Abstract:With the construction of ultra-high voltage(UHV)AC/DC interconnected power grid, the advancement of new energy and the deepening of electricity market reform, significant changes have taken place in the operation of power grid in China, which makes higher requirements on the dispatching and control functions. Based on the integrated distribution and concentration deployment scheme of application functions, this paper gives an overview of the new features of the application function, i. e. , global, rapid and accurate, analyzes the technological support to realize such application functions, and points out the urgency of promotion basic work such as power grid model, operation data and management rules.

Abstract:The theoretical basis of traditional power system state estimation is the law of large numbers based on traditional statistics, that is, when the number of measurements approaches infinity, the estimated value approximates to the true value with 1 probability. However, the number of measurements in the actual system is limited and sometimes even small, there is no theoretic guarantee for the evaluation index and estimation precision of the traditional state estimation. In view of this situation, the paper introduces the concept of uncertain measure under the uncertainty theory system, and explains the difference and relation among the uncertain measure, uncertainty of measurement and measurement error in the state estimation. Furthermore, the normal measurement, abnormal measurement and normal rate are redefined under the uncertainty theory system. And a new evaluation index of state estimation result considering the normal rate and the measurement deviation degree from the true value is proposed and the rationality is proved.

Abstract:The concept and calculation method of the dimension of distribution system security region(DSSR)are proposed. Firstly, the DSSR dimension is defined when the number of both constraints and variables are considered synthetically. Secondly, a DSSR dimension calculation method based on the main transformers connection vector is proposed. The key point in this method is to define a vector to describe the connection relationship among the main transformers and the DSSR dimension can be obtained without listing the security region expressions. Finally, an example is given to illustrate the proposed method. The relation between DSSR dimension and the distribution network is acquired by the preliminary analysis. The condition where the number of boundaries reaches the extreme value is also obtained. For different scale of distribution networks, the DSSR dimension is mainly affected by the number of transformers, feeders and sectionalizing switches. For the same scale of distribution networks, DSSR dimension may be different, and it is mainly affected by the connection modes of feeders. The concept of DSSR dimension proposed can rigorously describe the scale of DSSR and its corresponding N-1 security problems, which provides a necessary foundation for DSSR theory.

Abstract:With the development of smart distribution networks, the soft open point(SOP), as a sort of promising electronic device in place of traditional tie switch(TC), can improve the operating flexibility of a distribution network. This paper analyzes the SOP control modes of an active distribution network with SOPs under normal and fault conditions. For the special case that an arising serious fault prevents some regions from the contact with the grid through switches, each of the referred regions is called an unconnected subnetwork, the rest regions that can contact with the grid are called connected subnetworks. Then a service restoration strategy to restore the black-out loads by cooperating SOPs with distributed generators(DGs)is proposed. The power-transferring capability of the connected subnetwork is taken into account, which can provide the corresponding constraints for the restoration of the unconnected subnetwork. The SOP operates in a Vf mode during the service restoration. The objective function consists of the minimum weighted total loss of power load, the least DGs out of the grid and the minimum network loss. A service restoration model for the distribution network with SOPs is developed. The optimization variables are the output voltages of tie switch and SOP. Moreover, the ant colony optimization and chaos algorithm are combined to realize the restoration strategy. Finally, the case studies are conducted on a single IEEE 33-bus system with an SOP and a double IEEE 33-bus system, which show the effectiveness of the proposed service restoration strategy in both recovery schemes and load fluctuation adaptability. The simulation results show that the restoration strategy can not only realize the maximum restoration of the unconnected subnetwork, but also ensure the normal operation of the connected subnetwork.

Abstract:Currently, a lot of distributed generators and household electric vehicles are connected to the grid. The increase in capacity and the variety of loads have made it impossible to support the new load demand by the original microgrid. It is imperative to establish a new microgrid system that can meet a variety of needs. Taking into account the comprehensive constraints and the orderly charging, this paper develops a multi-objective optimal planning model in AC/DC hybrid microgrid system with wind turbine generators, photovoltaic generation, energy storage battery and electric vehicle. In order to reduce the cost of configuration and the load fluctuation, CPLEX optimal programming has been used to solve the objective function. The case study compares the difference between the new AC/DC hybrid microgrid and the original microgrid. A study is made on the rationality of electric vehicle orderly charging as well as the influences of interaction between the two types of microcrids on optimized configuration. The results verify the effectiveness and feasibility of the approach proposed.

