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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.

The transient stability of an offshore-wind farm connected to the main ac grid by a high-voltage direct current (HVDC) system depends on the control settings of the converters, the grid resonances and the system’s operating conditions. This paper focuses n identifying possible sources of resonance interactions that could endanger the overall stability of this kind of system. For this purpose, the ac-side input admittance/impedance of a full-power converter (FPC) wind turbine and an MMC-based HVDC system are derived and frequency-domain stability analysis tools are used to assess the impact of system and control parameters on stability. The analytical findings are verified through time-domain simulations and recommendations to increase system stability are given accordingly.

The aim of this paper is to investigate the risk for subsynchronous control interaction (SSCI) in Type 4 based wind farms connected to series-compensated transmission lines. For this purpose, the frequency-dependent input admittance of the turbine is derived and the power-dissipation properties of the turbine are used to identify those control parameters and operating conditions that mainly impact the behavior of the wind turbine in the subsynchronous frequency range. It is shown that the Type 4 wind turbine presents a non-dissipative behavior in the subsynchronous frequency range, which is mainly dependent on the speed of response of the synchronization loop, the reactive power controller, and the active-power output. For a radial connection, a high series-compensation level also negatively impacts the risk for SSCI. The generalized Nyquist criterion is used to assess the stability of the system and the theoretical findings are verified using detailed time-domain simulations.

The aim of this paper is to investigate the risk for subsynchronous control interaction (SSCI) in doubly-fed induction generator (DFIG) based wind farms connected to series-compensated transmission lines. For this purpose, a detailed analytical model of the frequency-dependent input admittance of the DFIG is derived. The developed admittance model is then used to get insights on the frequency characteristic of the DFIG wind turbine generator unit. In particular, the power-dissipation properties of the DFIG are used to identify those control parameters and operating condition that mainly impact the behaviour of the wind turbine in the subsynchronous frequency range. The admittance model of the wind farm together with the impedance model of the series-compensated transmission line are used to identify the risk for SSCI. Through the use of the Generalized Nyquist Criterion, it is shown that the closed-loop bandwidth of the current controller that regulates the rotor current has a major detrimental impact on the stability of the system. Furthermore, results show that the active power set-point and the level of series compensation also play an important role on the overall system's stability. Time-domain simulations are conducted to validate the theoretical findings.

The aim of this paper is to investigate the impact of steady-state deviations in the grid-frequency on the performance of grid-forming converter control strategies. In particular, the virtual synchronous machine (VSM) control structure has been the focus of study in this paper. Two alternative solutions to address the issue with the conventional VSM structure are suggested, and their impact on the active power response and the power dissipation properties of the converter are compared. For the latter, the ac-side input admittance of the grid-connected converter is derived for the different control structures under consideration. Finally, the dynamic performance of the different control structures is verified through the time-domain simulations.

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.

This paper investigates the stability of a modular multi-level converter (MMC) based HVDC system connected to a weak ac grid. Impedance analysis is used to study the interactions between the weak ac grid and the converter. For this, the impedance of the MMC is first derived considering various outer- and inner-loop controllers. The Nyquist-stability criterion is then used to analyze the stability of the MMC connected to the weak ac grid for various case studies. The impact of circulating current controller on the stability of the MMC connected to the weak ac grid is also validated for two different types of outer-loop controllers. Finally, time-domain simulations are carried out using MATLAB/Simscape Power System toolbox to validate the analytical results.

This paper deals with the derivation of the ac-side input admittance of a grid-connected modular multilevel converter (MMC) under both open- and closed-loop ac-side current control strategies. The contribution of the internal dynamics of the MMC on the input admittance depends on the type of ac-side controllers implemented. For this reason, the input admittance is first derived with the MMC operating in open loop on the ac side. The results are then modified to include various ac-side controllers in order to analyze the impact of the internal dynamics of the MMC on the total input admittance. Based on the findings, recommendations are given on modeling the input admittance of the MMC. Finally, the derived analytical models are verified using a detailed switching model of the MMC in the frequency domain.

The input admittance of a converter, connected to other dynamic subsystems, is a useful tool to investigate whether poorly damped oscillations or even unstable conditions might occur at certain frequencies. This is of interest in applications employing the modular multilevel converter (MMC), where the internal dynamics of the converter and the increased number of control loops greatly affect the MMC’s dynamic behaviour, compared to other types of converters. In this paper, the dc-side input admittance of the direct-voltage controlled MMC is derived analytically and verified via small-signal perturbation in a detailed nonlinear time-domain simulation model. The MMC’s input admittance is parametrically studied and compared to the dc-side input admittance of an equivalent two-level converter.

