High-voltage DC Transmission Lines Research Articles

Analysis of heating defects in extension rods of extra-high voltage direct current overhead transmission lines

This paper studies the overheating defect of an extension rod in a ± 800 kV extra-high voltage direct current overhead transmission line, and proposes various factors that affect the overheating of the extension rod. Establish an equivalent model of an extra high voltage DC transmission line based on equipotential point and line parameter analysis, and calculate the model parameters. The proposed model can also be applied to ultra-high voltage DC overhead transmission lines such as ± 500 kV, and has strong universality. Simulation experiments show that if the equivalent resistance value of the split wire side is reduced by 0.01Ω, the power loss on the extension tie rod can be increased to 380.951 W; if the equivalent resistance value of the jumper side is increased by 0.01Ω, the power loss on the extension tie rod can be increased to 273.41 W. Combined with theoretical analysis and simulation verification, it is concluded that the main factor affecting the overheating of the extension rod is the change in the resistance value on the jumper side caused by the increase in contact resistance. A systematic troubleshooting plan for related defect problems is proposed, which has important reference and guidance significance for such line fault problems.

Design of Series-Connected Novel Large-Scale Offshore Wind Power All-DC System with Fault Blocking Capability

The utilization of wind power all-DC systems with DC collection and transmission is an effective solution for the extensive development of wind power in deep-sea areas. However, in the event of faults occurring in wind power all-DC systems, the fault propagation speed is extremely rapid, with a wide-ranging impact, and to date, there are no complete DC engineering references available. It is crucial to research the topology and fault isolation methods applicable to large-scale offshore wind power all-DC systems in deep-sea areas. This paper proposes a novel series-connected all-DC system topology and presents corresponding fault isolation methods for internal faults in wind turbine units and faults in high-voltage DC transmission lines. The system simulation model was constructed using PSCAD/EMTDC (v4.6.3), and simulations were conducted for internal faults in the wind turbine units and DC transmission line short-circuit faults. The simulation results demonstrate that the proposed system can isolate various DC faults while maintaining stable operation, thereby validating the effectiveness of the control strategies and fault isolation methods proposed in this paper.

Adaptability analysis of travelling wavefront protection in LCC‐UHVDC transmission lines

AbstractThe existing relay protections for line‐commutated converter (LCC)‐ultra high voltage DC (UHVDC) transmission lines are insufficient in protection sensitivity and operation speed. The travelling wavefront fault distance information can be used to improve protection performance, which has been used in protection of voltage source converter ‐high voltage DC (HVDC) transmission lines. The adaptability of travelling wavefront protection in LCC‐UHVDC transmission line is analyzed in this paper. First, the conditions that need to be met for the travelling wavefront protections to be able to effectively recognize faults are analyzed. Eight applicability conditions of wavefront protection are revealed from the perspective of travelling wavefront generation and practical application requirements. Then the degree of match between the applicability conditions and the conditions of LCC‐UHVDC system is analyzed as a basis for assessing the adaptability of travelling wavefront protection in the LCC‐UHVDC system. It is pointed out that the index coefficient‐based wavefront protections are not available for LCC‐UHVDC transmission lines since the hardware sampling rate of protection device is insufficient, while first peak time of transient voltage (FPTV)‐based wavefront protection is available for LCC‐UHVDC transmission lines if it has enough anti‐noise ability. Finally, an enhanced anti‐noise algorithm is proposed for the FPTV‐based travelling wavefront protection. The adaptability of the travelling wavefront protection is verified by hardware‐in‐the‐loop test and filed recording experimental data.

