Voltage Direct Current Transmission Research Articles

Signal processing and artificial intelligence based HVDC network protection: A systematic and state-up-the-art review

In this modern era, the smart grid (SG) is progressively empowered for direct current (DC) power transmission, either at high voltage (HV) or at medium voltage (MV). The high voltage direct current (HVDC) transmission systems have a significant impact on the SG operation even though the integration of add-on green and renewable-based power generation (RPG). In this RPG era, the application of HVDC can be seen in the grid-connected remote offshore wind or photovoltaic (PV) based large RPG, and interconnections amongst the states or countries. In such a context, a smart grid inevitably is ridden with technical complexities (both control and protection viewpoints), and thus signals processing (SP) techniques along with artificial intelligence (AI) methods are considered very much important to understand, effectively plan, draw up plans, and drive the multifaceted electric power system. This paper presents an extensive review of HVDC network protection systems with special attention to SP and AI applications. This paper presents an extensive review of the suggested techniques in the past three decades and discussed the pros and cons of each method. Apart from that critical findings are discussed and analyzed along with the future scope of research toward the development of HVDC protection. This review tries to focus on the gap between the existing protection schemes and topology with the smart grid-based power system perspective and convey to the power engineers and researchers the possibilities of further research as a solution to the associated issues and challenges.

An Improved Suppression Method of AC Transient Overvoltage for Line Commuted Converter Based High Voltage Direct Current Considering AC-DC System Coupling

Commutation failures in line commuted converter-based high voltage direct current (LCC-HVDC) transmission systems leads to an increase in the converter bus voltage of the rectifier station, thus resulting in AC transient overvoltage in the sending-end grid. The transient overvoltage could lead to the disconnection of renewable energy generation and threaten the stable operation of the sending-end grid. However, the influences of the coupling between AC and DC systems caused by the interaction between the active and reactive power of the sending-end grid, the AC bus voltage of the rectifier station, and the DC current are ignored. The AC transient overvoltage cannot be accurately suppressed. Therefore, in this study, the transient voltage characteristics of the rectifier station under a commutation failure of the inverter station are analyzed. The influence of LCC-HVDC control on the AC bus voltage of a rectifier station through the active and reactive power of the rectifier station is analyzed. A dynamic model of the AC bus voltage of a rectifier station under an AC-DC system coupling is established. The calculation method of the command value of the DC current of the rectifier station is proposed by a predictive control model, and an improved suppression method for AC transient overvoltage is proposed. The case studies show that the accuracy and effectiveness of the suppression of AC transient overvoltage are improved by considering the coupling between AC and DC systems.

Review paper on multilevel converters topology for VSC-based HVDC transmission system connected offshore wind power plant

This review paper offers a comprehensive analysis of multilevel converters (MLCs) within Voltage Source Converter (VSC)-based High Voltage Direct Current (HVDC) transmission systems, focusing on their integration with offshore wind power plants. Through an exhaustive exploration, the paper investigates various MLC configurations and control strategies, assessing their potential to enhance the performance, efficiency, and reliability of offshore wind energy integration into the power grid. By examining the intricate interplay between MLCs, VSC-based HVDC systems, and offshore wind power plants, this review aims to provide valuable insights into the evolving landscape of renewable energy integration technologies. Key topics covered include an overview of offshore wind power generation, the significance of HVDC transmission for offshore wind farms, and the role of MLCs in overcoming integration barriers. Furthermore, the paper delves into control strategies, performance evaluation metrics, and future perspectives and challenges. Through its in-depth analysis, this review contributes to a deeper understanding of the potential of MLCs in advancing the efficiency and reliability of VSC-based HVDC systems, ultimately facilitating the seamless integration of offshore wind energy into the broader power grid.

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.

Characterization of electric field distribution in valve-side casing of 500 kV converter transformer considering AC and DC mixed voltage

The converter transformer, as one of the indispensable core devices in the ultra-high voltage direct current (UHVDC) transmission system, exhibits highly complex electric field distribution characteristics within the valve-side bushing. In-depth research into the electric field distribution characteristics and variation patterns of the valve-side bushing under both alternating current (AC) and direct current (DC) voltage conditions is crucial for ensuring the reliable operation of the transmission system. This study focuses on investigating the electric field distribution of the valve-side bushing under AC and DC voltage excitations. A full-scale finite element model of the valve-side bushing of the converter transformer is constructed using finite element simulation software for modeling and research purposes. The results indicate that the internal electric field distribution of the bushing is generally similar under both AC and DC voltage excitations. However, in the SF6-filled region, a steeper voltage drop is observed under DC excitation compared to AC. The space charges generated in the DC electric field due to field polarization have a significant impact on the distortion of the electric field on the inner and outermost layer electrode plates. These conclusions provide theoretical guidance for the insulation design of the valve-side bushing in converter transformers at the 500 kV voltage level and similar ratings.

