Abstract:Charging network is one of the typical scenarios of Energy Internet. The in-depth study of charging network operation mode and key technologies can effectively alleviate the impact of electric vehicle charging load on the power system. Meanwhile, the research can also provide support for promoting large-scale renewable energy consumption. Considering the policy background of renewable energy quota system, the definition and operation framework of charging load aggregator is proposed. On this basis, the driving force of charging load aggregators to participate in the green certificate trading market is analyzed, and then the two-level green certificate trading mode inside and outside the charging load aggregators is constructed. Then, in view of the coupling mechanism of green certificate trading, carbon emission trading and electricity market trading, a multi-market coordination mechanism is established. Finally, according to the green certificate trading mode, through the four dimensions of perception layer, network layer, platform layer and application layer, the concept of green certificate trading system and key technologies for charging load aggregator are put forward. The research results will support the development and construction of charging load aggregating system. This study can also provide a reference for the design and development path construction of Energy Internet business model in China.

Abstract:The multi-energy networks represented by power, gas and heat networks are one of the most complex physical networks in the world, which are also the key components connecting energy production and consumption, as well as the important way of the coupling of multi-energy systems. Based on the generalized electric circuit model of the branch layer in multi-energy systems, this paper proposes the generalized electric circuit analysis theory for the network layer. Firstly, according to the difficulty of representing the high-dimensional dynamic characteristics of multi-energy networks, a generalized modeling method for multi-energy networks in Laplace domain is proposed based on the generalized electric circuit model at branch layer, and the corresponding compact matrix model is also proposed. Secondly, external-port equivalent method of multi-energy networks is proposed, which can transform the complex internal information into equivalent boundary conditions to simplify the coordinated analysis of multi-energy networks and protect the data privacy of each energy system. Finally, combined with the practical characteristics of the heat network and the gas network, the generalized electric circuit model and the boundary equivalent method are proposed for the whole network with heat network and gas network.

Abstract:Soft open points (SOPs), which have powerful capability of power flow control, can make positive contribution under normal operation and fault conditions of distribution network. The benefits both from the improvement of power supply reliability and operation economy should be taken into account at the planning stage. Planning model of SOPs is proposed in this paper considering the effect of connecting SOP on supply reliability and operation economy in distribution network. The fast-search and the method of finding density peaks clustering are used to aggregate the annual load data, and the typical operation scenarios in distribution network are generated to calculate the improvement cost of operation economy. The calculation method of branch failure rate based on related constraints is adopted, considering the influences of load rate of line current, node voltage and length of line on branch failure rate, the dynamic prediction failure set is generated to calculate the benefits of power supply reliability. A hybrid optimization algorithm, which combines the simulated annealing method with the second-order cone programming, is used to solve the planning model mentioned above. Finally, the effectiveness of the proposed planning model is verified by the modified PG&E 69-node test system. The results show that the SOP planning method facing reliability and economy in distribution network can effectively reduce the comprehensive cost of distribution network and maximize the investment benefits.

Abstract:Considering the uncertainty of renewable energy and electricity, gas and heat load, this paper discusses the island partition of integrated energy microgrid system and establishes a robust island partition model based on continuous operation of island. This model uses energy conversion and storage equipment of integrated energy microgrid as scheduling resources, and targets to optimize islanding partition scheme after fault occurrence. In this model, the shortest notification time constraint against the blackout is taken as the target penalty term and a certain response time is reserved for the load to some extent. Meanwhile, it avoids repeated access and removal of island load caused by source power and load fluctuation. The robust discrete optimization theory transforms uncertain constraints into certain constraints, and a mixed integer linear programming model is constructed. Global robust tuning coefficient is used to improve the conservativeness of robust optimization. The economy and operability of the islanding scheme are well balanced to meet different decision-making needs. Finally, the case verifies the effectiveness of the proposed model and solutions.

