The global climate change has resulted in frequent extreme storm events in marine areas. Consequently, significant offshore wind power ramps have occurred, thus affecting the safe operation of onshore main grids. Through an electrolysis system, the offshore wind-electrolysis joint system (OWEJS) converts surplus power into hydrogen power, thereby reducing wind-power curtailment rates and penalty costs. Meanwhile, the OWEJS can sell hydrogen power and enhance its economic benefits. This study proposes an OWEJS ramp-control scheme that considers wind-power uncertainties. First, the ramp characteristics of conventional offshore wind farms and the OWEJS under extreme storms are compared, and the ramp-suppression characteristics of the OWEJS are investigated. Second, chance-constrained programming is performed to construct a ramp-control model that considers the “source-storage-load” operating characteristics and offshore wind-power uncertainty. Finally, the CRITIC-TOPSIS comprehensive evaluation model is used to analyze the additional operating costs and uncertain resource-usage rates at different confidence levels to obtain the optimal confidence level. Simulation results show that under the same grid-power ramp-control conditions, the comprehensive operating cost of the offshore wind and hydrogen co-generation system is 0.29 million yuan, which is significantly lower than the 1.33 million yuan for offshore wind farms. When considering wind-power uncertainty, a confidence level of 0.98 can offer balance between cost control and risk mitigation.
Owing to the large-scale development of renewable energy and the high proportion of grid connections, hybrid virtual power plants have gradually become one of the key technologies for fully exploiting the economic and environmental benefits of new power systems. This study focuses on the multi-objective optimization of an electric-hydrogen coupling virtual power plant. First, a virtual power plant with electric-hydrogen coupling, which comprises a distributed wind turbine, micro gas turbine, distributed energy storage, and flexible load, is constructed. Second, the carbon-emission coefficient of the components of the virtual power plant is measured using the multi-agent full life-cycle method, which is combined with the stepped carbon-trading mechanism in the optimization model. Third, considering the minimum operating cost and lowest carbon-dioxide emission in the system as the objective function, the multi-objective optimization is transformed into a single-objective optimization using the weighting method. Finally, three typical scenarios in a park in North China during summer, a transition season, and winter are selected for analysis to verify the effectiveness and feasibility of the model. The simulation results show that the model considers the economic and environmental benefits of operating the main equipment in the virtual power plant.
In this study, a photovoltaic (PV) hydrogen-production model using salt caverns for hydrogen storage is constructed and its economy is analyzed. First, salt caverns are physically modeled to calculate the hydrogen-storage capacity. Subsequently, a PV hydrogen-production system is constructed, pressure-vessel and salt-cavern hydrogen-storage schemes are set to be analyzed against each other, and the capacity of each scheme is optimized using a mixed-integer planning model for cost minimization. Next, the results of the optimization are analyzed via techno-economics analysis. Finally, a sensitivity analysis is performed for the salt-cavern storage scheme based on four factors. The results show that the levelized cost of hydrogen in the PV hydrogen system using salt-cavern storage is $32.18/kg-H2, which is 8.73% lower than the cost incurred by the pressure-vessel hydrogen-storage scheme. Additionally, the final net present value of the salt-cavern scheme is 20.65% higher than that of the pressure-vessel scheme, with a shorter payback period of 10.14% and a higher internal rate of return. The factors influencing the economy of the salt-cavern hydrogen-storage scheme for PV hydrogen production are, in the descending order, solar irradiation, capital cost of PV and electrolyzer, and salt-cavern burial depth. Therefore, establishing PV-to-hydrogen salt-cavern hydrogen storage in western irradiation-rich areas is more economically feasible in the future when the costs of PV and electrolyzers are lower.
