Voltage Drop on 1 of 3 Phases
Voltage Drop on 1 of 3 Phases
(OP)
I have a three phase, 480/277, 750kVA, Y-Y solidly grounded, transformer with 4 runs of 500MCM copper to the service entrance (approximately 60 ft). A voltage recorder at the meter shows several voltage blinks on 1 phase, while the other 2 phases seem normal. The voltage that drops on the single phase does not seem to be dropping with an increase in load current. For example, at one point, I gathered the following:
Aphase: 430A, 281V
Bphase: 460A, 155V
Cphase: 575A, 282V
This has occured several times over a 1 month period, but the single phase voltage that drops is not always the same phase.
Does anyone know a probable cause? Thanks.
Aphase: 430A, 281V
Bphase: 460A, 155V
Cphase: 575A, 282V
This has occured several times over a 1 month period, but the single phase voltage that drops is not always the same phase.
Does anyone know a probable cause? Thanks.






RE: Voltage Drop on 1 of 3 Phases
How long is this "blink"?
RE: Voltage Drop on 1 of 3 Phases
The transformer connections and primary connections were checked and everything looked ok. Is it likely that the problem could be resulting from the transformer itself?
RE: Voltage Drop on 1 of 3 Phases
What you are seeing may be voltage regulators operating. If there is a lot of current unbalance on your distribution circuit, there will be neutral currents and voltage drops on the neutral. That will give you phase shifts and unbalanced voltages. You may be seeing a voltage regulator changing taps in response to changing single phase loading..
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Voltage Drop on 1 of 3 Phases
Duration of the blinks, common time of day, for example.
Have you contacted the utility (or are you the utility?)
Do you know anything about the utility source?
RE: Voltage Drop on 1 of 3 Phases
RE: Voltage Drop on 1 of 3 Phases
I am also interested in what the distribution circuit looks like. If, as waross suggests, it is a long circuit there are distribution events that might explain it.
For what it is worth the transformer itself wouldn't be high on my list of suspects.
Just for clarity, there is only one service from this transformer?
RE: Voltage Drop on 1 of 3 Phases
The three phase portion of the ciruit is about 5 miles long, with most of the single phase taps protected by relosers or fuses. The three phase line feeds through the transformer to a three phase lift station.
RE: Voltage Drop on 1 of 3 Phases
Are you getting current spikes to accompany your voltage dips?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Voltage Drop on 1 of 3 Phases
The duration of the event would be helpful. Can you configure the power meter to trigger on voltage excursions and record some waveforms?
Or some funky voltage regulators as waross suggested.
RE: Voltage Drop on 1 of 3 Phases
What specific issues are these drops causing?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Voltage Drop on 1 of 3 Phases
RE: Voltage Drop on 1 of 3 Phases
RE: Voltage Drop on 1 of 3 Phases
your sub breaker inst setting is causing the breaker to open when ever you have these problems(trees, weather etc).
You may want to see if you can remove the inst relay and operate with out it, using only the time delay settings.
RE: Voltage Drop on 1 of 3 Phases
RE: Voltage Drop on 1 of 3 Phases
There could be faults on taps to the circuit resulting in fuse blowing and voltage dips, but this is unlikely because there are so many and fuse blowing would cause a permanent fault to other customers.
There could be single phase faults and recloser operations on other circuits from the same MV bus. The faults would cause the bus voltage to dip. It seems unusual that they are all on the same phase.
There could be intermittent tree contact that causes a high current on the circuit, but not high enough or long enough to trip the breaker. Considering the frequency and that they are all on the same phase, this seems like a likely culprit. Are there any single phase taps on the distribution feeder that go through trees?
RE: Voltage Drop on 1 of 3 Phases
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Voltage Drop on 1 of 3 Phases
RE: Voltage Drop on 1 of 3 Phases
Loose probes in elbows or bad bushing inserts can cause intermittent voltage dips not necessarily associated with high loads if the issue is determined to be localized.
Good luck.