Abstract:With large-scale asynchronous power supply replacing some synchronous motors in the traditional power grids, the power supply structure and grid skeleton are changing, and the power system will face the problems of the weak system, low inertia and weak voltage support. The flexible DC transmission system with grid-forming control can simulate the operation characteristics of synchronous motors and play the role of power grid support while realizing power transmission. This paper introduces the application scenario and technical framework of grid-forming control for flexible DC transmission systems and analyzes the key technologies of grid-forming control applied to the flexible DC transmission system from three perspectives of the grid-connected performance evaluation, broadband resonance stability and transient stability. This paper summarizes and analyzes the challenges in the three aspects of control parameter design, over-current capacity of primary equipment and energy sources as well as several technical directions.

Abstract:The configuration of the synchronous condenser (SC) in the renewable energy station is an effective measure to improve the transmission capacity of renewable energy, but it also brings the risk of transient angle instability to the system. First, through the network simplification, the transient analysis mathematical model of SCs in the renewable energy station is established. Then, the transient power angle instability mode of SC is analyzed under the dynamic influence of renewable energy. The instability mechanism is explained and the criterion of transient instability is proposed based on the equal area criterion. Finally, the effectiveness of the theoretical analysis is verified through the simplified model of the actual renewable energy sending system and the improved IEEE 39-bus system simulation. Results show that the SC deployed near a renewable energy station has the risk of transient power angle accelerating instability,no matter the control depth of renewable energy low voltage ride-through is low or high, which needs to be considered in the configuration planning and operation control of renewable energy sending system.

Abstract:With the large-scale integration of renewable energy generators to power grid, the voltage stability problem of modern power systems is increasingly prominent, and effective voltage stability analysis methods is urgently required. The voltage stability of the systems with photovoltaics is analyzed based on the monotone system theory. It is proven that the Jacobian matrix of the voltage subsystem of power systems has sign pattern of the interconnected monotone system through the modular derivation method. The stability of the voltage subsystem of the power system is analyzed based on the small-gain theorem of the interconnected monotone system, and the spectral radius of the Jacobian matrix of the gain function is adopted as the criterion for the asymptotic stability of the system equilibrium point. The influence of some parameters on the spectral radius of the Jacobian matrix of the gain function based on the special matrix theory is quantifiably analyzed as the theoretical basis for the setting of the control parameters of the converters. The above conclusions are verified by the 10-machine 39-bus system. The simulation results show that the grid-following converters running under the unity power factor will increase the sensitivity of the spectral radius to the parameters of the synchronous generators, and cause the weaker system robustness, while the increase of the reactive power-voltage droop gain and the outlet reactance reduction of the grid-forming converters can enhance the voltage stability of the power system.

Abstract:When direct-driven wind turbine is connected to a weak AC power grid, it will cause sub-/super-synchronous oscillation of the power system. The amplitude of the generated supersynchronous component is generally different from that of the subsynchronous component, and the specific generation mechanism is not clearly explained. In this paper, based on the oscillation mode of the positive and negative sequence coupling impedance loop in the frequency domain, the observability and controllability of the sub-/super-synchronous components are quantitatively analyzed, and the observability and controllability laws of the sub/super-synchronous components are explained. First, the positive and negative sequence loop coupling impedance equation of the grid connected with frequency-domain direct-driven wind turbine system is established, the relationship between the sub-/super-synchronous component, the positive and negative sequence component, and the dq component is analyzed, and the cause of the unequal amplitude of the sub-/super-synchronous component is explained. Then, the expression of the degree of observability and controllability of sub-/super-synchronous oscillation mode on positive and negative sequence components and sub-/super-synchronous components is derived from a quantitative perspective, and the analysis of the degree of observability and controllability of sub-/super-synchronous components is realized. Finally, the sensitivity method is used to analyze the parameters that affect the magnitude of sub-/super-synchronization components, and the simulation results are verified against the theoretical analysis results.

