DC Cables Research Articles

Interfacial space charge characteristic of PPLP insulation for HVDC cables

High-voltage DC (HVDC) cables show more efficiency than high-voltage AC cables over the long distance power transmission and have attracted significant attention recently due to rapid developments in off-shore wind farm. Polypropylene laminated paper (PPLP) insulation based HVDC cables have provided a reliable operation for many years and are the highest voltage level in operation at the moment. In this study, the authors intend to understand the electrical performance of PPLP insulation system by examining space charge in detail, especially at the interfaces. It has been found that the interface between polypropylene and Kraft paper acts as trap for charge carriers, resulting in an optimal electric field distribution in PPLP insulation system. The optimal electric field distribution enhances overall electrical performance of PPLP insulation, including higher dc breakdown strength and lower electrical conductivity.

Partial discharge identification of DC cross-linked polyethylene cables based on GA-BP algorithm

In this paper, XLPE DC cables were used to construct three kinds of defects of inner semiconducting layer breakage, internal air gap defect, and insulating surface scratching defect. And their DC PD characteristics were tested under different voltage amplitudes and polarities. The study found that these three defects exhibit different DC partial discharge characteristics and change with voltage amplitude and polarity. Subsequently, two methods were used to identify the type of defect. The recognition rate of the traditional defect type identification method reached 97.1%, and the recognition rate of the optimized defect type identification method based on GA-BP was increased to 98.9%, which solved the problem that the traditional method was affected by the voltage level and polarity, and improved the recognition rate.

Electrical Insulation Characteristics of LN2/CF4 Mixture at Cryogenic Temperatures

High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN2). LN2 is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN2 can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN2 at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF4) is another cryogenic refrigerant and electrical insulation material. The mixture of LN2 and liquid CF4 has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN2/CF4 mixture is used in the insulating layer of the superconducting energy pipeline to provide a low-temperature environment (85–100 K) for the HTS dc cable. However, the insulation characteristics of the LN2/CF4 mixture at cryogenic temperatures have not been studied, quantitatively. In this article, a test platform is developed to measure the breakdown voltage of the liquid mixture. The dc breakdown voltages of LN2/CF4 liquid mixture under varying mixing proportions are obtained at cryogenic temperature. The insulation failure probability is estimated using a two-parameter Weibull statistical method. The results reveal that the breakdown strength of the LN2/CF4 mixture is lower than that of pure LN2. As the proportion of CF4 increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF4 is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN2 (28.26 kV/mm). The results of this article can provide a reference for the applications of LN2/CF4 liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications.

Fault diagnosis of shipboard medium-voltage DC power system based on machine learning

This study proposed a fault diagnosis method of a shipboard medium-voltage DC (MVDC) power system based on Noise-Assisted Multivariate Empirical Mode Decomposition (NA-MEMD) and Multilevel Iterative – LightGBM (MI-LightGBM), which overcomes the limitations of the existing fault diagnosis methods in this regard, such as relies heavily on the relay or slow training process. MI-LightGBM is proposed to solve the problem of unbalanced training samples caused by the difficulty in obtaining fault samples in practical engineering. First, NA-MEMD was adopted to pre-process the voltage signals, which were decomposed into a set of components called Intrinsic Mode Functions (IMFs) according to the local characteristic time scales of the original signals. The energy moment of each order IMF was calculated as fault feature vector to train the MI-LightGBM model, which led to the development of a high-precision fault classifier. A model of a shipboard MVDC power system was established using the AppSIM Real-Time Simulator. Simulations were performed on earth fault and short-circuit fault at the generator output and DC cable. Compared with the existing fault diagnosis methods, the proposed method is simple to use and save more than half of the training time while maintaining high diagnostic performance, which is more suitable for engineering applications.

Design and testing of a 10 kV/1 kA superconducting energy pipeline prototype for electric power and liquid natural gas transportation

High Tc superconducting (HTS) DC cable is a promising solution for large-scale power transmission over long distance. However, the refrigeration system for HTS cable is indispensable but undesirable. Considering that LNG (liquid natural gas) is now one kind of clean energy that is used more and more widely, it is possible to integrate HTS cable and LNG transportation into a single pipeline, resulting in a new energy transportation system- a HTS energy pipeline. In this paper, the principle and structure of the energy pipeline are first discussed and then the basic physical properties of the HTS tapes and liquid insulation medium in the LNG temperature range are analyzed. On this basis, a 10 kV/1 kA energy pipeline is designed, fabricated and tested.

