High Voltage Direct Current Converter Research Articles

Optimised Congestion Management Using Curative Measures in Combined AC/DC Systems with Flexible AC Transmission Systems

Due to the increasing demand for transport of electrical energy, measures for power flow control, congestion management, and higher utilisation of the existing grid play a decisive role in the transformation of the power system. Hence, enormous efforts must be undertaken using measures of congestion management. Modelling and integration of corresponding measures in optimisation tools to support grid and system operation and therewith reduce the resulting efforts become more important. This is especially true because of the high intermittency and decentralisation of renewable generation leading to increased complexity of the power system, higher loading of assets, and a growing need for control over flexible alternating current transmission systems (FACTS) and high-voltage direct current (HVDC) converters. This work therefore describes the implementation of optimised congestion management in an A Mathematical Programming Language (AMPL)-based nonlinear optimisation problem. AMPL is an effective tool to deal with highly complex problems of optimisation and scheduling. Therefore, the modelling of assets and flexibilities for power flow control in AC/DC systems in combination with an innovative grid operation strategy using predefined curative measures for the optimised use of the existing grid is introduced. The nonlinear mathematical optimisation aims at the optimal cost selection of flexibility measures. The application of the optimisation technique in a combined AC/DC system shows the optimal preventive and curative use of measures in operational congestion management. Simulation results prove that, by using predefined curative measures, the volume of cost-intensive preventive measures can significantly be reduced, especially in association with power flow control.

Transient response mitigation using type-2 fuzzy controller optimized by grey wolf optimizer in converter high voltage direct current

Long high voltage direct current (HVDC) transmission link is commonly used to transmit electrical energy via land or under-sea cable. The long HVDC avoids reactive power losses (RPL) and power stability problems (PSP). On the contrary, the RPL and PSP phenomena occur in long high voltage alternative current-link (HVAC) caused by the high reactive component in the HVAC-link. However, the HVDC produces a high and slow transient current response (TCR) on the high value of the up-ramp rate. Interval type-2 fuzzy (IT2F) control on converter-side HVDC is proposed to mitigate this TCR problem. The IT2F is optimized by grey wolf optimizer (GWO) to adjust input-output IT2F parameters optimally. The performance of IT2F-GWO is assessed by the minimum value of integral time squared error (ITSE), peak overshoot, and settling time of the TCR. The IT2FC-GWO performance is validated by the performance of IT2F control that is optimized by genetic algorithm (IT2F-GA) and proportional integral (PI) controller. Simulation results show that the IT2F-GWO performs better with small ITSE, low peak overshoot, and shorter settling times than competing controllers.

Low-Frequency and Sub-Synchronous Power Oscillation Suppression and Analysis of Hybrid Line Commutated Converter for HVDC Grid

Building high-proportion renewable energy grid requires economical and reliable High Voltage Direct Current (HVDC) converters. Line Commutated Converter (LCC) is widely used because of its long transmission distance, low power loss, low manufacturing cost, and large transmission capacity, which modular multilevel converter (MMC) cannot achieve, but has commutation failure (CF) problems. Low-frequency and sub-synchronous power oscillation caused by CF will affect the safety of bulk power systems. This paper theoretically analyzes the power oscillation during CF and CF recovery, and then proposes a novel hybrid line commutated converter (H-LCC) based on integrated gate commutated thyristor (IGCT), which can effectively mitigate CF and suppress power oscillation. The principle of H-LCC suppressing power oscillation is analyzed. On this basis, the performance of LCC, MMC and H-LCC under the same fault conditions is compared by a simulation study, which proves that H-LCC can improve the security and stability of high-proportion renewable energy power systems.

Advancements and comparative analysis of high-voltage direct current transmission technologies

This paper outlines the fundamental principles of high-voltage direct current (HVDC) transmission, elucidating its two primary variants: current-source converter (CSC) HVDC and voltage-source converter (VSC) HVDC. It also undertakes a comparative analysis with high-voltage alternating current (HVAC) technologies, focusing on aspects such as power transmission efficiency and cost-effectiveness, drawing upon prior research findings. Additionally, the paper underscores the critical role of circuit-breakers (CB) as essential components for controlling HVDC systems. HVDC technology plays a pivotal role in augmenting AC transmission systems, facilitating the integration of large-scale renewable energy sources, and enhancing the efficiency of expansive power grids over considerable distances. Its continued evolution and refinement are highly probable, given its indispensable role in the energy landscape.

