混合直流输电控制策略研究

为充分利用传统高压直流输电(LCC-HVDC)与柔性直流输电(VSC-HVDC)各自的优点,建立了整流侧采用LCC换流站,逆变侧采用VSC换流站的混合直流(hybrid HVDC)输电系统。对直流系统两侧换流站主控制器进行了设计,并针对该系统设计出双侧功率/频率调制的辅助控制器。最后对混合直流输电系统与交流输电线路并行输电和单独的混合直流输电系统进行了仿真与分析,通过仿真验证了当系统受到扰动时所加辅助控制器能提高系统的稳定性。     

多馈入直流输电系统的辅助频率协调控制

在高压直流(HVDC)输电系统上加装辅助频率控制器具有显著的经济效益.首先阐述了HVDC功率调制的基本原理及辅助频率控制的实现方法,然后提出了一种基于PID控制的多回HVDC辅助频率协调控制器.采用一个3机2直流系统对该控制器进行了时域仿真计算.仿真结果表明,加装了辅助频率协调控制器的多回HVDC系统能在任一回HVDC输电线路两侧交流系统发生大扰动时,实现两侧系统间功率的相互紧急支援,显著改善了两侧交流系统频率特性.     

华中—华东多回HVDC辅助功率/频率控制

在华中-华东多回高压直流(HVDC)输电系统上加装辅助功率/频率控制器具有显著的经济效益。首先阐述了HVDC功率调制的基本原理,然后给出了常规辅助功率/频率控制方案,并通过大规模数字仿真和动模试验验证了该方案的可行性。研究结果显示,加装了辅助功率/频率控制器的华中-华东多回HVDC系统能在两侧交流系统发生大扰动时,实现两侧系统间功率的相互紧急支援,显著改善两侧交流系统频率,并可在多回直流输电线路中实现功率的平稳转移,明显提高了整个华中、华东交直流系统的安全性和经济性。     

华中—华东多回HVDC辅助功率/频率控制

在华中—华东多回高压直流（HVDC）输电系统上加装辅助功率/频率控制器具有显著的经济效益。首先阐述了HVDC功率调制的基本原理，然后给出了常规辅助功率/频率控制方案，并通过大规模数字仿真和动模试验验证了该方案的可行性。研究结果显示，加装了辅助功率/频率控制器的华中—华东多回HVDC系统能在两侧交流系统发生大扰动时，实现两侧系统间功率的相互紧急支援，显著改善两侧交流系统频率，并可在多回直流输电线路中实现功率的平稳转移，明显提高了整个华中、华东交直流系统的安全性和经济性。     

华中一华东多回HVDC辅助功率／频率控制

在华中一华东多回高压直流(HVDC)输电系统上加装辅助功率／频率控制器具有显著的经济效益。首先阐述了HVDC功率调制的基本原理，然后给出了常规辅助功率／频率控制方案，并通过大规模数字仿真和动模试验验证了该方案的可行性。研究结果显示，加装了辅助功率／频率控制器的华中一华东多回HVDC系统能在两侧交流系统发生大扰动时，实现两侧系统间功率的相互紧急支援，显著改善两侧交流系统频率，并可在多回直流输电线路中实现功率的平稳转移，明显提高了整个华中、华东交直流系统的安全性和经济性。     

基于互调原理的交直交变流系统中的间谐波分析

基于现代调制理论，首先对整流器和逆变器进行统一调制建模，通过分析其交直流侧的频率变换关系，揭示交流侧谐波、间谐波的产生过程。将谐波和间谐波的分析统一起来。并指出直流侧的纹波信号是变流器产生间谐波的直接原因。通过分析交直交变流系统两侧的交流信号在直流侧的互调产物，考虑两侧变流器之间的相互影响，针对不同的运行工况，特别是考虑系统三相不平衡、存在背景谐波等非理想情况下，详细推导各种情况下交直交变流系统的直流侧的纹波频率特征，利用交直流侧频率转换规则，很好地揭示交直交变流系统中的间谐波产生机理。仿真实验验证理论推导的正确性和有效性。     

