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The high penetration of renewables has sparked new oscillatory concerns in hybrid AC/DC grids. Knowledge of the oscillation regarding its source and propagation mechanism, is crucial to system operation. In this respect, the frequency-domain modal analysis (FMA) provides a way (e.g., participation factor) to extract these oscillation properties, and has been widely applied in converter-based AC grids. Compared to AC grids, oscillation properties of hybrid AC/DC grids are much more complex. Such complexity mainly originates from the diversity of oscillation propagation behaviors in hybrid AC/DC grids, where a distinctive local-area oscillation will emerge. Therefore, a dedicated oscillation propagation analysis for hybrid AC/DC grids is desired, which is the main focus of this paper. First, the mechanism of the distinctive local-area oscillation is analyzed considering AC/DC coupled dynamics of converters. Then, a systematic method for the oscillation propagation analysis is presented, where indices for characterizing the local-area oscillation and its impact area are established. Finally, some propagation properties are discussed, by which a strategy for the online oscillation source locating is designed. Case studies are carried out in a typical hybrid AC/DC grid to validate the propagation analysis in this paper. [ABSTRACT FROM AUTHOR]

Microgrids relying on cooperative control are supported by communications, which are highly vulnerable to cyberattacks. A significant amount of research is already carried out on the detection and mitigation of cyberattacks to secure the operation of dc microgrids. Although cyberattacks are fully capable of causing cascaded converter outages leading to full/partial system blackouts, disturbing the system stability can also be a viable target by the adversaries, which has been overlooked so far. Hence, this article focuses on addressing the instability caused by stealth cyberattacks, which can easily bypass the well-defined observability tests. In addition, this article also introduces a novel adaptive stabilization method to eliminate the unstable modes due to cyberattacks, which has been designed considering a previously defined cyberattack detection metric as an input. To investigate its feasibility, a detailed model of a stable dc microgrid is first developed. Then, considering stealth cyberattack as a nonlinear element, the describing function-based method is used to investigate system stability under attack conditions. Finally, theoretical analysis, simulation, and experimental results under various scenarios are presented to verify the effectiveness of the proposed stabilization scheme. [ABSTRACT FROM AUTHOR]

A type of peak-value dc voltage control (denoted as PK control) was proposed in the literature for supporting the use of a smaller dc-side capacitance when the single-phase voltage source converter is operated as a static synchronous compensator. Although it was demonstrated to operate stably under several conditions, it will be revealed in this article how the PK control will suffer from a more severe small-signal stability issue under nonideal grid conditions than the conventional method of controlling the average value of the dc voltage (denoted as Avr control). Especially, it will be shown how the PK control is sensitive to some of the control parameters. To obtain these results, a parameter-oriented stability analysis method is developed in the linear time-periodic framework. Then, it is utilized for parametric stability assessments of the Avr and PK control. Finally, both frequency- and time-domain experimental results verified the effectiveness and accuracy of the applied method in the presented analysis. [ABSTRACT FROM AUTHOR]

Second-order Generalized Integrator (SOGI)-based quadrature-signal-generator (QSG) together with either a phase-locked-loop (PLL) or a frequency-locked-loop (FLL) constitute two types of typical synchronization units (i.e., SOGI-PLL and -FLL) that have been widely used in grid-tied converter systems. This article will reveal and clarify the stability issue of these two synchronization units arising from different implementations of the frequency-feedback-path (FFP) connecting the SOGI-QSG and the PLL/FLL. In this regard, four types of FFP implementations will be discussed. Although different implementations of the FFP will not affect the steady-state frequency adaptation, their dynamical effects on the small-signal stability of SOGI-PLL/FLL remain concealed. To this end, this article will present a comprehensive stability assessment and comparative analysis of SOGI-PLL/FLL focusing on the FFP issue. To extend the applicability and accuracy of discussions, all the analyses will be fulfilled by using a parameter space-oriented stability assessment method formulated in the linear-time periodic (LTP) framework. The obtained results are verified by time-domain simulations, and the main findings are further interpreted by using appropriate analytical models. [ABSTRACT FROM AUTHOR]

The digitally controlled inverter is widely applied to the photovoltaic (PV) plant, however, the effects of inverter digital time delay on the harmonic characteristic of PV system which directly influences the power quality and even the stability of the system, have not been investigated yet. This study investigates the harmonic characteristic of a grid‐connected large‐scale PV system based on the equivalent Norton model of the system with considering the digital time delay of the inverter. According to the system equivalent model, the series harmonic resonance circuit and the parallel harmonic resonance circuit are divided to study the impacts of digital time delay in the cases of the inverter output harmonic exist and the grid voltage harmonic exit, respectively. Then, the harmonic resonance mechanism of a grid‐connected large‐scale PV system is studied and the effects of digital time delay on the harmonic characteristic of system output current is quantitatively analysed by defining the harmonic magnification coefficient. It turns out that frequency and magnitude of harmonic magnification points in the grid‐side current of the system are increased as the digital time delay increases. Reference to the actual 20 MWp grid‐connected PV system, a simulation model is built to verify the theoretical analysis results. [ABSTRACT FROM AUTHOR]

