It is standard to use the precept of symmetrical aspects in performing gadget format calculations, mainly those involving unbalanced loadings and system earth fault conditions. This machine is described in positive, bad and zero-sequence impedance values to the elements of the electrical system. For a three-phase transformer, the effective and poor sequence impedance values are equal to the price described above, however, the zero-sequence impedance varies substantially by the development of the transformer.
IEC60076-1 Clause 10.7 "Measurement of the zero-sequence impedance(s) on three-phase transformers
IEEE C57.12.90 Clause 9.5 "Zero-phase-sequence impedance
IEEE C57.12.90 Clause 9.5 "Zero-phase-sequence impedance
After definition, the zero-sequence impedance is the impedance measured between phase terminal (three phases connected) and neutral, which is only possible in star- or zig-zag- connected windings. The zero-sequence impedance, attributed to each phase, is three times the measured value.
The size has to be carried out at rated frequency and continually with the energetic phase in the tank, due to the fact of its excessive impact on the zero-sequence impedance. The zero-sequence flux may also purpose immoderate heating in metal structural components such as tank, cowl, or clamping construction. That's why the measuring contemporary ought to now not to be greater than 30% of the nominal current. Currents up to nominal modern are solely accepted for a very brief time. The utilized voltage needs to exceed now not the phase-to-neutral voltage in the course of ordinary operation.