A novel linear regression density peak clustering‐based transmission line protection for line‐commuted converter‐voltage source converter hybrid high voltage direct current system

AbstractLine‐commuted converter (LCC)–voltage source converter (VSC) hybrid high voltage direct current (HVDC) transmission system is an innovative technology, the existing LCC and VSC protection require setting, and it is difficult to be directly applied in the LCC–VSC hybrid HVDC, which is a serious problem in practical engineering. A novel linear regression density peak clustering (LRDPC) approach is introduced for transmission line protection. LRDPC employs Least Squares linear regression to compute the fault current slope, followed by density peak clustering for fault type identification. The proposed protection structure is straightforward and setting‐less, eliminating the need for fault pole selection elements and classification thresholds. Validation on the Kun‐Liu‐Long LCC–VSC HVDC RTDS system demonstrates the method's effectiveness in identifying diverse faults under varying conditions, including fault types, locations, resistances, and signal‐to‐noise ratios. Notably, it remains robust against fault impedance (600 Ω) and noise interference (20 dB).

Reactive power capacity allocation technology of shore‐based power supply system of offshore oil production platform under different network topology reliability measures

AbstractTo improve the economy of offshore oil production platform shore‐based power transmission mode, the reactive power capacity allocation technology of offshore oil production platform shore‐based power supply system based on different network topology reliability measures was proposed. The optimization model of power supply system network topology and the structure model of voltage source converter high voltage direct current transmission system were established, the start and stop control scheme of flexible direct current (DC) transmission system was designed, and the operation parameters of converter station and DC line were monitored in real time. The topological structure of offshore oil production platform shore‐based power supply system is constructed, the mathematical model of flexible DC transmission system is established when the alternating current side voltage is balanced, the reactive power capacity configuration of offshore oil production platform shore‐based power supply system is optimized, and the life cycle cost results are calculated. The experimental results show that the algorithm proposed in this paper can obtain the optimal solution only after 26 calculations, with high optimization efficiency and good convergence effect. Reactive capacity configuration of shore‐based power supply system for offshore production platform can reduce failure rate and life cycle cost.

New multi-injection commutation topology for circuit breakers of HVDC transmission lines

Power-sharing between countries has an essential effect on increasing the power system’s reliability and allowing a resilient energy market. High voltage direct current (HVDC) transmission systems are preferable for long-distance transmission to decrease power losses. However, HVDC transmission lines have many protection challenges including the differentiability between various types of HVDC circuit breakers (HVDC-CB). Although mechanical HVDC-CBs suffered from long response time, they superseded their solid-state counterparts in terms of price and power losses. In this paper, a multi-injection commutation system MICS-HVDC-CB is developed to provide economic and fast response HVDC-CB. The proposed breaker doesn’t add external elements to avoid any price increase but instead modifies the existing topology. The MICS consists of multiple L–C commutation circuits inserted sequentially following the receiving of the tripping signal. The proposed MICS-HVDC-CB was tested upon a real transmission line using ATP simulation software. The results emphasize that the developed MICS-HVDC-CB decreased the arcing time to 38.5% and 20% compared to passive and active DC-CBs. The impact of cooling power, arcing time constant, and fault resistance was also investigated. The results showed the effectiveness of the proposed MICS topology in reducing the arcing time while keeping a simple and economic breaker structure.

