This paper presents a frequency support strategy for the diode rectifier unit (DRU)-high-voltage direct current (HVDC)-based offshore wind power integration system, which coordinates multiple power sources without communication to reduce receiving grid frequency fluctuations. First, based on the deduced DRU's frequency transfer characteristic, a fine-designed ripple carrying frequency information is superimposed on the HVDC link, transferring the onshore frequency to offshore wind turbines (WTs) via the DC ripple and coupled AC harmonic without communication. Second, multiple power sources are utilized for frequency support, including HVDC capacitance and grid-forming WTs combined with energy storage systems, and appropriate sources are activated in the order specified by the designed thresholds. Finally, the effectiveness of the proposed frequency support strategy is verified by simulations in PSCAD/EMTDC.

Offshore wind power in deep and far offshore areas is one of the important sources of power supply increment along the eastern coastal regions of China. However, with the gradual increase in the grid-connected renewable energy represented by offshore wind power, the trend of “hollowing out” in the power system has become more pronounced. In order to provide solid and reliable frequency support for the onshore main AC power grid, this paper innovatively proposes a grid-friendly control strategy based on the system where offshore wind power is transmitted through a medium-frequency MMC. This paper first introduces the system structure and basic principles, then proposes a communication-less main AC grid frequency support control strategy, and conducts simulation verification in PSCAD. The research results show that this strategy can significantly reduce the frequency deviation of the onshore power grid and enhance the stability and resilience of the grid.

This study analyzes the stability and reactive characteristics of the hybrid offshore wind farm that includes gridforming(GFM) and grid-following(GFL) wind turbines(WTs) integrated with a diode rectifier unit(DRU) based high-voltage direct current(HVDC) system. The determination method for the proportion of GFM WTs is proposed while considering system stability and optimal offshore reactive power constraints. First, the small-signal stability is studied based on the developed linear model, and crucial factors that affect the stability are captured by eigenvalue analysis. The reactive power-frequency compensation control of GFM WTs is then proposed to improve the reactive power and frequency dynamics. Second, the relationship between offshore reactive power imbalance and the effectiveness of GFM capability is analyzed. Offshore reactive power optimization methods are next proposed to diminish offshore reactive load. These methods include the optimal design for the reactive capacity of the AC filter and the reactive power compensation control of GFL WTs. Third, in terms of stability and optimal offshore reactive power constraints, the principle and calculation method for determining the proportion of GFM WTs are proposed, and the critical proportion of GFM WTs is determined over the full active power range. Finally, case studies using a detailed model are conducted by timedomain simulations in PSCAD/EMTDC. The simulations verify the theoretical analysis results and the effectiveness of the proposed determination method for the proportion of GFM WTs and reactive power optimization methods.

The VSC-HVDC transmission systems are the typical solution for integrating far offshore wind power. Using diode rectifier units （DRUs） at the rectifier stations can further enhance the system’s economic efficiency and reliability. Therefore， a steady-state power flow calculation method of AC system transmission system for offshore wind power， based on grid-forming （GFM） wind turbines and DRU， is proposed. The topology of the offshore wind power integration system using DRU is described， and the GFM control strategy of the wind turbines， utilizing Q-f（reactive power-frequency） droop control， is introduced. The proposed steady-state power flow calculation method iterates the power flow using the reactive power imbalance in AC bus at the DRU-based converter station and considers the Q-f droop control characteristics of the GFM wind turbines. The steady-state operational characteristics of AC transmission systems for offshore wind power， based on GFM wind turbines and DRU， are analyzed for both the low-frequency and intermediate-frequency AC transmission systems. The effectiveness of the proposed method is validated.

The diode rectifier unit can improve the economy and reliability of the long-distance large-capacity offshore wind power DC transmission system.This paper presents an optimal configuration method for AC filters in offshore wind power integration systems based on the 24-pulse diode rectifier unit.Firstly,the structure of the offshore wind power transmission system based on the 24-pulse diode rectifier unit is defined,and the basic principle of the 24-pulse diode rectifier unit is expounded.Then,the reactive power requirement of the 24-pulse diode rectifier unit is calculated.For the 24-pulse diode rectifier unit,the complicated AC filter can be simplified into a capacitor.By scanning the filtering effect of capacitors with different capacities in the range of active power variation,the reactive power capacity optimization method of the capacitor is proposed,which takes the filtering effect and the filter capacity into consideration.Finally,in PSCAD/EMTDC,the harmonic analysis and AC filter optimization design of the offshore wind power test system based on the 24-pulse diode rectifier unit are carried out.

