With the increase of HVDC feed power, the "hollowing" of receiving-end power grid poses a serious threat to the transient voltage security. Therefore, a transient voltage security preventive control method for HVDC receiving-end power system is proposed in this paper. The preventive control method is decomposed into three sub problems: transient voltage security assessment, sensitivity analysis and optimal control. A transient voltage severity index suitable for sensitivity method is used to construct transient voltage security constraints. The accuracy and efficiency of the solution are improved by limiting the effective control interval of sensitivity and selection of control set. The method has been applied in Henan power grid. The application results verifies the effectiveness of the proposed method.

Aiming at the limitation of using trial-and-touch method which lacks of theoretical basis in setting the proportional integral (PI) controller parameters of HVDC transmission system with line ommutated converters, this paper proposes a PI controller parameter setting method. Firstly, the linearized state space model of HVDC transmission system is established, and then the linearized transfer function expression with PI controller parameter variables is obtained. Next, the influence results of PI controller parameters on time domain performance and frequency domain performance are obtained. The setting range of controller parameters considering only time domain performance index or frequency domain performance index is obtained. Then, using these two results, the time-frequency domain performance correspondence of the system is given. Finally, the stability margin for HVDC control parameter setting is established.

The constant active power control mode of MMC-HVDC system is based on the power order sent by dispatching operators, and cannot adjust the transmission power according to the status of AC system. According to the characteristics of power transmission in AC lines, an asynchronous automatic control strategy for MMC-HVDC system is designed, which enables the MMC-HVDC system to automatically adjust the transmission power and realizes the function of simulating AC tie lines. The basic principles of asynchronous automatic control strategy, the detailed value-taking method of virtual impedance, and the scheme of function on/off are elaborated. Then, the simulation model based on actual AC power network is built on RTDS platform according to the control and protection devices of practical MMC-HVDC project. Finally, the effectiveness and feasibility of the proposed control strategy is verified.

The valve cooling system is a key part to ensure the safe operation of the converter station. Its function is to discharge the heat formed by the components in the valve body, so the accurate evaluation of its cooling capacity is very important. Aiming at the above problems, a bidirectional interval constrained normal cloud model based on ISSO-PP combination weighting is proposed to evaluate the cooling capacity of the converter valve. Firstly, the collected data is preprocessed and the cooling capacity evaluation index system of the valve cooling system is determined. Secondly, AHP and improved CRITIC method are used to calculate the subjective and objective weights of evaluation indicators. And the comprehensive weight is obtained by the improved shark-optimized projection pursuit method （ISSO-PP）. Finally, a forward cloud digital feature calculation method with bidirectional interval constraints is proposed in this paper, based on the data grade evaluation standard as the intermediate type. At the same time, the reverse cloud digital feature calculation method is improved, and the final certainty degree is obtained by combining with the comprehensive weight calculation. The final evaluation result is determined according to the principle of maximum certainty degree.

After the pole-to-pole short circuit fault of DC region distribution system, the power electronic converter needs to be quickly locked, which seriously affects the reliability of power supply and causes the difficulty of low-voltage area fault location. In this paper, a mechanism of no locking VSC converter and DC/DC current limiting after fault is proposed, to solve the problem of fault area location, the fault characteristics of pole-to-pole short circuit fault in low voltage area are studied. On this basis, the fault characteristics of pole-to-pole short circuit fault in low-voltage area are studied. The result shows that the shortcircuit current, voltage drop and fault development are obviously different in different voltage levels. Taking advantage of the difference between inside and outside fault zones, a fast fault region location method based on the difference of fault characteristics is proposed. The simulation model of DC regional distribution network is set up in the simulation software PSCAD, which verifies the feasibility of the fault location method proposed in this paper.

Online evaluation of the ability of power system to resist power disturbance is of great significance for power dispatcher to understand the power grid frequency stability situation in real time. In this paper, a low-order simulation model for frequency security analysis based on dynamic characteristic node clustering is proposed for large-scale interconnected synchronous systems with obvious time-space distribution characteristics. Based on the electromechanical transient time-domain simulation results of contingency fault set, the coherency identification and clustering of node dynamic frequency characteristics are carried out. Based on the single machine equivalent model and frequency lead-lag characteristics of each coherency node set, a low-order simulation model for online analysis of frequency security is established, which can be used for online fast and refined evaluation of frequency security after contingency fault set. The accuracy and effectiveness of the proposed method are verified by an actual power grid.

