Simulation study on transformer remanence demagnetization methods under new energy grid-connected harmonics

Abstract

Due to the growing use of renewable-energy sources in contemporary power systems, harmonic distortion and challenging operating conditions have also increased the relevance of transformer remanence-related issues. The residual magnetism in the transformer core upon de-energization can enhance excitation inrush current, local magnetic stress, and even influence safety and stability of the power system. Hence, precise measurement of remanence and successful demagnetization is theoretically and engineering-wise important.

The present thesis aims at measuring and removal of transformer remanence in the context of renewable-energy grid-connected harmonics in the field of engineering. It first examines the hysteresis properties of ferromagnetic core materials as well as the physics of how remanence is produced. Using this, an evolution model of flux-linkage with harmonic-contaminated excitation is then discussed and the theoretical applicability of the adaptive constant-voltage variable-frequency (CVVF) demagnetization technique is elaborated based on the point of view of voltage integration and flux reconstruction. The study demonstrates that the integrated harmonic voltage elements are presented into the flux-linkage equation with coefficients inversely related to harmonic order and angular frequency, which explains why the proposed method can reduce the disturbance level theoretically.

Subsequently, a closed-loop simulation model is created in MATLAB/Simulink and SimPowerSystems. The single-phase saturable transformer model with a controlled voltage source, signal acquisition modules, and an S-function based adaptive control algorithm is designed considering the nameplate parameters and magnetization properties of the transformer. The model represents the whole process of bidirectional saturation calibration, initial remanence calculation and exponential decay demagnetization. The simulation outcomes at various initial remanence levels indicate that the suggested approach can offer relatively high accuracy of the measurement of the remanence and lower the remanence ratio to the final residual level to a very small value. Moreover, three-phase transformer models are built to check the flexibility of the method in case of asymmetric initial remanence conditions.

In order to better represent the thesis background of harmonics in the renewable-energy grid-connected systems, an additional single-phase verification case is added by overlaying third and seventh harmonic components on the control input of the controlled voltage source, and maintaining the original model and control logic. The findings show that even though the voltage and current waveforms are increasingly distorted, the reconstructed flux-linkage trajectory remains orderly shrinking and the demagnetization process will continue to be accomplished successfully. It also supports the fact that the suggested CVVF technique would be able to withstand a particular level of robustness in the conditions of harmonic-contaminated excitation.

The paper offers a fairly full simulation and analysis model of transformer remanence measurement and demagnetization, as well as can be used to provide practical assistance in the creation of demagnetization strategies, enhancement of maintenance procedures, and operation of transformers in complex power-system conditions.