The transient stability of a wind-farm connected to an ac grid depends on the control settings of the involved converters, the grid resonances and the system's operating conditions. This paper focuses on investigating the impact of converter control algorithms on possible high-frequency resonance interactions that could endanger the overall stability of these kinds of systems. Mainly, two control strategies namely the conventional cascade control structure, consisting of an inner current-control loop and various outer-loop controls, and the immediate voltage control structure, which does not include a current-control loop, are compared. The impact of control parameters for both methods will be investigated as well. For this purpose, the ac-side input admittance of a wind-turbine converter is derived for the two control strategies and passivity-based frequency-domain analysis is used to assess their impact on system stability. Finally, the analytical findings are verified through detailed time-domain simulations and recommendations to increase system stability are given accordingly.

A number of methods have been proposed and implemented to improve system damping against low-frequency electromechanical oscillations in the power system. Among these, flexible ac transmission systems (FACTS) can be used to provide power oscillation damping (POD) function to the power system. The design of the POD algorithms in these devices requires estimation of the power oscillation frequency components and this is mostly achieved through the use of various filter combinations. However, these filter-based solutions are characterized by low bandwidth to extract the required signal components accurately, and this limits the dynamic performance of the FACTS controllers. Moreover, the filters are designed for specific frequencies, and a change in the system would reduce the performance of the methods. Thus, there is a need for a better estimation algorithm with fast and selective estimation of the required signal that is robust against system parameter uncertainties. In this paper, this is achieved by the use of a recursive least square algorithm that uses variable forgetting factor and a frequency adaptation mechanism. The investigated method has fast estimation in transient conditions without compromising its selectivity in steady state. The effectiveness of the proposed method is proven through simulation as well as experimental verification.

The transient stability of power-electronic dominated systems such as wind farms and photo-voltaic power plants depends on the control settings of the involved converters, the grid resonances and the system’s operating conditions. This paper focuses on investigating the impact of converter control algorithms on possible interactions that could endanger the overall stability of these kinds of systems. Mainly, two control approaches namely the conventional cascaded control structure, consisting of an inner current-control loop and various outer-loop controls, and the immediate voltage control structure which generates the converter voltage directly from the power and voltage controllers without necessarily including a current-control loop, are compared. The latter control structure falls in the class of grid-forming control strategies and the impact of different variants of this control structure will also be investigated. For this purpose, the ac-side input admittance of a grid-connected converter is derived for the two approaches and passivity-based frequency-domain stability analysis is used to assess their impact on system stability. Finally, the analytical findings are verified through time-domain simulations and recommendations to increase system stability are given accordingly.

This paper deals with the design of an adaptive power oscillation damping (POD) controller for a static synchronous compensator (STATCOM) equipped with energy storage. This is achieved using a signal estimation technique based on a modified recursive least square (RLS) algorithm, which allows a fast, selective, and adaptive estimation of the low-frequency electromechanical oscillations from locally measured signals during power system disturbances. The proposed method is effective in increasing the damping of the system at the frequencies of interest, also in the case of system parameter uncertainties and at various connection points of the compensator. First, the analysis of the impact of active and reactive power injection into the power system will be carried out using a simple two-machine system model. A control strategy that optimizes active and reactive power injection at various connection points of the STATCOM will be derived using the simplified model. Small-signal analysis of the dynamic performance of the proposed control strategy will be carried out. The effectiveness of the proposed control method to provide power oscillation damping irrespective of the connection point of the device and in the presence of system parameter uncertainties will be verified through simulation and experimental results.

Power electronic converters are today an essential part of the electrical power systems, employed to improve the system performance and enhance its stability. At the same time, the introduction of power converters might negatively affect the overall dynamic behaviour of the power system. System stability can be investigated from different perspectives, e.g. time-domain observation, eigenvalue analysis, or frequency-domain analysis. The aim of this paper is to provide an in-depth overview on two of the most popular frequency-domain methods for stability assessment of systems comprising grid-connected converters: the passivity approach and the net-damping criterion. The importance of proper system modelling is highlighted while advantages and pitfalls of the two methods are investigated. The considered frequency-domain methods are applied to a two-terminal high voltage direct current (HVDC) system, to investigate its stability both from the ac and dc side.

The stability of grid-connected modular multilevel converter (MMC) under two control strategies is studied in this paper. For this purpose, the ac-side input admittance of the MMC is derived under open- and closed-loop ac-side control strategies. The derived analytical models are used to investigate the impact of the two approaches on the stability of the MMC and detailed time-domain simulations are used to the verify the theoretical findings.

