When the bipolar HVDC transmission line works in the operation mode where the ground is used as the loop line, the DC current will be generated on the power transmission line, which will result in the DC magnetic bias of the current transformer (CT). The DC magnetic bias may bring many disadvantages to a CT, such as the magnetic saturation and the change of its transfer characteristic. Then the accuracy of the CT will experience a degeneration, which affects the fairness of power trading and the reliability of the relay protection. Therefore, a method of compensating the DC magnetic bias is proposed in this paper and it is comprised of 4 steps: recognizing the second harmonic, identifying the optimal compensation, V-I transformation and creating the opposite magnetic field. Moreover, tests are carried out to verify the validity and the results indicate that the proposed method makes much sense in power trading, relay protection and the stable operation of the power system.
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