kV High Voltage Direct Current Research Articles

Optimization of insulators in ±550 kV HVDC GIS for offshore wind platform considering charge accumulation

High voltage direct current (HVDC) gas-insulated switchgear (GIS) plays a crucial role in developing far offshore wind power and attaining renewable energy and interconnection objectives. However, surface charge accumulation reduces the surface flashover voltage and restricts the development of DC GIS at the higher voltage. In this paper, the design criteria for a ± 550 kV DC insulator were proposed based on DC insulation testing. The influence of geometry on electric field and charge distribution of obtuse conical insulators, disk insulators, and conical insulators was systematically investigated. The results show that the threshold of the tangential electric field under the DC superimposed lighting impulse test for the ±550 kV DC insulator is 11.1 kV/mm. The simulation results indicate that the increasing inclination of the insulator enhances the normal electric field and surface charge density, while the increase in insulator size diminishes the tangential electric field dramatically. The obtuse conical insulator with an upper surface inclination angle of 30°, a lower surface included angle of 140°, and a diameter of 812.5 mm is ideal for ±550 kV DC GIS. The obtained results may provide guidance for the compact design of insulators and the development of DC GIS.

Comparison of Different Configurations for Shoreline Pond Electrode Station for HVDC Transmission Systems—Part I: Electric Field Study for Frames of Linear Electrode Arrangement Based on a Simplified Analytical Model

During the design of a shoreline electrode station for High-Voltage Direct Current (HVDC) interconnections, the location of the electrodes plays a critical part, especially in the development of the near-electric field. The basic structure is their linear placement, in the form of successive frames, parallel to the longitudinal axis of the breakwater, as proposed by CIGRE and implemented in existing projects. However, this arrangement requires a considerable breakwater length, which may not be permissible, as in the case of Stachtoroi, one of the two electrode stations being built for the 1 GW, ±500 kV HVDC interconnection between Crete and mainland Greece. This troubled the preliminary study team of the electrode stations, which investigated other possible configurations. In this paper, configurations of linear placements of electrode frames are studied and compared at the preliminary study level in terms of electric field effects (especially the near-field), using an analytical simplified model and the superposition method, to determine the most appropriate arrangement of electrodes that will cover the respective requirements of CIGRE directives B4.61/2017. These arrangements are practically evaluated for two different electrode station locations at Korakia in Crete and at Stachtoroi in Aegina for the Crete–mainland-Greece interconnection, resulting in interesting alternative solutions.

Non-unit protection for long-distance LCC-HVDC transmission lines based on waveform characteristics of voltage travelling waves

Currently, the voltage-derivative-based protection (VDBP) method has been applied in High-Voltage Direct Current (HVDC) lines as primary protection. But it has the problem of low reliability to high-resistance faults, and even more difficult or fail when applying to long distance lines. For such circumstances, current differential protection will play backup protection role with slow operation speed. Therefore, HVDC transmission lines require high reliable and high-speed protection scheme. In this paper, the complex frequency domain expressions of the line-mode fault component voltage (LFCV) at the line end as well as its waveform characteristics for internal and external faults are derived respectively. Based on the theoretical analysis of the LFCV at the line end, a novel non-unit protection scheme utilizing the waveform similarity coefficient is proposed. A ± 800 kV HVDC system built in PSCAD/EMTDC is used to verify the feasibility and validity of the proposed protection method. Comprehensive simulation results show that the proposed scheme can accurately identify internal and external faults, including high resistance faults (600 Ω) and long-distance faults (over 2000 km).

Impacts of Corona cage dimension on the Corona characteristics of HVDC applications

High Voltage Direct Current (HVDC) applications have been increasing in parallel with the development of semiconductor technology. Their corona performance is generally better than that of the traditional AC transmission. Corona cages and scaled transmission system models are preferred to determine the corona characteristics and the corona performance of the prospective alternative designs. On the other hand, cage dimension and conductor scaling should be optimized to simulate the real operational conditions. This paper presents the studies performed for corona cage design to identify the corona characteristics of a prospective 500-600 kV HVDC line in Turkey. Cage dimensions are determined using the simulation results for the real system at rated operating voltages and the scaled conductors mounted in a cage at reasonable voltage levels. Finally, corona inception voltages and corresponding maximum field strengths were determined by experiments and compared with the simulation results.

