Line Commutated Converter HVDC Research Articles

Transient stability margin assessment of AC/DC hybrid system with commutation failure involved

The commutation failure of Line Commutated Converter-High Voltage Direct Current (LCC-HVDC) caused by AC fault has an increasingly serious impact on the transient stability of AC/DC hybrid system. This paper presents a novel transient stability margin assessment approach based on energy function, where generators adopt 6th order electromechanical model and DC system adopts electromagnetic model. First, the network structure-preserving model of AC/DC hybrid system is established, and its energy function is deduced. Next, the energy function of post-fault system is approximately calculated by trapezoidal integral path, and energy function based transient stability criteria are put forward to determine whether the post-fault system is stable or not. Then, Bisection Method (BM) is employed to obtain Critical Clearing Time (CCT) quickly and effectively where CCT can be adopted as index of stability margin. Finally, the New England-39 bus system integrated with LCC-HVDC is used as test system to investigate the effectiveness of the proposed transient stability margin assessment approach.

An Improved AC System Strength Measure for Evaluation of Power Stability and Temporary Overvoltage in Hybrid Multi-Infeed HVDC Systems

We developed a new strength measure, the hybrid multi-infeed effective short-circuit ratio (HMESCR), for evaluating hybrid multi-infeed high-voltage direct current (MIDC) systems. The HMESCR defines the strength of hybrid MIDC systems with line-commutated converter (LCC) high-voltage direct current (HVDC) and voltage-source converter (VSC) HVDC systems. Unlike previous measures, the HMESCR considers the capacity of VSC HVDC systems to provide reactive power and thus, enhance the strength of AC systems connected at different buses with LCC HVDC systems. To demonstrate how the HMESCR reflects power stability (PS) and temporary overvoltage (TOV), we used MATLAB/Simulink software to perform quasi-steady-state analyses and dynamic simulations. The HMESCR can be used to determine the strength of hybrid MIDC systems.

A Dynamic Series Voltage Compensator for the Mitigation of LCC-HVDC Commutation Failure

To effectively mitigate the commutation failure (CF) of the line-commutated converter high-voltage direct current (LCC-HVDC) transmission system, a dynamic series voltage compensator (DSVC) scheme is proposed. The DSVC consists of three integrated gate-commutated thyristor (IGCT)-based full-bridge submodule (FBSM) chains to match the bulk power transmission of the LCC-HVDC. Inserted between the transformer and the AC port of the inverter, the DSVC assists the commutation process by superposing the transient voltage to the AC line voltage and increasing the commutation margin of the inverter valve. Operation and control strategies, which are two critical techniques for the DSVC, are designed based on the working states of the FBSMs, assisting the commutation process and balancing the DSVC capacitor dynamic voltages. The CF detection method is another critical technique of the DSVC control and is improved considering the reverse time of the LCC-HVDC thyristors. Compared with the topologies proposed in existing literature, the DSVC has better performances in safe and stable operation due to the urgent control strategy considering situations beyond its ability. A detailed EMT simulation shows that the DSVC can respond quickly to mitigate CFs and validate the effectiveness of the overall design and key techniques. Furthermore, the operation and control strategy can limit the current stress once it is beyond the capability of the DSVC, which guarantees its feasibility and practicability.

Analysis of fault-induced inrush current of converter transformer in LCC HVDC system considering DC control and protection

Converter transformers are vital devices in line-commutated-converter (LCC) high-voltage direct-current (HVDC) systems. This paper introduces a new inrush current induced by a valve-side asymmetric ground fault on a converter transformer. It is defined as a “fault-induced inrush current (FIIC).” Taking a valve-side single-phase-to-ground (VSSPG) fault on a rectifier-side converter transformer as an example, the FIIC induced by this fault and its characteristics are first analyzed. Subsequently, the differences of the FIICs caused by the VSSPG faults occurring at the transformers in wye-grounded-wye (Y0/Y) and wye-grounded-delta (Y0/Δ) connections are discussed. After that, the effect of HVDC control and protection (C&P) on the characteristics of FIICs is considered. Finally, the impact of the FIIC on the transformer differential protection is analyzed. It is concluded that the transformer differential protection has a risk of being inappropriately blocked because of the FIIC. A suggestion for the HVDC C&P to apply a shorter protection action delay is proposed to reduce the risk. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC. This study will be valuable for the research of inrush currents of converter transformers.

