Renewable Energy

Renewable Energy is energy generated from natural resources - such as sunlight, wind, rain, tides and geothermal heat - which are naturally replenished. In 2008, about 18% of global final energy consumption came from renewables, with 13% coming from traditional biomass, such as wood burning. Hydroelectricity was the next largest renewable source, providing 3% (15% of global electricity generation), followed by solar hot water/heating, which contributed with 1.3%. Modern technologies, such as geothermal energy, wind power, solar power, and ocean energy together provided some 0.8% of final energy consumption. The book provides a forum for dissemination and exchange of up - to - date scientific information on theoretical, generic and applied areas of knowledge. The topics deal with new devices and circuits for energy systems, photovoltaic and solar thermal, wind energy systems, tidal and wave energy, fuel cell systems, bio energy and geo-energy, sustainable energy resources and systems, energy storage systems, energy market management and economics, off-grid isolated energy systems, energy in transportation systems, energy resources for portable electronics, intelligent energy power transmission, distribution and inter - connectors, energy efficient utilization, environmental issues, energy harvesting, nanotechnology in energy, policy issues on renewable energy, building design, power electronics in energy conversion, new materials for energy resources, and RF and magnetic field energy devices

Harmonic domain modelling and analysis of the electrical power systems of onshore and offshore oil and gas field /platform

This thesis first focuses on harmonic studies of high voltage cable and power line, more specifically the harmonic resonance. The cable model is undergrounded system, making it ideal for the harmonics studies. A flexible approach to the modelling of the frequency dependent part provides information about possible harmonic excitations and the voltage waveform during a transient. The power line is modelled by means of lumped-parameters model and also describes the long line effect. The modelling depth and detail of the cable model influences the simulation results. It compares two models, first where an approximate model which make use of complex penetration is used and the second where an Bessel function model with internal impedance is used. The both models incorporate DC resistance, skin effect and their harmonic performances are investigated for steady-state operating condition. The methods illustrate the impotance of including detailed representation of the skin effect in the power line and cable models, even when ground mode exists. The cable model exhibit lower harmonics comparable to overhead transmission lines due to strong influence of the ground mode. Due to the application of voltage source converter (VSC) technology and pulse width modulation (PWM) the VSC-HVDC has a number of potential advantages as compared with CSC-HVDC, such as short circuit current reduction, independent control of active power and reactive power, etc. With these advantages VSC-HVDC will likely be widely used in future oil and gas transmission and distribution systems. Modular multilevel PWM converter applies modular approach and phase-shifted concepts achieving a number of advantages to be use in HVDC power transmission. This thesis describes the VSC three-phase full-bridge design of sub-module in modular multilevel converter (MMC). The main research efforts focus on harmonic reduction using IGBTs switches, which has ON and OFF capability. The output voltage waveforms multilevel are obtained using pulse width modulation (PWM) control. The cascaded H-bridge (CHB) MMC is used to investigate for two-level, five-level, seven-level, nine-level converter staircase waveforms. The results show that the harmonics are further reduced as the sub-module converter increases. The steady-state simulation model of the oil platform for harmonic studies has been developed using MATLAB. In order to save computational time aggregated models are used. The load on the platforms consists of passive loads, induction motors, and a constant power load representing variable speed drives on the platforms. The wind farm consists of a wind turbine and an induction machine operating at fixed speed using a back-to-back VSC. Simulations are performed on system harmonics that are thought to be critical for the operation of the system. The simulation cases represent large and partly exaggerated disturbances in order to test the limitations of the system. The results show low loss, low harmonics, and stable voltage and current. With the developments of multilevel VSC technology in this thesis, multi-terminal direct current (MTDC) systems integrating modular multilevel converters at all nodes may be more easily designed. It is shown that self-commutated Voltage Source Converters (VSC) is more flexible than the more conventional Current Source Converter (CSC) since active and reactive powers are controlled independently. The space required by the equipment of this technology is smaller when compared to the space used by the CSCs. In addition, the installation and maintenance costs are reduced. With these advantages, it will be possible for several oil and gas production fields connected together by multi-terminal DC grid. With this development the platforms will not only share energy from the wind farms, but also provide cheaper harmonic mitigation solutions. The model of a multi-terminal hypothetical power system consisting of three oil and gas platforms and two offshore wind farm stations without a common connection to the onshore power grid is studied. The connection to the onshore grid is realized through a High Voltage Direct Current (HVDC) transmissions system based on Voltage Source Converter (VSC) technology. The proposed models address a wide array of harmonic mitigation solutions, i.e., (i) Local harmonic mitigation (ii) semi-global harmonic mitigation and (iii) global harmonic mitigation. In addition, a computationally-efficient technique is proposed and implemented to impose the operating constraints of the VSC and the host IGBT-PWM switches within the context of the developed harmonic power flow (HPF). Novel closed forms for updating the corresponding VSC power and voltage reference set-points are proposed to guarantee that the power-flow solution fully complies with the VSC constraints. All the proposed platform models represent (i) the high voltage AC/DC and DC/AC power conversion applications under balanced harmonic power-flow scenario and (ii) all the operating limits and constraints of the nodes and its host modular converter (iii) three-phase VSC coupled IGBT-PWM switches

