Impedance-compensated grid synchronisation for extending the stability range of weak grids with voltage source converters

This paper demonstrates how the range of stable power transfer in weak grids with voltage source converters (VSCs) can be extended by modifying the grid synchronisation mechanism of a conventional synchronous reference frame phase locked loop (PLL). By introducing an impedance-conditioning term in the PLL, the VSC control system can be virtually synchronised to a stronger point in the grid to counteract the instability effects caused by high grid impedance. To verify the effectiveness of the proposed approach, the maximum static power transfer capability and the small-signal stability range of a system with a VSC HVDC terminal connected to a weak grid are calculated from an analytical model with different levels of impedance-conditioning in the PLL. Such calculations are presented for two different configurations of the VSC control system, showing how both the static power transfer capability and the small-signal stability range can be significantly improved. The validity of the stability assessment is verified by time-domain simulations in the Matlab/Simulink environment.Peer ReviewedPostprint (published version

Exploring the Potential for Increased Production from the Wave Energy Converter Lifesaver by Reactive Control

Fred Olsen is currently testing their latest wave energy converter (WEC), Lifesaver, outside of Falmouth Bay in England, preparing it for commercial operation at the Wavehub test site. Previous studies, mostly focusing on hydrodynamics and peak to average power reduction, have shown that this device has potential for increased power extraction using reactive control. This article extends those analyses, adding a detailed model of the all-electric power take-off (PTO) system, consisting of a permanent magnet synchronous generator, inverter and DC-link. Time domain simulations are performed to evaluate the PTO capabilities of the modeled WEC. However, when tuned towards reactive control, the generator losses become large, giving a very low overall system efficiency. Optimal control with respect to electrical output power is found to occur with low added mass, and when compared to pure passive loading, a 1% increase in annual energy production is estimated. The main factor reducing the effect of reactive control is found to be the minimum load-force constraint of the device. These results suggest that the Lifesaver has limited potential for increased production by reactive control. This analysis is nevertheless valuable, as it demonstrates how a wave-to-wire model can be used for investigation of PTO potential, annual energy production estimations and evaluations of different control techniques for a given WEC device

Management of harmonic propagation in a marine vessel by use of optimization

Advances in power electronics drive systems for variable speed operation has enabled extensive use of such solutions in the propulsion and thruster systems of marine vessels. These solutions however introduce current and voltage distortions that compromises the overall power quality of the onboard electrical system. This paper presents and discusses one approach for generating the harmonic current reference for an active filter based on optimization. Two relevant results are revealed by this study: 1) lower THD values are attained by performing system optimization compared to local compensation of one load, and 2) the lower THD values are achieved with a smaller active filter rating than the one required for local load compensation.(c) 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works

Modeling and Analysis of SOGI-PLL/FLL-based Synchronization Units: Stability Impacts of Different Frequency-feedback Paths

— Second-order Generalized Integrator (SOGI)-based quadrature-signal-generator (QSG) together with either a phaselocked-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 paper 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 that are frequently seen in the literature 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 paper 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. Index Terms— FLL, PLL, synchronization, SOGI, stability, LTP, frequency feedback.acceptedVersio

Optimal Compensation of Harmonic Propagation in a Multi-Bus Microgrid

This paper discusses how an Active Power Filter (APF) can be utilized for system-wide harmonic mitigation in a microgrid with multiple sources of harmonic distortion located at different buses. A two-bus microgrid system with independent nonlinear loads at both buses is first investigated analytically, and it is demonstrated that it is possible to derive a harmonic current injection from the APF that will minimize the harmonic distortion at both buses. However, analytical optimization of the APF current will be sensitive to parameter variations, will deteriorate when the APF reaches current saturation and cannot be easily extended to larger systems with many loads at different buses. A more practically applicable method for calculating the APF current references, by using the framework of Model Predictive Control (MPC) is instead proposed for the investigated system. Under realistic operating conditions, this approach can obtain further improvement in the system-level harmonic mitigation. The characteristics and performances that are obtained with the analytical solution and the MPC-based control are assessed by time domain simulations in the Matlab/Simulink environment. The results clearly indicate how an MPC-based system-oriented compensation can maximize the utilization of a single APF in a multi-bus Microgrid.© EA4EPQ. This is the authors’ accepted and refereed manuscript to the article

System-Wide Harmonic Mitigation in a Diesel Electric Ship by Model Predictive Control

Email Print Request Permissions This paper proposes a system-oriented approach for mitigating harmonic distortions by utilizing a single Active Power Filter (APF) in an electrical grid with multiple buses. Common practice for control of APFs is to locally compensate the load current harmonics or to mitigate voltage harmonics at a single bus. However, the operation of an APF in a multi-bus system will influence the voltages at neighboring buses. It is therefore possible to optimize the APF operation from a system perspective instead of considering only conventional local filtering strategies. For such purposes, Model Predictive Control (MPC) is proposed in this paper as a framework for generating APF current references that will minimize the harmonic distortions of the overall system within a given APF rating. A diesel-electric ship, with two buses supplying separate harmonic loads, with an APF located at one of the buses, is used as study case. The operation with on-line MPCbased optimization of the APF current references is compared to two benchmark methods based on conventional approaches for APF control. The results demonstrate that the MPC generates current references that better utilize the APF current capability for system-wide harmonic mitigation.2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other work

Understanding the Origin of Oscillatory Phenomena Observed between Wind Farms and HVDC Systems

Field experience has shown that sub-synchronous oscillation (SSO) 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) 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 wind farm and in the voltage source converter (VSC)-based HVDC transmission system. In order to understand this phenomena, the impedance of both the wind farm and the HVDC from the offshore ac collection point are analytically derived to identify potential resonance points. The impedance frequency responses of the wind farm 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 impedancebased 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

Impact of digital time delay on the stable grid-hosting capacity of large-scale centralised PV plant

Digital control system has been widely used for the inverters in PV plant. However, the effects of digital time delay of digital control system on grid-connected PV plants have not been fully studied. Therefore, in view of the stable grid-hosting capacity is a key parameter in the process of designing and operating of a grid-connected PV plant, this paper investigates in detail the influence of digital time delay on the stable grid-hosting capacity of large-scale centralised PV plant. Considering the practical situation of large-scale centralised PV plants, the stable grid-hosting capacity of plant is discussed with the conditions of inverters in PV plant have the same and different digital time delays. The analysis results revealed that stable grid-hosting capacity is varied with the digital time delay of inverter and the combination of the inverter with different digital time delays. Meanwhile, in order to realize a stable grid-connected PV plant in particular delay case, a procedure on how to determine the exact inverter number that can be used in a specific PV plant is provided. The theoretical analysis is verified by simulation and equivalent experiment results

A Gray-Box Method for Stability and Controller Parameter Estimation in HVDC-Connected Wind Farms Based on Nonparametric Impedance

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

Real-time stability analysis of power electronic systems

The paper presents a new method for performing online stability analysis of power electronic systems. The method is based on impedance analysis, where the only information required are measurements taken at a single point. The proposed method injects current composed by several frequencies. The impedance is estimated at these frequencies, and the Vector Fitting method is used to estimate a continuous impedance model. The Nyquist Criterion is then applied to the fitted models of source and load subsystem impedance. The stability analysis is performed continuously with an update rate up to 10 Hz. The proposed method has been compared with the conventional frequency sweep method in an experimental setup.acceptedVersion© 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works