Multi‐objective optimal design for AC filters in high voltage direct current system based on improved multi‐objective evolutionary algorithm based on decomposition

AbstractA triple‐objective optimal design method for AC filters in high voltage direct current (HVDC) system based on an improved multi‐objective evolutionary algorithm based on decomposition (MOEA/D) is proposed. A triple‐objective optimisation mathematical model including total harmonic distortion (THD) on AC side voltage, investment cost of AC filters and telephone harmonic form factor (THFF) is established. An improved MOEA/D is proposed to realise the triple‐objective optimal design for AC filters more efficiently. The improved MOEA/D adds a particular mutation operator, which can not only to prevent MOEA/D from getting stuck at local optimum but also accelerate the convergence to the optimum solution while the algorithm approaches the neighbourhood of the optimum solution. A three‐dimensional fuzzy evaluation system is designed to select the optimum solution of the triple‐objective optimisation. A case of ±500 kV HVDC system is simulated. Taking the ±500 kV HVDC system as the prototype system, a dynamic simulation experimental platform of the rectification system of the direct current transmission system with AC filters is designed and established based on the similarity principle. Both the simulation and experimental results verify that the proposed method have excellent performance.

Effect of Nano-MgO Doping in XLPE on Charge Transport and Electric Field Distribution in Composite Insulation of HVDC Cable Joint

The space charge characteristics of cross-linked polyethylene (XLPE) can be improved to some extent by doping the appropriate amount of nano-MgO. In this study, in order to explore the influence of nano-MgO on the space charge and electric field distributions of the composite insulation of high voltage direct current (HVDC) cable joints, the effect of nano-MgO concentration on the depth and density of the deep traps in MgO/XLPE was first analyzed. On this basis, the charge transport simulation model of a 320 kV HVDC cable joint was established with MgO/XLPE as the cable insulation, and the space charge and electric field distributions of the cable joint under different temperature conditions were simulated. It was found that the radial charge distribution in the joint shows different trends with the change of nano-MgO concentration. There is a significant difference in the charge density on both sides of the (MgO/XLPE)/EPDM interface, and the difference first decreased and then increased with the increase of concentration. When the nano-MgO concentration was 0.5 wt%, the number of charges in the radial direction is the fewest, and the maximum value is only 0.42 C/m−3. The radial electric field changed abruptly at the (MgO/XLPE)/EPDM interface, and it was homogenized to a certain extent with time. It was found that the highest electric field of the interface is at the root of the stress cone, which is the weakest point of the joint insulation. When the nano-MgO concentration was 0.5 wt%, the electric field at the root of the stress cone was found to be the lowest, with a value of 13.38 kV/mm. A comprehensive comparison shows that the joint can maintain better insulation when the concentration is 0.5 wt% compared to other concentrations. The results can provide a basis for further improving the insulation properties of HVDC cable joints through nano doping technology.

Development of High Voltage DC Power Supply from Tesla Coil Using Hybrid Energy Sources

Background: High Voltage Direct Current (HVDC) is a well-proven technology used to transmit electricity over long distances by overhead transmission lines or submarine cables. It is also used to interconnect separate power systems, where traditional Alternating Current (AC) connections cannot be used. Objective: In this paper, an HVDC supply has been proposed with the Tesla coil using hybrid energy sources such as solar panels, batteries, and AC main supply with an AC-DC converter. Method: DC voltage is obtained from various hybrid sources and is supplied to the input of the Cockcroft-Walton (C-W) voltage multiplier circuit through DC/AC inversion. Result: The produced HVDC is provided to the Tesla coil to obtain a stable HVDC output. Fractional Order PID (FOPID) controller is used in the system for control purposes to stabilize the output voltage due to load variations. The model was designed in Matlab / Simulink environment. The simulation results show that the proposed design successfully achieved a stable 15 kV HVDC. Conclusion: The advantage of the proposed system is the availability of variable output voltage as per load requirement compared to the fixed limited output voltage generated by conventional HVDC systems.

