Search Results (110)

This paper presents a comprehensive review of High-Voltage Direct-Current (HVDC) systems, focusing on their technological evolution, fault characteristics, and advanced signal processing techniques for fault detection. The paper traces the development of HVDC links globally, highlighting the transition from mercury-arc valves to Insulated Gate Bipolar Transistor (IGBT)-based converters and showcasing operational projects in technologically advanced countries. A detailed comparison of converter technologies including line-commutated converters (LCCs), Voltage-Source Converters (VSCs), and Modular Multilevel Converters (MMCs) and pole configurations (monopolar, bipolar, homopolar, and MMC) is provided. The paper categorizes HVDC faults into AC, converter, and DC types, focusing on their primary locations and fault characteristics. Signal processing methods, including time-domain, frequency-domain, and time–frequency-domain approaches, are systematically compared, supported by relevant case studies. The review identifies critical research gaps in enhancing the reliability of fault detection, classification, and protection under diverse fault conditions, offering insights into future advancements in HVDC system resilience. Full article

Low-frequency power oscillations (LFPOs) may occur in voltage source converter-based high-voltage direct current (VSC-HVDC) systems when providing frequency support to asynchronously interconnected power grids. This phenomenon has been observed in the LUXI back-to-back (BTB) VSC-HVDC project in China and results from insufficient damping, which may threaten the stability of the overall power system. To better understand and address this problem, this study investigates the root causes of LFPOs and evaluates how different parts of the system affect damping. A combined approach using small-signal modeling and the damping torque method is developed to analyze the damping behavior of DC power in VSC-HVDC systems. Results show that LFPOs are caused by the interaction between VSC-based frequency control and the dynamic response of synchronous generators (SGs). The turbine and governor systems in SGs help stabilize the system by providing positive damping, whereas the DC voltage-controlled VSC station introduces negative damping. The findings are supported by detailed simulations using a modified IEEE 39-bus test system, demonstrating the effectiveness of the proposed analysis method. Full article

Voltage source converter-based high-voltage direct current (VSC-HVDC) systems integrated into weak AC grids may exhibit oscillation-induced instability, posing significant threats to power system security. With increasing structural complexity and diverse control strategies, the stability characteristics of VSC-HVDC system require further investigation. This paper focuses on the stability of a receiving-end VSC-HVDC system consisting of a DC voltage-controlled VSC parallel-connected to a power-controlled VSC, under various operating conditions. First, small-signal models of each subsystem were developed and a linearized full-system model was constructed based on port relationships. Then, eigenvalue and participation factor analyses were utilized to evaluate the influence of control strategy, asymmetrical grid strength, power flow direction, and tie line on the system’s small-signal stability. A feasible short-circuit ratio (SCR) region was established based on joint power–topology joint, forming a stable operating space for the system. Finally, the correctness of the theoretical analysis was validated via MATLAB/Simulink time-domain simulations. Results indicate that, in comparison to the power control strategy, the DC voltage control strategy was more sensitive to variations in the AC system and demands a strong grid, and this disparity was predominantly caused by the DC voltage control. Furthermore, the feasible region of the short-circuit ratio (SCR) diminished with the increase in the length of the tie-line and alterations in power flow direction under the mutual-support power mode, leading to a gradual reduction in the system’s stability margin. Full article

Future power generation plants will be largely based on renewable energy sources such as wind or photovoltaic power. These plants are connected to the grid through power electronic converters, which may present stability problems, specifically in weak grids. Consequently, numerous stabilities studies have been conducted. In these studies, converters work as inverters; however, in power electronic interfaced loads, energy storage systems or High-Voltage Direct Current (HVDC) links, power converters can also function as a rectifier. Stability studies focusing on the rectifier operation have received little attention in previous research. In this paper, the Voltage Source Converter (VSC) stability is analysed for both the inverter and rectifier modes, with particular focus on the influence of the Phase-Locked Loop (PLL) and the current controllers’ bandwidths. Additionally, a Two-Degree-of-Freedom Proportional Integral (2DOF-PI) controller is proposed to expand the stable operating range. The stability study is carried out using a small-signal model validated through PSCAD simulations. The results show that for inverter operations, a slow PLL and fast current controllers yield better performance, whereas for rectifier operation, a fast PLL and slow current controllers are recommended. Finally, a robustness study based on the $H_{\infty }$-norm is carried out to provide some tuning recommendations for the controller parameters, confirming the different behaviour in inverter and rectifier operation. Full article

