# Maintenance schedules for smaller dry type transformers



## BillyMac59 (Sep 12, 2019)

I've worked as a plant electrician for 30+ years. Transformer maintenance has always been very basic. Is it clean/ well-ventilated? Accumulated dirt/dust can impede normal cooling paths. Thermal scans are where I'd start as part of a PM programme. I'm no expert on 70B, but I'm guessing that some of the tests you mentioned are more in line with commissioning than routine maintenance. (I'm sure I'll be corrected if this is an incorrect assumption.) Another avenue of information would be to consult the manufacturer of the trannies. Following their guidelines (and documenting them) would help you in the event of premature failure. I'll follow the thread. I'd be interested if others in similar positions have a more detailed maintenance schedule


----------



## gpop (May 14, 2018)

I have a rule based on what im willing to do live verses the risk. dry transformer cover removal is a solid 9 so im going to power it down to remove the covers which probably means a thermal scan that isn't worth the effort. Like ValeoBill says a quick years clean and check is about all its worth. 

Now if you are looking for low hanging fruit you need to start with checking things are sized correctly and have not been bypassed by a previous employee. 

Yesterday i was called to a small pump that gets used 3 days a year. I didn't even know it existed until i was called to see why it would not start.

pump is 3hp 480v 4.6 amp
Phase protection relay --- bypassed
breaker ---- 50 amp
fuses ---- 30 amp time delayed
overloads ------ 9 to 25 amp adjustable with the relay bypassed 
thermal switch on motor ------ disconnected

Poco dropped a leg earlier in the week so it smoked the motor. 

I had 362 days out of a year i could have planned and taken care of the problem before it became a even larger problem. Yesterday the panel had to be rebuilt and the motor replaced as a emergency. 

So back to the transformers. Are they sized correctly and are they protected by the correct sized breakers (fuses). Ive only lost 2 dry transformers over the years. One to lightning (practical a direct hit) and the other to being undersized (over the years additional load was added and no one bothered to redo the calculations)


----------



## mayanees (Jan 12, 2009)

mburtis said:


> I am the plant electrician for a water treatment department. Having an in house electrician is a fairly new thing so there hasn’t really been any electrical maintenance done in the past, pretty much wait for the smoke to roll and then figure out what doesn’t work. I’m slowly getting caught up on major fixes to the point I can start trying to develop a maintenance program. I would like to start some regular inspections/testing of our transformers. I have been going through NFPA 70B but would like some guidance from some experienced folks on the most useful tests to do to smaller dry type transformers are. The transformers I’m talking about are your standard 480v three phase primary ventilated dry type. Fairly small, probably all less than 150 kva.
> 
> Voltage/current tests are an obvious place to start, as are thermal scans and insulation resistance tests, looking into ultrasonic equipment as well. What about some of the other tests mentioned in 70B, such as turns-ratio tests, power factor tests, excitation current tests, etc. Will these tests be able to tell me anything about the condition of these smaller type transformers. Keep in mind that we are a public utility and there is no such thing as extended unscheduled down time due to public safety and government regulations. Also I am in Wyoming so it’s not exactly like I am going to run down to the local supply house and pick up a transformer. I also typically have a lot more time available than might be typical in a maintenance situation so the time required to do the tests is of little consequence. Appreciate any guidance on this from those more experience in the maintenance side of the industry or any references to books, standards, etc that can help in this area.


Good for you mburtis, for giving the electrical system maintenance attention.
Is the system labelled for arc flash? If not, I suggest getting a Power System Study performed, Fault, Coordination and Arc Flash analyses. That document will provide you with a Report of Electrical Protective Device Settings that would serve as a biddable document for an electrical testing outfit to come in and perform PM according to NETA Maintenance Testing Specification. And of course you could handle any of that work that you were comfortable with, or have time for. The Power Study would also check what gpop was referring to - NEC compliance for cable protection, xfmr protection, and would provide settings for compliance, along with settings to prevent the Utility fuse from tripping ahead of your on-site protection.
If you have a decent one-line diagram, complete with cable sizes and lengths, breaker types and settings, etc, you're half of the way there for a Power Study.
We're doing exactly what I'm suggesting here for a WTP in a County in Virginia. Their equipment was installed in the late 70s, and has never had any attention electrically. In the time since then service-entrance transformers were upgraded and much of the downstream equipment is overdutied from a fault current standpoint, but that's the kind of stuff a Power Study will uncover. Our objective is to provide arc flash labels along with the necessary analysis that a Power Study entails. Their requirement for installing arc flash labels is that the system be maintained per industry standards in compliance with NFPA 70 (NEC) and in accordance with NFPA 70E, Standard for Electrical Safety in the Workplace. 
Good luck with it. It's good work, and a good way to spend a career, in my biased opinion!


