Line Commutated Converter Based High Voltage Direct Current Research Articles

Transient overvoltage suppression of LCC‐HVDC sending‐end system based on DC current control optimisation

AbstractThe receiving‐end system AC fault of the line‐commutated‐converter‐based high voltage direct current (LCC‐HVDC) will lead to commutation failure of the inverter side. During the fault and its recovery, AC transient low voltage and transient overvoltage (TOV) will occur in the sending‐end system. The TOV has the risk of triggering the disorderly off‐grid of the nearby renewable power generations. Besides, in a serious situation, it will threaten the power system to maintain a secure and steady operation. Therefore, the authors analyse the mechanism involved in the AC transient voltage during the AC fault and the recovery period first. It reveals that the key factor causing the TOV of the sending‐end system is the setting of the DC current reference value. Then, a DC current reference value limit method based on the AC TOV sampling value is proposed, which is used to accelerate DC current recovery and suppress the TOV of the sending‐end system. Finally, the effectiveness of the designed control method has been confirmed through electromagnetic transient simulations using the CIGRE HVDC benchmark model and a ±800 kV HVDC transmission system model situated in Northwest China.

Transient characteristics and protection coordination strategy of hybrid AC / DC system based on modular multilevel converters under AC/DC transmission lines touching fault

Rapid development of hybrid AC/DC transmission systems has increased the probability of the touching faults between AC and DC overhead lines. The mechanism of coupling interaction is so complex that the adaptability of the existing hybrid system protection is affected. Previous studies mainly focus on the line-commutated converter based high-voltage direct current (LCC-HVDC) system and AC system, this paper concentrates on the fault coupling characteristics analysis and coordination protection of modular multilevel converter based on high voltage direct current(MMC-HVDC) and AC transmission systems. The analytical expressions of fault current are deduced under the different operating states of the converters and the valve-side AC circuit breaker. Subsequently, the fault current coupling characteristics and generation mechanism are revealed. On this basis, the adaptability of existing hybrid system protection is assessed. Further, by comparing and analyzing the influence and risk of mal-operation of AC transmission line protection, the coordination reclosing protection strategies for hybrid system is proposed. Finally, the simulation tests were carried out based on PSCAD/EMTDC. The simulation results show that the proposed coordination of AC and MMC-HVDC system reclosing strategy is effective for improving the reliability of hybrid system.

Investigation on grid-following and grid-forming control schemes of cascaded hybrid converter for wind power integrated with weak grids

The conventional line commutated converter based high voltage direct current (LCC-HVDC) system is no longer suitable for transmitting wind power integrated with weak grids. Therefore, a cascaded hybrid converter consisting of LCC and modular multilevel converter (MMC) connected in series is considered as a promising solution in this paper. And a grid-following (GFL) control scheme and a grid-forming (GFM) control scheme are designed for the cascaded hybrid converter. To investigate the stability performances of the proposed two control schemes, this paper develops a small-signal model of the cascaded hybrid HVDC system for wind power integrated with weak grids. Then, based on the eigenvalue analysis method, several issues of concern are investigated for the proposed GFL and GFM control schemes, including their tolerance to a weak grid and a low wind power output, significant participants and sensitivity to parameter detuning. Based on the analysis results, one of the two that is more suitable for weak grids is recommended. The time-domain simulations implemented in PSCAD/EMTDC confirm the validity of the model and analysis results.Index Terms—Cascaded hybrid converter, grid-following, grid-forming, wind power, weak grids.

Impact of Time Delay on the Control Link in Small Signal Dynamics of LCC-HVDC System

The time delay in the control link will affect the small signal stability of the line commutated converter based high voltage direct current (LCC-HVDC) system. This paper establishes the small signal model for the LCC-HVDC system considering the time delay in the control link. Then, based on the developed models, the movement feature of the eigenvalue loci with varying time delay under different AC system strength is investigated, the impact of the time delay on the system stability margin is quantitatively evaluated, and the frequency band affected by the time delay is identified. Subsequently, influence mechanisms of time delay on system stability are revealed from the perspective of the system damping characteristic by decomposing different damping paths. Moreover, impacts of time delay on DC current/voltage controller's stability margin are investigated. Theoretical analysis and simulation results indicate that, the increase of time delay at the rectifier-side will continuously deteriorate the system stability, as well as the system dynamic performances, while in most cases the increase of the time delay at the inverter-side imposes a non-monotonic effect on the system stability, which is weakened first and then slightly strengthened. Therefore, in certain scenarios, the larger time delay at the inverter-side may play a positive role in the system stability to some extent. The results can provide certain technical reference for the identification of key influencing factors causing the system oscillation and reasonable selection of control parameters in practical LCC-HVDC projects.