Evaluation of lightning overvoltage at neutral point of HVDC converter transformer based on EMTP

In order to ensure the safe and stable operation of high voltage direct current (HVDC) systems, this paper studies the lightning overvoltage at the neutral point of the HVDC converter transformer. The work first introduces the basic types of lightning overvoltage based on the CIGRE Benchmark 500 kV HVDC transmission system which is modeled in the electromagnetic transient program (EMTP) software. The high-frequency model of the HVDC transmission system is considered where lightning overvoltage at the converter transformer neutral point is studied at different values of the small reactance connected at neutral point grounding and different lightning strike locations. Although increasing the small reactance value can effectively suppress the lightning current, the lightning overvoltage will gradually increase. When the neutral point is grounded by 11 Ω small reactance, the lightning overvoltage is most serious, where the peak value can reach 415 kV. Also, the farther the lightning strike position from the converter transformer, the smaller the lightning overvoltage.

Evaluation of the Transient Overvoltages of HVDC Transmission Lines Caused by Lightning Strikes

High-voltage direct-current (HVDC) transmission systems are considered an outstanding solution due to high electrical losses emerging from long-distance transmission. However, HVDC transmission lines (TLs) are vulnerable to lightning strikes. In this work, the Yunnan-Guizhou 500 kV HVDC transmission system is used as a case study to evaluate the impact of lightning strikes on DC-TL overvoltages, as no research studies have been conducted to assess the lightning transient behavior of DC-TLs. A comprehensive investigation of the 500 kV DC-TL transient performance during lightning strikes is performed, taking into account different technical aspects that have not been studied in detail by previous researchers. Additionally, analysis of the back-flashover phenomenon has not been conducted well in previous work, and results on the effect of changing the lightning strike current peak and tower grounding resistance on shielding-failure flashover are quite limited. The distributed-parameter model is used to represent the DC-TL using the electromagnetic transients program (EMTP), considering real parameters of shielding wires and DC towers to study the lightning impact in the case of back-flashover and shielding-failure phenomena. Lightning strike is applied to the shielding wire, and the impact of increasing the peak value of lightning current is investigated on the back-flashover occurrence. Moreover, the influence of tower grounding resistance variation on the transient overvoltages across the tower body and back-flashover phenomenon is evaluated. From the simulation results, increasing the lightning current peak and grounding resistance results in higher overvoltages across the tower body, which increases the probability of back-flashover. Additionally, the shielding failure of the TL is assumed, and the variation impact of the lightning current peak and grounding resistance on shielding-failure flashover is investigated. The results show that the impact of the lightning current peak has a more significant impact than the grounding resistance in the case of shielding-failure flashover.

Temperature-Dependent Charge Dynamics of Double Layer Interface in 500 kV HVDC XLPE Cable Factory Joint with Different Interfacial Roughness

This article investigates the effect of different temperatures (30 °C, 50 °C, and 70 °C) on the interfacial and electrical insulation properties of 500 kV high voltage direct current (HVDC) cross-linked polyethylene (XLPE) cable factory joint having different roughness levels. The test samples with different interfacial roughness (#80, #400, #1000, #2000, and #0000) were prepared by surface polishing followed by an XLPE injection vulcanization process. At the interface, visible microporous defects were observed. The volume of pits on the sample surface and microporous defects at the interface can be reduced by a smooth interface. Moreover, the smoother interface facilitates space charge accumulation. The dc breakdown strength was nearly the same regardless roughness level at high temperatures. The results show that smooth interfaces help improving insulation properties of cable factory joints at low temperatures. However, the effect of interfacial roughness on insulating properties is weakened at elevated temperatures.

