Hybrid Multi-terminal HVDC System Research Articles

Fault identification scheme for protection and adaptive reclosing in a hybrid multi-terminal HVDC system

A fault identification scheme for protection and adaptive reclosing is proposed for a hybrid multi-terminal HVDC system to increase the reliability of fault isolation and reclosing. By analyzing the "zero passing" characteristic of current at the local end during the converter capacitor discharge stage, the fault identification scheme is proposed. The distributed parameter-based fault location equation, which incorporates fault distance and fault impedance, is developed with the injection signal and the distributed parameter model during the adaptive reclosing stage. The fault distance is determined using a trust region reflection algorithm to identify the permanent fault, and a fault identification scheme for adaptive reclosing is developed. Simulation results show that the proposed scheme is suitable for long-distance transmission lines with strong anti-fault impedance and anti-interference performance. Also, it is less affected by communication delay and DC boundary strength than existing methods.

Fault identification scheme for hybrid multi-terminal HVDC system based on control and protection coordination strategy

To further improve the capability of fault identification for protection and adaptive reclosing, a fault identification scheme based on a control and protection coordination strategy for a hybrid multi-terminal HVDC system is proposed. The protection principle for fault isolation is constructed based on the difference of the polarity characteristic of fault current between the fault line and non-fault line in the uncontrolled fault stage and fault current limiting stage. In the adaptive reclosing stage, an active fault identification scheme based on the amplitude distribution law of the injection signal along the line is proposed. Intensive simulation studies prove the fault identification scheme can identify the fault area in various fault scenarios and identify permanent faults to carry out adaptive reclosing.

Analysis of Fault Waveforms of Key Electrical Parameters in Hybrid Multi-terminal DC System

LCC-VSC hybrid multi terminal HVDC system combines the advantages of traditional HVDC and flexible HVDC technology, and becomes one of the new directions of HVDC development. In this paper, taking the Baihetan Jiangsu HVDC transmission project as an example, under the appropriate simplified model, the critical electrical stresses of the DC side faults of a typical LCC-VSC hybrid multi-terminal DC system are analyzed. The simulation results verify the correctness of the simplified analysis in this paper.

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 control parameter analysis method based on a transfer function matrix of hybrid multi-terminal HVDC system with flexible adaptability for different operation modes

To achieve better controller performance, the impact of control parameters on the stability and dynamic performance is essential to be clarified mathematically for hybrid multi-terminal HVDC (Hybrid-MTDC) systems considering different operation modes (OMs). In this paper, a generic small-signal model (SSM) is established, which can be modified flexibly for different OMs. Then the SSM can be converted into a transfer function matrix (TF-Matrix), which avoids deriving the transfer function separately for each controller of the system, and greatly improves the work efficiency. By inputting a step into the TF-Matrix, the time-domain mathematical model for the all system state variables can be obtained, and it is more efficient than iterative solutions of high-order differential equations. To this end, a quantitative control parameter analysis method based on a TF-Matrix is proposed. Utilizing the proposed method, a typical Hybrid-MTDC system is investigated, and the results demonstrate the practicality and accuracy of the proposed method.

Small-signal stability of hybrid multi-terminal HVDC system

The Hybrid Multi-Terminal High Voltage Direct Current (H-MTDC) transmission system can take advantages of Line-Commutated Converter (LCC) technology to reduce capital cost and Modular Multilevel Converter (MMC) technology to eliminate commutation failure (CF) issues, and recently attracts much attention in academic research and practical engineering. Based on the scheme of Wudongde H-MTDC project, which is under construction in China, this paper develops a small-signal dynamic model of the hybrid 3-terminal HVDC system, adopting an LCC station at rectifier and two MMC stations at inverter. Then, the small-signal model is validated by comparing its time-domain responses with the detailed electromagnetic transient (EMT) simulation results obtained from PSCAD/EMTDC. Based on eigen-analysis and participation factor methods, this paper identifies the oscillatory modes of the overall hybrid system. From the revealed oscillatory modes, it indicates that DC-side active power relevant state variables and MMC internal harmonics have major contributions to low-damped modes which may result in instability of the system. Then, the effects of power-type controllers, i.e., constant DC current controller of LCC station, DC-side voltage controller of MMC station and active power controller of MMC station, and circulating current suppressing controllers (CCSC) of MMC stations on the stability of the hybrid system are investigated. Finally, the effects of AC system strength on the stability of the hybrid system are analyzed. The results show that smaller proportional gains of constant DC current controller of LCC and DC-side voltage controller of MMC can enhance the system small-signal stability. And the active power controller gain of MMC station has slight influences on the system stability. Moreover, the CCSC gains are required to be properly selected based on the system setup to prevent from the possible internal harmonic instability of MMC stations.

Electromechanical Transient Modeling of Line Commutated Converter-Modular Multilevel Converter-Based Hybrid Multi-Terminal High Voltage Direct Current Transmission Systems

A method for electromechanical modeling of line commutated converter (LCC)-modular multilevel converter (MMC)-based hybrid multi-terminal High Voltage Direct Current Transmission (HVDC) systems for large-scale power system transient stability study is proposed. Firstly, the general idea of modeling the LCC-MMC hybrid multi-terminal HVDC system is presented, then the AC-side and DC-side models of the LCC/MMC are established. Different from the conventional first-order DC-side model of the MMC, an improved second-order DC-side model of the MMC is established. Besides considering the firing angle limit of the LCC, a sequential power flow algorithm is proposed for the initialization of LCC-MMC hybrid multi-terminal HVDC system. Lastly, simulations of small scale and large scale power systems embedded with a three-terminal LCC-MMC hybrid HVDC system are performed on the electromechanical simulation platform PSS/E. It is demonstrated that if the firing angle limit is not considered, the accuracy of the power flow solutions will be greatly affected. Steady state calculation and dynamic simulation show that the developed LCC-MMC hybrid MTDC model is accurate enough for electromechanical transient stability studies of large-scale AC/DC system.

Three-terminal Hybrid HVDC Transmissions Control Strategies for Bundled Wind-thermal Power Plants

This paper evaluates application feasibility of a Hybrid Multi-terminal HVDC system and wind-thermal-bundled plants simulated in DIgSLIENT PowerFactory environment. The proposed hybrid MTDC system consists of two line-communicated converters (LCC), which are connected to both wind farms and thermal power plants, and one voltage source converter (VSC) at the grid side. Control strategies for each converter are designed to handle this system under different disturbance conditions. Simulation results show that the wind power fluctuation can be compensated by the thermal-generated power. Results demonstrate the effectiveness of the proposed control strategies of the hybrid MTDC system compared to a conventional MTDC system. The proposed scheme combines advantages of both LCC and VSC HVDC systems and provides a new way to transmit wind power over long distances to the main grid.

Input/output selection for wide‐area power oscillation damper of hybrid multi‐terminal high‐voltage direct current to connect remotely located renewable energy resources

A hybrid multi-terminal HVDC (MTDC) scheme combines line commutated converter and voltage source converter technologies for HVDC transmission system. This study presents a comprehensive design of wide-area power oscillation damper (POD) controller for the hybrid MTDC system. The process consists of input/output selection and parameter tuning for the POD of the hybrid MTDC based on the linearised black-box model. The objective is to choose an effective control loop to damp the critical mode, which does not have negative impacts on the other modes in the system. For this purpose, a new index, named relative joint interaction index, is proposed for the input/output selection. Afterward, the parameter of POD is tuned using residue-based phase compensation technique because of its practical application. Finally, non-linear simulations are also carried out to verify the effectiveness of the designed POD in different loading conditions.