DC Wind Turbines Research Articles

Study on the Design of Series-Type All-DC Wind Farms Based on Half-Bridge Voltage Balancing Circuits

Offshore wind farms connected in series, with each wind turbine connected in series with one another, enhance the coupling between them. Significant differences in wind speeds between neighboring DC wind turbines (DCWTs) might result in a substantial disparity in the output voltage, hence posing a risk of overvoltage. Nevertheless, implementing voltage-limiting configurations for DCWTs might lead to the dissipation of wind energy, thereby diminishing the wind farm’s capacity to deliver electricity. This work introduces a half-bridge voltage balancing circuit (HVBC) topology as a solution to the issue of DCWT output voltage changes affecting the stable operation of wind farms. The proposed HVBC topology is designed specifically for large-capacity series-connected all-DC wind farms where wind speed variations occur. This design achieves power decoupling for series-connected all-DC wind farms by providing current compensation to the series-connected DCWTs. A control strategy is devised by examining the decoupling principle and operational characteristics of the HVBC. A 60 kV/48 MW tandem-type all-DC wind farm model consisting of six DCWTs in series is built in Matlab/Simulink. The model is then simulated to evaluate its performance under conditions of unequal wind speed, rapid changes in wind speed, and wind turbine failure shutdown. This research verifies the feasibility of the HVBC topology and improves the stability of the series-type all-DC wind farm.

Design of a Series–Parallel All-DC Power Generation System Based on a New DC Wind Turbine

Wind energy is a good alternative to fossil fuels, as the use of fossil fuels has seriously exacerbated the emission of greenhouse gases such as carbon dioxide and has greatly affected the environment. Conventional AC wind farms and AC transmission systems inevitably face problems involving reactive currents and overvoltage in the context of large-scale, large-capacity, and long-distance transmission. However, the use of all-DC wind turbines, together with DC convergence and DC transmission systems, has obvious advantages over AC transmission in terms of transmission losses and expandability. Such technology does not require bulky frequency transformers and can well solve the aforementioned problems of reactive currents and overvoltage. This paper proposes a new series–parallel structure for an all-DC wind power generation system. The series end uses a DC/DC converter based on the Cuk circuit to solve the current consistency and power balancing problems of the series wind turbine through current control, whereas the parallel end uses a large-capacity DC/DC converter based on the capacity transfer principle, to solve the problem of voltage consistency at the grid-connected end. The series part is used to increase the voltage level of the system, which can reduce the huge construction costs of offshore platforms, and the parallel part is used to increase the capacity of the system, which enables its incorporation into large-scale wind farms to achieve the replacement of fossil fuel energy.

Coordinated Predictive Control of Offshore DC Collection Grid and Wind Turbines for Frequency Response: A Scheme Without Secondary Frequency Drop

From both technical and economical perspectives, all-dc offshore wind farm (OWF) is a trend for future offshore wind energy applications. Furthermore, in an all-dc OWF, both the capacitors in dc collection grid and offshore wind turbines have the potential for onshore grid frequency response. In light of this, this paper presents a model predictive control scheme, which coordinates the operation of offshore dc collection grid capacitors (dc-grid capacitors) and offshore wind turbines for the onshore frequency support. With this scheme, offshore dc-grid capacitors provide fast inertia support right after the frequency event, while offshore wind turbines provide primary frequency response for a longer period. A key feature of the proposed scheme lies in the ability to suppress the secondary frequency drop issue, which is caused by sudden power reduction of offshore dc-grid capacitors during the frequency recovery phase. An analytical framework is derived to validate this ability. Finally, simulation results verify the effectiveness of the proposed scheme.

Enhancing Dynamic Stability Performance of Hybrid AC/DC Power Grids Using a New Robust Controller

This research investigates the dynamic performance of a multi-terminal DC (MTDC) network linked to a wind farm side converter. The suggested approach is based on loop shaping, mutual quality decomposition, and an effective H controller architecture (MQDM). The transfer function was built as the wind power plant shifted from the Master-Slave control approach to the conventional feedback control method. The singular value curves of the original open-loop system and the suggested solution are compared in order to derive the weighting functions of the robust H controller. The control gain is then calculated by utilizing the MQDM approach to get the system's transfer function from the state space. By using the single value order reduction approach and frequency response analysis to generate a robust controller's gain, the suggested methodology was put into practice. The suggested method for controlling the DC grid voltage on the MTDC converter on the wind farm side has now been implemented in the MATLAB environment. Applying various uncertainties to the DC grid, wind turbines, and AC grid using simulation data demonstrates how reliable the suggested approach is.

