PT ratings on impedance grounded systems

[rockman7892]

I know that for applying a PT on a 4.16kV impedance grounded system (non-rigidly grounded)the PT needs to be a 2400/4160Y PT as opposed to a 2400/4160GY PT due to the fact that the neutral bushing needs to be a fully rated bushing for when neutral floats to line voltage during certain fault conditions.

I have 2 questions here:

1) What is the L-G rating of the bushings on a 2400/4160Y PT? I believe they are only 2400V but wouldn't they need to be 4160V if L-N voltage can be elevated to L-L voltage during certain fault conditions?

2) In a 2400/4160Y PT I understand the winding to be rated at 2400V. If PT is connected L-N then during certain ground fault conditions the PT winding will see L-L voltage of 4160V. Can the PT winding sustain this temporary overvoltage or must a 4160/4160GY PT be used? I'm not familiar much with these but understand that during normal conditions they are meant to operate at 58% rated voltage and thus during ground fault would only see the voltage rating of the primary winding?

 

[scottf]

1) The H2 bushing is rated to an insulation class and not a specific voltage. So the 5kV class would be 75kVp BIL and 26kV PFWV.

2) Yes, the VT can withstand temporary over-voltage. Normally marked as the Over-Voltage Factor (OVF) on the VT's nameplate. Typically, units of this design would have an OVF rating of 1.73 - 1 minute (or longer).

 

[RRaghunath]

The international standards do specify (Over) Voltage factor for PTs. If the impedance earthed system has ground fault protection system that isolates the faulty feeder automatically, 1.9pu for 1min is stipulated. In case there is no automatic trip available, it shall be 1.9pu for 8 hours.

R Raghunath

 

[Kiribanda]

1)Yes, you are correct. It is 2400V to ground.
Therefore, no questions. You cannot use a 2400/4160Y PT on an impedance grounded system.
Instead you have to use DUAL bushing, 5kV class, indoor or outdoor
with a VTR=35 if your secondary system is 120V AC.
Therefore, rated primary accuracy voltage = 4200V.

 

[scottf]

Kiribanda-

I don't believe you are correct

You CAN use a 2400/4160Y V PT - which is a 2-bushing VT

You CANNOT use a 2400/4160GY V PT - which is a 1-bushing VT

The line-to-neutral voltage on an impedance grounded 4160V system is indeed 2400V.

 

[stevenal]

scottf,

Where do you connect H2 of that 2400/4160Y V PT?

 

[scottf]

stevenal...the to neutral if connected line-to-neutral or to the other line if connected line-to-line.

2400/4160Y means the unit can be connected L-N on a 4160V system or l-l on a 2400V system.