Relay protection rejection and misoperation exist in the existing distribution network, which will affect the fault diagnosis results. To diagnose faults in distribution networks, this paper presents a fault diagnosis method for the distribution network based on the D-S evidence theory Bayesian network. First, the collected relay protection information is divided into two categories, protection information and circuit breaker information; the corresponding Bayesian network model is established based on their respective action logic, and the corresponding component failure probability is obtained by Bayesian backward inference. Second, the fault probabilities obtained from the two Bayesian networks are fused by the D-S evidence theory, and the obtained fault probabilities are used to diagnose the faulty component. Then, using the Bayesian network corresponding to the faulty component to perform Bayesian forward inference, the protection devices and circuit breakers are identified for misoperation or rejection to achieve the fault diagnosis of the distribution network. Finally, the correctness and reliability of the proposed diagnosis method are verified through the analysis of arithmetic cases.

In view of the misoperation of the inter-turn protection of high voltage shunt reactor(HVSR) connected with submarine cable in offshore wind power project during the conventional opening and closing operation, a comparative analysis of the inter-turn protection principles of different manufacturers and analytical analysis of the misoperation process of the operation sceneis are carried out. Based on the analytical analysis, two scenarios of misoperation are summarized, namely, the zero-sequence source generated by the saturation of the iron core during closing is judged to be mistaken as an internal fault, and the measurement impedance decreases due to the increase of the current amplitude during the resonance period. The simulation reproduces the recorded waveform of the scene, and the correctness of the analytical analysis is verified. Finally, some countermeasures and suggestions are given, such as increasing appropriate harmonic restrain criteria, increasing resonant frequency identification criteria, and the break near reactor should close first and then open last during operation.

For the Inverter-Interfaced Distributed Generator (IIDG) access system, a ground fault occurred on the 110 kV contact line, and the zero-sequence current distribution on both sides of the line was analyzed by combining the characteristics of the controlled current source of the IIDG after the fault, the relationship between the sequence quantities, the neutral grounding method of the transformer and the access capacity of the IIDG. Factors. The analysis points out that the IIDG access side transformer neutral grounding mode determines the zero sequences current distribution on both sides of the line; when the system side circuit breaker is disconnected, the IIDG access capacity mainly affects the zero sequences current distribution, and the local load power level has a small effect on the zero sequence current distribution. The correctness of the theoretical analysis is verified by PSCAD/EMTDC software simulation.

High voltage shunt reactor((referred to as HSR, the same below)) is an important equipment used to absorb reactive power and limit overvoltage level in power system. Its frequent fault is inter-turn short circuit of winding. The traditional inter-turn protection based on the principle of zero sequence component has been maloperation for many times on site. Based on the analysis of the structure and electrical characteristics of HSR, this paper defines the root cause of the maloperation of traditional inter-turn protection, and puts forward a new type inter-turn protection scheme for HSR, which composed of core saturation criterion, zero sequence power direction criterion and zero sequence impedance criterion. The experimental and operation results show that no matter whether the core of HSR is saturated or not, this scheme can ensure that not only not operate reliably when air-drop without fault, but also operate sensitively and reliably when there is more than 2% inter-turn short circuit fault, so as to solve the problem of maloperation of traditional inter-turn protection due to core saturation.

In order to avoid the adverse effects of the access of renewable energy sources on automatic reclosing, reclosing is usually done by cutting out renewable energy power supply after the fault occurs and extending the reclosing setting time. But this will lead to the interruption of power supply to a large number of local loads, which is not conducive to the efficient use of renewable energy sources. In this paper, a new reclosing method for substation lines with inverter-type renewable energy source access is proposed. This method can keep the inverter-type renewable energy source running continuously during reclosing, and ensure the continuity of power supply of important loads. Firstly, it is pointed out that the control strategy of renewable energy source must be switched during reclosing, to make it have the basic island operation capability. Then, to maintain the power balance between renewable energy source and load, the method of using load shedding cooperating with unloading resistors during reclosing is proposed. Finally, the reclosing operation will be performed when the synchronization conditions are met. And simulation under different working conditions is carried out on PSCAD/EMTDC platform, which verified the feasibility of the proposed reclosing strategy.

