Transformer riddle No.11 - Yyn0 Transformer
I was carrying out a fault current distribution for an earthing design for a primary or zone substation when I found the transformer to be used was Yyn0 (33/11 kV). Apparently the lines company here have many substations with these transformers.

When I checked in the books the zero sequence in some books appear like a (i)"floating" impedance (with a value 3 to 10 times the positive impedance) while in other ones (ii) the equivalent circuit appears to be the same as a Dyn transformer.

Most of the books have the equivalent circuit as (i) in which case no zero sequence will flow for a phase to earth fault at the 11 kV feeders. Also when modelling this transformer with the software DigSILENT gives as result a very low value of current for a phase to earth fault.

The question is with this arrangement; Is there any current flowing for a phase to earth fault at the secondary? Having in mind that these substations are located mainly in semi-rural areas with overhead feeders is there any problems with this choice of transformer?

#1
Thu, September 18, 2008 - 15:09
a-Yes
b-Generally no, with regard existing overhead line you can benefit from some advantages of this connection.

1- The zero-sequence impedance of a star winding will be very high if no delta winding is present. The actual value will depend on whether there is a low reluctance return path for the third-harmonic flux. For three-limb designs without a delta, where the return-flux path is through the air, the determining feature is usually the tank, and possibly the core support framework, where this flux creates a circulating current around the tank and/or core framework. The impedance of such winding arrangements is likely to be in the order of 75 to 200% of the positive-sequence impedance between primary and secondary windings. For five-limb cores and three-phase banks of single-phase units, the zero-sequence impedance will be the magnetizing impedance for the core configuration. Should a delta winding exist, then the third harmonic flux will create a circulating current around the delta, and the zero-sequence impedance is determined by the leakage field between the star and the delta windings.
Five-limb transformers have their zero-sequence impedances substantially equal to their positive-sequence impedance between the relative star and delta windings.

2-Zero Sequence Circuit Modeling of Yyn0 and Dyn0 connection is very different as you see below:

3-According to above diagram, the Yyn0 connection is provide interconnection path for zero sequence current between two sides of transformer. Therefore each side zero sequence impedance (high impedance probably) can be transfer to opposite side via power transformer and that mentioned software can calculate them.

4- The most common characteristics of  three-phase transformer connection grounded wye – grounded -wye connections are:

Tank heating — Probable with three-legged core construction; less likely, but possible under severe unbalance with five-legged core construction. Impossible if made from three single-phase units.

Zero sequence — All zero-sequence currents — harmonics, unbalance, and ground faults — transfer to the primary. It also acts as a high impedance ground source to the primary.

Ferroresonance — No chance of ferroresonance with a bank of single phase units or triplex construction; some chance with a four- or five legged core construction.

Coordination — Because ground faults pass through to the primary, larger transformer services and local protective devices should be coordinated with utility ground relays.

The grounded wye – grounded wye connection has become the most common three-phase transformer connection. Reduced ferroresonance is the main reason for the shift from the delta – grounded wye to the grounded wye – grounded wye.
A grounded wye – grounded -wye transformer with three-legged core construction is not suitable for supplying four-wire service. Unbalanced secondary loading and voltage unbalance on the primary system, these unbalances heat the transformer tank. In a three-legged core design, zero sequence flux has no iron-core return path, so it must return via a high reluctance path through the air gap and partially through the transformer tank. The zero-sequence flux induces eddy currents in the tank that heat the tank.
A four- or five-legged core transformer greatly reduces the problem of tank heating with a grounded wye – grounded wye connection. The extra leg(s) provide an iron path for zero-sequence flux, so none travels into the tank. Although much less of a problem, tank heating can occur on four and five-legged core transformers under certain conditions; very large voltage unbalances may heat the tank. The outer leg cores normally do not have full capacity for zero-sequence flux (they are smaller than the inner leg cores), so under very high voltage unbalance, the outer legs may saturate. Once the legs saturate, some of the zero-sequence flux flows in the tank causing heating. The outer legs may saturate for a zero-sequence voltage of about 50 to 60% of the rated voltage. If a fuse or single-phase recloser or single pole switch opens upstream of the transformer, the unbalance may be high enough to heat the tank, depending on the loading on the transformer and whether faults still exist. The worst conditions are when a single-phase interrupter clears a line-to-line or line-to-line-to-line fault (but not to ground) and the transformer is energized through one or two phases.

5-The delta – grounded wye connection has several interesting features, many related to its delta winding, which establishes a local grounding reference and blocks zero-sequence currents from entering the primary.

Ground faults — This connection blocks zero sequence, so upstream ground relays are isolated from line-to-ground faults on the secondary of the customer transformer.

Harmonics — The delta winding isolates the primary from zero sequence harmonics created on the secondary. Third harmonics and other zero-sequence harmonics cannot get through to the primary (they circulate in the delta winding).

No primary ground source — For line-to-ground faults on the primary, the delta – grounded wye connection cannot act as a grounding source.

Secondary ground source — Provides a grounding source for the secondary, independent of the primary-side grounding configuration.

No tank heating — The delta connection ensures that zero-sequence flux will not flow in the transformer’s core. We can safely use a three legged core transformer.

Ferroresonance — Highly susceptible.