New Cascaded 1+PII2D/FOPID Load Frequency Controller for Modern Power Grids including Superconducting Magnetic Energy Storage and Renewable Energy

Having continuous decrease in inertia and being sensitive to load/generation variation are considered crucial challenging problems for modern power grids. The main cause of these problems is the increased penetration capacities of renewables. An unbalanced load with generation power largely affects grids’ frequency and voltage profiles. Load frequency control (LFC) mechanisms are extensively presented to solve these problems. In the literature, LFC methods are still lacking in dealing with system uncertainty, parameter variation, structure changes, and/or disturbance rejection. Therefore, this paper proposes an improved LFC methodology using the hybrid one plus proportional integral double-integral derivative (1+PII2D) cascaded with fractional order proportional-integral-derivative (FOPID), namely, the proposed 1+PII2D/FOPID controller. The contribution of superconducting magnetic energy storage devices (SMES) is considered in the proposed design, also considering hybrid high-voltage DC and AC transmission lines (hybrid HVDC/HVAC). An optimized design of proposed 1+PII2D/FOPID controller is proposed using a new application of the recently presented powerful artificial rabbits optimizers (ARO) algorithm. Various performance comparisons, system changes, parameter uncertainties, and load/generation profiles and changes are considered in the proposed case study. The results proved superior regulation of frequency using proposed 1+PII2D/FOPID control and the ARO optimum parameters.

A simple decision tree-based disturbance monitoring system for VSC-based HVDC transmission link integrating a DFIG wind farm

Fault detection and classification is a key challenge for the protection of High Voltage DC (HVDC) transmission lines. In this paper, the Teager–Kaiser Energy Operator (TKEO) algorithm associated with a decision tree-based fault classifier is proposed to detect and classify various DC faults. The Change Identification Filter is applied to the average and differential current components, to detect the first instant of fault occurrence (above threshold) and register a Change Identified Point (CIP). Further, if a CIP is registered for a positive or negative line, only three samples of currents (i.e., CIP and each side of CIP) are sent to the proposed TKEO algorithm, which produces their respective 8 indices through which the, fault can be detected along with its classification. The new approach enables quicker detection allowing utility grids to be restored as soon as possible. This novel approach also reduces computing complexity and the time required to identify faults with classification. The importance and accuracy of the proposed scheme are also thoroughly tested and compared with other methods for various faults on HVDC transmission lines.

Change detection filter technique-based fault analysis of HVDC transmission line

PurposeThis research paper aims to investigate the change detection filter technique with a decision tree-based event (fault type) classifier for recognizing and categorizing power system disturbances on the high-voltage DC (HVDC) transmission link.Design/methodology/approachA change detection filter is used to the average and differential current components, which detects the point of fault initiation and records a change detection point (CDP). The half-cycle differential and average currents on both sides of the CDP are sent through the signal processing unit, which produces the respective target. The extracted target indices are sent through a decision tree-based fault classifier mechanism for fault classification.FindingsIn comparison with conventional differential current protection systems, the developed framework is faster in fault detection and classification and provides great accuracy. The new technology allows for prompt identification of the fault category, allowing electrical grids to be restored as quickly as possible to minimize economic losses. This novel technology enhances efficiency in terms of reducing computing complexity.Research limitations/implicationsSetting a threshold value for identification is one of the limitations. To bring the designed system into stability condition before creating faults on it is another limitation. Reducing the computational burden is one of the limitations.Practical implicationsCreating a practical system in laboratory is difficult as it is a HVDC transmission line. Apart from that, installing rectifier and converter section for HVDC transmission line is difficult in a laboratory setting.Originality/valueThe suggested scheme’s importance and accuracy have been rigorously validated for the standard HVDC transmission system, subjected to various types of DC fault, and the results show the proposed algorithm would be a feasible alternative to real-time applications.

Causal theory on acceleration of seed germination in the vicinity of high voltage direct current transmission line

Seed germination is the primary stage of growth in a seed. A wealth of experiments exist in literature to support the existence of correlation between seed germination to the electric and magnetic fields. This becomes more important as researchers have suggested to develop technologies to build ecologically clean and environment-friendly solutions to agricultural practices. Although the literature supports the existence of seed germination acceleration, the lack of a definite causal theory has been observed by numerous researchers over decades. After considering all the existing experimental data, we have formulated a causal theory to explain the factors influencing seed germination around high voltage DC transmission lines. This work opens new avenues of research in this field.

