Method for detecting direct-current magnetic bias of transformers

Abstract

The invention discloses a method for detecting direct-current magnetic bias of transformers. The method comprises the following steps: firstly judging that the transformers are saturated through a differential protection device and calculating secondary harmonic wave of differential current of the transformers; then researching attenuation condition of the secondary harmonic wave; enabling the secondary harmonic wave to be rapidly attenuated in allusion to iron core saturation caused by switching-in of the transformers under general conditions, and appropriately reducing a threshold value if the harmonic wave is slowly attenuated by system parameters accessed to the transformers; and enabling the content of the secondary harmonic wave to be increased or become a constant value in allusionto iron core saturation caused by direct-current magnetic bias. The method is capable of rightly distinguishing iron core saturation caused by direct-current magnetic bias and no-load switching-in ofthe transformers and avoiding internal fault rejection actions caused by direct-current magnetic bias and other hidden risks that threaten the safe operation of the transformers; in addition, the reliable detection of the direct-current magnetic bias phenomenon can facilitate power plants or substations to make corresponding strategies for inhibiting direct-current magnetic bias; the power supplyreliability is improved; the economic and social losses are reduced.

Description

technical field [0001] The invention belongs to the field of power systems, relates to the technical field of transformer protection, in particular to a method for detecting DC bias magnetism of transformers. Background technique [0002] The operation mode of the DC transmission system includes two poles directly forming a circuit, and a single pole forming a circuit with the earth or metal. When the DC system is overhauled or a single-pole blocking fault occurs, the system will form a circuit through the earth. At this time, the direct current entering the ground will change the distribution of the surface potential, making the potentials of different grounding points of the AC grid different, and generating a potential difference between the two grounding points. The circuit will cause the DC bias of the transformer, which will have an adverse effect on the transformer and its protection equipment, and even cause some protection malfunctions or delayed actions. In additi...

AI Technical Summary

Problems solved by technology

For half a cycle with the same DC bias direction, the magnetic flux density of the total magnetic flux will increase greatly, and the corresponding excitation current will increase significantly, showing positive and negative half-wave asymmetry, severe distortion of the waveform, and sharp peaks in the positive half-wave and will produce overshoots. The excitation phenomenon will also have a bad influence on the work of the transformer.

In addition, when the commutation of the DC system fails, the current of the AC system can be regarded as a harmonic source at this time. When an internal fault occurs at this time, the differential protection of the transformer will also work with a delay due to the existence of the second harmonic in the differential current. refuse to move

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