Line-commutated Converter Research Articles

An Accurate Impedance Model of Line Commutated Converter With Variable Commutation Overlap

This paper proposes an accurate impedance model for line commutated converter (LCC) in the stationary frame. The commutation overlap process caused by the transformer leakage inductance complicates the modeling for LCC. Traditional impedance models are derived in positive sequence and negative sequence frame based on the simplified commutation overlap process, which degrades the modeling accuracy and limits the description of frequency coupling. This paper systematically analyzes the effects of the commutation overlap process on the impedance model, and the impedance model with the accurate commutation overlap process is established in the <i>dq</i>-frame. For clearly revealing the frequency coupling phenomenon of the LCC, the complex space vector is employed to extend the impedance model in the <i>dq</i>-frame to the complex <i>&#x03B1;&#x03B2;-</i>frame. A closed-loop model on PSCAD&#x002F;EMTDC is used to verify the proposed impedance model in the time-domain and the frequency-domain. The consistency between the simulation results and theoretical analysis effectively verifies the accuracy of the proposed model.

Fault Analysis and Traveling Wave Protection Based on Phase Characteristics for Hybrid Multiterminal HVDC Systems

By fully combining the advantages of line commutated converter (LCC) high-voltage direct current (HVDC) and modular multilevel converter (MMC)-HVDC systems, hybrid multiterminal HVDC (hybrid-MTDC) transmission technology has become a new technical means to overcome the shortcomings of conventional two-terminal HVDC systems. However, the conditions of high fault resistances and more sophisticated network topologies have brought considerable challenges to the existing traveling wave (TW) protection principles. In this article, a parallel hybrid-MTDC system is taken as the research object, and its mathematical analysis model is established. Based on the mathematical model, the TW characteristics that the phase of the high-frequency voltage TW during the occurrence of an internal and external fault approaches approximately 90° and 0°, respectively, are clarified. Therefore, considering the influences of reflection and refraction of TWs and line propagation, the phase-characteristics-based criterion is constructed. Using the proposed criterion, a protection scheme for hybrid-MTDC systems is proposed, which realizes the identification of line faults with high fault resistance. The proposed method is verified in detail with the electromagnetic transient simulation model using PSCAD/EMTDC, and the test results show that the proposed protection scheme is feasible and has high sensitivity, reliability, and adaptability to different operation modes.

Travelling wave fault location for LCC–MMC–MTDC transmission lines based on frequency-dependent characteristic

The paper studies the fault location principles of multi-terminal direct-current (MTDC) hybrid transmission systems based on line-commutated converter (LCC) and modular multi-level converter (MMC). Two issues should be addressed: i) the travelling wave (TW) fault location principle adaptable for the hybrid multi-terminal transmission system; and ii) the algorithm of obtaining frequency-dependent characteristic of the TW signal. In the paper, a novel TW fault location principle adaptable for the LCC–MMC–MTDC hybrid transmission lines is developed based on the frequency-dependent characteristic of TW signal. Firstly, the frequency-dependent characteristic of HVDC transmission line is studied. Secondly, a novel TW fault location principle is proposed, comprised of TW arrival time detection, TW propagation velocity calculation, fault section identification and fault location method. A typical ±400kV LCC–MMC–MTDC hybrid system is constructed using PSCAD/EMTDC for case study to validate the accuracy of the proposed principle. Its robustness against fault location, fault resistance and fault type is well demonstrated.

Capacitive Filter Based HVDC Converter for Reducing the Vibration and Noise of Converter Transformer

Harmonic currents generated by converters in the HVDC system flow into the converter transformer. The harmonic current and voltage in the converter transformer will cause electromagnetic vibration of windings and iron core, which result in serious noise pollution, power loss and electrical stressing. In this paper, a capacitive filter based HVDC converter is proposed to reduce the vibration and noise of converter transformer. First, the topology of the HVDC system is introduced, and its harmonic characteristics are analyzed. Then, the noise reduction mechanism by using capacitive filter is revealed by analyzing the relationship among harmonics, vibration, and noise of the converter transformer. Finally, the experimental platform of the capacitive filter based HVDC system is built. Moreover, the vibration and noise of the converter transformers are measured in a semi-anechoic chamber, with the use of line-commutated converter (LCC) and capacitive filter based HVDC converter respectively. The experimental results show that the vibration and noise of converter transformer using the capacitive filter are significantly suppressed compared with that using the LCC.

