Open Delta vs Wye Voltage Transformers 2

[KenAlmon]

I have noticed that on metering points, three single phase VT's connected in wye (usually) are used to ensure highest possible accuracy.

However, for protection issues, as inputs to relays requiring 3 phase voltage sources, oftentimes an open delta made up of two VT's is used, with the secondary "B" corner grounded (old Westinghouse standard I understand, to prevent dropping a generator on a secondary ground fault).

Can anyone provide the technical basis as to why this open delta is so popular? Are there some instances when the 3 VT arrangement may be required for protection issues?

TIA
Ken Almon

 

Ken, we see this in Mississippi alot. The only reason that I have ever really heard is more economic than technical. The utility is able to serve the facility or building with two transformers in lieu of three. The problem with this arrangement is that the single phase load can only be shared between two phases in lieu of three with a conventional arrangement. This can make for an unbalanced system if you are not careful.

Joe Legge

 

[scottf]

Joe is correct. The only advantage (perceived) is economic. Currently, it is sometimes actually more expensive to use 2 x 2-bushing VTs instead of 3 x 1-bushing VTs, especially at system voltages of 46 kV and above (normally 2-bushing VTs are only available up to 69 kV).

The change in economics is mainly based on supply and market conditions. There are not too many folks who make 2-bushing VTs above 34.5 kV (because most folks use 3 - L-G connected units) and hence not as much competition.

 

[redtrumpet]

Please correct me if I am wrong, but I believe you need three wye-connected VTs to implement directional metering or overcurrent protection. I don't think you can do this with two open-delta connected VTs.

 

[peterb]

You need 3 wye-wye VTs for directional ground fault relays (or perhaps wye-open corner (broken) delta for electromechanical ground directional relays).
Phase fault direction is sensed phase-phase and open delta VT connection is fine for this. For example, when a distance relay is used for generator phase backup protection it is connected to the generator open delta VTs, so is the reverse power protection.

 

[redtrumpet]

Thanks for clarifying, peter.

 

[scottf]

My experience is mainly with VTs in substation application and not so much on the generator side. My understanding is that peterb is correct about the generator protection applications. I do know that L-L connected VTs are quote common on generator-side applications.

In substation applications it is not very common to apply 2 VTs in open-delta, especially for metering applications.

 

[peterb]

Scottf -
I think that you're absolutely correct when dealing with outdoor substations. In my experience, the open delta arrangement is common in metalclad switchgear, whether feeding generation or industrial plant systems.
As my background is heavier on relaying rather than metering, I'll be guided by your experience there.

 

[busbar]

To add to good comments already— For metering, Blondel’s Theorem must be satisfied for all modes of system operation—load unbalance, grounding resistor dissipation or faults. Simply, if there is any connection (resistor, reactor or solid) to the XO busing of the serving transformer, then Grounded-Wye—Grounded-Wye PTs are in order.

C57.13 VT groups should be carefully understood, and It may be necessary that other than group 1 ratings be applied. Overvoltages from neutral shift during faults may dictate review of BIL ratings and fuse ratings, also. Nowadays, the cost and availability of 3-element metering should, in most all cases, obviate anything less. If there is likelihood of temporary system reconfiguration deengergizing PTs when remote or optical polling {or a readable display} is needed, auxiliary-power capability and consideration for a reliable source should be reviewed.

For relaying, if zero-sequence quantities are needed, Grounded-Wye—Grounded-Wye is the ticket. Thin specifications will just about guarantee open-delta PTs. The “secondary ‘B’ corner grounded” arrangement is not unique to Westinghouse.

 

[jbartos]

Suggestion: Traditionally, 2-CT and 2-VT approach to metering (kWh) is adequately applied to the 3-phase and 3-wire power supply system (Aaron's Connection of Watthourmeters). The 3-CT and 3-VT approach to metering (kWh) is mandatorily applied to 3-phase, 4-wire system power supply system.