DC Cables Research Articles

Insulation Characteristics of PPLP and Design of 250 kV Class HTS DC Cable

The development of high temperature superconducting (HTS) dc cable system has been promoted for the land long-term high-capacity power transmissions, submarine cables and renewable energies. Polypropylene laminated paper (PPLP) has been widely used as insulating material for HTS cable. To develop the PPLP-insulated dc cable, it is important to study the dc, dc polarity reversal and lightning impulse characteristics at a cryogenic temperature. In this paper, we discuss the electrical insulation characteristics of PPLP-insulated mini-model cable. We investigate the dc, impulse breakdown strength, dc polarity reversal, and dc voltage (V)-numbers of dc polarity reversal (N). Also, based on these results, the insulation design of 250 kV class HTS dc cable is performed.

A Study on AC Losses of HTS Pentagonal Conductors Consisting of 2G Wires Carrying DC Offset Transport Current

In a high-voltage dc (HVDC) transmission system, a high-temperature superconducting (HTS) dc cable is a feasible candidate due to its high power density. However, the dc in the grid includes ac ripple generated from switching devices and the environment, which can cause ac losses. In this paper, two pentagonal conductors consisting of second-generation (2G) wires are prepared, and their ac losses are investigated under the application of current with ac and dc components. Moreover, a numerical analysis is performed for both conductors to compare the measured losses. The results show that the measured ac losses correspond well with the numerical ones calculated by our models. The dc offset current does not affect the ac losses when the total current is smaller than the critical current, and the ac losses depend on the amplitude of the ac component. All measured losses are verified by evaluating the dependence on the arrangement of the voltage leads, the contact position, and the frequency.

Stability Analysis of the Cable Core of a 10 kA HTS DC Power Cable Used in the Electrolytic Aluminum Industry

High temperature superconducting (HTS) dc power cable shows a wide application prospect in the field of power transmission for its nearly lossless and rather high capacity. IEE has installed a 360-meter long high temperature superconducting (HTS) dc power cable at the self-supply power plant of Zhongfu Industrial Company Ltd. in Gongyi, Henan and the system has operated for two years. The cable connects a 19.5 MVA/1.5 kA silicon-controlled rectifier, which connects with a 110 kV/1 kV transformer, to the bus bar of an electrolytic aluminum cell. It is designed to carry 10-kA current and the voltage is 1300 V. The HTS dc power cable core consists of five conductor layers wound with the spliced Bi-2223 wires with the length of 40 km. The cable core has five layers and 23 HTS wires in each layer with the outer diameter of 45 mm. The HTS dc power cable is fabricated with the spliced superconducting wires which will have effect on the overall superconductivity. Also, since dc output of the rectifier contains a proportion of the ac harmonic ripple, the large dc and small ac will generate the loss in the cable core. In the operation of the 10 kA HTS dc power cable, anode effect will occur in electrolytic aluminum tank, which will lead to a large fault current in the cable and even lead to the power off protection. In this paper, stability of the spliced Bi-2223 wire, stability of the cable core under the cold shrinkage force, loss under the large dc and small ac ripple are analyzed by the theoretical and experimental methods. The test results of ac ripple loss, anode effect, and stable operation are also presented.

Fault-Current Limiting Properties in Innovative Force-Free Superconducting DC Cable

We proposed the innovative force-free superconducting cable with higher current-carrying capacity than conventional superconducting DC cables utilizing the longitudinal magnetic field effect that enhances the critical current density. That is, the critical current density can be enhanced by the longitudinal magnetic field that the conductor itself and the outer shield conductor produce. It means that, if the longitudinal magnetic field can be reduced, the critical current density is appreciably reduced, resulting in an accelerated resistive transition. Such situation can be realized by arranging the copper conductor layers in a suitable structure that transport currents when the superconductors are in resistive state. We propose to arrange the copper conductor layers in the inner and outer conductors to reduce the longitudinal magnetic field by twisting in opposite directions to respective superconducting layers in each conductor region. Various conditions of fault-current limiting can be achieved by arranging the winding angle of the copper layers. Discussion will be given on the fault-current limiting properties of the proposed cable.

