Hybrid HVDC Transmission Research Articles

A single-ended high-impedance fault protection scheme for hybrid HVDC transmission lines based on coordination of control objective

Hybrid HVDC transmission technology advancement has led to significant progress. A reliable and effective DC line protection scheme is crucial for DC transmission systems. The traditional protection schemes suffer from insufficient sensitivity and reliability during high-impedance faults on DC lines. To address the issue, this study introduces a hybrid HVDC line single-ended protection scheme based on the coordination of converter control objectives. In the fault-ride-through(FRT) stage, different control strategies and objectives are applied to the converter based on the fault pole and fault direction identification results. Internal and external faults are discerned by calculating measured voltage with the coordination of the converter control objective. This approach maintains heightened sensitivity to high-impedance DC line faults. Moreover, it operates independently of double-ended communication and demands a low sampling rate. Extensive simulation results robustly substantiate the efficacy of the proposed protection scheme.

A Fault Identification Method of Hybrid HVDC System Based on Wavelet Packet Energy Spectrum and CNN

Aiming at the shortcomings of traditional fault identification methods in fault information acquisition, In the scenario of hybrid HVDC transmission system, a new fault identification method is proposed by using wavelet packet energy spectrum and convolutional neural network (CNN), which effectively solves the problem of complex fault feature extraction of hybrid HVDC transmission system. This method effectively improves the accuracy of fault identification. Firstly, tThe frequency-domain characteristics of the fault transient signal are extracted by wavelet packet transform, and the feature differences are reflected in the form of energy spectrum. Secondly, according to the extracted energy feature information, the order of fault line and fault type is identified by CNN. Finally, through example verification and algorithm comparison, it is concluded that, the mentioned model has a strong ability to identify faults, and has strong anti-noise interference and tolerance to transition resistance.

Control and Protection Strategy Study of VSC Station Fault at Hybrid HVDC Transmission System

LCC-VSC UHV hybrid cascaded DC transmission system will face over-voltage problems caused by power surplus when receiving terminal VSC station fault occurs. An over-voltage suppression strategy based on the action input of a high-voltage controllable arrester and the stepwise blocking of non-fault VSC at the receiving end is proposed, and a dynamic simulation test system is built to verify that the stepwise blocking protection scheme can effectively reduce the DC voltage stress of fault VSC by about 25.9%. Based on the test results, the effectiveness of the over-voltage suppression scheme of VSC at the receiving end in practical projects can be demonstrated by analogy.

Single-end Protection Scheme Based on Transient Power Waveshape for Hybrid HVDC Transmission Lines

Single-end Protection Scheme Based on Transient Power Waveshape for Hybrid HVDC Transmission Lines

Hybrid HVDC system fault transient analysis considering traveling wave propagation and converter control response

Accurate fault analysis is essential for protection and control design of hybrid HVDC transmission system. This paper proposes a dc fault transient analysis method, which considers both the traveling wave propagation process and converter control response. Firstly, the multiple reflected and refracted traveling waves at line terminals as well as fault point are calculated by building propagation coefficient matrices. Secondly, the transfer function between the converter equivalent source and the traveling wave transients is built, then the fault current and voltage are calculated by compensating multiple traveling waves with corresponding control response. Compared with existing methods, the proposed method can calculate long-period traveling waves in a simplified way, and implement more accurate analysis for the converter response caused by the basic control strategy and the fault control strategy. An LCC-MMC HVDC system is built based on PSCAD/EMTDC, and the simulation cases are carried out to verify the correctness and superiorities of the proposed method. Finally, the impact of control response on the performance of traveling-wave-based directional protection is evaluated by the proposed fault analysis method.

Tech-Economic Assessment of Power Transmission Options for Large-Scale Offshore Wind Farms in China

China is taking initiative in energy transition to cope with the long-term controversy of its enormous energy consumption, aiming to use less carbon. Wind power, especially offshore wind energy, has become a prevailing alternative due to its low carbon emissions, renewability, competitiveness, and operation security. The layout of a transmission channel is a key consideration in marine project implementation. This paper investigates the technical characteristics, application status, and viable advantages of a conventional AC transmission, voltage source converter-based high-voltage direct current (VSC-HVDC) transmission, gas-insulated line (GIL) transmission, and hybrid HVDC transmission. A component-resolved evaluation model was proposed to estimate the costs to be incurred of four electrical transmission options for offshore wind power along the coast of Eastern China, with technical feasibility and economical considerations. Cost comparisons and component sensitivity analyses were developed with different transmission distances and capacities. Results suggest HVAC transmission and VSC-HVDC are the preferable solutions for present offshore wind farm development in Eastern China, and the economic potential of the hybrid HVDC makes it feasible for future deployment. Some conclusions can be applied in disparate regions across the globe.