The tap changer of converter transformers is a key component in UHVDC systems. Several instances of forced converter outages or equipment damage have been attributed to tap changer faults. Reducing the action frequency of tap changers can decrease the probability of equipment fault and increase the reliability of UHVDC systems. An optimized control strategy is proposed for the typical control of tap changers in UHVDC systems. For the sending converter station, the proposed strategy suggests appropriate expansion of the triggering angle deadband and adjustment of the tap changer before converter deblocking. For a receiving converter station based on alfa-max inverter control, dynamic DC voltage control and corresponding upgrading is proposed. For a receiving converter station based on constant DC voltage control, the proposed strategy suggests expanding the extinguishing angle deadband appropriately and regulating the tap changer before converter deblocking. The presented optimized control strategy of tap changers is theoretically concise and requires minimal modifications to existing control and protection systems, making it easily applicable. Theoretical analysis results, real-time simulations, and on-site operational data validate the efficacy of the proposed strategy, showing significant reductions in tap changer actions and promising prospects for UHVDC applications.
An LCC-DR hybrid HVDC transmission system that considers DC fault ride-through and economy is proposed herein to achieve large-scale centralized and long-distance transmission of island wind power in the Gobi Desert. First, the topology of the hybrid transmission system is introduced. The system uses LCC and DR in series to integrate a large-scale wind power base. Subsequently, a mathematical model of the system is established, and a control scheme is designed based on it. Under DC and AC faults, the operation of the system is analyzed, and a fault ride-through strategy is designed to ensure the stable operation of the system. To verify the effectiveness of the control strategy, an LCC-DR hybrid transmission system model is established in PSCAD/EMTDC, and the steady-state and transient operation characteristics are simulated. The results show that the system can effectively realize the large-scale and long-distance transmission of renewable energy in the Gobi Desert.
In the context of “carbon emission peaks and carbon neutrality,” electric heating has become the mainstream heating method. Electric heating loads typically increase the operating pressure of the distribution network; therefore, it is necessary to expand the distribution network. Traditional planning methods have difficulty in solving the problem of flexible distribution network planning with a large number of electric heating-electric heat storage devices. Therefore, a low-carbon expansion planning method for a distribution network considering access to electric heating-electric heat storage devices is proposed. First, based on the optimal room temperature for heating buildings, wind speed and other factors were comprehensively considered to derive a temperature time-varying equation and solve it to obtain the heat load demand. Next, the output characteristics of the heat load demand, wind power, and photovoltaic power were used during the planning process to optimize the operation mode of the thermal storage electric boiler to achieve the lowest carbon emissions. Finally, the range and variance of the load rate was used as the uniformity index of the distribution network, and the multiobjective expansion plan for the distribution network was carried out to achieve comprehensive cost and line load rate uniformity. A case study was solved using a nested hybrid particle swarm optimization algorithm, which introduces a dynamic inertia weight to enhance the optimization ability of particle swarm optimization. Actual data from a specific area in Northeast China were used in the case studies to verify the feasibility of the proposed method.
The dynamic characteristics and stability mechanism of a VSG grid-connected system are not sufficiently clear because of the interaction between the different timescale controls of the VSG. A time constant was selected as the leading feature to solve this problem. The critical factors influencing the interaction between the inner and outer rings were extracted, and a reduced-order model of a VSG grid-connected system was established. The established reduced-order model was further linearized at the equilibrium point to obtain a similar Heffron-Phillips stability analysis model. Based on the damping torque method, the oscillation characteristics of the VSG grid-connected system at different inner loop control parameters and grid strengths were analyzed. The oscillation modes introduced by the interaction between the inner and outer loops and the electromagnetic transient interaction of the line and its key influencing factors were further analyzed. Finally, a model was established in Matlab/Simulink, and the effectiveness of the reduced-order model and the correctness of the theoretical analysis of VSG grid-connected oscillation characteristics, considering the interaction between the inner and outer loops, were verified.
