Power System Oscillations with Different Prevalence of Grid-Following and Grid-Forming Converters

The oscillatory behaviour of the power system is an aspect that is significantly affected by the increasing integration of converter-based generation sources. Several works address the impact of non-synchronous generation on the operation of the system from different points of view, but only a few studies focus on power-frequency oscillations with a prevalence of generation sources interfaced through power electronics. A lack of research can be found in particular in the comparative analysis of the two main control strategies for power converters, namely grid-following and grid-forming. The article aims to contribute to this direction, starting from a theoretical analysis of the two control structures and then examining the case study of an existing transmission system. The research provides a specific insight into the fundamental aspects related to synchronisation mechanism and inertial capabilities of both grid-following with synthetic inertia and grid-forming controls. The difference in the relationship between synchronisation unit and inertial capability is recognised as the fundamental aspect determining the different impacts on the oscillatory characteristics of the system. The observation derived in the theoretical analysis is then applied to an actual power system with a high predominance of converter-based generation, considering the Colombian interconnected national system as a case study

Evaluation of the optimal renewable electricity mix for Lampedusa island: The adoption of a technical and economical methodology

Worldwide, the majority of small islands not connected to the main grid is still dependent on fossil fuels. From an economic and environmental point of view, this condition is no more sustainable given the high costs for electricity generation and the high level of pollutant emissions. Furthermore, the dependence on fossil fuel represents a risk for the security of the supply of several small developing Countries since they are obliged to import those resources from foreign Countries. The introduction of renewable energy sources in small islands represents a valid solution to solve these problems. In this context, the paper investigates the case of Lampedusa, a small Italian island whose electrical power system is currently totally supplied by diesel power plants. In the paper, the authors investigate the transition toward an economically and technically feasible generating system based on solar, wind and sea wave plants, to achieve specific targets of decarbonization. Commercial technologies are adopted for the exploitation of solar and wind sources, while sea wave plants are based on an innovative device, currently under development at the University of Palermo. A mathematical model is proposed to find the optimal energy mix that can satisfy a fixed share of annual electricity production from renewables, considering the Levelized Cost of Electricity. Finally, the proposed solution is analyzed in order to check the dynamic stability of the power system. The paper shows that, for replacing the 40% of the current electricity demand of Lampedusa, an optimal energy mix comprising 1509 kW from photovoltaic plants, 2100 kW from wind turbines and 640 kW from wave energy converters is needed. In this way, the actualized cost for the electricity production could be reduced to 0.260 €/kWh from the current value of 0.282 €/kWh

Flexibility Services to Minimize the Electricity Production from Fossil Fuels. A Case Study in a Mediterranean Small Island

The design of multi-carrier energy systems (MES) has become increasingly important in the last decades, due to the need to move towards more efficient, flexible and reliable power systems. In a MES, electricity, heating, cooling, water and other resources interact at various levels, in order to get optimized operation. The aim of this study is to identify the optimal combination of components, their optimal sizes and operating schedule allowing minimizing the annual cost for meeting the energy demand of Pantelleria, a Mediterranean island. Starting from the existing energy system (comprising diesel generators, desalination plant, freshwater storage, heat pumps and domestic hot water storages) the installation of solar resources (photovoltaic and solar thermal) and electrical storage were considered. In this way, the optimal scheduling of storage units injections, water desalination operation and domestic hot water production was deduced. An energy hub model was implemented using MATLAB to represent the problem. All equations in the model are linear functions, and variables are real or integer. Thus, a mixed integer linear programming algorithm was used for the solution of the optimization problem. Results prove that the method allows a strong reduction of operating costs of diesel generators also in the existing configuration

Power grid integration and use-case study of acid-base flow battery technology

There are many different types of energy storage systems (ESS) available and the functionality that they can provide is extensive. However, each of these solutions come with their own set of drawbacks. The acid-base flow battery (ABFB) technology aims to provide a route to a cheap, clean and safe ESS by means of providing a new kind of energy storage technology based on reversible dissociation of water via bipolar electrodialysis. First, the main characteristics of the ABFB technology are described briefly to highlight its main advantages and drawbacks and define the most-competitive use-case scenarios in which the technology could be applied, as well as analyze the particular characteristics which must be considered in the process of designing the power converter to be used for the interface with the electrical network. As a result, based on the use-cases defined, the ESS main specifications are going to be identified, pointing out the best power converter configuration alternatives. Finally, an application example is presented, showing an installation in the electrical network of Pantelleria (Italy) where a real pilot-scale prototype has been installed

