IEEE C57.158-2017 pdf free downlaod – IEEE Guide for the Application of Tertiary and Stabilizing Windings in Power Transformers.
For transformers that work under bidirectional power flow, either main winding (H or X) can act as primary or secondary. Multiwinding transformers can also have two primaries or two secondaries; however, that application falls outside the scope of this document. For the rest of this document, the term transformer will indistinctly indicate a two separate-winding transformer or an autotransformer, unless specifically required. For the case of wye-wye-connected transformers, either in the form of a three-phase transformer, or as a transformer bank, a third, delta-connected tertiary winding may be required for the main purpose of stabilizing the phase-to-neutral voltages under unstable condition. Unstable condition means that the line- to-neutral voltages of the transformer become unsymmetrical with respect to neutral.
The phenomenon described in the foregoing paragraph is not necessarily coincident with the so-called “neutral shift,” which can be seen when a wye-connected source, with high-impedance grounding in the neutral, gets one of the three lines unintentionally grounded, raising the voltage to ground of the other two “live” lines, but keeping the line-to-neutral voltages constant at the source (see Chapter 1 of IEEE Std 142™-2007 [B17], Chapter 15 of Evans, et al. [B7], and Chapter 15 of Schlabbach and Rofalski [B35]). Coming back to wye-wye-connected transformers, line-to-neutral voltages may become unstable when unbalanced line-to-neutral loads are applied to the secondary, load side, and the primary neutral is not directly connected to the neutral of the source (the most general case). Line-to-neutral voltages may also become unstable when the third harmonics of the exciting current have a significant magnitude and cannot flow through the primary or secondary windings, consequently inducing third-harmonic voltages in the line-to-neutral voltages of primary and secondary sides of the transformer. A more detailed explanation of these conditions is introduced in 5.1. For three-phase transformers (as opposed to transformer banks), if the terminals of the delta-connected winding are not brought out of the tank, this winding cannot be designated as a tertiary winding, in the proper sense, as it cannot be loaded or tested as a three-phase device.
That closed-loop winding array is frequently designated as a buried tertiary, properly referred in this guide as a stabilizing winding, which can only carry single-phase circulating currents, also known as zero-sequence, or homopolar currents, in terms of power systems analysis techniques. Line-to-ground faults on the primary or secondary side of the substation produce high transient circulating currents in the stabilizing winding, especially if the neutral of the transformer is directly grounded, and therefore this winding must be accordingly designed, from mechanical and thermal perspectives. If continuous (more than a few tens of minutes) loading of the neutral is expected for any specific operating conditions, the stabilizing winding shall have a kVA rating assigned for expected thermal duty in addition to the short-circuit withstand capability. In this case, the stabilizing winding will be loaded in a similar fashion as neutral grounding devices. Some system operation codes limit the amount of continuous load current in neutral of transformers (see for an example Chapter 7 of Schlabbach and Rofalski [B35]). As opposed to stabilizing windings, tertiary windings can be connected to external loads, and potentially subjected to high three-phase or other fault currents at their terminals, which impose mechanical and thermal stresses on these windings. Single-phase transformers cannot have stabilizing windings, in terms of the foregoing definitions, because the delta connection is always formed external to their individual tanks. However, it is important to realize that every delta-connected tertiary winding in a transformer bank, or in three-phase transformers, automatically performs as a stabilizing winding, just because of its closed-loop connection, besides other functions that the winding might be required to perform, such as supplying three-phase load to local station services.IEEE C57.158 pdf download.