Novel Controllable-Line-Commutated Converter for Eliminating Commutation Failures of LCC-HVDC System

To entirely eliminate commutation failure (CF) in conventional line-commutated converter based high-voltage direct current (LCC-HVDC) transmission system, a novel controllable line-commutated converter (CLCC) based on the combination of partially and fully controllable semiconductor devices is presented in this paper. The operating principle of CLCC and its control strategy are proposed. In addition, the voltage and current stress are analyzed. To verify correction and application of presented CLCC solution to the project, a CLCC-HVDC system model, adopting LCC at the rectifier side and CLCC at the inverter side, is developed in PSCAD/EMTDC software. Based on the model, the transient responses of the CLCC-HVDC system under various AC faults are simulated and compared with the pure LCC-HVDC. Simulation results show that the presented CLCC topology and its control strategy can not only eliminate CFs in various types of AC faults, but also improve the transient performance of the system after faults, while inheriting the attributes of traditional LCC such as large capacity, low cost and almost the same system control strategy. This is quite positive and meaningful for application of the CLCC-HVDC to multi-DC infeed system particularly.

An Improved Quantitative Analysis Method of Oscillation Mode for DFIG-Based Wind Power Base With LCC-HVDC Transmission Considering Frequency Coupling Characteristic

The line-commutated converter-based high-voltage direct-current (LCC-HVDC) transmission has become one of the main forms to deliver wind power in China. However, due to the connections of a large number of power electronic devices (PEDs), there emerges a risk of sub/super-synchronous oscillations in the DFIG-based wind power base with LCC-HVDC transmission. The existing multi-node system analysis methods have ignored the frequency coupling characteristic (FCC) of PEDs, unable to obtain accurate oscillation characteristics. Besides, the relationship between the oscillation and multi-level factors has not been revealed yet. Therefore, this paper proposes an improved quantitative analysis method of oscillation mode which considers the FCC on the basis of the system admittance network model. Firstly, the impedance model of the system considering the FCC is established. Then, the improved quantitative analysis method of the oscillation mode considering the FCC is proposed and its steps are as follows. 1) To establish the system transfer function matrix considering the FCC, and determine the system stability accurately by the dominant oscillation mode; 2) To define the node oscillation participation factor, considering the effect of the coupling frequency, and quantify the participation degree of each node and identify the origin and weak point of oscillation; 3) To carry out multi-level oscillation mode sensitivity analysis. Finally, the oscillation characteristics and laws of the DFIG-based wind power base with LCC-HVDC transmission are studied, and the time-domain simulations in MATLAB/Simulink validate the analytical results.

Continuous commutation failure suppression method based on dynamic compensation of firing angle deviation

In the line commutated converter based high voltage direct current(LCC-HVDC) transmission system, it has important meaning to suppress continuous commutation failure(CCF) for the safe and stable operation of the system. Aiming at CCF caused by poor dynamic performance of DC control system, this paper first analyzes that firing angle deviation is an important cause of CCF in the dynamic process of recovery after first commutation failure(FCF). Then, the reasons for firing angle deviation caused by the underlying firing control at this stage are analyzed: One is the phase deviation caused by the inherent response of the phase locked loop(PLL), which is a second-order system; the other is the phase deviation of the asymmetric commutation voltage. On this basis, a CCF suppression method based on dynamic compensation of firing angle deviation is proposed. The method dynamically and adaptively compensates firing angle deviation through the actual measurement value, which improves the controllability of actual firing angle in the dynamic process, thereby suppressing CCF. Finally, it is verified that the improved method has good suppression effects on CCF in the CIGRE HVDC benchmark model.

A novel partitioning method for the power grid restoration considering the support of multiple LCC-HVDC systems

The restoration of the modern alternating current-direct current (AC–DC) hybrid power grid after a blackout is difficult and complex. By using the partitioning restoration strategy and the support of high voltage direct current (HVDC) system, the restoration speed of the blackout power grid can be greatly accelerated. Aiming at this problem, this paper proposes a partitioning method for the power grid restoration with multiple line commutated converter-based HVDC (LCC-HVDC) systems. Firstly, by introducing the system strength indices to quantify the interaction between AC and LCC-HVDC systems in the restoration process, a partitioning model considering multiple LCC-HVDCs is proposed. Wherein, the transmission power is optimized to make full use of LCC-HVDCs’ support capability. Then, by combining the system strength characteristic and community structure characteristic of the power grid, a partitioning method based on an improved Girvan Newman (GN) algorithm is proposed. Finally, the results are studied based on a modified IEEE 39-bus system and a realistic partial power grid of southwest China. Compared with two typical partitioning methods for the AC power grid restoration, the results show that the proposed method can effectively use the DC support power and speed up the restoration progress.