As a line protection applied to various voltage levels, distance protection is of great value to ensure the reliable operation of the power grid. In this paper, taking a distance I section protection misoperation accident as an example, by analyzing its misoperation recording, the calculation principle is deduced, and the problems in the setting of the action area of the polygonal impedance element are pointed out. When the terminal fault is caused, the non-faulty phase-to-phase impedance falls into the action zone of the first section of the distance, causing misoperation. Therefore, a protection fixed value logic fixed value optimization scheme suitable for polygonal impedance elements is proposed, and an independent load limiting resistance limit condition is set for the distance I section to realize the self-adaptive optimization of the protection software. Finally, the fixed value verification and fault playback verification are carried out on the boundary of the optimized action area. The experiment proves the feasibility of the optimized scheme, which has great reference significance for improving the reliability of distance protection using polygonal impedance elements.

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.

For the automatic reclosing of power system under renewable energy source access, the current practice is usually to cut off the renewable energy source or block the reclosing gate after a fault occurs. This will lead to the shutdown of the renewable energy source and the interruption of power supply to a large number of local loads, which is not conducive to the safe operation of the grid and the efficient utilization of the renewable energy source. In this paper, a fast reclosing method is proposed for substation lines with inverter-type renewable energy source access, which can ensure the power supply of important loads and make the renewable energy source stable during the reclosing period without going off-grid. Firstly, it is pointed out that the inverter-type renewable energy source is switched to VSG-control during the reclosing period, so that it has the basic islanding operation capability. Then the method of load shedding with the startup of unloading resistor during the reclosing period is proposed to ensure the power balance between the new energy power and the loads. Next, adjusting the rated frequency of the renewable energy source to accelerate the simultaneous process, which can speed up the whole reclosing process. Finally, the simulations under different working conditions are carried out on the PSCAD/EMTDC to verify the feasibility of the proposed reclosing strategy.

Fractional frequency transmission system (FFTS) is a promising solution for integrating large-scale wind energy into power systems. In FFTS, the grid frequency is much less than the typical 50 Hz or 60 Hz. Therefore, the phase-locked-loop (PLL) designed for power converters used in traditional systems could not be suitable in FFTS. This paper proposes the design of dual second-order generalized integrator (DSOGI) based PLL, so as to achieve satisfactory dynamic and steady-state performance. The accurate small-signal model is firstly built. Then the optimal parameters are obtained to achieve fast dynamic responses with small transient oscillations. Simulation results have verified the superior performance of the designed DSOGI-PLL.

The short circuit test of distributed power flow controller (DPFC) is still a blank in the world. In order to test the effectiveness and adaptability of DPFC in case of line failure, it is necessary to carry out short circuit tests in DPFC projects. Based on the Hu Zhou DPFC Project, this paper proposes the control and protect strategy of DPFC which is coordinated with the protect strategy of AC line, designs single phase short circuit grounding test scheme of the AC line where DPFC is located. In the application, the action logic of DPFC under single-phase grounding fault is verified, the single phase short circuit test of DPFC were completed for the first time, and the transient voltage and current were obtained. It provides technical support for the popularization and application of DPFC project.

Distributed Power Flow Controller (DPFC) is a typical D-FACTS device for power flow control by distributed installation, which is installed on overhead transmission lines or substations. The voltage source converter (VSC) unit in DPFC is connected in series to the transmission line, which can adjust the line impedance continuously and quickly. DPFC obtains energy mainly through the AC line current. When the line current is too low, DPFC cannot obtain energy reliably. In this paper, the control strategy of DPFC under low AC line current condition is proposed to prevent the DPFC from losing control under low current condition to protect DPFC equipment. The rationality and engineering applicability of the proposed strategy are verified by experimental results of Hu Zhou DPFC project.

In an offshore flexible low-frequency power transmission system based on a modular multilevel matrix converter (M3C), ensuring that the module capacitor voltage follows a given value is a prerequisite for the stable operation of the system. In order to study the capacitor voltage control strategy, this paper first established the M3C mathematical model based on double alphabeta transformation, and designed the input and output dq transformation vector controllers and circulating current controllers, and then analyzed the power of the M3C bridge arm. For the frequency characteristics of the bridge arm power, the notch filter is used to separate the AC uncontrollable quantity and the DC controllable quantity, so as to obtain the expected value of the circulating current to control the circulating current. Finally, a model of the M3C converter station based on the nearest level approximation modulation is built for simulation verification. The simulation results verify the frequency characteristics of the bridge arm power, and the proposed control strategy controls the capacitor voltage within ±10% of the average value.

The modular multilevel matrix converter (M3C) based low frequency alternating current (LFAC) system is an attractive solution for the flexible interconnection in urban power grids. However, M3C stations show negative damping in certain frequency bands due to their fast control system dynamics. It is necessary to analyze the resonance stability when M3C stations are connected to the power grid. This paper aims at the resonance stability analysis of Hangzhou M3C based LFAC system. Firstly, port impedance models are built for both power frequency sides and low frequency sides by frequency domain scanning. Secondly, the resonance modes in the normal condition and some extreme conditions are calculated through the s-domain admittance matrix (s-DAM) method. The analysis indicate that all resonance modes have positive damping and no resonance instability risk exists in Hangzhou LFAC system.

