基于成本分析的柔性直流配电网综合规划方案研究

随着直流分布式发电的高渗透率以及敏感直流电子负载的不断增长,柔性直流配电网络的应用越来越广泛,为比较柔性直流系统与传统交流配电系统,提出基于成本分析的柔性直流配电网规划方案。该方案可确定最优的交直流变电站位置和规模、交直流馈线的布线路线以及中低压侧交直流馈线的长度和容量,其目标函数由投资成本和运营成本组成。提出的柔性直流配网规划方案是在没有预先分配配电系统的地区实施的,并与传统交直流规划方案进行了比较。最后算例结果表明,与常用的交流配电系统相比,采用柔性直流配电方案可降低敏感负载下的总成本,并能提高分布式电源的渗透率。     

Influence of AC-DC parallel interconnection on voltage stability of AC systems

The static and dynamic characteristics of power supply to each node in a hybrid ac-dc power system are analyzed by means of static and transient P-V curves. Three kinds of dc transmission control scheme are compared. DC transmission systems are more effective in delivering power to inverter nodes. The constant firing angle, constant current control techinque provides more power to inverter nodes than the conventional constant current, constant voltage and constant power, constant extinction angle techniques. Transient digital simulations indicate that dc interconnection improves voltage stability to both the opening of a dc interconnected line and one-line opening of ac double-circuit interconnected lines.     

The Newton?Raphson Load Flow Applied to AC/DC Systems with Commutation Impedance

Existing conditions in industrial ac-dc power systems may result in significant commutation resistances. If the resistance to reactance ratio of the commutation impedance is high, significant errors can result in load-flow studies which neglect the resistive portion. A method is presented for inclusion of a resistance-inductance fed bridge rectifier in a Newton-Raphson (N-R) load flow program. Polar form of the power flow equations is assumed. An example problem is provided for comparison.     

Analysis Techniques in AC/DC Power Systems

Many present-day industrial power systems have combined ac/dc power distribution systems. A typical example is a coal mine power system, where ac is used for running the mining equipment and dc is used for track haulage, etc. Also, the topology of the dc network is constantly changing due to the movement of locomotives along the track. Methods are described that can be used to carry out load-flow and short-circuit analyses of such a system.     

Single-phase PWM converter using balanced two-phase rectification

Energy storage devices, such as inductors or capacitors, are needed in single-phase ac to dc converters because instantaneous power flow of input single-phase ac is inherently pulsating and output dc power must have as little ripple as possible. Energy storage devices (such as smoothing inductors or capacitors) are usually inserted on the dc side. In those converters, energy storage ability of the devices cannot be used effectively, especially when dc power ripple is required to be very small. To solve such a problem, this paper presents novel single-phase ac/dc conversion circuits, in which energy storage devices are used at the ac rather than at the dc side. That is, after converting single-phase ac into balanced (rectangular) two-phase ac using variable reactance element, reconversion of it into dc is carried out. This paper presents a control method for the converters and the variable reactance devices, and performance of the proposed circuits is confirmed using computer simulation.     

Hybrid-model transient stability simulation using dynamic phasors based HVDC system model

A novel hybrid-model transient stability simulation algorithm for ac/dc power systems is suggested in this paper, where dynamic phasors theory is applied for HVDC transmission system modeling, and traditional electromechanical transient models are used for ac system. A detailed dynamic-phasors-based HVDC system model is derived first, and the algorithm for interface of the dc dynamic phasors model to ac network is proposed next. Computer simulation results show that the HVDC dynamic phasors model has very good accuracy as compared with its electromagnetic transient model; the test results from a 2-area ac/dc power system and a multi-infeed HVDC power system show clearly that the suggested interface algorithm works effectively in system transient stability analysis. The proposed hybrid-model simulation algorithm provides a new approach for dynamic simulation of large-scale ac/dc power systems.     

Power-Factor Improvement of Single-Phase Converter by Means of Bias Voltage Control

Power-factor improvement and harmonic reduction in ac line current, as well as stable and reliable operation, are very significant areas in the high-power ac-dc converter. The improvements of these characteristics of the single-phase converter may be generally difficult compared with the polyphase type. A new converter scheme suitable for the high power single-phase converter to overcome the above problems is presented. The basic configuration and principle of operation of the new converter scheme, theoretical characteristics and design indexes, and the drive system and characteristics of a dc series motor by this scheme are described. The basic principle of operation is verified and the feasibility of this converter system is demonstrated by experiments.     

Integration of HVDC control dynamics into transient energy functions

A common approximation used in the formulation of transient energy functions (TEFs) for ac/dc power systems involves the omission of dc control dynamics. The differential equations representing the dc system dynamics are used to track the fault-on trajectory, but not to derive the TEF. This approach leads to an error in TEF-based stability prediction. In this paper, a method is presented to derive the TEF with dc system dynamics explicitly integrated. By using this comprehensive transient energy function, the accuracy of the stability prediction in ac/dc systems is improved. In addition, because differential equations representing dc dynamics are included directly in the TEF, the calculation of the post-fault equilibrium point is done only once. This leads to a considerable reduction in CPU time. The method is validated by comparing results from 12-bus and 30-bus ac/dc test systems with those obtained from time simulation.     

