Zero Sequence Voltage from VT

[dfdt]

Hi- We all know that in order to generate zero seq voltage from a VT/PT there need to be a path for zero sequence flux to flow i.e. in other words the VT needs to be either 3 x Single phase VTs or a five limb and star point of the primary winding needs to be grounded.
My question is can numerical relay compute zero seq voltage internally if we connect three limb/three phase VT secondary to the relay?

 

[davidbeach]

Three single phase VTs, connected grounded wye-grounded wye, allow a decent relay to correctly calculate 3V0. I’ve never run across multi-phase VTs so I don’t know how they’d respond.

 

[dfdt]

Thanks - and its pretty common to use three phase VT on on metal clad switchgear at 11/22/33 kV voltage level.Regardless of that my query is - will modern numerical relay connected to secondary of three phase three limb constructed VT be able to calculate zero seq voltage internally from three phase voltage?

 

[davidbeach]

If the secondary accurately mimics the primary, yes the relay can calculate 3V0. If the secondary isn’t a perfect mimic of the primary find a different VT.

 

[dfdt]

David- I am interpreting you answer as NO i.e. relay will not be able to compute zero seq voltage if it's connected to three phase three limb VT
and i also think so

 

[davidbeach]

I don't know. I've never encountered a situation where the wye connected VTs couldn't provide the necessary information to the relay to calculate 3V0. I don't know the configuration of the switchgear VTs (transmission VTs are always three singles) but I've never seen an event report where the secondary didn't correctly mimic the primary. If any of them have been three limb there isn't a problem; if none of them have been three limb I don't know. I'd go after the design engineer if I ever found I didn't get a true replica of the primary voltage.

 

[scottf]

3-phase VTS in the North American market, are typically only found at low-voltage/600V levels and, in general, they are just 3 - 1-phase VTs assembled in a common case (or 2 x 1-phase VTs in the case of open-delta configuration).

I'm not aware of standard 3-phase VTs in the MV level and certainly not at HV levels. The one exception to that is there used to be some 3-phase meter sets (CTs and VTs) used on distribution lines for primary metering applications.

I'm not a relay guy, but I do know, from the instrument transformer perspective, it is quite common for modern relays to be able to calculate 3VO from wye-connected VTs.

 

[cranky108]

I have seen three phase VT at 138kV, and it was just three single phase VT's in a single package. Always a concern that if it failed, what to replace it with. Very compact instal.

I don't remember the brand.

One set of the windings were connected in a broken delta for relay directionality. EM relays can't calculate 3V0, but could measure it.

 

[LionelHutz]

Open delta 5kV class PT's are readily available in NA. The relay can't calculate 3V0 using one.