Study For Characteristics Of Protections With DC Magnetic Bias

For DC bias impact on power system, the scholars mainly study suppressionmeasures for DC bias, but in the operation of all the measure that can not completelyavoid the DC current flowing into the grid, and the DC bias current will affect theprotection. So, this article studies the DC magnetic bias impact on protection, DCcurrent flows into the grid will impact the current transformer (CT), firstly, study theDC bias effect on CT Transfer Characteristics then analyzes impact of relayprotection.In this paper, the impact protection of DC bias on a detailed theoretical analysisand experimental research, first deduced the basic model of the CT under DC biasconditions, CT excitation current and the magnetic flux of expression, indicating thatDC bias all for under direct current excitation, the flux will also appear in themagnetic field flux, and then use PSCAD/EMTDC simulation software built CTsimulation model, the results show that CT secondary current distortion, defects, andthe secondary current contains large number of harmonics, and with the increase ofDC current the fundamental becomes small, the harmonic is increasing, when the DCcurrent increases to a certain value, even harmonic amplitude will be greater than theodd harmonic amplitudes, which will lead to protection algorithm error increased, andprotection incorrect action. Meanwhile set up experimental circuit in the laboratoryunder DC bias conditions, through the experimental verification of the theoreticalanalysis, simulation analysis is correct, then analyzes the distance protection anddifferential protection action characteristics conditions under DC bias, and concludedrelay refuse operation under DC bias.Finally, this paper presents an improved Prony algorithm is used to eliminate theDC bias on the impact of protection, this method SVD reduced order to reduce thecomputation time, through the right side of the sampling frequency of the CTsecondary current is proposed together, restore the CT secondary side current DC bias influence before this current is used to protect the DC bias to eliminate the impact onprotection, then use PSCAD/EMTDC simulation software to verify that this methodcan effectively eliminate DC magnetic bias impact on the protection.