Real-Time LCC-HVDC Maximum Emergency Power Capacity Estimation Based on Local PMUs

The maximum active power that a Line-Commutated-Converter High Voltage Direct Current (LCC-HVDC) can provide instantly, called LCC-HVDC-MC Maximum Emergency Power Capacity, is an important information for determining the emergency control strategy of a power system subject to a large disturbance. However, it is a challenging task to calculate this capacity accurately due to system model uncertainties and contingencies. To address this problem, we propose to estimate this capacity by real-time tracking the Thevenin equivalent (TE) parameters of the AC system based on local PMUs. This new TE estimation method can handle the large disturbances and yield an accurate HVDC-MC while considering various constraints. Specifically, we first investigate the impacts of the TE potential changes on the TE impedance estimation under a large disturbance. To reduce the estimation error caused by the TE potential change, we develop an adaptive current screening process of the current measurements. Moreover, we use the total least square estimation method to further filter out the PMU errors. Finally, we calculate LCC-HVDC maximum emergency power capacity while accounting for the limitations of the control performance, voltage dependent current order limit, the converter capacity, and the AC voltage. The simulations show that the proposed method can accurately track the TE parameters and the real-time LCC-HVDC maximum emergency power capacity following a large disturbance.

Switching System's MLE Based Transient Stability Assessment of AC/DC Hybrid System Considering Continuous Commutation Failure

The commutation failure of Line Commutated Converter-High Voltage Direct Current (LCC-HVDC) cannot be avoided. This paper presents a novel transient stability assessment approach for AC/DC hybrid system considering continuous commutation failure. First, the conception and mathematical background of Maximum Lyapunov Exponent (MLE) of switching system is introduced, and the switching compensation matrix is derived. Next, the switching system model of AC/DC hybrid system considering continuous commutation failure caused by AC faults is established. On the basis of switching compensation, the MLE of switching system is calculated accurately, and the MLE based criteria are proposed to assess the transient stability of the switching system. Finally, the modified 39-bus system with LCC-HVDC added and the real power system are used as test systems to investigate the validity and effectiveness of the proposed transient stability assessment approach.

Response Mechanism of DFIG to Transient Voltage Disturbance Under Commutation Failure of LCC-HVDC System

Commutation failure is one of the common faults in the line commutated converter high voltage direct current (LCC-HVDC) transmission system, which will lead to a transient voltage disturbance with the characteristic of “decrease first and then increase” at the feed end. Such a transient voltage may cause severe dynamic responses on the wind generator adjacent to the converter station, resulting in the tripping of large-scale wind turbines and even cascading failure. By analyzing the principle of the transient voltage, this paper investigates the response mechanism of doubly-fed induction generator (DFIG), involving the variation of electrical parameters and fault ride through (FRT) ability. Further verifications through simulations and experiments prove that under the disturbance of commutation failure, DFIG may suffer greater transient shocks compared to a single sag or swell disturbance event. Finally, some suggestions are put forward to mitigate the transient voltage and improve the transient response of DFIG.

Study on the reactive power coordinated control in hybrid parallel HVDC system

Hybrid parallel HVDC system adopts line commutated converter high voltage direct current(LCC-HVDC) and voltage source converter high voltage direct current(MMC-HVDC) parallel structure, which combines the advantages of LCC-HVDC and MMC-HVDC. Based on the existing reactive power control of HVDC system, the reactive power coordination control strategy of hybrid parallel HVDC system is innovatively proposed in this paper. Firstly, the reactive power control of LCC-HVDC and MMC-HVDC are studied respectively. And then the voltage stability of LCC-HVDC fed weak AC system is analyzed. Furthermore, a reactive power coordination control strategy between reactive power compensation device of the converter station and MMC-HVDC is proposed. Finally, based on PSCAD/EMTDC simulation platform, a hybrid parallel HVDC simulation system is built. The reactive power coordination control strategy under steady and transient conditions is simulated and analyzed respectively. The analysis results show that the reactive power coordinated control strategy can suppress the voltage fluctuation of converter bus in steady state. At the same time, the reactive power coordinated control strategy can restrain the commutation failure of LCC-HVDC in the fault of receiving power grid and improve its fault recovery ability.