An integrated method for generating the system's periodical steady-state (PSS) conditions and the harmonic-state-space (HSS)-based impedance model is presented, referred to as the automatic-model-generation (AMG) method. This method is efficient for parametric impedance-based stability analysis, since it can precisely take the varying PSS into account and the presented algorithm can be readily implemented by the frequency-domain iteration. Application of this AMG method to the impedance acquisition and stability analysis of a single-phase grid-tied voltage-source-converter (VSC) along with experimental verifications is presented as an example. The presented results demonstrate how the AMG method can facilitate parametric stability assessments (e.g., under varying control parameters) in an efficient and accurate manner. [ABSTRACT FROM AUTHOR]

This article presents a harmonic-domain single-input single-output (SISO) equivalent modeling technique for the impedance modeling and stability analysis of a single-phase grid-connected voltage-source converter (VSC). The basis is a conversion technique that transforms a harmonic transfer function (HTF)-based model into a SISO equivalent model while preserving all the information of frequency couplings. The proposed SISO modeling concept is useful for understanding the meaning and consequence of SISO impedance measurement of an interconnected system with frequency couplings, which further enables a simpler impedance measurement and impedance-based analysis. Applications of this method for the VSC model reduction and stability characteristic analyses are presented. From these results, useful conclusions regarding the accuracy of three types of reduced-order VSC impedance models and the stability effects of the VSC control with and without compensation for dc voltage variation are obtained. The presented examples of applications demonstrate how the proposed SISO modeling technique facilitates a simpler and efficient impedance-based analysis. Finally, experimental results verify the validity of the proposed VSC-SISO admittance and corresponding analyses. [ABSTRACT FROM AUTHOR]

Estimation of critical control parameters is a desirable tool feature for stability analysis and impedance shaping of high voltage dc (HVdc) connected wind farms. Accurate estimation of such parameters would be enabled by access to detailed models, which is not always the case in real wind farms. Industrial secrecy is one of the main factors hindering the access to such models. This paper proposes a gray-box method that, with basic assumptions about the control structure of the wind energy conversion system (WECS), can estimate the parameters of its controllers. The method is based on the measurements of frequency domain equivalent impedance combined with nonparametric impedance identification used in the solution of an inverse problem. The method makes possible to specify which part of the equivalent WECS impedance has a major impact on the stability of the system and according to this, reshape the impedance to enforce stability. Once the critical controller bandwidth is identified with this method, an instability mitigation technique is proposed based on reshaping the impedance by retuning the critical controllers of the interconnected converters. In order to avoid interaction between the HVdc rectifier and the WECS inverter, the controllers of both converters need to be retuned in such a way that the q-axis impedance magnitude of the HVdc system is kept lower than the q-axis impedance magnitude of the wind farm at the frequency of the phase-locked loop bandwidth. The results show that the method ensures the stability of the system by retuning only the critical controller parameters. [ABSTRACT FROM AUTHOR]

A subsynchronous oscillation (SSO) phenomenon in a wind farm integrated with a modular multilevel converter (MMC)‐based high‐voltage direct current (HVDC) transmission system has been recently observed in the real world. An attempt is made in this paper to contribute to the understanding of the root cause of the SSO in the MMC‐HVDC connected wind farms. For that, the small‐signal impedance model of the MMC is first developed based on the harmonic state‐space (HSS) modelling method. An inherent low‐frequency resonance peak in the MMC excluding any control influence is identified by its terminal impedance. Arguably, this could be the reason why the SSO occurs in the MMC‐HVDC connected wind farms. In addition to that, the influence factors, such as main circuit parameters, controller parameters, and power level, on the stability of the interconnected system are examined, which can provide guidelines for the system design in order to guarantee the stability of the interconnected system. Based on the mechanism analysis, a stabilization control for suppressing the SSO in the MMC‐HVDC based wind farms is also proposed. Finally, the theoretical analysis and stabilization control are validated by both time‐domain simulations and field measurements in a real MMC‐HVDC connected wind farm in China. [ABSTRACT FROM AUTHOR]