The hybrid solar energized back-to-back high voltage direct current modular converter for distributed networks

The hybrid solar energized back-to-back high voltage direct current modular converter for distributed networks

DC protection coordination for multi‐terminal HB‐MMC‐based HVDC grids

AbstractFast progress of High Voltage Direct Current (HVDC) technology has been considerable over the past decades. Emerging Modular Multilevel Converter (MMC) provides power systems with promising solutions to integrate large‐scale renewable energy sources, as well as multiple asynchronous power grids. Consequently, implementation of a multi‐terminal HVDC transmission system has been easier using MMC converters. However, protection of multi‐terminal MMC‐based HVDC grids under DC fault conditions is a challenge for the industry. This study sheds light on concept of system‐level DC protection coordination for multi‐terminal HVDC grids. Although, there are different protection methods for HVDC transmission systems including multi‐terminal ones, there are no well‐defined protection coordination strategies for transmission links protection and belonging HVDC converters under DC fault conditions. The proposed protection coordination prevents unnecessary tripping of MMCs in the multi‐area HVDC grids and supports the system with minimum effect on healthy parts. For the first time, different HVDC Converter Tripping (HCT) profiles are presented based on a fully selective protection strategy and their characteristics are discussed comprehensively. Simulation results demonstrate the effectiveness of the proposed strategy for multi‐terminal MMC‐based HVDC grids. The proposed coordination can be applied to DC grid code requirements of meshed HVDC grids in the future.

A parameter optimization method of multi-layer nested slit resonator for the converter transformer noise in high voltage direct current converter station

We propose a parameter optimization method of multi-layer nested slit resonator (MNSR) for the converter transformer noise in a high voltage direct current (HVDC) converter station. With the optimization objective of achieving quasi-perfect sound absorption at the target frequencies and the structural parameters of MNSR as the optimization variables, the optimization model of the structural parameters of MNSR is established and then solved by the sequential quadratic programming algorithm (SQP). The theoretical and simulation results of the four validation cases show that the optimized MNSR can achieve quasi-perfect sound absorption at all target frequencies, which is consistent with the optimization objective. The parameter optimization design method provides an efficient method to design a dual-frequency quasi-perfect sound absorber for the converter transformer noise.

Analysis of Onshore Synthetic Inertia and Primary Control Reserve Contributions of Alternating Current-Side Meshed Offshore Grids with Voltage-Source Converter and Diode Rectifier Unit High-Voltage Direct Current Connections

The increasing use of renewable energy sources in place of conventional generation units is leading to a reduction in onshore inertia and to the development of offshore wind park grids connected by multiple high-voltage direct current (HVDC) connections to the onshore alternating current (AC) grid. For AC-side meshed offshore grids with voltage-source converter (VSC) and diode rectifier unit (DRU) HVDC connections towards onshore grids, this study focuses on the energetic feasibility of synthetic inertia (SI) and primary control reserve (PCR) contributions triggered locally at the onshore converters of both connection types. To this end, the obstacles preventing contributions for VSC HVDC connections and the mechanisms allowing contributions for DRU HVDC connections are identified first. Based on these findings, the article proposes an enhancement of the offshore HVDC converter controls that is continuously active and allows locally triggered onshore contributions at all onshore HVDC converters of both connection types without using communication and requiring only minimal system knowledge. Additional simulations confirm that, although the enhancement is continuously active, the operational performance of the offshore HVDC converter controls for normal offshore grid operation and its robustness against offshore AC-side faults are not affected.

Influence of Rotor Damping Bars on Rotor Temperature Rise of Synchronous Condenser After Single-Phase Short-Circuit Fault

A synchronous condenser (SC) can provide the transient reactive power support for the Ultra High Voltage Direct Current (UHVDC) converter station. However, the rotor temperature rise is generally the main factor which limits the transient operation ability of the SC. In this paper, a rotor structure of the SC with the rotor damping bars made of different materials in the rotor large teeth is proposed to reduce the rotor temperature rise. In order to obtain the influence of the rotor damping bars on the temperature rise, the rotor loss and temperature distribution of a 300-MVar SC under the single-phase short-circuit fault are calculated by coupling the electromagnetic field, loss and temperature field models of the SC with the model of the power grid. For the damping bars made of the copper, aluminum and stainless steel, the rotor maximum temperature rises are calculated and compared. According to the maximum permissible temperature of the rotor core, the transient operating ability of the SC with the rotor damping bars made of different materials is obtained under the single-phase short-circuit fault. This study can provide the theoretic basis for the improvement of the transient reactive power support capacity of the SC under the single-phase short-circuit fault.

