Abstract Model predictive direct power control (MPDPC) has strong robustness and fast dynamic response, so it is widely applied in the control system of grid‐connected converter. However, the traditional FCS‐MPC bears with variable switching state, which will reduce the current tracking accuracy, accidentally produce current ripple and electromagnetic noise. Here, a double closed‐loop control strategy with constant switching frequency based on optimal switching sequence synthesis of model predictive direct power control (MPDPC‐CF) in the inner loop and SMC control in the outer loop is proposed for the non‐linear system of Vienna rectifier. Firstly, the Vienna rectifier is modelled to obtain the predicted values of input power. Then, the given values of active power and reactive power are obtained through the outer loop. Three adjacent voltage vectors with the lowest cost function in the sector are selected to synthesize the optimal voltage vector, and the pulse time of the corresponding vector is calculated according to the cost function of the voltage vector. To verify the correctness of the theoretical analysis, the Vienna rectifier is taken as the research object, and the comparison with the traditional MPDPC shows that the proposed constant frequency model predictive control has good steadystate and dynamic performance.

According to the shortcomings of the current traveling wave ranging method and the problems existing in the wave head measurement algorithm, by analyzing the variation of traveling wave velocity with frequency, we can deduce the frequency region coverage of the wave velocity stabilization time, so as to overcome the negative impact of the wave velocity variation on the ranging accuracy conclusion. In the further study of high-voltage direct current transmission line fault traveling wave arrival time and traveling wave must be based on the instantaneous amplitude correlation theory, analyzes the instantaneous amplitude directly affects the mechanism of the traveling wave ranging, and gives a transmission line fault distance scheme based on the instantaneous amplitude, according to using the Hilbert huang transform (HHT), an iterative fashion, low voltage high frequency transmission failure is established According to the time-lapse diagram, the traveling wave head time is determined to be the instantaneous amplitude at a certain time, so as to establish the fault distance calculation of HVDC transmission line. A lightning point location method for transmission line based on Hilbert Huang transform (HHT) is proposed. When OPGW is struck by lightning, the transmitted light in its internal communication fiber generates polarization state mutation signal due to Faraday effect. Firstly, the polarization state signal is denoised, and then the polarization state signal after noise reduction is Hilbert Huang transformed and processed to obtain the spectrum of amplitude and frequency changing with time, Thus, the time when the polarization state mutation signal reaches the polarization state demodulation equipment is obtained to locate the lightning point.

This paper develops a Sobol sensitivity analysis model of loss of the distribution transformer and line considering complex power quality, to quantitatively analyze the interactive effects of voltage deviation, harmonic and three-phase imbalance on specific component loss, and the most sensitive factors are sought. Firstly, the calculation model of distribution network loss with complex power quality is given. Using the Sobol sensitivity analysis method, the first sensitivity coefficient (FSC) and total sensitivity coefficient (TSC) are deduced according to the loss model, and the sobol sequence is selected for sampling. In addition, FSC and TSC results of the transformer and line are obtained, and the quantitative interaction of among power quality factors on loss is given and analyzed.

有限集模型预测控制(finite control set model predictive control,FCS-MPC)由于便于植入、可实现多目标跟踪等优点,在并网变换器控制系统中获得了广泛关注.然而,电感参数失配将直接影响FCS-MPC的预测精度.此外,传统FCS-MPC开关频率不固定导致并网电流纹波大、权重因子取值困难、迭代寻优过程繁琐.基于此,该文针对Vienna整流器预测控制存在的控制误差大、权重系数整定难、运算量大的问题,在建立滤波电感对并网性能的解析基础上,提出一种引入电感参数在线辨识的直接功率快速模型预测恒频控制策略(fast model predictive with constant frequency based on parameters online identification,F-MPCCF).通过优选对中点电位平衡有利的冗余矢量,构建了基于直接功率的单目标代价函数,有效避免了权重因子选取问题,结合零矢量调节的方式进而实现类SVPWM调制,实现了开关状态的平滑切换.最后,从静态、暂态、中点电位平衡控制等多个维度进行了仿真与测试分析验证,结果表明所提出的F-MPCCF具有良好的稳态和动态性能.

面向并网变换器的传统模型预测直接功率控制(model predictive direct power control,DPMPC)常利用增加电压矢量数量或矢量区间的精确划分以实现定频控制.针对其导致处理器运算量增加、处理时间长的问题,提出一种基于优选开关矢量合成的Vienna整流器模型预测直接功率控制(direct power mode predictive control with constant frequency,DPMPC-CF)与滑模(sliding-mode control,SMC)外环控制的的双闭环复合控制方法.通过对Vienna整流器建模得到输入功率的预测值,利用SMC外环计算有功功率的给定值.在每个扇区内,通过优选代价函数最小所对应的3个电压矢量,采用代价函数计算相应电压矢量的占空比,进而实现类空间矢量脉宽调制(space vector pulse width modulation,SVPWM).最后,从静态、暂态、调制信号等多个维度与传统的DPMPC进行对比验证,结果表明,所提出的DPMPC-CF具有良好的稳态和动态性能.

The control system design of Vienna rectifier is inseparable from the parameters of grid voltage, which will increase the cost and complexity when using voltage sensors. Therefore, this paper proposes a grid voltage sensorless model predictive control strategy for Vienna rectifier. Firstly, the virtual flux mathematical model of Vienna rectifier is established to estimate the grid voltage. Due to the mathematical model contains integral operation, the integrator is modified to a second-order low-pass filter (LPF) by solving the problems of integral initial value and dc bias to improve the observation effect. Then, the given values of active power and reactive power of the system are obtained through the outer loop. Lastly, combined with direct power and model predictive control (DPMPC), it can quickly find the switch sequence of tracking target. A simplified vector selection is also used to the system to reduce the computational complexity. Thus, it is verified the correctness of the theoretical analysis, and the results shows that the proposed model predictive direct power control has good steady-state and dynamic performance.