Small-Signal Modeling and Interaction Analysis of LCC-HVDC Systems Based on Harmonic State Space Theory

High-voltage direct current (HVDC) technology based on Line Commutated Converter (LCC) is widely used in modern power systems due to its numerous benefits. However, as a typical power electronic device, LCC-HVDC has caused significant changes in the dynamic behavior of modern power systems due to largely replaced conventional power supplies in power systems. The interactions among sending-terminal and receiving-terminal system become stronger and have caused some stability issues, but the mechanism behind this has yet to be fully revealed. In order to analyze the interactions among sending-terminal and receiving-terminal system of LCC-HVDC, a small-signal model of modular HSS modeling is proposed based on Harmonic State Space (HSS) theory. Comparison of time-domain response with detailed switching model shows that the proposed can well reflect the dynamic behavior of the detailed switching model. Based on the model, the influence of controller’s parameters on system stability is analyzed, as well as the dynamic interactions of LCC-HVDC among sending-terminal and receiving-terminal system are investigated. Analytical results demonstrate that there exists interactions among sending-terminal and receiving-terminal system which have an influence on the stability of the system.

Optimization and Configuration of Control Parameters to Enhance Small-signal Stability of Hybrid LCC-MMC HVDC System

This paper investigates the small-signal stability of the hybrid high-voltage direct current (HVDC) transmission system. The system is composed of line commutated converter (LCC) as rectifier and modular multi-level converter (MMC) as inverter under weak AC grid condition. Firstly, the impact of short-circuit ratio (SCR) at inverter side on the system stability is investigated by eigen-analysis, and the key control parameters which have major impact on the dominant mode are identified by the participation factor and sensitivity analysis. Then, considering the quadratic index and damping ratio characteristic, an objective function for evaluating the system stability is developed, and an optimization and configuration method for control parameters is presented by the utilization of Monte Carlo method. The eigenvalue results and the electromagnetic transient (EMT) simulation results show that, with the optimized control parameters, the small-signal stability and the dynamic responses of the hybrid system are greatly improved, and the hybrid system can even operate under weak AC grid condition.

The On-site Verification of Key Technologies for Kunbei-Liuzhou-Longmen Hybrid Multi-terminal Ultra HVDC Project

Hybrid high voltage direct current transmission (Hybrid HVDC) is a new type of HVDC technology developed in recent years. It combines the characteristics of large-capacity and low cost of the line commutated converters (LCC) and non-commutation failure and dynamic reactive power support of voltage sourced converters (VSCs) in one HVDC system. It has technical advantages in the fields of unidirectional power transmission and in the application of improving the stability of multi-infeed HVDCs, giving it broad application prospects. The Kunbei-Liuzhou-Longmen (KBL) project is the first hybrid ultra HVDC project. It was put into service on Dec. 27 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> , 2020. This paper introduces the KBL project, analyzes the operation characteristics of hybrid HVDC system, and provides on-site verification of KBL project, including operation of LCC-VSC hybrid system, ultra-high voltage VSC technology using the series of two valve groups, the DC fault ride-though of hybrid VSC without using DC breaker, the connection and disconnection of a third station, and etc.

Improved Identification Method and Fault Current Limiting Strategy for Commutation Failure in LCC-HVDC

Line-commutated converter based high-voltage direct-current (LCC-HVDC) transmission systems are prone to subsequent commutation failure (SCF), which consequently leads to the forced blocking of HVDC links, affecting the operation of the power system. An accurate commutation failure (CF) identification is fairly vital to the prevention of SCF. However, the existing CF identification methods cause CF misjudge or detection lag, which can limit the effect of SCF mitigation strategy. In addition, earlier approaches to suppress SCF do not clarify the key factor that determines the evolution of extinction angle during system recovery and neglect the influence. Hence, this paper firstly analyzes the normal commutation process and CF feature based on the evolution topology of converter valve conduction in detail. Secondly, the energy in the leakage inductance of converter transformer is presented to characterize the commutation state of the valves. Then a CF identification method is proposed utilizing the leakage inductance energy. Thirdly, taking the key variable which is crucial to the tendency of extinction angle during the recovery process into account, a fault current limiting strategy for SCF mitigation is put forward. Compared with the original methods, the proposed methods have a better performance in CF identification and mitigation in terms of detection accuracy and mitigation effect. Finally, case study on PSCAD/EMTDC validates the proposed methods.

