Grid-forming wind power plants

With growing concerns over climate change, the power system is witnessing an unprecedented growth in electricity generation from intermittent renewable energy sources (RES) such as wind and solar, which are commonly interfaced to the grid by power-electronic converters. However, increasing the penetration level of converter-interfaced generation units reduces the number of synchronous generators (SGs) in the grid that provide system services to support voltage and frequency, either inherently or through mandatory requirements and market products. This brings several challenges for the grid operators, which include increasing risk of harmonic interactions, decreasing system inertia and reduction in the short-circuit power of the grid, which all together might jeopardize the security and availability of the power systems. As a countermeasure, it is necessary that the power-electronic-based generation units not only provide grid support services that are originally provided by the SGs, but also operate in harmony with other generation units in all kinds of grid conditions. As a result, the concept of grid-forming (GFM) control, which mimics the beneficial properties of the SGs in converter systems, has emerged as a viable solution to allow effective and secured operation of power systems with increased penetration of converter-based resources.\ua0\ua0 This thesis investigates the application of GFM control strategies in wind power plants (WPPs). In particular, the focus of the work will be on developing an effective GFM control strategy for the energy storage systems (ESS) in WPPs that not only supports the operation of the WPP in various grid conditions, but also offers a certain degree of GFM properties to the overall WPP. To start with, the selection of the most suitable GFM control strategy for wind power applications is made by evaluating and comparing various control strategies available in the literature. The comparison is based on their influence on the frequency characteristics of the converter and robustness of the controller in varying grid strength. To address the transient stability problem of GFM converters during current limitation, a novel strategy based on the limitation of converter\u27s internal voltage vector is developed, which effectively limits the converter current to a desired value and retains the GFM properties of the converter at all times. An experimental setup is used to validate the effectiveness of the proposed limitation strategy in case of various grid disturbances. By implementing the proposed GFM control strategy for the ESS in a test WPP model, it is shown using detailed time-domain simulation results that the GFM behaviour can be offered to the overall WPP. The Network Frequency Perturbation (NFP) plots are used to verify the GFM behaviour of the considered WPP. Furthermore, an overview of various energy storage technologies (ESTs) suitable for providing ancillary services from WPPs is presented. With a focus on the two most suitable ESTs, i.e., batteries and supercapacitors, recommendations are given for design and sizing of the ESS for a given application. Finally, a coordinated control strategy between the WPP and SGs is developed, which facilitates the provision of frequency support from the WPP and at the same time reduces the energy storage requirements for the converter system

Capacitive energy balancing of multilevel submodules for cascaded converter in STATCOM applications

Cascaded converters also known as modular converters are used for STATCOM applications due to their small footprint, capability to achieve high voltage levels, modularity and reduced losses. A commonly used modular converter consists of a series connection of full - bridge also known as H - bridge cells or submodules. There is a wide area of resea rch that focuses in various topologies of these submodules. In this project a new multilevel submodule has been proposed. This project investigates operational princip les and capacitive energy balancing of this multilevel submodule that can be used in any type of modular multilevel converter. The investigation consists of analysis of the submodule circuit and its switching states, design and simulation of the modulation strategy as well as the capacitive energy balancing algorithm in the case when multiple submodules are connected in series forming a cascaded converter. Various methods of capacitive energy balancing that employ different ways of sorting capacitor voltage s and deciding which capacitors/submodules should be inserted in the current path are investigated in this thesis. These energy balancing methods have been simulated and evaluated in terms of their impact on the capacitor voltage ripple and switching frequ ency of the new multilevel submodule. This evaluation results in the selection of the most promising energy balancing method, which is embedded in a simulation model of a three - phase STATCOM constructed with the proposed multilevel cells. Finally, the simu lation model is used for evaluating the performance of the proposed multilevel cell and capacitive energy balancing method under steady - state and fault conditions, which would typically occur in a utility application

Impact of Control Loops on the Passivity Properties of Grid-Forming Converters with Fault-Ride through Capability

Due to the increasing integration of renewable energy sources (RES) and a corresponding reduction of conventional generating units, there is nowadays a demand from the power-electronic converters to provide grid-forming properties through proper control of the converter systems. This paper aims to evaluate the impact of various control loops in a grid-forming control strategy equipped with a fault-ride through capability on the passivity properties of the converter system. Through the analysis of the frequency-dependent input admittance of the converter, the main factors affecting the passivity properties are identified. A simplified analytical model is derived in order to propose possible control modifications to enhance the system’s passivity at various frequencies of interest and the findings are validated through detailed time-domain simulations and experimental tests

Coordinated control of grid-forming converters and hydro generators to enhance frequency quality of future power system

