Abstract:Flexible distribution networks are promising solutions to improve the operation performance by coordinating the diverse resources of distributed generators and loads. Soft open point (SOP) is considered as the essential equipment for realizing flexible distribution networks. In recent years, researches on the SOP operation control have been widely conducted and achieved abundant progress. This paper starts from the development trend of flexibility of the distribution network. The technical forms and application forms of SOP and its derived flexible interconnection concepts in multiple scenarios are reviewed and sorted out. On this basis, the key technical issues on SOP integration in flexible distribution networks for operation optimization, self-healing control, and configuration planning are explained, followed by a summarization of the current works.The development trend of SOP in the future is prospected, which provides ideas for the further expansion and deepening of the research and application of SOP technologies.

Abstract:Typhoon disasters often result in multiple physical failures in the distribution network. A reasonable rush repair strategy can reduce the time of outrage and improve the reliability of the distribution network. The existing research mainly focuses on the dispatch of repair resources, including repair crews, materials and the operation of the distribution network. However, the current repair strategy has not considered the effect of traffic condition, which would affect the implementation effect of the strategy. With the improvement of power grid information collection capabilities, power grid has the ability to obtain traffic information. Therefore, this paper proposes a method to optimize the post-typhoon rush repair strategy for the distribution network, which considers the information integration of the traffic network and the distribution network. Based on the construction of an information integration framework, a two-layer optimization model for rush repair strategies considering the interruption of the traffic network is established to coordinate and optimize repair crews, repair materials, mobile power sources, network reconfiguration and other resources to reduce the economic losses. Finally, the effectiveness of the proposed method is verified by example simulation.

Abstract:The exploration and utilization of the reactive power response potential of electric vehicle charging piles can optimize the operation of the distribution network and increase the utilization rate of infrastructure capacity. In this paper, a distributed model predictive control strategy in which residential charging piles participate in reactive power response is proposed. On the premise of not relying on control and communication centers, peer-to-peer (P2P) communication and edge computing are used to realize the coordinated reactive power response of charging piles and improve the voltage quality of the distribution network. First, based on the instantaneous power theory and the sensitivity matrix, an optimal model of the reactive power response of the charging pile is established. Model predictive control algorithm is used to solve the optimal setting value of control voltage for the charging pile. Then, a neighborhood P2P communication strategy is established based on the asynchronous replacement of timestamps. The voltage and power prediction information of each other is exchanged to realize the decoupling of optimization problems for multiple piles and improve the overall voltage regulation effect. Finally, an example simulation shows that the proposed reactive power control strategy can effectively reduce the grid voltage deviation on the premise of meeting the active charging demand of the charging pile. In addition, in the P2P communication interaction mode, the voltage regulation effect of the proposed strategy is significantly improved compared to the disorder response of the charging pile in the non-communication mode, and it has better communication delay robustness.

Abstract:For the optimal scheduling problem of the aggregation in large-scale battery charging and swapping stations, a real-time optimal scheduling strategy is proposed for battery charging and swapping loads based on soft actor-critic (SAC) deep reinforcement learning. The strategy fully considers the user, system and market factors in the load control process, and can realize the friendly interaction between large-scale electric vehicles and various types of power system subjects. Firstly, considering the development scale and scheduling performance of battery charging and swapping stations, a joint operation framework is established. Secondly, an adjustable identification model considering multi-user characteristics is proposed to judge the actual adjustable performance of electric vehicles. Furthermore, considering the multiple spatio-temporal characteristics of the optimal scheduling of battery charging and swapping stations, the optimal scheduling models of adjustable battery charging and swapping loads in different scenarios are constructed. Then, the real-time scheduling scheme of battery charging and swapping loads is solved based on SAC algorithm. Finally, the case of virtual power plant load for electric vehicle load aggregation optimization verifies the economy and high efficiency of the SAC algorithm applied in real-time optimal scheduling of electric vehicle loads for large-scale battery charging and swapping.