A High-Speed Protection Strategy for Bipolar CSC-Based HVDC Transmission System

This paper introduces a novel approach to high-speed fault protection strategy for bipolar Current Source Converter (CSC) based high voltage DC transmission line. The Bagged-Tree Ensemble-Classifier (BTEC) method is employed to detect the presence of a fault and categorizes the various types of faults in the DC transmission line. The implementation of the Wavelet Transform (WT) technique assists in diminishing the effect of deviation in the fault resistance value, fault location, and disturbances created due to noise on the performance BTEC method. The development of the predictive model through concatenating individual models is the prime objective of the ensemble schemes. Ensemble procedure assists to enhance the performance accuracy of the fault protection strategy. The local measurement at the rectifier end side is enough for the proposed strategy to realize the protection of the entire line with no use of communication channels. The AC sinusoidal voltage, DC current and DC voltage are recorded at the rectifier end terminal and sampled at 1 kHz, which is very less as compared to the other existing techniques. The proposed strategy detects the fault in the least possible time of 4 ms and able to classify all the tested faults with reasonable accuracy. The results convey that the proposed strategy offers high-speed detection of fault and preciseness in the classification of all types of faults, despite variation in the fault resistance up to 180 Ω. The proposed scheme is applicable for DC faults and also detects the presence of AC faults in either rectifier end as reverse fault and in inverter end as sub-sequent forward section fault.

Design and Model of Series-Connected High-Voltage DC Multipliers

The generation of high dc voltages is necessary for feeding many technical applications and testing the insulation material of high-voltage dc transmission lines and ac power cables with long lengths. The article proposes a modular multiplier high-voltage dc generator with integrated Cockcroft-Walton (CW) circuits. In this new topology, CW circuits are connected in series to produce a high dc output voltage with low regulation factor (<; 2%) and a negligible ripple factor without the need for costly filters to produce a nearly constant output voltage. The modules are fed by low input ac voltage sources that have the same magnitude and frequency. Numerical simulations using MATLAB/Simulink confirm the viability of the proposed generator. The generator is finally validated by experimental tests on a kV-size prototype.

Cyber Security in Control of Grid-Tied Power Electronic Converters—Challenges and Vulnerabilities

Grid-tied power electronic converters are key enabling technologies for interfacing renewable energy sources, energy storage, electrical vehicles, microgrids, and high-voltage dc transmission lines with the electrical power grid. As the number of power converters in modern grids continually increases, their monitoring and coordinated control in a way to support the grid have become topics of increased practical and research interest. In connection with this, latest standards have also defined a mandatory set of control parameters for grid-tied converters, which should be adjustable by a remote entity that sends commands through a communication network. While such a remote control capability allows many new control functions in grid-tied converters, it also renders them vulnerable to cyber-attacks. The aim of this article is first to shed light on the portions of the power converter control systems that are vulnerable to cyber-attacks. Next, typical cyber-attacks are overviewed by considering different applications of the grid-tied converters. Further, the impact of different types of cyber-attacks on grid support functions is studied. Finally, this article is concluded with summary and recommendation for further research.

Assessment of Overvoltage and Insulation Coordination in Mixed HVDC Transmission Lines Exposed to Lightning Strikes

Many geographical constraints and aesthetic concerns necessitate the partial use of cable sections in the High Voltage DC (HVDC) transmission line, resulting in a mixed transmission line. The overhead sections of mixed lines are exposed to lightning strikes. The lightning strikes can not only result in flashover of overhead line (OHL) insulators but can enter the cable and permanently damage its insulation if adequate insulation coordination measures are not taken. In this work, we have analyzed the factors that affect the level of overvoltage inside the cable by simulating a fast front model in PSCAD. It has been determined that surge arresters must be provided at cable terminals when the length of cable sections is less than 16 km to limit the core-ground overvoltage within the lightning impulse protective level (LIPL). The level of sheath-ground overvoltage is independent of the length of cable; however, it can be limited within LIPL by lowering the sheath grounding impedance to 1.2 Ω. Insulation coordination measures do not impact the likelihood of OHL insulators’ flashover. The flashover performance of OHL can be improved by lowering the footing impedance of the second tower closest to the cable terminals, which is otherwise most likely to flashover.