Abstract:For the existing active power control (APC) methods based on the rotor with passive speed variation, the pitch regulation is only activated at the boundaries of variable-speed range and the pitch angle is set by feedback control laws. Therefore, the setting of the pitch angle of the rotor with passive speed variation has strong randomness. This paper reveals that the pitch angle significantly affects the dynamic characteristic of the rotor with passive speed variation. The randomly setting pitch angle is difficult to adapt to the fluctuation range of the actual wind speed, resulting in over-speeding of the rotor or electromagnetic power drop. Hence, based on the wind speed range of non-pitch regulation, the paper establishes the matching relationship between the pitch angle of the rotor with passive speed variation and the fluctuation range of wind speed, and the APC method of wind turbines considering the wind speed range of non-pitch regulation is proposed. This method can overcome the randomness of the pitch angle setting in the existing APC method by adding the pitch angle setting link and dynamically set the pitch angle that adapts to the wind speed fluctuation range, thereby reducing the frequency of the rotor reaching the rotational speed boundary, and alleviating wind turbine overspeed and electromagnetic power drop. Finally, experiments based on the experimental platform for the dynamic simulation of wind turbines verify the effectiveness of the proposed method.

Abstract:Accurate dynamic state estimation results are of great significance for monitoring the operating states of doubly-fed wind turbines in real time. Appropriate measurement set selection is the foundation for the realization of accurate state estimation. Therefore, the observability matrix of nonlinear system states is derived based on the Lie derivative, and the fast calculation of the observability matrix is implemented based on the automatic differential technique. The smallest singular value of the observability matrix is used to quantitatively evaluate the observability of different measurement sets. Considering that the prediction step of the Kalman filter does not use the measurement value, a cubature Kalman filter (CKF) method based on adaptive multi-step prediction is proposed. The observability analysis of the doubly-fed wind turbines is carried out by using the modified IEEE 39-bus system, and the measurement set is selected to make the states of doubly-fed wind turbines have high observability. On this basis, the effectiveness of the proposed cubature Kalman filter method based on multi-step prediction is verified.

Abstract:To achieve quantitative improvement of the resilience of the electricity-gas interconnection system, this paper proposes a resilience improvement method based on global sensitivity analysis. First, the impact-increment-based global sensitivity is derived for quantitatively analyzing the joint impact of simultaneous changes in component failure probability parameters on the resilience index. Then, the global sensitivity is introduced into the optimization model of the component enhancement scheme in the scenarios of the budget limitation and the resilience qualified constraint, so as to design a cost-effective resilience enhancement scheme. Finally, the effectiveness of the proposed method is verified by taking an electricity-gas interconnection system in an earthquake disaster scenario as an example.

Abstract:After an emergency disaster, considering the coupling influence of urban power and transportation systems and the evacuation demand, the fast evacuation of people and the fast restoration of critical loads can be achieved by coordinating the two departments, which is conducive to reducing casualties and losses of the disaster. First, this paper proposes a coordinated emergency restoration framework of power and transportation systems considering the evacuation demand. To minimize the emergency evacuation time, an optimal evacuation model of the transportation network is built with consideration of dynamic traffic flow characteristics of charging stations and the restoration of critical power facilities, and the electricity demand of critical transportation power facilities can be obtained. To maximize the power restoration effect, a mixed-integer linear programming model for emergency restoration of the distribution network is established, in which the availability of mobile emergency resources and the energy demand of key power facilities affecting evacuation are considered. The numerical example test uses the improved power and transportation coupling system for comparative test, which verifies the effectiveness and superiority of the proposed coordinated emergency restoration method.

Abstract:In the context of the digital economy, the energy consumption of data centers is increasing day by day. The flexibility of data center energy consumption in the spatio-temporal scale has attracted widespread attention. In order to fully exploit the flexible resources of data center energy consumption and promote the optimal resource configuration of integrated energy system, based on the complementarity and multi-energy conversion of electricity, gas, heat, and cooling, multi-regional interconnection and mutual assistance, and deep coupling of energy flow and data flow, this paper proposes a collaborative planning model of multi-regional energy station considering the spatio-temporal adjustment of data center energy consumption. First, the data load and data center energy consumption are modeled, and the spatio-temporal adjustable characteristics of data center energy consumption are analyzed. Then, a planning model of multi-regional energy stations considering the spatio-temporal adjustment of data center energy consumption is constructed with the goal of minimizing the total cost. Finally, three scenarios are set up for comparative analysis with or without consideraton of the spatio-temporal adjustable characteristics of data center energy consumption. Based on an actual example in Tianjin municipality of China, the positive effects of considering the spatio-temporal adjustable characteristics of data center energy consumption in improving equipment utilization and reducing investment cost are verified.