A Switching Frequency Reduction Strategy of Modular Multilevel Converter for VSC-HVDC

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2013. 8. 설승기.본 논문에서는 모듈형 멀티레벨 컨버터가 고압 직류 전송 시스템에 적용될 때, 필수적으로 요구되는 스위칭 저감 방법을 제안한다. 모듈형 멀티레벨 컨버터는 경제성 및 동작의 연속성을 보장하는 모듈화 구조를 극대화시킨 시스템으로 현재 주요 제조사에 의해 상용화되어 고압 직류 전송 시스템에 실제 적용되고 있다. 모듈형 멀티레벨 컨버터는 직류단 전압에 비례하여 셀의 개수가 증가하기 때문에 고압 직류 전송 시스템의 운전 효율을 높이기 위해서 평균 스위칭 주파수를 낮추는 방법이 필수적이다. 본 논문에서는 개별 셀 캐패시터 전압 정보를 이용한 새로운 전압 합성 방법을 제안한다. 고압 직류 전송 시스템에 적용되는 모듈형 멀티레벨 컨버터는 운전 효율을 위해 낮은 스위칭 주파수에서 동작해야 한다. 하지만, 스위칭 주파수가 낮아질수록 셀 캐패시터 전압 맥동이 증가하여, 기존 전압 합성 방법을 사용할 경우 그 전압 합성 오차가 커지게 된다. 이를 위해 본 논문에서는 스위칭 주파수에 영향을 받지 않는 전압 합성 방법을 제안한다. 스위칭 주파수가 감소할수록 개별 셀 캐패시터 전압의 맥동은 증가하게 된다. 이를 억제하기 위해서 큰 용량의 셀 캐패시터가 사용되어야 한다. 이는 전체적인 초기 구성 비용의 증가를 의미하므로 본 논문에서는 별도의 2고조파 순환 전류 주입을 제안한다. 본 논문에서는 모듈형 멀티레벨 컨버터의 전도 손실과 스위칭 손실을 구하기 위한 방법을 제안한다. 손실 분석을 통해 모의한 모듈형 멀티레벨 컨버터의 효율, 스위칭 주파수 및 셀 캐패시터 용량을 선정하였다. 또한, 제안한 2고조파 순환 전류 주입 방법으로 인한 손실이 기존 방법과 큰 차이를 보이지 않음을 확인하였다. 모의한 시스템에 제안한 2고조파 순환 전류 주입을 사용할 때, 기존 방법보다 0.05 % 감소된 99.25 % 의 효율로 운전하면서 33 %의 셀 캐패시터 용량이 저감될 수 있음을 보였다. 제안된 방법들의 유효함을 입증하기 위해 400kV의 직류단 전압, 암 당 셀의 개수가 220개인 400MVA 모듈형 멀티레벨 컨버터를 모의하여 결과를 분석하였다, 또한 암 당 셀의 개수가 6 개인 축소형 실험 장치를 이용하여 제안된 방법들의 결과를 분석하여 그 타당성을 실험적으로도 검증하였다.This paper describes a switching frequency reduction method for the application of the MMC in HVDC systems. Because of modularity of the MMC, it has several advantages such as cost effectiveness and fault tolerant features, which is critical to HVDC transmission. Major electric manufacturers in the world have already commercialized MMC and applied it in several HVDC transmission lines. As the number of cell in a MMC increases in proportion to the DC-link voltage, the reduction of the average switching frequency of each cell is critical to enhance the operating efficiency of MMC. In this thesis, a novel voltage synthesis method is proposed based on the voltage of the individual cell capacitors. As the average switching frequency of MMC is getting reduced for the higher operating efficiency, the voltage ripple of each cell capacitor is getting larger. To overcome this problem, a voltage synthesis method which is not influenced by the switching frequency has been proposed and discussed. To reduce the voltage ripple of the cell capacitor, larger capacitance of the cell capacitor is usually used at higher material cost. To lessen this issue, additional 2nd order harmonic circulating current injection method is suggested in this thesis. In addition, the conduction and the switching losses of MMC have been analytically derived. Through the