pittengineer
Electrical
- Aug 23, 2009
- 31
We are having an issue where an 120V branch circuit is dropping to 112V during the starting of a motor and causing problems with electronics. This voltage drop is causing several problems and we are searching for answers as to what we are seeing and where to make corrections.
Our system has a utility transformer with a secondary of 4.16kV controlled by an automatic tap changer. 4.16kV is distributed to main switchgear which is then distributed to different sections of plant through feeder breakers. The area where I am seeing this drop is off one of the feeder breakers which serves a second 4.16kV switchgear bus which has a 5kV motor and a 4.16kV - 480V transformer. On the 480V bus there is a 480V - 240/120V transformer that has the 120V issue that I am referring to which is a dip lasting about 1s in duration. Looking at this I have a few questions.
1) Should we look at our main incoming utility voltage to verify that it is within limits as a starting point? What is the range of acceptable or ideal voltages for this incoming voltage?
2) When looking at meters on incoming switchgear I see voltages of about 4035V, 4079V, and 4122V. However when I look at the voltages on the second downstream switchgear I notice that they are all upwards of 4130V. I dont believe that the voltages downstream could be larger than the upstream ones. Do you think this could just be an error in measurment? Both meters are connected in an open Delta arrangement to the PT's.
3) Should this momentary voltage drop be a concern for re-evaluating my system and possibly re-tapping some of these transformers. We have no other problems with motors or anything, and just this one electronic piece of equipment seems to be the problem?
4) Would it be more logical to focus on the downstream 480V and 120v circuits then looking way upwards in the system? Wont each one of these transformers present a voltage drop of some sort due to impedance.
Thanks in advance for any advice on this.
Our system has a utility transformer with a secondary of 4.16kV controlled by an automatic tap changer. 4.16kV is distributed to main switchgear which is then distributed to different sections of plant through feeder breakers. The area where I am seeing this drop is off one of the feeder breakers which serves a second 4.16kV switchgear bus which has a 5kV motor and a 4.16kV - 480V transformer. On the 480V bus there is a 480V - 240/120V transformer that has the 120V issue that I am referring to which is a dip lasting about 1s in duration. Looking at this I have a few questions.
1) Should we look at our main incoming utility voltage to verify that it is within limits as a starting point? What is the range of acceptable or ideal voltages for this incoming voltage?
2) When looking at meters on incoming switchgear I see voltages of about 4035V, 4079V, and 4122V. However when I look at the voltages on the second downstream switchgear I notice that they are all upwards of 4130V. I dont believe that the voltages downstream could be larger than the upstream ones. Do you think this could just be an error in measurment? Both meters are connected in an open Delta arrangement to the PT's.
3) Should this momentary voltage drop be a concern for re-evaluating my system and possibly re-tapping some of these transformers. We have no other problems with motors or anything, and just this one electronic piece of equipment seems to be the problem?
4) Would it be more logical to focus on the downstream 480V and 120v circuits then looking way upwards in the system? Wont each one of these transformers present a voltage drop of some sort due to impedance.
Thanks in advance for any advice on this.