三峡—华东HVDC辅助频率控制的动模试验

通过大规模动模试验分析了三峡—华东高压直流（HVDC）输电系统加装辅助频率控制器后的动态特性。试验结果表明，加装了辅助频率控制器的HVDC系统能在两侧交流系统发生大扰动时，实现两侧系统之间功率的相互紧急支援，可显著改善两侧交流系统频率，并能在多回直流输电线路中实现功率的平稳转移，经济效益显著。通过试验也初步揭示了HVDC辅助功率调制对交流系统稳定性以及对HVDC系统自身稳定性的影响，为设计综合性能良好的辅助控制器提供了具有较高参考价值的试验数据。     

MMC－MTDC输电系统频率协调控制策略

针对交流系统通过MMC-MTDC实现异步联网后无法实现区域间频率支撑的问题,参照交流系统功-频特性和柔性直流输电系统直流电压下垂特性设计了3种频率协调控制策略:固定频率控制、频率偏差PI控制和功率估算频率控制。固定频率控制将频率参考值设定为常数来获得功率支撑量;频率偏差PI控制是将频率事故换流站的频率与参与协调控制换流站的频率平均值之差作为PI控制器的输入量来获取功率支撑量;功率估算频率控制根据事故换流站交流侧频率的实时变化,利用功-频静特性系数估算功率变化量来决定换流站的功率支撑量。在PSCAD仿真软件中搭建了异步联网交直流系统进行仿真验证,仿真结果表明了所提3种利用MMC-MTDC实现区域间频率协调的可行性和有效性。     

三峡—华东HVDC辅助频率控制的动模试验

通过大规模动模试验分析了三峡一华东高压直流(HVDC)输电系统加装辅助频率控制器后的动态特性。试验结果表明，加装了辅助频率控制器的HVDC系统能在两侧交流系统发生大扰动时，实现两侧系统之间功率的相互紧急支援，可显著改善两侧交流系统频率，并能在多回直流输电线路中实现功率的平稳转移，经济效益显著。通过试验也初步揭示了HVDC辅助功率调制对交流系统稳定性以及对HVDC系统自身稳定性的影响，为设计综合性能良好的辅助控制器提供了具有较高参考价值的试验数据。     

海上风电接入多端柔性直流输电系统中换流站退出运行时直流功率再分配策略

大容量海上风电场通过多端柔性直流输电系统接入主网是未来极具前景的并网方式。研究了岸上换流站故障退出运行后,基于电压源型换流器的多端直流输电系统(voltage source converter based multi-terminal HVDC,VSC-MTDC)直流功率的优化再分配策略。岸上健全换流站间的功率再分配可分为自消纳情景与无法自消纳情景。首先,针对自消纳情景,以减小潮流重分配对交流电网频率稳定影响为优化目标,合理地重新配置各受端换流站的控制器,使转移功率完全被故障端换流站所在交流电网消纳;其次,针对无法自消纳情景,保持故障端换流站所在交流电网内的其余健全换流站满发,并利用源侧风电场的桨距角控制以及虚拟惯量控制减小换流站退出功率冲击对岸上电网频率稳定影响。最后,在PSCAD/EMTDC中搭建改造的39节点系统,所设计的直流功率再分配策略对系统频率稳定提升得到了验证。     

Isolated Ac/Dc Converter Using Soft Switching Technique

This paper presents a high‐frequency isolated ac/dc converter using the soft switching technique. The ac/dc converter consists of a matrix converter for transforming three‐phase ac voltage to high‐frequency ac voltage, a high‐frequency transformer, and an ac/dc rectifier. In order to reduce the switching loss, soft switching at every commutation of the matrix converter and ac/dc converter is achieved. The effectiveness of the proposed converter and the control scheme was verified by experiments.     