Abstract:The security and economic operation of the power system is impacted or even challenged by the disordered charging of large-scale electric vehicles, especially for the distribution system. Aiming at the defects of centralized optimization and control method and the fixed pricing strategy, a decentralized optimization strategy of ordered charge scheduling is proposed based on the Lagrange relaxation method. The traditional centralized charging optimization problem of electric vehicle charging station is decomposed into N sub problems(N is the number of electric vehicles demanding charging). The maximum profits of the charging station are used as the objective function in the optimization model. The constraints of power demand of users, charging time, capacity of transformer and time-of-use(TOU)price of charging station are taken into consideration. In order to verify the effectiveness of the proposed method, Monte Carlo method is adopted to simulate the charging demand of electric vehicles. The profits of charging station, the effect of clipping load peak and the calculation efficiency under the fixed price and TOU price strategy of the charging station in situations of ordered charging and disordered charging when adopting centralized and decentralized optimization respectively are simulated and analyzed. The simulation results show that the proposed method can significantly increase the profits compared to use the disordered charging and fixed price strategy. By contrast with the centralized optimization, the computational efficiency of the proposed decentralized optimization is much higher. Though the charging station using TOU price has the effect of filling valley, the effect is not too ideal in stabilizing the load fluctuation.

Abstract:Decision-making and risk assessment of electricity purchasing and selling business is the major activities of electricity retailers in electricity market. In order to enrich the electricity purchasing and selling business modes, the possible power purchasing and selling business are classified and combined to acquire the different business mode in the medium and long term and spot electricity market. Some random variables such as spot prices and electricity demand are simulated by the scenario method. The conditional value at risk(CVaR)of profit is used as the financial risk assessment indicator. A comprehensive model of risk decision and assessment for purchasing and selling business is built to maximize the comprehensive utility of the electricity retailer. The influence of different factors on the electricity retailers risk decision and assessment is analyzed with different business models. Three different power purchasing and selling business mode are presented as examples in this paper. The validity of the model is verified in the example 1 by analyzing the influence of the risk preference of retailer and different types of selling contracts on the selling behavior and the financial risk. Based on the power purchasing and selling business modes of example 1, example 2 and example 3, the influence of the unit cost and scale for the decision and risk of electricity retailers is assessed when respectively introducing medium- and long-term renewable power business and energy storage leasing business.

Abstract:To reveal the internal dynamics mechanism of blackouts in Chinese power grid from the macro and global perspective, the correlation of blackouts in Chinese power grid is analyzed. Firstly, based on the relative value and the R/S method, the power loss correlation of blackouts in Chinese power grid from 1981 to 2014 is researched. It is proved that the power losses of blackouts in Chinese power grid have the strong long-range correlation, to determine the direction of correlation, and to effectively measure the degree of correlation according to the Hurst exponent. Secondly, over time, the change of the power loss correlation of blackouts in Chinese power grid is analyzed. It is revealed that the positive correlation is becoming stronger while the degree of correlation is becoming higher than the power losses of blackouts in Chinese power grid. Finally, on the macro level, the trend of the absolute value continues to be increase and relative value continues to be decrease for the power losses of large blackouts in Chinese power grid in the next few years can be predicted.

Abstract:For the problem of the classified loads in different cells with different development degrees that may cause the unbalanced cellular load distribution liable to lead to undesirable effects on the accuracy of spatial load forecasting results, a method of spatial load forecasting based on the development degree of cell load is proposed. Firstly, the electric power geographic information system is established, on which cells are generated. And the basic information is integrated including the land use information, 10 kV feeder power supply area and classified load data. Secondly, the saturated load value of the total classified load density, and the development law of total classified load density are determined by the growth curve. Then the position each cell classified load density is in in the current year on the total classified load density development curve is found, as the development degree of the classified load density of each cell. Finally, the current year development degree and the total classified load density development law curve are used to forecast the size of the classified load density of each cell in the target year, then multiplied by the corresponding area, and the result is the cell load of the predicted value. An example analysis shows the correctness and the effectiveness of the method.

Abstract:Owing to the fact that the operating point on the de-loading power curve will shift when the actual rotor speed changed during the frequency droop control, doubly-fed induction generators(DFIG)cannot provide the desired primary frequency control ability. To solve this problem, firstly a novel low-order system frequency response model is established for distributed systems including DFIG and thermal generation unit based on the small signal analysis theory. And then the quantified formulas of the actual and desired primary frequency control abilities of DFIG are derived by using the proposed model. According to the derivation, this paper proposes that DFIG provide the desired primary frequency control ability after multiplying a reduction factor for the original set of droop coefficients. Finally, the capability of the proposed droop coefficient modification method is tested on an actual distributed system. The simulation results show that, with the proposed method, not only has the system quasi-steady-state frequency deviation reached the expectation level, but also the system maximum frequency deviation has further improved.

