Solutions to Improve Transient Stability of Universal Grid-forming Inverter-based Resources

In the future power grids, the share of grid-forming inverter-based resources must be increased in order to guarantee stability of the power system during rapid changes in generation, consumption and network topology. Therefore, it is important that the behavior and response of grid-forming inverters is also stable during different types of operation modes, events and faults. Previously, frequency stability improvement of grid-forming battery energy storages with universal frequency-locked-loop after load change in small HV network has been studied. However, in this paper the focus is on more severe disturbances i.e. unbalanced 2-phase and balanced 3-phase faults that are typically most challenging for the different synchronization methods. In this paper, based on multiple PSCAD simulations, new solutions are proposed to improve the transient frequency and voltage stability and fault-ride-through capability of grid-forming inverter-based resources with universal frequency-locked-loop during and after unbalanced and balanced faults.© 2023 The Authors. Published by Praise Worthy Prize S.r.l. This article is open access published under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Available online by June 30th, 2023.fi=vertaisarvioitu|en=peerReviewed

Improvement of Power System Frequency Stability With Universal Grid-Forming Battery Energy Storages

Large-scale integration of inverter-based renewable generation leads to the reduction of power systems’ natural inertia. Therefore, the dynamics of the future power systems will be more sensitive than in the traditional systems with high-inertia rotating synchronous generators. This development is a potential risk for frequency stability and requires utilization of rapidly controllable resources for dynamic frequency stability support. Simultaneously, development of new synchronization and control methods for inverter-based resources is needed in order to ensure the frequency and synchronization stability of future power systems. In this paper, a grid-forming and supporting universal frequency-locked-loop -based control and grid synchronization for inverter-based resources is utilized to improve the frequency stability of a small high-voltage network. The simulations are done with PSCAD software and the main focus is on the battery energy storages to evaluate the effect of their location, enhanced control schemes as well as operation mode on frequency stability. In the studies, for example, the effect of battery storages location, active power response related control parameters, communication time delay and input frequency determination on frequency support are studied during charging and discharging of the batteries. Based on the simulations, also new solutions to improve the frequency stability of future variable inertia power systems with universal grid-forming battery storages are proposed.©2023 Author. Published by IEEE. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/fi=vertaisarvioitu|en=peerReviewed

Technical solutions for low-voltage microgrid concept

fi=vertaisarvioitu|en=peerReviewed

Universal Grid-forming Method for Future Power Systems

Power system inertia typically refers to the energy stored in large rotating synchronous generators. Dynamics and stability of the traditional power system is closely linked to the natural inertia of these synchronous generators. In recent years, increasing amount of synchronous generators have been replaced by high amount of different type of inverter-based generating units connected at different voltage levels of the power system. Therefore, the dynamics and stability of future low-inertia power systems will be increasingly dominated by the control and synchronization of these inverter-based resources. One essential issue is that the typical grid-following control with phase-locked-loop (PLL) -based synchronization of inverter-based generation is not enough to guarantee frequency stability in future low-inertia power systems. Therefore, different grid-forming inverter control and synchronization methods have been proposed and developed. Currently there does not exist any universal grid-forming control and synchronization method. Therefore, this paper tries to propose a new universal frequency-locked-loop (U-FLL) -based synchronization method which is grid-forming for inverter-based generating units and grid-supporting for inverter-based loads. Advantageous operation of the new U-FLL synchronization and control strategy is confirmed by multiple simulations with different shares of inverter-based resources and synchronous generators in MV and HV hybrid power systems as well as with 100 % inverter-based LV, MV and HV networks.©2022 the Author. Published by IEEE. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/fi=vertaisarvioitu|en=peerReviewed

Islanding detection during intended island operation of nested microgrid

In the future, functionalities like islanding detection must also operate during intended island operation of (MV+MV or MV+LV) nested microgrids. In this paper, healthy and faulty islanding detection of LV network connected generation unit during nested (MV+LV) microgrid islanded operation were studied by simulations with PSCAD model from real-life smart grid pilot. Main focus in the simulations was on the study and comparison of usage possibilities of combined (high-speed communication based transfer trip & fault detection/direction + voltage vector shift) and multi-criteria (voltage total harmonic distortion & voltage unbalance) based islanding detection schemes also during intended island operation of nested microgrid consisting only from inverter based generation units.fi=vertaisarvioitu|en=peerReviewed

