Query on grounding of VT primary side

[kev2632]

Hi,

Is anyone able to explain to me why on this drawing where the withdrawable VTs are ? Why does the primary side of the VTs get grounded as well as the secondary side?

Thanks

 

[stevenal]

To provide a reference.

 

[kev2632]

To provide a reference.

Is it normal practice to do both sides of the VTs? As I've seen in some cases only the secondary side being done?

 

[waross]

Why does the primary side of the VTs get grounded as well as the secondary side?

So that you may determine the voltage to ground or neutral.
If you intend to measure the line to line voltage, grounding the primary side of the VT may be counterproductive to the point of immediate destruction.
(Check the interrupting capacity of the VT fuses against system ASCC before trying this.)
(PS; Don't try this at home. We are _untrained professionals.)

 

[kev2632]

So why does some VTs only do the secondary side and not the primary too?

 

[wcaseyharman]

There are various reasons why a designer chooses different VT grounding methods:
- the grounding of the power system neutrals can be different.
- the protection methods are different
- different sets of VTs are used for different things.
- design standards from different engineering groups favor one over other

I mostly work on utility scale generators, and often the Vt configuration goes with the manufacturer. GE for a very long time seemed to prefer 2 VTs in open delta. Other manufacturers including Westinghouse used 3 VTs in grounded Y on similar vintage machines. Designers choice.

 

[bacon4life]

It would be atypical to have ungrounded wye PTs. It is common to have ungrounded PTs in an open delta configuration.

In certain cases grounded wye PTs are used on capacitively (i.e. ungrounded) systems. In this case the PTs must be rated for full LL voltage even though they are connected LL. In this configuration, any unbalanced load on the PT secondary ends up causing a slight voltage shift on the primary side.