Power Transformers Blog – Expert Safety and Usage Info

One common concern with pad mount transformers is in the area of oil sampling. Companies with large pad mount transformers may sample and test the oil in the transformers annually to determine the reliability of the transformer. The condition of the oil reflects the current state of the transformer. Now here’s where it gets tricky. Large pad mount transformers are designed with two cabinet doors. One door exposes the high voltage cables and bushings and the other door exposes the secondary, low voltage, side. The sample valve can be located in either compartment but most of the time the valve is located in the secondary cable side. Years ago it was not uncommon for an intrepid technician with nerves of steel to pull an oil sample from an energized pad mount transformer, the valve being located at the bottom of the transformer while hot cable lugs were located only a couple feet above the oil valve. With safety becoming a paramount concern in industry today, it is no longer prudent nor standard practice to pull an oil sample on an energized pad mount transformer.  The hot lugs being within short reach of the technician create a shock hazard.  And the secondary side of a transformer is one of the most dangerous arc flash hazards there is. Oil sampling is now performed only during scheduled shutdown of the transformer.

However, there still remains somewhat of a challenge when it comes to pulling an oil sample from a pad mount transformer.  The secondary compartment of pad mount transformers can be a pretty fully house. There may be as many as six cables attached to each secondary bushing, making it very difficult to access the sampling valve through that jungle of vines. One way around this, literally, and we’re finding it used more and more in industry, is extending the sample value system to a small secure box on the exterior of the transformer enclosure. The valve would then be enclosed in a newly created box with a locked access door mounted on the exterior wall of the cabinet. In this way the transformer can be sampled at any time, while energized, without exposing the technician to shock or arc blast hazards.

During a highly technical conversation about the life expectance of old and new electrical transformers, MIDWEST was asked by the Consulting Team what the most common failure mode for outdoor oil filled power transformers was. The discussion involved 1000 kva to 10 Mva power distribution transformers typically found in the outdoor substations of manufacturing plants.  We were discussing Failure Mode and Effects Analysis (FMEA) and its value for reliability optimization.  The consultant was using military data for failure mode and frequency of occurrence. There was poor connectivity between that data and our world of big old oil transformers used by industry. It was even less appropriate to use historical military data on the new oil transformers used today to replace those old tubs.  The whole thing was rather ridiculous as every one struggled to find legitimacy in the analysis. The solution came with a twist from a reliable and too often forgotten source.

MIDWEST’s senior field service technicians and service engineers were asked what the most common failure mode was, based on their experience, experience that exceeded 100 years. Their one word answer was, “Raccoons.”  After the technical minds recovered and realized the answer was more than just a little jab and a lot hilarious, they realized it was true. So now they asked what should be done to lower the probability of this critical failure mode.  The answer, “No raccoons.”  Problem solved.   

MIDWEST was asked if we ever rejected a transformer even though it passed all our electrical tests.  The answer was “yes.”  We scrapped an old 1000 kva dry type transformer used in a fairly clean room.  We had purchased the old transformer after replacing it with a larger unit in a paper mill.  The transformer was trucked to MIDWEST.  But an outer high voltage winding had physical damage that was found during the “hard focus” visual inspection. The riggers had damaged the outside of one winding with the tie down chains.  All the standard test results were very good.  And we energized the transformer at full voltage.  Everything tested out great.  But our inspection told us the transformer could not be used.  It was junk. Not worth repairing. It had what we call an “Incipient Failure.”  It was damaged and was going to fail.  When, who knows.  A day, a week, two years?  We didn’t know.  But we did know it was not reliable.  The high voltage windings were going to fail, short out.  It was just a matter of time before the used transformer became scrap.  We just hurried it up and declared it junk.  Here is another great example of pure technical knowledge just not being enough.  Experience and Knowledge were needed. 