Abstract:Compared with energy storage with single-type units, the hybrid energy storage has better performance in improving the flexibility of thermal power units. However, the combination of hybrid energy storage with different resource endowment characteristics and thermal power units will significantly increase the difficulty in coordinated frequency regulation. Therefore, this paper proposes a two-stage coordinated frequency regulation control model for thermal power and energy storage based on stochastic model predictive control, which divides the entire control cycle into two different time scales: coarse granularity and fine granularity. On coarse granularity, the frequency regulation control model for the thermal power and energy storage considering the uncertainty of frequency regulation signal is proposed, and the frequent switching of energy storage is limited and its service life is prolonged by constructing the energy storage charging and discharging switching model. On the other hand, by introducing unexpected constraints, a feasible energy-based energy storage operation state scheme for the whole scenario is obtained. On fine granularity, with the goal of minimizing the cost of frequency regulation, taking into account the constraints of the equipment itself and frequency regulation deviation, the optimal solution of the rest of the control schemes for the thermal power and energy storage hybrid system is made. The frequency regulation effect of the proposed frequency regulation control model is evaluated in terms of regulation rate, accuracy and response time. Finally, taking the actual data of the generator set in a certain area as an example, the effectiveness and economy of the frequency regulation effect of the proposed model are verified.

Abstract:The energy transaction among prosumers is of great significance to promote the local consumption of distributed resources. To this end, by fully considering the interests of prosumers, a complete peer-to-peer transaction mechanism for prosumers without coordination entities is proposed to maximize the social welfare. Taking the microgrid as an example, the internal resources of prosumers are modeled, and the physical network constraints are introduced to propose the centralized optimal model for energy transaction of multiple prosumers. Through the branch tearing method and Lagrangian dual decomposition, the problem is transformed into a distributed optimization problem and solved by the alternating direction method of multipliers. It can be proven that this mechanism requires no coordination of third-party entities, and only needs to transmit information between interacting entities. The optimization can be completed through the resource allocation within each entity to achieve the fully-distributed solution. In addition, the price conforms to the operation law of the actual market better. Finally, four microgrids are taken as an example for case anlysis to verify the reliability and effectiveness of the proposed transaction mechanism.

Abstract:Standardized information model is a necessary way for the information exchange between information systems. Compared with traditional power applications, the renewable energy accommodation application needs to consider social factors such as heating information and environmental protection information. First, based on the experience of power system standardization, a two-column-two-row information architecture for renewable energy accommodation with the accommodation evaluation model layer and power market operation management layer is designed, which expounds the important role of the cyber-physical-social standardized information model in the construction of renewable energy accommodation applications. Then, the standardized modeling method for renewable energy accommodation based on phenomenon-data-provider pattern is proposed, which realizes the social information modeling. The accommodation interdependence event-driven analysis is also incorporated into the model. Cyber-physical-social standardized data model for renewable energy accommodation evaluation is formed. At the same time, the IEC 62325 MarketOperations Package is analyzed and expanded, and the measures to promote renewable energy accommodation in market environment are standardized modeled to form a standardized business information model for renewable energy accommodation. The cyber-physical-social standardized data model for renewable energy accommodation evaluation and the standardized business information model for renewable energy accommodation are built together to form the cyber-physical-social standardized information model for renewable energy accommodation. Finally, the standardized information research is applied in the northern Hebei region of China, which improves the efficiency and security of information exchange between the applications for renewable energy accommodation and simplifies the research & development and application processes.

