High-temperature Superconducting Cable Systems Research Articles

Abnormality Monitoring for Three-Phase HTS Cable via Time-Frequency Domain Reflectometry

With the advent of commercialization in high temperature superconducting (HTS) cable systems, maintenance techniques for the HTS cable systems are required. In the HTS cable systems, which must maintain the superconducting state during the operation, real-time monitoring is essential. Therefore, in this paper, a monitoring technique for three-phase HTS cables is proposed. The proposed technique, which is based on time-frequency domain reflectometry, has an efficient instrument configuration compared to conventional techniques. Also, by extracting a feature in the time-frequency domain of the monitoring signals, abnormal phase in a three-phase HTS cable can be identified. The proposed technique is demonstrated by experimental data using a real-world three-phase HTS cable. In order to simulate a local quench in the operation of three-phase HTS cable systems, heating elements are utilized in the experimental setup. It is expected that the proposed technique will improve reliability in operating the three-phase HTS cable systems.

Condition Monitoring of 154 kV HTS Cable Systems via Temporal Sliding LSTM Networks

High-temperature superconducting (HTS) cables are expected to be installed in cable tunnels that are already constructed in urban districts. Therefore, the installation of normal joint boxes is inevitable, and it is necessary to develop a diagnostic methodology that considers both the existence of the joints and the electrical characteristics of HTS cables. In this work, temporal sliding long short-term memory (TS-LSTM) is proposed to estimate the locations of the joints that can be hidden by multiple reflections. TS-LSTM includes short-term TS-LSTM and long-term TS-LSTM for analyzing various time dependencies. The reflected signals of the actual joints, which are distinguished from multiple reflections, are analyzed via the chirplet transform (CT) which is one of the time-frequency (TF) analysis methods. The proposed condition monitoring method is applied to an AC 154 kV 600 MVA HTS cable system (1 km) connected to a real power grid network in Jeju, South Korea. For the validation of the proposed methodology, the dielectric and electrical characteristics of the 154 kV HTS cable system are monitored during the cooling process.

Time–Frequency-Based Condition Monitoring of 22.9-kV HTS Cable Systems: Cooling Process and Current Imbalance

In this paper, new time–frequency-based anomaly detection methodology is proposed for the condition monitoring of high-temperature superconducting (HTS) cable systems. The time–frequency-based anomaly detection methodology includes two indices, which are obtained via cross Wigner–Ville distribution and scattering parameter. For the validation of the proposed methodology, the methodology is applied to an ac 22.9-kV/50-MVA HTS cable system connected to the real power grid network. Furthermore, the changes in dielectric properties of the HTS cable system during the cooling process and the current imbalance failure are monitored. The proposed anomaly detection indices are shown to be able to detect the heat and the current imbalance caused by the malfunction of the joint box. For the application of the two validated indices, new anomaly detection procedure for HTS cable systems is also presented.

Monitoring Electrical and Thermal Characteristics of HTS Cable Systems via Time–Frequency Domain Reflectometry

A high-temperature superconducting (HTS) cable system with the 22.9 kV, 50 MVA, and 410 m length is installed and operated at 154 kV Icheon substation of Korea Electric Power Corporation (KEPCO). Unfortunately, it is a difficult task to diagnose and monitor electrical and thermal characteristics of the HTS cable system in a real-time manner. In order to protect operational failures of grid-connected HTS cable systems, this paper proposes time-frequency domain reflectometry (TFDR) and analysis techniques, i.e., time-frequency cross correlation and instantaneous frequency estimation. To verify the performance of the proposed method, the temperature is changed via the cryogenic refrigeration system and the status of the grid-connected HTS cable is monitored via TFDR in a real-time manner.

