Primary frequency control considering communication delay for grid-connected offshore wind power systems

Offshore wind farms are becoming increasingly distant from onshore centralized control centers,and the communication delays between them inevitably introduce time delays in the measurement signal of the primary frequency control.This causes a deterioration in the performance of the primary frequency control and,in some cases,may even result in frequency instability within the power system.Therefore,a frequency response model that incorporates communication delays was established for power systems that integrate offshore wind power.The Padéapproximation was used to model the time delays,and a linearized frequency response model of the power system was derived to investigate the frequency stability under different time delays.The influences of the wind power proportion and frequency control parameters on the system frequency stability were explored.In addition,a Smith delay compensation control strategy was devised to mitigate the effects of communication delays on the system frequency dynamics.Finally,a power system incorporating offshore wind power was constructed using the MATLAB/Simulink platform.The simulation results demonstrate the effectiveness and robustness of the proposed delay compensation control strategy.

Optimization of the Gas Generator in Composite Power System with Tip-Jet Rotor

The key and bottleneck of research on the tip-jet rotor compound helicopter lies in the power system. Computational Fluid Dynamics (CFD) was used to numerically simulate the gas generator and rotor inner passage of the tip-jet rotor composite power system, studying the effects of intake mode, inner cavity structure, propellant components, and injection amount on the characteristics of the composite power system. The results show that when a single high-temperature exhaust gas enters, the gas generator outlet fluid is uneven and asymmetric;when two-way high-temperature exhaust gas enters, the outlet temperature of the gas generator with a tilted inlet is more uniform than that with a vertical inlet;adding an inner cavity improves the temperature and velocity distribution of the gas generator's internal flow field;increasing the energy of the propellant is beneficial for improving the available moment.

Electrical characteristics of new three-phase traction power supply system for rail transit

A novel three-phase traction power supply system is proposed to eliminate the adverse effects caused by electric phase separation in catenary and accomplish a unifying manner of traction power supply for rail transit.With the application of two-stage three-phase continuous power supply structure,the electrical characteristics exhibit new features differing from the existing traction system.In this work,the principle for voltage levels determining two-stage network is dissected in accordance with the requirements of traction network and electric locomotive.The equivalent model of three-phase traction system is built for deducing the formula of current distribution and voltage losses.Based on the chain network model of the traction network,a simulation model is established to analyze the electrical characteristics such as traction current distribution,voltage losses,system equivalent impedance,voltage distribution,voltage unbalance and regenerative energy utilization.In a few words,quite a lot traction current of about 99%is undertaken by long-section cable network.The proportion of system voltage losses is small attributed to the two-stage three-phase power supply structure,and the voltage unbal-ance caused by impedance asymmetry of traction network is less than 1‰.In addition,the utilization rate of regenerative energy for locomotive achieves a significant promotion of over 97%.

Power transfer efficiency in an air-breathing radio frequency ion thruster

Due to a series of challenges such as low-orbit maintenance of satellites, the air-breathing electric propulsion has got widespread attention. Commonly, the radio frequency ion thruster is favored by low-orbit missions due to its high specific impulse and efficiency. In this paper, the power transfer efficiency of the radio frequency ion thruster with different gas compositions is studied experimentally, which is obtained by measuring the radio frequency power and current of the antenna coil with and without discharge operation. The results show that increasing the turns of antenna coils can effectively improve the radio frequency power transfer efficiency, which is due to the improvement of Q factor. In pure N_2 discharge,with the increase of radio frequency power, the radio frequency power transfer efficiency first rises rapidly and then exhibits a less steep increasing trend. The radio frequency power transfer efficiency increases with the increase of gas pressure at relatively high power, while declines rapidly at relatively low power. In N_(2)/O_(2) discharge, increasing the N_(2) content at high power can improve the radio frequency power transfer efficiency, but the opposite was observed at low power. In order to give a better understanding of these trends, an analytic solution in limit cases is utilized, and a Langmuir probe was employed to measure the electron density. It is found that the evolution of radio frequency power transfer efficiency can be well explained by the variation of plasma resistance, which is related to the electron density and the effective electron collision frequency.

