HVDC technology has become an efficient transmission technology and is widely used in China power grids. However, when the AC section power of DC transmission is lost, overvoltage probelm is easy to occur. Based on the back-to-back VSC-HVDC project, overvoltage problem of VSC-HVDC is analyzed in the case of total power loss and single-phase loss operation of AC section. AC section power loss discrimination system architecture is proposed, the standard information of device is designed, and the power loss discrimination logic for single phase and three-phase AC line applying in LCC-HVDC and VSC-HVDC system is also developed. Finally, an actual fault event is demonstrated for the effectiveness and correctness of the AC section power loss discrimination system, avoiding the possible problem of overvoltage and equipment damage.

DC high speed switch (HSS) is the key equipment in multi-terminal DC transmission system. The power transmission capability and operation flexibility of multi-terminal DC system could be affected by the performance parameters of HSS. In order to develop ±800 kV HSS products with better performance parameters, this paper proposes the problems to be solved in long term DC withstand performance, small DC current breaking and DC arc withstand performance. In this paper, the 1 224 kV/1h wet withstand insulation capacity of the prototype is realized by combining the internal and external insulation simulation optimization and insulation verification of the prototype. Through the magnetohydrodynamic simulation of small current interruption, the temperature and pressure distribution in the arc extinguish chamber when HSS interrupts small DC current is obtained. A small DC breaking experimental prototype is trial produced, and the maximum breaking current of the prototype experiment reaches 350 A under 1 kV recovery voltage. Through the magnetohydrodynamic simulation of long duration arcing, the temperature and pressure distribution in the arc chamber during the 4 000 A DC current breaking process are obtained. The research in this paper lays a solid foundation for the development of ± 800 kV HSS products with better performance parameters, and is of great significance for improving the power continuity and the safe and stable operation of multi-terminal DC transmission systems.

Hybrid MMC consisting of full-bridge sub-modules (FBSMs) and half-bridge sub-modules (HBSMs) works as the HBSMs mode during normal operation. The charging behaviours of FBSMs and HBSMs are almost coincident. The loss distribution of sub-modules device is extremely uneven, eespecially the loss of IGBTs at the lower part of HBSMs is particularly prominent for MMC works as an inverter, which threatens the operation safety of the system. Thus, a loss distribution optimization method with the differential control of FBSMs and HBSMs is proposed. Firstly, the reason of loss imbalance is clarified by calculating the sub-module device loss. Secondly, to improve the device loss distribution, a full-bridge /half-bridge modules decoupling control method without affecting the external characteristics of the equipment output is proposed. Then, the influence mechanism of modules average switching function on the device conduction and switching losses is analyzed. Aming to reduce the IGBT loss in the lower part of HBSM, the optimal modules average switching function is derived. Finally, the feasibility of decoupling control strategy and the effectiveness of loss distribution optimization are verified by MATLAB/Simulink and PLECS simulation.

Aiming to enhance the damping of certain low frequency oscillation modes in voltage sourced converter based high voltage direct current （VSC-HVDC） systems, a coupling robust controller design method based on multi-input multi-output （MIMO） system model is proposed by utilizing the interaction between different control loops. Firstly, the optimal feedback signal is selected by using the dominant mode ratio （DMR） index and then sent to the controller. Then, the total least squares-estimation on signal parameters via rotational invariance technique （TLS-ESPRIT） is used to identify the MIMO system model. Finally, the mixed H₂/H_∞ robust control theory is applied to design the HVDC supplementary robust controller with regional poles placement. A simplified multi-infeed HVDC transmission system is simulated in PSCAD/EMTDC. Compared with the traditional lead-lag compensators without coordination design, the simulation results verify that the designed controller has better control effect and stronger robustness.

In order to study the ralationship between grid-forming （GFM） and grid-following （GFL） units that can support the stable operation of renewable systems, this paper firstly takes the single wind farm integrated system as an example to analyze the dynamic characteristics of the GFL and GFM units. Then, from the perspective of maintaining the oscillation stability of wind farms both in low frequency and subsynchronous frequency bands, the calculation principle of the GFM unit capacity to be deployed in wind farms is proposed. On this basis, based on the three-machine-nine-bus system, the feasibility of the stable operation of the 100% renewable system is explored, and the influence of the proportion and location of the GFM units on the dynamics of the system are studied. The results show that the subsynchronous oscillation （SSO） risk can be effectively improved by deploying a small number of GFM units in the wind farm, and the capacity of GFM units is mainly constrained by the low-frequency mode stability. Besides, in the 100% renewable system, the SSO due to inadequate support of GFM to the remote GFL units is the key constraint in determining the need for GFM units.