loss analysis, the efficiency of the MMC can be calculated and the switching frequency and the capacitance of the cell capacitor optimized. Moreover, it is identified that the 2nd order harmonic circulating current injection does not incur severe additional losses. The 2nd order harmonic circulation current injection is found to lead to the reduction of the cell capacitance by 33% compared to the conventional method with efficiency degradation of 0.05 %. To validate the effectiveness of the proposed methods, the 400 MVA MMC system at 400kV HVDC link consisted with 220 cells/arm has been simulated. For the experimental proof, a reduced scale version of MMC consisted with 6 cell/arm has been implemented. With the scaled version, the proposed switching frequency reduction method, voltage synthesis method and the 2nd order harmonic circulating current injection method have been verified by the experimental results.초록 i 목차 iii 제 1 장 서론 1 1.1 연구 배경 1 1.2 연구 목적 6 1.3 논문의 구성 7 제 2 장 모듈형 멀티레벨 컨버터 9 2.1 대표적인 멀티레벨 컨버터 11 2.1.1 NPC 컨버터 11 2.1.2 FC 컨버터 14 2.2 모듈형 멀티레벨 컨버터(Modular Multilevel ConverterMMC) 19 2.2.1 MMC의 구조 21 2.2.2 하위 모듈(sub module)의 구성 및 동작 25 2.2.3 MMC 시스템의 모델링 38 2.2 MMC 시스템의 전압 지령 생성 43 2.3.1 극 전압 합성을 위한 전압 지령 생성 43 2.3.2 순환 전류 제어를 위한 암 공통 전압 지령 생성 44 2.3.3 상 하단 암 전압 지령 생성 45 제 3 장 모듈형 멀티레벨 컨버터의 제어 방법 48 3.1 MMC 제어기 설계 48 3.1.1 암 캐패시터 전력 분석 48 3.1.2 레그 평균 전압 제어기 51 3.1.3 암 균형 전압 제어기 설계 54 3.1.4 순환 전류 제어기 설계 58 3.1.5 부하 전류 제어기 설계 63 3.2 전압 합성 방법 및 셀 간 균형 제어 67 3.2.1 위상 천이 PWM (phase-shifted PWMPSPWM) 67 3.2.2 레벨천이 PWM (level-shifted PWMLSPWM) 69 3.2.3 PWM 방법에 따른 선간 전압 고조파 특성 비교 73 3.3 셀 캐패시터 전압 맥동 저감 제어 79 3.3.1 2고조파 순환 전류 주입을 통한 전압 맥동 저감 제어 79 3.3.2 고주파 옵셋 전압 및 순환 전류 주입 88 제 4 장 HVDC 시스템 적용을 위한 모듈형 멀티레벨 컨버터의 스위칭 저감 및 전압 합성 방법 95 4.1 전압 변조 방법 96 4.1.1 선택적 고조파 제거(SHE) 변조 방법 97 4.1.2 근사 계단 변조(NLM) 방법 105 4.2 스위칭 주파수 저감 방법 108 4.2.1 스위칭 주파수 저감 방법 검증을 위한 개별 셀 캐패시터 전하량 계산 108 4.2.2 가상 캐패시터 전압을 이용한 스위칭 주파수 저감 방법 111 4.3 셀의 최대 전압 저감 방법 123 4.3.1 2고조파 순환 전류 주입 127 4.3.2 가상 전압을 활용한 최대 전압 변동량 저감을 위한 방법 135 4.4 제안 전압 합성 방법 141 4.4.1 제안 전압 합성 방법 141 4.4.2 개별 셀 사고 시, 제안 전압 합성 방법의 동작 151 4.5 모의 실험 결과 157 제 5 장 실험 결과 185 5.1 실험 장치의 구성 185 5.2 제안 스위칭 주파수 저감 방법 검증 189 5.3 제안 전압 합성 방법 검증 191 5.4 제안한 셀 캐패시터 최대 전압 저감 방법 검증 204 5.5 손실 분석 211 제 6 장 HVDC 시스템 운용을 위한 모듈형 멀티레벨 컨버터의 스위칭 주파수 및 셀 캐패시터 용량 선정 217 6.1 MMC의 손실 계산 방법 217 6.1.1 전도 손실 계산 방법 217 6.1.2 스위칭 손실 계산 방법 221 6.1.3 셀 캐패시터의 손실 226 6.1.4 게이트 드라이버(gate drive)의 손실 227 6.2 셀 캐패시터 전압 맥동 저감 방법에 따른 손실 분석 230 6.3 MMC의 동작 스위칭 주파수 및 셀 캐패시터 용량 선정 249 제 7 장 결론 및 향후 과제 257 7.1 연구 결과 257 7.2 향후 과제 261 참고 문헌 266 ABSTRACT 280Docto

