Short circuit ratio is an outdated measure of system strength in converter dominated systems as the useful behaviour of inverter-based resources is not represented. While still useful for determining fault current provision, converter connected generation contributes to short circuit level and system strength in a different way to traditional synchronous generation. Additionally, traditional measures only account for system strength at the fundamental frequency while converters possess complex interactions across a frequency range. A novel method of determining system strength across this frequency range from the network impedances is proposed in this paper. The approach known as the Grid Strength Impedance Metric, calculates the system strength independently from short circuit level. The unique contribution of grid-following and grid-forming converters is represented by including the multiple-input multiple-output converter output impedances in the calculation. Small-signal impedance models are employed, and the metric is largely concerned with short-term voltage stability, but some inferences can be made for longer timescales. The approach is validated using time domain simulations to check for any identified areas of weakness in the frequency range. The Grid Strength Impedance Metric successfully represents the useful behaviour of grid-forming converters, indicating an enhancement in voltage stiffness in locations of deployment.

This paper presents a small-signal analysis of different grid side controllers for full power converter wind turbines with inertia response capability. In real wind turbines, the DC link controller, the drivetrain damping controller and the inertial response might present contradictory control actions in a close bandwidth range. This situation might lead to reduced control performance, increased component stress and non-compliance of connection agreements. The paper presents an analysis of the internal wind turbine dynamics by considering different grid-side converter control topologies: standard current control used in the wind industry, standard current control with inertia emulation capabilities and virtual synchronous machines. Comments are made on the similarities between each topology and the negative effects and limits, and possible remedies are discussed. Finally, the conclusion poses that the inclusion of a DC link voltage controller reduces the ability of a converter to respond to external frequency events without energy storage. The degradation increases with the DC link voltage control speed.

The penetration of converter connected generation is increasing globally, bringing with it valid concerns over the stability of the modern electricity network. In terms of frequency stability, the provision of inertia and frequency support from converter interfaced generation has been the topic of significant research with a wide range of systems considered. One resource that has avoided significant attention is the GB rail electrical rolling stock. Everyday thousands of trains run on a strict schedule, travelling at high speeds with considerable mass all acting as one large energy store. The AC connected trains possess regenerative braking systems allowing for this energy to be harvested. With simple software modifications this energy can be extracted during large frequency events. This article investigates the power available for inertia and frequency response throughout a working day. A sensitivity analysis of parameters is conducted and the work looks to the future by considering increasing penetration of AC trains. A response between 300–850 MW is estimated for a one-minute frequency response. The calculated energy and response profile was then used to investigate the effect that the trains would have had on the 9th of August power cut that occurred in the U.K. in 2019.

This work investigates the effect of grid-connected converter topology on equivalent converter output impedance with a specific focus on the diagonal dominance of the impedance matrix across a frequency range. When considering multiple-input multiple-output systems most traditional stability techniques are reliant on the diagonal dominance of the studied system. Therefore, a rating of diagonal dominance is proposed based upon the correlation coefficient between row and column in the impedance matrix. This provides a scale that ranges from off-diagonally dominant (-1) through uniformly distributed (0) and up to diagonally dominant (1) across a range of frequencies. The scale is used to specify which control structures can be considered as diagonally dominant at certain frequencies and which control components have the greatest effect on the rating. A direct relation is found between system exhibiting a diagonal dominance rating of 0.7 and above and the efficacy of traditional stability margins. Traditionally strong systems with low network impedance where controllers can be tuned conservatively exhibit high degrees of diagonal dominance and can be analysed quickly with traditional margins with minimal error. For systems exhibiting a lower rating, disk margins are explored as an alternative which offer greater accuracy. Additionally, more realistic perturbations of gain and phase occurring simultaneously in multiple channels can be considered which is more applicable for the modern electricity network with a high penetration of grid-connected converters.