Abstract:This paper proposes a method of equipment optimal configuration for an integrated energy system (IES) considering the uncertainties of integrated demand response (IDR). Firstly, in order to improve the efficiency of energy use, the basic structure of IES considering IDR is constructed based on a combined cooling, heating and power system. Then, the aleatory and epistemic uncertainty of IDR are analyzed by the evidence theory, and the load curve under a certain price scheme is optimized by using a credible level constraint. On this basis, a bi-level collaborative planning model considering the optimal configuration and operation strategy of the equipment in IES is established. In the upper level, the equipment selection and capacity allocation are conducted with the goal of minimizing the total planning cost. And in the lower level, the equipment output is optimized with the lowest operation cost as the objective. By comparing the total costs of all tariff schemes, the optimal tariff and equipment allocation scheme is obtained. Finally, the proposed approach is illustrated on an example and the results demonstrate that the optimal allocation results considering the uncertainties of IDR are more resistant to risks. Meanwhile, evidence theory can be used to achieve the unification of probability theory and interval theory.

Abstract:To alleviate the system operation and reserve scheduling pressures caused by multiple levels of uncertainty, a two-stage robust optimization model of multiple reserve resources is proposed, which considers the reserve capacity of wind power and demand response. On one hand, to make full use of the wind farm and demand reserve capacity and improve the flexibility of system operation, the reserve capacity provided by wind farm and demand side is modeled， respectively. On the other hand, based on the robust optimization model, multiple reserve resources are co-optimized to ensure the safe and reliable operation of power grid in the worst operation conditions, thus improving the robustness of power system. The two-stage robust optimization problem is solved by the column and constraint generation (C&CG) algorithm. Simulation results on the modified IEEE RTS-79 test system verify the effectiveness of the proposed model and algorithm. The results show that the co-optimization of multiple reserve resources can improve the operation flexibility of power system and promote wind power consumption. At the same time, by adjusting the uncertainty set of the robust model, the balance between the robustness and economic efficiency of the system can be achieved.

Abstract:A Stackelberg game model between a power grid corporation and multiple users is constructed for demand response. The power grid corporation selects the appropriate time period to formulate the subsidy strategy of demand response based on the predicted load duration curve of next year. The construction costs of power transmission and distribution are decreased by reducing the peak load, thereby the overall revenue of power transmission and distribution is improved. Users choose the response in electricity consumption based on the subsidy price of demand response during the time period set by the power grid corporation to obtain additional profits. Moreover, the existence of the Stackelberg game model and the idea for solving the model are analyzed, and the process of solving the game equilibrium solution is presented. Case study shows that the power grid corporation can declares subsidized price during peak load period based on the developed model. Both the power grid corporation and users can benefit from demand response. In addition, the impact of the unit cost change of avoidable power transmission and distribution capacity on profits of demand response for the power grid corporation is analyzed.

Abstract:To improve the reserve service capability of electric vehicle (EV), this paper proposes a charging/discharging day-ahead scheduling strategy of EV based on the double-incentive mechanism. Firstly, a user-oriented double-incentive mechanism and market interaction mode considering the discharging mode of EV are designed. Then, a semi-managed user response mode is proposed. In this mode, the uncertainty of user response state is modeled and analyzed. A charging/discharging dispatching model of EV for responding the double-incentive is constructed by stochastic programming method. Based on the above models, the case study compares the reserve service capacities of EV with and without discharging incentive. Furthermore, the impact of some factors, such as charging and discharging incentive prices and dispatching time scale, on the reserve capacity of EV is analyzed through the sensitivity analysis of dispatching parameters.

Abstract:Droop coefficients of inverters for distributed generators in AC/DC hybrid microgrid can have great influences on feasibility region of power flow in island operation mode. While traditional droop coefficients are determined by capacity and frequency of inverters and voltage regulation range, which can not give full consideration of the above influence. To fill the gap, a calculation model for power flow of AC/DC hybrid microgrid considering control modes of inverter is firstly established. Then a fast calculation method of feasibility region of power flow in parameter space is proposed, the impact of droop coefficients on feasibility region of power flow is further considered. The optimal values of droop coefficients are suggested by overall considering both load stability margin and small-signal stability factors. Finally, an improved 12-node AC/DC hybrid microgrid is simulated to verify the effectiveness and applicability of the proposed method. Results show that the voltage stability margin is significantly increased compared with traditional methods.