Resilient DC Voltage Control for Islanded Wind Farms Integration Using Cascaded Hybrid HVDC System

To integrate large-scale islanded onshore wind power with different sizes, this paper proposes an integration system based on the cascaded hybrid HVDC transmission system, which consists of LCC and several MMCs in series connection at the DC side of the rectifier. A large-scale wind farm is connected with one LCC and one MMC while several small-scale wind farms are connected with MMCs directly. Owing to the hierarchical integration arrangement, the operating flexibility can be improved with reduced capacity and the number of step-up interfacing transformers. A resilient DC voltage control is proposed for the integration system to adaptively redistribute power among the converters during wind power fluctuations. Firstly, the topology and operating characteristics of the wind power integration system are introduced. Then, a resilient DC voltage control is proposed to ensure stable operation during wind power curtailments. Finally, a simulation model of the hybrid cascaded HVDC transmission system is built in PSCAD/EMTDC to verify the effectiveness. The research results show that the system provides a new option for long-distance transmission of large-scale islanded wind power.

Grid Forming Converters in Offshore Wind Farms for Grid Integration Through Hybrid HVDC Transmission System

Grid Forming Converters in Offshore Wind Farms for Grid Integration Through Hybrid HVDC Transmission System

Adaptive fault recovery strategy of LCC‐MMC based hybrid HVDC

Hybrid HVDC consisting of line commutated converter (LCC) on rectifier side and modular multilevel converter (MMC) on inverter side is the favourable future trend of HVDC projects utilising the advantages of both converter technologies. However, fault clearing and adaptive restart capabilities of such hybrid HVDC are not much investigated in the literature. In this paper fault clearing of hybrid HVDC transmission line is investigated using mechanical DC breaker at MMC terminal and forced firing angle retardation at LCC terminal. Then, the characteristics of the line voltages are analysed during the fault blocking states of converters to clear the transient fault. Characteristics of the line voltages of LLC rectifier have shown prominent difference under permanent and temporary fault states, and also dependent on the severity of fault. Therefore, based on line voltage characteristics, a restart strategy is proposed for hybrid HVDC which is adaptive to the fault characteristics. Several PSCAD/EMTDC based simulation studies are conducted to validate the proposed fault recovery strategy. Simulation results validate the effectiveness of the proposed strategy for hybrid HVDC under DC line faults.

Equivalent characteristic impedance based hybrid-HVDC transmission line fault location

Aiming at the transmission line fault in Hybrid-HVDC system, this paper divides the fault evolution process into different stages according to the difference of transition resistance at the fault point and the conduction state change of power electronic devices, emphatically analyzes the conversion conditions between different stages, and proposes an adaptive criterion for judging the moment when the fault evolves into AC feed-in stage. Based on the characteristic component of electrical quantities at AC feed-in stage, this paper defines the equivalent characteristic impedance at any point of transmission line, derives the functional relationship between the fault distance and the equivalent characteristic impedance at inverter side DC bus, and locates the fault point according to this relationship. In order to extract the characteristic component of electrical quantities, a characteristic component extraction algorithm is studied, it can effectively eliminate the influence of constant DC component and decaying aperiodic component. This paper builds the Hybrid-HVDC system simulation model and carries out simulation experiments on PSCAD/EMTDC, and compares the proposed fault location principle with the existing methods. The simulation results verify the effectiveness of the adaptive criterion for judging the AC feed-in stage starting moment and the proposed fault location principle.

Influence of coupling characteristics on commutation failure of new hybrid HVDC transmission system

Hybrid cascaded multi-terminal DC transmission provides a more flexible transmission method, improves the voltage stability of the inverter AC system, and reduces the probability of commutation failure. The hybrid LCC-VSC at the receiving end of the DC system is a new type of topology.In order to study the influence of coupling characteristics between VSC and LCC on commutation failure in AC power grid at receiving end,firstly,a hybrid LCC-VSC simulation model is built on PSCAD to analyze the operation characteristics of the system during the fault and verify the reactive power support ability of VSC to LCC, and the improvement effect of VSC on LCC-HVDC under different reactive power control modes is compared and analyzed. In order to study the influence of AC/DC coupling on commutation failure of LCC, commutation failure immune factor (CFII) was introduced to study the influence of AC system strength on commutation failure of LCC. Finally, considering that the influence of AC/DC coupling on commutation failure is closely related to the degree of coupling, the electrical distance between LCC and VSC is further studied. The results show that the conclusion can provide theoretical guidance for the design and analysis of related projects.