Abstract:Aiming at the difficulties of load transfer without power interruption between two distribution lines with 30° phase angle difference which are existed in a fair amount in present power system in China, a scheme of fast closing and opening loop is proposed to shorten the duration of loop current, and the sequence control of pro-assigned circuit breaker group is proposed to copy with the possible refusing action of circuit breaker. Taking the real network as case study, firstly, the loop current characteristics under the fault and normal condition are analyzed. The electrodynamic force can be ignored if the corresponding power system is not working in its maximum operation mode. Secondly, the allowable loop current time duration of main electrical equipment to bear the thermal effect is estimated as well as the maximum duration of over limit loop current considering fault scenarios without affecting relay protection. And the minimum loop current time duration will be selected as the maximum operation time window for sequence control of pre-assigned circuit breaker group. Thirdly, it is revealed that the minimum loop current can be obtained under the coordinated selection between angle of voltage on two sides of the closing circuit breaker and the time interval of closing-opening of pre-assigned circuit breaker group. Finally, the overvoltage effect due to the loop closing-opening operation has been studied. The whole studies provide a calculation support for the proposed scheme of high-speed and sequence control automatic loop closing-opening to finish the load transfer without interruption operation, and it is proved by the pilot real applications.

Abstract:As the further research of load transfer problem in high-voltage distribution network, the time-sequence of switch operation has great influence on the reliability of power supply and the safety of system. By analyzing the transfer patterns, a risk evaluating method is proposed based on the transfer principles and the risk measuring matrix is built up which is similar to the distance matrix in travelling salesmen's problem(TSP). The improved ant colony algorithm is used to solve the time-sequence problem of switch operation. Load transfer cases of high-voltage distribution network in a city are used to test the method, and several strategies have been obtained for different operational situations. By comparing with enumeration method and empirical method, the proposed strategies are proved effective.

Abstract:In concern of the instability of the complex transient power angle during the developing process of power grid, a concept of weak stability mode of transient power angle and its identification method are proposed to analyze and control the safety and stability of power system. The mechanism of weak stability mode evolving into dominant instability mode which is caused by the emergency control and its approximate necessary conditions are analyzed. Based on the theoretical analysis, a mathematical model of emergency control based on the transformation of system transient stability constraints is presented. Then a heuristic coordination emergency control strategy based on the dynamic sensitivity index is designed. The adaptability of emergency control algorithm to deal with the complex transient instability is improved and the automatic coordination control method to balance the multiple weak stability modes of power system transient power angle is realized. An example based on the actual grid is used to demonstrate the effectiveness of the presented mechanism of negative effects caused by the emergency control and the presented strategy search algorithm.

Abstract:The value set approach is a graphics-oriented method based on the characteristic polynomial of the system. It can be used to analyze the impacts of uncertain parameters on the system stability. This paper describes the implementation of the value set approach in the robust stability analysis of the large power system, including the treatment of large numbers, memory optimization, assessment and adjustment of system reduction, and application to Gamma-stability criterion. These techniques are vital for the actual application of the value set approach. Also the value set approach is compared with the widely used μ-based approach in terms of their robustness stability analysis, stability margin calculation, parameter impact analysis, conservativeness, and computation speed. Finally, the North China system is used to illustrate the implementation of the value set approach and test the two approaches, which reflects characteristics of the two approaches.

Abstract:The under-excitation limiter(UEL), as a limit part of the excitation system, has an important influence on the stability of the system. However, due to the unrational parameter setting of UEL, the positive damping effect provided by the power system stabilizer(PSS)is weakened after UEL acts. This causes the oscillation and even the serious fault of the generator frequently in the actual operation of the power grid. Based on the extended Heffron-Phillips model, a simplified model with PSS and UEL for excitation system is derived. The frequency response method is used to draw the Bode diagram of the open-loop transfer function of PSS and UEL. The frequency band of UEL and PSS is analyzed, and a specific parameter setting program of UEL is given, which can make UEL and PSS coordinated. Taking the data of Yunnan power grid for example, the single-machine infinite-bus system is developed to verify the correctness of the theory in simulation software of PSASP and MATLAB. The simulation results show that, after the UEL parameter is set by the frequency response method, the UEL action will not affect the output of the PSS, and UEL and PSS can cooperate to ensure the steady-state operation of the system.

