PT connection for an ungrounded system
PT connection for an ungrounded system
(OP)
The question is very similar as one below. A set of three PTs are connected to a short 27 kV bus. This bus has no reference to ground, being connected between two transformers, 230/27 kV and 27/0.6 kV - both having 27 kV windings connected in DELTA.
However, secondary PT windings are connected to WYE (star) and used for ground fault detection by a newer generation of relays that do not necessarily require an open corner delta as they can calculate 3Vo from the directional element and 4-wire WYE connection.
Those 16 kV rated PTs at primary side have no firm reference, other than one they will form themselves being connected to a WYE and then ground, creating so a "virtual" (artificial) 27 kV ground reference. However, I wouldn't expect this reference to remain very firm and will likely "wonder around" (float a bit) in case of any system unbalance or disturbance.
Does anyone have experience with this, i.e., how likely is that this may cause some problems as the reference is not so firm and relays actually calculate 3Vo rather than measure it?
Thank you.






RE: PT connection for an ungrounded system
RE: PT connection for an ungrounded system
Described is a conventional generator/unit-transformer arrangement, with MV system not solidly grounded—preventing serious stator/bus damage in ground-fault conditions. A VT broken-delta burden resistor is included to prevent VT-core saturation from neutral-shift overvoltage or ferroresonant {or arcing-transient} conditions. North American relay texts suggest values for the application, like on the order of 85 ohms for some voltage levels. Ratios on the order of 300:1 may be appropriate for the subject application.
Note that VT characteristics may dictate thermal-voltampere capacities for broken-delta resistor use.
The same VTs can also used for ø-ø/ø-g voltage sensing/relaying/polarizing {imputed 3V0} like wuth C57.13 §7 group 3 components. Secondaries (X-terminals) are grounded-wye [like primaries] with a broken-delta configuration of tertiaries (Y-terminals.)
It’s important to preserve [e.g., 0.3%] ratio accuracy of the secondary-winding set in addition to the VT-thermal rating of the tertiary-winding set.
Aside—unlike for grounded-wye secondaries, the broken-delta loop must remain unfused, given the serious overvoltages that nay develop with the resistor our of service.