Analysis and Prevention of Commutation Failure Caused by Line Coupling Effect in LCC-HVDC

In actual line-commutated converter based high- voltage direct-current (LCC-HVDC) projects, multiple HVDC transmission lines are usually installed on the same tower to save transmission corridors and reduce construction costs. However, electromagnetic coupling effect between adjacent lines can cause commutation failure (CF) in the non-fault poles when a line or pole fails. Hence, this paper deeply investigates the mechanism and the prevention strategy of CFs caused by line coupling effect in single- and double-circuit HVDC systems. The influencing factors of line coupling effect and the coupling characteristics under fault conditions are analyzed first based on the modeling of parallel transmission lines and the traveling wave theory. Then the mechanism of CF caused by line coupling effect is investigated with the consideration of different fault conditions, different line structures, and different system operation modes. Based on the above analyses, prevention strategies for CFs of non-fault poles in single- and double-circuit systems are proposed. Finally, the simulation results verify the correctness of the theoretical analyses and the effectiveness of the proposed control strategies.

Commutation failure suppression method considering chain reaction in multi-infeed LCC-HVDC systems

In multi-infeed HVDC (MIDC) systems, in which HVDC means high-voltage direct current, the complex interactions between AC/DC systems and different line-commutated converter-based high-voltage direct current (LCC-HVDC) produces a special commutation failure (CF) chain reaction phenomenon. The existing methods are mostly aimed at the transient process of the single LCC-HVDC, which reduces the CF suppression effect but may become the driving force of the CF chain reaction. In view of this situation, this study proposes a new CF suppression method based on the CF chain reaction security region. First, the CF chain reaction process of multiple LCC-HVDC systems is analyzed, and the boundary of the CF chain reaction is characterized. Second, by establishing the CF chain reaction security region, the control method of the coordinating DC current and firing advance angle to suppress the CF chain reaction is proposed. Simulation results show that the proposed method avoids the subsequent CF of the LCC-HVDC while maximizing the suppression effect of the successive CF in the adjacent LCC-HVDC, thereby blocking the cascading reaction of CF that usually caused large-scale security problems.

Admissible HVDC infeed capacity evaluation of receiving-end power grids

To fulfil the cross-regional bulk power delivery of renewable energy and alleviate the power shortage pressure of load centers, a number of line-commutated converter-based high voltage direct current (LCC-HVDC) transmission projects have been deployed in worldwide. Due to the system-wide operating risk sharply escalated with the intensively infeed of multiple large-capacity HVDC links, a vital knowledge gap exists in understanding the maximal admissible HVDC infeed capacity (AHIC) of receiving-end power grids. To tackle this issue, this paper proposes a novel AHIC evaluation method, which determines the maximal allowed total number and capacity of HVDC links that the receiving-end grid can accommodate while taking into account multi-preconceived fault security requirements. An improved dynamic system strength criterion is developed to assessing the immunity of post-contingency HVDC commutation failures (CF), which creatively considers the support effect from all AC buses based on the voltage sensitivity. The original two-stage model is decomposed into a construction master problem and an operation optimization subproblem, and then being iteratively solved using the generalized Benders decomposition algorithm. The effectiveness of the proposed approach is demonstrated on a modified IEEE RTS-79 system and a practical large-scale system.

An improved reactive power control strategy for LCC-HVDC to mitigate sending end transient voltage disturbance caused by commutation failures

The sending end transient voltage disturbance (TVD) caused by commutation failure of line commutated converter-based high voltage direct current (LCC-HVDC) systems, which is characterized by “first reduce then rise”, has received increasing attention due to its threat to the stable operation of renewable energy integrated modern power systems. To mitigate the TVD, this paper devises an improved reactive power control strategy that takes into account the signal delay between converter stations. Firstly, the mechanism of the TVD is investigated. It is found that the voltage dependent current order limiter (VDCOL) and signal delay between the rectifier and the inverter stations are two vital factors that impact the TVD, while the VDCOL cannot well cope with commutation failure suppression and TVD mitigation at the same time. On this basis, an improved closed-loop reactive power control (CLRPC) strategy is proposed for the LCC-HVDC to mitigate the TVD. The CLRPC can regulate the reactive power demand of the rectifier station by adjusting the DC current order, which replaces the conventional VDCOL during the disturbance. Furtherly, a delay compensation unit is augmented in the proposed scheme to suppress the adverse effect of signal delay. Simulation results conducted in PSCAD/EMTDC verify that the CLRPC can not only alleviate the TVD effectively but also suppress the occurrence of successive commutation failures, thus, enhancing the overall performance of the sending end power system.