Fault detection and localization method for modular multilevel converters in offshore DC wind turbines

The offshore DC wind turbine, consisting of direct-drive permanent magnet synchronous generator (PMSG) and modular multilevel converter (MMC), has become a promising candidate for the large-capacity wind energy conversion system. Submodule (SM) failure detection and localization are crucial for reliable operation of the MMC. However, existing techniques mainly focus on the rated operation condition, which may not be suitable for the MMC in wind turbine due to varying wind speed. Moreover, proper injection of second-order harmonic circulating current (SHCC) is desirable to reduce capacitor voltage ripple and power loss, which may also affect the accuracy of fault detection. This paper proposes a fault-tolerant strategy immune to variable operation conditions, as well as SHCC injection, for the MMC with phase-shifted carrier modulation. Firstly, the inherent switching harmonics of circulating current with SM failure are analyzed, based on which the fault detection method is developed. Then, the localization approach is presented by measuring the difference of SM capacitor voltages. Compared with existing techniques, the proposed strategy requires neither estimator nor additional sensor, which is robust, simple, and economical. Finally, both simulation and experimental results demonstrate the effectiveness of the proposed method, which is not influenced by SHCC and varying wind speed.

Grid-Connected Adaptive Analysis and Control of the DC Wind Turbine Under the Grid-Side Transient Faults

Grid-Connected Adaptive Analysis and Control of the DC Wind Turbine Under the Grid-Side Transient Faults

Design and Control of Series Resonant DC-DC Converter for DC Wind Turbines

High power reliability and high energy density are always in demand, and this is driving the evolution of power conversion technologies. People have begun to examine the potential positions of dc systems in both current and future power equipment due to the rise in the use of renewable energy and the implementation of energy storage in recent decades. Because of its efficiency and power density, dc voltage representation has found use in a wide range of fields, including data centres, the aerospace industry, and dc micro-grids. Resonant DC-DC converters, with their gentle switching and minimal EMI, provide a promising solution to these problems. Finding and examining the various modes of operation of the converter while employing the pulse-removal approach is the primary objective of this study. The new method of operation utilises variable frequency and variable phase displacement in sub-resonant mode, which promises to reduce the transformer size while also facilitating the soft-switching change of the insulated gate bipolar transistors (IGBTs) and line frequency diodes on the rectifier side. The pulse elimination approach, a revolutionary mode of operation for the converter, involves phase and frequency shift modulation at varying rates. Using the results of MATLAB/SIMULINK simulations, the suggested control technique may be utilised to establish the baseline switching function configuration necessary to attain high power efficiency.

Design and Control of Series Resonant DC-DC Converter for DC Wind Turbines

High power reliability and high energy density are always in demand, and this is driving the evolution of power conversion technologies. People have begun to examine the potential positions of dc systems in both current and future power equipment due to the rise in the use of renewable energy and the implementation of energy storage in recent decades. Because of its efficiency and power density, dc voltage representation has found use in a wide range of fields, including data centres, the aerospace industry, and dc micro-grids. Resonant DC-DC converters, with their gentle switching and minimal EMI, provide a promising solution to these problems. Finding and examining the various modes of operation of the converter while employing the pulse-removal approach is the primary objective of this study. The new method of operation utilises variable frequency and variable phase displacement in sub-resonant mode, which promises to reduce the transformer size while also facilitating the soft-switching change of the insulated gate bipolar transistors (IGBTs) and line frequency diodes on the rectifier side. The pulse elimination approach, a revolutionary mode of operation for the converter, involves phase and frequency shift modulation at varying rates. Using the results of MATLAB/SIMULINK simulations, the suggested control technique may be utilised to establish the baseline switching function configuration necessary to attain high power efficiency.