Ladder Time Stepwise Inertia Coordinated Control Method of Multiple Wind Farms to Suppress System Frequency Secondary Drop

Currently,both regulated and deregulated power trading exist in China’s power system,which has caused imbalanced funds in the electricity market.In this paper,a simulation analysis of the electricity market with wind energy resources is conducted,and the calculation methods of unbalanced funds are investigated systematically.In detail,the calculation formulas of unbalanced funds are illustrated based on their definition,and a two-track electricity market clearing model is established.Firstly,the concept of the dual-track system is explained,and the specific calculation formulas of various types of unbalanced funds are provided.Next,considering the renewable energy consumption,the market clearing model based on DC power flow is constructed and solved;by combining fitting methods of mid-and long-term curves,the unbalanced funds are calculated based on clearing results and formulas.

Influence of flue gas cleaning system on characteristics of PM_(2.5)emission from coal-fired power plants

This study investigated the influence of precipitators and wet flue gas desulfurization equipment on characteristics of PM_(2.5)emission from coal-fired power stations.We measured size distribution and removal efficiencies,including hybrid electrostatic precipitator/bag filters(ESP/BAGs)which have rarely been studied.A bimodal distribution of particle concentrations was observed at the inlet of each precipitator.After the precipitators,particle concentrations were significantly reduced.Although a bimodal distribution was still observed,all peak positions shifted to the smaller end.The removal efficiencies of hybrid ESP/BAGs reached 99%for PM_(2.5),which is considerably higher than those for other types of precipitators.In particular,the influence of hybrid ESP/BAG operating conditions on the performance of dust removal was explored.The efficiency of hybrid ESP/BAGs decreased by 1.9%when the first electrostatic field was shut down.The concentrations and distributions of particulate matter were also measured in three coal-fired power plants before and after desulfurization devices.The results showed diverse removal efficiencies for different desulfurization towers.The reason for the difference requires further research.We estimated the influence of removal technology for particulate matter on total emissions in China.Substituting ESPs with hybrid ESP/BAGs could reduce the total emissions to 104.3 thousand tons,with 47.48 thousand tons of PM_(2.5).

Distributed Model Predictive Load Frequency Control of Multi-area Power System with DFIGs

Reliable load frequency control LFC is crucial to the operation and design of modern electric power systems. Considering the LFC problem of a four-Area interconnected power system with wind turbines, this paper presents a distributed model predictive control DMPC based on coordination scheme. The proposed algorithm solves a series of local optimization problems to minimize a performance objective for each control area. The generation rate constraints GRCs, load disturbance changes, and the wind speed constraints are considered. Furthermore, the DMPC algorithm may reduce the impact of the randomness and intermittence of wind turbine effectively. A performance comparison between the proposed controller with and without the participation of the wind turbines is carried out. Analysis and simulation results show possible improvements on closed-loop performance, and computational burden with the physical constraints. © 2014 Chinese Association of Automation.

Neural-Network-Based Terminal Sliding Mode Control for Frequency Stabilization of Renewable Power Systems

This paper addresses a terminal sliding mode control(T-SMC) method for load frequency control(LFC) in renewable power systems with generation rate constraints(GRC).A two-area interconnected power system with wind turbines is taken into account for simulation studies. The terminal sliding mode controllers are assigned in each area to achieve the LFC goal. The increasing complexity of the nonlinear power system aggravates the effects of system uncertainties. Radial basis function neural networks(RBF NNs) are designed to approximate the entire uncertainties. The terminal sliding mode controllers and the RBF NNs work in parallel to solve the LFC problem for the renewable power system. Some simulation results illustrate the feasibility and validity of the presented scheme.

Effect of power frequency excitation character on ferroresonance in neutral-grounded system

In most earlier ferroresonance studies the traditional excitation characteristic of iron core, in which the traditional excitation characteristic contains harmonic voltages or currents, has been used as if it were made up of pure fundamental voltage or current. However, this is not always true. In comparison with traditional excitation characteristics, this paper introduces the power frequency excitation characteristic of the iron core, which contains no harmonics. The power frequency excitation characteristic of iron core has been obtained by Elector Magnetic Transient Program, resulting in discrete voltage and current pairs. Extensive simulations are carried out to analyse the effect of power frequency excitation characteristic on potential transformer ferroresonance. A detailed analysis of simulation results demonstrates that with power frequency excitation characteristic of iron core inclusion at certain excitation voltage the ferroresonance may happen, conversely it may not happen with traditional excitation characteristic inclusion.

