Control and protection scheme of HVDC system with self‐commutated converter in system fault conditions

For extending self‐commutated converter application to future trunk power systems, it is important to develop a stable operation scheme as well as to realize substantial cost reduction through coordinated system and control design. Suppression controls of converter overcurrent and dc overvoltage in various system fault conditions are essential in order to ensure stable operation and cost reduction of HVDC systems with voltage source type self‐commutated converters. Converter control and protection schemes which include such suppression controls have been developed, employing CRIEPI's ac/dc Power System Simulator test and EMTP analysis. This paper first discusses the cause of converter overcurrent at ac system faults, considering the effect of PWM pulse number and converter control speed. Continued operation has been achieved by adding a new overcurrent suppression scheme to the converter control. In the case of a dc line grounding fault, the selection of the grounding circuit constant and the adoption of a high‐speed converter control practically ensure the reduction of dc overvoltage while suppressing converter overcurrent. The converter block and restart sequence after a dc fault, which is coordinated with dc circuit breaker operation, enables stable recovery of HVDC transmission as fast as the usual line‐commutated HVDC system. © 2000 Scripta Technica, Electr Eng Jpn, 132(2): 6–18, 2000     

An active‐passive capacitor‐commutated converter for HVDC systems with a three‐phase voltage‐source PWM converter

This paper introduces a new approach to the capacitor‐commutated converters (CCCs) for HVDC systems. A small‐rated three‐phase voltage‐source PWM converter is connected between a series commutation capacitor and thyristor converter through matching transformers. The PWM converter acts as auxiliary commutation‐capacitor for the thyristor converter while the series passive capacitor acts as the main commutation capacitor. The capacitance, which is the sum of the small‐rated active and series passive capacitors, is variable, so that stable commutation is obtained. In CCCs, commutation failure occurs when the AC bus voltage is recovered whereas the proposed combined commutation‐capacitor can achieve successful commutation for both rapidly decreasing and increasing AC bus voltages. The basic principle of the proposed active–passive capacitor‐commutated converter is discussed in detail. Then, constant margin angle control with a constant firing angle of the thyristor converter is proposed using a function generator block. Digital simulation demonstrates the novelty and effectiveness of the proposed active–passive capacitor‐commutated converter. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 151(1): 66–75, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20030     

Low‐Voltage‐Ride‐Through Performance of an HVDC Transmission System Using Two Modular Multilevel Double‐Star Chopper‐Cells Converters

This paper presents an intensive discussion on a long‐distance high‐voltage direct‐current (HVDC) transmission system that combines two modular multilevel cascade converters based on double‐star chopper cells (MMCC‐DSCC) with DC power cables. Hereinafter, a single MMCC‐DSCC is referred to as a DSCC converter or just as a DSCC for the sake of simplicity. The HVDC transmission system is required to provide low‐voltage‐ride‐through (LVRT) capability to enhance transmission system availability. This paper proposes a new LVRT method without any direct information exchange between the two DSCC converters. The validity of the method is verified, using simulated waveforms from the software package of “PSCAD/EMTDC” and experimental waveforms from a three‐phase 200‐V, 400‐Vdc, 10‐kW, 50‐Hz downscaled HVDC system with a set of 300‐meter‐long DC power cables.     

Direct‐connected multiple current‐source converters for SMES system

The superconducting magnetic energy storage (SMES) system would be a valuable system for power line stabilization. It would increase stability by supporting peak energy and compensating generation shortage. With the development of a large‐capacity GTO thyristor suitable for 20‐MW‐level power conversion systems, it is possible to control the four‐quadrant real power (P) and the reactive power (Q) condition by self‐commutated power converters with high performance. This paper describes a new direct‐connected multiple current‐source converter, and also explains suitable control strategies for the pilot plant of an SMES system. The feasibility of the control strategies is demonstrated by EMTP simulation results. © 2000 Scripta Technica, Electr Eng Jpn, 133(1): 87–96, 2000     

