Power Transformers Blog – Expert Safety and Usage Info

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. 

MIDWEST is frequently asked if it is safe to take an oil sample from a sample valve under the secondary cables of a pad mount transformer while the transformer is still energized?  No, it is not safe. Do not do it.  We think it is extremely dangerous.  This is the location of possibly one of the most dangerous arc flash hazards in a facility.  You can’t wear enough PPE to protect yourself.  Keep away from the secondary.  Just opening the door to inspect it can be very dangerous.  In addition, you would be exposed to unsafe shock hazard.  Our recommendation is to sample the transformer during a scheduled outage.  We know very well in years past this was common practice for many service companies.  Those days are gone.  As they should be.

It’s starting to catch on ­ this energy efficiency stuff. For once, it seems, the numbers tell the story.

A man in Iowa buys a former manufacturing facility and changes it to a warehouse facility. He no longer needs the amount of power from the production days as the facility is now used to store portable generators. Building load is now mainly just for lighting. Starting in January 2007, the Federal Energy Policy Act of 2005 mandates new minimum energy efficiency standards for low voltage dry type transformers per the NEMA TP-1 2002 Standard.

The new owner swaps out the old 500 KVA dry type transformer which powered the building for a new Jefferson 150 KVA 480/240 dry type transformer at a purchase cost of $3,375.00. Given equal loads, the new Jefferson high efficiency 150 KVA transformer translates into substantial savings due to lower “no load core loss”. The new transformer saves up to approximately $175.00 per month in electricity used. The recovery cost break even point at that rate was 19 months. Another important factor when considering a new replacement transformer is the size of the transformer footprint. The new Jefferson had an 80% smaller footprint even though the transformer KVA capacity was reduced by 70%. These are numbers you can depend on. “Things are in the saddle…ride mankind ride.”

By Larry Dahlgren

So someone told you should obtain a transformer.

Before you open your wallet and buy the first transformer you meet, let’s consider the situation a bit.

Buy Dry Type Transformers

You might have just moved your equipment to a new facility and the new location seems to be electrically different from your former location.  You do not see the familiar types of disconnects and plugs your former location had or your machines used to be hard wired and now, at their new home, you have a collection of machines with pigtail wires and you are unsure where to connect them.  Maybe you just picked up a spare to one of your existing machines or added a machine because there are many available at auction.

You get them to your shop and now find the nameplate voltage is different than your building voltage.

A.        AVAILABLE VOLTAGE

Is the actual voltage at the new location any different from the old location?

It could be the same and it might just be referred to by some other name.

Maybe that machine you just picked up at the fire sale has a voltage tolerance that will mean it can work just fine on your available supply.

Consider this single phase example:

There are those who refer to the voltage that comes from a typical wall outlet as 120 Volts.  A generation ago that same voltage used to be called 115 Volts or 110 Volts.

Today the do-it yourselfer may connect an electric water heater or electric dryer or range to 240 Volts from a breaker fed from both sides of the common household circuit breaker cabinet.  Thirty years ago the water heaters were connected across 230 Volts from both sides of the fuse box if the thinking is that the wall outlets were good for 115 Volts.  Maybe that water heater used to be wired to 220 Volts coming from both sides of the fuse box that powered all of the 110 Volt outlets. 

This can become complicated even more in multi family buildings, served by the utility with 208Y/120 volts, where each unit is served from two hot legs and the neutral.  In that case the wall outlets are still at 120 Volts but there is only 208 Volts between the two hot legs so the dryer and water heater are fed from 208 Volts.  Trouble is, a lot of folks just say the specialty socket for the stove or range has 220 Volts and then you have to investigate further and measure safely so you know the actual voltage that is present.

The same type of confusion can extend into the three-phase industrial setting. 

People refer to one type of nominal, three phase, three wire voltage as 440, 460 or 480 Volts.  Some refer to another type of nominal, three phase, three wire voltage as 220, 230 or 240 Volts.

The two, common, three phase, four wire, nominal voltages are 480Y/277 Volts and 208Y/120 volts.

Measuring (safely) you may discover that the actual voltage, over time, could vary above and below these nominal values.  The voltage at times could reach as high as 504 Volts or drop to only 456 volts.  Other times the 240 Volt nominal voltage could swing as high as 252 Volts or as low as 228 Volts.  Watch out because occasionally 208 volts can swing as high as 218 Volts.  You are then left to wonder if a measurement of 216 Volts could mean a stiff 208 Volt system or a very soggy 240 Volt system.  Determining the actual voltage and the nominal voltage are critical in the selection of a transformer. 

The same holds true for your wall outlets where the acceptable voltage, according to your local power company, can swing from 95% of 120 volts (114 volts) to 105% of 120 Volts (126 volts).  Should the voltage drops to 90% of 120 Volts (108 Volts), call your power company.  

Hey 114 Volts is only a volt shy of 115 Volts.  This means your parents were right again; this time about wall outlets.

Next time we will discuss the equipment you energize with the voltage you have.

See Part 2

Good Luck, be safe and happy transforming.