The aim of this paper is to propose a coordinated control strategy between grid-forming converters equipped with energy storage, and hydro generators to facilitate frequency support from the converters in future power systems. In this way, it is possible to take advantage of the fast dynamic properties of the converter system, and at the same time minimize the energy storage requirements associated with the converter system. The proposed tuning criterion for the frequency controller of the grid-forming converter facilitates a natural coordination between the converter system and hydro generators. The effectiveness of the proposed control strategy is compared against the conventional droop-based approach available in the literature. Finally, the analytical findings are validated using detailed time-domain simulation model in PSCAD

Capacitive energy balancing of multilevel submodules for cascaded converter in STATCOM applications

Cascaded converters also known as modular converters are used for STATCOM applications due to their small footprint, capability to achieve high voltage levels, modularity and reduced losses. A commonly used modular converter consists of a series connection of full - bridge also known as H - bridge cells or submodules. There is a wide area of resea rch that focuses in various topologies of these submodules. In this project a new multilevel submodule has been proposed. This project investigates operational princip les and capacitive energy balancing of this multilevel submodule that can be used in any type of modular multilevel converter. The investigation consists of analysis of the submodule circuit and its switching states, design and simulation of the modulation strategy as well as the capacitive energy balancing algorithm in the case when multiple submodules are connected in series forming a cascaded converter. Various methods of capacitive energy balancing that employ different ways of sorting capacitor voltage s and deciding which capacitors/submodules should be inserted in the current path are investigated in this thesis. These energy balancing methods have been simulated and evaluated in terms of their impact on the capacitor voltage ripple and switching frequ ency of the new multilevel submodule. This evaluation results in the selection of the most promising energy balancing method, which is embedded in a simulation model of a three - phase STATCOM constructed with the proposed multilevel cells. Finally, the simu lation model is used for evaluating the performance of the proposed multilevel cell and capacitive energy balancing method under steady - state and fault conditions, which would typically occur in a utility application

Capacitive energy balancing of multilevel submodules for cascaded converter in STATCOM applications

Cascaded converters also known as modular converters are used for STATCOM applications due to their small footprint, capability to achieve high voltage levels, modularity and reduced losses. A commonly used modular converter consists of a series connection of full - bridge also known as H - bridge cells or submodules. There is a wide area of resea rch that focuses in various topologies of these submodules. In this project a new multilevel submodule has been proposed. This project investigates operational princip les and capacitive energy balancing of this multilevel submodule that can be used in any type of modular multilevel converter. The investigation consists of analysis of the submodule circuit and its switching states, design and simulation of the modulation strategy as well as the capacitive energy balancing algorithm in the case when multiple submodules are connected in series forming a cascaded converter. Various methods of capacitive energy balancing that employ different ways of sorting capacitor voltage s and deciding which capacitors/submodules should be inserted in the current path are investigated in this thesis. These energy balancing methods have been simulated and evaluated in terms of their impact on the capacitor voltage ripple and switching frequ ency of the new multilevel submodule. This evaluation results in the selection of the most promising energy balancing method, which is embedded in a simulation model of a three - phase STATCOM constructed with the proposed multilevel cells. Finally, the simu lation model is used for evaluating the performance of the proposed multilevel cell and capacitive energy balancing method under steady - state and fault conditions, which would typically occur in a utility application

Impact of control loops on the low-frequency passivity properties of grid-forming converters

The aim of this paper is to evaluate the low-frequency passivity properties of grid-forming converters. Through the analysis of the frequency-dependent input admittance of the converter, the impact of various control loops is investigated. A simplified analytical model that is valid in the low-frequency range is also derived in order to identify possible control modifications to enhance the passivity of the system

Comparison of Grid-Forming Converter Control Strategies

The aim of this paper is to compare three different categories of grid-forming converter control strategies based on their structural differences, and ability to fulfill the requirements that have been recently identified by the European Network of Transmission and System Operators for Electricity (ENTSOE). Among these requirements, the ones associated with the frequency characteristics of the converter system, such as its ability to prevent the risk for adverse control interactions, and act as a sink for harmonics are in focus. These properties are assessed through the frequency-dependent input-admittance models of the converter system. In addition, robust performance of the converter system at various grid strength is paramount, which is investigated both analytically and through detailed time-domain simulations in PSCAD

Passivity characterization of grid-forming converters with fault-ride through capability

Due to the increase in the use of renewable energy sources (RES) and a corresponding reduction in the conventional energy generation systems, there is nowadays a demand from the power-electronic converters to provide grid-forming properties through proper control of the converter systems. Thispaper aims at evaluating the impact of various control loops of a grid-forming control strategy equipped with a fault-ride through capability on the passivity properties of the converter system. Through the analysis of the frequency-dependent input admittance of the converter, the main factors affecting the passivity property are identified. A simplified analytical model has been derived in order to propose possible control modifications to enhance system’s passivity at various frequencies of interest and the findings are verified through detailed time-domain simulations and experiments