Research on Equivalent Modeling Method of AC-DC Power Networks Integrating with Renewable Energy Generation

Along with the increasing integration of renewable energy generation in AC-DC power networks,investigating the dynamic behaviors of this complex system with a proper equivalent model is significant.This paper presents an equivalent modeling method for the AC-DC power networks with doubly-fed induction generator(DFIG)based wind farms to decrease the simulation scale and computational burden.For the AC-DC power networks,the equivalent modeling strategy in accordance with the physical structure simplification is stated.Regarding the DFIG-based wind farms,the equivalent modeling based on the sequential identification of multi-machine parameters using the improved chaotic cuckoo search algorithm(ICCSA)is conducted.In light of the MATLAB simulation platform,a two-zone four-DC interconnected power grid with wind farms is built to check the efficacy of the proposed equivalentmodelingmethod.Fromthe simulation analyses and comparative validation in different algorithms and cases,the proposed method can precisely reflect the steady and dynamic performance of the demonstrated system under N-1 and N-2 fault scenarios,and it can efficiently achieve the parameter identification of the wind farms and fulfill the equivalent modeling.Consequently,the proposed approach’s effectiveness and suitability are confirmed.

Renewable energy power generation projects started construction in Tibet

On March 19, the construction of a 10-MW photovoltaic power plant and a 1 000-kW new type geothermal power generation project were started by Guodian Longyuan Group in Yanbajing Town, Dangxiong County of Tibet.

Large-Scale Renewable Energy Transmission by HVDC: Challenges and Proposals

Renewable energy transmission by high-voltage direct current(HVDC)has attracted increasing attention for the development and utilization of large-scale renewable energy under the Carbon Peak and Carbon Neutrality Strategy in China.High-penetration power electronic systems(HPPESs)have gradually formed at the sending end of HVDC transmission.The operation of such systems has undergone profound changes compared with traditional power systems dominated by synchronous generators.New stability issues,such as broadband oscillation and transient over-voltage,have emerged,causing tripping accidents in large-scale renewable energy plants.The analysis methods and design principles of traditional power systems are no longer suitable for HPPESs.In this paper,the mechanisms of broadband oscillation and transient over-voltage are revealed,and analytical methods are proposed for HPPESs,including small-signal impedance analysis and electromagnetic transient simulation.Validation of the theoretical research has been accomplished through its application in several practical projects in north,northwest,and northeast region of China.Finally,suggestions for the construction and operation of the future renewable-energy-dominated power system are put forward.

Control and Stability of Large-scale Power System with Highly Distributed Renewable Energy Generation:Viewpoints from Six Aspects

Power systems are moving toward a low-carbon or carbon-neutral future where high penetration of renewables is expected.With conventional fossil-fueled synchronous generators in the transmission network being replaced by renewable energy generation which is highly distributed across the entire grid,new challenges are emerging to the control and stability of large-scale power systems.New analysis and control methods are needed for power systems to cope with the ongoing transformation.In the CSEE JPES forum,six leading experts were invited to deliver keynote speeches,and the participating researchers and professionals had extensive exchanges and discussions on the control and stability of power systems.Specifically,potential changes and challenges of power systems with high penetration of renewable energy generation were introduced and explained,and advanced control methods were proposed and analyzed for the transient stability enhancement of power grids.

Photovoltaic,thermoelectric and electromagnetic generation technologies applied in power systems for mobile unmanned systems

Unmanned systems are increasingly adopted in various fields,becoming an indispensable technology in daily life.Power systems are the lifeblood of unmanned systems,and affect the working time and task complexity.However,traditional power systems,such as batteries and fuels have a fixed capacity.Therefore,once the power supply is exhausted and cannot be replenished in time,the unmanned systems will stop working.Hence,researchers have increasingly begun paying attention to renewable energy generation technologies.The principles,advantages,and limitations of renewable energy generation technologies are different,and their application effects in different unmanned systems are also uneven.This paper presents a comprehensive study of the application and development status of photovoltaic,thermoelectric,and magnetoelectric generation technologies in four kinds of unmanned systems,including space,aviation,ground,and water,and then summarizes the adaptability and limitations of the three technologies to different systems.Moreover,future development directions are predicted to enhance the reliability of renewable energy generation technologies in unmanned systems.This is the first study to conduct a comprehensive and detailed study of renewable energy generation technologies applied in unmanned systems.The present work is critical for the development of renewable energy generation technologies and power systems for unmanned systems.