Control strategy for DC series-connected wind farm without energy curtailment

With lower construction costs and power losses, the DC wind farm (DCWF) based on series-connected DC wind turbines (DCWT) is a potential collection scheme for large-scale offshore wind power. However, energy curtailment caused by unequal wind speed exists in DCWTs and results in energy waste, weakening the advantages of DCWTs. The energy curtailment mechanism is analyzed for DCWF, where single active bridge (SAB) is used as DC/DC converter in DCWTS and a full-bridge modular multilevel converter (FB-MMC) as the receiving end converter. The linear relationship between input and output of single active bridge (SAB) is interpreted, and a novel coordinate control strategy for DCWF is proposed to solve the wind energy curtailment caused by unequal wind speed. In addition, an auxiliary control strategy for receiving end converter is proposed to reduce the loss on HVDC link. A complete DCWF is built in PSCAD/EMTDC software package, and the results show that the proposed control strategy makes DCWF can operate well without energy curtailment under unequal wind speeds.

Control strategy of series–parallel DC wind farm with fault isolation

The series–parallel DC wind farm composed of a large number of DC wind turbine (DCWT) is a potential scheme for offshore wind power collection with lower construction costs and power losses. However, enhanced coupling issues, such as energy curtailment and fault isolation, exist in the DC wind farm. To overcome the difficulties, a series–parallel DC collection system is proposed, where DCWTs based on the single active bridge (SAB) are connected in series and then are connected in parallel via DC/DC converters. The coupling mechanism among series–parallel strings is analyzed first, and a coordinate control strategy for the whole system is proposed based on the characteristics of SAB. DC fault located at different strings is analyzed and strategies are proposed to protect the whole system. Simulation results in PSCAD/EMTDC verifies the proposed control strategy with the less coupling among series–parallel DC wind farms under unequal wind speeds and fault.

Control Strategy for Offshore Wind Farms with DC Collection System Based on Series-Connected Diode Rectifier

The DR-HVDC (Diode rectifier-based HVDC) transmission topology was recently proposed for integration on large offshore wind farms due to its low investment cost and high reliability. To further reduce the investment, a DC collection topology based on the series-connected diode rectifiers (DR) is proposed, where no offshore platform is needed. However, units of series-connected topology (SCU) show coupling issues, such as overvoltage, energy curtailment, and fault isolation. First, the coupling mechanism is analyzed, and a suitable operation mode for SCUs is selected to ensure the safe operation of the DC system. Then, the linear relationship of active power and output DC current and DC voltage of SCUs is analyzed, and a novel coordinate control strategy for DC wind farms is proposed, where an onshore converter adapts a DC current controller and wind turbines adapt a mediate output voltage control strategy. The mediate output voltage control strategy includes a triple loop with power loop, mediate output voltage loop, and current loop. Also, the DC open line fault, DC grounding fault, and AC grounding fault of the onshore grid are investigated, and a protection strategy is proposed. A 160 MW wind farm with a DR-SCU DC collection system is built in PSCAD/EMTDC to verify the validity of the proposed control strategy under unequal wind speeds, DC fault, and onshore AC fault, and the results validate the performance of the proposed strategy.

A second-order harmonic circulating current injection method for MMC capacitance reduction in offshore DC wind turbine

The offshore DC wind turbine, consisting of direct-drive permanent magnet synchronous generator (PMSG) and modular multilevel converter (MMC), has become a promising candidate for the large-capacity wind energy conversion system. However, due to the low-frequency output current of PMSG, submodule (SM) capacitors suffer from large voltage ripple especially at rated wind speed. Second-order harmonic circulating current (SHCC) injection is effective for ripple reduction, but conventional techniques only suppress the fundamental component, leading to large SHCC at low wind speed, where the ripple is naturally small. The main contribution of this paper involves that a SHCC injection method, considering all voltage ripple components, is proposed to maintain the ripple within a reasonable limit, which decreases the SHCC amplitude. The proposed method not only mitigates the voltage ripple, but also reduces the MMC power loss over the entire wind speed range. Firstly, the voltage ripple is modeled with SHCC. Then, the injected SHCC for defined ripple limit is derived, which facilitates capacitance selection. Finally, a case study of MMC connected with 10-MW and 10-kV PMSG is conducted, based on which the voltage ripple and power loss without SHCC, with conventional and proposed injection methods, are analyzed and compared. The results demonstrate that, the maximum voltage ripple of conventional method can be more than 2 times that of the proposed method. Moreover, the SM capacitance can be reduced by 40% with the proposed method, which also features power loss reduction over the entire wind speed range.