交流不对称故障下的LCC-MMC混合直流系统输送功率提升策略研究

将常规两端直流输电系统逆变站的电网换相换流器(LCC) 替换为模块化多电平换流器(MMC)所构成的混合直流输电系统,可结合两种换流器的优点而具有广阔的应用前景。在研究其基本稳态控制特性的基础上,重点分析了交流电网不对称故障引起的直流输送功率下降及中断问题。通过分析混合直流系统的交流故障特征,发现交流不对称故障发生在整流侧时易引起直流电压下降甚至输送功率的中断,发生在逆变侧时易引起直流系统电压异常。鉴于此,提出了基于MMC典型控制的附加直流电压控制策略,在其调制范围内通过降低故障时逆变侧的参考直流电压以提高直流系统的输送能力。若检测到本站直流电压的交流分量大小超过限定值,则附加控制策略自动投入,无需依靠换流站间的通信。最后,通过PSCAD/EMTDC电磁暂态仿真验证了所提控制策略的可行性。     

Tapping on HVDC lines using DC transformers

This paper investigates a new topology for tap terminals on HVDC transmission, which is based on high-power DC/DC converters. The use of DC transformers brings benefits in terms of better resilience from tap system faults, more flexibility in power reversal, an additional control channel and lower costs for main AC/DC converter and transformer. The paper studies both options: using line commutated and voltage source converters as the main AC/DC tap converters. PSCAD simulation results are given for a 0.025 p.u. bidirectional tap on the 1000 MW CIGRE HVDC benchmark system. The simulation responses with a line commutated converter indicate ability to operate the tap terminal at neutral reactive power exchange over wide range of power levels and also excellent resilience to disturbances on the tap AC grid. The simulations with voltage source AC/DC converter also show excellent responses for wide range of inputs but the DC/DC converter becomes more complex. It is concluded that the proposed topology may provide means for wider access to the existing HVDC transmission links.     

A self‐commutated back‐to‐back system and its performance under a single line‐to‐ground fault condition

This paper deals with a self‐commutated BTB (Back‐To‐Back) system for the purpose of power flow control and/or frequency change in transmission systems. Each BTB unit consists of two sets of 16 three‐phase voltage‐source converters, and their AC terminals are connected in series to each other via 16 three‐phase transformers. Hence, the BTB unit uses totally 192 switching devices capable of achieving gate commutation. This results in a great reduction of voltage and current harmonics without performing PWM control. Simulation results verify the validity of the proposed system configuration and control scheme not only under a normal operating condition but also under a single line‐to‐ground fault condition. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(3): 68–78, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10124     

A 21‐level (line‐to‐line) BTB system based on series connection of 16 converter cells. Experimental verification by a 200‐V, 20‐kW laboratory system

A 21‐level (line‐to‐line) self‐commutated BTB (Back‐To‐Back) system based on series connection of 16 converter cells has attractive features as follows: (i) Both active and reactive powers can be controlled independently even in transient states. (ii) The BTB system produces almost sinusoidal voltage at the AC side without performing PWM (Pulse‐Width‐Modulation) control, so that no harmonic filter is required. (iii) The BTB system provides an active power reason as high as 3 ms. This paper presents experimental verifications of the self‐commutated BTB system intended for achieving power flow control in transmission systems. Experimental results obtained by a 200‐V, 20‐kW laboratory prototype confirm effectiveness and validity of both the system configuration and the developed control strategy. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 160(1): 61– 70, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20324     

New control for HVDC system connected to large windfarm

HVDC consisting of self‐commutated inverters is able to be applied for power transmission connecting from a remote large windfarm to a weak AC system. Most self‐commutated HVDC is applied for connection between two AC systems that have synchronous power sources, but it is not suitable for a windfarm that consists of induction generators without synchronous power source. This paper presents new control for the self‐commutated HVDC system connected to a large windfarm of induction generators. The effect of the proposed control is shown by EMTP simulation. ©2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(4): 31–39, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20539     

HVDC transmission system using multilevel power converters based on dual three-phase two-level inverters

Multilevel converters are now an attractive solution for high-voltage direct-current (HVDC) electrical energy transmission systems. Unlike the well-known two-level voltage source converters, multilevel converters use 3 or more voltage levels or steps per leg to modulate the ac voltages, decreasing voltage distortion and reducing electromagnetic interference. This work presents a HVDC transmission system based on a new multilevel structure using a dual two-level converter topology. This structure attains multilevel operation and advantages using two well known three-phase voltage source two-level inverters connected to one three-phase open windings transformer. The proposed dual converter structure has two independent dc links allowing each inverter to process half of the total power. This arrangement is fitted with a control system designed to control the active and reactive power towards their specific set point values, while balancing the voltages of the two dc link capacitors in real time. Obtained results show the effectiveness of the proposed HVDC transmission system.     

