Research On The Efficient Modeling Methods For Cascaded H-Bridge Based Power Electronic Transformer

With the construction and development of the Energy Internet,as the core equipment of the "energy router",power electronic transformer(PET)has become a research hotspot in the academic and engineering field in recent years.Particularly,the cascaded H-bridge(CHB)based PET(CHB-PET)based on high-frequency transformer magnetic coupling has both high voltage AC port and low voltage DC port,indicating broad prospects in the fields of photovoltaic integration and AC/DC hybrid power distribution network.CHB-PET has three typical characteristics including "high frequency","isolation type" and "multiple modules",of which the "high frequency" and"multiple modules" lead to extremely slow offline simulation speed of the electromagnetic transient detailed model,curbing the progress of the simulation research and large-scale engineering application on the PET-contained power distribution network.Therefore.it is of great theoretical and engineering significance to study and develop an efficient PET modeling method in the premise of making sure sufficient simulation accuracy.Regarding the issue above,the main research work of this paper is as follows:Firstly,based on the Thevenin’s(Norton’s)equivalent and nested fast simultaneous solving algorithm,this paper proposes a CHB-PET electromagnetic transient simulation accelerated model,which is suitable for non-blocked condition.More precisely,by arranging flexibly the decoupling companion network of the primary/secondary side of the high-frequency transformer.the cascaded sub-modules were divided into two single-port networks,and then the equivalent part of the Thevenin’s(Norton’s)equivalent parameters were forwardly solved.Then,the equivalent part was substituted into the electromagnetic transient program for overall solution and inverse solution to initialize the electrical network for the next simulation step.Since the external equivalent form is a Thevenin’s(Norton’s)equivalent circuit with only 4 external nodes,the computational complexity of the proposed model in the overall system solution process hardly increases with the number of submodules.A 10kV/3kV CHB-PET system was built in PSCAD/EMTDC.and the simulation results validates the simulation accuracy and acceleration effect of the model proposed.Secondly,this paper puts forward an equivalent modeling method suitable for the blocking state of CHB-PET.A practical example was used to explain the circuit characteristics of the blocking state.After reasonable simplification,the Thevenin’s(Norton’s)equivalent parameters in the partially blocking and totally blocking states were derived.Then,three implementation methods of the blocking equivalent model were analyzed,and an integrated method of sharing the state variable storage unit with the non-blocked model was proposed.The simulation results of the startup process,AC voltage sag,and DC fault in PSCAD/EMTDC showed that the model proposed has sufficient simulation accuracy.Finally,aiming at further lifting the simulation efficiency and the utilization rate of CPU,by making use of the high parallelizability of the equivalent model proposed,this paper established a multi-processing parallel simulation framework for the equivalent model,put forward an equivalent modeling and parallel computing method for the CHB-PET with multiple active bridges at isolation stage,and evaluated and analyzed the factors affecting the acceleration performance of the parallel computing method.Based on the validation of the PSCAD/EMTDC simulation,the equivalent model proposed can achieve high-degree multiple-condition matching to the detailed model,the simulation speedup factor is able to reach two or even three orders of magnitude.Meantime,the parallel equivalent model based on the optimal number of threads can realize further acceleration 1.3 to 3 times faster than the serial equivalent model.