Charge Transport in Full-Size HVDC Cable Joint with Modeling of XLPE/EPDM Interface

Charge transport in high-voltage direct-current (HVDC) cable joint with complicated structure has not been profoundly investigated. In this study, a two-dimensional electrothermal coupled charge transport model combining finite element method (FEM) and bipolar charge transport algorithm is proposed, and the effects of temperature gradient and stress cone tilt angle on charge transport and electric field distribution in a 320 kV HVDC cable joint is investigated by this model. In the simulation, the interfacial area between the cross-linked polyethylene (XLPE) cable insulation and the ethylene-propylene-diene monomer (EPDM) joint insulation is specially modeled with different trap parameters from XLPE and EPDM. To obtain the simulation parameters, simultaneous measurements of space charge and relaxation current are performed on XLPE/EPDM double-layer samples, XLPE single-layer samples, and EPDM single-layer samples. It is found that with the increase of temperature gradient, more positive charges are injected and migrate from the inner side of XLPE insulation to the interface, thus, the charges accumulated at the interface change from negative to positive under large temperature gradient. At the same time, the position of the maximum electric field changes from the inner side of XLPE insulation to the root area of the stress cone at the interface. With the increase of the stress cone tilt angle, both the positive charges in XLPE insulation and the negative charges near the stress cone become less, and the overall maximum electric field first decreases and then increases under 20°C temperature gradient.

Optimization of parabolic trough based concentrated solar power plant for energy export from Saudi Arabia

In this work optimization of the parabolic trough (PT) based concentrated solar power (CSP) system is analyzed for solar energy export from Saudi Arabia to European and Asian countries based on their peak load hours. Initially, Al-Ahsa, in east, and Tabuk, in the west, are compared for a PT based CSP plant for electrical energy generation as these locations are used to export solar energy from Saudi Arabia to Pakistan, India, and China in east and Greece, Germany and UK in the west. The analysis is performed to sell electrical energy generated by the CSP plant to the customers in their peak load hours to reduce their load factor and keep the capital cost minimum. A high voltage direct current (HVDC) transmission system is employed for this study as an undersea cable for Asian or European countries. The feasibility of 2 GW solar energy export by PT based CSP system transmitted by 2 GW bipolar 660 kV HVDC transmission line is based on the net present value of the project (NPV), real levelized cost of energy (LCOE), and the payback period of the project where Karachi, Pakistan has the highest NPV of 1866 MUSD with LCOE of 10.76 cents/kWh and a payback period of 12.7 years.

Optimal Design of Excitation Systems of Synchronous Condensers for HVDC Systems in Power Grid Environment Based on Variable Universe Fuzzy Adaptive PID Controller

In a gesture to optimize the reactive power properties of synchronous condensers and improve capabilities of condensers to support the voltages of AC systems, the outer loop control of the reactive power of condensers and the outer loop control of the voltages of AC systems are introduced into the conventional design of the main excitation systems of condensers in high voltage direct current (HVDC) systems in this study. Moreover, variable universe fuzzy adaptive PID controller is proposed to serve excitation systems of condensers to improve the response ability of condensers in power grid faults and the recovery speed of condensers after power grid faults, and to reduce overvoltage of power grids generated by condensers in the process of fault recovery. To verify its experimental validation, the model of ±100 kV HVDC system containing a condenser is set up in MATLAB/Simulink. The verified results reveal that the optimal design strategy of excitation systems of synchronous condensers in the proposal can ensure rapid regulation of the voltages of AC systems arising from condensers, and reduce overcompensation of the voltages and reactive power of AC systems arising from condensers when faults recover.