A single-phase PWM current source type converter coupled with an ac chopper

A current source type converter can easily generate a sinusoidal current on the ac side by employing a PWM strategy. In the converter system for a single-phase supply, however, the dc current pulsates because the dc output voltage of the converter contains an ac component with twice the ac supply frequency. The dc current pulsations cause the ac current waveform to be distorted. Although the use of a dc reactor with large inductance reduces the dc current pulsations, the size and the weight of converter equipment including a dc reactor is large. Two-phase rectification using two full-bridge converters and a converter system with an ac chopper circuit has been proposed for elimination of the dc pulsations. In these converters, the number of switching devices comprising the circuit will unavoidably increase. To solve this problem, we propose a novel single-phase current source type converter coupled with an ac chopper circuit and the PWM method. In this circuit, two switching devices in the main bridge are used to form an ac chopper bridge with two added devices and a capacitor. This paper gives the experimental and theoretical waveforms and the steady-state characteristics. The results prove that a smooth dc current and a sinusoidal ac current are obtained, and that a great reduction of the dc inductance can be achieved by using the proposed converter. © 1998 Scripta Technica. Electr Eng Jpn, 123(3): 36–45, 1998     

Operation analysis of quasi‐resonant,high‐frequency transformer link dc–ac converters applied to phase‐shift PWM control on the secondary side

In recent years, the soft‐switching techniques have attracted attention for their peculiar advantages such as low switching loss, high power density, EMI/RFI noise reduction, and so on. The authors have previously reported on a quasi‐resonant dc–dc converter using new phase‐shift PWM control scheme. By using the proposed control scheme, circulating current is eliminated and ZVS (Zero Voltage Switching) is achieved with small commutating current. As a result, the conduction losses caused by their currents are substantially reduced. In this paper, the authors apply a proposed control scheme to a quasi‐resonant high‐frequency transformer link dc–ac converter. As a result, all switching devises in this dc–ac converter can achieve soft switching with small commutating current irrespective of inverter mode and rectifier mode. Its operating principle and unique features are described as compared with the symmetrical control scheme of dc–ac converter. Operating performance of this dc–ac converter in the steady state is illustrated by means of simulation results. © 1999 Scripta Technica, Electr Eng Jpn, 130(2): 88–98, 2000     

Three-Phase AC/DC PWM Converter with Sinusoidal AC Currents and Minimum Filter Requirements

A pulsewidth modulation (PWM) control technique suitable for fully controlled three-phase ac/dc converters is analyzed, which gives sinusoidal input currents and ideally smoothed dc voltage. The technique allows four-quadrant operation and full-range control of the input power factor. An extension to a simplified converter scheme, capable of one-quadrant operation, is also considered. Operation of the converter is analyzed under both ideal and actual conditions. Control implementation and design criteria are discussed and experimental results are reported.     

A Sinusoidal Pulsewidth Modulated Three-Phase AC to DC Converter-Fed DC Motor Drive

The external performances such as power factor, displacement factor, harmonic factor, and ripple factor of a three-phase ac to dc converter-fed separately excited dc motor drive employing sinusoidal pulsewidth modulation (SPWM) control technique are obtained for different speeds and modulation indexes. Since separately excited dc motors with armature voltage control provide constant torque operation, the external performance is also determined for the drive motor operating at different values of constant load torque. Motoring and regenerating operations of the dc drive machine are considered. The three-phase PWM converter-motor drive system is analyzed, taking commutation effects into account in motoring and regenerating operations. The analysis has revealed 27 common modes in one repetitive period of the output voltage. The sequence of modes for all pulses in one period of the output voltage is established. Experimental oscillograms of typical waveforms from a laboratory-sized dc motor are illustrated to verify the basic principles of operation. Although the converter circuit requires some additional components in comparison with the commonly used phase-controlled converter, the improved performance characteristics make it attractive for industrial applications involving large power ratings.     

The High-Frequency Base Converter?A New Approach to Static High-Power Conversion

A new approach to high-power conversion in which two naturally commutated converters (cycloconverters) are used in tandem, each with its input connected to a (passive) high-frequency (HF) source (``base') is presented. The arrangement approximates an ideal converter; its ``input' and ``output' frequency can be equal or different, and the power factor at both external terminals can be maintained at unity or any other value. Potential utility applications in which the HF base converter functions as an asynchronous intertie between two ac power systems and as a tie between a dc transmission line and a weak ac system are discussed, and technical and economic comparisons with conventional approaches are given.     

