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The use of renewable power generation brings new challenges related to power quality issues. Furthermore, with the changing power system nature due to the presence of new components such as power electronics in large numbers and distributed generation systems, the tools used for more than a century to analyze signals in this type of systems are no longer providing accurate information with a good resolution in time and frequency domain. To contribute with a new view of the problem, this paper presents a hybrid technique for the analysis of oscillations in low voltage distribution systems considering photovoltaic generation. The aim is to characterize the behavior of the system in a time–frequency domain and get the different instantaneous frequencies that appear. The results obtained with this technique are compared with three well-known methods of analysis. The validation of the methodology is carried out in a real-time digital simulator of a distributed system with photovoltaic generation. [ABSTRACT FROM AUTHOR]

The small-signal impedance modeling of a modular multilevel converter (MMC) is the key for analyzing resonance and stability of MMC-based power electronic systems. The MMC is a power converter with a multifrequency response due to its significant steady-state harmonic components in the arm currents and capacitor voltages. These internal harmonic dynamics may have great influence on the terminal characteristics of the MMC, which, therefore, are essential to be considered in the MMC impedance modeling. In this paper, the harmonic state-space (HSS) modeling approach is first introduced to characterize the multiharmonic coupling behavior of the MMC. On this basis, the small-signal impedance models of the MMC are then developed based on the proposed HSS model of the MMC, which are able to include all the internal harmonics within the MMC, leading to accurate impedance models. Besides, different control schemes for the MMC, such as open-loop control, ac voltage closed-loop control, and circulating current closed-loop control, have also been considered during the modeling process, which further reveals the impact of the MMC internal dynamics and control dynamics on the MMC impedance. Furthermore, an impedance-based stability analysis of the MMC-high-voltage direct current connected wind farm has been carried out to show how the HSS-based MMC impedance model can be used in practical system analysis. Finally, the proposed impedance models are validated by both simulation and experimental measurements. [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]

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 introduces, explains, and investigates the application of a transformerless series-connected reactive/harmonic compensator. The proposed compensator is applied to compensate the high reactive-power/harmonic demand of a 12-pulse line-commutated HVDC rectifier terminal. Simulation results reveal that the proposed strategy compensates the reactive power and harmonics of the HVDC converter satisfactorily. All of this is achieved using significantly smaller capacitance at the compensator dc link than the capacitance required in the state-of-the-art shunt compensators employing passive components. The compensator thus would eliminate the need of fundamental-frequency reactive power supply and minimize (or eliminate altogether) the requirement of passive shunt harmonic compensation. This would result into smaller shunt passive filters, smaller or no shunt capacitor banks, and smaller converter transformers, leading to a more compact HVDC terminal, the current size of which is considered to be the main hurdle in its application for integration of large-scale offshore wind energy. Experimental results from a small-scale setup validate the effectiveness observed in the simulations. [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]

Abstract: This paper presents the stability analysis based on impedance representation of a single phase inverter operating as voltage source, where the Nyquist criteria can be applied. The method can be used to study the stability of a micro-grid dominated by power electronics in isolated operation mode. First, a small signal input impedance model is described. The second methodology applies no energy storage at the AC and DC side of the inverter. For two methods a comparative evaluation with the measured impedance was realized. The obtained results show that the proposed method of no energy storage can be employed in the studies of stability for micro grid systems. [Copyright &y& Elsevier]

In wave energy converters (WECs), the maximum power extraction would be achievable at the expense of a very high rating of the electric and power electronics equipment. The goal of this paper is to show how a convenient tradeoff between high-power extraction and viable electrical device rating can be achieved by a proper choice of the WEC control strategy. Referring to a direct coupled point absorber in heave operating in regular waves, it will be analytically shown how most common control techniques impact on both the power performance and the power takeoff (PTO) rating. Thus, a tool that can assist in the preliminary PTO sizing by taking into account the main constraints imposed by the application is obtained. Following, an adaptive control strategy including a reactive component is proposed, whose goal is to improve the overall system performance when the WEC is already operative in the sea. Its effectiveness in increasing the average power extraction while respecting the PTO peak power constraint is proved by computer simulations in both regular and irregular waves, and specific analyses also including the PTO force/torque limitation are finally developed. [ABSTRACT FROM PUBLISHER]

The choice of the most suitable control strategy for wave energy converters (WECs) is often evaluated with reference to the sinusoidal assumption for incident waves. Under this hypothesis, linear techniques for the control of the extracted power, as passive loading and optimum control, are well known and widely analyzed. It can be shown, however, how their performances are fundamentally different when irregular waves are considered and the theoretical superiority of optimum control is questionable under real wave conditions. Moreover, the global optimization of WECs implies a rational design of the power electronics equipment. This requires the analysis of the instantaneous extracted power in addition to the average one. In this paper, the impact of irregular waves on the power extraction when using different control techniques is analyzed in the case of a point absorber in heave. It is also shown how a convenient tradeoff between high average power extraction and limited power electronics overrating can be obtained by applying simple power saturation techniques. Moreover, the impact of power conversion efficiency on the control strategy is analyzed. [ABSTRACT FROM AUTHOR]

Abstract: Use of power electronic equipment has increased and introduced new dynamical phenomena in power systems. For example, new electric rail vehicles (locomotives) equipped with modern power electronic traction chains have caused situations of low-frequency power oscillations and instability in single-phase railway power supply systems. This paper presents the development and implementation of an instantaneous value model and simplified fundamental frequency (RMS) models of such an advanced electric rail vehicle in order to investigate their representation of low-frequency dynamics. The dynamical behaviour is studied by use of both time-domain simulations and linear analysis (eigenvalues) and the degree of simplifications regarding controller dynamics and power system dynamics are presented and discussed. An enhanced RMS model is tested in order to account for the impact of fast current dynamics on the low-frequency behaviour. The results show that this enhanced model is corresponding more closely to the instantaneous value model than what can be obtained by the traditional RMS simplifications and indicate that current dynamics should be included in stability studies involving power electronic inverters. [Copyright &y& Elsevier]