With the need of national energy transformation, offshore wind farms are developing rapidly. Submarine cable plays a key role in trans-sea networking, and its safe and stable operation is very important, which requires the corresponding monitoring technology. This paper summarizes the commonly used techniques for monitoring the submarine cable, including four aspects: temperature and capacity monitoring, strain monitoring, disturbance monitoring and routing monitoring, and the principle and engineering application are briefly described. Finally, the applications of domestic submarine cable comprehensive monitoring platform are exemplified. The development status of the key technologies of cable monitoring technology is summarized, and the development trend in the future is explored.

The faults at the ports of star common DC bus multi-port power electronic transformer (SBM-PET) are independent from each other and thus the SBM-PET has the characteristics of multiple operation state, but there is no mathematical analytical model that can effectively characterize its multi-state reliability. A SBM-PET multi-state reliability model is proposed and applied to the reliability evaluation of AC/DC distribution network. Firstly, the structure and working principle of SBM-PET are analyzed. The functional modules are divided according to the characteristics and fault influence range of each component module of SBM-PET. On this basis, considering the redundant configuration of components, the reliability model of each functional module of SBM-PET is established. Then, according to the outage of each functional module of SBM-PET, the multi-state reliability model of SBM-PET is established based on Markov process, the reliability indexes of probability, frequency and average duration time under different states of SBM-PET are solved, and the reliability evaluation model of AC/DC distribution network is established based on the solutions. Finally, the multi-state reliability of SBM-PET and the reliability of AC/DC distribution network are calculated by cases analysis, and the results verify the effectiveness of the proposed model.

Thyristor controlled phase shifting transformer (TCPST) has fast response characteristic, and can improve system stability. In order to realize the transient stability calculation of large-scale power system with flexible AC transmission system (FACTS) equipment, an electromechanical transient simulation model of TCPST is proposed in this paper. Considering the electrical part and control structure of TCPST, the mathematical expression of TCPST node power injection model is deduced through model equivalence and equivalence transformation. And the gear control model is implemented with supplementary damping controller. Furthermore, in view of the numerical oscillation problem caused by the discrete switching characteristics of gear in the TCPST simulation model, a suppression scheme using the filter to process the injected power is proposed. Finally, the electromechanical transient model is built by using the AC/DC power system calculation and analysis software, and the model is applied to the case of China Southern Power Grid. Through the comparison of the simulation results of the example system by DSP, BPA and PSCAD respectively, the correctness of the proposed transient model is validated, and the electromechanical transient modeling method provides reference for the simulation and modeling of other FACTS equipment.

During the process of the interactive frequency regulation between offshore wind farm and island microgrids, the sufficiency of the inertia level of island microgrids is in a time-varying state, which will seriously affect the stability of microgrid running after the faults, such as the tie-line interrupt. In order to ensure the safe and steady operation of island microgrids in the process of the interactive frequency regulation, this paper further proposes the interactive frequency regulation strategy which takes minimum inertia requirement into accout on the basis of the interactive frequency regulation strategy between offshore wind farm and island microgrids. Firstly, based on the national demonstration project of Wanshan island renewable energy microgrid in Zhuhai, the assessment system of minimum inertia requirement of island microgrids is built. Secondly, minimum inertia requirement of island microgrids is solved by genetic algorithm and particle swarm optimization（GAPSO）. The stability margin of the inertia supporting of the island microgrids is refined. The inertia control level of island microgrid on offshore wind field is constrained. Finally, the frequency response characteristics of island microgrid are analyzed under severe disturbance and faults by the example simulation.

In order to solve the problems of many boards and plug-ins, high failure rate, large size of devices faced by relay protection technology in intelligent substations, a chip-based protection design is proposed, and the architecture design of line differential protection and each functional module are developed based on ZYNQ platform. In the sampling value synchronization module, the Lagrangian quadratic interpolation method is chosen to resample and interpolate the sampling messages, and the effect of primary frequency change is eliminated by time scale correction; in the design of the Fourier transform module, the hardware advantage of field programmable gate array (FPGA) is fully utilized to ensure the speed and real-time calculation, and the function of the line differential protection is verified by simulation and dynamic mode test. The chip-based line protection device designed based on this scheme has been put into trial operation in a 500 kV substation, and it is in good operation.

This paper studies on the droop control of non-simultaneously cut-in inverters in islanded microgrid, which is aimed at achieving balanced power between inverters and voltage (frequency and amplitude) without static error by no communication. Firstly, this paper indicates an issue with non-simultaneously cut-in inverters based on current droop control scheme, i.e., unable to ensure balanced power and voltage without static error at the same time. Next, in order to ensure balanced power between inverters, a general rule is derived. And then an improved droop control scheme based on the rule is proposed. This scheme adopts a specific correction method of no-load voltage frequency and amplitude, which can control the voltage to be without static error and the output power of inverters to be balanced without communication. The conclusions of this paper have been verified by the final simulations, which demonstrate that the proposed droop control scheme reaches the expected effect and solves the issue with non-simultaneously cut-in inverters.

