Active superconducting DC fault current limiter based on flux compensation

This paper presents a new topology of the active superconducting DC fault current limiter (DC-SFCL) applied in the hybrid AC/DC power supply systems. The proposed DC-SFCL is composed of superconducting transformer, DC chopper and voltage source converter (VSC). The primary winding of the transformer is connected in series with DC transmission line, and the second winding is connected with AC power system through the VSC and the DC chopper. The active DC-SFCL can suppress the fault current of DC system through power exchange between the DC and the AC system. Moreover, the DC-SFCL can act as an active power filter for both the DC system and the AC system to solve the problem of power quality. Using MATLAB SIMULINK, the simulation model of the hybrid AC/DC power supply systems with the proposed DC-SFCL is created. Simulation results validate the dynamics of system, and the performance of the active DC-SFCL is confirmed.

Assessment on the influence of resistive superconducting fault current limiter in VSC-HVDC system

The superconducting fault current limiter (SFCL) which has many advantages is being chosen as an attractive method for limiting fault current. To upgrade the performance of the SFCL, various configurations are reported. Among them, the double-quench type SFCL has been studied in order to reduce the power burden of the high-temperature superconducting (HTSC) elements. The double-quench type SFCL which consists of the two HTSC elements and the normal resistance is a method to improve the performance of the HTSC elements. The design of the normal resistance in the double-quench type SFCL for the control of distributed fault currents on both HTSC elements to reduce the power burden is very important. However, the double-quench type SFCL is difficult to change the specification of the component depending on changing the power system. To improve the disadvantage on the double-quench type SFCL, this paper proposed the double-quench-flux-lock type SFCL which combine the double-quench type SFCL and the flux-lock type SFCL. The conventional flux-lock type SFCL which consists of an iron core, two coils, and the HTSC element could control the operation current and the impedance of the SFCL depending on the turn ratio between two coils. The operational characteristics of the flux-lock type SFCL could flexibly respond to changes in the power system. The double-quench-flux-lock type SFCL proposed in this paper was verified through PSCAD/EMTDC.

This paper introduces a methodology to study converter-based power system stability using immittance-based frequencydomain models. The approach is based on modeling individual converters by their impedance and admittance (collectively referred to as immittance) measured at their grid interfaces. Together with network immittance, they enable a system model to be formulated in the frequency domain for stability study. The immittance-based models capture system dynamics from subsynchronous to EMT (electromagnetic transient) frequency range, thereby overcoming the limitations of fundamentalfrequency models used in traditional power system studies. The method is a generalization of the impedance-based stability analysis method for individual converters connected to the grid. It can be applied to ac, dc and hybrid ac-dc power systems with any number of converter-based resources. The paper reviews the method and use a practical wind power system to demonstrate the applications.

Comparative study on current limiting characteristics of flux-lock type SFCL with series or parallel connection of two coils

In view of the development of high-temperature superconducting materials, more applications on electric utility devices are used in the power industry. In this paper, a modified flux-coupling-type superconducting fault current limiter (SFCL) is suggested to mitigate the effect of DC short circuit in a voltage source converter-based high-voltage direct current (VSC-HVDC) system. The SFCL’s structural principle is firstly presented. Then, considering that the SFCL is placed in series with a VSC-HVDC transmission line, its influence mechanism to the DC fault current’s dynamic process is analyzed, and some brief discussions on the design of the SFCL are carried out. Further, the detailed model of a 100-kV-class VSC-HVDC system integrated with the SFCL is built in MATLAB/SIMULINK, and a simulation analysis is performed. From the demonstrated results, applying the SFCL can help to suppress the DC fault current, compensate the DC voltage sag, and maintain the power balance. Consequently, the VSC-HVDC system’s robustness against the DC short circuit fault can be well enhanced.

The modern power system is experiencing transformation from the rotational-generation-equipment-dominated system to a power-electronics-converter-dominated system, with the increasing penetration of renewable energy resources such as wind and photovoltaic. The power-electronics-based renewable generation, as well as energy storage system, can lead to the reduction of system inertia. As dc systems such as dc microgrids are attracting more attention, the low-inertia issues will challenge their stability. In this paper, a comprehensive review of inertia-enhancement methods in dc power systems is presented. The concept and significance of the inertia in dc systems is firstly introduced, and then the types of inertia-providing sources in dc systems are discussed. After that, the different virtual inertia control strategies applied in power electronics converters are classified and investigated. These virtual inertia control methods are proven to have a great ability to enhance the inertia of a dc system. The challenges and future research direction are discussed at the end of the article. In this paper, the previous research work on the inertia of dc power systems is summarized in detail, the inertia-enhancement methods of DC systems are comprehensively introduced, and the future research directions are prospected.

Superconducting fault current limiters (SFCL) have been successfully demonstrated to effectively curb electric faults. This chapter provides a comprehensive overview of the existing SFCL technology. All major types of SFCLs will be introduced, including the most popular resistive and saturated-core SFCL, as well as magnetic-shielded, coreless, transformer, flux-lock, bridge, resonance, and novel hybrid SFCLs. Resistive SFCLs have simple structure and they respond to faults automatically. Iron cores are essential for saturated-core and magnetic-shielded SFCLs, which enable quick response to fault with a significant increase of impedance. Due to the existence of different types of three-phase faults, a new type three-phase SFCL was designed to solve both symmetrical faults as well as unsymmetrical faults. In unsymmetrical faults such as single-line-to-ground faults, the three-phase balance is lost, and the magnetic flux in the iron core is not zero any more.