Qualification Test of a 80 kV 500 MW HTS DC Cable for Applying Into Real Grid

Until now, some countries including South Korea have made considerable progress in the development of HTS (High Tc Superconducting) power equipment. Superconducting power cable systems are the strongest candidate from the viewpoint of their use in the real grid owing to their high current capacity. Specially, in the future, high-current long-distance HTS DC cables will play an important role in power transmission systems because, in contrast to HTS AC cables, HTS DC cables experience nearly no loss. In this paper, the authors suggest a new testing method for the load cycle and suitable thermal cycles to meet the requirements for HTS DC cables to be applied to the real grid. A prototype 100 m/3.25 kA/80 kV HTS DC cable system was developed for the qualification test in South Korea. The qualification test was carried out, based on HTS experience, the international standard for HTS AC cables, and the standards for conventional HVDC cables. It was performed in the same manner as that for a conventional HVDC cable for 6 months in the KEPCO PT (Power Testing) Center. Since its successful qualification test, the 500 MW/80 kV HTS HVDC cable system will have been operated in the KEPCO real grid since 2015, to evaluate its practical requirements and confirm its technical feasibility. This paper presents the recommendations and results of the qualification test for HTS DC cables.

A novel VSC-HVDC link model for dynamic power system simulations

This paper introduces a new RMS model of the VSC-HVDC link. The model is useful for assessing the steady-state and dynamic responses of large power systems with embedded back-to-back and point-to-point VSC-HVDC links. The VSC-HVDC model comprises two voltage source converters (VSC) linked by a DC cable. Each VSC is modelled as an ideal phase-shifting transformer whose primary and secondary windings correspond, in a notional sense, to the AC and DC buses of the VSC. The magnitude and phase angle of the ideal phase-shifting transformer represent the amplitude modulation ratio and the phase shift that exists in a PWM converter to enable either generation or absorption of reactive power purely by electronic processing of the voltage and current waveforms within the VSC. The mathematical model is formulated in such a way that the back-to-back VSC-HVDC model is realized by simply setting the DC cable resistance to zero in the point-to-point VSC-HVDC model. The Newton–Raphson method is used to solve the nonlinear algebraic and discretised differential equations arising from the VSC-HVDC, synchronous generators and the power grid, in a unified frame-of-reference for efficient, iterative solutions at each time step. The dynamic response of the VSC-HVDC model is assessed thoroughly; it is validated against the response of a detailed EMT-type model using Simulink®. The solution of a relatively large power system shows the ability of the new dynamic model to carry out large-scale power system simulations with high efficiency.

AC/DC 하이브리드형 혼합송전계통에서 DC 케이블의 유도전압평가에 의한 적정배열 선정

Hybrid type combined transmission systems is being operated by AC and DC line at the same space will be expanded instead of the overhead line. However, such hybrid type combined system has problem like the arrangement selection of DC cable for effective system operation. In this paper, to select the proper arrangement of DC cable, induced voltage of DC cable influenced by AC cable was analyzed in case of several type arrangement of DC cable. Such induced voltage is in detailed analyzed not only in case of steady, but transient state. The arrangement which has the lowest induced voltage is selected as the proper one. EMTP/ATPDraw is used for modeling and analysis of hybrid type combined transmission system.

Life Estimation of HVDC Cables Under the Time-Varying Electrothermal Stress Associated With Load Cycles

In this paper, a new procedure is presented for life estimation of HVDC cables subjected to the time-varying electrothermal stress associated with load cycles and voltage. The electric-field inversion phenomenon in the insulation of dc cables is accounted for by updating the field profile as the temperature profile changes with the load. The thermal transients due to load cycles are modeled via the CIGRÉ transient thermal network adapted to dc cables. The life fractions lost during each cycle because of the cyclic variation of the electrothermal stress are computed via an adequate electrothermal life model for the insulation of HVDC cables. The Weibull hypothesis is made for relating cable life to failure probability. The procedure is applied to the load cycles of the prequalification test for HVDC extruded cables after CIGRÉ Technical Brochure 496 that does not consider the effect of thermal and synergistic electrothermal aging. It is thus shown that this test might be less severe than it should be.