Abstract:The arm current control method for modular multilevel converter (MMC) can simultaneously accomplish AC-side current control and circulating current suppression control. Continuous control set-model predictive control (CCS-MPC) is an optimal control method based on system model in time domain. It has fast dynamic response and can accurately track multi-band compound signals. In this paper, an arm current control strategy based on CCS-MPC is presented. The designed model predictive control （MPC） controller can simultaneously realize the accurate tracking of DC component and fundamental-frequency AC component of bridge arm current, and suppress multiple-frequency circulating currents. The interaction problem between different bandwidth controllers during transient state of conventional arm current control is eliminated. Besides, there is no need to design a separate controller for each frequency signal, therefore the control structure is simplified. On this basis, a comprehensive control strategy of MMC is proposed, which includes reference value calculation of arm current, MPC based arm current control and balancing control of sub-module capacitor voltage. Finally, a three-phase MMC simulation model is built in MATLAB/Simulink to verify the feasibility and effectiveness of the proposed control strategy.

Abstract:Multi-terminal flexible DC transmission system with integration of wind farms is prone to instability, it is necessary to establish an accurate small-signal model to analyze the stability of small disturbances in the system. However, most of the existing research papers do not consider wind farm models when a small-signal model of DC system with source is established. According to the fundamental theory mentioned above, in time domain, a small-signal model of the DC system integrated with wind farm is established based on the average value model. In order to reflect the power losses of converter, the impedance of equivalent bridge arm on DC side is considered and proposed to simplify the DC-side model derivation of the converter by using parts of the DC line capacitance. At the same time, for the purpose of reflecting impacts of the wind farm integration, point of common coupling (PCC) filter capacitors is also considered. In view of wind farm modeling, a small-signal model of the full-powered wind farm is established, which can constitute a single state space with the converter on wind farm side. In the case, the small-signal model of the three-terminal flexible DC transmission system with integration of wind farms is built. Step response and stability margin of the model is compared with the PSCAD model. The results indicate that the proposed small-signal model can accurately simulate the dynamic response of small disturbance, and the superior performance of the model in system stability analysis is shown.

Abstract:Transient energy flow method is a new method for locating oscillation sources. Firstly, according to the calculation formula of transient energy flow, combined with the shafting model, the grid-side converter model of doubly-fed wind turbine and the third-order simplified model of doubly-fed induction motor, the transient energy flow for doubly-fed wind turbine is deduced and the energy flow power is defined as an index of the energy flow direction. Secondly, the frequency characteristics of the doubly-fed wind turbine are analyzed after the forced sources are added. Finally, based on the PSCAD/EMTDC electromagnetic transient simulation platform, a single-machine infinite system with forced oscillation sources and a grid connection system of several wind turbines via series compensated line are built for time-domain simulation. Transient energy flow and energy flow power are calculated using the collected data. The validity of the transient energy flow method for locating the forced oscillation sources of doubly-fed wind turbines is verified.

Abstract:The increasing ratio of DC power supply in the eastern coastal region puts forward the higher requirements for the safe and stable operation of the receiving-end power grid. Thus, it makes the problem of determining the minimum generator number using the voltage stability as the objective is becoming the research hot topic in the new situation. Firstly, this paper proposes the optimization target for determining the minimum generator number, which can be treated as complex nonlinear and integer optimization problem. Secondly, based on the classical non-cooperative game theory, game power flow distribution model is established. At the same time, the established model is improved by considering the power capacity reserve, weak line, critical loads, and special injection power. Converting minimum generator number optimization model into calculating game balance problems, the determination method of minimum generator number is proposed. Finally, the minimum generator number is determined in the test of IEEE 162-node system and Shandong power grid of China, and the rationality of the game power flow distribution model is verified by sensitivity analysis and the comparison between the calculation results from proposed method and Newton method.

Abstract:Cyber-attack by the global navigation satellite system has become a real threat with the intensification of attack in cyber space. The slow sustained satellite time synchronization attack can bypass the anomaly detection mechanism of existing satellite clock, trick the timed device to output an incorrect clock signal, and cause serious damage via operation mechanism of the power system. A Kalman filtering based protection method of sustained satellite time synchronization attack is proposed. Firstly, a Kalman filtering based satellite timing model considering sustained malicious interference is developed. Then, clock bias of time synchronism device in malicious attack is optimized by energy functional regularization. Finally, the clock bias is compensated according to the constructed compensation model to achieve precise time synchronization in attack. Simulation results show that this method can effectively prevent the impact of slow sustained satellite time synchronization attacks on the time synchronism system.