Dynamic characteristics of hydraulic power steering system with accumulator in load-haul-dump vehicle

Using hydraulic power steering system of model EIMCO 922 load-haul-dump vehicle as a simulation example, the dynamic characteristics of hydraulic power steering system in load-haul-dump vehicle were simulated and discussed with SIMULINK software and hydraulic control theory. The results show that the dynamic characteristics of hydraulic power steering system are improved obviously by using bladder accumulator, the hydraulic power steering system of model EIMCO 922 load-haul-dump vehicle generates vibration at the initial stage under the normal steering condition of pulse input, and its static response time is 0.25 s shorter than that without bladder accumulator. Under the normal steering working condition, the capacity of steering accumulator for absorbing pulse is directly proportional to the cross section area of connecting pipeline, and inversely proportional to the length of connecting pipeline. At the same time, the precharge pressure of nitrogen in steering accumulator should be 60%80% of the rated minimum working pressure of hydraulic power steering system. Under the abnormal steering working condition, the steering cylinder piston may obtain higher motion velocity, and the dynamic response velocity of hydraulic power steering system can be increased by reducing the pressure drop of hydraulic pipelines between the accumulator and steering cylinder and by increasing the rated pressure of hydraulic power steering system, but the dynamic characteristics of hydraulic power steering system in load-haul-dump vehicle have nothing to do with the precharge pressure of nitrogen in steering accumulator.

Study on Characteristic of Electric Power Assist Steering System

A pinion-type electric power steering (EPS) equipped on a sedan is reached in this paper. A three-freedom dynamic model of this system is created. The variables affecting assist character is analyzed. The formulas of simpled steering resistance force and the relationship between assist gain and vehicle speed are presented for the first time. Assist character is found based on the parameters of a sedan at last. This assist character is fit for the control rule of the EPS system through analyzing this character. The assist character figure offers reference for system design and control. Furthermore, this research method has generality for assist character of different kinds of vehicles.

Frequency Regulation of Power Systems With a Wind Farm by Sliding-Mode-Based Design

Load frequency regulation is an essential auxiliary service used in dealing with the challenge of frequency stability in power systems that utilize an increasing proportion of wind power.We investigate a load frequency control method for multiarea interconnected power systems integrated with wind farms,aimed to eliminate the frequency deviation in each area and the tie-line power deviation between different areas.The method explores the derivative and integral terminal sliding mode control technology to solve the problem of load frequency regulation.Such technology employs the concept of relative degrees.However,the subsystems of wind-integrated interconnected power systems have different relative degrees,complicating the control design.This study develops the derivative and integral terminal sliding-mode-based controllers for these subsystems,realizing the load frequency regulation.Meanwhile,closed-loop stability is guaranteed with the theory of Lyapunov stability.Moreover,both a thermal power system and a wind power system are applied to provide frequency support in this study.Considering both constant and variable external disturbances,several numerical simulations were carried out in a two-area thermal power system with a wind farm.The results demonstrate the validity and feasibility of the developed method.

Robust H_∞ Load Frequency Control of Multi-area Power System With Time Delay:A Sliding Mode Control Approach

This paper is devoted to investigate the robust H∞sliding mode load frequency control(SMLFC) of multi-area power system with time delay. By taking into account stochastic disturbances induced by the integration of renewable energies,a new sliding surface function is constructed to guarantee the fast response and robust performance, then the sliding mode control law is designed to guarantee the reach ability of the sliding surface in a finite-time interval. The sufficient robust frequency stabilization result for multi-area power system with time delay is presented in terms of linear matrix inequalities(LMIs). Finally,a two-area power system is provided to illustrate the usefulness and effectiveness of the obtained results.

A probabilistic power flow calculation method considering the uncertainty of the static frequency characteristic

In a power system, power generation and load have frequency response characteristics, which randomly fluctuate with changes in operating status. This study investigates a probabilistic power flow method that considers the unit and load uncertainty of the static frequency characteristic. Firstly, a calculation model is established on the basis of the characteristics of the frequency modulation performance of the unit and load. Then a calculation method is developed using the concept of dynamic power flow in order to determine the probability distribution of the active power flow of each line under the occurrence of a fault in the system. In the method, Monte Carlo sampling with the semi-invariant method is applied for analysis and calculation. The IEEE-30-buses system is taken as an example to analyze the impact of different responses of units on the power flow distribution of various branches. The method discussed herein is compared with the Monte Carlo simulation method to verify its effectiveness.