Topology assessment for 3 + 3 terminal offshore DC grid considering DC fault management

This study assesses four DC grid topologies where each of the terminals can exchange power with any other terminal and a DC fault on any DC cable can be isolated. The DC grids are built using the following components: hybrid DC circuit breakers (CB), mechanical DC CBs, DC/DC converters and DC hubs. The aim is to compare DC fault performance, technical feasibility/readiness, power transfer security, expansion and costs. The base case for comparison are three separate 300 km offshore HVDCs connecting 3 1 GW offshore wind farms with three onshore VSC terminals. The rating of DC CBs is limited by the state of technology and costs which introduces limitation on the length of DC cables (because of communication delays) and on the DC bus fault level (limiting the number of connecting DC lines). DC/DC converters inherently block propagation of DC faults and their rating is not sensitive to DC fault level. DC hubs have considerable cost advantages over multiple DC/DC converters in cases where multiple DC systems are connecting at the same DC station. The cost assumptions for all major components are analysed, including the offshore platform costs. The study concludes that overall DC grid costs are similar (within 8%) for all four topologies. Although power security is similar for all technologies, the expansion options are best with DC/DC converters or DC hubs. DC hubs nevertheless suffer from highest on-state losses.

Evaluation of electromagnetic radiation from the DC side of a photovoltaic power generation system

The radiation mechanism from the DC side of a photovoltaic (PV) power generation system is investigated at frequencies between 150 kHz and 30 MHz. To determine the source of the outbreak of the common mode noise, the balancing of the PV measurement model, which is composed of a PV panel and a 2-m-long DC cable, is measured by the mixed-mode S-parameter method. Results showthatthe PV model has good balance. Then, a balanced PV simulation model is proposed. To express the common mode noise, the input signal is supplied to only one of the two wires of the PV simulation model. To confirm the validity of the PV simulation model, the current flowing in the DC cable and the magnetic field from the PV simulation model are compared with those of the PV measurement model. A good agreement between the measured and simulation results is obtained. The PV simulation model is valid for evaluating the radiation noise from the DC side of the PV power generation system. Moreover, the effect of the frame wire of the PV power generation system on the radiation noise is evaluated. The radiation noise from the PV power generation system will strengthen or weaken depending on the layout of the frame wire, especially at low frequency. The radiation noise from the PV power generation system with a plurality of PV panels is also investigated. The peak of the frequency spectrum of the radiation noise shifts to lowfrequency when the number of PV panels is increased.

A Study on the Thermal Characteristics of the Epoxy Insulator in the Stop Joint Box of HTS Power Cable

The HTS power cable is cooled by the circulation of liquid nitrogen. The cooling capacity of refrigerator increases with cable length. As the length of power cable increases, a joint box should be installed to reduce the cooling capacity and pressure drop for unit cooling system. The type of joint box can be divided into two groups. There are NJB(Normal Joint Box) and SJB(Stop Joint Box). Generally, SJB can separate each cooling system. In case of long distance DC cable, it is necessary to separate the cooling line in the regular distance of cooling system so SJB should be used. However, SJB, which has a difference from the insulation method of existing joint box, uses solid electrical insulation method. At the primary cooling time, thermal stress is generated by the temperature difference between the internal and external epoxy. So to prevent the damage stress analysis is required for the electrical insulation structure of SJB. In this paper, using the FEM analysis we study the cooling method and optimal shape of SJB to reduce the thermal stress result from temperature difference during the cooling time.