Abstract:Risk assessment and visualization of power system under typhoon disasters has scientific significance and engineering application value for disaster prevention and mitigation of power systems. In order to predict high-risk areas and optimize the emergency material allocation and risk-based dispatch of power flow, the data layer, knowledge extraction layer and visualization layer are used to construct the risk assessment system for power transmission towers under typhoon disasters. Firstly, based on equipment operation information, meteorological information and geographic information, a spatial multi-source heterogeneous information database is built. Then, based on parameter optimization, six machine learning algorithms are used to establish intelligent models for tower damage risk prediction, and a relative optimal model is selected through index comparison. At the same time, a combined model based on goodness of fit method with unequal weight is proposed. The tower damage risk in a Chinese coastal city under the typhoon “Mujigae” is assessed and visualized with dimension of 1 km×1 km. The relative optimal model is compared with the combined model in detail. The results show that both relative optimal model and combined model can identify the most severely damaged area, but the combined model has better prediction with the same risk threshold, which verifies the feasibility and rationality of the proposed method. Finally, the model universality and the influence of sample magnitude on prediction effect are analyzed.

Abstract:When the grid voltage is unbalanced, since the control bandwidth of the virtual synchronous generator (VSG) is limited, it has almost no effective control over the double-frequency fluctuation component of the power caused by the negative sequence voltage of the power grid. Therefore, the long-term voltage imbalance will cause problems such as stator and rotor current distortion, power and torque oscillation in doubly-fed induction generator (DFIG), which seriously affect the output power quality and operation performance in the system. In order to solve these problems, this paper proposes a method to quantitatively control the electromagnetic torque and the double-frequency pulsation of reactive power for DFIG by using the second-order generalized integrator (SOGI). This method enables the VSG control system of DFIG to achieve three control objectives under unbalanced power grid conditions, namely balanced and sinusoidal stator current, sinusoidal stator current and constant active power, sinusoidal stator current and constant reactive power and electromagnetic torque. At the same time, each control target can be flexibly switched according to the real-time requirements of the power grid, which improves the control performance of VSG control system for DFIG. Finally, the effectiveness of the proposed strategy is verified by simulation results.

Abstract:Microgrid is a nonlinear and strong coupling system with multiple constraints and large load disturbance. The conventional proportional-integral (PI) dual-loop control has been unable to meet requirements. The active disturbance rejection control (ADRC) technology can significantly improve the performance of microgrid inverter control system by compensating the disturbances. This paper proposes a time-frequency voltage control strategy of microgrid inverter based on linear active disturbance rejection control (LADRC). In order to improve anti-disturbance and dynamic performances of microgrid inverter, the decoupling of dq axis, the dimension-reduced extend state observer (ESO) with capacitive current and the linear state error feedback control law are designed and analyzed in the time domain. In order to improve the tracking accuracy and anti-disturbance performance of microgrid inverter at various harmonic frequencies, the frequency response characteristics of time-domain LADRC system are analyzed. Based on it, the real and imaginary decoupling links and the time-frequency domain LADRC strategy are designed and analyzed. Finally, the proposed strategy is verified by experiments for the microgrid inverter operating in the island mode. Experimental results show that the time-frequency voltage control strategy of microgrid inverter based on LADRC has better decoupling, anti-disturbance and dynamic performance when it is compared with PI double-loop control. Moreover, it can accurately control harmonic voltage to achieve harmonic suppression.

Abstract:The small-signal model of grid-connected inverter is an important tool for judging system stability and parameter design. The complete state equation of the grid-connected inverter will be accompanied by more redundant information. It is convenient to tune and debug system parameters by adopting appropriate reduced-order model. However, some of the existing typical reduced-order models have narrow applicability, low accuracy under specific conditions or loss of generality with the change of parameters. Firstly, this paper develops a general power-loop model based on the concepts of equivalent inertia constant and equivalent damping coefficient. Through single-side Fourier analysis, it is pointed out that the traditional third-order model is only suitable for the analysis of the systems with large inertia and high damping. Through the analysis of output impedance of grid-connected inverter, it is pointed out that the accuracy of the electromagnetic fifth-order model is sensitive to the control parameters. In order to solve the accuracy problem of reduced-order model, a model order-reduction method based on time constant is proposed by using the characteristics of natural separation of the time scale of inverters. The proposed method is simple and effective. And a simple reduced-order model of the inverter is derived. The obtained model has high accuracy and wide application. Finally, the correctness of the theoretical analysis and the effectiveness of the order-reduction method and the reduced-order model are verified by comparative experiments.

