Zig zag transformers 1

[peetey]

Could someone explain quickly how these work to create a neutral? Are they only tied to a ground? Are these considered to be grounding transformers? Under balance loading is the principle to carry zero current?

--peetey

 

[dpc]

These are generally used as grounding transformers. If you look at how they are wound, you'll see that in the presence of zero sequence voltage and current, in which all three phases are in phase, the fluxes of each winding tend to cancel each other out and transformer presents a very low impedance to zero sequence current. For normal phase voltages, the transformer looks like a very high impedance, carrying only a small amount of excitation current.

They are three-phase transformers connected to each phase and to ground.

You can get the same effect by using a delta-wye transformer, leaving the delta unconnected (or connected, actually).

Zig-zag transformers are rarely used these days in the US.

 

[waross]

A question dpc;
By connected, do you mean connected to and supplying a load, or connected in delta rather than leaving the delta open or "Broken delta"?
respectfully

 

[dpc]

The delta must be closed to provide a path for the zero sequence current to flow.

The delta-wye transformer provides a good ground source (on the wye side) regardless of what load, if any, is connected to the delta side.

When we need to add a grounding transformer these days, we normally recommend a delta-wye transformer instead of a zig-zag due to availability and cost.

 

[cuky2000]

a) How these work to create a neutral? The neutral is connected to ground. Two scenarios may be considered:

* Normal operation: The positive and negative sequence voltages are unaffected since the excitation impedance is very large.

* SLG fault: The system will see only small leakage impedance. Therefore, zero sequence impedance current will flow trough the ground as much as the rated neutral current and preclude current to flow in the phase conductors.

b) Are they only tied to a ground? No. The primary terminals are connected to the 3 wires ungrounded system in one end. The primary and secondary winding are interconnected as an autotransformer in a 1:1 ratio. The secondary windings are connected in a common neutral point and this is connected to the ground as shown in the enclose link.

http://electrical-contractor.net/theory/zzaxsche.gif

c) Are these considered to be grounding transformers? The zigzag described above is for grounding applications. If another winding connected in delta is wound on the same core as the zigzag connected winding, the transformer is capable for power transmission with stable grounding and not phase shift.