Pulsewidth Modulation of Neutral-Point-Clamped Indirect Matrix Converter

An indirect matrix converter is an alternative “all-semiconductor” energy processor proposed recently for converting an ac source with fixed magnitude and frequency to a variable voltage and frequency supply that can meet the requirements of a particular industry application. In principle, an indirect matrix converter is constructed by connecting a front-end bidirectional rectification stage to a conventional two-level inversion stage with no bulky capacitive or inductive energy storage connected to their intermediate dc link. Through the proper modulation and compensation of the resulting converter, sinusoidal input current and output voltage can be achieved with minimized rectification switching loss, rendering the indirect matrix converter as a competitive choice for interfacing the utility grid to, e.g., defense facilities that require a different frequency supply. As an improvement, an indirect matrix converter assembled using a three-level energy processing stage has briefly been mentioned in the literature, but has neither been tested in simulation nor experimentally because its operational principles, pulsewidth-modulation (PWM) control, and index compensation have not yet been discussed in the existing literature. Addressing the previously described issues, this paper focuses on the operational mode analysis of a three-level indirect matrix converter implemented using a neutral-point-clamped inversion stage and the design of a number of PWM and modulation ratio compensation schemes for controlling the converter with improved waveform quality. The performances and practicalities of the designed schemes are verified in simulation and experimentally using an implemented laboratory prototype with some representative results captured and presented in this paper.      

模块化多电平变换器全电平产生原理及电容均压策略

模块化多电平变换器每个桥臂有N个模块,根据调制方法的不同,输出可以产生N+1电平(基本电平)和2 N+1电平(全电平),文中研究了这两种输出的工作机理,以及对桥臂电感的影响,研究了电平产生、环流及电容电压控制之间的关系。在此基础上,针对载波重叠脉宽调制(PWM)策略,提出了一种基于PWM信号轮换的模块电容均压策略,该策略可以根据需要调整策略的执行时间,以减少因均压带来的开关状态变化。通过一个560V直流输入的实验和仿真平台验证了所提出均压策略的有效性。     

Design of a Pulsewidth-Modulated Resonant Converter for a High-Output-Voltage Power Supply

The design and fabrication of a parallel resonant converter circuit and a high-frequency step-up transformer used to supply an adjustable dc voltage to a load is described. The 500-W system is operated from 115/230 V single-phase 60-Hz power, which is rectified and filtered to form a 310-V dc link. A two-transistor half-bridge circuit operating at a fixed frequency above the circuits resonant frequency converts the dc voltage to an ac voltage at approximately 20 kHz. This high-frequency voltage is transformed with a low-capacitance oil-impregnated ferrite transformer. The output voltage is rectified to form a dc voltage with a maximum value of 90-kV peak. The output voltage is adjustable using pulsewidth modulation of the conduction time of the two transistors in the power circuit. The energy stored in the resonant circuit provides a sinusoidal transformer voltage at fixed frequency over a wide range of control. The system is provided with a closed-loop peak-voltage regulator and an on-off capability from the control electronics. The transformer is designed for a specific value of inductance and capacitance to operate at the desired resonant frequency and characteristic impedance.     

A novel utility‐interactive electrical energy storage system making use of electrical double‐layer capacitors and an error tracking mode PWM converter

A novel electrical energy storage system interacting with a commercial ac network was designed and experimentally evaluated. It consists of an ac/dc converter modulated by a constantly sampled PWM scheme called Error Tracking Mode and electrical double‐layer capacitors paralleled by monitor circuits. The ac ratings are 100 V, 500 W, and 1.0 power factor. The storage capacity is designed to be 500 Wh. Since the voltage of the capacitor bank decreases to one‐fourth with discharge, a reversible step‐down chopper is inserted between the bank and the ac/dc converter. ac/dc converter efficiencies of 95.6 and 95.8% were obtained in rectifier operation and inverter operation, respectively. The average switching frequencies were set between 10.4 and 11.2 kHz for all operating conditions. Chopper efficiencies of 94.6% and 93.1% were measured in the charge and discharge modes, respectively. The storage efficiency of the capacitor bank was 89.8%. This novel utility‐interactive energy storage system has demonstrated good and rapid performance. © 2000 Scripta Technica, Electr Eng Jpn, 133(2): 52–62, 2000