A Full-oder Model Construction Method Of D-Q Electrical Network And Power Flow Computation

Power system modeling is the basis of power system calculation,analysis and control.If the power system model is not accurate enough,it cannot reflect the system characteristics,and may change the qualitative conclusions or cover up some important phenomena in the critical state.The traditional power system model is usually based on the phasor model assumed as "quasi-steady state".The abc three-phase AC time-domain dynamic differential equation is transformed into an algebraic equation with power frequency characteristics for solving and analysis,which greatly simplifies the calculation.With the rising proportion of new energy,the problem of power system wide-frequency oscillation caused by large-scale low inertia power electronic equipment connected to the power system has become increasingly prominent.Since the quasi-stationary phasor is strictly used to represent the sinusoidal quantity with constant amplitude and frequency,it cannot reflect the high frequency characteristics of the power system,and it is impossible to model and analyze the wide-band oscillation problem of power system.Moreover,power electronic equipment is usually modeled and analyzed in dq reference frame.Due to the inconsistency of coordinate system between grid side and power electronic equipment side,it will affect the speed and accuracy of joint modeling,calculation and analysis between grid side and power electronic equipment side.To sum up,it is necessary to break through the limitations of the original quasi steady-state phasor modeling ideas and seek new modeling methods.To solve the above problems,this paper proposes a full-order model construction method of electrical network based on dq reference frame.In the three-phase symmetrical electrical network,the abc three-phase AC instantaneous value is transformed into d-axis and q-axis components by Park transformation.Therefore,the network model using dq reference frame is more concise in overall form,easy to analyze and solve,and more suitable for the application of large-scale complex power network;And abc/dq0 is based on the equivalent transformation between instantaneous values in time domain.Therefore,the full-order model of D-Q electrical network constructed by dq reference frame can reflect the real-time dynamic relationship between voltage,current and electrical network elements,and has a wide application prospect in power system dynamic numerical simulation.In addition,only the steady-state components in the full-order model of D-Q electrical network are taken into account,and the D-Q electrical network power flow equation is derived,which can solve and analyze the power flow.The specific work of this paper is as follows:Firstly,by analyzing the constitutive relations of RLC circuit,the dynamic model of electrical network components in the dq reference frame was established.The mathematical model of the electrical network was constructed through algebraic graph theory.That is,the full-order model of D-Q electric network including coupling terms,differential terms and coaxial algebraic terms was established,which provides a new idea and reference for power system dynamic numerical simulation and power flow computation.Secondly,when the three-phase symmetrical system operates stably,the d-axis and q-axis components of the full order model of D-Q electrical network are constant and the differential is0.Based on the node voltage equation of D-Q electrical network,the d-axis and q-axis components of the node injected current are taken as the unbalanced quantities for PQ and the active power and voltage amplitude are taken as the unbalanced quantities for PV nodes to construct the D-Q electrical network power flow model.Finally,the NR（Newton-Raphson）method was used to solve the D-Q power flow model iteratively.In the iterative process,because the off-diagonal elements of the Jacobian matrix corresponding to PQ nodes and voltage amplitude of PV nodes remain constant,the calculation efficiency can be improved.The proposed power flow method was tested and analyzed by IEEE-118,IEEE-300-bus system and other examples corresponding to different scenarios.The results show that the power flow method has the advantages of fast computing speed and insensitive to initial iteration values and loading levels.