feroresonance resistor

[vreijb]

What would be the size of the ferro resonance resistor for PT 100VA, with 4200/69/69 V. The plant is 7.2KV with high resistance grounding
any assisstance is aprrecviated

 

[davidbeach]

You have a far more fundamental problem. In a high resistance grounded system your PT needs to be rated for line-line voltage not line-ground voltage. You will have saturation problems during a line-ground fault. During saturation you will be seeing secondary voltages that are not replicas of the primary voltages.

 

[vreijb]

pt is actually rated for line voltage with ratio 7.2KV/root3:120/root3

 

[scottf]

A PT rated for line-to-line voltage would be rated 7200:69 V, meaning that it can take 7200 voltage across the primary winding. If the PT is rated 7200/rt3, then it is rated for 4200V across the primary winding. The correct PT would be a 2 bushing unit rated 7200:120V, which would given you 69V on the secondary with 4200V on the primary.

 

[scottf]

Forgot to add....a unit rated 7200/rt3V with a overvoltage factor of 1.73 continuous, would also work, but that kind of overvoltage factor is not common.

 

[waross]

What exactly does it say on the transformer nameplate?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[vreijb]

The swicthgear still under factory assembly. However the PT specs is as follows:
Potential Transformer indoor use 15KV
Ratio 7200/1.73:120/1.73-120/1.73V
Accuracy Class 0.5 100VA
Type UCL-7
Manufactured by ARETECHE

 

[vreijb]

To add to the above, we will trip the breakers in earth fault eventhough High Resistance Grounded

 

[davidbeach]

You need one with a ratio of 7200:120-120. Blackburn has a section on sizing loading resistors to avoid ferroresonance, but until you change PTs your resistor won't help much.

 

[scottf]

To add to David's post, the easiest way is connect one winding of each PT in a broken delta with a resistor sized to present the rated thermal burden of the PT at the rated secondary voltage.

For instance, if the thermal burden is 1000VA at 120V, then use approx. 14 ohms of resistance.

 

[vreijb]

THE PT IS 100VA. 1000VA IS THE THERMAL BURDEN?

 

[scottf]

100VA is probably the accuracy burden. You'll have to look at the nameplate to see the thermal burden.

If you're in the IEEE world, accuracy burden will be a letter like 0.3 WXYZ and thermal burden will be spelled out in VA and labeled as such.

If you're in the IEC world, accuracy burden will be listed in VA after the class, i.e. Cl 0.5 - 100VA and thermal burden may or may not be on the nameplate.

From the voltages you listed, I assume IEEE world.

 

[vreijb]

does it make difference if it was low resistance grounded

 

[davidbeach]

It depends. What is the maximum voltage to ground of an unfaulted phase during a ground fault? Is the transformer rated for that overvoltage condition?

 

[vreijb]

"Voltage transformers are normally connected phase to earth. In the event of a disturbance in the network the voltage across the VT's (CVT's) will be increased in the healthy phases. IEC specifies the voltage factors:
1,9 for systems not being solidly earthed.
1,5 for solidly earthed systems.
The saturation is specified to be 30 sec for systems with tripping earth fault protection and 8 hours if no Earth fault tripping protection is used.
The VT's must not be saturated at the voltage factor."

Does the factor here represents the overvoltage

 

[noing]

I THINK YOU MAY FIND THIS DOCUMENT INTERESTING.

 

[vreijb]

I have attached the catalog of the PT that we intend to use in 7.2 KV switchgear where we have high resistance grounding. it looks like the PT is suitable for 7.2KV
Any comments

 

[davidbeach]

Looks like a maximum of 5kV to me. No idea what the basis of the IEC rating is, but I certainly wouldn't recommend exceeding the IEEE ratings.

 

[waross]

Is 7200/1.73:120/1.73 another way of saying that the ratio is actually 4157:69V?
Do I correctly understand that under IEEE ratings this transformer is suitable for use on a solidly grounded 7200V system (wye voltage 4157V), but not suitable for a resistance grounded 7200V system where the voltage across the transformer under fault conditions may approach 7200V which is in excess of the maximum IEEE rating of 5000V.?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

I believe you've got it Bill.