The aim of this paper is to investigate the implementation of an adaptive generalized delayed signal cancellation (DSC) method for estimation of phase-sequence components. The proposed approach overcomes the intrinsic problems of the digital implementation of the classical DSC algorithm due to nonideal selection of the sampling frequency and variations in the nominal system frequency. The derived algorithm is extended for estimation of harmonic components in the measured signal. The effectiveness of the proposed estimation method is validated through experimental results.

Signal estimation is important for protection, system study, and control purposes. This paper deals with the application of a modified recursive least squares algorithm, based on a variable forgetting factor, for estimation of frequency components in a generic input signal. Application areas such as synchronization and control in three- and single-phase systems for various grid conditions as well as estimation of harmonic and subsynchronous components will be discussed. Advantages of the proposed method over the available techniques in terms of estimation speed, frequency selectivity, and noise rejection capability will be described. Finally, simulation and experimental results will be used to evaluate the dynamic performance of the proposed method.

The application of two control strategies for shunt-connected FACTS devices to enhance the transient stability of a power system will be discussed in this paper. The main focus will be on the use of active power in the FACTS device and therefore a STATCOM with active power injection capability will be considered. The first approach represents the classical cascade controller that consists of an inner vector-current controller and various outer-control loops. The alternative approach controls the converter in a similar fashion as a synchronous generator, where the mechanical behavior of the machine is emulated in the control structure using the active power injection capability of the device. Using various simulation and experimental tests, the two methods will be compared in terms of their advantages and disadvantages, and a recommendation will be made on the use of the methods for transient stability enhancement.

One of the problems encountered in a power system and that impacts its stable operation is low-frequency electromechanical oscillations. Depending on the system configuration, its operating conditions and the nature of the disturbance, one or more oscillation modes can be excited in the system. This paper deals with the design and analysis of an independent damping controller for multiple oscillation modes using shunt-connected FACTS devices. This is achieved using a selective and adaptive estimation of the various oscillation modes from locally measured signals. The effectiveness of the proposed control method to provide independent damping for the various oscillation modes in the presence of system parameter uncertainties will be verified through small-signal analysis and simulation using both single and multiple compensators.

Loads constitute a major part of the power system and their characteristics greatly influence the stable operation of the system. One of the problems that are encountered in the power system is low-frequency electromechanical oscillations following disturbances. This paper deals with analysis of the impact of different static load characteristics on the performance of Power Oscillation Damping (POD) controllers by shunt-connected FACTS devices. From the results of the analysis, a modification to the POD controller will be proposed to deal with the impact of the load characteristics and its performance will be validated through small signal analysis and time domain simulation.

Signal estimation is important for protection, system study and control purposes. This paper deals with the application of a Recursive Least Square (RLS) algorithm with variable forgetting factor for estimation of frequency components in a generic input signal. Application areas such as synchronization and control in three-phase and single-phase systems for various grid conditions as well as estimation of harmonic and subsynchronous components will be discussed. Simulation and experimental results will be used to evaluate the dynamic performance of the algorithm.

This paper deals with improvements to the classical Proportional-Integral (PI) current controller for Voltage Source Converter (VSC) connected to a grid in the presence of harmonics. A discrete PI current controller implemented in the synchronous reference-frame has simple design procedure and robust performance in ideal grid conditions. However, low-order harmonics in the connecting grid will result in a steady state harmonic current flow between the converter and the grid. A method based on harmonic estimation to avoid the steady state harmonic injection to the grid will be discussed. The effectiveness of the described control method will be validated through simulation and experimental verification.

This paper deals with a control strategy for power oscillation damping using Static Synchronous Compensator (STATCOM) with energy storage. First, analysis of power oscillation damping by only active and reactive power injection into the power system will be carried out using a single machine infinite bus and two machines system model. A control strategy that optimizes the use of active and reactive power injection for power oscillation damping based on locally measured signals will be derived. Small signal analysis of the dynamic performance of the proposed control strategy will be made. The effectiveness of the proposed control method to provide power oscillation damping irrespective of the connection point of the device will be verified by simulation.

In a voltage source converter (VSC) based HVDC system, the modulation scheme used is an important factor in achieving a desired harmonic performance with allowable semiconductor losses. In this paper, the use of selective Harmonic Elimination Pulse Width Modulation (HEPWM) for a three-level Neutral Point Clamped (NPC) converter in VSC based HVDC application will be discussed. Steady state performance of the converter system in terms of harmonics and losses will be evaluated using MATLAB and PSCAD/EMTDC simulation. Simulation results show a 37% improvement in total semiconductor loss with better harmonic performance by using the three-level solution compared to the two-level solution with the stated modulation scheme.