rex2704
Electrical
- Nov 22, 2006
- 14
I work for a utility in Canada. I will first give some background on our network in question…
We have 9MVA, 66kV/6.6kV, YND1 transformer. The HV side has a transmission line with the circuit breaker about 100kM away. The breaker probably performs only 3-pole operation. The LV side of this transformer is connected to 6.6kV switchboard with about 50ft cable. On this LV side we have 3 single phase voltage transformers (VT) connected Y0Y0. These VTs have single secondary winding and are very lightly loaded (1500VA VTs with burden of <5VA connected on them). The VTs have primary fuses mounted on top. These are indoor type VTs with metering accuracy (0.3WXYZ) and rated for 7.2kV. I do not have their knee point voltage information.
My questions are…
Should we be concerned about ferroresonance issues with these VTs when energizing the main transformer from the HV side breaker? I am asking this question specifically for the VTs and not the main transformer. Main transformer, by the way, is energized without any load connected to it.
If there is a likelyhood of ferroresonance, then would it suffice to have VTs rated for line-to-line voltage with a knee point of about 2p.u.? or do we have to replace these VTs with an additional open delta winding and damping resistors?
There are two 6.6kV generators connected to the same switchboard. They have high impedance grounding on them. So when any of the two generators is running, the 6.6kV side is not completely ungrounded.
I have read quite a few papers published on this subject (I found was Schneider's Cahier Technique document to be the best) and I am getting an impression that ferroresonance should not be an issue at 6.6kV voltage levels, especially with such a small ungrounded system.
I have always found this forum to be very helpful and there are some real experts here who are willing to share their knowledge. I am looking forward to some good responses.
Thankyou everyone in advance!
We have 9MVA, 66kV/6.6kV, YND1 transformer. The HV side has a transmission line with the circuit breaker about 100kM away. The breaker probably performs only 3-pole operation. The LV side of this transformer is connected to 6.6kV switchboard with about 50ft cable. On this LV side we have 3 single phase voltage transformers (VT) connected Y0Y0. These VTs have single secondary winding and are very lightly loaded (1500VA VTs with burden of <5VA connected on them). The VTs have primary fuses mounted on top. These are indoor type VTs with metering accuracy (0.3WXYZ) and rated for 7.2kV. I do not have their knee point voltage information.
My questions are…
Should we be concerned about ferroresonance issues with these VTs when energizing the main transformer from the HV side breaker? I am asking this question specifically for the VTs and not the main transformer. Main transformer, by the way, is energized without any load connected to it.
If there is a likelyhood of ferroresonance, then would it suffice to have VTs rated for line-to-line voltage with a knee point of about 2p.u.? or do we have to replace these VTs with an additional open delta winding and damping resistors?
There are two 6.6kV generators connected to the same switchboard. They have high impedance grounding on them. So when any of the two generators is running, the 6.6kV side is not completely ungrounded.
I have read quite a few papers published on this subject (I found was Schneider's Cahier Technique document to be the best) and I am getting an impression that ferroresonance should not be an issue at 6.6kV voltage levels, especially with such a small ungrounded system.
I have always found this forum to be very helpful and there are some real experts here who are willing to share their knowledge. I am looking forward to some good responses.
Thankyou everyone in advance!