In power system black start and restoration plans, VSC HVDC transmission systems can and have been used to re-energise AC networks. This is a valuable black start resource particularly considering increased interconnection between the UK and other European countries and the reduced conventional synchronous generation on the system. However, as the short circuit current of a VSC is limited by its control system, this limit may affect grid short circuit and dynamic characteristics during the energisation of an AC grid from a VSC HVDC system; thus impairing protection performance at both transmission and distribution levels, with the potential for mal-operation of protection devices under this unique grid scenario. In such cases, the sensitivity and reliability of the protection schemes will be degraded, which may pose dangers to public safety and grid assets as well as prevent or delay system restoration.
In this project EPRI will bring world-leading experience in protection, HVDC and operation of grids with low short circuit strength to evaluate system restoration conditions and relay/protection types where the issues are credible. This will be achieved through dynamic simulations of areas of the UK grid, detailed RTDS based simulation of HVDC interconnections and hardware-in-the-loop based evaluation of existing protection relays under system restoration scenarios. In addition to providing results of the analysis; EPRI, in collaboration with the National HVDC Centre will develop guidance for operators on determining AC grid strength and requirements for protection adjustments during energisation of the AC system from HVDC and during system conditions where low short circuit strength may occur.
Key Outputs [Please click links below to access technical reports]
- Public Webinar: Black Start from VSC HVDC and its impact on AC Protection Coordination
- Final Report: Coordination of ac protection settings during energisation of ac grid from a VSC HVDC interconnector
This paper presented at the IET DPSP conference, 2020: “COORDINATION OF AC PROTECTION SETTINGS DURING ENERGISATION OF AC GRID FROM A VSC HVDC INTERCONNECTOR”
Abstract—The Relatively low short circuit current limit of VSC converters in comparison to conventional synchronous generation can affect grid short circuit and dynamic characteristics during a black start. This can impair protection performance at both transmission and distribution levels. This paper investigates the protection issues related to black start and system restoration using VSC HVDC as the black start resource. A case study on the Scottish transmission system is used to examine the implications for protection devices operating in the unique scenario where the network is restored from HVDC. Given the greater flexibility in its black start operation, both traditional step by step energisation and soft energisation from the VSC HVDC are examined finding that soft energisation can mitigate the potential for large transformer inrush currents exciting resonances on the network.
To access this paper: https://www.hvdccentre.com/wp-content/uploads/2020/06/DPSP2020.pdf