Control and stability analysis of VSC-HVDC based transmission system connected to offshore wind farm

Offshore wind farms (WFs) with significant capacities have been installed recently all over the world. In order to transmit the WF power to the onshore grid, high voltage direct current (HVDC) transmission system is appropriate technology. This paper analytically studies the impact of system parameters, controllers and operating conditions on the dynamic behavior of HVDC transmission systems based on three-level neutral point clamped voltage source converters (VSC). Also, it investigates modeling, control and stability analysis of VSC-HVDC system connected to the offshore wind farms. The VSC-HVDC system comprises offshore and onshore converters and high voltage dc transmission lines. The paper extracts VSC-HVDC system dynamics at the dc-side and argues the interaction between onshore converter control and HVDC transmission line dynamics. Moreover, the paper presents controller design for the dc-link voltage regulation by the onshore converter and examines the impacts of HVDC line length and dc voltage control bandwidth (BW) on the system stability by the modal analysis and time domain simulations.

Pollution agglomeration characteristics on insulator and its effect mechanism in DC electric field

To obtain the pollution agglomeration characteristics of insulators and its effect mechanism of Ultra High-Voltage DC transmission lines under natural sedimentary condition, a natural pollution accumulation system was set up and used to analyze the micro-shape features of the surface insulators on different conditions. The experimental results show that the surface pollution is respectively distributed as chain and granular shapes on the DC charged insulator and electrically neutral insulator, which indicates that the pollution agglomeration characteristics are influenced by the DC electric field significantly. Meanwhile, the radius of the pollution particles on the DC charged insulator is larger than that on the uncharged insulator. Based on the thermodynamics and electrostatics theory, this paper proposed the model of electric dipole between aerosol groups to explain the characteristics of the pollution distribution on the surface of the DC charged insulator and revealed the mechanism of aerosol particles radius affected by DC electric field. The conclusions of this paper can provide an experimental and theoretical basis for the UHV DC external insulation pollution characteristics and insulator flashover.

A two-stage stochastic optimization planning framework to decarbonize deeply electric power systems

In 2015, 195 countries signed the Paris Agreement under the United Nations Framework Convention on Climate Change. To achieve the ambitious greenhouse gas-reduction targets therein, the electric power sector must be transformed fundamentally. To this end, we develop a two-stage stochastic optimization model. The proposed model determines the optimal mix of generation and transmission capacity to build to serve future demands at least cost, while respecting technical constraints and climate-related considerations. The model uses a mix of AC and high-voltage DC transmission lines, conventional and renewable generation, and different types of energy-storage units to meet these objectives. Short- and long-term uncertainties are modeled using operating conditions and scenarios, respectively.We demonstrate the model using a case study that is based on the Texas power system, with 2050 as the target year of the analysis. We include explicit carbon-emissions constraints. Doing so allows us to examine the effect of carbon-reduction targets and deep decarbonization of electricity production on investment decisions. As expected, we find that thermal-dominated power systems must transition toward having a renewable-dominated generation mix.

