Open Delta vs Wye Voltage Transformers
Open Delta vs Wye Voltage Transformers
(OP)
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
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






RE: Open Delta vs Wye Voltage Transformers
Joe Legge
RE: Open Delta vs Wye Voltage Transformers
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.
RE: Open Delta vs Wye Voltage Transformers
RE: Open Delta vs Wye Voltage Transformers
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.
RE: Open Delta vs Wye Voltage Transformers
RE: Open Delta vs Wye Voltage Transformers
In substation applications it is not very common to apply 2 VTs in open-delta, especially for metering applications.
RE: Open Delta vs Wye Voltage Transformers
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.
RE: Open Delta vs Wye Voltage Transformers
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.
RE: Open Delta vs Wye Voltage Transformers