Flexible DC Transmission Research Articles

Flexible DC transmission line fault type identification scheme based on Pearson correlation coefficient

Abstract Faults occurring on the DC side of the line are very harmful to the MMC-MTDC system, and with the popularization of flexible DC transmission technology based on overhead lines, how to discriminate the type of fault is the difficulty of MTDC DC line protection. This paper gives a fault kind identification approach based totally on the Pearson correlation coefficient, which discriminates the fault kind with the aid of calculating the Pearson correlation coefficient of the line voltage at each pole after the fault.

Compact Valve Hall Layout of DC Transformer for Offshore Wind Farm All-DC Collection Platform

Abstract To solve the problems of high investment cost, difficulty in improving AC collection efficiency, and broadband resonance in flexible DC transmission of offshore wind power, researchers have proposed an all-DC collection system. To further optimize the economic efficiency of offshore wind farm engineering, this paper studies on the compact valve hall layout of DC transformer for offshore wind farm all-DC collection platforms. The minimum air clearance inside the valve hall was determined based on the insulation level of the DC transformer, and a compact valve hall layout plan for the DC transformer was proposed, taking into account factors such as the electrical main wiring scheme, weight distribution, and symmetrical balance. The proposed compact valve hall layout scheme for DC transformers can reduce the volume of offshore all-DC collection platforms, save engineering construction costs, and improve the efficiency of offshore wind farms. This article can provide reference and guidance for subsequent research on all-DC collection and economic transmission of large-scale offshore wind farms.

Fault Distance Measurement Method Based on Wavelet Energy Spectrum and BWO Algorithm Optimized CNN-GRU Hybrid Neural Network

Aiming at the problems such as noise interference in the current fault ranging methods for flexible DC transmission lines, a single-ended intelligent fault ranging method is proposed as a hybrid neural network based on wavelet energy spectrum and BWO algorithm to optimize the multi-head attention mechanism of CNN combined with gated recurrent unit GRU. First, the correlation between wavelet spectrum energy and fault distance is analyzed, and wavelet packet decomposition is used to extract the wavelet packet energy spectrum feature vector as the model input of the neural network. Second, the hybrid neural network model is constructed and trained to mine the deep fault information in the time series, and the parameters of the hybrid neural network model with added multi-head attention are optimized using the iterative optimization search of the beluga algorithm to achieve fast and accurate positioning of the fault distance. Finally, the network is trained using the constructed four-terminal Zhangbei flexible DC transmission system model to collect data, and the experimental results prove that the method has high distance measurement accuracy, strong anti-interference ability and generalization ability, and a certain degree of resistance to transition resistance.

Steady state analysis of Multiport Interline DC Current Flow Controller integrated meshed MTDC grids

Flexible DC Transmission Systems (FDCTS) devices offer the most promising solution for Power Flow Management (PFM) problem in complex meshed MTDC grids. This paper proposes a new formulation for steady state analysis of a MTDC grid, integrated with the multiport version of the FDCTS device called Multiport Interline DC Current Flow Controller (MIDCCFC). MIDCCFC can regulate the current flow of two or more lines, thereby achieving line power flow regulation and aid for DC line breaking in a MTDC system. The proposed formulation integrates the steady state operating point of MIDCCFC with the MTDC system. The proposed formulation is tested on a 5 bus 320 kV meshed MTDC grid with 5-port MIDCCFC. A criterion is evolved to ensure the feasibility of the operating point obtained from the proposed formulation. The line current regulation range offered by MIDCCFC for line-12 in the test system is (30-3086) A. The impact of line-12 current variation on the system variables over the obtained range is investigated. The application of MIDCCFC for DC breaking is also investigated. The results are validated by performing steady state simulation of the test system in MATLAB/Simulink and real time simulation of the test system in Real Time Digital Simulator.