Abstract:With the accelerating process of electricity spot market construction in China, it has become a trend for renewable energy sources such as wind power to participate in spot market competition. As a development mode of future wind power stations, integrated wind power and energy storage stations can maximize the revenue through reasonable regulation and control. Based on the long-term coexistence of dual-settlement mode and deviation assessment in the early stage of the electricity spot market in China, this paper comprehensively considers the bidding revenue in the day-ahead market and the opportunity revenue in the real-time balance market of power suppliers. Considering the impact of energy storage life, a bidding model in the day-ahead market and a rolling self-dispatching model in the real-time balance market are established with the participation of integrated wind power and energy storage stations, which realize the decision-making optimization of the spot market during all clearing scales. Then, the stochastic-robust modeling method is used to describe the day-ahead and real-time market prices and the uncertainty of the wind power output to optimize decision-making effects. On this basis, the effects of the energy storage life model, robust control coefficient and forecasting capabilities of power generation on the revenue are studied. The case results verify the effectiveness of the proposed model and method.

Abstract:As for the main trend of the development of energy markets in China, it lies in the construction of a multi-agent regional integrated energy market. However, the integrated energy system is generally small in scale, which thus has difficulty in mastering the behavior of energy consumption. The key problem of market transaction is how to deal with the energy deviation caused by inaccurate energy demand prediction. The participation of the integrated energy system group in the day-ahead and real-time market through energy sharing is the powerful way to deal with energy deviation. An integrated energy day-ahead and real-time market framework based on multi-agent bidding interaction on the supply side and multi-agent energy sharing on the demand side is designed. Based on this, the internal business of the park operators on the demand side adopts the energy hub model to carry out the operation optimization, while adopting energy sharing mode in the external end to carry out the transaction, and uses the Shapley value method of cooperative game to reallocate the benefits of multi-agent joint operation, so that the real-time collaborative optimization of multi-agent can be realized. Finally, it indicates that the proposed trading mechanism and energy sharing strategy can effectively enhance the interests of the market participants while stabilizing the energy deviation according to the analysis of the case.

Abstract:With the increasing integration of power electronic devices into the power system, the strength of the power grid is relatively weakened, and the system faces higher risk of small-signal instability. Synchronous condensers can provide voltage support for the power system fed by large-scale power electronic devices, which reduces the risk of the system instability. However, the quantitative research on the effect of synchronous condensers on system stability improvement is not mature, and the siting planning method is inadequate. To this end, the dynamic model of multi-infeed system integrated with synchronous condensers is first established. Then, the mechanism that synchronous condensers improve the strength and the stability margin of the system is theoretically deduced. Finally, the optimal siting method of synchronous condensers is studied from the perspective of improving the stability margin of the system. The effectiveness of the proposed method is verified through simulations on a 2-area 4-machine system and a 39-bus system.

Abstract:With the continuous expansion and development of the power grid, the structure of hydropower unit integration through the high voltage direct current (HVDC) transmission system appears in the power grid. In order to ensure the safe and stable operation of the power grid, it is necessary to explore whether the system has the risk of multi-frequency oscillation. First, the independent models of the hydropower unit and the HVDC system are established and the interface dynamic equation is deduced. Then, the state space model of the whole system is established and the correctness of the model is verified. Based on this model, the eigenvalue analysis method is adopted to identify the multi-frequency oscillation mode characteristics of the system and analyze the influence. The analysis results show that the integration system has ultra-low frequency, low frequency and sub-synchronous oscillation modes. The ultra-low frequency oscillation modes are not only strongly related to the hydropower unit governor, but also weakly related to the constant current controller of the rectifier station. The ultra-low frequency oscillation is prone to occur in the scenario of high proportion of hydropower unit. The low frequency oscillation modes are related to the phase-locked loop (PLL), which will not be excited if the PLL control parameters change within a reasonable range. The sub-synchronous oscillation modes are affected by multiple electrical and control links between the hydropower unit and the HVDC system. When the grid strength is weak at the receiving end, one of the sub-synchronous oscillation modes is excited. The modal damping can be improved by adjusting the excitation gain and the control parameters of the rectifier/inverter station.

Abstract:Compared to the traditional centralized control, the distributed control for microgrids offers higher flexibility and scalability. But the distributed control is short of a central controller for monitoring operation of generation units. When attackers maliciously tamper with measured data and input the controllers, the system cannot effectively detect it, and the fault control signal is still used for system control, which affects the control accuracy and even destroys the stability of the overall system. To solve the above problems, this paper proposes a control method of distributed resilient secondary voltage restoration based on an intermediate estimator. When the controller of islanded DC microgrids is under FDI attack, the control method can effectively observe the attack signal and compensate it in the system. The experimental results verify that when the system is under the time-varying FDI attack, the proposed method can achieve voltage restoration and current accurate distribution.