Modelling and analysis of LCC‐HVDC converter station for harmonic coupling of AC/DC power grid during geomagnetic storm

AbstractIn this paper, the problem of harmonic coupling between the converter transformer and the converter under the action of geomagnetically induced current (GIC) is proposed to be investigated. Firstly, a model of a 12‐pulse line commutated converter high voltage direct current (LCC‐HVDC) converter station based on harmonic state space (HSS) theory is established. Secondly, the harmonic components of the interaction between the AC‐side harmonic current and the DC‐side harmonic voltage are qualitatively analyzed by using the harmonic modulation theory. Through the simulation model and harmonic state space (HSS) calculation model, the harmonic coupling characteristics of power grid are analyzed. Finally, the harmonic current and voltage amplitude of AC / DC power grid under different GIC are calculated, and the characteristics and rules of AC / DC harmonic propagation are revealed. Among them, the amplitude of the 2nd harmonic current on the AC side and the 3rd harmonic current on the DC side increases linearly with the GIC. In addition, the 3rd and 9th harmonics on the AC side are also greatly affected by the magnetic bias of the converter.

Control strategy used for AC fault ride-through in VSC–HVDC transmission systems

Fault ride-through (FRT) is a very important requirement for power grids based on voltage source converters (VSCs) to improve its operational availability of the AC grid. Grid codes require VSC stations to incorporate fault handling capabilities to prevent disconnection of converter stations from the alternating current (AC) grid for certain fault characteristics. In this paper, an FRT strategy is used in the two converter stations to handle the faults that may occur in the AC part of the grids and ensure their stability. The method improves the performance of the FRT compliance strategy during a voltage drop in the AC side. In addition, a current control that operates simultaneously on positive and negative sequence current is adopted. It also allows the injection of a reactive current to maintain the availability of the high-voltage direct current (HVDC) converter station during a fault. The performance and stability of the suggested FRT control is tested considering the direction of power flow with possible faults in balanced and unbalanced regime. For the validation of the efficiency of the FRT capacity, simulation tests are carried out with MATLAB-Simulink software, and experimental tests with a Smart-grid test bench.

Energy Efficiency Evaluation of HVDC Converter Station Based on Deep Neural Network Model

Abstract This paper introduces a new idea of energy efficiency evaluation of high voltage direct current (HVDC) converter based on deep neural networks. This paper analyzes the leading operating indicators and finds the main influencing factors in constructing the energy-saving hands of each converter table. Then this paper uses the deep neural network method to establish the converter table's energy efficiency model. The deep neural network for energy efficiency evaluation of the converter table is obtained. In this paper, PSCAD/EMTDC was used for simulation to evaluate the energy-saving effect. The research shows that the method of energy efficiency evaluation based on deep neural networks is feasible.

Influence of Thermal Aging on Dielectric Properties of Natural Ester Oil-Impregnated Paper

Mineral oil is widely used as a dielectric liquid in high voltage direct-current (HVDC) converter transformers because of its excellent electrical insulating and cooling properties. Due to the low biodegradability and high fire risk of mineral oil, natural ester oil is gaining considerable interest as a potential alternative for HVDC converter transformers. This requires more information thorough understanding of the electrical behaviours of natural ester oil under various conditions. This paper compares the impact of thermal ageing on the space charge characteristics of natural ester oil-impregnated paper to mineral oil-impregnated paper. During thermal ageing, although the increased acidity caused by the natural ester oil led to more charge accumulation in natural ester oil-impregnated paper compared to the mineral oil, the natural oil-impregnated paper showed a higher DC breakdown voltage.

TransFNN: A Novel Overtemperature Prediction Method for HVDC Converter Valves Based on an Improved Transformer and the F-NN Algorithm.