Research on Overvoltage Suppression by Blocking Diode and DC Circuit Breaker in LCC-VSC Hybrid HVDC System

In this study, a LCC-VSC hybrid HVDC transmission system was taken as an example. The valve and neutral overvoltage of the hybrid HVDC system were simulated and then calculated at the VSC converter station by installing a blocking diode and a DC circuit breaker, as well as by grounding after neutral bus outage. The isolation effect of the blocking diode and the DC circuit breaker on the VSC converter station in the protection and the effect on the DC overvoltage were compared, and the difference was analyzed by complying with the mechanism of overvoltage. The effect of different locations of the blocking diode on the overvoltage suppression was compared, and it was concluded that different locations of blocking diode could raise different requirements for the insulation coordination of the converter valve. By analyzing energy sources of the neutral bus overvoltage at different stages, it was further concluded that the energy stored in the distributed inductance of the metal return line could be a vital source of neutral bus overvoltage energy. On that basis, an overvoltage suppression strategy for grounding neutral bus directly after the stop of the converter station was proposed.

Hybrid LCC-AAC HVDC transmission system

A hybrid HVDC transmission system based on a line-commutated converter (LCC) and an alternate arm converter (AAC) is developed in this article. The basic operation principles and control methods of the proposed LCC-AAC hybrid HVDC system are demonstrated following with appropriate system design and parameter selection. In addition, this paper proposes a set of control schemes for riding through ac faults and handling dc fault. The operation and performance of both converters in such a hybrid HVDC configuration are verified through a detailed set of results based on a 3000 MVA LCC-AAC hybrid HVDC system including steady state operation, ac-grid harmonics & dc-side ripple analysis, reference tracking (active power change at LCC station, reactive power change at AAC station) and system performance under ac-grid faults without blocking converter at both terminals of the HVDC system. Also, the response of the hybrid system under dc fault presents potential dc fault handling capability compared with the hybrid system based on LCC and modular multilevel converter (MMC) with half-bridge submodules (HBSMs). The hybrid LCC-AAC HVDC transmission system lays preliminary steps for research on complex mixed converter multiterminal dc configurations in future dc super grids.

Research on Coordination Control Strategy of LCC-MMC Hybrid HVDC Transmission System under Fault

High voltage direct current based on modular multilevel converter (MMC-HVDC) has a good application prospect in engineering, but it is expensive and cannot handle DC faults and other disadvantages. So it is still not suitable for long-distance transmission. To solve these problems, this paper designs multi-terminal hybrid DC transmission system with a line commutated converter based on the rectifier side (LCC) and a modular multilevel converter based on the inverter side (MMC). Besides, for the asymmetric fault on the AC side of the system, the minimum trigger angle control strategy based on the rectifier side LCC and the maximum modulation ratio control strategy based on the inverter side MMC are proposed to improve the DC system transmission power and reduce the possibility of power transmission interruption. Finally, a model of hybrid DC transmission is built in PSCAD/EMTDC, the proposed strategy is verified its effectiveness.

Coherence Analysis of System Characteristics and Control Parameters for Hybrid HVDC Transmission Systems Based on Small-Signal Modeling

Hybrid high-voltage direct current (HVdc) system, which comprises a line-commutated converter (LCC) at rectifier side and a voltage source converter (VSC) at inverter side, is drawing wide attention due to its high reliability and economic benefit. For such a new transmission technology, the system characteristics are not comprehensively studied, and inappropriate control parameters may be coherent to system instability. This article puts forward a novel coherence analysis procedure that analyzes system stability based on a verified hybrid HVdc small-signal model. For the first time, the procedure is proposed to capture the coherence relationships between the system characteristics and the control parameters. The analysis results reveal that the circuit and control parameters can highly affect the oscillatory modes and even cause instability. Moreover, the variation of grid short-circuit ratio (SCR) at LCC rectifier side trades off the response speed and damping of the oscillatory modes—under low SCR, large proportional and small integral coefficients of the LCC dc current control are suggested to enhance response performance. Also, the increase of VSC dc capacitance reduces the mode response speed—for a large capacitance, large proportional and small integral coefficients of the VSC dc voltage control shall be set to guarantee system stability.