Voltage and current THD in microgrid with different DG unit and load configurations

After microgrid transition from the normal operation to the islanded operation the system impedance changes considerably and it affects on the harmonic voltages of the microgrid. In this paper the voltage and current THD before and after islanding are studied with different microgrid configurations. Simulations are also made by applying negative sequence filtering in control system of converters to reduce the voltage and current THD inmicrogrid during unsymmetrical load. Based on the studies recommendations for technical solutions which ensure high power quality in islanded microgrid are given.fi=vertaisarvioimaton|en=nonPeerReviewed

Sensitivity analysis of frequency and voltage stability in islanded microgrid

This paper studies the voltage and frequency stability of an islanded microgrid and the sensitivity of these quantities to certain changes in system configuration. In conventional power systems the system frequency is coupled with the rotor speed of the directly grid connected large synchronous generators and power unbalance can be seen as changed system frequency. But in an islanded microgrid it is possible that all generation units are connected to grid via converters and there is no inertia of rotating masses to affect the frequency. In that case the frequency has to be created by a power electronic device and the frequency is more of less fixed and power unbalance cannot be detected in the classical way. The studied urban low voltage (LV) network based microgrid consists of three converters and one synchronous generator based distributed generation (DG) units. The studies are made with PSCAD simulation software.fi=vertaisarvioimaton|en=nonPeerReviewed

Applications of Probabilistic Forecasting in Smart Grids : A Review

This paper reviews the recent studies and works dealing with probabilistic forecasting models and their applications in smart grids. According to these studies, this paper tries to introduce a roadmap towards decision-making under uncertainty in a smart grid environment. In this way, it firstly discusses the common methods employed to predict the distribution of variables. Then, it reviews how the recent literature used these forecasting methods and for which uncertain parameters they wanted to obtain distributions. Unlike the existing reviews, this paper assesses several uncertain parameters for which probabilistic forecasting models have been developed. In the next stage, this paper provides an overview related to scenario generation of uncertain parameters using their distributions and how these scenarios are adopted for optimal decision-making. In this regard, this paper discusses three types of optimization problems aiming to capture uncertainties and reviews the related papers. Finally, we propose some future applications of probabilistic forecasting based on the flexibility challenges of power systems in the near future.© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).fi=vertaisarvioitu|en=peerReviewed

Fault type and location detection in islanded microgrid with different control methods based converters

In this paper the fault type and location detection in an islanded low voltage (LV) distribution network based microgrid with converters applying different control methods is studied. In the future most of the distributed generation (DG) units in microgrids will be connected to network through power electronic interfaces (e.g. converters) which usually have limited capability to feed fault current due to power devices protection reasons.Also converter control methods affect to the converter fault behavior. This means that during the faults the converter cannot feed larger current than the nominal current of it or it tries to keep active power fed to grid on the reference value, so that the current of converter increases to a certain limit. Simulations show that protection of islandedmicrogrid with converters could be based on changes in phase voltages and voltage RMS values at the connection points of DG units. But, due to different control methods of converters, the fault location estimation is not that simple in every case.The simulation studies are made with PSCAD simulation software package.fi=vertaisarvioimaton|en=nonPeerReviewed

Protection system for future LV microgrids

Traditional protection of low voltage (LV) network will not be applicable for future LV microgrids and new adaptive protection system must be developed. In this paper adaptive protection system for LV microgrid during normal and island operation will be presented. Adaptivity means that protection system will e.g. change settings of LV feeder protective devices (PDs) according to the operation state of the LV microgrid, i.e. normal or island operation, and in the settings of LV feeder PDs also the number and type of DG units connected to the corresponding LV feeder and also their fault current feeding capabilities will be taken into account. Fast and selective operation between different PDs is achieved by intelligent utilization of high-speed communication.fi=vertaisarvioimaton|en=nonPeerReviewed