Domain of stability of ac/dc power systems

The domain of stability (DOS) of a dynamic system is the sub-state space within which the system converges to a stable equilibrium state if the operating point varies because of a disturbance. The cell mapping technique has been widely used to find the DOS of second-order systems. However, there is no generic method reported in the literature for finding the DOS of any system of order greater than two, other than for ac power systems or for ac/dc power systems with dc dynamics omitted. For these two systems, the DOS can be found using the transient energy function (TEF) because a TEF has been available only for ac-dynamics-only systems. The DOS for ac/dc systems with dc dynamics has not been found because a TEF for dc systems with dc dynamics was not available until the authors recently derived one. In this paper, the existing cell mapping technique is improved to find the DOS of systems of order greater than two. This improved cell mapping technique and the recently developed TEF for dc systems have been used to find the domains of stability of ac/dc systems.     

The power flow algorithm for balanced and unbalanced bipolar multiterminal ac–dc systems

Bipolar dc terminals are used for hvdc transmission in an ac–dc system. The sequential ac–dc power flow algorithm has been proposed for an ac–dc system consisting of balanced and unbalanced bipolar multiterminal in this paper. The algorithm has two superiority. Firstly, as dc system variables are not included in Jacobian matrix the convergence time of the algorithm and necessity of computer memory is little. Secondly, both this algorithm has been developed for (balanced and unbalanced) bipolar system (not only monopolar) and this algorithm also can be run with current controlled, voltage controlled and power controlled terminals without making any changes in proposed method.     

Unity-power-factor PWM converter with DC ripple compensation

The generation of harmonics and their subsequent propagation into power lines is a topic of increasing concern to power-supply authorities. To prevent obstacles in the power system, a unity-power-factor PWM converter will be applied at ac-dc power conversion plants. However, the PWM converter, especially at single-phase circuit, has some serious defects, including low-frequency ripple current that flows into the dc line and gives rise to a low-frequency ripple voltage that appears on the dc output. In usual cases, it is necessary to connect a very large capacitor or a passive L-C resonant circuit to the dc line for reduction of low-frequency ripple voltage. However, when batteries are connected to the dc output, most of the dc ripple current flows into the battery even if the above circuits are used, because the impedance of the battery is very low compared to that of the circuits. The low-frequency ripple current causes power loss on the battery and the temperature rises. It is well known that the life of a battery is deeply influenced by the temperature. The ripple current, therefore, should be reduced as low as possible. To accomplish reduction of the low-frequency ripple current, a novel topology for the PWM rectifier is presented in this paper. The main circuit is constituted by adding only a pair of switching devices to the conventional PWM converter circuit. With a simple control technique, the ripple energy on the dc line is converted into stored energy on the input ac capacitors through additional switches. The theoretical characteristics are obtained by using the state-space averaging method. The effect of ripple reduction is confirmed by experiments using a breadboard setup. © 1998 Scripta Technica, Electr Eng Jpn, 123(1): 51–62, 1998     

Modeling and simulation of ac–dc buck-boost converter fed dc motor with uniform PWM technique

PWM controlled rectifiers can efficiently and economically be employed in low and medium power applications of dc drives and in front-end converters of rectifier–inverter systems while maintaining the advantages of design simplicity and operation reliability of naturally commutated schemes. Due to the high dc voltage that is produced which is greater than the peak voltage of the utility supply, the ac–dc buck-boost converter is especially suited as a front-end power source in variable-speed drive systems to convert the utility supply voltage into a variable dc link voltage where a single-phase or a three-phase utilities power supply is available. In this paper, the dynamic model and steady state equivalent circuit of a single-phase ac–dc buck-boost converter fed dc motor with uniform PWM control is presented. The waveforms of voltage and current, the input and output characteristics of the converter are discussed and verified. Measured, computed and simulated results are shown to be very close and the model is proved to be efficient and accurate.     

基于电池储能的MMC-HVDC系统的建模与仿真

由于交直流混联电网中的交流系统和直流系统之间存在有功功率耦合,因此基于模块化多电平换流器(MMC)的高压直流输电(HVDC)系统无法完全隔离故障时交直流侧之间的相互影响。文中提出将基于电池储能系统(BESS)的MMC应用于HVDC系统以实现交直流侧功率解耦控制的方法。首先分析了基于BESS的MMCHVDC系统的基本结构和工作原理。然后,基于数值积分和嵌套的快速仿真方法,推导了基于BESS的子模块和基于BESS的MMC换流器的戴维南等效电路。最后,搭建了±400 k V的两端MMC-HVDC系统对所提建模方法和系统的解耦效果进行了验证。结果表明,基于BESS的MMC-HVDC系统交流侧有功功率和直流侧功率可以彼此解耦,在交流或直流故障期间可以保持功率的正常输送。     

