The contribution of virtual inertia through grid-connected converters is essential to tackle the stability issues that arise from the substitution of traditional synchronous generators by converter-interfaced systems. In the last decade, different adaptive virtual inertia controllers have been proposed, but the contribution of negative inertia to the transient response of grids has not been addressed yet. In this paper, an adaptive positive-negative inertia (A-PNIE) control is proposed, where the inertial contribution of the converter is optimised according to grid conditions. In addition, a Dual A-PNIE control is proposed for interlinking converters (IC), which adapts the bidirectional inertia support by switching from positive to negative inertia. A key advantage of the proposed controller is that it can be used for ac and dc grids interchangeably, using a per-unit representations of the frequency and voltage. The simulation results at a single-grid and two-grid scenario, and the experimental results on the single-grid scenario emulated at a Power-HIL testbench demonstrate the effectiveness and feasibility of the proposed controller, as well as the role of the negative inertia concept in accelerating the recovery of the frequency towards its nominal value after a power perturbation.

A more renewable energy generation model entails the replacement of synchronous generator-based power plants by converter-interfaced renewable sources, resulting in more susceptible power systems in terms of frequency and voltage oscillations under sudden power perturbations. In addition, dc-based power systems are becoming very popular thanks to the advantages they offer compared to classical ac systems. This has created a need for developing new time-domain simulation tools to represent the dynamic behaviour of converter-dominated, hybrid ac/dc power systems. This paper presents a Simulink® based simulation tool, named DFPF, that can be used to study the dynamic performance of ac, dc and hybrid power system scenarios. The DFPF tool consists of combining the dynamical models of grid-connected elements with the iterative solving of a static power flow algorithm of each part of the power system. The Simulink interface simplifies the implementation of element models and power system test scenarios, and since fast electromagnetic transients are not considered, medium-term simulations can be carried out rapidly for relatively complex scenarios. To test the tool, a hybrid scenario comprised by four ac grids interconnected by a multi-terminal dc grid is implemented and simulated. The results demonstrate that the DFPF tool can be employed to evaluate the transient response of interconnected ac and dc systems under different grid conditions.

This paper proposes an interoperable energy management system (EMS) for grid-connected HESSs, enabling the provision of ancillary services to the grid. Power systems are evolving towards a more renewable and decentralised structure, where energy storage systems (ESS) have emerged as a key energy asset to ensure the power balance and the system stability. In this context, hybrid ESSs (HESS) are an interesting solution because they take advantage of the dynamic properties of different ESS technologies. The proposed EMS structure ensures an adequate internal power allocation between the different ESS packs even when operating at power or state of charge limits. The proposed structure can be easily adapted and combined with specific control functions to provide a wide variety of grid services. Moreover, a power dispatch algorithm is included to allocate the power between the parallel power converters to maximise the system efficiency. The results from two representative use cases demonstrate the effectiveness of the proposed EMS to determine the operating setpoints of a modular HESS and to provide different grid-oriented services.

This paper presents a comparative eigenvalue analysis of the stability characteristics and small signal dynamics of four different control strategies for Synchronous Machine Emulation (SME) by power electronic converters, considering a Synchronous Machine (SM) as the benchmark system. The four SME techniques are selected to represent the most established general approaches for emulating the inertial characteristics of SMs in the control of power electronic converters. The small-signal stability assessment is based on the analysis of system eigenvalues, including evaluation of participation factors and parametric sensitivities. All the investigated techniques can be tuned to obtain similar inertial dynamics under grid frequency variations, but exhibit differences in other small-signal characteristics due to the distinct control system implementations. Among the analyzed cases, the current-controlled virtual synchronous machine has the highest damping of the most oscillatory mode. However, the study shows that the most oscillatory modes of the other techniques are associated with the LCL impedance, and could be further attenuated by active damping techniques.