When planning stand-alone microgrids with a high proportion of renewable energy penetration, the uncertainty of wind and solar outputs poses difficulties in achieving the goal of lean planning. To this end, a stand-alone microgrid optimization planning method based on the information gap decision theory(IGDT) is proposed for hydrogen-containing energy storage. First, the basic structure of a stand-alone microgrid containing a hydrogen energy storage system is constructed. Based on the developed structure, we explain the basic working principle of the hydrogen energy storage system is explained and a simplified mathematical model is derived. Second, an uncertain scene set is established based on the robust optimization idea of IGDT, and a two-level robust optimization planning model for a stand-alone microgrid considering the incentive DR is established. The outer model aims to minimize the annual comprehensive cost of the microgrid, while the inner model aims to minimize the annual operating cost of the microgrid. A genetic algorithm with an elite retention strategy is employed to solve the bi-level robust optimization programming model. The results validate the rationality and effectiveness of the established model and provide a reference for the optimization planning of stand-alone microgrid containing hydrogen energy storage.
Considering the characteristics of a flexible DC transmission system, in which the fault current increases rapidly and features a large amplitude when a line fault occurs, a complete fault identification of the line must be accomplished within a few milliseconds to ensure the safe and stable operation of the system. Rapidly and reliable identifying faulty lines remains challenging in protecting the lines of flexible DC transmission systems. Hence, this study proposes a new transient protection principle based on corrected voltage. First, a voltage-correction algorithm is proposed that simultaneously utilizes the amplitude and rate of change of voltage after a short circuit to identify faults more sensitively. Subsequently, an integral based on a corrected voltage is used to effectively distinguish between internal and forward external faults. Second, a new directional element is proposed to construct a pilot backup-protection criterion, which solves the problems of low sensitivity under high-resistance faults and the reverse fault misoperation of single-terminal protection. Finally, a four-terminal flexible DC system model is constructed based on the PSCAD/EMTDC simulation platform to verify the effectiveness of the scheme via simulation.
A large proportion of distributed generation and electric vehicles are connected to distribution station areas. This results in several problems, such as insufficient hosting capacity, unbalanced load rates, and out-of-limit voltage. The flexible interconnection of the station areas is an effective solution to these problems. This paper proposes a master-slave game planning model for the flexible interconnection of distribution station areas, considering economic and power supply capacities. The proposed model considers economic decision-making as the main player and power supply capacity decision-making as the subordinate player, jointly participating in the planning decision-making process. First, a power flow model of the substation area is established using a DC bus-segmented chain flexible interconnection structure. Subsequently, a planning model based on a master-slave game is established. The interconnection schemes and installed capacity of the interconnected devices are used as the main player strategy, the minimum annual comprehensive cost is the payment. The load apparent power of the substation areas is used as the subordinate player strategy, and the maximum power-supply capacity under N-0 security constraints is the payment. The power-flow constraints of the substation areas operation are considered in the model. The particle swarm optimization algorithm is used to solve this problem. Finally, an IEEE-33 node distribution network connected to a high proportion of photovoltaic power generation in the substation area is analyzed. The results show that the proposed model can balance the operational economy and power supply capacity of the system.
In this paper, a controller “black/grey box” identification method for a direct-drive wind turbine generator (WTG) based on port measured impedances is proposed to overcome the modeling difficulties caused by the control “black/grey box” problem of wind power converters. First, the analytical impedance models of the DC port of the machine-side converter and the AC port of the grid-side converter of a permanent magnet synchronous generator (PMSG)-based WTG are derived and validated under different control modes. Second, the parameter sensitivities of the DC and AC port impedances are analyzed to demonstrate the identifiability of the controller parameters, and the features of the DC port impedance under different control modes are extracted based on the parameter sensitivity. On this basis, a “black box” control mode identification method is proposed for a PMSG-based WTG based on frequency-domain impedance feature matching. Further, a “grey box” control parameters identification method, in which all control parameters are accurately identified by minimizing the sum of squares of the differences between the measured impedance and the theoretical impedance over the studied frequency range based on the damped least squares algorithm, is proposed for a PMSG-based WTG based on frequency-domain impedance characteristic consistency. Finally, the effectiveness of the proposed method is verified through a case study.
