Electromagnetic Transient Equivalent Modeling And Parallel Simulation Technology Of Medium-Voltage DC Transformers

The medium-voltage direct current distribution network represents a highly promising form of power distribution within both the current new energy system and the future energy internet.This is due to its large capacity,high power quality,and flexibility when it comes to connecting various types of renewable energy sources and loads.One of the critical devices for enabling interconnection between multiple devices and flexible power regulation within this system is the direct current transformer(DCT).With multiple AC/DC ports containing different voltage levels,the DCT can be widely connected to distributed renewable energy sources such as wind power,photovoltaics,and energy storage.Its power flow is multi-directional and controllable,enabling flexible power allocation coordination between different ports,rapid adjustment of the distribution network operating status,and improvement of energy distribution efficiency and quality.The DCT widely used at present generally adopts a cascaded modular structure,which can be quickly adjusted in module topology according to the application scenario and is also convenient for expansion and maintenance.However,it is precisely due to the DCT’s comprehensive functionality and high degree of modularization that its topological structure and control method complexity are also high.In terms of topology,the DCT power module is a multilevel and multi-port structure,with modules connected in series and parallel through different ports to meet the voltage level requirements of different renewable energy sources such as distributed wind power,photovoltaic power,and energy storage.In terms of control,the DCT needs to modulate highfrequency square waves within each power module to achieve DC voltage level transformation and power transmission functions,while also considering the voltage and power control requirements of the AC power grid.The complexity of the DCT’s topology and control makes equivalent modeling of its electromagnetic transients(EMT)face difficulties in modeling and extremely slow simulation speeds,thus requiring suitable equivalent modeling methods to break the dilemma of simulation accuracy versus simulation speed.This paper focuses on the electromagnetic transient(EMT)equivalent modeling of DCT,with the main goal of preserving accuracy and improving speed.The research content of this paper is as follows:(1)Proposing a novel low-precision loss equivalent modeling method for DCT.The intricate topology of the DCT topology structure is composed of numerous electrical nodes and diverse node-branch connection modes,leading to a sluggish solving speed for its electromagnetic transient simulation.To address this issue,this paper analyzes the topological characteristics of basic electrical components,power modules,and bridge arms in DCT and proposes the discretization equivalent and internal node elimination methods for each layer structure,which markedly reduces the dimension of nodal admittance matrix in electromagnetic transient simulation solving and accelerates the simulation speed.Given that the proposed equivalent method is based on the fully equivalent transformation of the analytical expression of circuit equation and does not simplify or modify the circuit itself,it can maintain high simulation accuracy.Additionally,this method can also retain all controller trigger signals,ensuring accurate simulation under various control modes and operating conditions.(2)Proposing the optimization method for improving the performance of DCT model.Although the circuit structure has been simplified,there are still a large number of matrix operations in the process of obtaining the equivalent parameters of the power module and the bridge arm,which affects the efficiency of simulation solving.To address this issue,this article optimizes the modeling process of the power module by using cut-set voltage equations to avoid matrix operations when converting node voltages to branch voltages.The high-frequency link port decoupling method of the power module is proposed to further simplify the circuit structure of the equivalent model and improve the simulation efficiency.At the bridge arm level,a parameter matrix cascading method is proposed to change the matrix operations of node elimination and inversion calculation,which repeatedly appears in recursive calculations,into direct addition of parameter matrices,reducing the number of matrix operations.In addition,an integrated blocking circuit in the bridge arm is proposed to avoid the state variable inheritance and circuit switching behavior during blocking-state simulation.After optimization,the DCT model maintains accuracy while achieving higher simulation efficiency.(3)Proposing the efficient parallel simulation and solving framework of DCT.In order to maintain the high precision of the DCT model,it is necessary to obtain simplified circuits with equivalent values through identity mathematical transformations.However,there is a large fixed computational overhead,making it difficult to further improve the simulation speed.To address this issue,this paper focuses on the parallel computing thechnique and designs a parallel computing framework for DCT based on the numerical independence of each power module in the simulation solving process.With the multi-core parallel computing ability of CPUs,the simulation speed of the proposed model can be increased by several times on the basis of the initial speedup,without affecting accuracy.