A machine learning-based assessment model for defect diagnosis in XLPE power cables

Cross-linked polyethylene (XLPE) power cables are commonly used in flexible DC transmission applications. The diagnosis of the defects of XLPE power cable has consistently been a focal point of research as it is an essential equipment for power delivery. Previous research works have seldom employed machine learning methods to deal with this problem. Therefore, this work presents a machine learning-based assessment model that combines the Categorical Boosting (CatBoost) algorithm and Aquila optimizer. The CatBoost model is developed to examine the stages of the cable defect. The load condition, corrosion condition, operating life, and partial discharge are the inputs into the CatBoost model. The Aquila optimizer is employed to optimize parameters in an automated manner during the training of the CatBoost model. The performance comparison is made between the proposed model and the existing models. The proposed model demonstrates a high level of accuracy, precision, recall, and F1-score, all higher than 99 %. The model exhibits at least 1 % more achievement in accuracy, precision, recall, and F1-score when compared to the next best model, XGBoost. These results suggest that the proposed model is capable of effectively assessing the stages of cable defects. In addition to this, these experimental results of the proposed model are superior to those of the existing models, which demonstrate that the proposed model is superior to the existing models in terms of its ability to assess the defect stages of cables.

Research on the mechanism and transmission path of the oscillation in DFIG-connected flexible DC transmission system

Concerning the oscillations in DFIG (doubly-fed induction generator)-connected flexible DC transmission system caused by large disturbances, an analysis method based on subsystem partitioning and dynamic energy modeling is put forward. Firstly, the complex high-order system is partitioned into several low-order subsystems, and the dynamic energy model of each subsystem is built using the first integration method. Then, by analyzing the directions and variation patterns of the interaction energy flows between different subsystems in the oscillation process, the redistribution of the primary flow paths of the system’s oscillation energy after large disturbances occur is determined. And then, an oscillation tracing method based on the response characteristics of the oscillation energy links of the subsystems is proposed. In the oscillation process, the interaction energy links between different subsystems are categorized into 'oscillation gaining links', 'oscillation maintaining links' and 'oscillation damping links', according to the contribution of each interaction energy link to the system’s oscillation energy. By analyzing the ‘oscillation gaining links’ corresponding to the dominant interaction energy links of different subsystems, the transmission paths by which the dominant oscillation energy links keeps gaining can be obtained, and the energy source that induces oscillation can be located. Finally, hardware-in-loop tests are conducted on the RTLAB semi-physical simulation platform, the results of which verify the effectiveness of the proposed method. This research paper can provide theoretical support for suppressing the oscillation by adding compensation branches between the subsystems (i.e. damping control strategies based on supplementary compensation branches).

The Influence of Additional Virtual Synchronous Generator Technology in VSC-MTDC Systems with Wind Power on System Frequency

Introduction: A frequency control strategy is proposed based on additional virtual synchronous generator technology for voltage source converter-based multi-terminal high voltage direct current systems with wind power. Method: This strategy addresses the system's inertia reduction and frequency stability issues caused by integrating large amounts of wind power through multi-terminal DC transmission. Firstly, the virtual synchronous generator mathematical model is constructed based on the system structure. Secondly, for the problem of zero rotational inertia of voltage source converter in a flexible DC transmission system, based on the P-U droop control method of the converter station, additional virtual synchronous control generation technology is applied to simulate the P-f droop characteristics of the synchronous generator by adding virtual rotational inertia, so that the converter has the inertial response of synchronous generator to realize primary frequency regulation. Result: Finally, the simulation is verified on the PSCAD/ EMTDC platform with an example of a three-terminal parallel MTDC transmission system. Conclusion: the analyzed results demonstrate that the virtual synchronous generator control strategy is very valuable and useful for improving the frequency performance of the system.

Improved coordinated control strategy for VSC-MTDC system with DC voltage secondary regulation

Droop control is widely used in multi-terminal flexible DC (VSC-MTDC) transmission systems by virtue of the advantage of multi-station cooperative unbalanced power dissipation, however, the essence of the droop control strategy is to change the DC current to realize the unbalanced power dissipation, and the resulting DC voltage deviation will affect the normal operation of the system. Firstly, this paper theoretically analyses the working characteristics of the conventional droop control and proposes a control method to realize the quasi-differential-free regulation of DC voltage by translating the droop curve. Second, according to the power margin of the converter station, the feedforward compensation amount of each converter station is reasonably set to avoid the power impact on the converter station. Finally, for the problem that the actual value of DC voltage still deviates from the rated value, a control strategy containing secondary regulation of DC voltage is proposed to further restore the DC voltage to the initial value on the basis of ensuring the effect of power regulation, which improves the stability of the operation of the VSC-MTDC system. The final simulation results verify the effectiveness of the proposed method.