A method for determining the layout of a yaw system based on the Delphi method and the analytic hierarchy process

Wind power technology has been widely used due to its characteristics of environmental protection,sustainability and low cost.The yaw system plays a vital role in improving the energy capture efficiency of a wind turbine.However,the method of layout determination is lacking in the yaw system.To solve this problem,a method that combines the Delphi method and the analytic hierarchy process was proposed in this study.Twelve evaluation indexes,including transmission efficiency,ratio range,operating temperature range and others,were identified by screening 18 technical indicators using the Delphi method.Subsequently,the evaluation system of the yaw system was established.Then,six configuration schemes were selected.Experts’scores of schemes were collected according to the evaluation system and the score matrix of evaluation indexes was obtained.The hierarchical model of the evaluation indexes of the yaw system was established and the comprehensive weight was obtained by using the analytic hierarchy process.After calculating the comprehensive evaluation score,the comprehensive evaluation result was obtained.The 2Z-X(A)negative mechanism,which achieved the highest score of 0.9227,is the optimal scheme.A new method and specific process are provided for designers.The research gap in the scheme selection method for yaw systems is filled.

Modular System of Cascaded Converters Based onModel Predictive Control

A modular system of cascaded converters based on model predictive control(MPC)is proposed to meet the application requirements ofmultiple voltage levels and electrical isolation in renewable energy generation systems.The system consists of a Buck/Boost+CLLLC cascaded converter as a submodule,which is combined in series and parallel on the input and output sides to achieve direct-current(DC)voltage transformation,bidirectional energy flow,and electrical isolation.The CLLLC converter operates in DC transformer mode in the submodule,while the Buck/Boost converter participates in voltage regulation.This article establishes a suitable mathematical model for the proposed system topology,and uses MPC to control the system based on this mathematical model.Module parameters are designed and calculated,and simulation is built in MATLAB/Simulink to complete the simulation comparison experiment between MPC and traditional proportional integral(PI)control.Finally,a physical experimental platform is built to complete the physical comparison experiment.The simulation and physical experimental results prove that the control accuracy and response speed ofMPC are better than traditional PI control strategy.

Overview of Energy Storage in Renewable Energy Power Fluctuation Mitigation

The integration of renewable energy,such as PV and wind power,has exerted great impacts on the power system with its rapid development.If the corresponding energy storage system is configured,the power system could be able to hold a higher proportion of renewable energy.Focusing on energy storage application for the output fluctuation mitigation of renewable energy,this paper first analyses the reason for renewable energy power fluctuation mitigation from the four aspects of frequency,unit ramp,low frequency oscillation and cascading failure.In addition,the fluctuation rate standard of grid-connected renewable energy,the energy storage type and the mitigation topology are introduced.Then a summary and analysis on mitigation strategy and hybrid energy storage allocation strategy are presented.Finally,the demonstration application and development trend of energy storage are analyzed to provide reference for the promotion of energy storage in renewable energy.

Real-time Dispatchable Region of Renewable Generation Constrained by Reactive Power and Voltage Profiles in AC Power Networks

A large amount of renewable energy generation(REG)has been integrated into power systems,challenging the operational security of power networks.In a real-time dispatch,system operators need to estimate the ability of the power network to accommodate REG with a limited reserve capacity.The real-time dispatchable region(RTDR)is defined as the largest range of a power injection that the power network can accommodate in a certain dispatch interval for a given dispatch base point.State-of-the-art research on the RTDR adopts a DC power flow model regardless of the voltage profiles and reactive power,which can overlook potentially insecure operational states of the system.To address this issue,this paper proposes an AC power flow based RTDR model simultaneously considering the reactive power and voltage profiles constraints.The nonlinear constraints in our model are approximated using a linear power flow model together with a polytope approximation technique for quadratic constraints.An adaptive constraint generation algorithm is used to calculate the RTDR.Simulation results using the IEEE 5-bus and 30-bus systems illustrate the advantages of the proposed model.