Abstract:In the non-intrusive load monitoring, the monitoring method based on the original (voltage-current) V-I trajectory characteristics is difficult to effectively identify the loads with similar trajectory characteristics. Therefore, a color coding method based on V-I trajectory characteristics is proposed, and the K-means clustering algorithm and AlexNet neural network are used to identify load characteristics. First, the K-means clustering algorithm is used to preliminarily classify the active power and reactive power characteristics of the load. Then, construction and color coding of the V-I trajectory are performed on the unclassified loads to generate V-I trajectories with color characteristics. Finally, the AlexNet neural network is used to train and classify the load to achieve a fast and refined classification effect. For the public data sets PLAID and WHITED, the monitoring effect of original V-I trajectory characteristics and color-coded V-I trajectories are compared and analyzed. It is known that the proposed method not only saves calculation time but also improves the monitoring accuracy, and the improvement effect is obvious.

Abstract:The life assessment and decommissioning decision of relay protection devices in power systems play an important role in ensuring the safety of power systems. However, there is no very effective method for life modeling and decommissioning decision of relay protection devices. It is difficult to reflect the large fluctuation of operation statistical data by single line fitting of the bathtub curve. In engineering practice, the theoretical basis for the decommissioning decision of relay protection devices is not sufficient. This paper proposes a band fitting life model to describe the failure of relay protection devices. The failure interval of relay protection devices is described by fitting two boundary lines of failure data, focusing on the area of the failure data set and its change trend, so as to reflect the life change law of relay protection devices. This failure interval based model not only has clear boundary characteristics, but also has a certain range of elasticity when used in decommissioning decision. It can guide the operation department to make cooperative decisions between the decommissioning interval and the maintenance and renewal plan of primary equipment according to the actual operation of relay protection devices, which has stronger engineering guiding significance. Combined with the operation data of a provincial power grid in China, the effectiveness of the proposed method is verified.

Abstract:Ceph system has been widely applied in the storage of electric power data. The existing data center relies on the human experience to optimize the storage configuration parameters. However, the human experience is unable to adapt to the dynamic changes of the power grid, and the accuracy is insufficient. Therefore, the rapid storage and processing demands at the edge of the power grid cannot be satisfied. Targeting at the problems above, the data load-aware adaptive configuration parameter recommendation (DACR) method based on reinforcement learning is proposed in the electric power data storage system. This method allocates the parameter sample set and relevant cluster performance by Markov chain Monte Carlo sampling algorithm based on reinforcement learning, and builds the performance prediction model by the hierarchy modeling method. Also, the fast perception of load and continuous optimization of configuration parameters are provided by adopting the combined algorithm of cluster performance cost function and load similarity estimation. The experiment results show that the sampling efficiency is effectively improved and the prediction accuracy is better than that of existing prediction models. This method can satisfy the operation requirements of power data acquisition systems and its optimization time is shorter than that of the existing black box parameter tuning method.

Abstract:As the penetration rate of distributed generators (DGs) keeps increasing, the islanding effect of multiple DGs is likely to occur in the distribution network. In order to ensure power quality and power safety, the inverter must have the islanding detection function. To solve the problem of dilution effect caused by the opposite direction of injected disturbance of the multi-DG inverter, which would lead to the failure of the traditional islanding detection method, this paper proposes a new islanding detection method based on the fixed frequency control of inverters. This method uses the sinusoidal pulse width modulation technology to control the frequency of the output voltage of one of the inverters to 50 Hz. The islanding frequency is stabilized by the master inverter controlled to the fixed frequency, so that each slave inverter controlled by the phase-locked loop can be restored to the normal operation state to eliminate the dilution effect, and the islanding of multiple DGs is detected. The effectiveness of the method is verified by the simulation on the MATLAB/Simulink.