This paper investigates the small-signal stability of power electronics-based power systems in frequency domain. A comparison between the impedance-based and the eigenvalue-based stability analysis methods is presented. A relation between the characteristics equation of the eigenvalues and poles and zeros of the minor-loop gain from the impedance-based analysis have been derived analytically. It is shown that both stability analysis methods can effectively determine the stability of the system. In the case of the impedance-based method, a low phase-margin in the Nyquist plot of the minor-loop gain indicates that the system can exhibit harmonic oscillations. A weakness of the impedance method is the limited observability of certain states given its dependence on the definition of local source-load subsystems, which makes it necessary to investigate the stability at different subsystems. To address this limitation, the paper discusses critical locations where the application of the method can reveal the impact of a passive component or a controller gain on the stability. On the other hand, the eigenvalue-based method, being global, can determine the stability of the entire system; however, it cannot unambiguously predict sustained harmonic oscillations in voltage source converter (VSC) based high voltage dc (HVdc) systems caused by pulse-width modulation (PWM) switching. To generalize the observations, the two methods have been applied to dc–dc converters. To illustrate the difference and the relation between the two-methods, the two stability analysis methods are then applied to a two-terminal VSC-based HVdc system as an example of power electronics-based power systems, and the theoretical analysis has been further validated by simulation and experiments. [ABSTRACT FROM AUTHOR]

The stability of an offshore wind power network connected through a high-voltage dc (HVDC) transmission line can be challenging since a strong ac collection (ACC) bus might not be available, when there is no rotating machine connected in that bus. In addition, the synchronization unit (phase-locked loop, PLL) has shown to have a significant impact in achieving satisfactory performance. To tackle this problem, this paper has proposed a wind energy conversion system (WECS) controller for such an ACC bus based on the synchronverter concept. A synchronverter—an inverter without PLL that mimics the synchronization mechanism inherent to synchronous generator—is introduced in the grid side of WECS voltage-source converter (VSC), where the wind farms are connected to ac network through a modular multilevel converter (MMC)-based HVDC system. In order to determine the stability of the interconnected system, an impedance-based stability method is adopted. The impedances of both the wind power inverter and the MMC-HVDC converter are analytically derived, and the analytical model is verified by comparing the frequency responses obtained from numerical simulation. The detailed analysis and the results presented show the benefits of this controller and its potential for stability. The results highlights the synchronverter's ability in keeping better performance compared to PLL-based dq-domain control in point of stability and control in integrating offshore wind farm through the MMC-based HVDC system, since the impedance of the synchronverter reflects a simple RL characteristic. On the other hand, the impedance of PLL-based dq-domain control impedance is inductive above 2 kHz and reflects composite characteristics below 2 kHz with different resonance points and with higher impedance magnitude at low frequencies, making it more vulnerable to voltage instability. Finally, time-domain simulation results are presented to validate the theoretical analysis and to show how the self-synchronization impacts on the system performance. [ABSTRACT FROM PUBLISHER]

Future multiterminal HVDC systems are expected to utilize dc voltage droop controllers, and several control structures have been proposed in the literature. This paper proposes a methodology to analyze the impact of various types of droop control structures using small-signal stability analysis considering all possible combinations of droop gains. The different control structures are evaluated by the active power transfer capability as a function of the droop gains, considering various possible stability margins. This reveals the flexibility and robustness against active power flow variations, due to disturbances for all of the implementations. A case study analyzing a three-terminal HVDC VSC-based grid with eight different kinds of droop control schemes points out that three control structures outperform the remaining ones. In addition, a multivendor case is considered where the most beneficial combinations of control structures have been combined in order to find the best performing combination. [ABSTRACT FROM AUTHOR]

DC distribution power systems are vulnerable to instability because of the destabilizing effect of converter-controlled constant power loads (CPLs) and input filters. Standard stability analysis tools based on averaging linearization techniques can be used only when the switching frequency of the converter is significantly higher than the cutoff frequency of the filter. However, dc distribution systems with a reduced size filter, and consequently a high cutoff frequency, are common in transportation applications. Conventional methods fail to detect instabilities in the system because they do not take into account the switching effect. To overcome this drawback, this paper proposes a discrete-time method to analyze the stability of dc distribution systems. This model is applied here to a dc power system with a CPL. The switching effects and the nonlinearities of the system model are taken into account with a simple discretization approach. The proposed method is able to predict the dynamic properties of the system, such as slow-scale and fast-scale instabilities. An active stabilizer is also included in the system model in order to extend the stability margin of the system. Finally, these observations are validated experimentally on a laboratory test bench. [ABSTRACT FROM AUTHOR]

This paper presents the stability analysis of dc distributed power systems with multiple converter-controlled loads. The load converters are tightly controlled, behaving as constant power loads with low-damped $LC\;$ filters. The dynamic behavior of the system in high frequency range is often not studied with the classical tools based on conventional averaging techniques. However, dc power systems with the reduced size filter, and consequently, the high resonant frequency, are widely used in transportation applications. In this paper, the stability analysis of the system is established based on a discrete-time model of the system, taking into account the switching frequency and intrinsic nonlinearities of the system model. The impacts of the filter parameters and interactions among the constant power loads are investigated with the proposed discrete-time method. Moreover, an active stabilizer is developed and included in the dynamic model of the system, in order to extend the stability margin. The theoretical observations are then validated experimentally on a laboratory hardware prototype. [ABSTRACT FROM AUTHOR]

