Search

Converters are the key for increasing the development of renewable energy generation but their dynamic interaction with the grid has an important impact on stability. Oscillatory instabilities in different grid‐connected converter systems at several frequency ranges are reported. In particular, sub and supersynchronous oscillatory instabilities in AC power systems with type‐3 and type‐4 wind turbine generators (WTGs) were recently registered at several wind farm areas. A number of works based on eigenvalue analysis and frequency domain approaches are carried out to analyse this new stability issue and more research is going on to analyse in detail the electric and control parameters that may affect this phenomenon. This study contributes analysing in detail the influence of system parameters on the subsynchronous oscillations (SSOs) in AC power systems with type‐4 WTGs. This study is based on a new approach for subsynchronous stability assessment which could also be used for analysing other types of SSOs (e.g. subsynchronous control interactions between doubly‐fed induction generators and series‐compensated networks) as well as supersynchronous oscillations. A representative example of weak AC grids with type‐4 WTGs is used to illustrate the contributions of the study. Results are validated with PSCAD/EMTDC time‐domain simulations. [ABSTRACT FROM AUTHOR]

The occurrence of unstable subsynchronous control interaction (SSCI) between wind farms and series‐compensated networks poses a considerable threat to the safety of power system. Different from the conventional eigenvalue analysis and Nyquist method, the black‐box‐model based RLC fitting technique becomes more attractive for the assessment of the SSCI risk. However, the method has merely been verified by radial networks whose SSCI mode can be well explained by an RLC circuit. A practical wind power system, however, can be meshed networks comprising multiple geographically distributed wind farms and series capacitors. Thus, it is questionable whether the application of the RLC fitting technique to such systems can still yield a sufficient accuracy. To clarify this aspect, in this work, the performance of RLC fitting in various test systems was examined. The results indicated that the aggregation technique considerably influences the accuracy of the assessment, and the RLC fitting cannot be applied when multiple series capacitors are involved in the SSCI mode. To address this problem, a more advanced frequency‐domain fitting technique known as vector fitting was adopted, and its robustness and effectiveness were verified considering a modified The Electric Reliability Council of Texas (ERCOT) system. [ABSTRACT FROM AUTHOR]

This study proposes a generic method for modelling and comparison analysis of grid‐connected double‐fed induction generator (DFIG)‐based wind farms in a weak grid. A detailed model of DFIG in a weak grid is established and used as a benchmark. Specifically, the detailed model consists of the mechanical part, generator part, rotor side converter, and phase‐locked loop (PLL). Two simplified models of grid‐connected DFIG‐based wind farms in a weak grid are compared with the detailed model to investigate the mechanism of sub‐synchronous oscillation (SSO) as well as the influential factors through linear system analysis. The first simplified model ignores shafting dynamics, stator flux dynamics, and PLL dynamics. The same assumptions are used in the latter, except that PLL dynamics are considered. Time‐domain simulation and eigenvalue analysis are used to verify the accuracy of the simplified models. The block diagrams of the linearised system are derived based on the proposed model, and the oscillation mechanism is explained by linear system analysis. Through eigenvalue analysis and the root locus method, the influences of the parameters of the DFIG and its controllers on the system stability are identified and analysed. [ABSTRACT FROM AUTHOR]

This study presents the impact of the doubly fed induction generator (DFIG) with power system stabiliser (PSS) on critical low‐frequency oscillations (LFOs) caused by the synchronous generators (SGs). These LFOs arise due to uncertainties in a system such as generating/loading conditions, intermittent wind power and may cause instability. To improve the stability, the PSS is added. This study studies the effect of varying wind speed, DFIG locations and its capacity with and without PSS on stability. The system is analysed by replacing SGs with DFIG. The sensitivity analysis is carried out with wind power penetration and voltage gain as sensitivity parameters. This identifies electromechanical modes of oscillations that have positive and negative impacts on the system. The transient and small‐signal stability (SSS) investigations are done using non‐linear simulation and eigenvalue analysis, respectively. The system uncertainties are modelled using inverse output additive perturbation structure to elude the mathematical difficulty. The location and selection of best local input signals for PSS are evaluated from residue method and time‐domain simulation analysis. The effectiveness and robustness of the proposed approaches are verified on IEEE 9‐bus test system. The PSS improves the transient stability and SSS of the system. [ABSTRACT FROM AUTHOR]

