured line currents may cause protection stability issues, whereby mal-operation of relays may occur as a result of faults external to the protected zone. This paper investigates the impact of time synchronization errors for high-speed differential protection in DC distribution systems. It then proposes a multi-sample differential (MSD) scheme that performs multiple differential comparisons over a sampling window to ensure the stability of high-speed differential protection schemes for external faults whilst maintaining sensitivity to internal faults