A New Magnetic-Linked Converter for Grid Integration of Offshore Wind Turbines Through MVDC Transmission

A medium-voltage ac network is commonly used to interconnect wind turbines to the offshore substation, which can create significant power losses, as the distance between wind turbine and offshore substation can vary from 5 to 9 km. The copper-conductor-based high-voltage dc (HVDC) or high-voltage ac (HVAC) transmission lines are generally used to transmit the electrical power from the offshore substations to the onshore grids. With the recent advancement of the high-temperature superconducting (HTS) cables, it is now possible to replace the traditional HVAC/HVDC transmission lines with a medium-voltage dc (MVDC) cables. A new MVDC converter is proposed in this paper to convert the generator low ac output voltage to MVDC directly. The proposed new technology not only eliminates the requirement of low-frequency heavy and large step-up transformers and line filters, but can also eliminate the voltage conversion stages to allow the entire power transmission networks to use MVDC HTS cables. A compact, lightweight, and efficient high-frequency common magnetic link is used in the proposed converter. The magnetic link can inherently solve the isolation problem. In this paper, a scale-down prototype converter is developed in the laboratory to validate the feasibility of the proposed MVDC converter.

Closed loop control of voltage source converters during faults in high voltage DC transmission system

High voltage DC transmission is very cost effective alternate solution for high voltage AC transmission. High voltage DC transmission towers occupy less space and poles carry more current compared to high voltage AC system. High voltage DC transmission lines are emerging as an alternate solution for flexible AC transmissions (FACTS) technology. Earlier either thyristor valves or mercury valves were used in converters. Voltage source converter has replaced conventional converter in high voltage DC system because of their flexible control, black start capability, and they can work with weak AC system. Switching at higher frequency is also an added advantage. In voltage source converter-based high voltage DC transmission, both real and reactive power transmitted can be controlled whereas in conventional line commutated converter-based high voltage DC transmission system, only the real power transmitted was controlled. There may be chances of grid faults in sending end and receiving end stations. The typical faults are single line fault, double line to ground fault and three phase fault. The faults in one high voltage DC station should not affect the other end station. Necessary control should be initiated to take care of AC faults on both the sides that are before rectification in the sending end station and after inversion in the receiving end station. To handle such faults, the voltage source converters should be controlled to regulate the power transmitted through the line in such a way that the fault in one grid will not affect the parameters of the other grid. This paper presents closed loop control of both voltage source converters to take care of line to ground fault. The controller uses PQ theory. The behavior of high voltage DC system during fault without and with controller is presented in this paper. This simulation study is used to educate electrical transmission and faults detection to the trainees, they can easily understand.

Transient signal identification of HVDC transmission lines based on wavelet entropy and SVM

High-voltage DC (HVDC) transmission plays an important role in power transmission projects due to its advantages of large transmission power and good control performance. As the main protection of the DC transmission line, transient protection uses the high-frequency signal generated by fault transient to detect faults, having the characteristics of fast response and high accuracy. However, the HVDC transmission line has complex conditions along the route and is vulnerable to lightning strikes and other accidents, resulting in the occurrence of a variety of transients in the line, which increases the difficulty of fault identification. Being able to reveal signal time-frequency characteristic, wavelet entropy is an effective tool of signal recognition. This study proposes a method of transient signal identification based on the wavelet entropy and support vector machine (SVM). Firstly, the transient processes of three kinds of signals, including unipolar faults, lightning strike faults, and lightning disturbances, are briefly introduced. Then the time−frequency features of three kinds of transient signals under different scenes are analysed by wavelet entropy. Finally, the training set was used to train the SVM classification model with the signal wavelet entropy being taken as the eigenvector, and the test results validate the effectiveness of the proposed method.

Analysis of Transient Behavior of Mixed High Voltage DC Transmission Line Under Lightning Strikes