In recent years, the subsynchronous oscillation (SSO), which occurs in direct‐drive permanent magnetic synchronous generators (D‐PMSGs)‐based wind farm, has great impact on the stability of power system. The different parameters and operation conditions of D‐PMSGs lead to the subsynchronous interaction (SSI) among D‐PMSGs. What is more, the SSI will influence the SSO characteristics of the system of D‐PMSGs‐based wind farm integrated to AC network. Here, considering the difference of parameters or operation conditions among D‐PMSGs in the wind farm, eigenvalue analysis, based on the small‐signal state‐space model of two aggregated D‐PMSGs which are equivalent to wind farm, is carried out. Then the stability and the SSI between D‐PMSGs of the equivalent system can be studied, and the dominant D‐PMSG can be determined based on the participation factors. Lastly, the detailed electromagnetic transient simulation model of D‐PMSGs equivalent system is built in PSCAD/EMTDC. The simulation is conducted to corroborate the eigenvalue analysis. The result shows that an obvious SSI exists between two aggregated D‐PMSGs. Furthermore, the different SSO characteristics among D‐PMSGs in the wind farm indicate the study of SSO problem based on an aggregated D‐PMSGs model may leave out some useful information. [ABSTRACT FROM AUTHOR]

A hybrid microgrid has numerous decentralised control loops. Therefore, coordination among hybrid microgrid subsystems with desired performance is essential. This study presents a practical control approach for efficient tuning of proportional–integral (PI) controllers and leads compensators in islanded hybrid microgrids. This method is based on the frequency response characteristic and root‐locus trajectory. It is used to minimise the frequency deviations of an AC hybrid microgrid. The presented well‐tuned controllers are tuned based on droop mechanism, and coordination among hybrid microgrid subsystems with desired damping coefficient and stability margin. Then, the system performance is analysed under several disturbances. The results are compared with PI controllers tuned by Ziegler–Nichols method. As well, the robustness of the proposed approach in a wide range of parameter changes is investigated. Eigenvalue analysis and simulation results show that the minimum frequency deviations and desired relative stability of the hybrid microgrid subsystems are achieved by the proposed controllers. To show generality and efficiency of the proposed approach, the presented method is applied to a different hybrid microgrid model used in the literature. For this purpose, in order to control the frequency deviations in the stand‐alone mode, presented well‐tuned controller is compared with intelligent fuzzy and particle swarm optimisation‐fuzzy controllers. [ABSTRACT FROM AUTHOR]

In this study, small‐signal stability of the power system integrated with ancillary‐controlled large‐scale doubly fed induction generator (DFIG) based wind farm (WF) is studied. A model which considers grid code requirements and ancillary controllers is presented to indicate important DFIG dynamics for the study. The ancillary control of the WF which aims to improve power system voltage/frequency stability is evaluated from the small‐signal stability perspective. It is shown that the ancillary controller deteriorates power system low‐frequency oscillations and/or induces new lightly damped oscillation modes especially in a weak grid. The potential risks may restrict the WF to fulfil gird code requirements and threaten small‐signal stability of the power system. Meanwhile, this study reveals that wind power penetration level and WF connection impedance are two main factors which affect dynamic interactions between the WF and the power system and thus affect system small‐signal stability. The principle of the influence on the small‐signal stability with different grid weakness and ancillary control schemes is evaluated in this study by eigenvalue analysis and verified with time‐domain simulations. [ABSTRACT FROM AUTHOR]