Three answers, NO and NO and NO.  This is a big deal for us. MIDWEST does not sell old, used, or obsolete circuit breakers, bus plugs, transformers or other electrical equipment to an end user, ie the consumer, unless it has been properly serviced, reconditioned, tested, and passed Quality Controls.  We strongly recommend no one buys old electrical equipment “as is.”  That could be very dangerous. There are many “listing” companies and auction type sites that sell “as is.”  For example, eBay does not warranty the used and old electrical equipment sold through their service.  They provide a great service, but they do not provide a warranty on the specific used electrical equipment sold on eBay.  The same may be true with services that just list equipment.  The only people MIDWEST will sell electrical equipment “as it,” are other qualified electrical dealers that have proper Quality Controls.  Old, used, or obsolete circuit breakers, bus plugs, transformers, and switchgear have too many dangerous failure modes to be sold without proper testing.  MIDWEST does not just process orders.  Anybody can do that.  And many do.  We actually have a name for that.  We call it TC, Trained Chimpanzee, work.  You could train a chimpanzee to put old circuit breakers into a box. 

We add value.  Our staff of engineers and technicians service, repair, recondition, and test electrical equipment for a living.  We’ve done it for over 30 years.  That is what we do day after day. And it is what we think others should do if they are going to sell something.  “As Is” is just scary.  When electrical equipment gets unhappy, it can fail with a boom and a flash.  And that can hurt.

MIDWEST bought a 500 kva general purpose dry transformer that was taken out of service as part of an expansion job.  The transformer was well secured to a large wood pallet.  It was wrapped in plastic and looked great when it came in our door.  It was a used transformer, but looked like a brand new transformer, except for two knockout holes and a strange bulge on one side. When our switchgear shop checked it out, they found the shipping bolts had not been replaced before shipping.  The transformer was now a boat anchor.  A very large boat anchor.  New dry type transformers are shipped from the manufacturer with bolts holding the core and coil assembly to the base frame. After installation, these shipping bolts are removed so the transformer sits on absorbent pads that minimize vibration transfer from the energized transformer windings to the transformer enclosure.  But, if you move the transformer without replacing these hold down bolts, the transformer core and coils are just setting on a frame and they will trip over, inside the enclosure, the first time the truck goes around a sharp corner or stops suddenly. As soon as the outer high voltage windings hit the inside of the enclosure, the transformer becomes expensive junk.   

Frequently electricians and plant engineers ask how long dry type transformers last.  The easy answer is, “Until they fail.”  But that answer might result in a ‘click’ at the other end of the phone line.   In this case we are talking about general purpose dry type air cooled transformers, 208, 240, and 480 volts.  These transformers are located everywhere in manufacturing facilities.  For most people they are just metal enclosures that are warm in the winter and make a funny humming sound.  We were asked if they were good for 25 years or more.

The biggest enemy of air cooled dry type transformers are roof leaks and fork trucks.  The transformers are good for many more than 25 years.  But don’t let them get wet, dirty, hit, or buried under “stuff.”  They do not like water or heat.  If the ventilation is blocked, they may overheat and that will shorten their reliable life expectancy.  So, if your general purpose, dry type, air cooled transformer is kept dry, clean, and well ventilated, it may last 30, 40, even 50 years.  It should last until you retire, which has become the newest criteria for many electricians and plant engineers.  Also, these transformers look innocent, but they are very dangerous, fatal, if they blow up or if something gets stuck inside the enclosure.

We are talking about transformers that might be 1000 kva with a primary voltage of 4160 volts or 13,200 volts and a secondary voltage of 480 or 208.  They are in a metal enclosure with vents to allow air to circulate and cool the transformer. These transformers are indoor and usually found in locked electrical rooms.  We see these transformers 60 years old and still humming away.

Buy 45 KVA Jefferson Electric Dry Type Transformer

The small dry-type transformer – what can one say about these marvels?  They come close to achieving the perfect combination of form and function.  Designed to be cooled by movement of air, not moving water, the dry type transformer is perfect for indoor use or anywhere that it can be sheltered from the elements: porches, below lean-tos or beneath small structures.  There are some with optional rain ‘gutters’ intended to direct water away from the transformer coils.  Wherever they are found, the dry type transformer, one of the most common type of transformers seen today.  It is the ubiquitous work horse of the industry.  The three phase 480 Volt Delta to 240 Volt Delta, from between 50 to 150 KVA, are among the most popular.  As many utilities have moved to away from offering 240 Volt services, they are commonly used to transform the incoming 480 Volts to power 240 Volt equipment.  Most are designed so you can usually run them backwards to produce 480 volts from 240 volts. What happens when this very reliable workhorse fails?  No spare readily available, you obtain a replacement.  Right?