The growing demand for electricity, integration of renewable energy sources, and recent advances in power electronics have driven the development of HVDC systems. Multi-terminal HVDC (MTDC) grids, enabled by Voltage Source Converters (VSCs), provide increased operational flexibility, including the ability to reverse power flow and independently control both active and reactive power. However, fault propagation in DC grids occurs more rapidly, potentially leading to significant damage within milliseconds. Unlike AC systems, HVDC systems lack natural zero-crossing points, making fault isolation more complex. This paper presents the implementation of a wavelet-based protection algorithm to detect faults in a four-terminal VSC-HVDC grid, modelled in MATLAB and SIMULINK. The study considers several fault scenarios, including two internal DC pole-to-ground faults, an external DC fault in the load branch, and an external AC fault outside the protected area. The discrete wavelet transform, using Symlet decomposition, is applied to classify faults based on the wavelet entropy and sharp voltage and current signal variations. The algorithm processes the decomposition coefficients to differentiate between internal and external faults, triggering appropriate relay actions. Key factors influencing the algorithm’s performance include system complexity, fault location, and threshold settings. The suggested algorithm’s reliability and suitability are demonstrated by the real-time implementation. The results confirmed the precise fault detection, with fault currents aligning with the values in offline models. The internal faults exhibit more entropy than external faults. Results demonstrate the algorithm’s effectiveness in detecting faults rapidly and accurately. These outcomes confirm the algorithm’s suitability for a real-time environment. Full article

The worldwide promotion of carbon-neutral policies is leading to a continuously growing percentage of electricity being derived from renewable energy, which makes it feasible to design power systems composed of 100% renewable energy in the future. The question of how to realize stable transmission for 100% renewable energy-integrating grids under different operating conditions needs to receive more attention. Voltage source converter-based high-voltage direct current (VSC-HVDC) technology is one of the prospective solutions for large-scale renewable energy integration due to its unique dominance in areas such as independent reactivity and active control. In this study, we design a novel, 100% renewable energy system through grid integration via a VSC-HVDC system structure and a control strategy. Unlike in other research, a mixed control strategy based on grid-forming control (PSL) and grid-following control (GFL) is developed to realize smooth switching in order to ensure secure transmission and consistent operation when the operating conditions of the 100% renewable energy-integrating grid changes. The simulation results indicate that the proposed system structure and control could stabilize renewable energy transmission under normal operation conditions and provide necessary grid support under different system disturbances. Full article

Over the past few decades, wind energy has expanded to become a widespread, clean, and sustainable energy source. However, integrating offshore wind energy with the onshore AC grids presents many stability and control challenges that hinder the reliability and resilience of AC grids, particularly during faults. To address this issue, current grid codes require offshore wind farms (OWFs) to remain connected during and after faults. This requirement is challenging because, depending on the fault location and power flow direction, DC link over- or under-voltage can occur, potentially leading to the shutdown of converter stations. Therefore, this necessitates the proper understanding of key technical concepts associated with the integration of OWFs. To help fill the gap, this article performs an in-depth investigation of existing alternating current fault ride through (ACFRT) techniques of modular multilevel converter-based high-voltage direct current (MMC-HVDC) for OWFs. These techniques include the use of AC/DC choppers, flywheel energy storage devices (FESDs), power reduction strategies for OWFs, and energy optimization of the MMC. This article covers both scenarios of onshore and offshore AC faults. Given the importance of wind turbines (WTs) in transforming wind energy into mechanical energy, this article also presents an overview of four WT topologies. In addition, this article explores the advanced converter topologies employed in HVDC systems to transform three-phase AC voltages to DC voltages and vice versa at each terminal of the DC link. Finally, this article explores the key stability and control concepts, such as small signal stability and large disturbance stability, followed by future research trends in the development of converter topologies for HVDC transmission such as hybrid HVDC systems, which combine current source converters (CSCs) and voltage source converters (VSCs) and diode rectifier-based HVDC (DR-HVDC) systems. Full article

