基于辅助优化问题的安全约束机组组合约束削减方法

安全约束机组组合(Security-constrained Unit Commitment, SCUC)问题作为制定发电计划的核心环节,在电力系统优化调度等方面具有十分重要的意义。针对考虑故障态约束后SCUC问题规模庞大、难以求解的情况,提出了一种基于辅助优化问题的故障态安全约束削减方法。首先引入与具体故障态安全约束相关的辅助优化问题,从而建立判别相应故障态安全约束是否冗余的充分必要条件。然后探究冗余故障态安全约束辨识过程的具体加速方法,包括松弛辅助优化问题方法,使用可行性判据进行故障态安全约束预分类方法,以及多线程并行计算方法。最后,在IEEE118测试系统上对所提方法的正确性和有效性进行了仿真验证。     

Online detection of time-variant oscillations based on improved ITD

An online detector for time-variant oscillations in univariate time-series is proposed. This paper is motivated by the fact that it is still an open problem to design a real-time oscillation detector which is applicable to non-linear, non-stationary and intermittent oscillations. The proposed procedure is based on intrinsic time-scale decomposition (ITD) and contains (i) an improved iteration termination condition for ITD with on-line back-redecomposition and (ii) a novel hypothesis test with a robust statistic of variation coefficient which enables online monitoring of time-variant oscillations. The proposed approach is computationally efficient, does not require a priori supervision window and is better applicable for online detection of time-variant oscillations. In addition, it preserves nonlinear features in process variables which facilitates subsequent oscillation diagnosis. Simulation examples and industrial applications are provided to demonstrate the effectiveness of the proposed online oscillation detector.     

Towards reactive power dispatch within a wind farm using hybrid PSO

With the increasing penetration of wind power generation, the stringent grid codes are imposed by the system operators insisting the wind turbines to behave similar to that of synchronous generators. As a part of fulfilling the grid code in terms of steady state reactive power injection at point of common coupling, in this research work, the problem of optimal reactive power dispatch within a wind farm is formulated and a hybrid particle swarm optimization algorithm is proposed to get the optimal solution. The effectiveness of the proposed method is tested by simulations of the power collection grid of practical off-shore wind farm. The test results are compared with interior-point method based non-linear constrained optimization tool, fmincon, in Matlab. The analysis is carried out considering with and without wake effects on the wind farm at various grid bus voltage conditions. The test results demonstrate the effectiveness of the proposed method in achieving optimal solution for various operating conditions.     

A fault location criterion for MTDC transmission lines using transient current characteristics

Multi-terminal DC (MTDC) transmission system is applied in modern power systems due to its technical features. In an MTDC system, the converter stations are generally connected in parallel. To smooth the DC current output from the converter, the smoothing reactors are integrated in the converter outputs, instead of in the DC transmission line. When a fault occurs on the DC transmission line of the MTDC system, the fault current will flow along the whole DC line. As a result, the protection scheme may not locate the fault accurately. Based on the characteristics of DC filter, this paper analyzes the impedance characteristics of the combinational circuits, and proposes a method to identify the fault location by the characteristic harmonic. A protection criterion of the non-integer harmonics comparison is composed with phase-frequency characteristics curve derived from complex wavelet transform. The inevitable blind zone is analyzed in theory which exists on the terminal of transmission lines just like distance protection in AC systems. The algorithm is successfully tested and compared with other techniques.     

Confocal three-dimensional micro X-ray scatter imaging for non-destructive detecting foreign bodies with low density and low-Z materials in food products

Confocal three-dimensional (3D) micro X-ray scatter imaging technology based on polycapillary X-ray optics was used to carry out a point-to-point image analysis of foreign bodies in food products which were consisting of low density materials with low-Z elements. This confocal 3D micro X-ray scatter imaging method was based on a polycapillary focusing X-ray lens (PFXRL) in the excitation channel and a polycapillary parallel X-ray lens (PPXRL) in the detection channel. The PFXRL and PPXRL were placed in a confocal configuration. This let only the X-rays scattered from the confocal micro-volume overlapped by the output focal spot of the PFXRL and the input focal spot of the PPXRL be detected by the detector. The aim of this study was to demonstrate the ability of confocal micro X-ray scattering technology based on polycapillary X-ray optics to discriminate foreign bodies which adulterated in food products, and also utilize this confocal micro X-ray scattering technology to present scattered X-ray imaging for them. This confocal 3D micro-beam X-ray scattering imaging technology has potential applications in such fields as food safety, agriculture, bio-pharmaceutical, Meat Science, and so on.     

Wavelet technique based islanding detection and improved repetitive current control for reliable operation of grid-connected PV systems

Recent years have witnessed a thrust towards the use of solar energy as the major renewable energy source for distributed power generation. The proposed system requires reliable detection technique and suitable current control strategy for proper operation. This paper focuses on plug-in repetitive current (RC) control strategy for grid connected inverter system and wavelet technique for electrical grid status identification. The performance of proposed current control technique employed for grid connected inverter system under distorted and unbalanced grid voltage is compared with the existing conventional methods like PI and PR controller. This controller uses the feedback control system for attenuating periodic disturbances, improving high quality sinusoidal output current and high power factor. The proposed scheme employs fourth order infinite impulse response (IIR) filter for maintaining its resonance frequency, output frequency matching with grid fundamental frequency and reduction of harmonics. The DC-DC boost converter implements incremental conductance based (INC) maximum power point tracker (MPPT) algorithm. The effects of LCL filter for improving disturbance rejection capability and dynamic performance of the proposed system is also demonstrated. Grid connected PV inverter employs wavelet technique for an islanding detection functionality in order to determine the status of the electrical grid. In order to show the effectiveness of the proposed algorithm, modeling and simulation for grid connected PV system is performed using MATLAB/SIMULINK and its PowerSim toolbox.     

海上风电集群与火电打捆外送系统低电压穿越特性

海上风电集群与火电打捆外送系统的低电压穿越问题的研究具有重要的理论和实际意义。基于此,针对某海上风电集群与火电打捆实际规划场景,首先提出了风电集群中非故障风电场会恶化故障风电场低电压穿越的电网环境,风电集群对电网的低电压穿越支撑能力的要求更高。然后,从火电机组对系统短路比的提升作用和火电机组的无功支持作用2个方面,揭示了火电机组对海上风电集群的低电压穿越提升作用。最后,在PSCAD/EMTDC中搭建了某实际规划的海上风电集群与火电打捆外送场景,从时域角度证明上述理论分析的合理性。     

A feasible coordination protection strategy for MMC-MTDC systems under DC faults

Modular multilevel converter based multi-terminal HVDC (MMC-MTDC) technology is increasingly adopted as a promising option in the power transmission and distribution fields. However, how to isolate dc fault currents and ensure the continuous operation of the healthy parts of the MMC-MTDC system under dc faults presents a huge challenge. Based on a comprehensive analysis on the dynamic behavior of the dc fault current of a MMC, along with the operation characteristics of hybrid dc circuit breakers (CBs), a protection strategy will be developed. It coordinates the system parameters to ensure the hybrid dc CBs cut off the dc fault current before triggering MMC blocking in bipolar dc faults. The mathematical approximation of the dc fault current without converter blocking is achieved through theoretical analysis and numerical calculation, and the smoothing inductance at the dc side of MMC station has shown a major influence on the dynamic behavior of the dc fault currents. The smoothing inductance can be determined based on the mathematical expression of the dc fault current, as well as the breaking capacity of hybrid dc CBs. Data analysis and simulation are performed based on a three-terminal dc test system to validate the effectiveness of the coordination protection strategy.