Analysis of high frequency oscillations based on MMC-HVDC system

Modular multilevel converter is widely used in new energy grid-connection and power grid asynchronous interconnection by virtue of its advantages of flexible and easy to expand structure and no commutation failure problem. In recent years, high-frequency oscillations have been observed in several flexible DC transmission projects at home and abroad, while the internal harmonic dynamic characteristics and control system of the MMC system are complex and the mechanism of high-frequency oscillations has not been sufficiently analysed at present. In this paper, the detailed DQ impedance model and the equivalent circuit impedance model on the AC side are firstly derived based on the state-space model of the MMC system, and the impedance sweeping method with superimposed small perturbations is proposed. Meanwhile, the dynamic characteristics of the detailed DQ impedance model are compared with those of the electromagnetic transient model. The amplitude-frequency characteristics of the system impedance under different system delays are further compared, and the mechanism of the high-frequency oscillation of the MMC-HVDC system is revealed.

A Topology and Control Method for Operational Testing of ±800 kV/8 GW Flexible DC Transmission Modular Multilevel Converter Valve

A Topology and Control Method for Operational Testing of ±800 kV/8 GW Flexible DC Transmission Modular Multilevel Converter Valve

A Flexible DC Ranging Scheme Based on Variational Mode Decomposition and Teager Energy Operator

In this paper, a new Cable fault location scheme is proposed for the existing flexible DC Cable fault location methods, in which the fault signal is mixed with noise and the traveling wave speed is affected by line parameters. This scheme is based on the propagation characteristics of fault line mode voltage traveling waves in flexible DC transmission. Firstly, the fault voltage is effectively decomposed using Variational Mode Decomposition (VMD). Then, the decomposed intrinsic mode functions (IMF) are reconstructed to obtain the high-frequency components of the recorded fault information. Finally, the traveling wave head is calibrated using the Teager Energy Operator (TEO). A dual terminal ranging algorithm that is not affected by wave velocity has been proposed, which can effectively avoid the impact of fault traveling wave velocity uncertainty on the ranging results. A dual ended flexible DC power grid model was built in PSCAD/EMTDC, and a large number of simulation results show that the algorithm proposed in this paper can accurately calibrate the fault traveling wave head, has little interference from transition resistance, strong noise resistance, and can obtain high-precision fault location results in different fault locations.

DC line fault discrimination based on maximum information coefficient

In DC networks, conventional line differential protection has the disadvantages of low reliability, weak resistance to transition resistance, and vulnerable to harmonic interference. In this paper, a fault identification method based on maximal information coefficient (MIC) is proposed for non-high resistance faults in the DC side of a flexible DC distribution network containing a modular multilevel converter (MMC) with single pole grounding DC protection inside and outside segments. The failure prediction is performed based on the reciprocal information at two sides of the line current. At the same time, for single-pole grounded high-resistance faults, the MIC is compared using the zero moduli of the current at two sides of the line for the recognition of high-resistance faults. The MMC-based flexible DC transmission network model is built and validated in the PSCAD/EMTDC platform. The results show that the method can identify faults protection inside and outside segments on the DC line area quickly and reliably. It also has the ability to resist harmonic interference and has an endurance of resistance for single-pole faults on the DC side; meanwhile, for high-resistance faults, it can also be detected accurately.

Research on protection adaptability and optimization scheme of VSC/LCC DC parallel hybrid system

With the continuous development of high-voltage direct current (HVDC) technology and the relative concentration of power load and power, the HVDC parallel hybrid system will inevitably appear again. Because conventional DC transmission (LCC HVDC) and flexible DC transmission (VSC HVDC) in existing hybrid multi-infeed HVDC systems operate independently and are not coordinated with each other, and there is a gap in the research on the protection coordination relationship between VSC/LCC parallel hybrid systems and their near-area AC systems, it is necessary to study them systematically. This paper studies the protection coordination relationship between the VSC/LCC DC parallel hybrid system and its near area AC system, simulates and analyzes the typical faults of the hybrid multi-feed DC system, and puts forward a protection optimization scheme suitable for the VSC/LCC DC parallel hybrid system, which improves the safe and stable operation ability of the power grid, eliminates the hidden dangers of subsequent operation and maintenance, and provides a reference basis for further research.