Energy saving and consumption reduction is one of the important means for power supply enterprises to improve their economic efficiency. However, with the widespread access of distributed generation and non-linear loads, the power quality problems make it difficult to neglect the additional losses in the distribution network. Firstly, the additional loss caused by three-phase unbalance is theoretically derived; secondly, according to the superposition principle, the formula for calculating the additional loss generated by distribution line and transformer when harmonics exist alone is derived; then the formula for calculating the additional loss of each part when the two are considered together is derived. Finally, the losses of the line and distribution transformer are simulated for each case by using the Matlab platform. The results show that the theoretical derivation is in good agreement with the simulation results.

Three-phase LCL-Filter NPC-type grid-connected inverter has been widely used in the renewable energy generation systems with the advantages of high efficiency, high power and low cost.Finite control set model predictive control (FCS-MPC) has strong robustness and fast dynamic response, so it is widely used in grid-connected inverter control. However,the traditional FCS-MPC bears with irregular switching state,which will reduce the current tracking accuracy, produce current ripple and electromagnetic noise.Aiming at the above shortcomings, an efficient and fixed operation frequency model predictive without weighting factor is presented to ensure fixed operation switching frequency.The voltage vector of different voltage vectors is selected according to the location of the voltage vector reference, and the switching time of the selected are directly calculated by the inversely proportional with cost function value of the vectors. To verify the correctness and effectiveness of the proposed control strategy, a complete simulation model and experimental prototype platform under the digital control is established. The simulation and experimental results show that the steady and the transient performance of the grid current with the proposed control strategy are effectively improved.

A three-level T-type grid-connected inverter has been widely used in the current medium power distributed PV inverters, charging station and active power filter system with the advantages of high equivalent switching frequency, high efficiency, small filter inductance and grid current harmonic. However, the grid-connected inverter with LCL-filter is a third-order and multi-variable system, to meet the high-performance requirements for grid-connected current, claiming a higher requirement for the grid-current control system. Aiming at this, a new control strategy named capacitive current feedforward control based on the carrier-based PWM modulation scheme on is put forward. Further, to improve the power density, the capacitor current is generated indirectly calculated by the capacitor voltage. Finally, to verify the correctness and effectiveness of the proposed control strategy, a complete digital simulation model and experimental prototype platform is established, the detailed theoretical analysis and design method of the proposed control strategy are presented. The simulation and experimental results show that the steady and the transient performance of the grid current with the proposed control strategy are effectively improved, and no additional sensors are needed.

Three-phase NPC-type grid-connected inverter has been widely used in the renewable energy generation systems with the advantages of high efficiency, high power and low cost. Finite control set model predictive control (FCS-MPC) has strong robustness and fast dynamic response, so it is widely used in the control system of grid-connected inverter. However, the traditional FCS-MPC bears with irregular switching state, which will reduce the current tracking accuracy, accidentally produce high current ripple and electromagnetic noise. Aiming at the above shortcomings, an efficient and fixed operation frequency model predictive based on the discrete space vector modulation (MPC-DSVM) is presented in this paper. Firstly, the mathematical model and voltage vector of NPC-type converter are given according to the principle of deadbeat control. By applying the virtual vectors which are linear synthetized with the real vectors, the MPC-DSVM method can output more voltage vectors with fixed switching frequency. The results obtained show that the current tracking accuracy is improved, the current distortion and imbalance of neutral point potential are reduced.

Load flow calculation for droop-controlled islanded microgrids (IMGs) is different from that of transmission or distribution systems due to the absence of slack bus and the variation of frequency. Meanwhile considering the common three-phase imbalance condition in low-voltage systems, a load flow algorithm based on the direct Newton–Raphson (NR) method with step size optimisation for both three-phase balanced and unbalanced droop-controlled IMGs is proposed in this study. First, the steady-state models for balanced and unbalanced droop-controlled IMGs are established based on their operational mechanisms. Then taking frequency as one of the unknowns, the non-linear load flow equations are solved iteratively by the NR method. Generally, iterative load flow algorithms are faced with challenges of convergence performance, especially for unbalanced systems. To tackle this problem, a step-size-optimisation scheme is employed to improve the convergence performance for three-phase unbalanced IMGs. In each iteration, a multiplier is deduced from the sum of higher-order terms of Taylor expansion of the load flow equations. Then the step size is optimised by the multiplier, which can help smooth the iterative process and obtain the solutions. The proposed method is performed on several balanced and unbalanced IMGs. Numerical results demonstrate the correctness and effectiveness of the proposed algorithm.

Based on the analysis of fault current characteristics of distribution network branch with DG, matrix algorithm for fault location is optimized. The branch contained with distributed generations is improved. To begin with, fault associated area is located by the initial mutations quantity from sum of positive-sequence component and negative-sequence component rms. What’s more, fault line is located by comparing both sides of the line current amplitude variation characteristics. Comprehensive amplitude is simplified, protection scheme is improved for distributed generations branch, and other branches remain unchanged in order to reduce equipment, keep cost saving, and locate fault line. The typical model is built on PSCAD/EMTDC platform for proving the protection algorithm in both symmetric and asymmetric fault. The simulation results of different fault type results show the effectiveness of this scheme.