A Backup Protection Algorithm for Bipolar Line-Commutated Converter HVDC Lines

This article proposes a new backup protection algorithm for bipolar high voltage alternative current (HVdc) lines using two-end synchronized sampled currents. Unlike traditional differential protection, the proposed algorithm can be applied to detect and recognize the faulted poles directly without the requirements for long time delay. The currents at both line ends are sampled at 1 kHz and the noise in the current signals is removed utilizing the stationary wavelet transform. To activate the proposed backup protection algorithm, initialization unit is proposed. In addition, the internal faults can be discriminated from external faults and the faulted poles can be recognized using adaptive signed correlation indices. Comprehensive studies are simulated on PSCAD/EMTDC software considering several cases for external and internal faults. In addition, the frequency dependent phase model is employed to simulate the HVdc transmission line. The simulation results assure the satisfactory performance of the proposed backup protection algorithm for several fault locations, all fault types, and several fault resistances up to 1000 Ω.

Multirate and Mixed Solver Based Cosimulation of Combined Transient Stability, Shifted-Frequency Phasor, and Electromagnetic Models: A Practical LCC HVDC Simulation Study

The design of control and protection system of the line-commutated converter (LCC) in a meticulous way should consider both dynamics of the LCC and large-scale ac grids. Consequently, a highly accurate and efficient simulation method is desirable. In order to capture the high-frequency dynamics between ac/dc grids, the multirate and mixed solver based cosimulation by combining transient stability (TS), shifted-frequency phasor (SFP), and electromagnetic transient (EMT) models is proposed. The idea of the proposed multirate and mixed solver based cosimulations is to naturally decouple the whole system into the respective TS, EMT, and SFP subsystems. The accuracy of the proposed multirate cosimulation is guaranteed by the proposed interface models, i.e., the TS-SFP interface model (TS-SFP-IM) and the SFP-EMT interface model (SFP-EMT-IM). The efficiency is guaranteed by the natural partitioning strategy, decoupled manner based communication structure and the overall realization of time sequence. The performance of the proposed multirate cosimulation has been validated on practical ac/dc system in China.

Mitigation of commutation failure method in LCC converter based on HVDC systems by mean of modeling and simulation

Commutation failures is the commonest perturbation can appear in the inverter valves during or after the faults. Therefore, understanding the system fault is very important for regulation and protection design. Simulation by means of digital real time simulator can provide quickly the simulation results, and also represents the most effective methods to find the system fault behavior. This paper proposes a simplified method to detect and avoid the commutations failures in an HVDC inverter. Besides, the inverter is connected to the AC system which is considered as a weak AC grid having a low short circuit ratio. The results are provided by means of the digital real time simulator using the RT-lab platform and achieved by the simulation in the loop approach. Although the real time simulation allows for testing the behavior of these kinds of systems under adverse conditions that rarely occur in the field but are important to evaluate. 12 pulses line-commutated converter HVDC system is used in this study (both stations the rectifier and the inverter are based on thyristor valves). The function of the protection function is validated against the real time simulation of the HVDC inverter following different kind of faults, both kind of fault are the single phase to ground AC fault and a short circuit at the inverter valve. The obtained results show clearly the effectiveness of the proposed method to avoid the commutation failure by ensuring and maintaining a safe commutation process on the inverter station in the worst case.