The Research of Voltage Sag and Power Frequency Overvoltage on 110kV Resistance Grounding System

This paper researches the voltage transfer characteristics when one-phase ground fault occurred in the resistance grounding system, by using the theory of the asymmetric variable characteristics and the sequence network analysis of the -11 transformer, and concludes the scope of voltage sag and swell and the degree of power frequency overvoltage and their influencing factors in the 110 kV resistance grounding system. Accordingly this paper puts forward the resistance choosing principle: the resistance grounding coefficient must be equal to or greater than 10. So it can not only wipe out the voltage sag and voltage swell but also make sure the overvoltage is limited to electrical equipment allowing range. The method mentioned above is verified by simulation results of a 110 kV power system in ATP.

Analysis of Nonlinear Characteristics of Turbine Governor and Its Impact on Power System Oscillation

In this paper, the model of turbine governor?based on the physical principles is constituted to improve the accuracy of power system dynamic simulation, making the results of simulation consistent with the actual situation. The unit and grid coupling model which reflects the interaction between thermal system of power plant and power system is built using EMTDC/PSCAD. The influence of nonlinear characteristics on the valve opening and the steam turbine mechanical power is also analyzed in this paper. The results of simulation show that the improper setting of parameters reflecting nonlinear characteristics of turbine governor can lead to the cyclical oscillations of the valve opening and the?steam turbine mechanical power, and even can lead to the power oscillation persistently. The research achievements in this paper have a certain reference value on exploring the causes of power oscillation from the prime mover.

Frequency Control of Power System with Solar and Wind Power Stations by Using Frequency Band Control and Deadband Control of HVDC Interconnection Line

In recent years, environmental problems are becoming serious and renewable energy has attracted attention as their solutions. However, the electricity generation using the renewable energy has a demerit that the output becomes unstable because of intermittent characteristics, such as variations of wind speed or solar radiation intensity. Frequency fluctuations due to the installation of large scale wind farm (WF) and photovoltaics (PV) into the power system is a major concern. In order to solve the problem, this paper proposes two control methods using High Voltage Direct Current (HVDC) interconnection line to suppress the frequency fluctuations due to large scale of WF and PV. Comparative analysis between these two control methods is presented in this paper. One proposed method is a frequency control using a notch filter, and the other is using a deadband. Validity of the proposed methods is verified through simulation analyses, which is performed on a multi-machine power system model.

Frequency Control of Power System with Renewable Power Sources by HVDC Interconnection Line and Battery Considering Energy Balancing

Recently, introduction of renewable energy sources like wind power generation and photovoltaic power generation has been increasing from the viewpoint of environmental problems. However, renewable energy power supplies have unstable output due to the influence of weather conditions such as wind speed variations, which may cause fluctuations of voltage and frequency in the power system. This paper proposes fuzzy PD based virtual inertia control system to decrease frequency fluctuations in power system caused by fluctuating output of renewable energy sources. The proposed new method is based on the coordinated control of HVDC interconnection line and battery, and energy balancing control is also incorporated in it. Finally, it is concluded that the proposed system is very effective for suppressing the frequency fluctuations of the power system due to the large-scale wind power generation and solar power generation and also for keeping the energy balancing in the HVDC transmission line.

The research of power saving frequency conversion speed regulation experiment system

A power saving frequency difference controlling method was introduced by the double inverter and motor experiment system.The characters of the system under differ- ent loads were investigated.The theoretical analysis and experiment results show the frequency difference method is a ideal power saving speed regulation method for the dou- ble inverter and motor system.The experiment system is simply structured,convenient to operate and provides a new way of character testing for frequency conversion speed regulation.

JOINT ESTIMATION OF CARRIER FREQUENCY OFFSET AND POWER DELAY PROFILE FOR MIMO-OFDM SYSTEMS

This paper develops a Cyclic Prefix(CP)based joint Maximum-Likelihood(ML)estima-tion algorithm of Carrier Frequency Offset(CFO)and Power Delay Profile(PDP)for Multi-InputMulti-Output Orthogonal Frequency Division Multiplexing(MIMO-OFDM)systems.However,theexact solution of the joint ML estimation is very complex since it needs a search over amulti-dimensional domain.Thus a simplified method is proposed to estimate the CFO and the PDPiteratively via the alternating-projection method which could induce the multidimensional searchproblem to a sequence of simple one-dimensional searches.Simulations show that the proposed algo-rithm is more accurate and robust than the existing algorithms.