Studies On Systemic Analysis Methods And Application Technologies Of Power Quality Of Smart Distribution Network

With the integration of DGs and the improvement of power-electronized level of network equipments and loads,power quality issues of smart distribution network(SDN),such as harmonic(harmonic resonance,harmonic instability,harmonic propagation),negative sequence and voltage sag,are becoming more and more complex.As important functions of SDN,online prevention and control,self-healing recovery and fault locating,are of great significance to improve the operation reliability as well as ensure the safe and stable operation of SDN.Under the above background,to meet the needs of prevention and control of SDN,two novel method are firstly proposed.On the one hand,for the more complex harmonic resonance,which is caused by the frequent change of harmonic node admittance matrix(HRAM),a novel fast construction method of harmonic resonance parameter security region(HRPSR)is proposed.On the one hand,for the increasingly prominent issue of wideband oscillation,which is caused by the interaction of DG control systems and the power grid components,a novel fast construction method of the wideband oscillation parameter stable region(WOPSR)is proposed.Then,to meet the need of self-healing recovery of SDN,considering the threats to the safe operation from power quality issues,such as harmonic and negative sequence components,an island division strategy considering the effects of power quality factors is proposed.Finally,to meet the need of fault locating in SDN,considering that the short-circuit current distribution of SDN is different from that of the traditional single power distribution network,a novel accurate locating strategy of voltage sag source based on limited measurement data is proposed.Detailed illustrations are as follows.1)For prevention and control of harmonic resonance,a fast construction method of HRPSR is proposed.At first,the mathematical essence of harmonic resonance is summarized via the modal analysis method.Then,from the singularity of HRAM,the explicit sufficient condition that the elements of HRAM should meet to avoid harmonic resonance are derived using the extended disk theorem,which combines the basic Gerschgorin disk theorem and matrix similar transformation(MST).Further,based on the non-unique characteristic of the MST,an optimal programming model with linear constraints is established and solved to obtain the optimal similar transformation matrix(STM)to make the conservatism of the estimated security domain as small as possible.The proposed method does not need eigenvalue calculations and has high analysis efficiency.It shows strong applicability to the fast construction of HRPSR for SDN,whose operation mode is complex and changeable.The test results based on 4 different scale examples show that the larger the scale of the system or the more the variables to be analyzed,the more obvious the advantage of the proposed in analysis efficiency will be.2)For the prevention and control of wideband oscillation,a fast construction method of WOPSR is proposed.From the view that all the eigenvalues of the system state matrix should locate at the left side of the virtual axis,the explicit sufficient condition that the state matrix elements should meet to ensure the stability of the system is derived via the extended disk theorem.Furthermore,based on the non-unique characteristic of STM,a linear constrained optimization programming model for searching the optimal STM is established and solved to obtain the WOPSR with the minimum conservatism.The proposed method does not need eigenvalue calculations.The results of 2 different scale examples show that the proposed method has much higher efficiency compared with the method using point by point eigenvalue calculation.It shows strong applicability to the fast construction of WOPSR for SDN,whose operation mode is complex and changeable.3)For the issue of active island division,an strategy considering the effects of power quality factors is proposed.For each initial island determined via fast network connectivity test,a two-layer optimization model of load-shedding and structure reconstruction is established to effectively decompose the dimension of the optimization space to avoid combination explosion and get a feasible solution more quickly.The constraint set considering the uncertainty of the negative sequence and harmonic components are established to optimize the uncertainty propagation characteristics of negative sequence and harmonic components to avoid the potential island operation failure caused by the unreasonable negative sequence and harmonic indexes.The proposed strategy can obtain a more reliable active island division scheme faster.The test results based on a 94-node complex distribution network show that the proposed strategy has better power quality indexes than the traditional strategy,and has the higher operation reliability and efficiency.4)For the issue of voltage sag source locating,a novel accurate locating strategy based on limited measurement data is proposed.First,based on the directions of sequence power increments,the relationship between the sag source and the observation points is judged to determine the candidate region of the sags.Then,the cause of the sag is identified.For the voltage sag caused by electrical fault(short circuit fault),on the one hand,based on inverse theory,a step-by-step optimization model to locate the nearest node and fault location is established and solved.Its optimization variables are information of fault location and transition resistance,and the object is to minimize the total error between the actual observation values and the theoretical values of voltage and current phasors at the limited observation points.On the other hand,the short circuit calculation method considering the effects of inverter interfaced DGs(IIDGs)is analyzed.By these ways,the applicability of the proposed strategy to SDN can be guaranteed,and the side effects of the inaccurate calculation of short circuit calculation and the inaccurate estimation of transition resistance on locating accuracy can be overcome.Based on the improved IEEE 33-node benchmark and the improved IEEE 69-node benchmark,it shows that the greater the capacity of IIDG or the greater the transition resistance value,the more prominent the advantage of the proposed strategy in locating accuracy.