• Geomechanics and Engineering
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• 제37권2호
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• pp.167-178
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• 2024

The natural frequency shift under cyclic environmental loads is a key issue in the design of monopile-based offshore wind power turbines because of their dynamic sensitivity. Existing evidence reveals that the natural frequency shift of the turbine system in sand is related to the varying foundation stiffness, which is caused by soil deformation around the monopile under cyclic loads. Therefore, it is an urgent need to investigate the effect of soil deformation on the system frequency. In the present paper, three generalized geometric models that can describe soil deformation under two-way cyclic loads are proposed. On this basis, the cycling-induced changes in soil parameters around the monopile are quantified. A theoretical approach considering three-spring foundation stiffness is employed to calculate the natural frequency during cycling. Further, a parametric study is conducted to describe and evaluate the frequency shift characteristics of the system under different conditions of sand relative density, pile slenderness ratio and pile-soil relative stiffness. The results indicate that the frequency shift trends are mainly affected by the pile-soil relative stiffness. Following the relevant conclusions, a design optimization is proposed to avoid resonance of the monopile-based wind turbines during their service life.

• Journal of Power Electronics
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• 제16권5호
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• pp.1743-1751
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• 2016

The increasing demand for high voltage DC grids resulting from the continuous installation of offshore wind farms in the North Sea has led to the concept of multi-terminal direct current (MTDC) grids, which face some challenges. Power (current) flow control is a challenge that must be addressed to realize a reliable operation of MTDC grids. This paper presents a reduced switch count topology of a current flow controller (CFC) for power flow and current limiting applications in MTDC grids. A simple control system based on hysteresis band current control is proposed for the CFC. The theory of operation and control of the CFC are demonstrated. The key features of the proposed controller, including cable current balancing, cable current limiting, and current nulling, are illustrated. An MTDC grid is simulated using MATLAB/SIMULINK software to evaluate the steady state and dynamic performance of the proposed CFC topology. Furthermore, a low power prototype is built for a CFC to experimentally validate its performance using rapid control prototyping. Simulation and experimental studies indicate the fast dynamic response and precise results of the proposed topology. Furthermore, the proposed controller offers a real solution for power flow challenges in MTDC grids.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2018년도 전력전자학술대회
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• pp.372-373
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• 2018

HVDC(High Voltage Direct Current)전력망은 발전소에서 생산한 교류를 직류로 변환한 이후 송전하여 부하에 가까운 전력변환기를 이용하여 교류로 변환한 후에 사용하는 방식이며, 전압형 HVDC에 MMC(Modular Multilevel Converter)가 널리 사용되고 있다. 본 논문에서는 MMC가 적용된 HVDC를 이용한 해상 풍력 계통 제어 성능을 테스트하기 위해 실시간 시뮬레이션 툴인 RT-Lab에 적용한 모델에 대해 설명하고자 한다.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2013년도 전력전자학술대회 논문집
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• pp.421-422
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• 2013

본 논문에서는 3-레벨의 NPC(Neutral Point Clamped) VSC와 T-type VSC를 시뮬레이션용 열 모델링 기법을 이용하여 스위칭과 도통 손실을 비교 분석하였다. 두 가지 토폴로지의 인버터동작과 회생동작에서의 주파수에 따른 효율을 시뮬레이션을 통하여 고찰 하였다.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2013년도 전력전자학술대회 논문집
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• pp.445-446
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• 2013

최근 대규모 해상풍력단지 조성을 위한 프로젝트 등이 진행되고 있다. 매우 튼 전력을 생산하는 풍력발전기를 계통에 연계시키기 위해서는 du/dt 필터, LCL 필터 등의 요소가 반드시 필요하다. 이러한 필터 설계를 위해서는 선제작한 후 실제 실험을 통해 그적합성을 시험 받는 수밖에 없다. 작은 규모는 상관없지만 이렇게 큰 용량의 필터를 설계하기 위해서는 적지 않은 노력과 비용이 필요하다. 따라서 이러한 설계를 위해서는 많은 시뮬레이션 수단이 강구될 수 있는바 본 논문은 FEM 프로그램을 이용하여 LCL 필터용의 3상 리액터를 설계하고 그 효과를 검증하고자 한다.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2019년도 전력전자학술대회
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• pp.414-415
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• 2019

본 논문은 HVDC(High Voltage Direct Current)에 적용된 MMC(Modular Multilevel Converter)의 제어 알고리즘 개발을 위한 HILS(Hardware In the Loop Simulation)을 위한 모델링 및 HILS 시스템 구축 예를 보인다. 전력 계통, MMC, 풍력 발전 등의 HILS 적용 MATLAB/SIMULINK 모델 및 FPGA(Field Programmable Gate Array)를 이용한 제어기 개발 내용을 보인다. 시뮬레이션 모델과 FPGA 제어기를 이용하여 구축한 OPAL-RT 기반의 실험 결과를 보인다.