Abstract:To solve the difficult problem of monopolar grounding fault detection in flexible medium-voltage DC power distribution network, this paper proposes a new grounding mode with high resistor and arrester in parallel and a protection strategy including adaptive amplification factor. Firstly, advantages and disadvantages of the existing grounding modes in flexible DC distribution network are discussed. A new grounding mode with high resistor and arrester in parallel via the neutral point of the coupling transformer is proposed. Secondly, the overvoltage mechanism and fault characteristics of the new grounding mode are analyzed by theoretical derivation with AC/DC grounding faults. According to voltage and current characteristics in different fault conditions, the DC line protection strategy is developed. It realizes the accurate and rapid fault identification of differential protection with faults in DC network. Finally, the effectiveness of the new grounding mode and the reliability of the proposed protection method are verified by simulation example.

Abstract:The new generation of condenser is gradually used in the ultra-high-voltage direct current (UHVDC) station. Because the excitation voltage criterion of loss of excitation protection of the condensers in operation uses the average value of excitation voltage as its protection action value, it is difficult to distinguish the deeply leading phase operation and complete loss of excitation of the condenser reliably, and cannot distinguish most of the partial loss of excitation with pulse missing. The root mean square (RMS) criterion of excitation voltage is proposed to solve the problem of distinguishing deeply leading phase operation and complete loss of excitation of the condenser with the coordination of the average value criterion. The pulse RMS criterion is proposed to solve the problem of distinguishing partial loss of excitation with pulse missing. The method to calculate the settings of the RMS criterion, average value criterion and pulse RMS criterion is expounded. Results of the dynamic simulation test and on-site test verify the feasibility and validity of the criterion and the method of setting calculating.

Abstract:In order to realize the interoperability between the smart terminal units (STUs) for distributed feeder automation (FA) and ensure the communication safety between them, this paper proposes an IEC 61850 communication mapping scheme using built-in secure extensible messaging and presence protocol （XMPP） and generic object oriented substation event (GOOSE) based on user datagram protocol (UDP). The security protection scheme based on Hash obtain random subset （HORS） one-time signature is used to ensure the communication safety of GOOSE messages. The agent-based distributed FA test system is developed with the STUs.The test results show that the scheme can meet the requirements of the quick distributed FA.

Abstract:With the increasing penetration of electronic devices in power systems，electrical quantities are increasingly exhibiting wide-frequency characteristics, and thus new stability issues of power grid have arisen. New requirements for monitoring frequency range and response speed of power system signals are put forward. In this paper, a fast wide-frequency measurement method is proposed to realize the fast and high-precision measurement of the wide-frequency signals. The super-resolution characteristics of spatial spectrum estimation are used to achieve the wide-frequency measurement in a short time window. In order to improve the algorithm stability in low signal noise ratio （SNR） conditions, a kurtosis based method for estimating the number of signal frequency components is proposed， which guarantees the measurement accuracy. Finally, the proposed method is implemented on hardware. The results of simulation, hardware and field tests verify the validity of the proposed method.

Abstract:First of all, this paper analyzes the demand of current distribution network planning, and describes the importance of inclusion of the flexible load resources into planning. Furthermore, the classification methods and results of flexible load are proposed from three aspects, i.e. energy interaction mode, scheduling response mode, user and equipment type. Then, aiming at the influence of flexible load access on the distribution network planning, the adaptability and shortcomings of the existing planning methods are analyzed from four aspects, including load forecasting, power balance, planning optimization and planning evaluation, and corresponding improvement ideas and suggestions are put forward. Finally, the new key technologies such as flexible load modeling, big data mining and application are summarized, and the future development of distribution network construction is summarized and prospected.