Multi-terminal high voltage dc (HVdc) grids can eventually became a feasible solution to transport energy to remote and/or distant areas and its exploitation depend, among other things, on the performance of the converter terminals. Therefore, to optimize the power transmission strategy along such a grid, it is necessary to recognize the efficiency of all the converters in all points of operation, namely with the different load conditions. In this vision, the aim of this work is to provide the methodology to model the modular multilevel converter (MMC) efficiency by means of a mathematical expression that can describe, over a broad range of active and reactive power flow combinations, the power losses generated by the semiconductors. According to the presented methodology, a polynomial-based model with a reduced number of coefficients is deducted, in such a way that can be directly used for optimal power flow (OPF) studies. The accuracy of the proposed model is characterized by an absolute relative error, at the worst scenario, approximately equal to 3%. (C) 2017 Elsevier B.V. All rights reserved.

High-voltage DC (HVDC) connections enable integration of wind power plants located very far from shore. The decoupled AC offshore grid comprises multiple WT converters, and the voltage magnitude and frequency is primarily controlled by the offshore high-voltage DC voltage-source converter (VSC-HVDC). Faults in,the offshore grid challenge the connected converters to provide an adequate response improving the overall fault behavior. Of special interest are asymmetrical faults due to the resulting unbalanced voltage conditions. This article addresses such conditions in the offshore grid and analyzes the impact on the offshore grid behavior for different converter contributions. Four fault ride-through strategies are studied for the WT converters. The effect of over-modulation of the converter voltages during such voltage conditions is highlighted. A test system is defined to analyze the fault and post-fault behavior. It is found that voltage support from the WT converters in both positive and negative sequence shows the best performance compared to controlled negative sequence current suppression. This scheme helps additionally the VSC-HVDC AC voltage control to return quickly to normal operation. To validate this statement simulations are performed for line-to-line (LL) and single line-to-ground (SLG) faults in immediate vicinity of the VSC-HVDC. (C) 2017 Elsevier B.V. All rights reserved.

Resonance instabilities in power systems can be assessed with the positive-net-damping stability criterion. This criterion is a review of the complex torque coefficients method, but it does not provide the frequency of the closed-loop oscillatory modes. This paper presents an alternative approach of the positive-net-damping stability criterion to analyze electrical resonance instability. In this approach, resonance instabilities are identified in feedback systems derived from impedance-based equivalent circuits. The proposed approach is used to characterize the frequency of closed-loop oscillatory modes and identify the physical and control parameters of the system that increase or reduce the damping of these modes. The extension of the proposed approach to study the stability of single-input single-output and multiple-input multiple-output feedback systems is analyzed, and the approach is also compared with other stability methods in the literature. An example of an offshore wind power plant illustrates the theoretical study and compares the proposed approach with different methods to evaluate stability. Time-domain simulations in Power System Computer Aided Design/Electro Magnetic Transient Design and Control (PSCAD/EMTDC) are shown to validate the stability study.

This paper presents a control solution to damp the torque oscillations that appear in a generator during an unbalanced stator fault. The solution proposed is applicable to a multichannel configuration where a generator with several stators is commanded by parallel-connected power converters. The multichannel structure is introduced by some authors with the goal of increasing the reliability of high-power wind turbines cost effectively. Experimental results are obtained proving the effectiveness of the proposed solution in a nine-phase permanent magnet synchronous generator controlled by three back-to-back converters. The controller shows a good performance and a rapid response canceling the pulsating torque of the short-circuited stator by commanding properly the remaining healthy stators. In this way the risk of damaging the drive train is minimized while the ability to perform a safe stop is guaranteed. The proposed control solution does not require significant-modifications of the existing control system since it acts in parallel with the regular controller and reacts instantly when the fault triggers.

The present article assesses the study of the PV generator capability curves for use in large scale photovoltaic power plants (LS-PVPPs). For this purpose, the article focuses on three main aspects: (i) the modelling of the main components of the PV generator, (ii) the operational limits analysis of the PV array together with the inverter, and (iii) the capability curve analysis considering variable solar irradiance and temperature. To validate this study a PVPP of 1 MW is designed, modelled and simulated in DIgSILENT PowerFactory (R). The results for each case scenario shows that the capability curve and the limitations are directly affected by: the solar irradiance, temperature, dc voltage, and the modulation index. (C) 2016 Elsevier Ltd. All rights reserved.