Test Method of Secure and Controllable FPGA Chip Used in Flexible DC Transmission Control and Protection System

Flexible DC transmission control and protection system is a high-performance distributed computing system, which has the characteristics of high-speed data acquisition and fast processing, and uses a large number of high-performance FPGA (Field Programmable Logic Array) chips. FPGA has the characteristics of high integration and complex manufacturing process, and its security and controllability is faced with many challenges. The reliability of FPGA is an important standard to measure its performance, so reliability test fault screening is very important to ensure its normal operation. Although the automatic test equipment (ATE) can test the fault of FPGA, it is not convenient for field testing due to the price, test environment and other factors. In this paper, a design scheme of built-in self-test (BIST) for fault detection and fault diagnosis is proposed, which can detect faults of look-up table (LUT) and interconnection resources (wires and programmable switches). At the same time, considering the effect of rising temperature on fault probability, a controlled self-heating element (SHE) is constructed by using the internal resources of FPGA to provide the required temperature for BIST testing. Finally, an integrated layout scheme of BIST and SHE is proposed. An experiment of self-heating of FPGA chip is completed by TSINGHUA PGL22G, which verifies the correctness and feasibility of the theory.

MMC-HVDC High-Frequency Resonance Suppression Strategy Based on Multi-Band Band-Stop Filters

Renewable energy generation is a manifestation of global economic and societal advancement and serves as a fundamental assurance for humanity’s pursuit of sustainable development. However, recent years have witnessed several instances of high-frequency resonance events in high-voltage direct current (HVDC) transmission systems based on modular multilevel converters (MMC), which have resulted in converter station tripping and significant repercussions on the alternating current (AC) grid. This paper addresses the mid-to-high frequency resonance issues prevalent in flexible DC transmission systems employing modular multilevel converters (MMC-HVDC). To tackle these concerns, an impedance model for MMC’s AC side is established. Utilizing impedance analysis, the essential factors contributing to the negative damping characteristics of MMC are identified as delay and voltage feedforward loops, predominantly causing negative damping in the frequency range exceeding 400 Hz. In response, a suppression strategy is proposed, involving the incorporation of a multi-band stop filter and virtual impedance. This strategy ensures that within the 0–2000 Hz frequency range, only the impedance phase within 230–430 Hz slightly surpasses 90°. Consequently, the phase difference between MMC’s positive-sequence impedance and the AC system impedance is reduced from 222° to 174.7°, thus guaranteeing secure grid operation. Lastly, the accuracy and effectiveness of the theoretical analysis and suppression methodology are verified through the development of an electromagnetic transient model in MATLAB/Simulink, considering delay fluctuations of ±10%.

Research on operation characteristics and control strategy of hybrid multi-feed HVDC system

Flexible DC transmission technology based on voltage source converter (VSC-HVDC) and high voltage DC transmission technology based on grid commutation converter (LCC-HVDC) have complementary characteristics, so the hybrid DC technology combined with LCC-HVDC and VSC-HVDC can make them complement each other and become the development trend of power grid technology in the future. In the existing hybrid multi-infeed DC system, conventional DC transmission (LCC-HVDC) and flexible DC transmission (VSC-HVDC) operate independently and are not coordinated with each other, but there is a blank in the research on the protection coordination relationship between VSC/LCC parallel DC hybrid system and its near area AC system. Therefore, it is necessary to theoretically analyse and study the operation characteristics, control strategy and protection coordination relationship between near-area AC systems of hybrid multi-infeed HVDC systems. Due to the limited space, this paper mainly analyses the operation characteristics and control strategy of hybrid multi-infeed HVDC system, and verifies the correctness of the theory through simulation research. In the next step, the corresponding protection is configured in the above simulation model, and the protection adaptability analysis and protection optimization scheme of hybrid multi-infeed HVDC system are obtained.

