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.

Field experience has shown that subsynchronous oscillation and harmonic resonance can occur between wind farms (WFs) and high voltage dc (HVdc) systems. The oscillations can appear in the presence of background harmonics due to the interaction between the wind energy conversion system's (WECS's) converter controller, HVdc converter controller, and the impact of the interconnection system impedance. However, the root causes of these oscillations observed in the field are not entirely understood and they can be attributed to various sources within the components and controllers of the interconnected system. This paper explores the possible causes of these oscillations by investigating the impact of controllers and components in the WF and in the voltage-source converter (VSC)-based HVdc transmission system. In order to understand this phenomenon, the impedance of both the WF and the HVdc from the offshore ac collection point is analytically derived to identify potential resonance points. The impedance frequency responses of the WF and the HVdc converter indicate the potential resonance at low frequency. The origin of these oscillations can be attributed to the propagation of the WECS resonance through the WECS full converter dc link and the interaction between the WECS and the HVdc system. Once the source and the load impedance are identified, an impedance-based stability method is adopted in order to determine the stability. In an attempt to improve the oscillatory phenomena, an active damping scheme is implemented on the offshore HVdc rectifier. An analysis and time-domain simulation results with its respective harmonic spectra show that the implemented active damping is very effective in eliminating the oscillations observed in the interconnected system. Moreover, this paper presents the role of the ratio between the bandwidths of the interconnected areas, as having an essential role in the root cause of the instability. The general rule is observed that when the bandwidth of the HVdc rectifier (which is the source) is faster than the bandwidth of the load (WFs inverter); the system operates stably.

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.

The high-voltage dc (HVDC) systems are appearing more and more, and it is becoming a requirement that the HVDC voltage-source converters (VSCs) operate both as an inverter and a rectifier without changing the controls to provide the flexibility of having power flows in both directions. It is observed that the HVDC system operates stably when the power flow direction is from the power-controlled converter to the dc-voltage-controlled converter, and it becomes unstable when the power flow direction has been altered. In order to analyze such an instability problem and to design the local control, an impedance-based method is proposed. Identifying the source and the load impedance are prerequisite to apply the impedance-based method. The existing method of determining the source and the load impedance cannot predict the stability when the power flow direction is altered; therefore, a method based on the power flow direction has been presented to determine the source and the load impedance. The converter that injects power to the dc system is the current source represented with its Norton equivalent parallel impedance, while the other converter impedance is considered as the load impedance. The stability of the system is determined by the ratio of the load impedance to the current-source impedance. Once the source and the load impedance are analytically obtained, the impedance-based Generalized Nyquist stability criterion is applied to determine the stability. The system stability for the two power flow directions is well predicted from the Nyquist plot of impedance ratio. A two-terminal HVDC system has been developed in MATLAB/Simulink to demonstrate the application of this method, and the results are compared with the experimental results.

Microgrids (MGs) have become an integral part of smart grid initiatives for future power system networks. Networked microgrids consist of several neighbouring microgrids connected in a low/medium distribution network. The primary objective of a network is to share surplus/shortage power with neighbouring microgrids to achieve mutual cost‐effective operation, utilising green energy from renewable energy resources in the network and increasing the reliability of customer service. This article classifies networked microgrids on the basis of network formation and provides an overview of recent research on control of networked microgrids. In addition, a state‐of‐the‐art review of optimisation methods is provided to solve the energy optimisation problem in networked microgrids. Furthermore, the advantages and challenges of the networked operation of microgrids are presented as for possible research directions in the future.

Estimation of critical control parameters is a desirable tool feature for stability analysis and impedance shaping of 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 Grey-box method that, with basic assumptions about the control structure of WECS, can estimate the parameters of its controllers. The method is based on the measurements of frequency domain equivalent impedance combined with non-parametric impedance identification used in the solution of an inverse problem. The method will make possible to specify which part of the equivalent WECS impedance has a major impact on the stability of the system and according to this, re-shape the impedance to enforce stability. Once the critical controller bandwidth is identified with this method, an instability mitigation technique is proposed based on re-shaping the impedance by re-tuning the critical controllers of the interconnected converters. The results show that the method can ensure the stability of the system under changing operating conditions, by re-tuning only the critical controllers parameters by the above re-tuning criterion.