The GB electricity network is setting a fast pace of transition towards a net-zero carbon energy network. The system inertia and fault level are expected to decline continuously over time. Furthermore, the level of power electronic converters integrated into the electricity transmission network from renewable energy (e.g., wind turbines and photovoltaics), HVDC (High-Voltage Direct Current) links, traction loads, battery storage and FACTS (Flexible Alternating Current Transmission System) devices is going to increase dramatically and this will pose new risks to the electricity transmission network in terms of instability in the sub-synchronous and near synchronous frequency range. This is associated with transmission network resonant modes with power electronic converters time delay and control dynamics. The aim of this project is to understand the phenomena and assess the risk of sub-synchronous and near synchronous instability resulting from controller interaction with the sytem in future low carbon energy scenarios and develop an innovative mitigation measure to address it.
Objectives
The objective of the proposed work is to develop an indepth understanding of the different forms of sub and near-synchronous oscillations and the associated instability risk in the future GB sytem. The work will provide recommendations on a suitable modelling approach and analysis methodology. Nevertheless, the work is also aimed at exploring an active mitigation measure which is capable of reducing multiple frequency oscillations to mitigate the instability risk in the transmission network.
Learnings
Outcomes
The key outcomes of the project are listed below:
- The literature review summarised the interdependencies between power system components in different sub/super-synchronous forms and their nature of participation in oscillations. Available tools and analysis methods and their pros and cons are also summarised in the project technical report. The impedance-based screening method is recommended, particularly for initial design stage screening. If a potential risk of SSO is identified, more prudent EMT based technique is preferred.
- An impedance measurement toolbox was developed in the PSCAD to measure the impedance of the VSCs and GB network for screening analysis. This toolbox avoids complex model building and delivers efficiency in screening analysis of SSO instability in the GB network.
- Factors that may induce SSO instability are identified from the GB network and controls of VSCs. It is identified that: for PLL-based vector control, the fast-response current control and phase-locked loop cause the resonance and instability; for power synchronization control, the slow-response current control causes the resonance and instability. Furthermore, PLL-based vector control suffers instability when the grid is weak; power synchronization control suffers instability when the grid is strong.
- A method to identify effective network reduction boundary for oscillation instability analysis is provided to simplify the GB network modelling approach as well as to maintain the same network performance characteristic.
- An active resonance mitigation method for the VSC has been proposed to avoid instability and enhance the damping capability of the GB network. The measure is based on inserting a virtual impedance in the converter control. If the grid impedance is estimated accurately, the impedance enhancement PLL using a combination of virtual resistance and virtual negative inductance achieve the best stabilization effectiveness and damping capability. If the grid impedance is uncertain or varies excessively, the impedance enhancement PLL using virtual resistance only is recommended.
Future works of the project
Power electronic device with fast-response control may introduce high-frequency oscillation in the electricity transmission network. This needs to be thoroughly investigated to eliminate risks in future. We are continuing our research in this area. We are in process of creating a few projects on this subject in T2.
Lessons Learnt
The dissemination event received good interest and positive responses from stakeholders from both NGET and NGESO. It might be more appropriate to involve ESO in the beginning for stability related projects. We have used this learning for two new projects we are creating in this area, where ESO is involved from the initiation stage.
Dissemination
- A dissemination event with key industry stakeholders (i.e. NGET, NGESO and a SME) was held in June 2021.
- A journal paper with findings from the project work is under second round review in the IEEE transaction on power delivery.
