what do you know about ZERO-SEQUENCE IMPEDANCE for transformer

Zero-sequence impedance is a crucial parameter in power system analysis, and it is particularly important when analyzing unbalanced faults. Transformers are key components of the power system, and their zero-sequence impedance has a significant impact on power system performance. Zero-sequence current and voltage values are important quantities to provide proper relay protection of power lines and substation equipment. Zero-sequence quantities having any real significance generally occur only when a power system becomes unbalanced due to nonsymmetrical power loading or because of a fault.

Transformers are designed to transfer power from one voltage to another voltage level. During unbalanced conditions, zero-sequence currents are induced in the system, and these currents can circulate within the transformer. The zero-sequence impedance of the transformer determines the magnitude of these currents and the resulting voltage drop across the transformer.

If the zero-sequence impedance of the transformer is low, then the flow of zero-sequence currents will be high, resulting in a large voltage drop across the transformer. On the other hand, if the zero-sequence impedance is high, then the flow of zero-sequence currents will be restricted, limiting the voltage drop across the transformer. Additionally, low zero-sequence impedance can lead to saturation of the transformer core, which can cause significant damage to the transformer.

For a three-phase power system, zero-sequence current flows when one or two phases are faulted to the ground. The correct operation of ground relays requires proper zero-sequence relay current and (for directional relays) polarizing quantities during the fault. Delta-wye transformers with grounded wye neutrals are the major sources of zero sequence-current flow within a power system. Depending on a transformer's configuration, zero-sequence currents can flow through the transformer, from the transformer, or both.

To describe zero-sequence impedance testing on a transformer, a grounded wye (sometimes called a star configuration) connection is used. Zigzag transformers are tested for zero-sequence impedance by the same test connections. Measuring the zero sequence impedance of a transformer is relatively easy. A variation of the impedance formula is required to compute the impedance for zigzag or other grounding transformers.