Generic Analysis Framework for Modular Multilevel Converter HVDC with Multi-Infeed Line-Commutated Converter HVDC System

This paper proposes a generic analysis framework for a grid supporting modular multilevel converter (MMC)-high voltage DC (HVDC) in a multi-infeed of line commutated converter (LCC) and MMC (MILM) system. MMC-HVDC can support the grid by compensating for the exact reactive power consumptions within the MMC-HVDC system and the varying power system conditions in the MILM system. Maximum active/reactive power capability (MPQC) curve and PQ loading curve comparison process is introduced to properly design a grid supporting MMC-HVDC. While the MPQC curve presents the maximum PQ range of the MMC-HVDC system based on the submodule capacitance value and the modulation index, the PQ loading curve presents the reactive power requirement from the power system that MMC-HVDC needs to compensate. Finally, the comparison of these two curves yields the proper value of submodule capacitance and the modulation index for sufficiently supporting the MILM system. The proposed framework is validated with detailed PSCAD/EMTDC simulation; it demonstrated that it could be applied to various power system conditions.

CALCULATION METHOD FOR THE CONTRIBUTION OF HVDC STATIONS WITH LINE-COMMUTATED CONVERTES (LCC) TO THE AC SHORT-CIRCUIT CURRENT

Short-circuit currents are a key measure for the planning of AC high voltage electrical networks. In this paper, the short-circuit current contribution of a HVDC system with line-commutated converters (LCC) to the short-circuit current in the AC system is investigated. Based on simulations in PSCAD/EMTDC the influence of different factors on the short-circuit current contribution is studied. These factors include the rating of the HVDC system, its operating point, different DC links such as back-to-back links, cables and overhead lines with varying DC line lengths and the type of short circuit and its distance from the HVDC converters. Based on the findings relevant factors are identified. On the basis of the simulation results a simplified analytical approximation for the short-circuit current contribution is derived, which takes into account the identified relevant factors.

Transient stability emergency control for AC/DC grids considering successive commutation failures

The prevalence of Line-Commutated Converter (LCC) HVDC makes the coupling and interaction phenomenon between AC system and DC system of particular complicated. There are significant challenges to determine control strategy using state-of-the-art approaches especially when discrete events (e.g. switching and state resetting) happen frequently due to successive commutation failures (CFs). An enhanced transient stability emergency control (TSEC) for AC/DC grids based on the optimal control is presented to maintain the transient stability and mitigate the successive CFs caused by AC faults. In the proposed TSEC method, an extinction advance angle constraint is added to the TSEC model to mitigate the successive CFs. The emergency power control of LCC-HVDC is employed as an additional control measure to reduce generator-tripping and load-shedding amount. The direct sequential approach is adopted to obtain the optimal solution of TSEC to improve the calculation efficiency. In the direct sequential approach, the modified adjoint sensitivity analysis (MASA) with the consideration of adjoint jump conditions is utilized to increase the gradient calculation accuracy. The effectiveness of the control strategies and the accuracy of the sensitivity calculation are verified on two test systems.

Modeling and Design of Parallel LCC-VSC Interlinking Converters’ Unified Controller in AC/DC Network

While both line-commutated converters (LCCs) and voltage-source converters (VSCs) have been adopted in ac/dc grids, they are mostly used individually despite some point-to-point dc transmission. The combination of both technologies as a single interlinking converter (IC) unit in an ac/dc grid can bring advantages, such as capacity expansion, bidirectional power flow, reactive power support, and harmonics compensation. For such a parallel LCC-VSC unit, a suitable control scheme and careful control parameters’ design are important to ensure that the system can maintain the above advantages in different operation modes. In this article, the control scheme and parameter design for a parallel LCC-VSC unit for use in an ac/dc grid are conducted with a focus on capacity expansion scenario, where a unified control scheme that reserves most of the original LCC terminal’s control while also smoothly incorporates the VSC operation is considered. The operation states of a hybrid ac/dc network are modeled. The proposed unified control scheme is considered for each state for the stability study. Then, a set of control parameters for the unified control scheme suitable for all operation states with good dynamics and stability is designed. Real-time simulations and experiments are conducted to verify the design and study.

