Due to the traditional control strategy used in modular multilevel converters and power electronic transformers, it is difficult to ensure the dynamic performance of the system during operation, which further increases the difficulty of control when the power grid is in imbalance. In order to improve the dynamic performance of the system, a nonlinear feedback linearization sliding mode composite control strategy is proposed. This composite control strategy combines the advantages of feedback linearization control （FLC） and sliding mode control （SMC） to achieve complementary advantages. Firstly, an overall simulation model for modular multilevel converters power electronic transformers （MMC-PET） is established. Secondly, a control model for feedback linearization sliding mode control is established through a mathematical model. Finally, a simulation comparison experiment is conducted using a simulation and experimental platform to compare the proposed composite control strategy with a single feedback linearization control, sliding mode control, and commonly used PI control, in order to verify that the proposed control strategy has good dynamic performance and robustness under external disturbances.

When high voltage direct current （HVDC） transmission system works in mono-polar operation mode, DC bias will occur in the transformers near the grounding polar. The installation of a blocking device in a single station may cause new DC bias in the other adjacent stations. Considering the high installation and operation cost of the blocking device and the difficulty of substation transformation renovation, it is unreasonable to install the blocking device in all substations around the DC grounding electrode. Therefore, the installation strategy of DC bias blocking devices based on particle swarm optimization （PSO） that considers the cost and the treatment effect is proposed. Firstly, an optimization model of a capacitor DC blocking device is established with the installation and maintenance cost and the weighting of the transformer neutral point current as the optimization objective function, and the DC currents at all transformer neutral points are not exceeded as the constraint condition. Finally, the particle swarm algorithm is used to obtain the optimal installation strategy. To verify the effectiveness of the proposed installation strategy, the actual parameter model of the Jinhua Power Grid is constructed, considering different parameters such as the comprehensive costs and importance. The results show the installation strategy of DC bias blocking devices based on the PSO algorithm can quickly and effectively obtain the optimal configuration scheme of the blocking devices of all the substations in the regional power grid.

Wudongde hybrid multi-terminal UHVDC is nowadays the world's highest voltage and largest capacity flexible DC transmission project. It has the ability of non-blocking DC fault ride through. However, the project is a mixed structure of full bridge and half bridge.When the AC circuit breaker of uncontrolled rectifier asynchronous charging is not closed, the full bridge modules of the valve group are charged passively, and the DC port of the valve group presents induced voltage. In some connection modes, the induced voltages are very high, which cause the system to trip. Aiming at the problem of asynchronous charging, the partial voltage characteristics of valve group asynchronous charging from several main connection modes and the influence of induction charging on the system are analyzed. The method of isolating valve group by DC short circuit charging is proposed to solve the problem of valve group partial voltage, and according to the characteristics of asynchronous charging, the method of shortening asynchronous charging time interval is proposed to solve the problem of valve group partial voltage.

In order to improve the economy of lightning protection transformation of 10 kV overhead distribution lines, a joint simulation calculation method for lightning stroke characteristics based on MATLAB and ATP-EMTP is proposed. Based on the actual lightning protection configuration and terrain characteristics of the line, the renovation effect of each differential lightning protection strategy and the change in lightning stroke tripping rate per unit ground flash density are analyzed. A lightning protection strategy evaluation model is proposed based on the calculation results of trip rate and the economic cost of renovation, to comprehensively evaluate each differentiated lightning protection strategy. The results indicate that the lightning protection strategy of adding insulators throughout the line is the most cost-effective, but the transformation effect is limited and only applicable to areas with low lightning risk; The lightning trip-out rate of the line can be reduced by 99 % by installing lightning arresters on the whole line, which is suitable for high lightning risk areas, but the renovation cost is relatively high. Taking the actual 10 kV distribution line as an example, the optimal lightning protection transformation schemes for all the high-risk line sections are given by using the evaluation model proposed in this paper.

According to the design parameters of air core reactor with equal high and heat flow, a three-dimensional fluid field-temperature field coupling simulation model is established, and the detailed temperature field and flow field distribution characteristics of reactor are obtained.The simulation results show that the temperature rise of each encapsulated coil in reactor is basically the same. The heat dissipation characteristics between the coil and the air duct are obtained by extracting the temperature and velocity distributions along the axial direction of the encapsulated coil and the air duct. On this basis, the internal encapsulated coil-air duct unit is selected to reflect the overall temperature rise of the reactor, combined with the experimental correlation of vertical pipe convection heat transfer, the reactor temperature rise calculation method is deduced. Based on the inductance conservation and structural equation of reactor, the linkage of the amount of reactor metal conductor with the overall shape proportion of encapsulated coil，air duct width and encapsulated number is constructed. The influences of all structural parameters on the amount of reactor metal conductor are also analyzed. Taking the amount of reactor metal conductor as the optimization objective, the optimal structural parameters of the coil are obtained by multi-island genetic algorithm.The optimization results show that the optimization method can significantly reduce the amount of metal conductor under the condition of ensuring the constant inductance and temperature rise of reactor and meeting the requirements of loss, which provides important guiding significance for the optimal design of reactor.