混合级联多端直流输电系统的功率协调控制策略

针对一种整流侧采用电网换相换流器（LCC）,逆变侧采用LCC与多个模块化多电平换流器（MMC）串联的混合级联多端直流输电系统进行了研究。为解决目前已有的控制策略无法对逆变侧各换流站输送功率进行独立控制的问题,为逆变侧换流器设计了附加功率的协调控制策略,实现了对有功功率的独立灵活控制和MMC之间的功率互相支援,并为系统设计了故障控制策略。最后,在PSCAD/EMTDC中搭建了该直流输电系统模型并对所提出的协调控制策略进行了仿真验证。结果表明,附加功率的协调控制策略能够实现对逆变侧各换流器输送有功功率的独立控制,并且在系统发生故障后具有良好的故障恢复特性。     

The Proposal of Isolated Single‐Stage AC‐DC Converter

An isolated ac‐dc converter has been used in various applications, such as power supply and as a battery charger for electric vehicle. In conventional converters, a loss in each conversion stage can be reduced by applying a soft switching method. However, a conventional converter has many conversion stages including the rectifier stage, power factor correction, and dc/dc converter stages; thus, it is difficult to reduce the total converter loss and size. In this paper, we propose a novel isolated‐type ac‐dc converter with only one conversion stage; it can realize a zero‐voltage switching operation in all switching devices.     

Voltage balancing control of chain link converter application to interconnection system

An application of the chain link converter (CLC) system, which consists of multiple single‐phase voltage source converters (VSCs) connected in parallel or series, is studied as interconnection systems. In a CLC‐high voltage direct current transmission (HVDC) system, single‐phase converters must be connected in series on the DC side to make the DC voltage high, and the DC voltage of each converter must be controlled to the same value to get an appropriate capacity. This paper describes a DC voltage balancing control (DVBC) method between series converters. Simulations and simulator tests in steady states and in transient states were carried out to confirm behaviors of the CLC‐HVDC system. Those results confirmed the viability of CLC applications to interconnection systems. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Controller saturation nonlinearity in doubly fed induction generator‐based wind turbines under unbalanced grid conditions

Doubly fed induction generator (DFIG) is widely used in wind energy generation systems due to its cost‐effective, partially rated back‐to‐back power converters, variable rotor speed operation, and maximum wind power capture. The conventional design assumes balanced grid voltage and utilizes power protection for the power converters. The DFIG wind turbine is naturally one of the major components in distributed generations of the smart grid system. However, newly developed smart grid system is rich in unbalanced loads. This paper summarizes the limiter settings of controllers and explores the nonlinear behaviors of the DFIG‐based wind power generation system with unbalanced loads. The generator rotor speed and an unbalanced load resistance are chosen as variation parameters. An emerging low‐frequency linear‐modulated oscillation at line second harmonic frequency with DC drifting is identified on the DC link voltage of the back‐to‐back power converters. In terms of second harmonic and the usually reported hazardous low‐frequency oscillation, the saturation nonlinearity and over‐modulation of the back‐to‐back power converter and its power flow are investigated and analyzed. The built‐in detailed model of the DFIG wind energy generation system in Matlab with SimPowerSystems library is used in this study. Copyright © 2015 John Wiley & Sons, Ltd.     

HVDC滤波换相换流器的阻抗频率特性

高压直流输电(HVDC)换流器的阻抗频率特性是分析和解决谐波不稳定的一个重要因素,滤波换相换流器(filter commutated converter,FCC)是一种具有阀侧谐波抑制兼无功功率补偿功能的换流器,文章简要论述了FCC的接线方案和工作机理,并基于开关函数法对计及换流器换相过程影响下的FCC交、直流等值阻抗计算式进行了理论推导。以直流输电开发平台为例,对传统电网换相换流器(line commutated converter,LCC)与FCC的阻抗频率特性计算结果进行对比,仿真结果表明换流器的阻抗频率特性对交流系统的谐振频率有着不可忽略的作用,FCC在一定程度上提高了交流系统强度,改善了系统稳定性,有效降低了系统谐振频率下的交流等值阻抗,从而更好避免直流输电系统谐波不稳定现象的发生。     