Modified iterative learning controller for efficient power management of hybrid AC/DC microgrid

In this paper, modified ILC is proposed for maintaining stable voltage, frequency and for efficient power management in a hybrid micro grid (HMG). The system is modelled with solar, battery, DC loads at DC bus, wind turbine, utility grid and AC loads at AC bus. An interlinking converter (IC) is connected between the AC and DC bus to facilitate bidirectional power flow. The control of voltage at AC/DC bus and management of power are obtained in the modelled hybrid micro grid with set point weighting iterative learning controller (SPW-ILC) by controlling the interlinking converter both in autonomous and grid connected mode of operation. To minimise the error signal to the controller, the classical optimisation method of sequential quadratic programming (SQP) has been employed to improve the performance of the controller. The simulation results show that the proposed controller have better performance than other controllers under variable source and load conditions.

Cost and reliability optimization of modular multilevel converter with hybrid submodule for offshore DC wind turbine

The hybrid modular multilevel converter (HMMC) consisting of half- and full-bridge submodules (SMs), with DC fault ride-through capability, have become a promising candidate for offshore wind energy conversion system, especially all-DC output wind turbines. The topology of HMMC needs to be optimized for lower cost and higher reliability with active redundancy. This paper presents an optimization method for HMMC topology with cost and reliability objectives. The reliability is defined to be not only reliable under normal operation, but also able to ride through DC fault even with faulted SMs, which is different from the former model. Firstly, reliability and cost models with power loss, considering active redundancy configuration, are derived. Then, cost and reliability, with different numbers of original and redundant SMs, are quantified by taking a case study of 10-MW and 10-kV HMMC for offshore DC wind turbines. Finally, a multi-objective optimization approach that regards maximum reliability and minimum cost is proposed, and the optimal solution from Pareto-optimal set is selected according to fuzzy membership function, which determines the optimal topology for HMMC. The results show that reliability can be improve with redundant full-bridge SMs (FBSMs) but weakened with excessive half-bridge SMs (HBSMs). Furthermore, the incremental cost with a redundant FBSM is approximately 3 times that of HBSM. The optimal topology, 10 original SMs per arm (5 HBSMs and 5 FBSMs) with 1 redundant HBSM and 4 FBSMs, is validated by comparing with the results of former reliability model and single-objective optimization.

Characteristics Analysis of Inertia Damping of Grid-Connected System of Direct-Drive Wind Power Generation

As large-scale direct-drive wind turbine generator set is connected to the grid, the power system will face problems such as reduced inertia and insufficient frequency modulation capability. The control of wind turbine virtual inertia is an important way to solve this problem. Therefore, this paper takes the direct-drive wind power generation system as the research object, draws lessons from the multi-time scale modeling idea, and based on the electrical torque analysis, establishes the DC voltage time model with the wind turbine virtual inertia control. Based on this, inertia damping characteristics of direct-drive wind power grid-connected system are analyzed. The results show that the dynamic characteristic parameters of the system are affected by many factors, among which the equivalent inertia coefficient of the system is mainly affected by DC capacitance, DC bus voltage and wind turbine virtual inertia control parameters. Damping coefficient is mainly affected by steady-state operating point and DC voltage proportional control parameter K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pu</sub> . The synchronization coefficient is mainly affected by steady-state operating point and DC voltage integration control parameter K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">iu</sub> . The correctness of mechanism analysis of inertia damping characteristics of the whole system is verified by simulation, which provides a certain theoretical reference for inertia damping research of electronic power system.