Appropriate cooling of the converter valve in a high-voltage direct current (HVDC) transmission system is highly significant for the safety, stability, and economical operation of a power grid. The proper adjustment of cooling measures is based on the accurate perception of the valve's future overtemperature state, which is characterized by the valve's cooling water temperature. However, very few previous studies have focused on this need, and the existing Transformer model, which excels in time-series predictions, cannot be directly applied to forecast the valve overtemperature state. In this study, we modified the Transformer and present a hybrid Transformer-FCM-NN (TransFNN) model to predict the future overtemperature state of the converter valve. The TransFNN model decouples the forecast process into two stages: (i) The modified Transformer is used to obtain the future values of the independent parameters; (ii) the relation between the valve cooling water temperature and the six independent operating parameters is fit, and the output of the Transformer is used to calculate the future values of the cooling water temperature. The results of the quantitative experiments showed that the proposed TransFNN model outperformed other models with which it was compared; with TransFNN being applied to predict the overtemperature state of the converter valves, the forecast accuracy was 91.81%, which was improved by 6.85% compared with that of the original Transformer model. Our work provides a novel approach to predicting the valve overtemperature state and acts as a data-driven tool for operation and maintenance personnel to use to adjust valve cooling measures punctually, effectively, and economically.

Energy-balancing dual-port grid-forming control for VSC-HVDC systems

This work examines energy-balancing dual port grid-forming (GFM) control for high-voltage direct current (HVDC) transmission. In contrast to the state-of-the-art, HVDC converters controlled in this way do not require assigning GFM and grid-following roles to different converters. Moreover, this control enables primary frequency control and inertia support through HVDC links. A detailed stability and steady-state analysis results in conditions on the control gains such that i) the overall hybrid dc/ac system is stable, ii) asynchronous ac areas are quasi-synchronous, and iii) circulating power in cyclic topologies is avoided. Finally, a high-fidelity case study is used to illustrate and verify the analytical results.

Space Charge Characteristics of Natural Ester Oil-Impregnated Paper With Different Moisture Contents

Mineral oil is the most common dielectric liquid for high-voltage direct current (HVDC) converter transformers. However, natural ester oil receives much attention as an alternative dielectric liquid due to its environmental friendliness and low fire risk. Moisture is one of the most dangerous substances, which accelerates the degradation of oil-paper insulation systems. Thus, this article investigates the impact of moisture on space charge characteristics of the natural ester oil-impregnated paper by comparing it with the mineral oil-impregnated paper. Space charge accumulation is a typical phenomenon under dc conditions, and its presence can lead to electric field distortion and accelerate insulation aging. The space charge profiles in oil-impregnated papers with different moisture contents are obtained by the pulsed electroacoustic (PEA) method. It has been found that the hydrophilic characteristic of natural ester oil leads to a slow process of forming heterocharges in the natural ester oil-impregnated paper. On the other hand, the hydrophobic characteristic of mineral oil results in faster heterocharge formation in the mineral oil-impregnated paper.

Diode rectifier‐based hybrid high‐voltage direct current converter for offshore wind farms

This paper presents a diode rectifier (DR)-based hybrid converter topology for high-voltage direct current (HVDC) transmission system with offshore wind farms (OWFs). The hybrid converter consists of a DR and an auxiliary converter. A high conversion ratio ac/dc converter is designed as the auxiliary converter, which can form the offshore grid voltage for OWFs to achieve black-start. Under steady state, all the active power output from OWFs is delivered through the DR. Meanwhile, the auxiliary converter is able to compensate the reactive power and harmonic currents of the system. Comparing with the conventional modular multilevel converter (MMC), the number of submodules (SM) of the proposed hybrid converter is significantly reduced, which decreases the construction cost and size of the system. The topology and operation principle of the hybrid HVDC converter are introduced. Then the corresponding control strategy is designed to achieve energy balance and power transmission. Based on the mathematic model, a design method for the parameters of the auxiliary converter is proposed. Finally, the feasibility of the proposed hybrid HVDC converter is verified by both the simulation and hardware-in-the-loop experiment results.

