Fei Liang

Design and application of superconducting fault current limiter in a multiterminal HVDC system

Voltage source converter based HVdc (VSC-HVdc) systems are prone to high short-circuit current during transmission line faults. The situation for multiterminal HVdc (MTDC) systems is worse. The characteristics of superconducting material are ideal to limit the fault current in HVdc systems. This paper presents a novel use of the resistive type of superconducting fault current limiter (SFCL) in the MTdc network with the function of limiting the high current. The working principles of fault current limiter and a three-terminal HVdc system are modeled in detail using PSCAD/EMTDC software. The hybrid operation of the SFCL in the three-terminal HVdc system is tested in this paper for the fault response of the MTdc system. The performances of SFCL under different fault conditions are analyzed. The simulation results show that the fault current is effectively restrained and the SFCL can act as an efficient protective device for VSC-based multiterminal HVdc systems.

A finite element model for simulating second generation high temperature superconducting coils/stacks with large number of turns

An efficient two dimensional T-A formulation based approach is proposed to calculate the electromagnetic characteristics of tape stacks and coils made of second generation high temperature superconductors. In the approach, a thin strip approximation of the superconductor is used in which the superconducting layer is modeled as a 1-dimensional domain. The formulation is mainly based on the calculation of the current vector potential T in the superconductor layer and the calculation of the magnetic vector potential A in the whole space, which are coupled together in the model. Compared with previous T-based models, the proposed model is innovative in terms of magnetic vector potential A solving, which is achieved by using the differential method, instead of the integral method. To validate the T-A formulation model, it is used to simulate racetrack coils made of second generation high temperature superconducting (2G HTS) tape, and the results are compared with the experimentally obtained data on the AC loss. The results show that the T-A formulation is accurate and efficient in calculating 2G HTS coils, including magnetic field distribution, current density distribution, and AC loss. Finally, the proposed model is used for simulating a 2000 turn coil to demonstrate its effectiveness and efficiency in simulating large-scale 2G HTS coils.

AC loss comparison between multifilament and nonstriated YBCO coils designed for HTS propulsion motors

In this paper, the properties of current capacity and ac transport loss of striated high temperature superconductor (HTS) coil is compared with ordinary HTS coil by using both experimental and simulation methods. The measurements were carried out by transporting a sinusoidal varying current at 77 K, with an amplitude range of 10–50 A in frequency from 70 up to 300 Hz. Measurement facilities and methods are explained in more detail in the paper. The critical current of the 4-mm width multifilament coil made with four filaments at a spacing 0.03 mm was found to be lower than that of the nonstriated coil. The frequency dependent characteristics agreed well in both experimental and simulated results. Reducing ac loss of HTS is one enabling factor for widespread adoption of the technology, and therefore, understanding its characteristics is important and discussed in this paper. Future plans based on this preliminary work are the testing of multifilament tapes in an axial flux motor field environment.

Experimental test of two types of non-inductive solenoidal coils for superconducting fault current limiters use

Growing electricity demand from a range of sources and the changes of the power grid structures open the possibility of more frequent and much higher fault current. Traditional solutions to the fault are difficult to satisfy the requirement of the new power grid due to many factors, such as high cost, additional impact to power grid in normal loading condition, which leads to the research for an efficient alternative solution of interest to both academia and industry: Superconducting fault current limiter (SFCL). In this paper, two types of low-inductance solenoidal coils, the braid type and the nonintersecting type, are built with 2G high-temperature superconductors. The current limiting performance, the recovery characteristics, and the ac losses of both types of coils are tested and compared in detail. Experimental results show that both types of coils can effectively limit fault current and recover in several seconds. Furthermore, comparison between the ac losses of both types of SFCLs shows that the ac loss of the braid type coil is lower than that of the single tape by about an order of magnitude in low-current regions.

AC loss modelling and experiment of two types of low-inductance solenoidal coils

Low-inductance solenoidal coils, which usually refer to the nonintersecting type and the braid type, have already been employed to build superconducting fault current limiters because of their fast recovery and low inductance characteristics. However, despite their usage there is still no systematical simulation work concerning the AC loss characteristics of the coils built with 2G high temperature superconducting tapes perhaps because of their complicated structure. In this paper, a new method is proposed to simulate both types of coils with 2D axisymmetric models solved by H formulation. Following the simulation work, AC losses of both types of low inductance solenoidal coils are compared numerically and experimentally, which verify that the model works well in simulating non-inductive coils. Finally, simulation works show that pitch has significant impact to AC loss of both types of coils and the inter-layer separation has different impact to the AC loss of braid type of coil in case of different applied currents. The model provides an effective tool for the design optimisation of SFCLs built with non-inductive solenoidal coils.

The Impact of Critical Current Inhomogeneity in HTS Coated Conductors on the Quench Process for SFCL Application

Growing electricity demand from a range of sources with higher loads and the industry structural changes open the possibility of more frequent and larger fault currents producible in liver power grids. Traditional solutions to the fault have difficulties in satisfying the requirement of the new power grid requirement due to many factors, such as high cost and additional impact to power grid in normal loading condition, which leads to the research for an efficient alternative solution of interest to both academia and industry: Superconducting fault current limiter (SFCL). Critical current (Ic) is used to describe the maximum current that a superconductor can transport and is one of the most important parameters to be considered while designing a SFCL. Guaranteeing the homogeneous distribution of the critical current density flowing in a superconductor is not possible due to manufacturing process limitations. In this paper, the impact of critical current inhomogeneity of coated high temperature superconductors (HTSs) during the quench process is studied experimentally. The results show that both the amount of Ic degradation and the size of Ic degraded segments have a great impact on the maximum temperature generated in the quench process of HTSs when the prospective fault current is low (1.13 Ic to 1.51 Ic). Higher amount of localized Ic degradation allows higher maximum temperatures under low voltage fault. Additionally, smaller Ic degraded segment allows higher maximum temperatures.temperatures.temperatures.

Forceful uniform current distribution among all the tapes of a coaxial cable to enhance the operational current

A coaxial cable made out of helically wound 2G HTS tapes is being widely implemented based on its low critical bending radius, ease of manufacturing and installation. Though the design technique is mature, the cable still lacks standard Ic definition. With increased usage of such cables, robustness in its operation is expected. HTS tapes are highly sensitive to externally applied mechanical stress and strain values. These limitations force the coaxial cables to be operated at 50–75% of its rated capacity. One of the major problems in coaxial cable design is the contact resistance of the tapes with the terminal, which affects the current distribution among different tapes in the same layer. Further current redistribution at constant intervals helps the applied current in bypassing the localized faults in the individual HTS tapes, thus making efficient use of the major length of the tape. This paper proposes the use of voltage shunts between HTS layers at constant intervals for regular current redistributions along the length of the cable. This method significantly helps to sustain the Ic value of the cable, even if it has got several minor defects in the tapes.