The underlying cause of commutation failures in traditional line-commutated converter (LCC) high-voltage direct-current (HVDC) transmission technology lies in the sensitivity of the thyristor devices, which are prone to turning off, thereby restoring the forward circuit breaker capability. This paper presents a coordination strategy between a controllable line-commutated converter (CLCC) and a voltage-sourced converter (VSC) and delves into the fault characteristics specific to CLCC damage. Our research focuses on CLCC topology, where fully controlled devices are incorporated to manage the thyristor’s turn-off time, ensuring its successful deactivation. This approach serves as a fundamental preventative measure against commutation faults. Furthermore, we employ a coordination strategy between the VSC and the CLCC to enhance the recovery time efficiency of the AC system. This strategy is simulated and validated using PSCAD software, and the results confirm its effectiveness in fault tolerance and AC system recovery. Full article

Due to the difference in output characteristics between the line-commutated converter-based high-voltage direct current (LCC-HVDC) and voltage-source converter-based high-voltage direct current (VSC-HVDC), the hybrid multi-infeed high-voltage direct current (HMIDC) presents complex coupling characteristics. As the AC side is disturbed, the commutation failure (CF) occurring on the LCC side is the main factor threatening the safe operation of the system. In this paper, the simplified equivalent network model of HMIDC is established by analyzing the output characteristics of VSC and LCC. Hereafter, based on the derived model and the control system of LCC-HVDC, the dynamic equations of the extinction angle are deduced. Consequently, by applying the phase portrait method, the causes of CF occurring in the HMIDC system as well as the impacts of control parameters on the transient stability are revealed. Furthermore, the stabilization boundaries for the reference value of the DC voltage are obtained via the above analysis. Finally, the theoretical analysis is verified by the simulations in the PSCAD/EMTDC. Full article

The DC-side response characteristics of the VSC-HVDC transmission system under AC-side sub-synchronous disturbances may lead to the propagation of AC/DC oscillations, and then there may be non-fundamental frequency oscillation on the DC-side. To clarify the interactive influence mechanism and propagation evolution law of the AC and DC side sub-synchronous oscillations in the VSC-HVDC transmission system, the frequency characteristics of the DC-side oscillation power are investigated by considering the effect of converter control links. First, the AC-side voltage and current frequency responses under single/multiple SSO components are analyzed. Secondly, the frequency characteristics of the DC-side power after the injection of single/multiple SSO components are investigated. Meanwhile, the effect of system control parameters on the sub-synchronous responses of the DC-side power is investigated. Finally, the theoretical analysis is verified with PSCAD/EMTDC simulations. The results show that under the influence of the AC-side SSO components, the DC-side of VSC-HVDC faces the risk of multi-frequency oscillations; when there are multiple SSO components on the AC-side, there is a coupling between the components, and the system control parameters affect the amplitude of the DC-side oscillation power. Full article

With the gradual emergence of trends such as the asynchronous interconnection of power grids and the increasing penetration of renewable energy, the issues of ultra-low-frequency oscillations and low-frequency stability in power grids have become more prominent, posing serious challenges to the safety and stability of systems. The voltage-source converter-based HVDC (VSC-HVDC) interconnection is an effective solution to the frequency stability problems faced by regional power grids. VSC-HVDC can participate in system frequency stability control through a frequency limit controller (FLC). This paper first analyses the basic principles of how VSC-HVDC participates in system frequency stability control. Then, in response to the frequency stability control requirements of the sending and receiving power systems, a droop FLC strategy is designed. Furthermore, a multi-objective optimization method for the parameters of the droop FLC is proposed. Finally, a large-scale electromagnetic transient simulation model of the VSC-HVDC interconnected power system is constructed to verify the effectiveness of the proposed droop FLC method. Full article