Extending protection selectivity in low voltage DC microgrids using compensation gain and artificial line inductance

Vulnerability of power electronic converters in DC microgrids in case of fault occurrence in DC cables necessitates using a fast fault detection and isolation scheme. In this paper, a modified, fast and selective protection scheme has been presented, which provides the required tripping of low voltage DC (LVDC) microgrids. This protection scheme has been developed based on the natural characteristics of the fault current, in which the first and second derivatives of the fault current have been employed to define thresholds for discriminating between faulted and non-faulted situations. To enhance fault detection capability of the protection scheme, definition of thresholds have been improved by introducing compensation gains, which makes the thresholds adaptive to amplitude of line current. Furthermore, in order to extend time validity of the thresholds which are calculated by approximated equations, artificial line inductance (ALI) technique has been proposed. Since no communication link is involved and all measurements are local, any types of fault can be detected without time delay. As a result, fault detection is achieved within 250 μs. To show the superiority of the proposed protection scheme, different case studies are investigated and results are compared with the earlier study.

Simulation and experiment on superconducting DC energy pipeline cooled by LNG

Superconducting DC energy pipeline is an energy channel that simultaneously transmits electrical energy as well as the liquid fuel. It improves the efficiency of energy transmission and reduces the overall cost by cooling high-temperature superconducting DC cables with the low-temperature liquid fuel, such as liquid hydrogen, liquefied shale gas, and liquified natural gas (LNG). In order to properly design this, it is very important to know the temperature distribution along the pipeline, which is yet to be carried out by the researchers. In this paper, a 10 m energy pipeline with LNG cooling and having a concentric nested structure is simulated and tested experimentally. The simulation results of the temperature rise along the pipeline were in good agreement with the experimental results. The effect of various parameters on the pipeline temperature were also obtained. Finally, based on the theoretical equations and simulation results, an empirical formula is proposed to calculate the temperature at the outlet of the energy pipeline.

Heat leak of cryogenic pipe for superconducting dc power transmission line (SCDC)

Main loss of the superconducting DC power transmission line (SCDC) comes from the heat leak of the cryogenic pipe because of no internal heat generation in the dc cable. It should be minimized to realize the high performance of SCDC. The heat leak of the cryogenic pipe depends on the ambient temperature, and when it is high, the heat leak is also high. Therefore, we estimated the sensitivity of the heat leak for the ambient temperature at first, and estimated the effect of this for the distance between the cryogenic stations. The major process of the heat leak is the heat transfer through the multi-layer insulation (MLI) and the second process is the thermal conduction of the support leg of the inner pipe in our design. The MLI is made from the multi-layers of the aluminium-coated thin film (ACTF) with the spacer, and the support leg is made of the glass fiber reinforced plastic (GFRP). The aluminium layer reflects the infrared light highly to reduce the heat transfer by radiation, and the spacer can prevent the direct contact with the ACTF. Therefore, the weight of the MLI should be light to minimize the thermal transfer of the spacer. We tested various types of MLI to find the optimum MLI structure experimentally. Usually, the measurement of the heat leak is not easy in an actual cryogenic pipe and we need the long experiment time and a relatively large instrument to evaluate the heat leak. However, since we need to evaluate many MLIs, we developed the different way to estimate the heat leak through the MLI by a small experimental device. To find the optimum MLI, we started to test three samples of the MLI supplied from Kaneka Corp., and measured the temperature of the MLI. Since the heat capacity of MLI is low, we used the 50-micron meters thermocouple (TC) and attached it to measure the temperature of the MLI carefully. The result of the experiment shows that we found that the net type of the spacer is better than the spacer used in Ishikari project, and the heat leak would be half of the Ishikari project. We also measured the thermal conductivity of two test samples of GFRP to reduce the heat conductions of the support leg. The thermal conduction of these two GFRPs are almost 1/2 to 1/4 of the recommended values of the NIST. Because of these efforts to reduce the heat leak of the cryogenic pipe, we can expect the target values of the heat leak is a half of the Ishikari project in the next project, and if we can reach this value, we can extend the distance of the cryogenic stations to ∼100km actually on the earth. The values of this distance is important to transfer the electricity for all over the world like the gas and oil pipeline in the present time.