But why wait till then?  It would be good practice if they were looked at occasionally, once installed, but small transformers of this type are typically not tested during routine plant shutdowns.  So they are frequently used as tables, workbenches, shelves, ladders or stationary hotplates to keep lunches and bakery warm.  There are usually indicators well in advance of a transformer’s doom.  Little warning signs that if left uncorrected will lead to failure; excessive noise, excessive vibration, the usually warm transformer gradually becomes hot to the touch transformer.  If you look underneath one and see only dusty clogged vents, chances are it is running at a higher temperature than recommended. 

At MIDWEST we perform an insulation resistance test and load test on all of our reconditioned transformers before they leave our door.

Meanwhile take care of the dry type transformers you are presently using.  Be smart and keep them free and clear from clutter, dirt and moisture…

The good news is if the need for replacement arises, you can usually find something to take the place of the one you just lost.  You can buy a new one from us because the manufacturers still make them or you can ask us for a quote on a reconditioned transformer.   

Transformers are placed in parallel by electric utilities when they want to provide a ‘stronger’ voltage source and will result in higher available fault current that can be delivered downstream.  Usually the utility transformer can serve its own load but two are put together to achieve with a ties primary and secondary bus to ‘stiffen’ the voltage to ride through system load changes.

Yes the utilities have to worry about automatic load tap changers that could hunt forever.

Some industrial customers try this method to serve a load that is too large for a single transformer.  This arrangement has to be done carefully or you can damage one or both of the transformers.  You need an exactly matched pair of transformers or transformers of different manufacturers with identical characteristics or circulating currents can build up in the parallel connection that consumes energy and does not pass to the load.  Routine maintenance of the parallel connected transformers in the non-utility world is nixed because neither transformer is big enough to serve the load on its own.  This would mean extended outages to the load during maintenance of either transformer.  Parallel connections of transformers in the industrial world are usually not attempted because the lack of maintenance makes for a less reliable installation.  Transformers come in various sizes and the best or optimum transformer size is one that can adequately handle its load all by itself.

Customers can be skeptical of purchasing a transformer used compared to buying a new one.  It is understandable since a transformer is an electrical device, there is always the possibility that the unit is on its last legs and could fail and then the buyer is faced with the costly disposal of a cumbersome item that has little or no value.  But look at the cost savings if you can find a quality unit on the secondary market that will do the job compared to a new one priced at full retail or wholesale from a supply house.  There are many ways to obtain a transformer that is not ‘new’.  The secondary market can include demolition companies, scrap yards, friends, closeouts, new surplus, scratched and dented etc.  You can also find reconditioned units that have been tested and carry a warranty that is usually as long as the one offered when the unit was new.  Shopping for a transformer requires some homework to know every last detail of the transformer you want so when you stumble on a ‘non-new’ transformer, the lead time is immediate and you can snatch it up before somebody else does.  In the event you can’t find a bargain, you can always hope someone has a new one in stock.

Written by Walt Powell, Professional Engineer

Sometimes an old substation will have old oil filled transformers that have no gauges.  These are usually 500 kva to about 2000 kva transformers.  We are not referring to pole top type transformers.  Some Utilities used oil filled transformers with no accessories to save costs.  They would consider these transformers to be small and basically disposal.  Many of these substations have been sold by Utilities to the consumer.  The consumer gets a cheaper primary rate and owns the substation, along with the responsibility to take care of it.  Sometimes we are asked how to determine the oil level in a transformer that does not have an oil level gauge, without turning the transformer off.  We safely feel the cooling tubes and the tank.  If the tubes are cool and the tank is hot, it is low on oil.  If the oil is properly circulating through the cooling tubes, the tubes will also be warm.  Not as warm as the tank, but fairly close.  Caution, only a qualified electrician or electrical engineer should do this.  Stay away from energized parts and make sure the transformer tank is properly grounded before you touch it.  Protect yourself from shock hazard and from an electrical arc blast hazard.  This may well require PPE.  Do not touch anything else while touching the transformer tank.