A current injection method for converter power loss representation in electromagnetic transient simulation

The power-electronics converters are increasingly applied in modern power system, which contains various sub-systems, such as renewable energy power plants and high voltage direct current transmission systems, etc. The electromagnetic transient simulation has been one of the important tools for the study of complex system containing large numbers of power converters. The coordination control of the power converters and corresponding power generation and load units is one of the challenges ensuring stability and obtaining optimal efficiency. However, the detailed modeling of the power converters considering the conduction and switching losses can cost a great number of computation resources, which makes it difficult to simulate a large system. This work proposed a current injection method for converter power loss representation, which can accurately consider the conduction and switching losses of the power-electronics switches with low computation complexity. A look-up table (LUT) of the power loss ratio is calculated before the simulation, and a current source representing the power loss is connected in parallel with the converter to track the reference loss generated by the look-up table. The proposed method can be combined with multiple converter modeling schemes, such as the two-resistance model, the switching-function model, and the average value model, etc. The topologies of the two-level converter and the modular multi-level converter (MMC) were used in the case studies to demonstrate the effectiveness of the proposed method. Compared to the device-level model, the simulation power loss deviation is 0.64% and 4.13% applying the current injection method model in the case studies of the two-level converter and the MMC.

Network based impedance analysis of grid forming based MMC-HVDC with wind farm integration

Grid forming control for modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system is favorable to support the AC systems. This paper presents the hybrid AC-DC impedance model for grid forming based MMC-HVDC with wind farm integration. Comparisons of the hybrid AC-DC impedance with the traditional AC/DC impedance are presented, and the hybrid AC-DC impedance does not contain right half-plane poles caused by DC/AC coupling. Then, the network based impedance analysis is carried out to analyze the stability of grid-forming based MMC-HVDC with wind farm integration. It is found that grid forming based MMC-HVDC may lack damping with two modes of oscillations. Large inertia, DC/AC virtual impedance and wind farm integration can dampen the first mode oscillation, while only AC virtual impedance is effective for the second mode oscillation. The stability comparison of grid forming and grid following based wind farm integration is also presented. Moreover, MMC-HVDC with grid forming control cannot improve the stability of grid following based wind farm integration in weak grid where grid forming based wind farm performs better. Therefore, both wind farm and MMC-HVDC with grid forming control are better in weak grid. Simulation results in PSCAD/EMTDC validate the corresponding analysis.

On the Commutation of Thyristors for High- Voltage Direct Current Transmission (HVDC)

On the Commutation of Thyristors for High- Voltage Direct Current Transmission (HVDC)

Influence Analysis and Control Method of Ultra-Low Frequency Oscillation in a Hydro-Dominant Sending Power System with Wind Power Integration

To make clear the influence of renewable energy on ultra-low-frequency oscillation (ULFO) in a hydro-dominant system via an high voltage direct current (HVDC) transmission system, this paper studies the damping characteristics when wind power is integrated into the sending power system. The damping torque method is applied for the mechanism study and risk evaluation of ULFO. The study of the inner cause and outer performance proves that ULFO belongs to frequency oscillation rather than conventional power oscillation. According to the theoretical analysis, the suggested control mode is identified to reduce the risk of ULFO. Moreover, a robust controller is designed for wind units based on mixed H2/H∞ robust control theory, and the control method makes use of the fast response ability of the converter without compromising the primary frequency regulation ability. Finally, a simulation model of a hydro-dominant sending power system with wind integration is established in PSCAD Version 4.6 software. The simulations verify the proposed control can suppress ULFO effectively.

A modified control strategy for LCC with IGCT‐based full‐bridge submodules to solve the commutation failure issues

AbstractTo solve the commutation failure (CF) issues in high voltage direct current (HVDC) transmission systems based on line commutated converter (LCC), this paper proposes a hybrid solution combining the LCC inverter with the continuously controlled IGCT‐based full‐bridge submodules (FBSMs). The scheme can provide the LCC inverter with an additional commutation area, and compensate for the phase‐shift of the grid voltage under faults. Therefore, it shows better CF resistance than the existing scheme. The characteristics of the proposed hybrid converter are modelled and analyzed in both normal and fault conditions. Then a decoupling control strategy is presented for it. With the ability to counteract negative‐sequence grid voltage, the strategy brings good voltage stability to the dc transmission. And when the grid fault occurs, it can switch to suppress CF by rapidly increasing the output voltage of the FBSMs and applying commutation failure prevention (CFPREV) control to the LCC. As a result, the CF issues can be maximally solved with a smaller number of FBSMs than the existing scheme. Simulations in PSCAD verify the effectiveness of the strategy. Furtherly, an experimental platform is built to show the capacity of the proposed scheme to solve the CF issues.