Design Studies of VSC HVDC Converter According to AC Voltage Tests

Since high-voltage direct current (HVDC) systems are very expensive and operationally critical, these systems must be tested before they are put into service. Insulation and performance tests are the two main subjects of these tests. AC voltage tests, as part of the insulation tests, should be performed after system installation is complete and before commissioning. However, in this study, the objective was to perform these tests during the prototype phase of VSC HVDC. Unlike other studies, this study attempted to use COMSOL Multiphysics to determine in advance the problems that may occur in the real system. In this regard, the busbars connecting the submodules of the VSC HVDC system were first modeled in 3D, and the tests to be performed were simulated using COMSOL Multiphysics software. During the simulation, the finite element method (FEM) was used to identify critical points that could cause partial discharge. To validate the simulation results, partial discharge tests on a real system were conducted, and the design changes made in response to each test result were explained. After the improvement actions, the targeted partial discharge values were achieved.

Research on Long-term Reliability of Silver Sintered Press-Pack IGBT Modules

Owing to the advantages of short-circuit failure mode, double-sided heat dissipation, and low thermal resistance, press-pack insulated gate bipolar transistors (PP-IGBTs) are widely used in high-power-density applications, such as high-voltage direct-current converters. As the core device of large-capacity power equipment, the reliability of a PP-IGBT is directly related to the security of the power system. In recent years, a silver-sintered package has been proposed to improve the electro-thermal performance of PP-IGBTs. However, the long-term reliability of this package has not been proven. In response to this problem, 3300-V/50-A silver-sintered PP-IGBT (SPP-IGBT) modules are used in long-reliability research. First, electro-thermal-stress finite-element models of PP-IGBTs and SPP-IGBTs were proposed and the accuracy of models verified by experiments. Through a simulation comparison, the results show that under the rated operating conditions, the on-voltage and maximum temperature of SPP-IGBTs dropped by 9.3% and 3.7%, respectively. In addition, the temperature and stress of each component were reduced, among which the surface stress of the IGBT chip emitter dropped by as much as 24.7%. Subsequently, a power-cycle test platform was established, and three PP-IGBTs and three SPP-IGBTs were tested. Finally, the experimental results were compared and analyzed, and the reasons for the sharp increase of on-voltage and metal melt were explored. The results show that the silver-sintered package improves the electrical-thermal performance and long-term reliability of the module. • Electro-thermal-stress finite-element models of PP-IGBT and SPP-IGBT were established separately. The accuracy of the model is verified by the temperature in the experiment, and the error is less than 5%. • Electrical, thermal, and stress indexes of PP-IGBTs and SPP-IGBTs were analyzed and compared. The on-voltage, maximum temperature and IGBT chip emitter surface stress of SPP-IGBTs dropped by 9.3%, 3.7% and 24.7%, respectively. • Three PP-IGBTs and three SPP-IGBTs were used for power-cycle test under same condition. The results also show that the SPP-IGBT has better electro-thermal performance, less surface wear of the chip emitter, and better long-term reliability.

A life-cycle cost analysis of High Voltage Direct Current utilization for solar energy systems: The case study in Turkey

With technological advancements in High Voltage Direct Current, solar energy transmission over long distances and between countries is now possible. As a result, this paper conducts a thorough Life-cycle cost analysis in Turkey. To that end, a 1000 km long High Voltage Direct Current Overhead Transmission Line between Mersin, Balikesir, and Istanbul is being built, which will transport energy generated by solar power plants from potentially high solar regions to the most energy-deficient regions. The motivation is testing the hypothesis that any investment should be contingent on the investment cost, being it amortized and generating profits for the interest rate of any country. This hypothesis is tested in this paper with a comparative cumulative cash flow for High Voltage Direct Current converters using the net present value. Data is gathered from various studies as well as the Central Bank of the Republic of Turkey. The 27-year Life-cycle cost projection has shown that the Voltage Source Converter-High Voltage Direct Current-Overhead Transmission Line outperformed the best with a 448,61 M€ net present value, and a break-even 9 years after the beginning of the project pertained to a 1500 MW power rating option. This investment satisfies Turkey's condition in terms of opportunity cost and positive externality. The innovation of this paper is the proposal of an efficient operation of increasing solar energy systems with increasing High Voltage Direct Current transmission systems as an alternative energy solution to meet energy demands in the context of a growing economy. Meanwhile, this research can serve as a model for developing countries, not only for economic reasons, but also for environmental concerns.