Protection of Low-Voltage DC Microgrids

In this paper, a low-voltage (LV) dc microgrid protection system design is proposed. The LV dc microgrid is used to interconnect distributed resources and sensitive electronic loads. When designing an LV dc microgrid protection system, knowledge from existing dc power systems can be used. However, in most cases, these systems use grid-connected rectifiers with current-limiting capability during dc faults. In contrast, an LV dc microgrid must be connected to an ac grid through converters with bidirectional power flow and, therefore, a different protection-system design is needed. In this paper, the operating principles and technical data of LV dc protection devices, both available and in the research stage, are presented. Furthermore, different fault-detection and grounding methods are discussed. The influence of the selected protection devices and grounding method on an LV dc microgrid is studied through simulations. The results show that it is possible to use available devices to protect such a system. Problems may arise with high-impedance ground faults which can be difficult to detect.      

Analysis of power flow in a power converter

Power converters have been widely used in many industrial applications. On the ac side of such converters, the current is distorted because of nonlinearity of the power devices even when the source voltage is sinusoidal. This implies that the power converter is regarded as a harmonic-current source. The harmonic current is one of the most important factors in the analysis of converters. This paper analyzes power flow of a power converter that behaves synchronously with the line frequency. Power-flow analysis in a three-phase diode converter with a resistance on the ac side is presented. The analysis is compared with conventional analysis with the assumption that the ratio of the resistance on the ac side to that on the dc side is very small. The proposed power flow concept enables electrical engineers and students to discuss rationally the power flow of a converter. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(1): 70–76, 1997     

Using the DC-bus current to improve the power factor in low-cost electric drives

In this paper, a new approach is presented aimed at improving the power factor of pulsewidth-modulation inverters that equip low-power electric motor drives for household appliances. The key feature of the proposed approach consists of exploiting the dc-bus current as a suitable dither generator by means of a high-frequency transformer. Such a strategy is used to force the diode rectifier to draw current from the mains in those periods when, in conventional systems, the diodes are reverse biased, i.e., when the dc-bus capacitor voltage exceeds the ac input voltage. The proposed method allows for elimination of the power-factor controller (PFC) front end, traditionally used to comply with power quality requirements. In comparison with standard PFCs based on a dc/dc converter, the proposed approach represents a cheaper and more feasible solution, not using additional power devices and control circuits. Simulation and experimental results confirm the consistency of the proposed approach.     

Measurement Problems in Determining the Efficiency of Thyristor-Supplied Motor Drives

The reasons why simple ammeter and voltmeter measurements cannot be used to determine accurately the power flow into the armature of a dc motor supplied from a thyristor circuit are reviewed. Ratios of average to effective value of voltage are considered as an aid to determining the conditions under which approximate methods may give reasonable results. Ratios of fundamental to effective value in the case of the ac part of a motor drive are used to determine the appropriate frequency range for the ac measurements on a motor drive system. The implications on the problem of measuring power and losses are considered, and some criteria for the necessary instrumentation are proposed.     

SUPERCONDUCTING LOW VOLTAGE DC TRANSMISSION NETWORKS

ABSTRACT

A new paradigm for electric power transmission based on high temperature superconducting dc. networks is studied as a feasible alternative to ac power transmission systems. The dc network operates at generation voltages allowing for direct connection of the generators to the rectifiers, eliminating the need for high voltage insulation and transformers. The overall system consists of a mesh connected, low voltage, high current dc superconducting transmission system, supplied by unit connected generators and feeding many small inverters to pass controlled levels of real and reactive power to ac loads. This paper introduces the concept of a superconducting dc mesh. Inverter topologies and control strategies required for interfacing the ac distribution systems to the dc mesh. The paper presents a conceptual overview of the operation of the system based on simulation studies.     

含VSC-HVDC的交直流混合系统潮流统一迭代求解算法

介绍基于电压源换流器(VSC)的新一代高压直流输电(VSC-HVDC)技术,具有可向无源网络供电、不会出现换相失败等众多优点。分析VSC-HVDC输电系统的原理及其中VSC的控制方式。针对不同控制方式下的VSC,分别推导其交流母线及直流系统相应的潮流修正方程式。提出VSC-HVDC交直流混合系统潮流的统一迭代求解算法,并以修改后的WSCC-9节点交直流混合系统的潮流计算为例,验证统一迭代求解算法的有效性。通过该潮流算法分析VSC-HVDC输电系统的稳态特性和有功功率损耗特性。     

Evaluation of Power Losses in a Boost PFC Unit by Temperature Measurements

High power conversion efficiency is an important requirement of the front-end power-factor-corrected (PFC) boost rectifier that is used in shaping the ac input current in a typical modern switch-mode power supply. A reasonably accurate estimate of the power losses in individual components is essential in order to improve the efficiency of the PFC rectifier. In this paper, difficulties in the measurement of individual component power losses with particular reference to an ac-dc converter are brought out. A method of loss evaluation by measurement of temperatures of individual components and surrounding ambient is presented. Experimental results that are carried out on the front-end boost PFC rectifier of a commercial ac-dc converter are presented to validate the loss estimation method.