Tomislav Baškarad

The increasing share of renewable energy sources in electricity generation has reduced the presence of conventional rotating machines while increasing the share of converter-connected units. As a result, the overall system strength and inertia are reduced, making power systems more vulnerable to instability. To ensure reliable and secure operation, new technologies and novel, fast-acting ancillary services are being considered to replicate the inherent behavior of synchronous generators. Among the potential solutions, supercapacitor-based energy storage systems stand out due to their unique characteristics, particularly their ability to deliver rapid power support. This paper provides a comprehensive review of supercapacitor technology with a focus on applications in power systems. It examines their advantages and limitations, evaluates the shortcomings of existing studies, and summarizes the main performance metrics and integration possibilities. In addition, this paper also highlights research contributions that explore the use of supercapacitors for frequency-related as well as non-frequency ancillary services. Key challenges are identified, and directions for future research are proposed, with the aim of advancing the role of supercapacitors in enhancing power system stability.

This article presents a measurement-based approach to estimate the frequency response of power system areas during island operation by utilizing measurements from the interconnected power system. The approach relies exclusively on actual measurements of previous frequency responses, eliminating the need for prior knowledge of system parameters. Initially, a mathematical relationship is established between the frequency response of the interconnected power system and that of individual areas during island operation. Subsequently, the method is applied to the Nordic power system. Prior to its application to the entire Nordic power system, particularly the Finnish power system, validation and feasibility tests were conducted using simulation models. The focus of the analysis in this study is on estimating the frequency response of the Finnish power system in island operation mode, assuming its separation from the Nordic power system. Four real disturbance events from the year 2020 in the Nordic power system were analyzed and utilized as input data. The obtained results align well with expectations for both under-frequency and over-frequency events, thereby confirming the effectiveness of the proposed method.

In a power system with a high-share of photovoltaic systems, the frequency response can be improved by the participation of photovoltaics in frequency control. If the photovoltaic system operates at reduced power, in a so-called de-loaded mode, and maintains a specific amount of power reserve, then participation in system frequency control is usually realized by a droop control method. The implementation of this method is very simple, but it does not provide the recovery of PV power reserve before the secondary frequency control reacts. This means the PV system cannot provide additional support to the grid if a new disturbance occurs in a short time following the first one. This paper presents a novel approach for PV system control in providing support to system frequency. The novel control algorithm aims to enable rapid recovery of PV power reserve which makes the PV system capable of providing support to the cascading disturbance events. The development of such PV system control model is based on a detailed mathematical analysis of the dynamic frequency response. The effectiveness of the proposed method was tested on a two-area multi-machine power system model and through 4 different study cases.

By integrating the new production technologies into the electric power system (EPS) which are mainly inverter-based, the characteristics of the conventional bulk EPS are significantly changed leading to a reduction of synchronous inertia which causes faster frequency dynamics. Synchronous inertia directly affects the maximum value of the rate of change of frequency (ROCOF). The importance of ROCOF is vital: ROCOF is the main parameter for Loss of Mains relay, it is used as an input parameter for Fast Frequency Response and Synthetic Inertial Response control algorithms and can be used as a metric parameter for underfrequency load shedding. In this paper, the importance of ROCOF in the inverter rich EPS will be described in detail, different grid code versions concerning ROCOF will be presented and the results of ROCOF analysis in Croatian EPS with respect to the future renewable energy source (RES) share scenarios will be shown.

The most commonly used method for including photovoltaic system in the control of system frequency is to operate a PV power plant at reduced power while maintaining a specific amount of available power reserve. Such operating regime, often referred to as de-loaded, can cause additional costs and, consequently, determining a required power reduction becomes vitally important. This paper introduces a method for determining the required minimum de-loaded reserve of a photovoltaic system. To achieve the desired result a parabolic approximation of the system frequency response is developed to derive an explicit function of the frequency nadir. Taking this approximation into consideration, a method for determining the required minimum de-loaded margin was developed and tested in a case-study which observed a small power system consisting of several power plants. To demonstrate the accuracy of the proposed parabolic approximation, the calculated approximate values of the frequency nadir were compared to the values of the frequency nadir obtained from the frequency response simulations performed on the power system model developed in MATLAB/Simulink.

This paper presents a high-level overview of the integration of renewable energy sources (RES), primarily wind and solar, into the electric power system (EPS) in Croatia. It presents transmission system integration aspects for the particular case of this country. It explains the current situation and technical characteristics of the current conventional generation units and currently installed wind energy capacities. Based on the current situation future development scenario is determined and used to evaluate the impacts of the wide-scale integration of renewables. Grid connections aspects, power balancing, market participation, and inertia reduction aspects are considered. Furthermore, some specifics of both solar and wind integration are discussed identifying problems and potential solutions. Primarily through the provision of the inertial response of both solar and wind and through better forecasting of wind production. Finally, the outlook for the Croatian power system is given, that will most probably double its RES capacity in the coming 3-year period and a certain level of investments and changes of current operational practices will need to be provided.

In a power system, increased wind energy penetration changes the dynamic system response to active power disturbances. Due to the lack of natural inertial response from converter connected wind sources, an advanced converter control is required for maintaining a transient frequency stability of modern power systems with high shares of renewable energy sources. This research examines the new wind turbine fast frequency response control capability on the addressing electrically near and distanced active power imbalances considering transient dynamics system parameters. The focus of this research is to analyse the effectiveness of wind turbine fast frequency response control in reduced inertia power system modelled as a four-inertial centre system. The study is done on the case of Croatian power system with “real life” data for power generation, wind production, primary frequency regulation, and load shedding schemes. The simulation and scenario analysis are conducted under the MATLAB/Simulink environment using detailed power transmission system model.

This research examines the impact of wind penetration in the Croatian power systems on system frequency stability. The main focus of the research is the simulation of system frequency response on the mathematical model of Croatian power system. With these models the possibility of providing wind inertial response and primary frequency response will be examined. Characteristics of frequency response such as the value of frequency nadir, the steady-state frequency deviation and the rate of change of frequency will be compared for all investigated cases. This paper uses low-order system frequency response models to simulate power system frequency changes. Also, most of the block parameters are provided and are accurately calibrated to model “real life” processes. The models are developed and implemented in the MATLAB/Simulink and tested on “real life” case in Croatian power system.