Analysis and Simulations of the Primary Frequency Control during a System Split in Continental Europe Power System

The occurrence of system separations in the power system of Continental Europe has been observed in recent decades as a critical event which might cause power imbalances higher than the reference incident specified per system design, representing an actual challenge for the stability and safe operation of the system. This work presents an analysis and simulations of the primary frequency control in the Continental Europe synchronous area in conditions of system separation. The adopted approach is based on fundamental aspects of the frequency-containment reserve process. The analysis takes an actual event into consideration, which determined the separation of the system in January 2021. The main purpose of the work is the development of specific models and simulations able to reproduce the actual split event. Due to specific arrangements discussed in detail, it is possible to obtain a substantial match between the simulations and the frequencies registered after the system split. The work also provides insight into the importance of the temporal sequence of power imbalances and defensive actions in the primary frequency control process. The models developed in the work are finally used to investigate the separation event under different operating conditions, such as missing defensive actions and low inertia scenarios

Frequency dynamics of power systems with temporally distributed disturbances

The frequency dynamics and stability of power systems is essentially affected by nature and characteristics of the disturbances occurring in the system. Conventionally, frequency transients are examined assuming a single disturbance applied at a given time. However, actual incidents in the power systems can be generally composed of a temporal sequence of events, and thus characterized by multiple power imbalances of different magnitudes and time offsets. The consideration of the effects of the temporal distribution of power imbalances is important for two main reasons: the impact on the frequency dynamics of the system in terms of frequency metrics such as minimum instantaneous frequency and maximum absolute rate of change of frequency, and the correct representation of the dynamic behaviour of the system also for complex events such system separation. The work provides an analytical approach for the theoretical study of the frequency dynamics with temporally distributed power imbalances. The analytical approach is then used to examine the impact of multiple disturbances having different magnitudes and time offsets on the typical frequency metrics used to characterize the transient performances of the system. The concepts derived in the work are finally applied to the case of an actual event occurred in the Continental Europe power system, showing the fundamental role of considering temporally distributed power imbalances for a correct and accurate assessment of the dynamics of the system

Improving Angle Stability by Switching Shunt Reactors in Mixed Overhead Cable Lines. An Italian 400 kV Case Study

Stringent environmental constraints make the construction of new transmission overhead lines more and more difficult. Alternatively, today it is possible to use cable lines for high (HV) and extra-high (EHV) voltage systems. The configuration of the so-called mixed lines can create some problems in the operation of the electrical system, both during steady-state and transient conditions. In particular, the system stability is one of the main concerns when analyzing the dynamic response of power systems. In this paper, the study of angular stability of a system containing a mixed line is presented: a specific control logic applied to the shunt reactors of the mixed line is proposed as improvement of the overall system stability. The proposed switching logic is first discussed from a theoretical point of view and validated with two different testing systems. Then, the existing overhead-cable lines connecting Sicily to the rest of continental Europe 400 kV power system are taken as case study for the application of the proposed switching strategy. Several simulations are performed in the power system analysis software NEPLAN360: the results show the fundamental role of the timing of the control actions applied on the shunt reactors in helping the system to keep the stability. The proposed control proves to be an effective support to the system subjected to critical contingencies, contributing decisively to avoid the angular separation between areas and therefore to preserve the stability of the system

Synchronizing Interactions Between Different Types of Grid-Forming Converters in Smart Grids

The work investigates the transient synchronizing interactions between different types of grid-forming controls. Two well-known grid-forming schemes are implemented and considered for analysis: a droop-based control and a swing-based control. The two controls are examined for a temporary loss of synchronism. The analysis is performed considering two different conditions for the grid interconnecting the converters: a resistive-inductive grid and a mainly inductive grid. The first case corresponds to actual conditions in low and medium voltage applications. The second case is realized with the implementation of a virtual reactance in the grid-forming control scheme. The analysis of the results provides a specific insight on the characteristics of the synchronizing interactions between the different types of grid-forming converters, indicating the possibility of steep reactions, mutual oscillations and unsymmetrical interactions