This paper analyses, from a steady state point of view, the potential benefit of a Wind Power Plant (WPP) control strategy whose main objective is to maximise its total energy yield over its lifetime by taking into consideration that the wake effect within the WPP varies depending on the operation of each wind turbine. Unlike the conventional approach in which each wind turbine operation is optimised individually to maximise its own energy capture, the proposed control strategy aims to optimise the whole system by operating some wind turbines at sub-optimum points, so that the wake effect within the WPP is reduced and therefore the total power generation is maximised. The methodology used to assess the performance of both control approaches is presented and applied to two particular study cases. It contains a comprehensive wake model considering single, partial and multiple wake effects among turbines. The study also takes into account the Blade Element Momentum (BEM) theory to accurately compute both power and thrust coefficient of each wind turbine. The results suggest a good potential of the proposed concept, since an increase in the annual energy captured by the WPP from 1.86% up to 6.24% may be achieved (depending on the wind rose at the WPP location) by operating some specific wind turbines slightly away from their optimum point and reducing thus the wake effect. (C) 2015 Elsevier Ltd. All rights reserved.

This article introduces a novel power coordination method for the operation under restricted conditions of offshore wind power plants connected with VSC-HVDC without the use of communications between converter stations. The proposed method consists of the coordination of the Dynamic Braking Resistor (DBR) located in the Grid Side Converter (GSC) and the wind power plant in order to maintain the DC voltage stability. The coordination is achieved by means of two droop controllers, one for the GSC-DBR and another one for the offshore wind power plant. These droop gains are selected to avoid limit cycles using the describing function approach. The proposed power coordination scheme is tested and verified by means of dynamic simulations. (C) 2017 Elsevier Ltd. All rights reserved.

Offshore wind power plants (WPPs) built near each other but far from shore usually connect to themain grid by a common high-voltage DC (HVDC) transmission system. In the resulting decoupled offshore grid, thewind turbine converters and the high-voltageDCvoltage-source converter share the ability to inject or absorb reactive power. The overall reactive power control dispatch influences the power flows in the grid and hence the associated power losses. This paper evaluates the respective power losses in HVDC-connected WPP clusters when applying 5 different reactivepowercontrol strategies. Thecase study ismadefor a1.2-GW-rated cluster comprising3 WPPand is implemented in a combined load flow and converter lossmodel. Alarge set of feasible operating points for the system is analyzed for each strategy. The results show that a selection of simulations with equal wind speeds is sufficient for the annual energy production comparison. It is found that the continuous operation of theWPPswith unity power factor has a superior performance with low communication requirements compared with the other conventional strategies. The optimization-based strategy, which is developed in this article, allows a further reduction of losses mainly because of the higher offshore grid voltage level imposed by the high-voltage DC voltage-source converter. Reactive power control in HVDC-connected WPP clusters change significantly the overall power losses of the system, which depend rather on the total sum of the injected active power than on the variance of wind speeds inside the cluster.

This study presents a new sensorless control for small wind turbine clusters with a single power converter with a direct torque control algorithm. The proposed system consists of a wind farm connected to a back-to-back power converter that interfaces the wind farm with the AC grid. The studied wind turbines are based on fixed-speed wind turbines equipped with squirrel cage induction generators with individual pitch control. The presented structure permits to reduce the number of converters and allows to accomplish the grid codes (fault ride through capability and reactive power support). Furthermore, the generated active power can be reduced according to grid operator requirements. The presented control scheme can be applied to wind turbine repowering projects, wind farms connected to a microgrid, even, new small onshore and offshore power plants. The system performance and stability is studied and validated by means of dynamic simulations.

As a consequence of technological progress, wind power has emerged as one of the most promising renewable energy sources. Currently, the penetration level of wind energy in power systems has led to the modification of several aspects of power system behaviour including stability. Due to this large penetration, transmission system operators have established some special grid codes for wind farms connection. These grid codes require wind farms to provide ancillary services to the grid such as frequency regulation and reactive power regulation. In the near future, the capability of damping system oscillations will be required. For this reason, the influence of grid-connected wind farms on system oscillations is reviewed in this paper, focusing on the contribution or damping of power system oscillations, and on inner wind turbine oscillations. (c) 2012 Elsevier Ltd. All rights reserved.