Safety and Reliability of 66-kV Class HTS Cable Systems in Short-Circuit Current Accidents-Experimental Results on 40-m Cable System

High-temperature superconducting (HTS) cables can transmit large amounts of electricity in a compact size with minimal losses. Therefore, it expects to save the construction cost of underground lines in urban areas and decrease transmission losses. The stability and reliability of HTS cable systems in normal operation has been verified by real grid interconnection tests around the world. While general practical use of HTS cable systems requires the verification of their stability not only in normal operation but also in the event of various accidents (ground fault, short-circuit current, liquid nitrogen leakage, etc.) and understanding these phenomena and their impact. However, a few studies have been conducted to investigate the safety of HTS cable systems in the event of the accidents so far. Regarding the short-circuit current, the maximum current depends on the voltage class and grid conditions, and it was set as 31.5 kA for the 66 kV systems in Japan. In order to verify the phenomena and impact to the cable systems, a 40-m cable system was constructed and cooled by circulated subcooled liquid nitrogen (LN2). After the initial critical current (Ic) measurement of the cable system, the short-circuit current tests were conducted with ac 28.5 kA for 0.6 s and 16 kA for 8.4 s. The former testing condition is almost simulated as the current of 31.5 kA and the latter one is as equivalent heat generation in the HTS cable to 31.5 kA for 2 s. As the initial state of LN2 , temperature and pressure of the cable inlet were around 68 K and 0.26 MPaG. The measured temperature and pressure rises of the coolant were not so high to make some damage to the HTS cable. After these tests, the cable Ic was measured again and no damage was found.

Analysis of Wave Propagation of HTS Cables for Compensation of Thermal Loss on Connectors

High temperature superconducting (HTS) cable operates under relatively low temperature and its unique operating condition brings new challenges in the area of maintenance and diagnostics of the cable. As an example, a temperature difference occurs on connection systems between instruments on room temperature and HTS cables which eventually leads to thermal energy loss. The thermal energy loss affects frequency characteristics of the cable which are critical aspects for simulation and maintenance of the HTS cable systems. In this paper, an attempt to reduce thermal loss is introduced with an improved connector applying a Peltier module. The performance of the designed connector is verified with analysis of electromagnetic wave propagation properties of HTS tapes and a cable. The investigation of S -parameters and the proposed connection method is expected to be further applied to various HTS systems in future.

Transient Thermal Finite Element Modeling of HTS Cable Systems Cooled With Gaseous Helium

Intensive thermal modeling for gaseous helium cooled high -temperature superconducting (HTS) power devices is necessary for understanding the dynamic thermal behavior during potential system contingencies. This paper presents a transient analysis of an HTS cable system that uses helium gas as a cryogen in a closed loop. Several system contingencies in which the system undergoes a transition that may lead to quenching are simulated. These contingencies include: a vacuum breach, cryocooler failure, and a combination of cryocooler and gas circulation impeller failure. Additionally, the thermal mass of the cable termination is varied to analyze its importance and effect on the timescale for the system to reach a critical temperature of 80 K.

Surface Potential Behavior of Direct-Fluorinated PPLP Under Quench Condition

With the emergence of superconductor technology, high-temperature superconducting (HTS) cable is receiving considerable attention for application in power systems. Polypropylene laminated paper (PPLP) and liquid nitrogen (LN 2 ) have been widely used as insulation and coolant, respectively, in HTS cable systems, where the insulation is threatened by surface charge and quench. Direct fluorination is an effective method to improve the electrical properties on the material surface. To restrain the surface charge accumulation and prolong the insulation life, the surface potential behaviors of fluorinated PPLP under the quench condition has been studied to clarify the effects of fluorination time and low temperature. PPLP specimens were fluorinated in a reaction kettle using a F 2 /N 2 blend with an 80% volume fraction of nitrogen and pressure of 0.1 MPa for respectively 5, 10, and 15 min. Corona charging was performed with dc voltage at a relative humidity of ~15% in atmospheric air on room temperature specimens and after the specimens had been cooled by LN 2 for 20 s before the corona charging. Then, the surface potential was measured with an electrostatic voltmeter. The results indicate that the accumulation and decay behaviors of the surface potential for PPLP are affected by the charge polarity, specimen temperature, and fluorination time.