DC Power Control Strategy of MMC for Commutation Failure Prevention in Hybrid Multi-Terminal HVDC System

This article presents a control strategy for a modular multilevel converter (MMC) to prevent commutation failure of a line-commutated converter (LCC), forming a three-terminal hybrid HVDC transmission system, where one LCC sending end is connected to the large generation and two receiving ends (LCC inverter and MMC) are located near the load center. This configuration, one of the potential options, has been proposed to strengthen Korea electric power transmission system through the optimized use of existing assets and rights-of-way, extremely challenging to secure. The MMC power control strategy has been developed to regulate the AC voltage and the extinction angle of the LCC inverter. This indirect yet effective active and reactive power control of the LCC inverter terminal helps prevent the commutation failure (CF) of the LCC in emergency and maximize the benefits of the costly planning option. By establishing a theoretical foundation for this power control problem and relationship among the control parameters, we quantify the active power reference for MMC to secure the desired LCC extinction angle. A coordinated strategy has been developed for the AC filter, the on-load tap changer of a transformer, along with the MMC control to lower the risk of CF and its catastrophic impact on the whole power system. The validity and performance of the proposed control methods are demonstrated for the real Korea electric power planning cases using a real-time power system simulator.

A New Protection Principle of Hybrid Bipolar HVDC Line Based on Voltage Fault Component Rate of Change

At present, hybrid HVDC transmission has become a research hotspot. In this paper, the fault-added state characteristics of the hybrid bipolar HVDC transmission system with different faults on the DC transmission line are analyzed by connecting small inductors on both sides of the HVDC transmission line. A single-ended quantity protection method using voltage transient components is obtained. By analyzing the rate of change of voltage fault component, the transient energy component is used to calculate the instantaneous energy magnitude of the transient voltage component to distinguish the fault in the region. The energy function is constructed by the transient voltage of the positive and negative rectifiers to fault selection. The simulation results verify the correctness of the proposed protection principle and can achieve fault pole selection.

Research on coordinated reactive power control strategy of hybrid‐HVDC

Based on the hybrid HVDC transmission system, which is composed by line-commutated converter (LCC) in parallel with voltage source converter (VSC), a coordinated reactive power control strategy of hybrid-HVDC in both steady and transient states is proposed. The operation characteristics in situations like low load operation, filter switching, and de-locking process are analysed. Influence of the method to AC bus voltage and system power factor is taken into account. Three kinds of reactive control methods of VSC and LCC are studied, and the logical relationship between the filter and VSC reactive power controller is also given. Considering both the system voltage drop and the risk of commutation failure caused by AC faults, the VSC in voltage control mode is utilised to maintain AC bus voltage stability by providing emergency power support. Simulation results show the feasibility and effectiveness of presented control strategy, then the proposed strategy is applied in practical DC project, whose results achieve the desired effect by engineering field test.

Research on starting control strategy of hybrid-DC transmission system based on line polarity switching

The paper presented two kinds of typical topology structures of hybrid-DC transmission system along with the principle of the voltage source converter for the flexible DC transmission system. After that, the paper found that the flexibility of DC voltage in the start-up process should be restricted to an extent due to the unidirectional mobility of the HVDC transmission system. Therefore, the traditional start-up control method can not be directly applied in the starting process of the hybrid-DC transmission system. To solve this problem, the paper proposed a starting control strategy based on DC line polarity switching, which is reliable for hybrid HVDC transmission. Finally, the effect of the control strategy is verified by simulation.

Grid Integration of offshore wind farms using a Hybrid HVDC composed by an MMC with an LCC-based transmission system

This paper presents a hybrid HVDC-transmission system composed by a Full-Bridge Modular Multilevel Converter (FB-MMC) and a Line-commutated Converter (LCC) from the grid integration of offshore wind farms. The operational characteristics of a three-terminal hybrid-HVDC system, e.i. two LCC stations plus one MMC station, are investigated using PSCAD/EMTDC. This paper mainly focuses on the performance of the system under ac faults at the LCC inverter. This condition is critical for the entire system because LCC is prone to commutation failure, which can be translated into dc faults on the hybrid system. Numerical results show that FB-MMC can help to alleviate ac faults conditions in the LCC, and the system is able to restart after clearance of the fault.