False data injection attack is one of the important factors that threaten the safe and stable operation of power grid. The research on attack strategy and the in-depth analysis of attack mechanism can provide a basis for improving system defense strategy. Aiming at the state estimation process in the smart grid, the attacker can calculate the attack vector that satisfies the power flow constraint when he has certain resources and the attack conditions, so as to avoid bad data detection. Compared with the DC state estimation model, the attack conditions are more severe and the success rate is relatively low under the AC state estimation. This paper considers the situation that the attacker does not know the exact state variate under the AC state estimation. Firstly, the state variate is approximately calculated according to the known measurements and parameters, and then the attacker finds out the leverage measurements by the projection statistics and the hat matrix diagonal element method. Finally, an optimization model is established to attack the leverage measurements, and simulations on IEEE 14-node system verify the effectiveness of the proposed attack method.

As the construction of new power system gradually takes shape, the trend of source-load separation is becoming more and more obvious and its randomness is increasing, making the problem of voltage stability increasingly prominent. Under the new circumstances, the grid urgently needs a voltage stability assessment method with high accuracy, fast response speed and good extensibility. The static voltage stability assessment problem is defined as regression problem and artificial neural network is constructed to assess the problem online. Firstly, the training sample set is obtained by scenario simulation, power flow calculation and local voltage stability index calculation. Then the RReliefF method is used to sort the features and eliminate the attributes with low weight to improve the training efficiency. Then the mapping relationship between key features and voltage stability is obtained by artificial neural network training. Finally, taking the modified IEEE39-node system as an example, six groups of experiments are set and a simple linear weighting method is introduced to calculate a comprehensive evaluation index about the speed and accuracy of the model to verify that the proposed method has ideal modeling speed and high accuracy, and can meet the requirements of voltage stability assessment of power system under the new situation.

The problem of inrush current becomes more prominent by changing the arrangement order of the inner windings of the transformer with built-in high voltage winding. The zero-sequence over-current protection of bus tie circuit-breaker is faced with a high risk of maloperation because of the high amplitude zero-sequence inrush current due to the unloaded transformer energizaiton, which threatens the safe operation of the power grid. Combining with the setting principle of zero-sequence overcurrent protection of bus tie circuit-breaker, the maloperation mechanism of the protection under inrush current conditions of unloaded energization of the transformer with built-in high voltage winding is revealed in this paper. Based on the difference of waveform characteristics between zero-sequence inrush current and fault zero-sequence current, and considering the influence of saturation of current transformer (CT), the restraint criterion of zero-sequence overcurrent protection against maloperations applying the variation characteristics of symbol sequence of zero-sequence current is proposed. The maloperation mechanism of the protection and the validity of the proposed criterion are verified by simulation tests.

The power supply mode of closed-loop design and open-loop operation is usually adopted in the distribution network. In the actual power grid, the voltage amplitude difference and phase difference of the loop closing point of two transmission lines are often large. If the switch is directly switched on or off, it will bring large opening/closing shock. In serious cases, it will even lead to misoperation of the protection device and overcurrent of the transmission line, which will affect the safe and stable operation of the distribution network. In this paper, a loop closing control device and its control method for 10 kV distribution network are proposed. The phase shifting transformer and voltage regulating transformer are combined to adjust the voltage amplitude and phase of the loop closing point, so as to meet the technical requirements of no impact closing and low current opening in the process of line exit from maintenance and entry after maintenance of distribution network. Besides, the offline simulation model is built on the PSCAD/EMTDC simulation platform, and the detailed simulation test is carried out on the process of line maintenance exit and post-maintenance entry. Moreover, the hardware in-loop test platform of the RTDS with control device prototype is built. The test results show that the distribution network loop closing control device and its control method proposed in this paper can smoothly complete the maintenance exit and post-maintenance entry of the line, the simulink results of PSCAD are consistent with those of RTDS.

A lightning warning method based on atmospheric electric field features and deep learning algorithms is proposed to address the problem that the existing lightning warning method based on atmospheric electric field is ineffective. Firstly, the frequency domain features of the atmospheric electric field are extracted using ensemble empirical mode decomposition （EEMD）, and the above features are further dimensionally compressed by sparse auto encoder （SAE）, and combined with LSTM neural network to distinguish whether it is a thunderstorm weather or not. Finally, the effectiveness of the method is tested by using the lightning location data. The results show that the lightning warning method can predict lightning more accurately, and the highest accuracy of 81% can be achieved by using 45-minute time series of electric field. Compared with the traditional method, the proposed algorithm can be adapted for training and debugging for different regional conditions and has universal applicability. As the model runs longer and the amount of data increases, it can be updated and iterated to ensure the forecast performance is continuously improved. Thus, the method proposed in this paper has reference value and indicative significance for further improving the lightning protection level in Guangzhou.