家庭用太陽光発電用パワーコンディショナの入出力EMIフィルタ設計手法

Electro-magnetic interference (EMI) noise radiated from the DC cable of photovoltaic power conditioner systems (PCS) is a serious problem for radio transmission equipment. In order to prevent the interference, the International Special Committee on Radio Interference (CISPR) is discussing to determine a noise regulation value for this issue. To reduce the radiated noise, this paper studies the attenuation characteristics of EMI filters that are connected to both the DC- and the AC-sides of the PCS. In this paper, it is pointed out that the EMI filters do not show their own attenuation characteristic if those are connected to the PCS. In particular, when EMI filters are connected to both the DC- and AC-sides of the PCS, a specific series resonant frequency is observed and deteriorates the attenuation characteristics of both EMI filters. In order to prevent this problem, a design method for EMI filters which can reduce the common-mode EMI noise effectively, is proposed in this paper. Further, the effectiveness of the proposed method is confirmed through experiments.

Current-carrying capacity of single layer cable using superconducting Bi-2223 tapes in a parallel magnetic field

It was theoretically shown by the authors that the current-carrying capacity of superconducting dc power cable can be enhanced by choosing a force-free configuration under a parallel magnetic field produced by the current flowing back in the outer shielding conductor. This was experimentally checked for a single layer cable using Bi-2223 tapes in an applied parallel magnetic field. It was found that the current-carrying capacity took on a peak value under the force-free condition for the total magnetic field, including the self-field. This shows that the proposed structure is suitable for practical dc power transmission. The possibility of the innovative dc superconducting power cable with multi-layers with higher current-carrying capacity is discussed.

Analytical Calculation of the Electrical Energy Losses on Fixed-Mounted PV Plants

This paper presents analytical expressions for the annual energy losses on dc and ac cables, the annual energy losses on the step-up transformer, and the optimum transformer size for fixed-mounted photovoltaic (PV) plants of any size. These expressions are based on some unknown parameters, which can be easily estimated from data provided by the equipment (inverters or transformer) manufacturer and from available data in any meteorological database. The analytical expressions are meant to be used by the design engineers for comparing-from the energetic point of view-different cable cross sections and transformer sizes with various power loss classes or cooling medium, without having to perform multiple detailed simulations, as the current practice is. The validity of the derived expressions was tested against measurements taken in two existing 1003.52- and 491.4 kW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> PV installations during the past 2 years. The proper use of the derived expressions during the design phase of a PV system is demonstrated through a detailed working example.

Integrated SMES Technology for Modern Power System and Future Smart Grid

Superconducting magnetic energy storage (SMES) technology has been progressed actively for use in modern power system. Principal application schemes of a sole SMES system, a hybrid energy storage system (HESS) consisting of small-scale SMES and other commercial energy storage systems (ESSs), a distributed SMES (DSMES) system, and a distributed HESS (DHESS) are presented and compared. The concept of the SMES-based HESS provides an economical way to apply medium-scale SMES systems in daily load leveling and to apply small-scale SMES systems in power quality improvement. A dc power distribution network case with superconducting dc cables and SMES devices is conceptually designed to evaluate the performance of the proposed four SMES application schemes. The results show that the novel concept of the SMES-based DHESS can be particularly expected to achieve efficient and economical power management. Finally, the application prospects and possible schemes implanting SMES devices for power generation, transmission, distribution and utilization are explored for use in future smart grid.