General design of ±100 kV/1kA energy pipeline for electric power and LNG transportation

High Tc Superconducting (HTS) DC cable is a promising solution for large-scale power transmission over long distance. However, the indispensable refrigeration system for HTS cable might be undesirable. Considering that LNG (liquid natural gas) now is one kind of clean energy that is used more widely and widely, it is possible to integrate the HTS cable and LNG transportation into an energy pipeline, resulting an acceptable new energy transportation system. In this paper, the principle and structure of the energy pipeline are discussed firstly, and then the basic physical properties of HTS tapes and liquid insulation media in LNG temperature range are analyzed. On the base as above, the general design of ±100 kV/1kA energy pipeline is made.

Considerations of Photovoltaic System Structure Design for Effective Lightning Protection

Photovoltaic (PV) systems are susceptible to lightning strikes. During a lightning strike, an induced overvoltage is generated in the PV system. This overvoltage can damage the inverters connected to the dc cables. In this article, a comprehensive study is presented to address the installation issues that will influence the induced voltage between the +dc and -dc cables in the PV system. By using the partial element equivalent circuit method, the skin effect and ferromagnetic property of the mounting structures are well modeled. The wiring in the PV module is also considered in the simulation. The influences of the mounting systems, lightning protection systems, PV frames, and dc cable arrangements are thoroughly investigated. The simulation results and discussions provide guidance for PV structure design for maximizing lightning protection performance without adding additional protective devices.

Condition Assessment of Low Voltage Photovoltaic DC Cables under Thermal Stress Using Non-Destructive Electrical Techniques

The output power of the photovoltaic system is heavily dependent on the low voltage (LV) DC cables which are exposed to multiple stresses such as climatic, mechanical, electrical, and thermal stress, hence makes them more exposed to aging as compared to other components in the system. Accordingly, it is essential to monitor the state and know the real cause of the insulation degradation of the cable. The physio-chemical changes inside the insulation during service is attributed to the thermal stress, which the cable has to endure constantly. Traditionally, destructive test techniques have been adopted to study the aging phenomenon in the cable insulation, making them unsuitable for on-line condition monitoring. This research work has been aimed to study the degradation in LV photovoltaic DC cables under thermal stress by measuring the dielectric properties; complex permittivity, tanδ with the change in frequency and decay and return voltage slopes using extended voltage response method. The non-destructive diagnostic methods used are based on the phenomenon of polarization and conduction in the insulation material. The noteworthy change in the values of the imaginary part of permittivity, tanδ at low frequencies, and the overall decrease in the values of return voltage slope showed the change in the structure of the polymer matrix under the stress which was related to the cross-linking based chemical reactions. The results show that the techniques can be adopted for the on-line condition monitoring of the cable for the PV system and the dielectric parameters can be used to study the chemical and physical changes happening inside the material effectively.

One-end protection algorithm for offshore wind farm HVDC transmission based on travelling waves and cross-alienation

This paper develops a novel one-ended non-communication protection scheme for offshore wind farm connected to the public grid through HVDC cables. The relay is installed at the DC side of the inverter station. The proposed technique for both fault detection and location along HVDC cable is based on evaluating the arriving instants of fault induced backward travelling wave and its subsequence. These instants are determined using the cross-alienation coefficients between the backward travelling waves and a unity reference function. Cross-alienation coefficients values are used for fault detection, while the difference time between two sequential coefficients peaks after fault inception are used for fault location. The required fault detection and location time is very small of milliseconds range. This will enhance the system reliability due to fast DC cables protection. The proposed protection scheme is based on continuous tracking of sampled current and voltage signals. Thereby, the proposed technique is applied on sampled movable windows of the calculated backward travelling wave and the reference function. The simulation results show that the proposed protection scheme is simple, fast, accurate, and not sensitive to fault resistance or load variations. Also its implementation is economically as no need for extra devices or communication links.