Modelling and control of multi-terminal HVDC networks for offshore wind power generation

Due to the recent developments in semiconductors and control equipment, Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) becomes a promising technology for grid connection of large offshore wind farms. The VSC-HVDC provides a number of potential advantages over the conventional HVDC, such as rapid and independent control of reactive and active power, black-start capability and no restriction on multiple infeeds. Therefore, VSC-HVDC will likely to be widely used in the future transmission networks and for offshore wind power connections. Multi-terminal VSC-HVDC (VSC-MTDC) system, which consists of more than two voltage source converter stations connecting together through a DC link, is able to increase the flexibility and reliability of transmission systems. It allows connection of multiple offshore wind farms to the AC grid. In this thesis, a three-terminal MTDC system was investigated using simulations and experiments. MTDC system with its control was implemented in PSCAD/EMTDC. The control strategy developed through simulation was verified using experiments. The results of PSCAD/EMTDC simulation and laboratory demonstration were then compared. Additionally, a scenario of four-terminal MTDC transmission system for Modelling and Control of Multi-Terminal HVDC Networks for Offshore Wind Power Generation IV offshore wind power generation was investigated. A control system was designed considering the operating characteristics of VSCs and wind farms. An open loop control method was used for the wind farm side VSCs to establish a constant AC voltage and frequency. Droop control was used for the grid side VSCs to generate DC voltage reference by measuring the DC current. When the system was under fault operation condition, the output power of wind farm was reduced by reducing the DC voltage reference. Simulation results show that good coordination was achieved among VSCs for voltage control and power sharing. The system is able to recover to the normal operation status automatically when subjected to AC balanced fault (three phase fault) and unbalanced fault (single phase fault) on the grid. Keywords: control system, modelling, MTDC, Multi-terminal, offshore, VSC-HVDC, wind power generatio

Power control, fault analysis and protection of series connected diode rectifier and VSC based MTDC topology for offshore application.