Small hydropower-containing microgrids are susceptible to large power fluctuations caused by source-load uncertainty, and off-grid conditions can easily lead to power outages and power abandonment of the microgrid system. To fully consider the power and energy balance characteristics of microgrids containing small hydropower sources and improve their economy and robustness, this study proposes a method for optimizing the configuration of energy storage in microgrids containing small hydropower using the power and energy characteristics. First, the power-energy balance mechanism of a microgrid containing small hydropower is analyzed, and a power-energy characteristic model of the microgrid is proposed based on the power-energy characteristic model of the microgrid containing small hydropower and the source-load uncertainty model. A stochastic scenario decomposition clustering method based on the power-energy characteristic and the energy storage capacity configuration method is also proposed. Finally, the effectiveness of the proposed method is verified by comparing the results with those of existing methods and analyzing the influencing factors using an off-grid microgrid containing small hydropower in a region as a simulation example.
Sub-synchronous oscillation (SSO) in a grid-connected doubly fed induction generator (DFIG) threatens the stable operation of the system. Traditional control strategies may not be effective when the operating conditions of a system change. To solve this problem, a sub-synchronous suppression strategy based on linear active disturbance rejection control (ADRC) is proposed. On the basis of the traditional dual loop proportional-integral (PI) control on the rotor side, an improved auto disturbance rejection control is used to replace the PI part of the original inner loop, in which a linear extended state observer is used to estimate the part of compensation sub-synchronous control interaction (SSCI) in real time. The proposed method involves simple parameter tuning and is strongly robust. By building a DFIG grid connection model and comparing it with PI control, it is verified that the control strategy has good control performance in both steady-state and transient states. By comparing with additional damping control and nonlinear ADRC, it is concluded that improved active disturbance rejection control can suppress sub-synchronous oscillations under multiple operating conditions, converging the distorted current more effectively compared to the simple tuning of existing control parameters, thereby stabilizing the output power of the doubly fed fan system. Finally, the effectiveness of the proposed method is verified using an RT-LAB experimental platform.
With the significant increase in the use of renewable power generators, future power systems face stability challenges, including subsynchronous oscillation (SSO), which is a crucial issue. This study proposes subsynchronous damping control (SDC) strategies based on a grid-forming controlled battery energy storage system (BESS) to effectively suppress the SSOs induced by the interaction between nearby direct-drive wind farms and a weak AC grid. The BESS not only offers a range of grid services, including load shifting, peak shaving, and frequency regulation, but also provides positive damping for possible unstable SSO modes from grid-connected wind power. Different SDC structures were designed based on a low-pass filter, notch filter, and phase shifter, and their installed locations and filter/gain parameters were selected. Finally, the results were verified based on electromagnetic simulations, the damping performances of the three SDCs were compared, and the influence of the BESS capacity on oscillation damping was investigated. Specifically, the NF-SDC with a second-order notch filter and the PS-SDC based on a phase shifter exhibited superior performance under the given system conditions. The damping target SSO modes required at least 5% BESS capacity.
Existing measurement devices cannot synchronously monitor the internal electrical quantities of renewable energy stations in real time, and it is difficult to provide data support for dynamic process monitoring, inertia evaluation, frequency control, and other applications of renewable energy stations. Therefore, we propose a synchronized measurement algorithm suitable for renewable energy stations and the development of a synchronized measurement device that can be installed at the outlets of wind turbines. A wide-band fundamental phasor measurement algorithm based on the complex bandpass filter, which extends the fundamental measurement frequency band to 20-80 Hz, was developed to solve the narrow measurement frequency band of the existing synchrophasor measurement algorithm and the inability to track the dynamic process of the renewable energy stations, such as sub/super synchronous oscillation. A broadband measurement algorithm based on fast Fourier transform (FFT) spectral line interpolation fitting was proposed to minimize the barrier effect and spectrum leakage of the FFT. Based on this algorithm, the synchronized measurement device for renewable stations was developed, and the first batch of more than 80 devices have been installed in more than 80 wind turbines of JiHong No. 4 Power Station in Ulanqab, Inner Mongolia. Static, dynamic, and recorded data tests verified the proposed algorithm and developed device.
