Purpose/Aim
To precisely model the transient behavior of the 4-limb converter transformers used in the Kunliulong VSC-LCC HVDC project during faults or breakers operations, it is required to rewrite a new 4-limb transformer model instead of the built-in 2-limb single-phase UMEC model for use in PSCAD/EMTDC.
Experimental/Modeling methods
An equivalent 220V 4-limb transformer is built to get the required data for modeling the transformer in PSCAD.  The modified UMEC model takes the structure and the magnetic paths of the 4-limb iron core into account. In each time step, the model calculates the inductance matrix, and interfaces diretly to the eletric network so that nonliner power electronic load can be simulated correctly. Considering that the UMEC and converter model requires heavy computation load, it is suggested to improve the UMEC algorithm to meet the HVDC simulation requirements. In this paper, after the permeance and inductance matrixes in the UMEC algorithm are fully analyzed, only non-zero matrixes elements need to be calculated, therefore the simulation time is greatly reduced. The model is verified by the factory experiment datasheet and field experiment data of both 220V 4-limb prototype and 310kV transformer. The model is finally integrated into the Kunliulong HVDC PSCAD model and the results show that the model can precisely model the transient behavior of the power system.
Results/discussion
Compared to fully matrix calculations, the improved UMEC model can be used to model a non-linear transformer behavior in real-time. It means that the CPU time needed is smaller than the set simulation duration given an acceptable solution step (20us-50us) with INTEL I7 4790k. The research shows a great correlation between the model and the experiment results.
Conclusions
Since the UMEC model takes the structure of the transformer into account, the permeance calculation of the iron cores affects the results of the simulation the most. Therefore, the data derived from the factory datasheet must carefully be pre-processed and avoid unpredictable numerical oscillations in the simulation. The research shows that the accelerated UMEC algorithm developed for the 4-limb transformer model can precisely model the transient behavior of the converter transformers under various working conditions, such as no-load, magnetic inrush current, non-linear load(power electronics load), and provide technical support for fault transient, insulation coordination, protection analysis of the HVDC transformers.
 

non-linear,transformer,HVDC transmission,PSCAD/EMTDC,UMEC