Analyzed some serious issues, including power energy quality and large area power interruption, require complex method to overcome in 3-phase power system, so, suggested to go other way, proposed independent full DC power system to become one of main power systems in the future. The authors emphasized the necessity and capability of full DC power systems in the significance of technology and energy saving, so that safer, more reliable, stable, flexible and controllable DC power grids could serve the people better than that of the present AC power systems. Presented brief structure of full DC power system where DC power would be produced from multi-phase generator-rectifier group in power station, consumers take power from DC bus, which benefits motor control and energy flow between loads connected the same DC bus without 3 phase balance problems, and DC-DC converters based Duty-Cycle control implement commutations at two terminals of HVDC transmission lines, which reduce the technical difficulties than that in 3-phase AC commutation station. Non-crystal material with about 15 kHz of frequency could be selected for advanced isolators, so as to get smallest power consumption. Some new technology lifting DC voltage in geometrical progression was suggested to be available at the sent terminal of HVDC line. With the development of renewable power systems and their energy storages, DC interconnection grids could be a better choice than AC interconnection grids because no frequency and phase shift problems need to be considered. Bulky 3-phase transformer with industrial frequency and vary kinds of compensators for AC power qualities could be saved. The authors suggested scientific policy for economically implementing full DC power systems step by step. It could be predicted that people would see 3 kinds of power systems, full AC, full DC and hybrid AC-DC systems operating simultaneously by the middle of 21 century.

As one of the countermeasures to solve the increase problem of the fault current in a power system, the superconducting fault current limiter (SFCL) has been noticed as the attractive device because it has no power loss in a normal time and can limit the fault current rapidly without additional device to detect the fault current. However, the introduction of the SFCL in a power distribution system affects the protection coordination between the existing protective devices such as the over-current relay (OCR) and the recloser, which causes the operation of the protective devices to get out of their original setting values. Therefore, the analysis on the effect of the SFCL introduction on the existing protective devices is required and the method to keep the protection coordination between the protective devices in a power system with the SFCL should be first considered. In this paper, the protection coordination experiments between the OCR and the recloser in the simulated power distribution system with the SFCL were performed. Based on the experimental results, the influence of the SFCL on the operation of these protective devices was analysed and the methods to keep the protection coordination between the OCR and the recloser in a power distribution system with the SFCL were discussed.

Comparison between inductive and resistive SFCL in terms of current limitation and power system transient stability

Superconducting fault current limiters (SFCLs), which are used to restrict the maximum fault current level in a power system, can contribute to the economic operation of a power system by reducing congestion costs and expected energy not served costs. However, various power system factors such as load and fuel price need to be taken into account because these factors result in an economic risk when SFCLs are installed in a power system. Therefore, it is needed to carry out an economic assessment of SFCLs considering both the expected benefit and the risk. This paper presents a risk-based economic assessment method for an SFCL installation under power system uncertainties. In the paper, the conditional value-at-risk (CVaR) is used to estimate the economic risk due to power system uncertainties when installing SFCLs. The proposed risk assessment method for SFCL installation can be useful as a decision-making tool.

In this paper, a flux-lock-type superconducting fault current limiter (SFCL) with the uninterruptible power supplying (UPS) function during the fault period was suggested. The structure of the suggested flux-lock-type SFCL is the same as one of the conventional flux-lock-type SFCLs using magnetic coupling of two coils except for the third winding and two power switches. The third winding is wound on the same iron core as two coils of the flux-lock-type SFCL are wound, and two power switches are connected with the third winding and the load, required for the uninterruptible power supply, respectively. To investigate the operational characteristics of the suggested flux-lock-type SFCL with the UPS function, the short-circuit tests were carried out. Through the analysis for the experimental results, the fault current limiting and the UPS operations of the flux-lock-type SFCL could be confirmed to be performed through the proper design of the suggested flux-lock-type SFCL.

To minimize the impact of dc faults in modular multilevel converter based multi-terminal high voltage transmission system (MMC-MT-HVDC, MMC-MTDC), HVDC circuit breakers (HVDC CBs) combined with superconducting fault current limiters (SFCLs) was a preferred choice. For MMC-MTDC with HVDC CBs and SFCLs, this paper firstly analyzed the dc fault characteristics based on the equivalent electrical circuit for different operation stages. Then, this paper introduced the basic principle of resistive SFCLs and inductive SFCLs, and their effect in limiting the dc fault current. Based on the main purposes for installing SFCLs, this paper proposed a dc side main circuit parameters selection method for determining the parameters of the smoothing reactor and the SFCLs. The case studies are performed on a ±500 kV 3-ternimal MMC-MTDC system. The time-domain simulation results showed that both the resistive SFCLs and the inductive SFCLs could limit the dc fault current to a reasonable extend by selecting the proper parameters of the smoothing reactor and the SFCLs, and the energy stress on the voltage limiting element of the HVDC CBs is more severe when using the inductive SFCLs.

This paper addresses the stability and excessive fault current issues which arise due to the integration of distributed renewable energy sources with the main grid. The main focus of this work is to study the effects of the application of bridge type SFCL (Superconducting fault current limiter) on the suppression of excessive fault current and stability enhancement of solar integrated power system under fault condition. In this regard, we found scarce literature about the optimal position of SFCL for solar integrated power system. Thus, to bridge this gap, a typical power system with generation, transmission and distribution has been modeled in MATLAB/Simulink with solar power plant as a dispersed energy source. SFCL device has been connected at different positions while three phase fault has been introduced at three different locations. Results are evaluated to determine the best possible position of SFCL device according to fault current suppression and PV stability requirements. Consequently the strategic position of bridge type SFCL in solar connected power system is proposed and its effective performance is presented.