Growing urbanization coupled with increased power demands have led to increasing use of mixed power transmission lines with sections of overhead lines (OHL) and underground cables. Due to differences in surge impedance of cables and OHL, voltage surges experience reflections and refractions at their boundaries which make the transient behavior of mixed high voltage direct current (HVDC) line quite peculiar. Lightning strikes on overhead sections of lines induce voltage surges that travel along OHL and enter the cable section. Lightning overvoltage can cause OHL insulators to flashover and stress cable insulation or cause its permanent breakdown. In this paper, we simulated a fast front model of a mixed HVDC transmission line using an electromagnetic transient simulation program (PSCAD) to analyze its transient behavior under a lightning strike. The leader progression model has been used to predict the dielectric performance of OHL insulators. It has been shown that transition towers adjacent to the cable section are much less vulnerable to flashover than subsequent towers. The length of a riser section (connecting OHL and cable) and tower footing impedance have shown to significantly influence the flashover performance of OHL insulators. In addition, the length of cable segments and sheath grounding impedance has been found to influence cable overvoltage. This paper can be used to evaluate the insulation coordination and overvoltage protection requirements for a mixed HVDC transmission line.

Power system decarbonisation with Global Energy Interconnection – a case study on the economic viability of international transmission network in Australasia

The electric energy system in Indonesia is undergoing with the challenges of fast-increasing electricity demand, carbon constraints, and rising costs. Using our model of the Australian and Indonesian electrical grids (either separately or interconnected) that incorporates operational flexibility in capacity expansion planning, we first show that meeting the projected demand for Java and Bali—the main Indonesian grid, with 100% locally integrated renewables by 2050 would be challenging. However, a submarine high-voltage DC (HVDC) link connecting Indonesia’s Java-Bali power grid to the Australian National Electricity Market (NEM) grid through the Northern Territory would help alleviate this situation, given Australia’s abundant renewable energy resources. Then, our model reveals that the Australian NEM could also profit from additional renewables if connected to the Northern Territory through a ground HVDC transmission line to gather intermittent wind and solar generation, which would be curtailed otherwise if unused by Indonesia through the submarine link. Despite the expensiveness of long HVDC links, the wholesale electricity cost of the integrated 100% renewable Australasia power system could be reduced by over 16%, from $AUD177/MWh with only local renewables to $AUD148/MWh with integrated HVDC transmission. The model retrieved the optimal international HVDC link with capacity of 43.8 GW, and the optimal regional HVDC transmission line with a capacity of 5.5 GW. To the best of our knowledge, this is the first detailed model on power system decarbonisation planning for both Australian NEM and Indonesian Java-Bali power grid considering HVDC interconnections.

Characteristics of C3F7CN/CO2 as an alternative to SF6 in HVDC-GIL systems

The application of environmentally friendly gases to insulated transmission pipelines could potentially improve the flexibility of transmission line corridors and reduce the environmental impact of such lines. Recently, C 3 F 7 CN has been considered as a potential replacement for SF 6 . The present work assesses the feasibility of applying C 3 F 7 CN to high voltage DC transmission lines to attain a lower global warming potential (GWP). The GWP is estimated through calculation of C 3 F 7 CN mixtures with other gases and based on the weighted average method. The liquefaction temperature of these mixtures is between 0.4 and 1.2 MPa as determined by combining the Peng- Robinson equation of state with the van der Waals mixing rule. The insulating characteristics of C 3 F 7 CN/CO 2 mixtures in conjunction with DC voltage in a uniform electric field was also studied. Based on the present research, the viability of C 3 F 7 CN/CO 2 mixtures was evaluated by examining their heat transfer properties. The results show that, at operating temperatures of -20 and -30 °C, and at a pressure of 0.7 MPa, the maximum allowable mole fractions of C 3 F 7 CN in the mixture to avoid condensation is 7.8 and 5.1%, respectively. The insulation characteristics of 4% C 3 F 7 CN/96% CO 2 and 8% C 3 F 7 CN/92% CO 2 mixtures at 0.7 MPa is equivalent to that of SF 6 gas at 0.5 MPa. The heat-transfer capabilities of 4% C 3 F 7 CN/96% CO 2 and 8% C 3 F 7 CN/92% CO 2 mixtures meet the requirements of the GIL temperature rise standard. It is concluded that C 3 F 7 CN/CO 2 represents an environmentally friendly insulation gas suitable for use in HVDC-GIL equipment.