Considering reactive power coordinated control of hybrid multi‐infeed HVDC system research into emergency DC power support

According to the characteristics of line-commutated converter high-voltage direct current (LCC-HVDC) and voltage source converter high-voltage direct current (VSC-HVDC), respective additional emergency DC power support (EDCPS) controllers are designed, and a coordinated control strategy based on a hybrid multi-infeed HVDC system for EDCPS is proposed. Considering the differences in system recovery performance between LCC-HVDC and VSC-HVDC in providing EDCPS, the emergency power support priority issues and power allocation schemes for LCC-HVDC and VSC-HVDC are discussed in detail. To maintain the stability of the receiving-end AC system during boosting power process and to ensure the active power is raised to the specified value, this study also designs an additional reactive power controller that can make full use of the ability of VSC-HVDC to support the reactive power of the AC system. Finally, a hybrid three-infeed HVDC system consists of two parallel LCC-HVDCs and one VSC-HVDC is built on PSCAD/EMTDC and simulated. The effectiveness of the proposed approach is verified based on this hybrid three-infeed HVDC system.

A multi-area Thevenin equivalent based multi-rate co-simulation for control design of practical LCC HVDC system

The line commutated converter (LCC) based HVDC transmission is often adopted to transmit the large capacity of renewable energy, however, it is threatened by several critical issues, such as, commutation failures (CFs). In order to reduce CFs and improve the recovery characteristics of the HVDC, improved control and protection strategies are always desirable. On the other hand, an accurate and efficient electromagnetic transient (EMT) simulation method is highly necessary to design the so-called control strategy, where the traditional transient stability and EMT models cannot meet the accuracy and efficiency requirements simultaneously. To resolve these issues, a Multi-Area Thevenin Equivalent (MATE) based multi-rate co-simulation method is proposed, where the accuracy is guaranteed by the proposed MATE based transmission line model (MATE-TLM) and the efficiency is significantly improved by adopting several accelerating techniques, such as, the MATE-TLM and multi-rate co-simulation scheme. Further, a novel improved control strategy based on the concept of virtual impedance is proposed to reduce CFs and to increase the recovery speed of DC system. Simulation results of a practical AC/DC grid by the improved control strategy have well validated the effectiveness of the proposed MATE based multi-rate co-simulation method.

A novel AC filter system for the line commutated converter high voltage DC transmission system

This paper proposes a novel AC filter system for a line commutated converter high voltage DC (LCC-HVDC) transmission system. Through the coordination of the hybrid active power filters (APF) and the existing reactive compensation devices, the proposed filter system can not only enhance the suppression performance for LCC-HVDC harmonics, but also optimize the AC yard layout with reduced reactive power subbanks, reducing the cost of HVDC projects. The novel filter system adopts a serial passive resonance topology obtained by careful comparison of different APFs. A proper control scheme is then designed integrating the control strategy of the APF and impedance characteristics of the HVDC system, which is able to realize harmonic suppression and dynamic reactive power support simultaneously. In addition, a novel self-adaption digital low-pass filter algorithm is presented, which is used in the APF harmonic detecting step, enhancing both high precision and fast dynamic response. On the basis of a real HVDC project, the advantages of proposed filter system in harmonic suppression, reactive power regulation, and sub-banks reduction are simulated and demonstrated.

Comparison of Current Limiting Effect of an HTS DC Cable Under Different Short-Circuit Conditions

Power transmission with HTS dc cables has the characteristics of high capacity, high efficiency, more compact size, and environmental friendly. It has become an alternative and revolutionary upgrade of the traditional dc power transmission. The self-acting limit function during its quenching for fault current is unique to HTS dc cables. Fault-current-limiting capability of the HTS DC cable in a line-commutated converter (LCC) HVDC system is therefore analyzed with HTS current-limiting features in different short-circuit situations to identify the HTS dc transmission system design and operation features and verification achieved with the HTS dc cable characteristics and parameters, especially the peak value of the fault currents in the LCC HVDC systems.