Analysis of the Seismic Performance of ±500 kV Flexible DC Converter Valves

The converter valve is the core equipment of flexible DC transmission, but it has been severely damaged in previous earthquakes. Therefore, the seismic performance of its structure has a significant impact on the safety and reliability of power transmission. To study the seismic performance of ±500 kV flexible DC converter valves and identify weak links in the supporting valve tower, this article first established a refined finite-element model of a valve tower based on its physical structure; then modal analysis on the valve tower was conducted; finally, time-domain analysis of the valve tower under three seismic excitations, namely Wenchuan, EI Centro, and artificial, was conducted, and the equivalent stress and directional displacement cloud maps of the key parts of the valve tower were obtained. The research results indicate that the dynamic characteristics of the valve tower are relatively complex, with low fundamental frequency and diverse natural vibration modes. Under earthquake action, the predominant frequency of EI Centro waves is close to the fundamental frequency of the valve tower, which is prone to resonance and causes significant damage to the valve structure. The improved design of the valve tower meets the seismic strength requirements of level 8 seismic intensity.

High-Frequency Oscillation Suppression Strategy for Flexible DC Transmission Systems Based on Additional Joint Damping

Flexible DC transmission systems experience negative damping characteristics in the high-frequency range due to the delay in the system control link. This can lead to interactions between the impedance of modular multilevel converters (MMCs) and AC transmission lines, resulting in high-frequency oscillation (HFO) issues. To resolve this problem, a simplified MMC control model was developed, taking into account the current inner loop control link. The Sobol method was employed to analyze the factors influencing the impedance characteristics of MMC. Additionally, a combined approach involving a virtual passive filtering method and an equivalent current sampling method was proposed to provide additional damping. The study compared and analyzed the impact of additional damping control on the impedance characteristics and system stability of MMCs. Lastly, a flexible direct current grid connection simulation model was constructed using PSCAD to validate the effectiveness of the proposed additional damping control strategy.

Coordinate Fault Ride-Through Strategy for Connection of Offshore Wind Farms Using Voltage Source-Converter-Based High-Voltage Direct-Current Transmission under Single Polar Fault.

In a system where wind farms are connected to the grid via a bipolar flexible DC transmission, the occurrence of a short-time fault at one of the poles results in the active power emitted by the wind farm being transmitted through the non-faulty pole. This condition leads to an overcurrent in the DC system, thereby causing the wind turbine to disconnect from the grid. Addressing this issue, this paper presents a novel coordinated fault ride-through strategy for flexible DC transmission systems and wind farms, which eliminates the need for additional communication equipment. The proposed strategy leverages the power characteristics of the doubly fed induction generator (DFIG) under different terminal voltage conditions. By considering the safety constraints of both the wind turbine and the DC system, as well as optimizing the active power output during wind farm faults, the strategy establishes guidelines for the wind farm bus voltage and the crowbar switch signal. Moreover, it harnesses the power regulation capability of the DFIG rotor-side crowbar circuit to enable fault ride-through in the presence of single-pole short-time faults in the DC system. Simulation results demonstrate that the proposed coordinated control strategy effectively mitigates overcurrent in the non-faulty pole of flexible DC transmission during fault conditions.

Optimization Design of Voltage Level of Flexible DC Transmission with Offshore Wind Power Based on Genetic Algorithm

Voltage level selection is the foundation of planning and designing for flexible DC transmission with offshore wind power. Traditional voltage level selection method is largely dependent on the manufacturing level of DC cable and converter with having standardization or project experience. For flexible DC transmission with large-scale offshore wind power, the voltage level using the traditional selection method may be not the best. By studying calculation methods of main devices and system cost, under the condition of system capacity and length of DC cable known, system cost function with DC voltage and sub-module capacitor voltage as variables was built. Taking the lowest cost as a target, aiming at the low stability of the optimization results of the traditional genetic algorithm, an optimization design method of DC voltage level based on subsection searching genetic algorithm was proposed. The simulation model of flexible DC transmission with offshore wind power was built based on PSCAD/EMTDC. The AC fault ride-through of the converter station and the safety of the main equipment under the optimal DC voltage level was verified.