This research proposes an efficient configuration method for the AC filter in an offshore wind farm via a 24-pulse diode rectifier unit. First, the fundamental theory of the 24-pulse diode rectifier unit is explained, along with the construction of the offshore wind power delivery system via this unit. Second, the transmitted active power variation is taken into account while calculating the reactive power needed by the 24-pulse diode rectifier unit. The structure of the AC filter is simplified from the single-tuned or double-tuned filter to a capacitor. Then, the optimization method of the reactive capacity of the AC filter is proposed with the comprehensive consideration of the filtering effect and the filter capacity under full active power variation. Lastly, the offshore wind test system based on a 24-pulse diode rectifier unit is designed with the AC filter optimized, and the harmonic voltage is examined in PSCAD/EMTDC to confirm the efficacy of the suggested AC filter configuration technique.

Abstract The diode rectifier unit (DRU) is promising in offshore wind power integration due to its reliability and economy. Offshore AC voltage control is the key technology of the DRU‐based scheme. In this paper, a low‐cost offshore wind power integration system based on the DRU and the modular multilevel converter (MMC) in parallel on the rectifier side is proposed. The low‐capacity MMC rectifier is controlled to establish the offshore AC voltage amplitude and frequency, maintain all offshore wind active power transmitted by the DRU, and provide reactive power compensation and AC harmonic filtering for the DRU. Besides, the fault ride‐through strategies of both the offshore fault and the onshore fault are proposed. Moreover, the calculation method of the rated capacity of the MMC rectifier is derived. The time domain simulations of the hybrid system under typical operating conditions are investigated in PSCAD/EMTDC, and the feasibility of the proposed scheme is verified.

Offshore wind farms are the main trend of future wind power exploitation. The diode rectifier unit (DRU) shows great potential in offshore wind power integration due to its economy and reliability benefits, and stable AC voltage control of the offshore gird is the key technology of the DRU based scheme. In this paper, the sensitivities of the DRU based system are defined and the sensitivity calculation method is proposed. According to the sensitivity characteristics analysis, the relationship between the wind turbine (WT) active power (or reactive power) and the WT output voltage (or system frequency) is clarified. On this basis, a grid-forming control strategy for WT converters is proposed. The stable operation mechanism of the grid-forming WTs in the DRU based system is demonstrated according to the analysis of steady state operation point existence and small disturbance stability. The time domain simulations of two DRU based offshore wind farm integration systems are carried out in PSCAD/EMTDC, and the feasibility of the proposed grid-forming control is verified.

The modular multilevel matrix converter (M3C) is the core component in low-frequency AC (LFAC) transmission, which is a competitive scheme for offshore wind power integration. In this paper, the M3C control strategy with the reduced switching frequency SM voltage balancing method is proposed. First, based on the conventional αβ0 and dq transformations, the M3C mathematical model is derived. Then, the dual-loop control structure with outer loop and inner loop controllers commonly used in voltage source converters is applied to the M3C. The inner loop controller consists of the current tracking controller in the dq reference frame and the circulating current suppressing controller in the αβ0 reference frame; the outer loop controller is proposed for the offshore wind farm LFAC integration scenario. Additionally, according to the operating characteristics of full-bridge sub-modules (FBSMs), three characteristic variables are defined and a reduced switching frequency SM voltage balancing method based on the nearest level control (NLC) is proposed. Finally, time-domain simulations in PSCAD/EMTDC demonstrate the feasibility of the proposed control strategy.

Focusing on the three technical features related to the offshore wind turbine and the offshore transmission channel, eight schemes of offshore wind power transmission and their corresponding key technologies are discussed. The first technical feature describes the grid-forming capability of the offshore wind turbine; two types of wind turbines are discussed, namely, the grid-following wind turbine and the grid-forming wind turbine. The second technical feature describes the frequency characteristics of the output voltage and current of the offshore wind turbine, including DC, low-frequency AC, power-frequency AC and medium-frequency AC. The third technical feature describes the power transmission modes adopted by the offshore high-voltage main transmission channel, which are HVDC, high-voltage low-frequency AC and high-voltage power-frequency AC. Finally, the technical characteristics and maturity of the eight offshore wind power transmission schemes are reviewed, and a technical development direction is recommended.

The low-frequency AC (LFAC) transmission is a competitive scheme for offshore wind power integration, in which the modular multilevel matrix converter (M 3 C) is the key equipment. In this brief, the selection methods of the M 3 C main circuit parameters are proposed. First, the maximum arm voltage is derived and the selection method of the sub-module (SM) number is established. Then, according to the switching function approach, the analytical expressions of the average SM capacitor current and the average SM capacitor voltage are derived. On this basis, the selection method of the SM capacitance is proposed. Additionally, the design principle of the arm inductance is proposed. Finally, a case is elaborated to illustrate the proposed M 3 C main circuit parameter selection methods and provide a reference for practical projects.