Abstract:In the situation of load tension or intervention schedule, the transmission capacity of the high voltage transmission line is easy to be increased excessively, which will lead to the risk of overheating. However, the current overheating risk assessment method of the transmission line using aluminum conductor steel-reinforced cannot be directly used for the transmission line using aluminum conductor composite core(ACCC). Hence it is difficult to fully exploit the transmission capacity of the double capacity conductor, and the rate of return on investment is also reduced. The development of ACCC transmission technology is limited. In case of that, this paper proposes a method to assess the overheating risk of the high voltage transmission line using ACCC. Firstly, the thermal circuit model of ACCC is developed based on the thermal balance principle and thermoelectric analogy theory. The parameters of the model are identified based on the off-line experiment and the multi-objective optimization method. Then, the environment thermal resistance sequence is simulated randomly by the online measurement and the Markov chain method. The overtemperature probability of the conductor is obtained under different current levels so that the on-line assessment of the overheating risk of the conductor is realized. Finally, by taking the domestic ACCC as an example, the proposed method is validated by the experimental results.

Abstract:In the energy storage system without communication interconnection line, it is difficult to maintain power balance and voltage stability of DC bus without the increase of cost and complexity. To solve this problem, a coordinated control strategy for DC bus is proposed. The DC/AC converter is in constant DC voltage control mode or constant AC voltage control mode. An improved two-stage DC voltage deviation control method including battery charging/discharging control is used in two DC/DC converters. The typical operating modes are analyzed to show how these converters work together in the system. A microgrid experimental platform is set up to verify the proposed control strategy. The effectiveness and practicability of control strategy are verified by the experimental results.

Abstract:The doubly-fed induction generator(DFIG)on the power grid changes the transient performance of sources in power system. A rotor Crowbar is usually used to realize low voltage ride-through(LVRT)during short-circuit fault. The transient performance of DFIG is determined by the fault inception and the system fault severity. It is hard to achieve LVRT in various degrees of fault severity using the fixed resistance Crowbar. The expression of transient rotor current with Crowbar resister input is deduced. A LVRT scheme of DFIG by dynamically adjusting Crowbar resistance is proposed. An adaptive control strategy and the resistance setting method are given. Applying the scheme proposed, the LVRT performance of DFIG is simulated during different voltage dips. The results show that it is effective to suppress rotor current and DC bus voltage using the scheme proposed. Meanwhile the working time of Crowbar is shortened and action times are cut down.

Abstract:The earth fault in the high-voltage capacitor of direct current(DC)filter will greatly affect the security and stable operation of DC transmission system. The existing high-voltage capacitor earth fault protection of DC filter has the problems of poor selectivity, low sensitivity, difficult setting of definite value and so on. A new type of high-voltage capacitor earth fault protection criterion and protection scheme, which is independent of the system operating conditions, is proposed based on the requirements of DC transmission system and DC filter high-voltage capacitor protection. The proposed fault protection criterion and scheme is acquired by combining with the structural characteristics of H-type high-voltage capacitor and simplifying the earth fault model. The problems in the existing protection are solved by the proposed scheme. The actual ultra-high-voltage direct current(UHVDC)engineering simulation model is used to simulate the high-voltage capacitor earth fault. The validity and correctness of the proposed protection criterion and scheme are verified.

Abstract:To evaluate the location performance of single phase to earth fault in distribution systems, the corresponding test technology is investigated. According to an analysis of the drawbacks of the secondary injection test method, it is pointed out that it is not suitable for the evaluation of the location performance of single phase to earth fault. A new approach to test the behavior of single phase to earth fault location featuring a controllable electric arc generating device is put forward. With the new device, the phenomenon of permanent and temporary single phase to earth without arc, with stable arc or intermittent arc through various transitional resistance can be generated in the field. The composition of moveable complete equipment for the single phase to earth fault location test in the field is described. The application of the approach proposed is briefly treated, which shows its feasibility and effectiveness for evaluation of the location performance of single phase to earth fault.

Abstract:The adjustment of the power grid by breaking parts of specified voltage grade transmission lines can increase the electrical distance among over-standard nodes and source nodes in the network. This is a simple and effective measure of limiting short circuit current and widely applied in the power system operation. According to the application characteristics of breaking transmission lines in the adjustment of power grid structure, and considering the current limit requirements as well as maintaining the power grid security, an index fitness function based on impedance sensitivity weighted value is proposed to reflect the degree and proportionality of current limiting effect. The power grid adjusting model with breaking the transmission lines is proposed. The ant colony optimization is improved in the aspects of path selection strategy, pheromone updating and adjustment factor to obtain a rapid calculation and global optimization of power grid adjustment. The feasibility and effectiveness of the proposed algorithm is verified by the network adjustment scheme test of Guangdong power grid.

Abstract:For the confusion between the operators and planners about the contingency reserve, this paper firstly explains the concept of operation contingency reserve. On the basis of systematic survey on the contingency reserve standards of China, the United States, Europe and Australia, a standard framework is proposed, which consists of one reference accident, two performance indicators and three key parameters. The research results contribute to improving the operation contingency reserve standards and designing the ancillary service market.