The spatial electromagnetic field and transient ground potential rise level of the secondary equipment are measured. The AD7656, AD7606 and AD92883 analog-to-digital (A/D) converter chips commonly used in the electronic transformer acquisition unit boards are selected as the objects to study, and the analog boards for immunity test of these chips are developed. The electrostatic discharge and surge immunity tests are carried out, the severity degree of device damage is judged, and the failure characteristics of different chips are summarized. Electromagnetic compatibility (EMC) protection measures are added layer by layer on the circuit board, and the inhibition effect of different protection levels on various types of interference is obtained.

The wind power forecast error leads to the reduction of the operating economy of the power system and the increase of the wind farm assessment cost. An optimization strategy for wind farm power reporting based on minimization of assessment power is proposed. Firstly, the improved K-means algorithm based on the differentiation of distribution function is used to establish the probability distribution model of forecast error with obvious differences under different meteorological modes. Secondly, the support vector machine is used to realize the meteorological mode recognition, and then the Latin hypercube sampling method considering the time correlation is used to generate the day-ahead power curve scenario set. Finally, an optimization model of the day-ahead power reporting with the goal of minimizing the expected value of the assessment power is established, and the particle swarm algorithm is used to solve it. Taking the historical data of a wind farm in central China as an example, it is verified that the proposed method can effectively improve the operational economic benefit of wind farm compared with the strategy of directly reporting the forecast wind power.

Flexible DC transmission technology based on modular multilevel converter has gradually become the best grid connection scheme for large wind farms, and the system has the risk of broadband oscillation. In order to suppress this kind of oscillation, this paper calculates the dominant oscillation mode of the system based on eigenvalue analysis method, and further quantitatively analyzes the dynamic influence of various parameters of the system on the oscillation instability modes. It is concluded that the damping ratio of the system can be effectively improved by adjusting the control parameters of the grid-side converter of the wind farm, and a control parameter optimization algorithm of the grid-side converter of the wind farm based on eigenvalue sensitivity is proposed. In order to further reduce the oscillation risk of the system and make the system adapt to various operation conditions, a virtual resistance is added between the wind farm and the DC converter station, and an additional damping controller of the DC converter station sending-end converter based on the virtual resistance is designed. The time domain simulation model of the system is built based on PSCAD / EMTDC. The simulation results show that the compound oscillation suppression method combining the control parameters optimization of grid-side converter and the additional damping controller based on virtual resistor has better damping effect.

Mountain wind farms are limited to topography, wind energy distribution and other factors. They usually have a complex collector structure and diversified electrical connections, which leads to cumbersome harmonic modeling of wind farms and difficult harmonic resonance analysis. Therefore, based on the actual mountain wind farm topology, the current collection structure is divided into three types, and the transfer function model is established to systematically study the coupling resonance phenomenon and the mechanism of mountain wind farms. Firstly, it reveals that internal resonance, inner resonance, external resonance and cross-regional resonance widely exist in mountain wind farms. For the three collector structures, the control resonance band, the filter resonance band and the superimposed resonance band are distinguished in a wide frequency domain. Then, the harmonic amplification caused by each resonance band in the above four resonances is analyzed, and it is found that in the cross-regional resonance, the resonance frequency band is broadened and tends to high frequency. Further considering the change of the harmonic propagation path, the harmonic conduction coefficient is defined to describe the amplification mechanism of the coupled harmonics propagating through different paths. Finally, simulations verify the correctness of the theoretical analysis and modeling.

The high-proportion new energy connected to the grid will aggravate the pressure of power system frequency regulation. Participation of energy storages such as lithium batteries and supercapacitors in frequency regulation can effectively improve the frequency regulation performance and increase the new energy accommodation. Starting from the unbalanced components of the power system such as the net load fluctuation and the forecast deviation, and considering the complementarity characteristics of the wind and photovoltaic power forecasting errors, based on the variational modal decomposition and Monte Carlo simulation, a method for measuring the frequency regulation capacity and rate demand of the power system is proposed. The probability distribution statistical modall and the frequency regulation cost-benefit modall are used to evaluate the demand for hybrid energy storage, and the relationship between the frequency regulation capacity of the power grid and the demand for hybrid energy storage under different new energy installed ratios is compared. A provincial power grid in China is taken as an example to verify the effectiveness of the proposed method.