Discharge Breakdown Characteristics With Fluorinated Polyimide Films in High-Temperature Superconducting Cable Insulation

The application of high-temperature superconducting cable now has been in the process of being implemented in power system. Polyimide (PI) film has been used as insulation in the high-temperature superconducting cable systems with the advantage of good performance in low temperature, excellent mechanical properties, and dielectric properties. However, due to the organic properties of PI films, it can be aged even brake down in the service of HTS cable. It is found that partial discharge in HTS cable can lead to insulation material degradation and finally breakdown. Our recent researches have shown that fluorination can influence the surface charge behaviors thus influence the PD characteristics of modified PI films. In this paper, PI films were fluorinated in laboratory, the relationship between the fluorination time and discharge characteristics of the films were researched. The surface topography of direct-fluorinated samples were obverted by using an Atomic force acoustic microscope. It can be found from the experiment results that the discharge times and quantity both showed decreasing tendency with the fluorination time increasing. The discharge breakdown voltage shows increasing tendency with the fluorination time from zero minutes to forty-five minutes, and then shows decreasing tendency.

Time–Frequency-Based Insulation Diagnostic Technique of High-Temperature Superconducting Cable Systems

For the electrical insulation of a high-temperature superconducting (HTS) cable, wrapped polypropylene laminated paper (PPLP) tape is typically used. Unfortunately, it is possible that unexpected faults at insulation layers will be present in the cables as a result of either a problematic manufacturing process or an incomplete installation procedure. In order to protect against operational failures of grid-connected HTS cable systems, this paper proposes a nondestructive diagnostic technique, i.e., time-frequency domain reflectometry (TFDR), and focuses on the characteristic of HTS cable that caused the local insulation defects . To verify the performance of the proposed method, detection and localization of local insulation failure via TFDR are compared with traditional time-domain reflectometry. The experiments are conducted at room temperature and under liquid nitrogen in order to check the efficacy of the proposed method in varieties of HTS cable's conditions. In addition, to improve the accuracy of detection and localization, a methodology to analyze incident signals, which are composed of upchirp and downchirp signals, is presented.

New HTS Cable Project in Japan: Basic Study on Ground Fault Characteristics of 66-kV Class Cables

In July 2014, a new high-temperature superconducting (HTS) cable project supported by the New Energy and Industrial Technology Development Organization began in Japan. The aim of this project is to verify and improve the safety and reliability of HTS cable systems. The main verification targets are system safety in the event of the following accidents: (1) ground fault; (2) short-circuit current; (3) cryostat failure; (4) a low heat loss cryostat; and (5) a high efficiency cooling system. If a ground fault occurs, it is a matter of concern that the pressure in the cryostat increases due to the arc energy. It is an additional concern if the arc penetrates the cryostat so that the liquid nitrogen leaks out of the cable. It is important to know the amount of energy in the arc in order to numerically predict the outcomes of a ground fault. We performed basic ground fault tests using sheet samples immersed in liquid nitrogen while measuring the arc energy and also examining the structure of a protection layer that can prevent arc penetration to the outside of the cable core.

Reliability Analysis of HTS Cable Systems

In this study, a reliability model of a high-temperature superconducting (HTS) cable system is proposed. There are two versions of this HTS-system reliability model, depending on the scope of its application. As an independent system, there are three operating states in an HTS system, that is, a normal state, a postponable outage state, and an outage state. The probability and frequency of each state are calculated using the Markov model. However, when used as part of a distribution system, a derated state is added to the model because of the unique overcurrent behavior of the HTS cable system. The probability of this derated state depends on the condition of the HTS cable system. The proposed models are applied to an HTS cable project planned by the State Grid of China. The results illustrate the effectiveness of a backup refrigerator in the cooling system. The effects of a spare line in parallel with an HTS cable are quantitatively estimated with the model. The reliability of an HTS cable can be calculated using the proposed models if the required data are available.