Fault location of DC distribution network is an important approach to troubleshoot faults and ensure line safety. Based on this, a fault location method of DC distribution network based on the initial value of current differential is proposed in this paper. Firstly, the differential equations of the fault current of the DC distribution network based on the voltage source converter are derived when the bipolar short-circuit fault and the unipolar ground fault occur. Then, the fault types are determined by the difference of voltage and current of positive and negative lines in bipolar short-circuit fault and unipolar ground fault. Secondly, the fault current information is extracted by the discharge process of the DC side capacitor of the voltage source converter, and the differential equivalent expression of the fault current is obtained according to the voltage drop on the current limiting reactor. On this basis, the mathematical model of fault location is obtained by the differential initial value equation of fault current derived simultaneously, so as to achieve fault location. Finally, a simulation model is built using the PSCAD/EMTDC simulation platform to verify the effectiveness of the proposed fault location method.

As the scale of distribution network expands and the operation environment is complex and variable, the single-phase disconnection fault of arc suppression coil grounding system becomes more and more frequent. Due to the compensation function of arc suppression coil, the electricity quantity is weak when the disconnection fault occurs, so it is difficult to monitor the fault and construct the protection scheme. Aiming at the single-phase disconnection fault of the feeders of arc suppression coil system, Based on zero sequence admittance, a single-phase fault protection method for feeder of arc suppression coil grounding system is proposed in this paper. Firstly, the characteristics of the zero sequence current at the beginning of the line and the zero sequence voltage of the busbar are analyzed, Using the ratio of zero sequence current to zero sequence voltage to obtain zero sequence admittance, it is pointed out that the zero sequence admittance phase of non fault lines is in the first quadrant, while the zero sequence admittance phase of fault lines is in the second or third quadrant. Then, the zero sequence admittance of the faulty line in the case of arc suppression coil overcompensation is improved by multiplying the zero sequence admittance phase of the line by the corresponding symbol coefficient, which expands the difference between the zero sequence admittance phase pairs of the faulty line and the non faulty line. Based on the phase characteristics of the zero sequence admittance, a fault protection criterion is established. Finally, the reliability of the proposed method is verified by PSCAD/EMTDC simulation results.

At present, the power grid companies of China basically adopt expired replacement and batch sampling rotation for smart meters, which causes a waste of human and material resources. This paper proposes a method for identifying and recommending that the operating error of a smart meter is out of tolerance based on the electricity consumption data of the metering automation system. Firstly, the topology and mathematical model for analyzing the operating error of the smart meters under the distribution transformer are present. Secondly, considering the ill-conditioned problem, a solution method based on Tikhonov regularization （Tik) and generalized cross-validation （GCV) to optimize the regularization parameters is suggested. Then, a sorting recommendation mechanism for identifying out-of-tolerance smart meters is proposed. Finally, it is verified with actual distribution grid data in a certain district and compared with other methods. The results show that the proposed method is practical and effective.

Based on the transmission system interconnected by Chongqing-Hubei back to back voltage source converter based HVDC (VSC-HVDC) and Longquan-Zhengping line commutated converter based HVDC (LCC-HVDC), an additional frequency coordinated control strategy of VSC-HVDC and LCC-HVDC is proposed. The strategy can improve the frequency stability of VSC-HVDC transmission network by reasonable switching frequency-power additional frequency controllers of VSC-HVDC inverter station and LCC-HVDC rectifier station. And a frequency-power additional frequency controller is designed for VSC-HVDC inverter station to further improve the frequency transient characteristics of the system. Aiming at the problems of reactive power fluctuation and frequent switching filters in LCC-HVDC rectifier station during primary frequency regulation, an additional reactive power controller for VSC-HVDC inverter station is designed to reduce the number of switching filters of LCC-HVDC and improve the voltage stability of power network during frequency regulation. Finally, an equivalent simplified model of practical grid is built in MATLAB/Simulink, and the effectiveness and adaptability of the additional frequency coordinated control strategy are verified by simulation.

In recent years, the audible noise of overhead lines has gradually become an important electromagnetic environment problem limiting the construction of lines. Therefore, this paper summarizes the research status of audible noise in AC and DC transmission lines, including the generation mechanism, measurement and evaluation of audible noise, and focuses on the analysis of the main differences and research difficulties between AC and DC lines. The results show that some progress has been made in the study of audible noise, but the corona effect of AC and DC lines is very different and not well understood. The main reason for the difference is the influence of the ionic electric field of DC lines. The analysis shows that corona cage can be used to study the corona effect of DC circuit in the future, and the key is to establish the correlation between corona cage and actual circuit in the synthetic field.