This study aims to conduct the sub‐synchronous (SS) resonance analysis of a multi‐machine power system incorporating doubly fed induction generator (DFIG)‐ or permanent magnetic synchronous generator (PMSG)‐based wind farm. The linearised equations involving the dynamics of steam turbine generator, wind farm, shunt capacitor, transmission line, and load are derived, and the detailed state matrix formation process pertinent to each component is presented. Taking the IEEE 16‐machine‐68‐bus test system as example, the eigenvalue analysis along with participation factor is applied to investigate the impacts of the series compensation level of transmission line on the modal frequencies and damping ratios of two groups of oscillatory modes of interest: namely, the SS mode of the series‐compensated transmission line and the shaft modes of the studied steam turbine generator before and after one of the un‐studied steam turbine generators is replaced by the DFIG‐ or PMSG‐based wind farm. Some useful conclusions and comments are drawn. [ABSTRACT FROM AUTHOR]

Contribution to the damping of inter‐area and torsional oscillation modes, in doubly fed induction generators (DFIG) based wind farms, by power oscillation damping controllers (PODCs) based on two conventional structures and a fuzzy control strategy is investigated in this study. In this regard, a PODC with no lead/lag compensation is designed first and then a PODC with one lead/lag block is developed using eigenvalue techniques and the application of an iterative process based on the bat optimisation algorithm (BOA). Moreover, a fuzzy PODC, based on a simple fuzzy controller and tuned with the BOA according to the system transient response under a critical perturbation, is also designed. Comparative performance of the three PODCs is evaluated on a multi‐machine power system. It is observed that all three PODCs can contribute to improving the damping of inter‐area oscillations. However, eigenvalue analysis and non‐linear time domain simulations reveal that each of them may also impact to a lesser or greater extent the shaft torsional oscillation mode damping. Their relative impact in this regard is also investigated. [ABSTRACT FROM AUTHOR]

This study proposes an enhanced model of the doubly fed induction generator (DFIG)‐based wind farm (WF) for small‐signal stability (SSS) studies of weak power system. This model consists of a wind turbine, a drive train, a simplified generator/converter module and associated controls. As analysed and verified in this study, the dynamics of the phase‐locked‐loop and the rotor current loop are important for accurate SSS studies of the weak power system involving the DFIG‐based WF and thus should be modelled appropriately. The enhanced model is validated by eigenvalue analysis and time‐domain simulations. Compared with the detailed model, the WT3 model provided by General electric (GE) energy and the conventional model, the enhanced model presents high precision and satisfactory simplification for SSS studies. As the application of the enhanced model, a case study is carried out to explore the impact of WF ancillary voltage/frequency controls on SSS of the weak grid. As indicated in this study, the WF ancillary controls exert distinct impact on SSS. New lightly damped power oscillations can be introduced. Therefore extra measures should be considered if the ancillary controls are required by the WF in order to satisfy the future grid codes. [ABSTRACT FROM AUTHOR]

Grid synchronisation is crucial for integration of renewable energy sources and distributed generation systems into the grid. Accurate and fast estimation of the fundamental positive sequence component is a necessity for synchronisation of three‐phase distributed generation systems with the grid. In this study, a simple synchronisation technique based on the real‐complex‐coefficient filters is proposed. It has a very simple structure, and can directly estimate the fundamental positive and negative sequences under the distorted, unbalanced and offset conditions with no need of a phase‐locked loop. Furthermore, the dc offset can also be extracted precisely. For optimising the steady‐state and dynamic performance, a parameter tuning guidance is provided from the eigenvalue analysis point of view. In addition, a simple auxiliary unit is presented to remain its frequency‐adaptive feature. Finally, experimental results based on a 32‐bit fixed‐point TMS320F2812 DSP verify the effectiveness of the proposed synchronisation method. [ABSTRACT FROM AUTHOR]