Average-Value Modeling of Line-Commutated AC–DC Converters With Unbalanced AC Network

AC–DC line-commutated converters (LCCs) are widely utilized in existing and emerging ac–dc power systems. Analysis of such systems under balanced and unbalanced conditions is often carried out using electromagnetic transient (EMT) simulation programs. Therein, the detailed switching models of LCCs are often the computational bottlenecks for system-level studies. Recently, the parametric average-value models (PAVMs) have been developed to achieve fast and efficient simulations of switching converters. In this paper, the PAVM methodology is extended to consider operation of LCCs under unbalanced conditions in the ac network. This is done in the extended PAVM by formulating the ac-side harmonics in positive and negative sequences as well as the dc-side harmonics (i.e., ripples) with respect to the ac network imbalance. The new PAVM is validated experimentally and by simulations, and is demonstrated to be accurate in reconstructing the ac and dc waveforms under unbalanced conditions in the ac network. Meanwhile, the proposed PAVM is computationally much faster than its detailed switching model counterpart.

Analyses and Comparisons of Generic and User Writing Models of HVDC System Considering Transient DC Current and Voltages

Analyses and comparisons of generic models and a novel modeling method of line-commutated converter (LCC)-based high voltage direct current (HVDC) systems are presented for the power system simulator for engineering (PSS/E) simulation tool. The main purposes are to describe the dynamics of the DC voltage and current of HVDC system and to improve estimation of the maximum values using the generic and proposed HVDC models when AC line-to-ground faults are occurred. For the generic HVDC models, the characteristics and limitations as well as parameter investigations are conducted. Three modules are also are developed for the proposed HVDC model, which are composed of (a) AC/DC conversion; (b) controller selection; and (c) DC line model. Case studies were conducted considering the real-operated HVDC system in Korea under the PSS/E and power systems computer aided design (PSCAD) simulation environments. The simulation results were compared with measured data from the real-operated HVDC system and the results from the PSCAD HVDC models considering single- and three-phase line-to-ground faults. The case study results show that a specific generic HVDC model in PSS/E can simulate the dynamics of the DC voltage and current after the AC line-to-ground faults. The case studies also showed that the proposed modeling method is effectively improves the estimation of the maximum DC voltage and current values for the AC line-to-ground faults.

HVdc application: A valuable source for education and reference [Book Review

This book provides a valuable update to the first edition of High Voltage Direct Current Transmission. Jovcic is a professor at the University of Aberdeen, Aberdeen, United Kingdom, where he has been since 2004. He has a Ph.D. from the University of Auckland, New Zealand. Jovcic is a Fellow of IEEE. His research interests include high-voltage (HV) dc, dc grids, and control systems. High Voltage Direct Current Transmission provides comprehensive coverage of HVdc transmission and in-depth knowledge in key areas for the application of HVdc: power electronics, control engineering, and ac/dc power transmission systems analysis. The application of HVdc transmission is experiencing rapid growth and leading to new technology developments. This second edition provides new material to cover these developments with emphasis on developments going into new field installations. The book is divided into three major sections. The first section covers line commutated converter (LCC) HVdc. This edition includes system topologies and control schemes and adds a discussion of dc grid master controls. This section includes a discussion of dc/ dc converters for connecting systems at different voltage levels and HVdc circuit breaker technologies. The author provides many examples throughout the book. This book is suitable for graduate-level power engineering and power electronics students. It also serves as a valuable reference for researchers in academia and engineers in the industry.

Coordination of synchronous compensators and converters to improve the commutation failure recovery performance in HVDC systems

A novel control strategy of synchronous compensators and converters is proposed to increase the ability of line commutated converter (LCC) based HVDC systems to successfully recover from commutation failures (CFs). The dynamic var support is one of the most important factors, which impact the commutation process. Many synchronous compensators have been implemented to enhance the transient voltage stability for improving the CF recovery performance. However, the traditional automatic voltage regulator (AVR) of synchronous compensator may not be suitable for the commutation voltage support, because the transient var demand of converters during the CF recovery is neglected. In our work, the voltage control will be adaptively adjusted, depending on the state of converters. Case studies based on simulations in PSCAD/EMTDC show that the proposed strategy can improve the performance of fault recovery from CF.

Fast probability estimation of HVDC successive commutation failure caused by AC grid cascading failures

For line-commutated converter (LCC) based HVDC system, successive commutation failure (SCF) can result in continual cessations of power transmission or even forced blocking of the DC system. AC grid cascading failure near inverters is an important reason for the SCF. The SCF probability is utilized to represent the impact of AC grid cascading failures on DC systems. This paper proposes a fast probability estimation method of SCF based on sequential importance sampling (SIS) and stacked denoising autoencoder (SDAE). Firstly, the probability estimation of SCF caused by cascading failures is transformed into a statistical analysis problem. Secondly, considering AC line failures have different impacts on DC system, a cascading failure sampling method based on SIS is presented to efficiently generate cascading failure samples and evaluate the SCF probability. Thirdly, SDAE based assessment network is established to quickly assess the impact of AC failures on SCF. Topology features related to AC grid structure and short-circuit fault locations are selected as inputs of the assessment network. Finally, real-life hybrid AC/DC grids are adopted to demonstrate the validity of the proposed method.

