Dynamic stability is a major concern in maintaining the security of power grids with high shares of power electronics-based resources. Stability analysis tools are needed to evaluate the impacts of power electronics converters on system stability at slow and fast time scales. This project will investigate a new impedance scan method, which provides a more accurate impedance representation to investigate stability challenges associated with power electronics connections. If the project is successful, it will enable power engineers to do similar studies in the future and obtain a more accurate impedance representation.
Benefits
The net benefits for consumers from this project is expected to be delivered collectively from lower cost and risk for power electronic based devices connections, i.e. wind farm, solar farm and HVDC connections.
The outputs of this research will support energy networks building more understanding on interaction and instability issues associated with power electronics converters. It will allow the better planning for integration of power electronic devices. It will also help us in taking mitigating actions to avoid unplanned outages resulting from interaction issues.
Learnings
Outcomes
2023/2024
The project has initially developed a new impedance scan tool for a power-electronic power system, overcoming the limitations of traditional tools in accurately capturing converter control dynamics. This tool is poised to represent the dynamics of a power-electronic power system in terms of impedance and facilitate the understanding of wide-frequency oscillations.
A library for impedance scanning has been developed in PSCAD/EMTDC, including voltage/current injection, frequency analysis, impedance calculation, synchronization control, and transformation blocks for different frames. The developed tool has been initially validated by a PLL (phase-locked loop) based converter in PSCAD/EMTDC, and the results show good agreement.
The automation of impedance scanning in PSCAD/EMTDC has been achieved through Python Automation, allowing for the repetitive scanning process to execute automatically.
The tool was delivered to NGET on February 28th as Deliverable 1, utilising the dq impedance representation, while the development of the other mirror-frequency impedance representation for the tool is ongoing.
2024/2025
An impedance scan tool has been developed in PSCAD/EMTDC to measure the dq impedance and dq sequence impedance of a converter system. It has been identified that the impedance stability analysis method is a type of state-space matrix representation used for small-signal stability analysis.
Oscillations driven by converters are two-dimensional components that manifest in a mirror-frequency pattern in the AC system or as positive and negative sequence components in the dq frame. This oscillation pattern arises because converter control systems are inherently unbalanced. It can be interpreted as the interaction of positive and negative sequence oscillations through an unbalanced impedance. The impedance of a converter must be represented as a 2×2 matrix rather than a single-dimensional component.
Both the dq impedance scanning method and the dq sequence impedance method are considered for impedance analysis. It has been proven that both methods are equivalent in terms of stability analysis. However, the dq impedance method captures oscillations in a combined manner i.e. reflecting both positive and negative sequence components. Meanwhile, the dq sequence impedance method measures the positive and negative impedances separately.
The tool includes an interface to facilitate parameter setting for the desired impedance scan, and the scanning process is automated via Python. This stability analysis tool is tested on a HVDC system, where it can automatically measure the system impedance and perform stability analysis using a MATLAB script. The stability results show good agreement with the simulation results, effectively identifying stability and oscillation frequency.
Recommendations for further work
None.
Lessons Learnt
2023/2024
The integration of power-electronic converters has modified the impedance performance of power systems. Traditional impedance performance relies on the single-dimensional RLC (resistor-inductor-capacitor) representation, whereas converters introduce a two-dimensional impedance matrix performance due to the mirror frequency coupling effect of converter control. This results in the oscillation induced by converter control exhibiting two frequencies at mirror frequency positions.
In future projects related to oscillation analysis of power-electronic power systems, multiple-frequency oscillations need to be taken into consideration.
2024/2025
The project compared both the dq impedance scanning method and the dq sequence impedance method for the impedance analysis. It was shown that both methods are equivalent in terms of stability analysis. It should be noted that the term "sequence" here refers to components in the dq frame, not the traditional sequence components of the abc system.
The project aims to disseminate learnings to a wider group of stakeholders and provide training to relevant NGET personnel and external stakeholders. It’d be useful to plan this well ahead with the wider industry stakeholder group before completing all the study works required for the project. This will ensure the efficiency and smooth delivery of the knowledge transfer workshop.
Dissemination
2023/2024
A workshop on the 'Impedance Scan Project' was held at National Grid on 19th March 2024, to disseminate the effectiveness of the developed impedance scan tool. The automation feature for the scanning process with this tool was demonstrated.
2024/2025
An online workshop has been held on 24 March 2025 for demonstrating the developed impedance scan tool.
Dissemination workshops are planned to be held in early July.