A multiterminal high-voltage dc (MTDC) system is a promising method for transmitting energy generated from an offshore windfarm (OWF). The creation of MTDC systems became easier by the introduction of voltage source converter (VSC) due to the flexibility and controllability it provides. This technology is newer than the line-commutated converter technology (LCC). Power systems can include any number of windfarms together with converters for both offshore and onshore power conversion. Therefore, this thesis suggests a three-terminal MTDC model of two offshore windfarms and one onshore inverter. The electric energy generated by the two windfarms is rectified into dc and transmitted to the shore using dc cable. Although a VSC or a diode rectifier (DR) can convert ac to dc, a series connection of a VSC and two DRs was proposed at the windfarm side to convert the generated power to achieve controllability of the uncontrollable diode rectifiers and reduces the high cost of badditional VSCs. The proposed topology converts the ac power by dividing the windfarm power so that one-third is the share of the VSC and two-thirds is the share of the DRs. The same topology is used to convert the power produced from the other windfarm. Then, the dc power is transmitted via an undersea dc cable to the onshore location, and is then inverted into ac before it is supplied to the neighbouring ac grid using a grid-side VSC. The proposed topology has many advantages, including a significant save in windfarm VSC (WFVSC) capital cost and a significant reduction in the loss of power of the converter without losing the overall controllability. However, although this topology is suitable for windfarm applications, it might not be suitable for high-voltage direct current (HVDC) that requires bidirectional power flow unless making changes to the topology such as disconnecting the diode rectifiers. Furthermore, fault analyses were investigated, including dc faults and ac faults. Ac faults are categorised as symmetrical or unsymmetrical faults. For comparison purposes, a Simulink model was designed, implemented, and simulated as a reference model. The reference model can operate as VSC-, DR-based MTDC, or a mix of both in a way that any component can be added to or removed from the model at any time during the simulation run. The contribution to the dc fault current from various parts such as dc capacitor and the adjacent feeder was investigated thoroughly, and detailed mathematical formulae were developed to compute fault current from these contributors. In addition, the results of the system response due to both fault types are illustrated and discussed. Both symmetrical and unsymmetrical ac faults were initiated on the onshore grid side, and the system response results are presented for those faults. A generalised control scheme (GCS) was proposed in this thesis, which add the ability the model to control the reactive power and is suitable for both balanced and unbalanced ac faults conditions. A protection against faults was investigated and implemented using dc circuit breakers. The protection system was built to ensure safe operation and to fulfil the grid code requirements. Many grid codes are available and presented in the literature, such as Spanish, British, and Danish; however, a grid code by E.ON was chosen. The protection scheme in VSC-based MTDC networks plays a vital role during dc faults. It is vital that this protection be sensitive, selective, fast, and reliable. Specifically, it must isolate the fault reliably from the system within a short time after the fault occurrence, while maintaining the remaining components of the system in a secure operational condition. For optimal performance, the protection scheme discussed in this thesis employs solid-state circuit breakers. A literature survey relevant to the tasks mentioned above was conducted.PhD in Energy and Powe

A study of the harmonic content of distribution power grids with distributed PV systems

A photovoltaic system transforms solar radiation into electrical energy using so-called PV panels. A key component of this system is the power electronics subsystem which enables maximum power extraction from the available solar irradiation, as well as con-nection to the AC power grid. However, the current and voltage waveforms at the point of common coupling (PCC) with the power grid contain a degree of harmonic distortion which, in some instances, may surpass that recommended by existing standards. The presence of high harmonic distortion in an electrical installation significantly decreases power quality and the renewable energy sources’ power electronics carries the potential to yield high harmonic distortion. This thesis reports on an investigation of some of the factors that impact adversely the quality of the current and voltage waveforms in an electrical power distribution network with distributed photovoltaic systems. These factors include irradiance levels, imperfect conditions of the filtering system, resonant conditions, load imbalances and selection of the inverter’s switching frequency. To quantify current and voltage harmonic injections, a two-stage model of a photovoltaic array was designed in Simulink in order to show the impact of a single photovoltaic system. The basic PV system model is then applied to a model of an electrical power distribution grid, with several distributed PV units. The study indicates that irradiance is the primary factor influencing THD and that at low PV power outputs, harmonic emissions may exceed the recommended harmonic distortion limits, particularly when resonant conditions exist at the output of connection of the PV plant. Different MPP control methods employed in the DC-DC conversion stage were also investigated and it is observed that they do not seem to have much impact on THD. This applies in the absence of partial shading, an issue which was not considered as part of this research project. As expected, the use of well-designed filters is the key to keeping harmonics emissions low. Nevertheless, perfect filtering does not exist in actual installations and the study also investigates the impact of imperfect filtering parameters and filter branch failure, on the voltage and current waveforms at PCC

Applications of MATLAB in Science and Engineering

The book consists of 24 chapters illustrating a wide range of areas where MATLAB tools are applied. These areas include mathematics, physics, chemistry and chemical engineering, mechanical engineering, biological (molecular biology) and medical sciences, communication and control systems, digital signal, image and video processing, system modeling and simulation. Many interesting problems have been included throughout the book, and its contents will be beneficial for students and professionals in wide areas of interest