The frequency domain response method has been preliminarily applied to the daily maintenance of cables by the virtue of its sensitivity to the weak defect of the characteristic impedance change of cables. This paper firstly briefly introduces the basic principle of the frequency domain response method, and then sorts out the research progresses of the frequency domain response method based on broadband input impedance spectrum and reflection coefficient spectrum and their improved localization algorithms. On this basis, the technical problems existing in the field application of the frequency domain response method are summarized and analyzed, and the research and application of the new defect localization method time-frequency domain reflection method in improving the sensitivity of cable defect localization are introduced. Finally, the future extension research direction of the frequency domain response method is pointed out.

As the proportion of wind power generation and photovoltaic power generation continues to increase, the inertia of the power system and the conventional reserve capacity for frequency regulation are decreased. This causes power system frequency stability to be prominent. From the point of view of frequency stability constraints, this paper studies the evaluation method of grid renewable energy carrying capacity based on the frequency response model. Firstly, the frequency response model of the power system including renewable energy power generation and other frequency-regulated power sources is established, and the validity of the model is verified by electromagnetic transient simulation. Then, taking the minimum frequency （fNadir) and the maximum rate of change of frequency （RoCoF) as frequency stability constraints, a method for evaluating the carrying capacity of grid renewable energy based on the frequency response model is proposed. This method can construct the stability domain of the power system frequency according to the proportion of renewable energy generation, and evaluate the carrying capacity of renewable energy generation by the power system. Finally, the effectiveness and applicability of the method are verified through the application of the proposed method in evaluating the power grid renewable energy carrying capacity considering the frequency stability constraints at the system level and assessment of new frequency control technology to improve power system frequency stability.

Aiming at the uncertainty of random factors of load level in the power system, a probability analysis method for small disturbance stability of the power system based on the unscented transformation method is proposed. The method transforms the probability analysis problem into a deterministic analysis problem through a symmetric sampling strategy, and then calculates the probability index of small disturbance stability according to the weight distribution of each sampling point by the unscented transformation method. The example analysis is verified based on the power system analysis toolbox (PSAT) 3-machine 9-bus system in MATLAB. Then, the probability analysis of small disturbance stability for a 10-machine 39-bus system is performed. The validity and practicability of the proposed method are verified by comparing the unscented transformation method with the two-point estimation method. The simulation results show that the relative errors among the unscented transformation, Monte Carlo and two-point estimation methods are small, but the unscented transformation method takes much less time than the Monte Carlo method, which enables the rapid evaluation of small disturbance stability probability analysis and meets the requirements of engineering applications.

There have been many literature comparisons between synchronous condensers and STATCOM in various aspects. Contradictory views on response times do exist at the same time because their definitions and corresponding calculation methods fail to reach agreement, and the influence factors of the power grid are not manipulated appropriately. Unfortunately, the response time is an important technical index, which can sometimes be dominant to determine whether the corresponding scheme becomes an engineering application candidate. Firstly the definition and corresponding calculation method are provided in this paper, then simplified and verifiable equivalent simulation systems are established in order to control the grid influence factors, step response times of a synchronous condenser and a STATCOM are compared in details through electromagnetic simulation in three separated fundamental factors. The results show that the synchronous condenser is slightly better than the STATCOM only in the step response of the grid voltage amplitude based on transient response time.

The uncertainty of electric vehicle charging demand and photovoltaic output leads to a certain degree of uncertainty in the revenue of photovoltaic energy storage charging stations. In order to quantify the impact of this uncertainty on the revenue of photovoltaic energy storage charging stations, namely the CVaR value of revenue, a multi-objective capacity optimization allocation model for of photovoltaic energy storage charging stations is established, including revenue and CVaR. The CVaR function in the model is the expected revenue of the investor quantified by conditional value at risk, and the expected revenue function is the sum of the revenue and its probability product under typical scenarios of charging demand and photovoltaic output, combined with augmentation ε- constraint method focuses on revenue as the main objective, with the expected revenue CVaR as the secondary objective as the constraint. The pareto frontier of expected revenue and expected revenue CVaR under different risk preferences are obtained by the solution model, as well as the corresponding allocation capacity, and the entropy weight TOPSIS method is used to screen out objective decision-making solutions. Compared with the linear weighting method in traditional risk management, the augumented ε-constraint method can obtain more distributed and boundary optimal frontiers after processing the objectives, which can map the actual pareto frontiers of multi-objective problems and provide more detailed investment schemes for dividing the expected revenue value and expected revenue CVaR.