With the continuous increase in electricity demand and the common problems of centralized FACTS, such as high cost, large operating loss, and low reliability. The Distributed Power Flow Controller (DPFC) technology, which has the advantages of cost-effectiveness and reliability, is developing rapidly, but the DPFC in the current research only works in a single control mode. This paper introduces the basic principles of DPFC multi-mode operation based on typical systems, studies the control principles and control ability of DPFC under different control modes; proposes the multi-mode control strategy of DPFC and presents the electromagnetic transient model of DPFC based on PSCAD/EMTDC, the simulation verifies the validity and reliability of the DPFC multi-mode operation strategy.

With the increase of the load, the proportion of receiving power and the proportion of new energy generation, the imbalance of power flow which is the mismatch between the line power flow and the current-carrying capacity becomes more and more serious in the process of power grid operation, resulting in the low operating efficiency of power grid assets. The Distributed Power Flow Controller (DPFC) is adopted to control the power flow, which has advantages of small size, light weight and low cost, etc. This paper introduces the working principle and development trend of DPFC and the configuration of DPFC in Gan Quan–Xiang Fu 220 kV line in Huzhou. Finally the simulation results is given to prove that DPFC can cooperate with AC line to operate stably.

Compared with HVAC or HVDC, low frequency AC (LFAC) technology demonstrates specific advantages on the offshore wind farm integration. Firstly, this paper discusses the selection principle of the upper limit of operating frequency of low-frequency system. Then the current along the submarine cable is calculated by MATLAB. According to the simulation results, the maximum transmission frequency of the cable line is determined as 20Hz. On this basis, taking into account the transmission capacity and difficulty of equipment manufacturing, 20Hz is finally selected as the optimal transmission frequency of offshore wind farm based on LFAC, which provides guidance for future designs of equipment and demonstration engineering.

This paper presented a method of fault-tolerance for secondary-system information in smart substations. It was to solve the problem of information redundancy. This article drew on a method of state estimation on the primary-system and used the calculation of measurement-equation residuals to identify and eliminate bad data to complete the information fault-tolerance function. The diagrams illustrated the process of information interaction in the secondary-system, and introduced the configuration mode of relay protection in smart substations. Through simulation and verification, using the method of residual identification to perform protection fault-tolerance can effectively prevent the protection against misoperation caused by bad data, improve the credibility of data, and promote the reliability of the protection.

Nowadays interconnections of non-synchronous AC grids with VSC-HVDC links become attractive in preventing cascading outages. To share the spinning reserves, the VSC-HVDC link is expected to balance the frequencies of the interconnected grids under tolerable disturbances. Based on the synchronous generator emulation control (SGEC) strategy, this paper presents a communication-free scheme for the frequency regulation of interconnected grids with a VSC-HVDC link. Under a tolerable disturbance, the scheduled powers of the rectifier and the inverter are changed to their actual powers, respectively. As a result, the grid frequencies could be balanced and the adjacent grid will provide frequency support to the disturbed grid. If the disturbance becomes larger, the scheduled powers of the two converters are changed until the maximum or minimum limits and the frequency support to the disturbed grid is limited. Thus, the large disturbance results in a deeper frequency drop in the disturbed grid, which could be arrested by its under-frequency load shedding. As a result, the disturbance propagating to the adjacent grid is suppressed, which does not jeopardize security. The frequency regulation scheme coordinates well with the automatic generation control (AGC) of the grids. Time-domain simulation studies are performed in the PSCAD/EMTDC software environment.

The voltage source converter (VSC) station is playing a more important role in modern power systems, but the dynamic behavior of the VSC station is quite different from that of the synchronous generator. This paper presents the synchronous generator emulation control (SGEC) strategy for the VSC-HVDC station. The SGEC strategy is divided into the inner control loop and the outer control loop. The inner controller is developed for fast current and voltage regulations. An inertia element is introduced into the frequency-power droop to determine the command reference of the frequency, and the inertia response and the primary frequency regulation are emulated. In addition, the secondary frequency regulation can be achieved by modulating the scheduled power in the SGEC strategy. The time-domain simulation results demonstrate the VSC station with the proposed control strategy can provide desired frequency support to a low-inertia grid. Therefore, the SGEC strategy provides a simple and practical solution for the VSC station to emulate the behavior of a synchronous generator.