A Study on the Application of DC HTS cable systems to enhance power transfer limits of a grid-connected offshore wind farm

This paper introduces two on-going projects for DC high temperature superconducting (HTS) cable systems in South Korea. This study proposes the application of DC HTS cable systems to enhance power transfer limits of a grid-connected offshore wind farm. In order to develop the superconducting DC transmission system model based on HTS power cables, the maximum transfer limits from offshore wind farm are estimated and the system marginal price (SMP) calculated through a Two-Step Power Transfer (TSPT) model based on PV analysis and DC-optimal power flow. The proposed TSPT model will be applied to 2022 KEPCO systems with offshore wind farms.

Characterization of Partial Discharge With Polyimide Film in $\hbox{LN}_{2}$ Considering High Temperature Superconducting Cable Insulation

High temperature superconducting (HTS) cable has now reached the stage where their application in power systems is in the process of being implemented. Liquid Nitrogen (LN 2 ) and polyimide film have been used as a coolant and insulation in HTS cable systems. Partial discharge (PD) events may lead to material degradation and total breakdown. PD testing is an important quality check for the insulation of HTS cable. Thus it is necessary to consider PD for the insulation testing in LN 2 which is generated in void defects of the insulation layers. The PD characterizations of polyimide film in different defects were investigated under 50 Hz AC voltages in LN 2 and at room temperature (298 K). The results show that the number of discharges and the discharge quantity increase with the increasing of the applied voltage and the defect size. The PD inception voltage decreases when the void defect diameter enlarged and it is higher in LN 2 than that at room temperature. The maximum field strength for HTS tape insulation increases with the defect close to the tape boundary and the addition of contaminant relative permittivity. Obtained results at cryogenic temperature provide necessary information for insulation design and pre-shipment inspections of HTS cable system.

Thermal Management of Long-Length HTS Cable Systems

Projections of electric power production suggest a major shift to renewables, such as wind and solar, which will be in remote locations where massive quantities of power are available. One solution for transmitting this power over long distances to load centers is direct current (dc), high temperature superconducting (HTS) cables. Electric transmission via dc cables promises to be effective because of the low-loss, high-current-carrying capability of HTS wire at cryogenic temperatures. However, the thermal management system for the cable must be carefully designed to achieve reliable and energy-efficient operation. Here we extend the analysis of a super-conducting dc cable concept proposed by the Electric Power Research Institute (EPRI), which has one stream of liquid nitrogen flowing in a cryogenic enclosure that includes the power cable, and a separate return tube for the nitrogen. Refrigeration stations positioned every 10 to 20 km cool both nitrogen streams. Both go and return lines are contained in a single vacuum/cryogenic envelope. Other coolants, including gaseous helium and gaseous hydrogen, could provide potential advantages, though they bring some technical challenges to the operation of long-length HTS dc cable systems. A discussion of the heat produced in superconducting cables and a system to remove the heat are discussed. Also, an analysis of the use of various cryogenic fluids in long-distance HTS power cables is presented.

The Results of Installation and Preliminary Test of 22.9 kV, 50 MVA, 100 m Class HTS Power Cable System at KEPCO

As high temperature superconducting (HTS) power cables have some merits over conventional cables, several demonstration projects on the HTS cable system are presently under way around the world. Korea Electric Power Corporation (KEPCO) also initiated an HTS cable project in 2002 with the Korean government's support. A three phase 100 m HTS cable system with a capacity of 50 MVA has been installed at Gochang test yard, located in Chonnbuk province, Korea. The HTS cable system is composed of a 100 m-long cable, two terminations and a cooling system. The rated current is 1,250 A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> and the rated voltage is 22.9 kV considering compatibility with the conventional power distribution system in Korea. Main purposes of this project are to verify the performance of an HTS cable system and to evaluate the potential of the HTS cable system from the viewpoint of power utilities. The real grid application of the HTS cable system requires the demonstration of system reliability, accumulated operation experiences, and it has to meet the practical needs of the utilities. In such a meaning, this project provides various challenges for KEPCO, and the feedback will be delivered to cable manufacturers. This user initiative test will facilitate the introduction of HTS cable systems into a real grid network. The installation process of the HTS cable system and some results of the preliminary test were presented in this paper.