Operation and Challenges of Multi-Infeed LCC–HVDC System: Commutation Failure, AC/DC Power Flow, and Voltage Stability

This paper presents a detailed analysis of commutation failure, AC/DC power flow, and voltage stability of multi-infeed high-voltage direct current (HVDC). The use of HVDC power transmission technology has become common in modern power systems. During the past two decades, HVDC technology has been extensively used for long-distance bulk power transmission to remote areas. Throughout the world, the demand for power has drastically increased in recent years due to industrialization; such situations make HVDC an economic candidate because the distance between power generation plants and load areas is significantly very long. The line-commutated converter (LCC) technology-based HVDC system is well more mature than other available conversion schemes (i.e., voltage source converters), and it is widely used in high-power projects. China had approximately 50 HVDC–LCC links in 2020, and a single LCC-based link with the highest capacity is 12 GW. The installation of several HVDC links in an existing power network has led to a situation where two or more HVDC links terminate in the electric vicinity of each other’s AC network or even in same AC busbar. Such scenarios are termed multi-infeed HVDC system. Multi-infeed HVDC systems bring various challenges related to voltage stability, local and concurrent commutation failure, and AC/DC power flow. Here, the literature available on these phenomena of LCC-based HVDC is discussed for future research. The assumptions and drawbacks of various techniques used for investigating the mentioned phenomena are also highlighted.

Hierarchical Voltage/VAR security boundaries and margin assessments in Multi-HVDC infeed systems

Operational security issues such as a lack of voltage/VAR support and cascading failures triggered by an initial fault present crucial challenges and issues in systems with multiple high-voltage direct current (HVDC) infeeds with both line-commutated converter (LCC) and voltage-source converter (VSC) links. Due to the complexity of the switching strategy for the filter banks in the converter stations, the modeling of the voltage control process inside converter stations is also complex; however, this complexity is not considered in the existing methods, leading to low calculation accuracy. Therefore, this paper proposes an approach based on hierarchical voltage/VAR security boundaries (HVVSBs) and margin assessments for multi-HVDC infeed systems to ensure high calculation accuracy and speed. The voltage/VAR control characteristics of LCC/VSC stations are appropriately formulated by means of a stepwise search method and boundary information, and an HVVSB-based method of dynamic reactive power reserve evaluation is proposed that is suitable for extreme scenarios. Finally, the proposed method is tested in a modified IEEE-118 system to verify its accuracy and effectiveness.

Feasibility Evaluation on Elimination of DC Filters for Line-Commutated Converter-Based High-Voltage Direct Current Projects in New Situations

The line-commutated converter (LCC)-based high voltage direct current (HVDC) technology has been widely applied on long-distance and bulk-capacity power transmission occasions. Due to the terrible interferences in the vicinity of communication lines, DC filters (DCFs) are always installed to mitigate the interferences within acceptable levels for almost all in-service overhead line transmission LCC-HVDC schemes. With the rapid development of the communication technology, however, the anti-interference capability of the telephone system has been remarkably improved. Thus, the original purpose of employing DCFs has been virtually absent, and the necessity of the DCFs shall be re-evaluated in sufficient depth not only for new LCC-HVDC projects, but also in the case of refurbishment of older projects. To demonstrate this constructive topic, this paper carries out a commercial ±800 kV/8000 MW LCC-HVDC project as an illustrative example to analyze and discuss those crucial aspects, which may influence the LCC-HVDC stable operation and reliability after removing the DCFs. Then, the paper studies the harmonic voltage/current stresses of the DC equipment, the DC loop low-order harmonic resonances, and the overvoltage under the switching surge and lightning stroke. Finally, it is concluded that the DCF elimination mainly affects the harmonic steady-state stresses of the DC equipment, but has little influence on the transient stresses. For the refurbishment of older projects, the evaluation on the cost between the DCFs’ maintenance cost and the equipment modification is needed for the DCFs’ elimination. For new LCC-HVDC projects, the DCFs’ elimination or at least simplification may be a more economical and attractive alternative, thereby reducing the footprint and cost.