In this paper, through AC and DC overcurrent tests on second generation high temperature superconducting tape (2G HTS tape), we respectively summarize the typical types of quenching resistance and corresponding quenching degree, in which there are three types under AC overcurrent and two types under DC overcurrent. According to experimental results, a rule was found that, when 2G HTS tape quenches to normal state, the relationship between quenching resistance and joule heat generated from 2G HTS tape presents a fixed trend line, and the influence of liquid nitrogen can be ignored. Then, the characteristics and rules of quenching resistance found in experiments are well explained and confirmed by a detailed 3D finite element model of 2G HTS tape including electromagnetic field and thermal field. Finally, based on above works, our group proposes a new equivalent method to estimate the quenching resistance, where the results of AC and DC overcurrent experiments can be equivalent to each other within a certain range. Compared with FEM, the method has the following advantages: (i) The method is simple and easy to implement. (ii) This method combines precision and computational efficiency. (iii) With superconducting tape quenching to normal state, this method presents a good consistency with experimental results.

A resistive type DC superconducting fault current limiter (SFCL) is developed for the fault current limiting at the Suzhou Nami substation. The rated voltage and current are 20 kV and 400 A, respectively. The prospective peak value of the short circuit current is about 3.5 kA, and the current could be limited to less than 2.1 kA by the SFCL. The SFCL uses about 1800 m stainless steel reinforced YBCO conductor. According to the testing results, the developed SFCL has passed the DC withstand voltage test at 50 kV/1 min, the full wave lightning impulse voltage at 95 kV, and the DC power switching test with a peak value of 2.26 kA and a fault duration of 16.5 ms.

The MMC-HVDC technology plays a great role on multilevel DC systems and large capacity transmission systems gradually. Therefore, the protection for MMC-HVDC system becomes particularly important, for the impact acreage of DC short faults is wider and damage is more serious. In this paper, the MMC-HVDC system electromagnetic transient characteristics of fault are analyzed and superconducting fault current limiter (SFCL) resistance model is established for a resistive SFCL (R-SFCL) during the short fault. The thermal-electrical process of R-SFCL limiting short current is calculated by iteration method. Then a R-SFCL based on noninductive bifilar pancake is designed for $\pm$ 160kV bipolar MMC-HVDC system. The correctness of the theoretical calculation method and design scheme is verified by the simulation in software PSCAD. The results show that the response time of the R-SFCL based on noninductive bifilar pancake is 0.1 ms and the short circuit current suppression rate is 50% ~ 35% in 3~8 ms after the DC short fault.

There are many disadvantages in the conventional method of limiting the short-circuit of the flexible HVdc transmission system such as a series-connecting electric reactor on the dc side. This paper presents a short-circuit current limiting method for adding a resistive-type superconducting fault current limiter. Considering the flexibility of various HVdc operating conditions and short-circuit characteristics, as well as the performance requirements to the superconducting fault current limiter, the system scheme of the 160-kV/1-kA resistive-type superconducting dc fault current limiter was designed, and the system topology and the main parameters of the current limiter were proposed. According to the mutual restriction relationship between the current limiting resistance, the fault current, and the thermal stability, an iterative algorithm for optimizing the fault current-resistance curve was proposed, and the optimal parameters of the solution were obtained. The rationality of the designing scheme for the 160-kV/1-kA resistive-type superconducting dc fault current limiter was verified by the system simulation based on the prototype of the flexible dc transmission system at Nan'ao in Guangdong province proves. This paper provides a new technical solution for the stable operation of the flexible dc transmission system.