University of Strathclyde
Assessment and Mitigation of Converter Interactions
Summary: The rapid growth in the amount of converter-interfaced renewable sources and HVDC transmission links is significantly changing the characteristics of the GB grid. The wide timescale control dynamics of converters located in close vicinities can result in coupled effects among the converters and power networks and can lead to oscillations across a wide frequency range. Accurate assessment of potential system interactions is critical for ensuring stable operation of future and evolving GB network. The project will develop frequency domain models of HVDC converters and representative network considering future HVDC deployment in the GB network for stability analysis. Time-domain RTDS simulation will be used to validate the theoretical analysis and assessment. Remedial converter control actions that aim to mitigating instability and resonance will be investigated. Recommendations for specifications of converter models and advice to specific HVDC projects on stability will be proposed.
Improving Grid Code compliance of HVDC schemes
(Improve Requirements Specification of HVDC schemes)
Summary: High voltage direct current (HVDC) schemes are used in Great Britain (GB) power systems to facilitate flexible power flow and cross-border power exchange. New HVDC connections are in the pipeline to be installed in the GB system to carry clean and cheap renewables within the country and across the borders. In order to maximise their benefits and to ensure a safe and stable integration into the existing power grid, certain requirements are set-out by the operators, known as Grid Codes. HVDC schemes are required to meet these specifications under all operating conditions.
However, with the increased penetration of renewable resources, the network strength is declining in many regions of the GB system causing operational constraints to accommodate new HVDC connections. Stringent requirements are required and are foreseen to be imposed on HVDC schemes to address this situation, especially at electrically weak grid connection points.
On one hand, too negligent requirements may cause reliability or stability issues. However, on the other hand, too onerous requirements can prevent reaching energy policy and climate change targets. To address these scenarios, in this project we will investigate and test improved specifications and requirements for the operation and integration of HVDC schemes at different zones of GB system with respect to their power transfer capability under varying grid electrical strength.
The outcome of the project will be a set of guidelines for the recommended operational regions for a safe and stable HVDC interconnection without jeopardising the power system security and reliability. This will further enable a faster integration of low-carbon renewable resources from within the country and outside the GB system.
Coordination of protection settings and HVDC during energization of grid forming HVDC in low short circuit ratio grids
Summary: 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 energization 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.