Operation characteristics of DC transmission system with large‐scale renewable energy integration

Marked with flexible interconnection and control, the high-voltage direct current (HVDC) gird has captured much attention of industries and academics. Hybrid dual-infeed or multi-infeed HVDC composed of line-commutated-converter HVDC (LCC-HVDC) and voltage source converter HVDC (VSC-HVDC) will form the main pattern in a further power grid; meanwhile, the new gird pattern will bring new opportunities and challenges to security and stability control in the power system. First, research works on the control strategies and operation performances of LCC-HVDC and VSC-HVDC are stated in this paper; then, a model of wind power integration into a dual-infeed DC transmission system is established in PowerFactory, and case studies are conducted in both steady and transient states. On this basis, a new control strategy for variable-speed constant-frequency wind power generators to promote voltage characteristics of the DC network is designed in this paper, and two additional active power control segments are designed in the traditional control system; thus, DC voltage stability can be improved by fast regulation of active power output due to quick power adjustment of wind power generators; simulations are implemented and the results will lay a foundation for safe and stable operation in the DC transmission system with renewable energy integration.

Dynamic Phasor Modeling of LCC-HVDC Systems: Unbalanced Operation and Commutation Failure

This paper develops a dynamic phasor model of a line-commutated converter high-voltage direct current (LCC-HVDC) system that includes unbalanced operation and commutation failure. This model is beneficial in studying the behavior of the HVDC systems under asymmetrical faults, and it complements the conventional transient simulation-based methods. Equally, importantly, the model includes the representation of commutation failure that may arise as a result of severe faults. Due to its selectivity of the considered harmonics, the developed model has markedly less computational intensity than an electromagnetic transient (EMT) simulation model. This gives the model a distinct advantage in the study of large-signal transients of the LCC-HVDC systems, particularly where repetitive simulations are required. Validation of the proposed model is done using a detailed EMT model of the CIGRE monopolar LCC-HVDC benchmark.

Study on the Influence of Inconsistent Valve Parameters on LCC-HVDC Commutation and Operation

Line commutated converter high voltage direct current (LCC-HVDC) transmission has unique advantages and plays an irreplaceable role in the power system. It is of great significance to study the commutation process. The traditional ideal model of commutation process neglects many valve parameters. In this paper, the consistency problems of valve parameters and early slight faults of converter station are considered. The commutation process is studied and a more accurate module is built, which revises the traditional ideal commutation model. Based on the parameters of Haminan-Zhengzhou HVDC transmission project, the analysis results of the theoretical model are given. The results show that the consistency of valve parameters and the early slight fault of valve bridge arm have obvious influence on LCC-HVDC commutation process and stable operation. Finally, this model is verified by Matlab/Simulink simulation. Based on the model proposed, the control angle of the inverter valve can be calculated and controlled more accurately. Besides, the reactive power demand can be optimized and the commutation failure can be mitigated. What's more, the slight fault and aging of the valves can also be detected by monitoring of the commutation process. The paper aims to provide theoretical support for the economical and reliable operation of LCC-HVDC transmission.

Reliability and Power Density Increase in a Novel Four-Pole System for Line-Commutated Converter HVDC Transmission

Reliability and Power Density Increase in a Novel Four-Pole System for Line-Commutated Converter HVDC Transmission

New Insights Into Coupled Frequency Dynamics of AC Grids in Rectifier and Inverter Sides of LCC-HVDC Interfacing DFIG-Based Wind Farms

Coupling between frequency dynamics of the ac systems on both inverter and rectifier sides of the line-commutated converter HVDC with the rectifier station operating in frequency control is studied, along with the presence of large DFIG-based wind farms on the weak rectifier-side grid. An averaged model with 79 states, which includes dynamic models of grids on the rectifier and inverter sides, phase-locked loop, and the wind farm is derived. To develop a deeper understanding of the frequency dynamics, a simplified four-state nonlinear model is proposed, which, in turn, reveals strong coupling between frequency and ac voltage at the HVDC rectifier terminal. A firing angle correction strategy is proposed to decouple frequency–voltage interactions, thereby improving the frequency dynamics on the rectifier side. The four-state model is linearized to ascertain the interaction between rectifier- and inverter-side frequencies, and an analytical expression for the frequency dynamics in terms of gains of the frequency controller at the rectifier station is derived. Moreover, the proposed reduced-order model shows the implications of frequency droop control of the wind farms in improving frequency dynamics on both rectifier and inverter sides. Expressions for “synchronizing” and “damping torque” contribution from HVDC and wind farm are also established. The analytical expressions and the effectiveness of the proposed strategies are validated through nonlinear time-domain simulations.