The design of microgrids at the level of distribution systems requires a stable behavior for multiple operation states. The tools used to study the stability of such systems require the estimation of the grid impedance. By the use of the grid impedance estimation around an operation point, it is possible to define a space variable-parameter to obtain a qualitative or quantitative measure from the operation to the unstable boundary. This study presents a comparison of the Kalman filter and the recursive least squares method for the estimation of the grid impedance. The grid impedance is estimated by the technique based on two neighbor operation points. The results were validated by a hardware in the loop and an experimental setup. Finally, the estimated values of the grid impedance of a microgrid are used with a large signal stability study of a dc constant power load. [ABSTRACT FROM AUTHOR]

The introduction of distributed generation (DG) into low voltage (LV) systems demands that the generation system remain grid connected during voltage sags to ensure the operational stability. The DG consisting of fixed speed squirrel cage induction generator (SCIG)-based wind turbines is unable to provide reactive power control and needs a dedicated compensating device. Under asymmetrical grid faults the negative sequence flux circulation in the airgap introduces the torque oscillations that lead to the reduction of lifetime of the generation system. This paper proposes the use of distributed constant power loads (CPLs) for asymmetrical fault ride through (FRT) instead of using a centralized STATCOM. It has also been observed that the compensation of negative sequence voltage improves the performance of SCIG by eliminating the torque ripples. The compensation of positive sequence voltage avoids a possible voltage collapse at the LV distribution level and improves the reliability and stability of the wind farm. Centralized compensation of the asymmetrical grid fault by a STATCOM is compared with the distributed compensation by CPLs. The results suggest that each individual CPL injects lower current for maximum FRT enhancement compared to a dedicated STATCOM. [ABSTRACT FROM AUTHOR]

The classical distribution system can evolve in a new concept of grid, where the distributed generation has a big impact. These changes take into account the use of power electronic devices. With these devices it is necessary to study the stability of the system in a new form due to their multi-current domain and the influence of the controllers applied to each device. The paper presents a methodology that takes into account the structure of a DC microgrid system and evaluates its stability. The stability analysis uses the computational continuation to obtain the conclusions. The real time simulation results present the grid behavior and validate the limits of operation. [ABSTRACT FROM PUBLISHER]

The stability of fixed-speed induction generator (FSIG)-based wind turbines can be improved by a StatCom, which is well known and documented in the literature for balanced grid voltage dips. Under unbalanced grid voltage dips, the negative-sequence voltage causes heavy generator torque oscillations that reduce the lifetime of the drive train. In this paper, investigations on an FSIG-based wind farm in combination with a StatCom under unbalanced grid voltage fault are carried out by means of theory, simulations, and measurements. A StatCom control structure with the capability to coordinate the control between the positive and the negative sequence of the grid voltage is proposed. The results clarify the effect of the positive- and the negative-sequence voltage compensation by a StatCom on the operation of the FSIG-based wind farm. With first priority, the StatCom ensures the maximum fault-ride-through enhancement of the wind farm by compensating the positive-sequence voltage. The remaining StatCom current capability of the StatCom is controlled to compensate the negative-sequence voltage, in order to reduce the torque oscillations. The theoretical analyses are verified by simulations and measurement results on a 22-kW laboratory setup. [ABSTRACT FROM PUBLISHER]

Reactive power is one of the power system ancillary services and can be supplied from either static or dynamic VAR sources. Provision of reactive power in the proximity of load is clearly beneficial for the efficiency of the overall system. Reactive power provision as an ancillary service by the load itself could greatly increase the margin to voltage collapse and, therefore, influence the stability of the power system. Constant power loads (CPLs), interfaced by active rectifiers, have the potential for providing reactive power to the ac distributed system they are connected to. These types of loads with negative resistance characteristic are, at the same time, the most susceptible to voltage stability problems under abnormal operating conditions. This paper first investigates the effect that a large share of CPLs will have on the voltage stability of distributed ac systems during voltage dips. It then investigates the influence of introducing reactive current control by the active rectifiers on the overall voltage stability of the system. The critical clearing time of the system is selected as a measure of the transient stability limit for investigating and comparing the cases of CPLs with and without reactive current control. Centralized reactive compensation by a STATCOM is used as reference to compare with distributed compensation by the CPLs. Results show that the total required distributed injection of reactive current is lower compared to the rating of a centralized STATCOM. The influence of reactive compensation on the total attainable CPL current is investigated analytically as well as through simulations. It is observed that an optimal reactive current injection by CPL reduces the total current to a minimum value. [ABSTRACT FROM PUBLISHER]