Abstract:In the flexible DC transmission system, the stable and reliable operation of the converter is of great significance to the whole transmission system. Considering the current modular multilevel converter (MMC) is more and more widely used in the flexible DC transmission, and the number of components in MMC is large and the operating conditions are complex, it is necessary to deeply verify the design and control of MMC before commissioning. In order to achieve accurate and effective functional and reliability testing, it is necessary to impose operating conditions similar to the actual operating conditions on the key components of MMC. However, the traditional MMC test method has high cost and limited test freedom. In contrast, the new operating condition simulation test method can apply stress conditions and control behavior close to the actual operating condition to less sub-module units of MMC based on simplified circuit, so it can greatly improve the test efficiency. First, the concept and significance of operating condition simulation test for sub-modules of MMC are introduced. Then, a variety of operating condition simulation methods suitable for MMC sub-modules are introduced, and the differences between each method in test cost and simulation accuracy are analyzed and compared. Finally, the feasibility and potential value of operating condition simulation method for sub-modules of MMC are verified by simulation and experiment.

Abstract:In order to study mid- and high-frequency resonance problems of flexible DC transmission systems with AC-side voltage control, i.e. V/f control, the AC-side positive and negative sequence equivalent impedance model of flexible DC converter is established by means of harmonic linearization considering the functions of positive and negative sequence AC voltage and current loop, voltage feedforward, system modulation and control link delay. Then, the influence of AC voltage control mode, voltage feedforward control mode, control system parameters and link delay on the mid- and high-frequency impedance characteristics of flexible DC converter with V/f control is analyzed. By optimizing the control system structure and parameters, the improvement measures on mid- and high-frequency equivalent impedance of flexible DC converter are proposed to suppress the negative damping of converters. Taking a flexible DC delivery system for a 66 kV offshore wind farm as an example, the influence of flexible DC converter on the mid- and high-frequency resonance characteristics is analyzed. Finally, the correctness of the equivalent impedance model and the effectiveness of impedance improvement measures are verified by impedance scanning and electromagnetic transient simulation, respectively.

Abstract:With the continuous development of DC transmission and distribution technology, the problem of DC fault protection is becoming more prominent. As an important device for fault protection, the circuit breaker is gradually developing in the direction of low loss and fast operation. The Z-source solid-state DC circuit breaker has the characteristics of simple structure, rapid response, and arc-free disconnection, which provides a new solution to DC protection. Therefore, this paper proposes a novel bidirectional Z-source solid-state DC circuit breaker. Based on the analysis of its working principle, its design method is given, and the protection characteristics of the existing typical Z-source DC circuit breakers are compared. The novel circuit breaker uses capacitors to replace the reverse blocking effect of diodes in the traditional structure, reduces on-state loss of the circuit breaker, and realizes bidirectional energy flow through reverse-parallel thyristors, without additional detection and control circuits. And the rapid and automatic response to short-circuit faults can be realized through the design of impedance network parameters. Finally, the validity of the designed solid-state DC circuit breaker is verified by simulations and principle prototype experiments.

Abstract:Small signal modeling is the basis for the quantitative analysis of stability characteristics and multi-controller parameter optimization of variable-speed pumped storage units with full-size converter (FSC-VSPSU). Since the response time scale of frequency converter systems is at millisecond level and that of hydraulic turbine system is at second level, the stability characteristics of the pumped storage unit in fast-power mode and generation state are largely restricted by the hydraulic turbine. Aiming at the problem that the conventional hydraulic turbine model cannot accurately reflect the operation characteristics of the pumped storage units, an improved small signal model for the hydraulic turbine considering the initial operation condition is proposed. It can reflect the influence of the initial operating load and water head of the hydraulic turbine on the stability characteristics, and the model parameters are easy to obtain. Combined with the generator, excitation system, governor system and converter models, the small signal model and electromagnetic transient simulation model of FSC-VSPSU in generation state are constructed. The small signal stability characteristics of the pumped storage unit are studied by eigenvalue analysis method. It is found that the unit has the risk of ultra-low frequency oscillation, and the key factors are extracted. Furthermore, the parameter setting principle of the governor system is given.