Enhancing wind power transfer and protection of actual Egyptian 220 kV HVAC transmission system with multi-terminal VSC-HVDC system

Transmitting power using high-voltage direct current (HVDC) systems instead of high-voltage alternating current (HVAC) ones becomes desirable for integrating remote wind farms. Nowadays, Egypt planned to increase the wind power generation in the Red Sea zone from 1400 to 4000 MW by 2022. Unfortunately, the full capacity of the existing 220 kV HVAC transmission line connecting the current wind farms with the main grid is 1500 MW only. This paper aims first to pinpoint the challenges and drawbacks of the existing AC system, including limited capacity, and service quality. Then, it provides a comprehensive, technical and economic evaluation study of upgrading the existing 220 kV HVAC transmission system to a ± 160 kV HVDC system as compared with expanding the existed system to a conventional 500 kV HVAC transmission line. Finally, a thorough investigation study is presented for the proposed multi-terminal HVDC line in conjunction with its modified protection scheme based on a detailed simulation in MATLAB. Based on a variety of fault cases, an assessment of the performance and suitability of the proposed solutions is visualized. The results ensure the ability of the proposed HVDC system to be a suitable solution for integrating the wind farms in Egypt.

Study on Coordination of Resistive SFCLs and Hybrid-Type Circuit Breakers to Protect a HVDC System With LCC and VSC Stations

Directing at the robustness increase of a high voltage direct current (HVDC) system with line commutated converter (LCC) and voltage source converter (VSC), this paper proposes to coordinate resistive superconducting fault current limiters (SFCLs) and hybrid-type DC circuit breakers (HDCBs) for dealing with the DC line fault. The theoretical modeling of the SFCLs is presented, and the DC fault current characteristics are analyzed. Considering that the fault interruption involves multi-stages, the coordination of the SFCL and the HDCB in the LCC/VSC station is elaborated. Using PSCAD/EMTDC, the simulation model of a 320 kV HVDC system is built. Changing the SFCL size and the HDCB operation delay is simulated, and the performance indexes such as the fault current level, breaking time and dissipated energy are compared. The proposed coordination is verified to be very effective in handling the fault. Especially for the VSC station, it enables to visibly limit the fault current rising, shorten the transient voltage duration, and facilitate a significant decrease in the dissipated energy of the HDCB. For the LCC station, the proposed coordination can serve as a competitive backup to ensure a timely fault breaking. Consequently, the DC line fault is removed more rapidly and reliably.

Optimization Design of AC Filters for HVDC Systems Based on Improved Particle Swarm Optimization Algorithm

In order to reduce the adverse effect of harmonic currents on AC systems and ensure the effective transmission of power in high voltage direct current (HVDC) transmission lines, AC filters (ACF) must be installed on AC buses. Aiming at the limitations of current design methods for AC filters in HVDC systems, this paper proposes a new optimization design method using improved particle swarm optimization (PSO), which aims at the cost of filter investment and constrains the total harmonic distortion rate of AC bus voltage and the telephone harmonic form factor. The optimization mathematical model of AC filters is established, then an improved PSO algorithm is proposed to achieve the optimal design of AC filters. This algorithm introduces a mutation operator and the constraint processing mechanism to improve the exploration ability of this algorithm. Lastly, the simulation model of the ±500 kV HVDC system is established, and the simulation results show that the AC filters optimized by improved PSO have the lowest investment cost while satisfying the harmonic restriction standard.

Analysis of Ion Flow Field Considering Dielectric Film Under HVDC Overhead Transmission Lines

Dielectric films are widely used in agricultural greenhouses. If the greenhouse is near the high-voltage direct current (HVDC) transmission lines, the space charges generated by the corona discharge of the lines will accumulate on the surface of the dielectric film. The surface charges will severly distort the ground-level electric field. In this article, the ion flow field under the HVDC line including the effect of the dielectric film is computed by the Upstream finite-element method (Upstream FEM). The charge accumulation algorithm is proposed to deal with the dielectric film’s effect on electric field and space charges. In this algorithm, the adsorption effect of the dielectric film on space charges is equivalent to the accumulation of space charges that move to the film surface nodes, and the film surface charges are corrected according to the tangential component of the electric field on the surface in every iteration. Therefore, the ion flow field near the film can be predicted. To verify the computational method, an experimental platform is set up, and the measured electric field on the ground is consistent with the calculated results. On the basis of the aforementioned algorithm, the influence of the dielectric film under the +800 kV HVDC transmission line on the electric field is analyzed, which is helpful to design the HVDC overhead lines in the vicinity of greenhouses.

