Load unbalance on the low side of a grounded wye-delta transformer

[substationguy]

If I have an unbalance condition (not a phase to ground fault but an unbalance, say an open phase conductor) on the low side of a grounded wye-delta distribution transformer (distribution system is impedance grounded through a zig-zag grounding transformer) how can I sense this unbalance? Will I see zero sequence current flow though the neutral on the high side of the transformer ? Does anyone know any good technical papers that address this ? Note the distribution system I am refering to is a three-wire system (not a four wire) with the zig-zag connected to the phase conductors.

 

[dpc]

An open phase can cause both negative and zero sequence current, but there must be a complete path for the zero sequence current. With only the one zig-zag transformer, there is not a path for Io. But if there was a ground source on either side of the open conductor, then zero sequence current could flow. But I don't think you would see zero sequence current on the wye side of the transformer, because there won't be any in the delta.

If you are trying to sense this from the wye side of your wye-delta transformer, I would think that the negative sequence current would be the most reliable.

But this is all of function of load current, since you don't have a shunt fault. If there is no load, there is no current and you won't see anything.

But then again, it's Friday afternoon, and I'm thinking more about beer than sequence currents.

 

[substationguy]

Turns out no zero sequence current flows on the high side for an unbalance or open conductor on the low side. The open conductor problem turns out to be a nasty symetrical components problem. Thanks everyone for their help.

 

[bahrra]

Can somebody draw the zero sequence diagram for Substationguy's example? I am investigating an open source incident and I cant figure out how to draw the zero sequence diagram. In my incident we were feeding load fed from a Y-delta-Y grounded bank with only two phases supplying the high side of the bank (the Y side). The load bus was grounded through the delta Y grounded part of the bank. When I draw the zero sequence diagram I get an open point in the zero sequence on the right side of the diagram and thus no zero sequence current flow. I thought the Y ground would try to sustain the voltage and carry the load.

 

[rbulsara]

Refer to the following data bulletin from s&C compnay for transformer protection. See page 22 in it.

http://www.sandc.com/edocs_pdfs/edoc_025854.pdf