• 전기전자학회논문지
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• 제25권2호
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• pp.399-407
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• 2021

본 논문에서는 계통망 사업자의 High Voltage Ride-Through(HVRT) 기능 요구조건을 만족하며, 정상상태에서의 손실을 최소화할 수 있는 Type 4 풍력발전기의 직류단 전압 설계 방법을 제안한다. 대용량 해상 풍력 발전에 사용되는 Type 4 풍력발전기는 전력 계통과 연계된 컨버터와 풍력 발전기와 연계된 컨버터가 직류단을 공유하는 Back-to-Back 컨버터 형태이다. Type 4 풍력발전기에서 HVRT 조건인 계통 고전압 사고가 발생한 경우 사고 전압 크기에 비해 직류단 전압이 부족하다면 과변조로 인해 계통측 컨버터의 전류 제어기가 원활하게 동작되지 못한다. 따라서 HVRT 기능을 만족하기 위해서는 고전압 사고의 전압 크기를 기준으로 직류단 전압을 설계해야 한다. 그러나 직류단 전압의 크기의 증가는 정상상태에서의 컨버터 손실 증가를 야기하므로, 직류단 전압을 크게 설계하였을 때 증가될 손실에 대한 고려가 포함되어야 한다. 본 논문에서는 사고 전압의 크기와 발생 손실이 고려된 직류단 전압을 설계하는 방법에 대해 설명하고, 제안하는 설계 방법의 타당성을 2MVA급 Type 4 풍력발전기의 PSCAD 모델 기반 HVRT 기능 시뮬레이션을 통해 확인하였다.

• Journal of Power Electronics
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• 제18권2호
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• pp.467-481
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• 2018

Modular multilevel converter (MMC)-based high-voltage direct current (HVDC) presents attractive technical advantages and contributes to enhanced system operation and reduced oscillation damping in dynamic MMC-HVDC systems. We propose an advanced small-signal multi-terminal MMC-HVDC based on dynamic phasors and state space for power system stability analysis to enhance computational accuracy and reduce simulation time. In accordance with active and passive network control strategies for multi-terminal MMC-HVDC, the matchable small-signal stability models containing high harmonics and dynamics of internal variables are conducted, and a related theoretical derivation is carried out. The proposed advanced small-signal model is then compared with electromagnetic-transient and traditional small-signal state-space models by adopting a typical multi-terminal MMC-HVDC network with offshore wind generation. Simulation indicates that the advanced small-signal model can successfully follow the electromechanical transient response with small errors and can predict the damped oscillations. The validity and applicability of the proposed model are effectively confirmed.

• 한국항해항만학회지
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• 제35권7호
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• pp.575-580
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• 2011

It is important to understand the trajectory of structure in launching process because of the short time of launching process may result in unexpected accidents or damage to structures. The high risk of structural failure is not avoidable without the fully comprehension of changing forces in launching procedure. The commercial software can evaluate the motion of launching event in calm water condition but there is the limitation of research application because of the programmed commercial software. The launching process of the spar hull is suggested with stage concept that is divided into 10 stages in time domain. A force equilibrium diagram is derived for each stage where the changes of force vector and motion characteristics take place. In particular, the effects of changes in buoyancy and drag force due to the progressive submergence of the spar hull are taken into account by means of a touch length concept. The results contained in this paper provide the valuable information of the trajectory motion evaluation with suggested methods in spar launching process with sliding barge. Furthermore, the presented stage concept and touch length concept will provide basic knowledge for understanding launching process and help to develop further research area for launching analysis.

• 한국해양공학회지
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• 제35권4호
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• pp.287-295
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• 2021

This paper describes a recurrent neural network (RNN) for the fault classification of a blade pitch system of a spar-type floating wind turbine. An artificial neural network (ANN) can effectively recognize multiple faults of a system and build a training model with training data for decision-making. The ANN comprises an encoder and a decoder. The encoder uses a gated recurrent unit, which is a recurrent neural network, for dimensionality reduction of the input data. The decoder uses a multilayer perceptron (MLP) for diagnosis decision-making. To create data, we use a wind turbine simulator that enables fully coupled nonlinear time-domain numerical simulations of offshore wind turbines considering six fault types including biases and fixed outputs in pitch sensors and excessive friction, slit lock, incorrect voltage, and short circuits in actuators. The input data are time-series data collected by two sensors and two control inputs under the condition that of one fault of the six types occurs. A gated recurrent unit (GRU) that is one of the RNNs classifies the suggested faults of the blade pitch system. The performance of fault classification based on the gate recurrent unit is evaluated by a test procedure, and the results indicate that the proposed scheme works effectively. The proposed ANN shows a 1.4% improvement in its performance compared to an MLP-based approach.