A 6.6‐kV transformerless cascade PWM STATCOM. Experimental verification by a three‐phase 200‐V, 10‐kVa laboratory system

This paper deals with a 6.6‐kV transformerless STATCOM cascading multiple single‐phase H‐bridge PWM converters in each phase. The AC voltage of the STATCOM is almost sinusoidal, so that it requires no harmonic filter. Each converter is equipped with a capacitor and a voltage sensor on the DC side, which are electrically isolated from each other. The STATCOM has the capability of self‐starting and voltage‐balancing without any external power supply or equipment. Experiments using a three‐phase 200‐V, 10‐kVA laboratory system, along with computer simulations, are carried out to confirm the viability and effectiveness of the STATCOM. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(1): 55–64, 2010; Published online in Wiley InterScience ( www.interscience.wiley. com ). DOI 10.1002/eej.20822     

Multipulse converters and controls for HVDC and FACTS systems

Contents  For very high power applications, it is imperative to limit switching losses in the power converter. Multipulse converters are made up from several converter modules, switching at the fundamental frequency, but properly phase shifted to produce a nearly sinusoidal output voltage. Robust controls for HVDC and FACTS systems based on multipulse converters were developed. The developed HVDC system makes the rectifier station to control the instantaneous active (real) power through the HVDC voltage link, whereas the inverter station controls the instantaneous reactive (imaginary) power generation. The control of real and imaginary powers is independent one from the other and there is no control signal that is shared by both converter stations. A complete model of a 100 MW, 33 kVac, 50/60 Hz, 24-pulse HVDC system was implemented in the electromagnetic transient program (EMTP/ATP). The static synchronous compensator (STATCOM) is a shunt type of FACTS controller. It regulates the voltage magnitude of a controlled ac bus by drawing variable reactive current from the power system. The fast dynamic of the STATCOM controller avoids high fluctuations of the dc voltage caused by transients. A complete model of a 100 MW, 33 kV, 60 Hz, 24-pulse STATCOM was implemented and the results have confirmed the robustness of the proposed voltage control. Received: 21 December 2000     

Simulation study of unified power flow controller

The increasing complexity of ac power networks requires a high‐performance power flow control system in order to obtain the desired power flow and enhance static and dynamic stability. One of the most effective power electronic systems to satisfy these requirements is a UPFC (Unified Power Flow Controller) employing self‐commutated converters. This paper presents basic control strategies and simulation results for the UPFC using the EMTP (Electromagnetic Transient Program). The simulations were carried out for start and stop operations, power flow change operations in normal system conditions, as well as operations during system fault conditions. Thyristor‐based bypass switches are used for the protection of the series compensator of the UPFC from system fault currents, and satisfactory protection capability was confirmed. © 1999 Scripta Technica, Electr Eng Jpn, 127(1): 23–30, 1999     

高压直流输电技术现状及发展前景

为满足国内直流输电工程的建设需要,紧跟直流输电设备制造水平的前沿技术,对直流输电技术发展的最新成果进行了总结。总结了特高压直流运行方式的特点,指出将电流自然换相技术与柔性直流技术相结合构成多端直流输电技术是未来直流输电技术的发展方向;介绍了多换流站共用接地极技术、换相直流输电技术、大功率半导体器件及换流变压器的应用现状;分析了3种光电式电流互感器的特点;总结了直流输电控制保护系统的发展现状;最后结合生产实际,阐述了跨区电网运行中采用区域直流集控技术的优势。     

定电压与预测型定关断角控制对HVDC小干扰稳定性的影响

定电压控制和预测型定关断角控制是高压直流（HVDC）逆变站常规的控制方法,合理整定控制器参数可提高交直流系统的小干扰稳定性。文中首先基于开关函数法建立了基于电网换相换流器的高压直流（LCC-HVDC）系统的小干扰动态模型,将其动态响应与电磁暂态模型的动态响应进行对比,验证了小干扰动态模型的正确性。接着,采用特征值及其灵敏度方法分析了LCC-HVDC系统的振荡模式和阻尼特性,对比分析了逆变侧采用定电压控制和预测型定关断角控制时,换流站控制器参数以及交流系统短路比对系统稳定性影响的异同。最后,整定了能维持系统稳定运行的控制器参数稳定域,并通过PSCAD/EMTDC仿真验证了稳定域的正确性。