| With the development of distributed generation technology,photovoltaic,wind power and other renewable energy equipment are connected to the power system.At the same time,due to industrial demand,a large number of nonlinear loads are also connected to the power grid.The increase of these devices increases the harmonic content in the power system,resulting in serious harmonic pollution in the power system.As an efficient harmonic dynamic compensation equipment,active power filter(APF)is widely used in power system harmonic compensation.At present,the research of high-capacity APF technology has made a breakthrough,but there are still some problems,such as high cost,large volume,difficult maintenance and so on.Therefore,the method of using small capacity APF cascade expansion to compensate harmonics is widely used.Using small capacity APF cascade to realize highpower power supply has become one of the important directions of power quality control.Taking the power system with harmonics as the main research object,this thesis considers the harmonic suppression method from the demand side and supply side of the power system,and puts forward the corresponding control strategy for the cooperative operation control of multiple APFs.The main work of this thesis is as follows:(1)The principle of a single APF is described and its main circuit mathematical model is established.Through the analysis of the mathematical model,it is found that there is inherent error in harmonic compensation of a single APF.The control strategy of multi modular APF cooperative operation is analyzed.For a single modular APF,its harmonic compensation mode is analyzed.Based on the instantaneous reactive power theory,the harmonic detection algorithms based on p-q theory and ip-iq theory are deduced;The common algorithms and Strategies of APF current tracking control are analyzed.(2)The harmonic compensation of the power system is analyzed from the demand side.For the harmonic compensation allocation of multiple small capacity APFs at a single node,based on the traditional proportional allocation strategy of compensation capacity,a compensation capacity allocation strategy considering the performance of APF is proposed.This strategy uses hierarchical distributed cooperative control strategy to control multiple APFs.On the basis of considering the capacity of each APF,the performance of each APF is also considered.The two work together to calculate the harmonic capacity to be compensated by each APF.At the same time,the communication topology and the performance allocation weight factor are introduced to control the weight of the instruction current allocated according to the performance in the algorithm.Finally,through stability analysis and simulation,it is verified that the strategy not only makes full use of the capacity of each APF in the system,but also makes use of their respective performance,and effectively improves the harmonic compensation ability of multi APF cascaded system to a single node without increasing the installation capacity.(3)The harmonic compensation is analyzed from the supply side of the power system.The harmonics of each node in the distribution network will be coupled and amplified,which poses a new challenge to the power quality control of the distribution network.In this thesis,the amplification mechanism of harmonics in distribution lines is analyzed,and APF is used to control harmonics in distribution network on the basis of this theory.Aiming at the balanced output of each APF,the communication topology is established between APFs and the information of their actual harmonic compensation power is transmitted to each other.The actual output power of APF participating in harmonic control in distribution line is reasonably optimized by using consistency algorithm.The distributed collaborative optimization of harmonics by multiple APFs in distribution network is realized.Finally,through simulation,three APFs with different rated capacity are configured on the established 10 node distribution line model to compensate harmonics.The simulation results show that this strategy can effectively dynamically coordinate the output capacity of each APF participating in harmonic compensation to achieve balanced output.The robustness and economy of multi APF harmonic compensation system in distribution network are improved. |