Nowadays, the high voltage direct current (HVDC) transmission system is the typical scheme for long-distance offshore wind power integration. The diode rectifier unit (DRU) can further improve the economy and reliability of the HVDC system. In this paper, the steady state characteristics of the offshore wind power AC system based on the grid-forming wind turbine (WT) and the DRU are analyzed. First, the topology of the DRU based offshore wind power integration system is introduced, and the grid-forming WT control strategy with the reactive power-frequency (Q-f) droop is elaborated. Then, the mathematical model of the DRU station is derived, and the power flow equations are expressed. On this basis, the steady state power flow calculation method of the offshore wind power AC system is proposed. The power flow iteration is according to the unbalanced reactive power of the DRU station AC bus, and the Q-f droop control characteristics of the grid-forming WT are considered. Finally, the steady state characteristics of the offshore wind power AC system are analyzed in the DRU based case system.

远海风电场采用跟网型风机配柔直送出系统是已得到广泛应用的典型远海风电送出方案,但是目前存在海上换流站平台造价较高的问题.跟网型海上风电中频汇集柔性直流送出方案可以减小换流器的体积重量和造价,是提高工程经济性的一种有竞争力的方案.针对跟网型海上风电中频汇集柔性直流送出系统的最优频率选择开展研究,首先计算了频率变化时海缆的单位长度参数和载流量,基于海缆典型接线研究了不同频率下海缆的选型方案.然后分别计算了不同运行频率下集电系统功率损耗和海上换流平台阀损耗.最后,分别计算了集电系统功率损耗和海上换流平台阀损耗的折现电价、集电系统和海上换流平台的投资成本,基于经济性综合结果表明最优频率位于150～200Hz范围内.

The high voltage alternating current transmission with submarine cables is the common scheme to integrate offshore wind farms close to the land. Due to the capacitive effect of AC cables, the association of the long AC cables and the onshore grid can cause resonances, which may amplify grid background harmonics. This paper focuses on the mitigation of background harmonic amplification caused by offshore wind farm integration. First, the background harmonic amplification mechanism in the offshore wind farm integration system is analysed qualitatively, and the mitigation approaches are given in principle. Then, taking a practical project as a study case, the harmonic modelling methods of the offshore grid and the onshore grid are elaborated, including the harmonic models of wind turbines, transformers, cables, the high voltage and the extra-high voltage transmission grid, and the low voltage distribution network. According to the established harmonic models, the cause of background harmonic amplification is analysed. On this basis, two mitigation methods of installing the series blocking filter and the shunt filter are compared and the recommended mitigation scheme is proposed. Finally, the feasibility of the proposed scheme is verified by calculation results under different grid operating conditions.

由于相同电压等级下电力电缆的电容效应比架空线路大出20倍以上,使得一般在陆上风电场并网中并不严重的谐波谐振放大问题,在海上风电场并网中可能会变得十分严重,为此研究了海上风电场并网引起的谐波谐振放大问题及其治理原理.首先,对海上风电场并网时的谐波谐振放大机理进行了分析,从原理上给出了治理的技术途径;然后,以某海上风电并网实际工程为研究案例,分别建立了海上风电电网和陆上电网的谐波模型,包括电缆、变压器和风电机组的谐波模型以及超高压大电网的等效谐波模型和低压配电网的等效谐波模型,并根据所建立的谐波模型分析了发生谐波谐振放大的原因,提出了解决谐波谐振放大问题的治理方案并进行了验算;最后,对海上风电场并网引起的谐波谐振放大问题的机理和治理原理进行了总结.

随着社会发展和科技进步,道路交通事故日益严重,已成为严重威胁人类生命财产安全的社会问题.本文分写了道路交通事故鉴定中小型普通客车与自行车碰撞典型案例.

随着海上风电的大规模开发,由海上风电接入电网引起的谐波问题越来越突出,迫切需要进行建模和分析,而在谐波频段对海底电缆进行建模是其中的一项重要工作.研究了海底电缆谐波频段电气参数的计算方法,为海底电缆建模提供了依据.首先给出了电缆电气参数的定义,然后总结了海底电缆电气参数的计算流程和计算公式,并对计算公式的适用性进行了论证,最后对3种典型海底电缆的电气参数进行了计算.结果表明,无论是电缆的正序参数还是零序参数,都会随频率有较大幅度的变化,在进行谐波分析时必须考虑电缆电气参数随频率变化的特性.

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分析了多元联动、五位一体"高职院校国际化汽修人才培养模式的相关含义以及该人才培养模式的特点,指出应通过校企合作产教深度融合人才培养模式的深入实践培养国际化汽修人才,以满足2025智能制造的需求.

本文根据对沈阳市交通现状的分析,总结了沈阳市智慧公共交通存在的主要问题,并提出了关于发展智慧交通的几点建议.