Grounding Transformer Has Large Positive And Negative Sequence Impedance
Jun 08, 2018

According to the commonly used ZNyn wiring specification, when a grounding transformer is in operation, when passing a certain magnitude of zero-sequence current, the currents flowing through two single-phase windings on the same core column are of opposite and equal magnitudes, so that the zero-sequence current is generated. The magnetic potential cancels out in the opposite direction, so that the zero-sequence impedance is also small. In the event of a fault in the grounding transformer, the neutral point can flow through the compensation current. Due to the small zero-sequence impedance, when the zero-sequence current passes, the resulting impedance drop should be as small as possible to ensure the safety of the system. Because the grounding transformer has the characteristics of low zero-sequence impedance, when phase C single-phase ground fault occurs, the phase I current I of the phase C flows into the neutral point and is equally divided into three parts into the grounding transformer. The three-phase current of the transformer is equal, so the displacement of the neutral point N remains unchanged, and the three-phase line voltage remains symmetrical. However, in the manufacturing process, the number of turns and the geometric dimensions of the upper and lower windings of the high voltage winding cannot be completely equal, so that the magnetic potential generated by the zero sequence current cannot be cancelled out in the opposite direction, or a certain zero sequence impedance is generated, usually around 6-10Ω. Compared with the star-connected transformer's zero-sequence impedance of 600Ω, its advantages are self-evident. In addition, the tortuous grounding transformer can also make the no-load current and no-load loss as small as possible. Compared with common star-connected transformers, since each phase core of the meandering transformer is composed of two core windings, combined with the vector diagram, compared with ordinary star-connected transformers, it takes more than 1.16 times when the voltage is the same. . In the case of a neutral point resistance grounding mode, the amplitudes of zero-sequence impedance and positive-sequence impedance are largely different when the urban distribution network is single-phase-grounded. When the three-phase positive and negative sequence currents flow, the magnetic potential on each core leg of the grounding transformer is the phasor sum of the magnetic potentials of the two windings belonging to different phases on the core leg. The magnetic force on the three legs is a set of three-phase balance, with a phase difference of 120°. The generated magnetic flux can form loops on each of the three core legs. The magnetic resistance of the magnetic circuit is small, the magnetic flux is large, and the induced potential is large. Shows a large positive and negative sequence impedance; therefore, the grounding transformer has the advantages of large positive and negative sequence impedances, and low zero-sequence impedance.

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