Due to the high cost and complex challenges faced by offshore wind power transmission, economic research into offshore wind power transmission can provide a scientific basis for optimal decision-making on offshore wind power projects. Based on the analysis of the topology structure and characteristics of typical wind power transmission schemes, this paper compares the economic benefits of five different transmission schemes with a 3.6 GW sizeable onshore wind farm as the primary case. Research includes traditional high voltage alternating current (HVAC), voltage source converter high voltage direct current transmission (VSC-HVDC), a fractional frequency transmission system (FFTS), and two hybrid DC (MMC-LCC and DR-MMC) transmission scenarios. The entire life cycle cost analysis model (LCCA) is employed to thoroughly assess the cumulative impact of initial investment costs, operational expenses, and eventual scrap costs on top of the overall transmission scheme’s total cost. This comprehensive evaluation ensures a nuanced understanding of the financial implications across the project’s entire lifespan. In this example, HVAC has an economic advantage over VSC-HVDC in the transmission distance range of 78 km, and the financial range of a FFTS is 78–117 km. DR-MMC is better than the flexible DC delivery scheme in terms of transmission capacity, scalability, and offshore working platform construction costs in the DC delivery scheme. Therefore, the hybrid DC delivery scheme of offshore wind power composed of multi-type converters has excellent application prospects. Full article

Voltage source converters (VSCs) have emerged as the key components in modern power systems, facilitating efficient energy conversion and flexible power flow control. Understanding the fundamental circuit model of VSCs is essential for their accurate modeling and analysis in power system studies. A basic voltage source converter circuit model connected to an LC filter is essential because it lowers the harmonic distortions and enhances the overall power quality of the micro-grid. This guarantees a clean and steady power supply, which is necessary for the integration of multiple renewable energy sources and sensitive loads. A comprehensive methodology for developing a basic circuit model of VSCs, focusing on the key components and principals involved, is presented in this paper. The methodology includes the modeling of space vector pulse-width modulation (SVPWM) as well as the direct quadrature zero synchronous reference frame. Different design controls, including the design of current control loop in the S-domain, the design of the direct current (DC) bus voltage control loop in the S-domain, and the design of the alternating current (AC) voltage control loop in the S-domain, are explored to capture the dynamic behavior and control strategies of VSCs accurately. The proposed methodology provides a systematic framework for modeling VSCs, enabling engineers and researchers to analyze their performance and assess their impact on power system stability and operation. Future studies can be conducted by using case studies and simulation scenarios to show the efficiency and applicability of the developed models in analyzing VSC-based power electronics applications, including high-voltage direct current (HVDC) transmission systems and flexible alternating current transmission systems (FACTS). The significance of this work lies in its potential to advance the understanding and application of VSCs, contributing to more resilient and efficient power systems. By providing a solid foundation for future research and development, this study supports the ongoing integration of renewable energy sources and the advancement of modern electrical infrastructure. Full article

Voltage Source Converter-based High-Voltage Direct Current Transmission (VSC-HVDC) is essential for integrating renewable energy sources and facilitating inter-regional power transmission. This study evaluates the reliability of control and protection devices within these systems, which are crucial for the stable operation of power grids. Humidity significantly affects both the operational conditions and lifespan of these devices. Previous studies, reliant on extensive full-condition fatigue testing, have lacked effective test models and detailed analyses of mechanisms. To address this gap, a humidity diffusion model was developed to comprehensively investigate moisture diffusion mechanisms. Using the insights gained, the Hallberg–Peck model was applied to predict the lifespan of these devices, quantitatively assessing how changes in humidity affect their reliability. This approach employs a stringent failure criterion, leading to a conservative predicted lifespan. This method achieved a prediction accuracy of 85.648% compared to the benchmarks in GB/T 2423.50-2012, validating the accuracy of our model and the effectiveness of our simulation technology under stringent conditions. This research provides vital theoretical data and serves as an essential tool for guiding the precise maintenance of equipment in varying environmental humidity levels. Full article

With the increase in the number of power electronic devices in power systems, various techniques for assessing their stability have emerged. Among these techniques, impedance model-based stability analysis techniques have been widely used. However, conducting such analyses across multiple operating points requires abundant impedance measurement data from power electronic devices. In this paper, we propose a method for constructing impedance models of equipment with fewer impedance measurement data in voltage-source converter (VSC) back-to-back high-voltage direct current (HVDC) systems using physics-informed neural networks. Furthermore, given the power system states, we present a neural network approach to estimate grid stability at different operating points. Validation via PSCAD/EMTDC simulations and a PyTorch neural network confirmed the adequacy of these models. Full article