Analysis of transient electromagnetic force on end windings of 300 Mvar synchronous condenser during dynamic reactive power compensation process

The uneven distribution of renewable energy supply and demand promotes the development of high-voltage direct current transmission. As the critical reactive power compensation equipment for high voltage direct current transmission, the synchronous condenser must have a strong overload capability. One of the critical factors limiting the improvement of its overload capacity is the reliability of the end winding, which depends on the magnitude of the force on end windings during dynamic large reactive power output. Therefore, this paper proposes a coupling model of transient electromagnetic force for the high voltage direct current transmission system and the end zone of the synchronous condenser. The transient response of the synchronous condenser under different fault degrees is calculated, analyzed, and verified by experiments. Based on the current response of the synchronous condenser, the transient electromagnetic force on end windings is calculated. The distribution law of the force on end windings of the synchronous condenser under different degrees of dynamic reactive power compensation process is revealed. Further, the regions on end windings with larger force during strong overload are indicated. The optimizing fixation scheme for end windings is proposed according to the distribution features of the force, which can improve the operational reliability of end windings.

A novel fault location strategy based on Bi‐LSTM for MMC‐HVDC systems

AbstractThe integration of modular multilevel converters (MMCs) with high voltage direct current (HVDC) transmission systems is an efficient method for transporting electricity from distant renewable energy sources to demand centres. However, MMC‐HVDC systems face reliability challenges during DC overcurrent faults, often caused by component failures that can lead to HVDC network shutdowns. Consequently, a reliable fault location approach is crucial for grid protection and restoration, aiding in fault isolation and alternate power flow identification. Conventional fault location methods struggle with manual protective threshold setting, susceptibility to fault resistance and noise, and the need for communication channels, resulting in signal delays. In multi‐terminal HVDC networks, fault location becomes even more complex due to poor selectivity and sensitivity in traditional schemes. This study proposes a robust fault location approach based on bidirectional long short‐term memory (bi‐LSTM). The method offers a simplified decision‐making model with low computational requirements, utilizing fault features from one end of the network, eliminating the need for a communication channel. Remarkably, this approach achieves high fault location accuracy, even with varying fault types, resistances, and noise levels, as demonstrated by an MSE of 0.006 and a percentage error below 1% in simulations conducted using a real‐time simulator with MATLAB/Simulink.

ON THE SUITABILITY OF MODULAR MULTILEVEL CONVERTER (MMC) IN HIGH VOLTAGE DIRECT CURRENT (HVDC) TRANSMISSION SYSTEM

Transmitting bulk electrical energy over long distance by overhead transmission lines or submarine cables using high voltage direct current (HVDC) transmission system offers more flexibilities such as controllability of power flow and robustness against disturbances. Both current source converter (CSC) and voltage source converter (VSC) configurations have been considered in HVDC system. The former is used in HVDC transmission system involving stiff grid networks of higher ratings while the latter is commonly used in connecting to a weak grid. As such, special applications of HVDC such as connection between two or more weak grids and connection of wind power plants require the VSC configuration. However, in the conventional VSC configuration, the converter arm would have to be designed with large number of semiconductor switches that can block the full DC-bus voltage. However, with the evolution of modular multilevel converter (MMC) as a member of VSC family, sub-module (SM) chopper cells are used. This paper investigates the suitability of MMC in HVDC transmission system. The paper shows that the MMC configuration can generate any desired voltage levels required to synthesize a smooth output waveform, thus eliminating the need of heavy filters, has modularity in both hardware and software and generates very low harmonics.

Critical Technical Issues with a Voltage-Source-Converter-Based High Voltage Direct Current Transmission System for the Onshore Integration of Offshore Wind Farms

Long-distance offshore wind power transmission systems utilize multi-terminal high voltage direct current (MT-HVDC) connections based on voltage source converters (VSCs). In addition to having the potential to work around restrictions, the VSC-based MT-HVDC transmission system has significant technical and economic merits over the HVAC transmission system. Offshore wind farms (OWFs) will inevitably grow because of their outstanding resistance to climate change and ability to provide sustainable energy without producing hazardous waste. Due to stronger and more persistent sea winds, the OWF often has a higher generation capacity with less negative climate effects. The majority of modern installations are distant from the shore and produce more power than the early OWF sites, which are situated close to the shore. This paradigm shift has compelled industry and professional researchers to examine transmission choices more closely, specifically HVAC and HVDC transmission. This article conducts a thorough analysis of grid connection technologies for massive OWF integration. In comparison to earlier assessments, a more detailed discussion of HVDC and HVAC topologies, including HVDC based on VSCs and line-commutated converters (LCCs), and all DC transmission systems, is offered. Finally, a selection criterion for HVDC transmission is advised, and its use is argued to be growing.