Abstract:The power supply system of the control rod driving mechanism in large-scale nuclear power plants consists of two electric generators with flywheels. The excitation modes include self shunt excitation and phase compound excitation. A serious accident occurred in an rod power supply system of self shunt excitation when one generator lost excitation and the overcurrent protection of the other generator in parallel operated first. In actual nuclear power plants, a large number rod power supply systems use phase compound excitation. In order to explore the cause of the accident and determine whether there is a common risk of protection mismatch in the rod power system, it is necessary to study fault characteristics of the rod power supply system of phase compound excitation. Based on the full compensation formula of the phase compound excitation system, this paper theoretically analyzes the relationship between the change trend of the generator terminal short-circuit current and the parameters of the phase compound excitation system, and then establishes the PSCAD model of the rod power supply system. It is found that the simulation calculation of the terminal short-circuit fault is consistent with the theoretical analysis, and the correctness of the simulation model is verified through the single-terminal short-circuit experiment of the dynamic simulation unit. In this paper, the fault characteristics of a single rod power supply system with loss-of-field fault are simulated and analyzed. Combined with the real machine experiment of the manufacturer, the effectiveness of the estimation method is verified, and the reason for the mismatch between over-current and loss-of-field protection of the accident rod power supply system is explained, which lays a foundation for the improvement of protection devices of nucleon rod power supply system.

Abstract:It is an ideal scheme for grid connection of large-scale renewable energy to adopt overhead-line flexible DC transmission with obvious technical and economic advantages. However, the failure rate of overhead lines is high and the DC system faults develop rapidly. Isolating the fault lines by DC circuit breakers alone cannot solve the fault ride-through problem of the system perfectly. To solve this problem, aiming at the DC line fault condition of grid-connected system of large-scale wind power through flexible DC transmission, a complete DC fault ride-through strategy is proposed from fault location and isolation to fault category judgment, and then to the coordinated control between energy storage system, converter station and wind farm by making full use of the extensive configuration of the energy storage system in the wind farm. Then, the fault characteristic is analyzed from the perspective of the energy. Compared with the energy consumption resistor method used in the existing projects, the proposed strategy has higher flexibility and better control effect in adjusting the unbalanced power of the system, and can reuse the stored energy to avoid energy waste. In addition, in order to reduce the power and capacity requirements of the energy storage system, the rotor acceleration control of wind turbines is proposed to reduce the output of wind farms. Finally, the effectiveness of the proposed method is verified on the PSCAD/EMTDC simulation platform.

Abstract:For the AC transmission lines of the offshore wind power transmission system through the flexible DC, there are electronic converters on both sides of the line. The fault current presents the characteristics of limited amplitude and uncertain phase under the influence of the converter control, which is difficult to adapt to the conventional relay protection. Two control strategies, negative sequence current suppression and negative sequence current injection, are adopted to analyze the adaptability of the current differential protection for offshore modular multilevel converter (MMC) converter station. When the control strategy of negative sequence current injection is adopted in the offshore MMC converter station, the reliability of current differential protection is closely related to the reference of negative sequence current. Therefore, in order to improve the reliability of differential protection, the principle of determining the reference of negative sequence control current in the offshore converter station is proposed, and the optimal selection range of the reference of the negative sequence current is determined considering the influence of the reference of negative sequence current on the overvoltage of AC transmission line. Finally, the corresponding electromagnetic transient simulation model is constructed in PSCAD/EMTDC to verify the validity and feasibility of the proposed principle of negative sequence current injection and reference selection for negative sequence current control.

Abstract:The onshore wind power full DC power generation system can effectively solve the problems of large-scale wind power AC collection and harmonic resonance caused by transmission, and the economical and reliable wind power DC collection system topology is the basis for its sound development. In this paper, the topology of the wind power DC collection system based on wind turbine grouping in series and parallel is given first. Secondly, the reliability and economy of different grouping schemes of wind turbines in the topology are evaluated. Then, considering the cost of power shortage loss caused by the the system economy and reliability problems, the optimization model for the topology of the wind power DC collection system is established with the objective of the lowest total cost. Finally, taking a 50 MW wind farm in northwest China as an example, the proposed optimization model of wind power DC system topology is verified by simulation. The results show that the proposed optimization model for the topology of the wind power DC collection system can consider the system economy and reliability as a whole, and obtain the optimal grouping scheme of wind turbines. The optimized topology is compared with the structure of the typical wind power AC collection system to verify the feasibility and advantages of the proposed optimal method for the topology.