An approach for evaluating the impact of an intermittent renewable energy source on transmission expansion planning

We propose a new robust optimization approach to evaluate the impact of an intermittent renewable energy source on transmission expansion planning （TEP）. The objective function of TEP is composed of the investment cost of the transmission line and the operating cost of conventional generators. A method to select suitable scenarios representing the intermittent renewable energy generation and loads is proposed to obtain robust expansion planning for all possible scenarios. A meta-heuristic algorithm called adaptive tabu search （ATS） is employed in the proposed TEE ATS iterates between the main problem, which minimizes the investment and operating costs, and the subproblem, which minimizes the cost of power generation from conventional generators and curtailments of renewable energy generation and loads. The subproblem is solved by nonlinear programming （NLP） based on an interior point method. Moreover, the impact of an intermittent renewable energy source on TEP was evaluated by comparing expansion planning with and without consideration of a renewable energy source. The IEEE Reliability Test System 79 （RTS 79） was used for testing the proposed method and evaluating the impact of an intermittent renewable energy source on TEP. The results show that the proposed robust optimization approach provides a more robust solution than other methods and that the impact of an intermittent renewable energy source on TEP should bc considered.

Switched-capacitor Based Seven-level Boost Inverter with a Reduced Number of Devices

The conventional multilevel inverters(MLIs)have the disadvantages of numerous devices,incapacity of boost,unbalance for capacitor’s voltage,high complexity for control,and etc.Motivated by this issue,a seven-level boost inverter(7LBI)based on a switched capacitor is presented for singlephase applications in this paper.The proposed 7LBI using only seven transistors can achieve seven output levels,1.5 voltage gain,and natural balance of capacitors’voltages without sensors or other auxiliary methods,which illustrates its suitability for the applications of renewable energy generation.The configuration of topology and operating principles are illustrated in detail.The natural balance of capacitors and capacitance calculations are deduced as well.Moreover,the comparative study is conducted for different types of MLIs.The results illustrate the merits of the proposed 7LBI with respect to reduced devices,lower voltage stress,and less power loss.Finally,a simulation for the proposed 7LBI with PWM modulation is realized based on the theoretical analysis;an experimental prototype is also implemented,verifying multilevel output,boost ability,natural balance for switched capacitors,and performance of transient response.

Local Energy and Planned Ramping Product Joint Market Based on a Distributed Optimization Method

High penetration of renewable energy generation(REG)in the distribution system increases both the power uncertainty at a given interval and the power variation between two intervals.Reserve markets addressing power uncertainty have been widely investigated.However,there is a lack of market mechanisms regarding the power variation of the load and REGs.This paper thus defines a planned ramping(PR)product to follow the net load variation and extends the local energy market to include the trading of PR products.Players are economically compensated for their PR products.Bidding models of dispatchable generators and flexible load aggregators in the joint market are investigated.To solve the market problem in polynomial time,a distributed market clearing method is developed based on the ADMM algorithm.The joint market is tested on a modified IEEE 33-bus system.It verifies that introducing the PR market can encourage flexible loads to provide more PR service to accommodate the net load variation.As such,the ramping cost of dispatchable generators is reduced by 29.09%in the test case.The planned energy curtailment from REG is also reduced.The computational efficiency of the proposed distributed clearing method is validated by comparing it with a centralized method.

Multi-stage Provincial Power Expansion Planning and Multi-market Trading Equilibrium

Decarbonization in the power sector is one of the critical factors in achieving carbon neutrality,and the top-level design needs to be carried out from the perspective of power planning.A multi-stage provincial power expansion planning(PPEP)model is proposed to simulate the power expansion planning at different stages of the power systems rich in renewable energy generation.This model covers 16 types of power supply,considering macro-policy demands and micro-operation constraints.The stand-alone capacity aggregation model for coal-based units within the PPEP model allows for accurate construction and retirement with different stand-alone capacities.Moreover,the soft dynamic time warping(soft-DTW)based K-medoids technique is adopted to generate typical scenarios for balancing the model accuracy and solution efficiency.Additionally,a multi-market trading equilibrium(MMTE)mechanism is proposed to address the differences in the levelized cost of energy between the coal-based and renewable-based units by participating in energy and ancillary service markets.Since the coalbased units take on the task of providing ancillary services from renewable-based units in the ancillary service market,the MMTE mechanism can effectively equalize the profits of both by having renewable-based units purchase ancillary services from coal-based units and pay for them,thus improving the motivation of coal-based units.A case study in Xinjiang province,China,verifies the effectiveness of the planning results of the PPEP model and the profit equilibrium realization of the MMTE mechanism.