Partial Discharge Behavior and Accelerated Aging Upon Repetitive DC Cable Energization and Voltage Supply Polarity Inversion

DC cables are supposed to withstand a number of operations involving supply-voltage peak-value time variations, such as energization, switch off and polarity inversion. It is known that voltage-supply time variations cause electric field transients in cable insulation that can last for a long time before steady-state conditions are achieved, and during this time the electric field distribution is not only driven by electrical conductivity, as in steady state, but also by permittivity, as in AC. This paper shows that highly energetic degradation phenomena, that is, partial discharges, PD, can occur during voltage transients that a DC cable has to withstand during life. This results in a cumulative damage which can cause premature insulation breakdown, at times well shorter than those expected based on a cable design which is PD free under steady-state conditions. Quantification of the damage caused by PD, based on aging and residual life models, is proposed in the paper, and it is shown that damage can be significant enough to pose a threat for cable life when operation transient rate is e.g., a few per day, and that the faster the slew rate of the supply voltage variation, the highest the cumulative damage due to PD.

Configuration and Model Order Selection of Frequency-Dependent π Models for Representing DC Cables in Small-Signal Eigenvalue Analysis of HVDC Transmission Systems

Eigenvalue-based analysis of small-signal dynamics in high-voltage direct current (HVdc) transmission systems requires cable models that are compatible with a state-space representation. While distributed parameter models accounting for frequency-dependent effects are inherently incompatible with a state-space representation, a conventional $\pi $ model can only represent the cable behavior accurately at a single frequency. Instead, a frequency-dependent $\pi $ (FD- $\pi $ ) model consisting of a lumped circuit representation with multiple parallel $RL$ branches in each $\pi $ -section can be utilized to reproduce the frequency dependence of the cable characteristics in a specified frequency range. Based on an evaluation of relevant error metrics for FD- $\pi $ models, this article demonstrates how the number of sections and the number of parallel branches in each section will influence the accuracy. From this starting point, an optimization algorithm for identifying the configuration that fulfills a specified set of accuracy requirements with the lowest possible model order is introduced. A similar algorithm for identifying the most accurate FD- $\pi $ model within a specified maximum model order is also proposed. Examples of numerical results for different cable lengths and cross sections are presented to highlight their effect on the model, and it is demonstrated how the cable model configuration can influence the results from small-signal eigenvalue analysis of HVdc transmission systems.

A Multi-Terminal HVdc Grid Topology Proposal for Offshore Wind Farms

Although various topologies of multi-terminal high voltage direct current (MT-HVdc) transmission systems are available in the literature, most of them are prone to loss of flexibility, reliability, stability, and redundancy in the events of grid contingencies. In this research, two new wind farms and substation ring topology (2WF-SSRT) are designed and proposed to address the aforementioned shortcomings. The objective of this paper is to investigate MT-HVdc grid topologies for integrating large offshore wind farms with an emphasis on power loss in the event of a dc grid fault or mainland alternating current (ac)grid abnormality. Standards and control of voltage source converter (VSC) based MT-HVdc grids are defined and discussed. High voltage dc switch-gear and dc circuit topologies are appraised based on the necessity of dc cables, HVdc circuit breakers, and extra offshore platforms. In this paper, the proposed topology is analyzed and compared with the formers for number and ratings of offshore substations, dc breakers, ultra-fast mechanical actuators, dc circuits, cost, flexibility, utilization, and redundancy of HVdc links. Coordinated operation of various topologies is assessed and compared with respect to the designed control scheme via a developed EMTDC/PSCAD simulation platform considering three fault scenarios: dc fault on transmission link connecting the wind farm to mainland power converters, dc fault within substation ring of VSC-HVdc stations, and ultimate disconnection of grid side VSC station. Results show that 2WF-SSRT is a promising topology for future MT-HVdc grids.

Insulation design of ±10kV bipolar coaxial HTS DC cable

The high-temperature superconducting cable has low loss and high current-carrying density, and has extensive research prospects in power system transmission. In recent years, the advantages of DC transmission over long distances have become more and more obvious, and superconducting DC transmission has also become a research hotspot. Bipolar coaxial DC cable has a good economy, compact structure, and greater transmission power density, but also requires higher insulation, so the insulation design is particularly important. In this paper, a 10 kV / 6000 A / 10 m bipolar coaxial superconducting DC cable with a cold insulation structure is considered. In consideration of DC withstand voltage and lightning shock, an insulation design is carried out and a type test is performed. The test results meet the cable insulation requirements.