Abstract:When the load converter is aggregated to the DC system with common bus through the line, the LC oscillation link composed of the line inductor and the input filter capacitance of the load converter falls into the control bandwidth of the load converter, which leads to the unexpected instability of the whole DC system with a common bus. Therefore, the load subsystem stability criterion is proposed. First, the constant power load (CPL) model considering the influence of the line is established and the influence of load converter control bandwidth on the stability of the DC system with a common bus is studied. Secondly, the control bandwidth selection range of the load converter is determined so that the damping of the CPL impedance considering the influence of the line is positive at the LC oscillation frequency, and the stability of the whole DC system with a common bus is ensured. Finally, the experimental results of the hardware-in-the-loop simulation verify the effectiveness of the theoretical analysis of the influence of the load converter control bandwidth on the system stability.

Abstract:The premise of judging the small-signal stability of the power system is to calculate all unstable eigenvalues without omission. This paper proposes a dual-layer Arnoldi method based on exponential transform for calculating all unstable eigenvalues of the power system. First, the exponential transformation is introduced to map the unstable eigenvalues of the system to the dominant eigenvalues. Secondly, the outer-layer implicitly restarted Arnoldi algorithm is used to solve all dominant eigenvalues on the transformed spectrum. And all unstable eigenvalues can be calculated after the the inverse transformation. Then, combined with an accelerated convergence technology, the inner-layer Arnoldi algorithm is applied to solve the multiplication of the exponential matrix and vector. Finally, numerical results of two real power systems, i.e. Xing6u and NF2016, indicate that the proposed method can calculate all the unstable eigenvalues of the power system quickly and accurately.

Abstract:For the problem that the measurement data in the power system is seriously polluted by noise, a fast robust regression smoothing denoising method is proposed. First, the background noise is suppressed by the local regression smoothing process. And then the outlier data is detected adaptively according to the global smoothing residual. Finally, the outlier data is denoised by the robust regression smoothing process, and the local smoothing distortions caused by the outlier data are repaired.The proposed method realizes the fast regression smoothing of global data, the adaptive detection of random impulsive noise and the fast repair of local smoothing distortions, which greatly reduces the computational complexity. The simulation analysis and the field measurement data analysis show that, compared with the traditional robust local regression smoothing method, the proposed method not only maintains a good denoising ability against the background noise and the high-intensity random impulsive noise, but also can effectively identify the pseudo impulsive noise caused by the step change due to disturbance. The proposed method improves the preservation ability of the disturbance response information, and it also shows the obvious advantage of computational efficiency.

Abstract:The modular multilevel converter based high voltage direct current (MMC-HVDC) transmission technology has become an ideal solution to the integration of large-scale and remote wind farms. When the penetration rate of wind power is high, the MMC-HVDC system for wind farm integration presents low-inertia characteristics. A possible solution to improving system inertia is adding the energy storage system and the appropriate control strategy. This paper proposes a quasi-full-bridge submodule topology for the MMC with energy storage based on the multiplexing arm and studies its modulation and control strategy. The quasi-full-bridge submodule is mixed with the half-bridge submodule to reduce the cost and loss of the system. Compared with the single-stage half-bridge submodule topology, the proposed topology has significantly lower requirements on battery voltage and can smooth the battery current. Compared with the two-stage half-bridge submodule, the proposed topology has DC fault blocking capability. Finally, the simulation and experimental results verify the effectiveness of the proposed topology and the modulation and control strategies.

Abstract:With the development of artificial intelligence and big data technology, reasonable and efficient analysis of power data can empower the business of power companies to improve their work effectiveness. At the same time, the intelligent acquisition device provides massive measurement data, laying a solid foundation for data empowerment in the lean operation of low-voltage power distribution and consumption systems. In order to better summarize the past research and enlighten potential ideas, the lean operation of low-voltage power distribution and consumption systems is mainly divided into five parts, i.e., the evaluation of measurement devices and monitoring of abnormal measurement data, the user-phase identification and three-phase unbalance regulation, the user-transformer and topology relationship identifications in low-voltage areas, the abnormal power utilization and electricity theft detection, and the user-side service evaluation. The key technologies and algorithms of data empowerment in the lean operation of power distribution systems are introduced in detail, respectively, and the shortcomings of current research and prospects are also given.