Molecular modeling and electron transport in polyethylene

Polyethylene is commonly used as an insulator for AC power cables. However it is known to undergo chemical and physical change which can lead to dielectric breakdown. Despite almost eighty years of experimental characterization of its electrical properties, very little is known about the details of the electrical behaviour of this material at the molecular level. An understanding of the mechanisms of charge trapping and transport could help in the development of materials with better insulating properties required for the next generation of high voltage AC and DC cables. Molecular simulation techniques provide a unique tool with which to study dielectric processes at the atomic and electronic level. Here we summarise simulation methodologies which have been used to study the properties of PE at the molecular level, elucidating the role of morphology in the trapping of excess electrons. We find that polyethylene has localised states due to conformational trapping extending below the mobility edge (above which the electrons are delocalised), at -0.1±0.1eV with respect to the vacuum level. These trap states with localisation lengths between 0.3 and 1.2nm have energies as low as -0.4±0.1eV in the amorphous and interfacial regions of polyethylene with more positive values in lamella structures. Crystalline regions have a mobility edge at +0.46 ±0.1eV, so we would expect transport by electrons excited above the mobility edge to delocalised states to be predominantly through amorphous regions if they percolate the sample.

Experimental and Model Based Studies on Current Distribution in Superconducting DC Cables

Current distribution among tapes in superconducting cables has been studied as a function of variations in contact resistance, individual tape critical current, and index (n)-value of individual tapes. It has been shown that besides contact resistances, variations in other superconducting parameters affect current distribution. Variations in critical current and n-value become important at low contact resistances. The effects of collective variations in contact resistances, individual tape critical current, and n-value were studied using Monte Carlo simulations method. Using a validated mathematical model, 1000 cables were simulated with normally distributed random values of contact resistances, individual tape critical current, and n-value. Current distribution in the 1000 simulated cables demonstrated the need for selecting tapes with a narrow distribution in the superconducting parameters to minimize the risk of catastrophic damage to superconducting cables during their operation. It has been demonstrated that there is a potential danger of pushing some tapes closer to their critical current before the current in the cable reaches its design critical current.

HTS DC Transmission Line for Megalopolis Grid Development

Using of HTS AC and DC cables in electric power grids allows increasing of the transferred power, losses diminishing, decreasing of exclusion zone areas, the enhancement of the environmental conditions and fire/explosion safety of electric power systems. However, the use of DC superconducting cable lines together with converters brings additional advantages as reduction of losses in cables and suitable lowering of refrigerating plant capacity, as well as the realization of the function of short-circuit currents limitation by means of the appropriate setting of converter equipment. Russian Federal Grid Company and its R&D Center started the construction of the DC HTS power transmission line which includes the cable itself, cryogenic equipment, AC/DC converters, terminals and cable coupling boxes. This line will connect two substations in Saint-Petersburg – 330 kV "Centralnaya" and 220 kV "RP-9". The length of this HTS transmission line will be about 2500 meters. Nowadays are developed all the elements of the line and technologies of the cable manufacturing. Two HTS cable samples, each 30 m length, have been made. This paper describes the results of cables tests.

Current-carrying capacity of HTS DC cables with the reduced Lorentz force

The structure of superconducting DC cable made with high temperature superconducting tapes is designed to improve the current-carrying capacity by reducing the Lorentz force acting on flux lines. For this purpose an axial magnetic field is produced by the outer shield conductor. The analysis was performed for the optimum structure for various superconducting tapes with different Jc(B,θ) characteristics. It is found that the current-carrying capacity increases appreciably when the difference in the critical current density of the tape is large enough between normal and parallel magnetic fields.

Critical current measurement of HTS tape relating with cable structure for a DC power cable

In the 200 m high temperature superconducting (HTS) cable test facility at Chubu University, a coaxial power cable is used and composed of two BSCCO (Bi-2223) superconducting layers. The tapes are wound closely to reduce effects on the critical current of BSCCO at self-field. Accordingly, each superconducting layer has a different number of BSCCO tapes. Previously, we have investigated dependence of the critical current (Ic) on the gap in order to optimize the HTS DC cable design. We have been studying the effect on the performance of HTS tapes for the superconducting DC power cables by critical current measurements. In the present experiments several HTS tapes are used and set as a similar structure in the cable with a two-layer structure. The critical current of HTS tapes are measured against the gap between the tapes in the same layer. The experiments show the improvement of the critical current by optimizing the tape arrangements due to magnetic field interaction between the tapes. We will present the experimental results and discuss the design of the HTS DC cable.