Abstract:Active power and frequency control process in the power system consists of primary frequency regulation and automatic generation control (AGC). In the category of frequency oscillation, the oscillation strongly related to AGC is called AGC oscillation. To study the damping contribution of different kinds of active power and frequency control to AGC oscillation, the damping torque method is applied to the analysis of AGC oscillation. The calculation method for mechanical power damping torque coefficient in AGC oscillation is proposed. In a single-machine single-load system, the damping torque coefficient can be obtained by traditional method of substituting the eigenvalue into the transfer function, which is not applicable in multi-area multi-machine systems. Therefore, a method for calculating the damping torque coefficient using the mode shape is proposed, and the relationship between the damping torque coefficient and the stability of AGC mode is derived. The results show that if the damping torque coefficient is positive, the mechanical power will provide positive damping for the power system, conversely, the mechanical power will provide negative damping. The applicability of the damping torque method in AGC oscillation is verified. The damping torque coefficient is decomposed into each active power and frequency regulation process. The results show that in AGC oscillation, because the phase lag of AGC is too large while that of primary frequency regulation is small, the damping torque coefficient provided by AGC tends to be negative on the whole while that provided by primary frequency regulation tends to be positive on the whole. However, for each single unit specifically, the damping torque coefficient provided by AGC and primary frequency regulation may be either positive or negative. Finally, the influence of some parameters on AGC oscillation is analyzed based on the damping torque method.

Abstract:With the large-scale operation of China's ultra-high voltage AC/DC power grids, the complexity of primary and secondary equipment of DC converter stations is increasing, so that in case of state transition or failure of the DC system, the converter station sequential events recording (SER) system generates a large number of SER events in a short time, and the loss of characteristic events in the SER event set is difficult to be detected for the operation and maintenance personnel in time. To solve this problem, a diagnosis method for typical operation and maintenance event set of DC converter station based on improved association rules is proposed in this paper. First, the original SER characteristic types of converter stations are analyzed and the main characteristic types of SER events are selected to realize data dimensionality reduction and the construction of SER event model of converter stations. Then, the SER strong correlation event groups and SER correlation events of similar faults and state transitions in converter stations are mined by the improved association rule algorithm. Finally, based on the data mining results, whether there are missing characteristic events in SER event set is diagnosed. Combined with the trial operation data of the converter stations of Kunliulong DC Project, the results show that the proposed method can mine association rules from a large number of heterogeneous and polymorphic SER events, and provide a reference for assisting operation and maintenance personnel to diagnose the loss of SER event set under the state transition and fault of converter stations.

Abstract:As the scale of market transactions becomes larger and the amount of transaction data increases, it becomes possible to conduct collusion analysis with data. Therefore, combining with the collusion early-warning indicator system of the power generation enterprises and the unsupervised variational autoencoding Gaussian mixture model (VAEGMM), the intelligent early-warning of the collusion between power generation enterprises is realized. Firstly, a complete indicator system for the collusion early-warning and a detailed indicator calculation method are proposed. Secondly, in view of the high-dimensional data characteristics of the index set and the imbalance of positive and negative samples, the VAEGMM is proposed based on the idea of anomaly detection. Then, the network structure of VAEGMM is described in detail, and the joint loss function is reconstructed, making the network better learn the low dimensional expression of the original data. Thus it is helpful for more accurate density estimation. Finally, the actual case study shows that compared with other traditional unsupervised learning models, VAEGMM can warn the risk of collusion more efficiently and accurately.

Abstract:The existing technologies of wireless power transfer are proposed and constructed based on the theory of integer-order system. However, the transfer characteristics are sensitive to the transfer distance and the change of system parameters, and the anti-interference ability is weak, so it is difficult to adapt to actual application demands. Therefore, exploring new mechanisms and technologies of wireless power transfer is a long-term research topic. Benefiting from the rapid development of fractional-order calculus, the mechanism and technology of fractional-order wireless power transfer have been proposed. Different from the traditional integer-order system, the fractional-order wireless power transfer system is based on its unique memory characteristics, negative resistance characteristics, and frequency characteristics to achieve the medium-distance wireless power transfer with high efficiency and high anti-interference. The proposed system with flexible design can be adapted to the demands of various occasions. To this end, this paper introduces the proposal process and research progress of the fractional-order wireless power transfer mechanism, which are expounded from the aspects of fractional-order calculus, fractional-order components and circuits, and fractional-order wireless power transfer mechanism. Finally, the proposed system is compared with the existing integer-order wireless power transmission system. The results show that the transmission efficiency and output power of the proposed system are insensitive to the changes of distance and resonance frequency.