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

Enhancing transient performance of microgeneration-dense low voltage distribution networks

In addition to other measures such as energy saving, the adoption of microgeneration driven by renewable and low carbon energy resources is expected to have the potential to reduce losses associated with producing and delivering electricity, combat climate change and fuel poverty, and improve the overall system performance. However, incorporating a substantial volume of microgeneration within a system that is not designed for such a paradigm could lead to conflicts in the operating strategies of the new and existing centralised generation technologies. So it becomes vital for such substantial amount of microgeneration among other decentralised resources to be controlled in the way that local constraints are mitigated and their aggregated response supports the wider system. In addition, the characteristic behaviour of connected microgeneration requires to be understood under different system conditions to ascertain measures of risk and resilience, and to ensure the benefits of microgeneration to be delivered. Therefore, this thesis provides three main valuable contributions of future attainment of sustainable power systems. Firstly, a new conceptual control structure for a system incorporating a high penetration of microgeneration and dynamic load is developed. Secondly, the resilience level of the host distribution network as well as the resilience levels of microgeneration during large transient disturbances is evaluated and quantified. Thirdly, a technical solution that can support enhanced transient stability of a large penetration of LV connected microgeneration is introduced and demonstrated. A control system structure concept based on “a cell concept” is introduced to manage the spread of heavy volumes of distributed energy resources (DERs) including microgeneration such that the useful features of DER units in support of the wider system can be exploited, and the threats to system performance presented by significant connection of passive and unpredictable DERs can be mitigated. The structure also provides simpler and better coordinated communication with DERs by allowing the inputs from DERs and groups of cells to be transferred as collective actions when it moves from a local to a wider system level. The anticipated transient performance problems surrounding the integration of microgeneration on a large basis within a typical urban distribution network are addressed. Three areas of studies are tackled; the increased fault level due to the present of microgeneration, the collective impact of LV connected microgeneration on traditional LV protection performance, and the system fault ride through capabilities of LV connected microgeneration interfaced by different technologies. The possible local impacts of unnecessary disconnection of large amount of microgeneration on the performance of the host distribution network are also quantified. The thesis proposes a network solution based on using resistive-type superconducting fault current limiters (RSFCLs) to prevent the impact of local transient disturbances from expanding and enhance the fault ride through capabilities of a high penetration of microgeneration connected to low voltage distribution networks. A new mathematical approach is developed within the thesis to identify at which condition RSFCL can be used as a significant device to maintain the transient stability of large numbers of LV connected microgeneration. The approach is based on equation solution to determine the minimum required value of the resistive element of RSFCL to maintain microgeneration transient stability, and at the same time additional headroom against switchgear short-circuit ratings is provided. Remote disturbances or a failure to clear remote faults quickly are shown to no longer result in complete unnecessary disconnection of substantial amount of microgeneration

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

Supporting transient stability in future highly distributed power systems

Incorporating a substantial volume of microgeneration (consumer-led rather than centrally planed) within a system that is not designed for such a paradigm could lead to conflicts in the operating strategies of the new and existing centralised generation technologies. So it becomes vital for such substantial amounts of microgeneration among other decentralised resources to be controlled in the way that the aggregated response will support the wider system. In addition, the characteristic behaviour of such populations requires to be understood under different system conditions to ascertain measures of risk and resilience. Therefore, this paper provides two main contributions: firstly, conceptual control for a system incorporating a high penetration of microgeneration and dynamic load, termed a Highly Distributed Power System (HDPS), is proposed. Secondly, a technical solution that can support enhanced transient stability in such a system is evaluated and demonstrated

Using real-time simulation to assess the impact of a high penetration of LV connected microgeneration on the wider system performance during severe low frequency

In addition to other measures such as energy saving, the adoption of a large amount of microgeneration driven by renewable and low carbon energy resources is expected to have the potential to reduce losses associated with producing and delivering electricity, combat climate change and fuel poverty, and improve the overall system performance. However, incorporating a substantial volume of microgeneration within a system that is not designed for such a paradigm could lead to conflicts in the operating strategies of the new and existing centralized generation technologies. This paper investigates the impact of tripping substantial volumes of LV connected microgeneration on the dynamic performance of a large system during significant low frequency events. An initial dynamic model of the UK system based on a number of coherent areas as identified in the UK Transmission Seven Year Statement (SYS) has been developed within a real time digital simulator (RTDS) and this paper presents the early study results

Evaluating the impact of superconducting fault current limiters on distribution network protection schemes

Rising fault levels are becoming increasingly problematic in the UK distribution network, with large sections of the network operating near to its designed fault level capability. With the increase in penetration of distributed generation that is expected in the coming years, this situation is becoming more pressing. Traditional methods of dealing with the issue may not be appropriate - upgrading plant is expensive and disruptive, while network reconfiguration can compromise security of supply. Superconducting Fault Current Limiters (SFCLs) are emerging as a potential solution, with installations now taking place in several locations worldwide. The integration of an SFCL into a network involves a number of challenges, particularly concerning the coordination of protection systems. The operation of existing protection schemes may be compromised due to the increased resistance in the network during a fault (in the case of a resistive SFCL). Furthermore, the reduction in fault levels, although desirable, can have a detrimental impact on protection operating times. This paper will consider an existing medium voltage network in the UK, which incorporates distributed generation capacity. The performance of IDMT overcurrent and distance protection schemes will be examined when an SFCL is installed in this network. In particular, the increased operating time of overcurrent relays will be discussed along with grading implications. The impact on distance protection reach will also be examined. A variety of network operational scenarios including SFCL placement and fault conditions will be considered and compared. Recommendations will be made in terms of protection settings and SFCL placement in order to mitigate the aforementioned issues

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

Multi-function DC protection scheme for an LVDC smart distribution network

Low voltage direct current (LVDC) distribution systems have the potential to support future realisation of smart grids functionality. They do however present significant protection challenges that existing schemes based on DC fuses and conventional circuit breakers cannot manage due to slow device performance. Therefore, this research introduces an advanced protection scheme that addresses the outstanding challenges facing realisation of last mile DC distribution

Experimental validation of an advanced DC protection scheme for enabling an LVDC last mile distribution network

LVDC distribution systems have the potential to support future realisation of smart grids functionality. They do however present significant protection challenges that existing schemes based on DC fuses and conventional circuit breakers cannot manage due to slow device performance. Therefore, this research introduces an advanced protection scheme that addresses the outstanding challenges facing realisation of last mile DC distribution. The developed scheme has been validated using experimental testing

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