Pump station Electical Problems

[Pax]

Hello all,

I work for The town of Paxton, Light Dept. We have a problem at a water pumping station that I,m having trouble with. We have one circuit in town, 13,800 volts grounded wye. The pump station has a 45kVA 3 phase pad transformer with a delta 240/120 secondary (Xo bushing is provided for 120 volt load in pump building, ie lights, scada, teleometry, comm. radios.)

Problem: Water dept. is consistently burning up variable speed drives, boards inside of scada control box, radios, isolation xfers. It seems like, when the town has a dip in power due to a fault/recloser operation, that these problems arise.

Could this occasional loss of power and/or delta secondary configuration with a wild leg be causing a surge when re-energized?

I will provide any needed info to help diagnose the problem.

Thankyou
Chris

 

[busbar]

Is the site distribution transformer’s "X0" bushing solidly grounded? Is anything besides 3ø motors served from the high leg? Can you detail the primary configuration? [id est, locally grounded, and to what in detail? 4-wire from the 13.8kV distribution circuit? multigrounded or unigrounded?]

 

[dpc]

As much as I dislike wild-leg deltas, there are a lot of them around that work OK. As busbar says, I would check the grounding of the secondary side and make sure it is in good shape.

Do the failures all seem consistent with a voltage surge or high voltage condition?

You might also consider installation of secondary side surge suppression at the service entrance.

 

[DanDel]

Monitoring the incoming power would allow you to see what was happening as far as voltage surges or ground potentials. How often do the problems/power dips occur?

 

[jbartos]

Suggestion: It could also be a combination of harmonics, surges, voltage sags, voltage dips, etc.

 

[Pax]

The Xo bushing is grounded to a ground grid surrounding the xfmr pad, and the disconnect, meter socket, and breaker panel (All mounted next to the Xfmr in water tight enclosures) are also bonded to this ground grid. We have introduced the customer to a electrical test firm, who will install some monitoring equipment.

As far as the frequency, problems seem to arise most often after the primary circuit has a operation (2 second outage), due to a fault. I'm curious if there is some kind of inrush, or transiant being created during re-energization.

The primary feeding this pad is 13,800 volts grounded Y. Looking at the pads name plate, the internal primary coils are wired wye with all the H2's connected and floating. Through my research, this is correct, as grounding the H2's could lead to a whole host of problems, leading to roastout of the pad. There is no load off the wild leg, other than three phase load. The only equipment being damaged is the single phase load.

 

[busbar]

IEEE Standard C57.105-1978 suggests that an ungrounded-wye/{grounded 4-wire} delta transformer configuration is susceptible to ferroresonance. It may be difficult to achieve stable secondary-side voltage balance with this configuration—compared to, for instance, a delta/delta bank.

IEEE Standard C62.92.4-1991 states, “…an open conductor in a three-wire primary results in a single-phase input and output of the bank.” It is conceivable that high-side faults and single-pole reclosers could exacerbate the situation through significantly unbalanced secondary voltages.

Although not a trivial solution, C57.105 suggests as an alternative to highside wye-point solid grounding, that a resistor may be applied to limit resonant conditions in the bank. An electrical-engineering consultant should be contacted for review and recommendations.

[Mapquest shows six states with “Paxton” in the US.]

 

[DanDel]

Sounds like monitoring would be the first and best step at this point. My company worked on a similar problem with the Millis, MA, Water Dept., in which we did some 3-point ground testing and, I believe, monitoring. I believe we ended up installing surge arrestors to fix their problem.

 

[peebee]

I agree. Put a PQA on there and find out what's happening on your system rather than randomly replacing parts and pieces.

 

[jbartos]

Suggestion: The 2-second reenergization can cause a voltage dip that affects the single phase motors more than three phase motors since they become overheated due to the voltage dip/sag after the reenergization.

 

[swgrmfg]

Pax:

If all else fails, you might want to consider replacing this transformer with one that has an electrostatic shield.