Traditional black start is mainly undertaken by thermal power units and hydropower units, photovoltaic power stations and energy storage systems are considered to jointly undertake the black start task in areas rich in photovoltaic resources. Therefore, it is necessary to evaluate whether photovoltaic power stations can complete the black start and determine the appropriate energy storage configuration. The feasibility assessment method of photovoltaic black start is designed, and the energy storage configuration scheme based on the feasibility assessment of photovoltaic black start is proposed. Firstly,the demand analysis of the black start load to be started is carried out, and the photovoltaic feasibility evaluation index is proposed. Combined with the evaluation index and photovoltaic output analysis, the photovoltaic black start feasibility evaluation is carried out while removing bad data. Then, the photovoltaic scenes are divided, different photovoltaic energy storage control strategies are designed according to the characteristics of the scenes, and the concept of energy storage output reliability is introduced to design the configuration scheme of energy storage.Finally, through MATLAB simulation software, the feasibility evaluation method and the effectiveness of energy storage to improve the implementation of photovoltaic black start are verified. The results show that the proposed method can effectively improve the feasibility of photovoltaic black start.

Accurate photovoltaic power prediction is an important means to ensure the safe and stable operation of new energy power system. According to the output characteristics of photovoltaic system, a combined prediction method based on integrated clustering and improved Markov chain model is proposed. Firstly, the trend sequences representing periodicity and random sequences representing randomness are divided by integrated clustering. Secondly, the trend sequence is predicted by the Markov chain model improved by first-order difference processing, and the random sequence is predicted by the differential integrated moving average autoregressive model. Finally, through modeling the historical operation data of photovoltaic power stations in Jilin Province and Qinghai Province, the results show that the accuracy rate of the method based on integrated clustering and improved Markov chain model is 7.76% higher than that of the traditional Markov chain model, which verify the applicability of the model proposed in this paper under different output types and different geographical locations.

Existing electromagnetic transient modeling methods are mostly aimed at the simulation of small-capacity units under typical voltage sag conditions, and rarely take the considerations of comprehensive simulation and verification of different voltage disturbances at various operating points simultaneously. Based on the "black box" model of a certain type of large-scale wind turbines, the white-box electromagnetic transient modeling method is proposed to obtain main circuit identifications, control circuit identifications and model verifications. For the main circuit, a main circuit topology and parameters of a large-capacity full-power wind turbine, consisting of a typical half direct-drive rotary system, a neutral-point clamped parallel three-level converter and a protection circuit module are identified. For the control circuit, the neutral point potential balance controller and H/LVRT controllers and parameters are identified. Through the comparison with the H/LVRT response characteristics of actual wind turbine of different voltage sags and rises at various operating points, it is found that they are consistent with actural wind turbine, proving the effectiveness of the proposed modeling method.

In the context of Energy Internet, as the technical core and main terminal of integrated energy system （IES）, the penetration rate of electric vehicle （EV） has been rapidly improved, and the coupling between transportation and energy network has been enhanced. The charging and swapping facilities for IES with EV high penetration will become an important carrier and effective way to deepen the development of energy transportation integration system. Firstly, the energy transportation integration system is analyzed and described, and then the centralized charging and unified distribution swapping pattern for IES with EV high permeability is summarized from different dimensions such as theory, model, and practice. Secondly, the technical performance, key characteristics, key scientific problems and challenges of EV high permeability IES oriented switching network are summarized and analyzed. Finally, the paper looks forward to the problems that need to be solved urgently in the operation and planning of the power exchange network for IES under the high penetration load environment of EV in the future, which provides reference for future research.

As a distributed infrastructure and computing paradigm, blockchain technology has the advantages of decentralization, openness, and transparency, and it can support the construction of Energy Internet in the aspects of peer-to-peer transaction, privacy protection, trading market management, demand management and home energy management. With the development of blockchain technology, the combination of blockchain and Energy Internet is expected to become the core and focus of China’s energy system construction under the new infrastructure construction in the next stage. At present, pilot work of “blockchain + energy internet” has been gradually launched in China, and some periodical achievements have been made. However, it has encountered difficulties in large-scale promotion.Emphasis will be placed on the in-depth analysis of the “defects” of pilot projects, and the hidden dangers of current relevant works are identified based on practical cases. A series of countermeasures and relevant suggestions on the construction of “blockchain + Energy Internet” projects proposed can be used as a supplement to the research works in this field, providing reference for large-scale commercial applications and the formulation of relevant policies and standards in the future.

The development of electric vehicles （EV） plays an important role in realizing the dual-carbon strategy. It is of great significance to study the charging characteristics of EV and the orderly regulation of EV charging load. This paper proposes a spatial-temporal load modeling method of EV load considering user behavior decision. Firstly, the regret theory is introduced to simulate the user's charging behavior decision. Then, the user's travel characteristics are modeled by combining travel chain theory and regret theory, including the randomness and flexibility of electric private car travel behavior and the uncertainty of charging load in time and space. Finally, based on the statistic data of the US Department of Transportation 2009 NHTS resident travel survey, the spatial-temporal distribution model of EV charging load is established with Monte Carlo method. The simulation results verify the rationality and effectiveness of the proposed model in this paper.