VT Secondary winding voltage rating

[veritas]

Hi

Could someone please explain the following from a VT spec.

Voltage ratio = (11kV/1.732):110V/1.732 : 110/3

There are two secondary windings. I understand that the primary has 11kV line-to-line volts and the 1st secondary winding has 110V line to line and 63.5V line to neutral. What puzzles me is why the 2nd winding (a broken delta one) is given as 110/3? does this mean the line to neutral volts = 36.67V?

I understand that the voltage at the broken delta would be 3V0 but am really puzzled by the 110/3.

Thanks.

 

[burnt2x]

Your VT is Y:Y.

 

[prc]

For the broken delta winding 110/3 V rating is given so that in the event of a LG grounding in one phase, a voltage of 110 V (3 V0) will appear across the broken corner of delta to actuate relays..

 

[veritas]

prc, burnt2x

I hear what you say but I still do not get it. The way I see it, the "broken delta" voltage is 3V0 = Va + VB + VC. Each phase voltage is 110/3 = 36.67V and 120degrees apart under normal healthy conditions.

With an earthfault, let's say VA is reduced to 0. Then 3V0 = VB + VC = 36.67<-120 + VC<120 = 36.67<180V which is a long way off from 110V???

What am I missing?

Thanks.

 

[davidbeach]

What you're missing is that those aren't the VB and VC voltages during the fault.

 

[burnt2x]

Your turns ratio between primary and delta tertiary will be:

    11,000/1.732
 = --------------  = 173.2
     110/3

During a single-line to ground fault, the worst voltage that can be measured at the broken delta terminals will be:

           3 X(V[sub]LN[/sub] (system normal)
V[sub]BD[/sub] = ----------------------------------
                 turns ratio
  = (3 X 11,000/1.732)/ 173.2 = 110 volts

what prc said. Crystal!

 

[veritas]

Thanks burntx2, David. I agree with you 100%. I have done some serious thinking myself and have come to the conclusion that this kind of delta is suitable for high resistance neutral earthing or ungrounded systems.

With solid earthing there is no appreciable neutral shift under earthfault conditions and so the delta voltages can be the conventional 110/1.732 to give a delta output of 63.5 during an earthfault.

If the latter delta was used for high resistance earthing the delta output could be as high as 1.732 * 110V during an earthfault.

Thanks again and regards.