Simulation and design of 500 kV DC cable terminal accessory based on ZnO varistor microsphere composites

DC transmission has advantages over AC transmission in terms of distance and capacity and has thus received considerable research attention and development. However, the development of high-voltage DC cables is limited by the development of the associated DC cable accessories. In view of the existing problems with DC cable terminal accessories, materials with nonlinear conductivity are being applied to cable terminal accessories. Using finite element simulations, the internal electric field distribution of a DC cable terminal under a thermoelectric coupled field is calculated for different temperature gradients. It is found that cable terminal accessories made of materials with nonlinear conductivity have a stronger electric field grading effect than that of traditional cable terminal accessories. Based on the characteristics of the material with nonlinear conductivity, a simpler cable terminal accessory structure that can achieve the same electric field grading effect is proposed. The results show that the use of a material with nonlinear conductivity facilitates the development of high-voltage DC cable accessories.

Simulation study on ±320kV DC power cable steady-state temperature field distribution and its influencing factors

Temperature distribution is an important parameter in the operation of DC cables, while there are few studies at present. In this paper, the steady-state temperature field distribution of DC cable was calculated by simulation, and the factors including conductor current and outer surface temperature were discussed and analyzed. The results showed that there is a temperature gradient in the radial direction of the cable; Both the conductor current and the outer surface temperature have a significant effect on the overall temperature of the cable; The increase in the conductor current and the outer surface temperature will significantly increase the conductor temperature. However, the insulation temperature difference changes little.

On Integration of Wind Power into Existing Grids via Modular Multilevel Converter based HVDC Systems

Due to the advent of advanced solid-state power devices and converters, it is now promising to transmit huge quantities of renewable energies to the main electrical grids. One of the major renewable energy resources is the wind power plant, which has the benefits of very low generation cost, simple infrastructure, lesser operating staff, lesser erection times, and negligible environmental pollution. Various far located power stations, including offshore and mainland wind power plants, as well as big industrial loads can be joined together with the existing AC grids by implementing multiterminal HVDC transmission systems. This recently established strong feature of HVDC systems is making them more feasible for future grids. The current source converter (CSC) is an established topology used in HVDC systems, worldwide. Rigorous research is being carried out on the voltage source converters (VSCs), especially on Modular Multilevel Converters (MMCs) based HVDC systems that are distinguished by their benefits of lower harmonics, lower maintenance, almost absence of bulky DC link capacitors and AC filters, black start capability, smooth control of powers, and addition of renewable energy using DC cables with the prevailing AC grids. In this paper, technical features associated with wind power plants and the interconnection of such plants via new MMC based HVDC systems with the present AC grids are discussed in detail. Mathematical modeling and performance analysis of advanced MMC technology is carried out by performing simulations in Matlab/Simulink ® . The deduced technical outcomes remarkably show the effective use of this converter in the near future grids.

Research on Optical Fiber Sensor Localization Based on the Partial Discharge Ultrasonic Characteristics in Long-Distance XLPE Cables

To improve the safety and antielectromagnetic interference performance of partial discharge detection in high-voltage DC cables, partial discharge of cables is used as a criterion for characterizing insulation defects. An ultrasonic detection method for the partial discharge of optical fiber detection is carried out in a long-distance high-voltage DC operating environment. By analyzing the partial discharge mechanism and ultrasonic characteristics of DC cables, a correlation model between partial discharge ultrasonic waves and discharge quantity is established. The partial discharge charge of several common insulation defects is obtained experimentally, and an ultrasonic characteristic Sagnac fiber detection system is realized. Standard ultrasonic time domain signal detection and fiber-optic sensing probe directional experiments are performed. The influence of fiber sensing probes with different lengths on the sensitivity of partial discharge signals is investigated based on the frequency distribution characteristics of partial discharges. The ultrasonic wave is measured based on a fiber-optic probe and can perform multipoint detection, and the partial discharge of the ±320 kV flexible DC transmission system was tested based on the developed optical fiber sensing system. The experimental results show that the partial discharge signal detection of 6 km long cable can be realized, and the localization accuracy is less than ±80 m. It provides a practical monitoring method for the perception of long-distance transmission equipment operating states.