Fast and accurate evaluation method of commutation failure in line‐commutated converter‐based high‐voltage direct current systems considering probability characteristics

Line-commutated converter-based high-voltage direct current (LCC-HVDC) systems suffer from commutation failure (CF) when receiving-end AC grid faults occur, which is a major drawback and threatens the security and stability of power systems. This paper presents a fast and accurate evaluation method of CF for the LCC-HVDC systems, considering the CF probability characteristics. Firstly, based on the analysis of the CF probability characteristics from the view of short-circuit impedance, the CF tolerance index (CFTI) and CF vulnerability index (CFVI) are proposed to quantitatively evaluate the LCC-HVDC systems' risk of the possible- and inevitable-occurrence CF, respectively. Secondly, four factors, that is, commutation voltage, fault occurrence time, DC current and firing angle command, are considered to calculate CF evaluation indices (CFEIs) and identify the CF risk area boundaries based on the theoretical analysis. Moreover, the relationships among short-circuit impedance, fault position, and extinction angle are established based on the AC/DC decoupling method, which helps realize the decoupling of multi-factors affecting CF during fault. Compared to earlier methods, this evaluation method combines the advantages of the accuracy of the electromagnetic transient simulation method and the rapidity of theoretical derivation. Finally, the validity and accuracy of the evaluation method are verified in the CIGRE HVDC benchmark model and IEEE 39-bus system.

Analysis of Transient Voltage Disturbances in LCC-HVDC Sending Systems Caused by Commutation Failures

Commutation failures (CFs) including repetitive CFs (RCFs) are frequently occurring faults in line commutated converter based high voltage direct current (LCC-HVDC) systems. Consequently, sending end system will suffer transient voltage disturbances (TVD) with a characteristic of “first reduce then rise and may repeat”. Due to the complex interaction process of TVD, the electromagnetic process behind this phenomenon cannot be easily explained, which imposes great challenges on the development of mitigation strategies. Given this, this paper provides a deep insight into the mechanism and the impact factors of the TVD caused by CFs and RCFs. The numerical relationship between the transmitting power and the TVD is derived. Research results revealed that the transient process after fault can be segmented into five stages. In each stage, the control interactions among HVDC controllers and power interactions at the PCC are analyzed. Besides, we discussed the TVD under RCFs in detail. Several indices are further proposed to facilitate the characterization of the TVD, and main control loops of HVDC that have impacts on the characteristics of TVD are thereby studied. Finally, the mathematical relationship between wind farm power and the TVD is derived to quantify the impact of wind farms on the TVD.

Faults identification in high voltage direct current transmission lines using adaptive neuro-fuzzy inference system

ABSTRACT This paper presents an innovative approach in line commutated converter-based high-voltage direct-current (LCC-HVDC) transmission line for recognizing the section of the faults. In this work, three adaptive neuro-fuzzy inference system (ANFIS) modules have been suggested to investigate the safety responsibilities in the LCC-HVDC line. The ANFIS-1 aims to ascertain the faults in both sections; ANFIS-2 is to distinguish the section of the fault; and ANFIS-3 is to recognize the characteristics of the fault. To ascertain the effectiveness of the ANFIS approach, it has been compared with the differential protection (DP), wavelet transform (WT), fuzzy and artificial neural network (ANN). Furthermore, the proposed methods have been tested by varying the fault resistance (up to 500 Ω), power angle (from 5 to 15 degrees), near and end faults, and changing the transmission line parameters (±15 %). The simulation outcomes are disclosed that the proposed ANFIS technique yields better performance than other studied methods in terms of percentage of accuracy, time of fault section recognition, and reach setting.