Experimental study and numerical analysis of thermal hazard for fire accident in converter transformers

Converter transformers are widely used in high voltage direct current (HVDC) transmission systems. Due to its large storage capacity and relatively high failure probability of the insulation, pool fires are a common type of accidents for this type of transformers, which could pose a significant threat to the safety of personnels and the integrity of nearby transformers. During normal operation, the temperature of the transformer oil can vary between 80 and 90 °C. It is known that the oil temperature can have a significant impact on its burning characteristics. However, most of previous simulations of transformer fires utilized the material properties of transform oil obtained at ambient temperature to determine the heat release rate (HRR). In this study, experimental data (D = 0.2–1.0 m) at an initial oil temperature of 85 °C were employed to determine the correlation between HRR and burner diameter, which was subsequently used to validate the numerical model. 14 typical fire scenarios were examined to understand the accident development process of transformer pool fires. The results showed that a merged flame was observed at the inner side for the case when the fire occurred both on the oil tank upper surface and in the oil storage pit. The radiation impact on (and thus the risk of failure of) key transformer components was found to vary considerably across the different fire scenarios. The radiative heat flux at the outlet of the enclosure where the fire is located was found to exceed 5 kW/m2 and the recommended safety distance for firefighting can reach 6.6 m. It was also found that the maximum heat radiative flux received by the high voltage bushing of an adjacent transformer can reach 46.2 kW/m2 in the worst-case scenario, with a probability of escalation to the adjacent transformer approximately 3.50×10−4. Three distinctive areas (safety area, risk area, and dangerous area) were identified to determine firefighting positions and strategies for transformer substation fires. The present results could not only enhance the understanding of the accidental development of converter transformer fires but also provide valuable guidance in firefighting in various fire accident scenarios.

Cooperative Voltage and Frequency Regulation with Wind Farm: A Model-Based Offline Optimal Control Approach

Converter blocking is a serious malfunction encountered in high voltage direct current (HVDC) transmission systems. During sending-end converter blocking, the resultant active power and reactive power surplus in the sending-end power system lead to a severe increase in bus voltage and grid frequency. Consequently, this poses a substantial threat to the stability of the power system. Traditional control techniques generally control the frequency and voltage separately, which makes it challenging to regulate them jointly. This research paper introduces a collaborative approach for optimal control of voltage and frequency to address this issue. State space models for converter bus voltage and grid frequency prediction are developed using the bus voltage sensitivity matrices and system swing equation. The regulation of the converter bus voltage and grid frequency are intrinsically integrated using the explicit model predictive control (EMPC). When blocking occurs and results in an increase in the converter bus voltage and grid frequency, the EMPC controller regulates the output of active power and reactive power from the wind farm to realize the cooperative regulation of the converter bus voltage and grid frequency. The applicability and effectiveness of this strategy have been confirmed through simulation studies.

Multi-Mode Active Inertia Support Strategy for MMC-HVDC Systems Considering the Constraint of DC Voltage Fluctuations

The Modular Multilevel Converter based High Voltage Direct Current Transmission (MMC-HVDC) systems can provide active inertia support for the connected ac grid for fast frequency recovery. However, the submodule (SM) capacitance voltages will be affected by the transient power flow and there exist dc voltage fluctuation issues during the active inertia support process. This paper proposes a multi-mode active inertia support strategy for MMC-HVDC systems considering the constraint of dc voltage fluctuations. The analytical relationship of dc voltage fluctuations and ac frequency variations when MMC adopts the virtual inertia control is firstly established, and then different inertia support operation modes are divided according to the allowable dc voltage fluctuation ranges. Next, the multi-mode active inertia support strategy for MMC-HVDC is proposed, which can provide flexible active inertia support on the premise of reasonable dc voltage fluctuations according to different frequency variations and recovery requirements. Besides, the inertia support process of MMC is equivalent to the zero-input response of first-order circuit to calculate the maximum sustainable supporting time, which ensures that the dc voltage fluctuation constraint can be well satisfied in different operation modes. Finally, several simulation results verify the effectiveness of the proposed control strategy.