Voltage Drop on 1 of 3 Phases

[dlmartin]

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.

 

[dpc]

A remote line-to-ground fault somewhere on the utility system?

How long is this "blink"?

 

[dlmartin]

The blinks are instantaneous. The attached graph shows the blinks. I think the sample interval is set for 1 minute, so it's hard to tell on the graph.

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?

 

[waross]

Are you on a long distribution circuit and are there a lot of single phase loads or unbalanced three phase loads on your circuit?
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

 

[UtilityDave]

Can you give more information?
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?

 

[UtilityDave]

Sorry for the redundant questions, I should have refreshed before posting

 

[UtilityDave]

Any weather correlation?

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?

 

[dlmartin]

I work for a utility. As shown in the attachment, the blinks occur randomly. However, I am able to relate a couple of the events to weather.

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.

 

[waross]

An answer that would make perfect sense on one system may sound foolish on another system. On a soft system, recloser operation may cause voltage dips. On a firm system recloser operation ma not make much difference. Your dips may be orginating at the pumping station, on the distribution line ahead of the pumping station or on the distribution line past the station.
Are you getting current spikes to accompany your voltage dips?

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

 

[dpc]

Pretty tough to tell much with that time scale. If this feeder is overhead, it could just be a tree limb hitting the lines, a squirrel, or some other type of transient event.

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.

 

[itsmoked]

I notice you have events every hour on your first chart. Also two lines have simultaneous drops in one case.

What specific issues are these drops causing?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dlmartin]

The dipping of the voltage is causing lights, gas pumps, and coolers, etc, to blink at a local gas station.

 

[UtilityDave]

The blinks are affecting customers on other transformers?

 

[wareagle]

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.

 

[dlmartin]

The feeder breaker is set with a time delayed setting of 150A grd trip and 420A phase trip. I'm not aware of an instantaneous trip.

 

[jghrist]

Since the minimum voltage does not reach zero, the dips are not caused by trips on the circuit where the measurements were taken.

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?

 

[waross]

I agree with jghrist. Look for recloser operations.

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

 

[dlmartin]

Yes, the distribution feeder has some single phase taps that go through trees. I am going to field inspect the circuit again to get a closer look. Thanks guys.

 

[apowerengr]

A poorly maintained load-tap changer or voltage regulator (or set for that matter) while changing taps where the contacts are eroded or the drive mechanism is damaged can "break-before-make" on its contacts which will cause blinking lights for all customers downstream of the device on the phase(s) with bad contacts. In an LTC, different tap positions can affect different phases. Testing at other customers on the same phase(s) can pinpoint such an issue. Shortening the recording interval can provide better information.

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.