The excessive short-circuit current restricts the development of the power grid. By using the fast switch to dynamically change the system topology structure during faults, the short-circuit current flowing through the conventional circuit breaker can be limit under the switchable current range, but the application of fast switch will affect the relay protection. In this paper, the flexible short-current suppression technology using the fast switch is introduced firstly. Then, the influence of fast switch on line protection, differential protection, distance protection and zero-sequence protection is discussed, and the optimized protection strategies are proposed. Finally, the simulation result is verified in RTDS.

There are many hybrid transmission lines consisting of power cables and overhead lines. When a fault occurs, it is difficult to determine the exact location of the fault. In order to prevent further damage caused by reclosing when the cable faults, this paper proposes to build a differential protection for the cable line to realize the judgment of the internal fault of the cable line based on 5G communication technology. If it is judged that the fault is inside the cable line, then send the blocking signal to the relevant protection devices in the substations on both sides to block the function of reclosing, thereby preventing reclosing from cable faults. The fault discrimination algorithm of the system in different types of hybrid transmission lines and the key technology of the system are introduced.

During the energization of transformer with high-impedance (HT), the zero-sequence overcurrent protection (ZSOCP) often malfunction with the inrush current of primary winding (iP-inrush) shows a large unbalance. In order to clarify the cause of the misoperation and prepare for a solution, the imbalance characteristic is studied in this paper. The imbalance is visually displayed by dynamic phasor diagram. Through the method of numerical simulation, it can be obtained that the peak value of the zero-sequence current always appears near characteristic opening and switching angles (COSAs). At this time, the two phases are deep saturated and one phase is not saturated. The iP-inrushof the saturated phase is large, and that of the unsaturated phase is numerically the same as the circulating current (iD). According to the parameters characteristics of HT, the iDis small, so iPpresents a large imbalance. The research conclusions provide a theoretical basis for zero-mode inrush current suppression.

For the Y/D-connected transformer, if the circulating current in D-winding can be obtained to compensate the primary current, the accuracy of the inrush current identification method will be improved. At present, the constructing algorithm of the circulating current is still insufficient. The mapping relationship between circulating current and unbalanced quantity (zero-mode current, zero-mode voltage, and neutral point voltage) is obtained by deriving the transformer differential equation in this paper. A constructing algorithm for circulating current based on unbalanced quantity is proposed. In the algorithm, only the unbalanced quantity of the primary winding needs to be measured, and there is no requirement for the grounding mode and the system voltage. This method has strong applicability and high reliability. The simulations verify the correctness and effectiveness of the method.

The circulating current in the D-winding may cause primary current waveform distortion, and the reliability of the restraint criterion based on the typical magnetizing inrush current characteristics will be affected. The magnetizing inrush current with typical characteristics is the sum of primary current and circulating current. Using the circulating current to compensate the primary current can improve the reliability of the differential protection. When the phase is not saturated, the magnetizing inrush current is about zero. Therefore, the primary current of unsaturated phase can be replaced by the opposite of the circulating current. Based on this, an engineering practical calculation method for circulating current is proposed. In the method, the segmented primary currents are used to replace the circulating current. Phasor analysis is used to demonstrate the application effect of this method when remanence coefficients are different. The method is simple and practical, and has strong applicability and high reliability. Simulation and recorded waveforms have verified the effectiveness of the method.

The accurate leakage inductance value of the transformer primary and secondary side leakage inductance is the key to the theoretical and simulation analysis of the transformer inrush current transient characteristics. First, the transformer differential equation is deduced, and two calculation expressions for solving leakage inductance under system balance and unbalance are obtained. Aiming at the scenario where the circular current of the delta winding cannot be measured, a parameter acquisition method based on current coupling characteristics in unsaturated zone is proposed. This method takes advantage of the mutual coupling between the circular current of the delta winding and the non-saturated phase current of the primary side. First judge the unsaturated phase, and then use the opposite of the unsaturated phase current instead of the circulating current to solve the quasi-leakage inductance under the two expressions. The quasi-leakage inductance values in the unsaturated zone are almost equal, and the average value in this zone is used as the calculated value. Simulation data and field record data verify the effectiveness of the method.

To solve the failure operation of remote backup protection for adjacent stations caused by the substation secondary DC power supply loss, numerous secondary dc loss(SDL) protection schemes have been carried out. However, there are few researches to evaluate the influence of SDL protection on the reliability of the conventional relay protection system of the substation, which may cause certain risks to the engineering application. In order to solve the above problem, a reliability model for the conventional relay protection system, which contains links of sampling, protection device and tripping, is established based on the fault tree analysis method. On this basis, a reliability model of the novel relay protection system including SDL protection is also built. The sequential Monte Carlo algorithm is used in this paper to access the protection system reliability. It obtains the failure sampling time and repair sampling time of one state switch through simulated sampling. With numerous simulations, the normal working time and working times of the system can be counted, and the system's Mean Time Between Failures(MTBF) will be obtained. The simulation results indicate that SDL protection will not reduce the overall reliability of substation relay protection system. On the contrary, it can improve substation protection reliability in the case of secondary DC power loss. This paper provides powerful theoretical support for the engineering application of SDL protection.

Precise current tracking is one of the important techniques to ensure the compensation performance of shunt active power filter (SAPF). Repetitive control is favored for its superior steady-state accuracy, but it has the problem of poor adaptability when frequency changes. This paper proposes a fractional-order internal model based frequency self-adaption control strategy. Fractional-order theory is introduced and Lagrange interpolation algorithm is implemented to achieve the superimposed fractional models. Parameter of the fractional delay link is designed and frequency characteristics of different methods are compared. Finally, this paper's strategy is used on a SAPF prototype and experiments are carried out.