A novel non-unit transient based boundary protection for HVDC transmission lines using synchrosqueezing wavelet transform

Since high voltage direct current (HVDC) transmission system takes on significant tasks in coordinating the power grids asynchronous interconnection and the comprehensive utilization of energy resources, the reliable and fast faults clearance to guarantee the safety and stability of the power system should be attached great attention. This paper, based on the analysis of the HVDC boundary characteristics, proposes a novel non-unit transient based boundary protection as the main protection for HVDC transmission lines, and uses the novel synchrosqueezing wavelet transform (SWT) for time-frequency (T-F) representation. By synchrosqueezing and reassigning the T-F spectrum of the continuous wavelet transform (CWT), SWT can obtain a high resolution T-F spectrum, which brings higher reliability to protection. Based on the transmission differences between tuning voltage and low-frequency voltage by the boundary, this paper proposes the SWT-based non-unit transient boundary protection algorithm, and the ratio of the positive and negative voltage micro-increment is used to select the faulty line. Extensive PSCAD simulations of ±800 kV HVDC project and some field data are used to test the proposed protection scheme. Compared with the CWT-based boundary protection, experimental results verified the proposed method could perform rapidly and reliably in various types of fault, especially can be immune to commutation failure and shows good performance for switching operation as well as fault closer to the very begging of the line, and with high selectivity and sensitivity.

Non-Contact Voltage Monitoring of HVDC Transmission Lines Based on Electromagnetic Fields

The high-voltage direct current (HVDC) transmission network is envisioned to advance further as a supplement to AC transmission in a new era of renewable energy and smart grid development. Voltage monitoring of HVDC transmission lines is critical for evaluating the power quality in power delivery and realizing the stability control of the power system. Unfortunately, the traditional potential transformers can only work with AC voltage while the other existing voltage sensors (e.g., Hall-effect voltage sensors, and optical-fiber voltage sensors) are still not practical for large-scale deployment for monitoring the HVDC transmission lines, owing to the fact that they are either invasive or expensive. In this paper, a non-contact electric-coupling-based voltage sensing with the assistance of magnetic-field sensing for monitoring the voltages of HVDC transmission lines is proposed. The voltages of HVDC transmission lines can be reconstructed from measuring the induced voltage of induction bars, and their correlation coefficients are attained by the magnetic sensing with an optimization program which determines their relative spatial positions. The sensitivity and stability of the proposed platform, and the corona effect of the HVDC transmission lines are discussed. The performance of the proposed sensing technique was investigated by simulation on a ±100 kV HVDC transmission line system, and was further experimentally validated on a scaled HVDC transmission-line testbed in the lab. This sensing technique can be implemented at low cost by adopting copper induction bars and magnetoresistive sensors. Considering its non-contact nature and cost-effectiveness, this technique is suitable for a large-scale deployment over HVDC transmission networks for wide-area monitoring.

Design and test of a new kind of coupling mechanical HVDC circuit breaker

In this paper, a new kind of coupling mechanical high-voltage direct current (HVDC) circuit breaker topology based on pulse transformer is proposed and its working principle is analysed in detail. On the basis of the requirement of 160 kV rated voltage, and 9 kA breaking current, the varying trend and design principle of parameters are given by the theoretical calculation. The prototype design of 160 kV HVDC circuit breaker is completed and type test passes. Results prove that the prototype of 160 kV HVDC circuit breaker has natural bi-directional breaking capacity. It can interrupt the current below 9 kA and withstand transient interruption voltage 272 kV. The breaking time is <5 ms.