----------



## mburtis (Sep 1, 2018)

Thanks for the input so far. I do have an arc flash study that was done a few years ago, I need read the report some more to see how accurate it is or what has changed. One plant was built in the 60s and is a nightmare, a mix of 480V and some left over intentionally ungrounded 240v delta systems, the one line drawing is completely wrong, random wiring going everywhere etc, etc. The other plant was built in the early 90s and hasn't really been touched, only issue is everything is old and starting to fail. I know none of the transformers have ever been cleaned so that might be a good place to start. I got the boss to hook me up with the NETA MTS standard so Ill have to read through that and see what sort of rabbit hole that leads me down. I have a great interest in learning the maintenance and testing side of things, and typically have good support on buying equipment and training. In the mean time I will continue digging into the sizing of everything and see what I turn up. We already replaced one transformer that was not proper, leaving half the 120V circuits with no bond between ground and neutral. I also have an air compressor with number 8 wire on a 70 amp breaker, I'm only 30 and figure maybe ill be caught up by the time i retire.


----------



## SWDweller (Dec 9, 2020)

How can a fault study be done with out accurate drawings? 

I am going to come at you a bit differently. I did maintenance at a university where a scheduled outage could take a year. 

We dealt with dry type transformers that were over 1000kva once a year. We had temp readings on all of them taken once a month and recorded. There were gauges in the doors. If there were fans they were running. Not for the increased load but for keeping the coils cleaner. 
During the once a year the coils were TTR'd the feeders were megged or highpoted depending on voltage. All values recorded in a book for each transformer and kept online in a data base.
General cleaning and torquing to MFG specs. General visual inspection button up and return to service. Under 1000 wait till they break. We always reversed two of the middle screws so you had something to hang the doors on for R and R. We never did power factor testing.
I find thermal scans a waste of time and money. VERY FEW places will allow you to install viewing ports for your equipment. They FLATLY refuse you to pull a cover when energized. Sound reasoning behind this. If you shut down the transformer and the remove the covers and then try to do your scan how long are you going to wait for the load to impact the transformer? Again the sound reasoning applies per the arc fault study. 
The most important thing the boss created was a numbering system for every piece of equipment.

We found a vibration tester to be more valuable than a lot of equipment. We had our motor shop align ALL motors. A washer was epoxied on to the motor and related equipment so that the readings were the same each month. We had a lot of rotating equipment and this got us to fixing before grenading which is so much more fun.


----------



## mburtis (Sep 1, 2018)

The fault study was done before my time. I'm not sure if it was done before the last big upgrade, during the last upgrade and we never got the right drawings, or if its just completely wrong. I need to read the report again now that I am learning a little more about it. All I know is what I was told was the up to date one line drawing, didn't even have the wrongly installed transformer we replaced on it. Vibration monitoring is something else that I have been looking into, along with ultrasonic. I don't know much about either yet, but both look useful. I agree with you that fixing stuff before it blows up is sure a lot less stressful, especially if its something important. Don't make for nearly as cool of stories/pictures as when something comes apart in spectacular fashion though.


----------



## SWDweller (Dec 9, 2020)

We installed the Cutler Hammer monitoring system on all of the motor starters in our MCC's ran the com cable to an stand alone computer and recorded volts and amps and operations .
We very quickly realized that by monitoring and paying attention to the information we could plan outages and work relaxed with all of the part. Instead of wile coyote.


----------



## Easy (Oct 18, 2017)

I would think you would have your hands full just keeping the place running and not have much time for preventative maintenance. I commend you for being organized and trying to stay ahead of things. I would concentrate on standardizing equipment and look into getting tools that will make your job easier. Keep records of all the equipment not just power distribution but other things like emergency lighting. Repair anything that is unsafe. Monitoring temperatures, voltages, vibration and other aspects would be an investment but would be a great addition to the plant. Installing spy ports on switch gear for your thermal camera would be simple if you had a scheduled shut down. You might also do cleaning at the same time. You could also install lube systems on your motors that have bearings that required periodic lube. IMO safety is the first thing to look at. Open junction boxes and other low hanging fruit. It sound like your doing a great job and trying to improve things rather than just patch them up and that's what I call a good employee.


----------



## SWDweller (Dec 9, 2020)

Standardization will help you immensely. Great point Easy. We had a lot of medium voltage switches, and found out that we could force new projects into the same switch mechanism that S&C used. (had over 45 of them)
That way all of our medium voltage fuses and holders were the same type, sure made it a bunch easier keeping spare parts on the shelf.


----------