DC fault current approximation and fault clearing methods for hybrid LCC-VSC HVDC networks

The DC-side fault analysis of Line commutated converter based high voltage direct current (LCC-HVDC) and Voltage source converter based high voltage direct current (VSC-HVDC) networks has been studied by most researchers and achieved fruitful results. However, the traditional fault analysis method does not consider in detail the change process of the trigger angle of the LCC converter station and the AC current feeding of the VSC converter station caused by the control strategy. Therefore, this paper proposes a new DC side fault transient analysis method suitable for hybrid multi-terminal HVDC transmission networks. In addition, considering the deficiency that the active fault current suppression method only relying on the control strategy cannot fundamentally clear the DC fault, this paper proposes a new Z-source current-limiting circuit (ZCLC) with reference to the topology of the Z-source dc circuit breaker (DCCB). Then, ZCLC will cooperate with DCCB to improve the fault clearing capacity of the whole system. Firstly, this paper establishes the equivalent Laplace transform model of the three-terminal LCC-VSC-VSC, and analyzes the short-circuit current expression on both sides. Then, the change process of trigger angle is approximately linearized by the least square method, which obtains a new expression of fault transient current on the LCC side. Similarly, the composition of AC current fed to DC side is deduced by using the commonly used inner-loop PI controller model, which also obtains a new expression of fault transient current on the VSC side. Secondly, referring to the traditional crossed-type Z-source DCCB, series-type Z-source DCCB, and parallel-type Z-source DCCB, this paper also proposes a new ZCLC and analyzes its working principle and parameter design. Then, the newly proposed ZCLC is combined with DCCB to be applied to the hybrid multi-terminal HVDC network. Finally, the simulation results based on Matlab/Simulink verify the effectiveness of the fault analysis and passive fault clearing methods, and the semi-physical experimental platform based on PLECS/RTbox verifies the superiority of the new ZCLC.

A Calculation Method of DC Current Reference Value for Suppressing Commutation Failure in LCC-HVDC System

Line commutated converter based high voltage direct current (LCC-HVDC) transmission system is prone to suffer commutation failure, which can lead to the increase of DC current and the interruption of DC transmission power. Furthermore, the increase of DC current might lead to subsequent commutation failure. Based on the reactive power balance of the inverter AC bus, this paper studies the functional relationship between DC current and commutation voltage. The DC current reference value is obtained by using the DC current acquired from the above functional relationship, and combining with the DC current order given by voltage dependent current order limiter (VDCOL). This calculation method can adjust the DC current reference value in time according to the commutation voltage, thus limiting the increase of DC current. In order to verify the commutation failure suppression ability of the proposed method, several simulations are conducted based on CIGRE benchmark HVDC model in PSCAD/EMTDC. Simulation results show that the proposed method can not only suppress commutation failure successfully, but also improve the fault recovery performance of the LCC-HVDC system.

Multiple commutation failure suppression method of LCC-HVDC transmission system based on fault timing sequence characteristics

Aiming at the problem of commutation failure and the poor transient response of line commutated converter-based high-voltage direct-current (LCC-HVDC) transmission systems, this paper starts from the harmonic components of an AC system fault on the inverter side. The quantitative relationship between the harmonic wave and the trigger angle of the inverter side is derived in detail, and the effect of the trigger on the commutation of the LCC-HVDC transmission system is qualitatively analyzed with a phase-locked trigger. On this basis, an improvement scheme for an anti-harmonic control system is proposed. First, a virtual separated phase-locked control method is used to determine the fault occurrence time. Additionally, the characteristics of the trigger angle of the rectifier side and extinction angle of the inverter side after the fault are used to determine the fault position and generate the corresponding fault timing sequence. Then, the generated fault timing sequence is used to control the notch filter and modify the input voltage of the voltage-dependent current order limiter to improve the robustness of the control system in terms of the harmonics. Finally, based on CIGRE benchmark LCC-HVDC transmission system model, the advantages of the improved method are verified. The results can provide a reference for actual projects.

Research on transient stability control method for LCC‐HVDC receiving‐end system based on the gradient of dynamic energy

A new transient stability control strategy for the Line-Commutated-Converter based High-Voltage-Direct-Current (LCC-HVDC) receiving-end system based on the gradient of dynamic energy is proposed to deal with this problem. Firstly, in the whole process from fault occurrence to fault removal, a dynamic energy function representing the stability of the system is constructed. On this basis, considering the influence of Direct Current (DC) inverter side controller, the dynamic energy gradient expression under three-phase and single-phase short-circuit fault scenarios is deduced in detail. The influence of control parameters on dynamic energy accumulation or consumption is analytically described. Then, the function term related to the inverter side controller is extracted from the dynamic energy gradient expression, the trigger pulse on the inverter side is controlled to keep the dynamic energy gradient of the DC port at the minimum. Finally, the hardware in the loop test is carried out on RT-LAB. The test results show that in the process of receiving end fault, the DC port will inject dynamic energy to the receiving end system, making the system tend to be unstable. This method is able to reduce the dynamic energy accumulation rate of system and increase the critical clearing time of system.

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.