Design of a High-Power Resonant Converter for DC Wind Turbines

This paper presents a design procedure and loss estimation for a high-power, medium-voltage series resonant converter (entitled SRC#), intended for application in megawatt medium-voltage dc wind turbines. The converter is operated with a novel method of operation, entitled pulse removal technique, characterized by variable frequency and phase-shift modulation, below the resonant point of the LC tank. A step-by-step design procedure is proposed and circuit component ratings and efficiency are compared for two variants. The new method of operation reduces transformer size to below 50%, whereas losses are kept below 1.5% from zero to nominal power, due to soft-switching characteristics. An experimental setup, rated for 10 kW and 5 kV output was assembled to extract losses and validate the semiconductor loss model.

Operation of DC Series–Parallel Connected Offshore Wind Farm

A dc series–parallel wind farm was proposed as an alternative to an ac wind farm. But, the operation of such a wind farm results in significant wind turbine (WT) output voltage variation, which may lead to severe stress being seen by the turbine capacitors. The output voltage variation problem is demonstrated and rigorously analyzed with proposed power flow calculation. In order to mitigate the voltage variation a dc WT with integrated storage along with strategies for control and storage sizing is proposed. The time domain simulation results for 120 MW 2 $\times$ 6 dc series–parallel wind farm shows that a large voltage variation, with maximum value reaching close to 2 p.u and low voltage of about 0.2 p.u, is obtained when WT output voltage is not limited. The power input variation of a single turbine affects voltages and currents of all the WTs in the wind farm. A 48 kWh storage integrated with the WT appears to significantly alleviate the WT voltage variation.

Model-Based Control Design of Series Resonant Converter Based on the Discrete Time Domain Modelling Approach for DC Wind Turbine

This paper focuses on the modelling of the series resonant converter proposed as a DC/DC converter for DC wind turbines. The closed-loop control design based on the discrete time domain modelling technique for the converter (named SRC#) operated in continuous-conduction mode (CCM) is investigated. To facilitate dynamic analysis and design of control structure, the design process includes derivation of linearized state-space equations, design of closed-loop control structure, and design of gain scheduling controller. The analytical results of system are verified in z-domain by comparison of circuit simulator response (in PLECS™) to changes in pulse frequency and disturbances in input and output voltages and show a good agreement. Furthermore, the test results also give enough supporting arguments to proposed control design.

Analysis of a High-Power, Resonant DC–DC Converter for DC Wind Turbines

This paper is introducing a new method of operation for a series resonant converter, with intended application in megawatt high-voltage dc wind turbines. Compared to a frequency controlled series resonant converter operated in subresonant mode, the method (entitled pulse removal technique) allows the design of the medium frequency transformer for highest switching frequency, while being operated at lower frequency without saturation. The main focus of this paper is to identify and analyze the operating modes of the converter with pulse removal technique. With the use of variable frequency and variable phase displacement in subresonant mode, the new method of operation promises transformer size reduction and facilitates soft-switching transition of the insulated gate bipolar transistors (IGBTs) and line frequency diodes on rectifier side. Four modes of operation are identified, while equations for output power, voltage, and current stress are identified. Experimental results are concluded on a 1 kW, 250 V/500 V prototype.

A resilient DC community grid with real time ancillary services management

Abstracts The restructuring of the electric supply industry has prompted the situation where customer is a critical business player. This has also made the maintaining of reliability and stability in the power system a monetarily crucial task. Self-resilient community grid is a realization of reliable and stable system at the customer's level as individual households with solar photovoltaics or other renewable system can sustain itself even if the power grid fails. In this paper, a novel DC community grid has been proposed which is capable of providing ancillary services to the main distribution grid. The proposed DC community grid consists of 100 houses with individual solar installation along with a centralized DC-wind turbine and EV charging stations. Load flow analysis was conducted on the community grid using DC power flow method. The individual homes were restructured into a novel hybrid AC/DC home grid which was proved experimentally to have better harmonic reduction, more power resilience and efficiency. A bidirectional three phase voltage source inverter is used for grid interconnection whenever there is an ancillary service request from the distribution system operator or there is a shortage of power within the community. Fault ride through capability of the community grid tied inverter was tested for line–line and three phase bolted fault which is a common interruption in the distribution network. The analysis was done using MATLAB/Simulink. This paper succeeded in establishing the feasibility of connecting individual homes into a community grid for better efficiency and distribution grid reliability.