Power Transformers Blog – Expert Safety and Usage Info

MIDWESTwas recently asked again why we don’t sell some of our rental transformers. This is a question often asked after power has been restored as fast as possible using temporary equipment instead of permanent repair involving a long time delay.  There are many reasons we try not to sell some of our rental power transformers. The transformer may have an attached high voltage fused switch with lightning arrestors and cabling from the switch to the transformer already installed. A rental pad mount power transformer may be mounted on a skid and already have openings for conduit for the high voltage cables and for the temporary low voltage cables. There may be a main breaker already mounted on the secondary. Some rentals have unusual voltages or many voltage taps not found on typical power transformers, making them much more valuable as a rental than for permanent installation. Some transformers are too old to justify the cost of permanent installation. Some may have unusual primary or secondary physical configurations that are okay for a temporary transformer installation, but too difficult or expensive for permanent installation. Older transformers are physically much larger than same kva and voltage new power transformers, which may make the installation of the older rental power transformer not practical or too expensive. Most of all, many requests to rent a transformer, are an urgent need to get power back up as fast as possible, which frequently means temporarily. Then the owner can more calmly figure out what they want for permanent repair, considering cost and future needs.

An Electrical Contractor called MIDWEST with an emergency. A customer lost a 1500 kva dry type power transformer. The voltage was 13,200 to 480.  Two of the fuses in the primary load break switch had blown. They checked things out and couldn’t find anything wrong, but wanted MIDWEST to test the transformer anyway. Meanwhile they checked out the secondary switchgear and a 480 volt to 208 volt dry type transformer in the same room as the switchgear. They found no smell and no smoke, so they figured they might get lucky, although we both knew that was not very probably.  When 15kv, 100 amp fuses blow, a lot of energy went somewhere.  The 1500 kva dry type transformer was a typical indoor unit substation type, with LAs, cooling fans and a fan control package. When we got there, the electrical contractor already had the panels off the switchgear and transformer, and everything properly grounded.  They offered to help haul up the test equipment, which is always nice, but we said to hold off for a minute.  It took 15 seconds to get our flashlight and then it took another 15 seconds to inspect the transformer and tell them the transformer had failed and they needed to get a replacement transformer or a rental transformer. Their skepticism was obvious, and for good reason. But we showed them the debris that had blown out the bottom of the center high voltage winding. Then had them look down through ventilation ports of the blown high voltage winding so they could see the blown out turns within the high voltage winding.  They understood immediately.  We ran a quick turns ratio test to document the failure. A case of seeing is believing.

What do you do if your manufacturing company suddenly loses its main 5000 kva, 34.5 kv to 480 volt transformer? You have orders pending. And if you can’t fill them, a key account may go elsewhere. You have employees suddenly out of work.  You need to heat the buildings.  And you are told it will take 10 weeks to get a new replacement transformer. This is one of a manufacturer’s worse nightmares. What to do? This happens more frequently than one may realize. MIDWEST actually recommends a “Disaster Action Plan,” on the shelf, ready to go, in case of such a nightmare.

MIDWEST receives these calls all too frequently. You might be fortunate and MIDWEST have a replacement reconditioned or used 5000 kva transformer that can be shipped immediately. No one has new transformers this size in stock. And many companies will not accept anything but a new replacement power transformer. In that case, the best solution is to rent a temporary transformer that will provide power until a new replacement transformer can be specified and manufactured. A new replacement transformer may take 10 weeks.

Renting transformers, and other electrical equipment, is a common solution to this critical problem. The first big question to ask when such an emergency occurs is, “Do you want power back on permanently as quickly as possible or do you just want power back on as quickly as possible?”  There is a huge difference in the time to perform either of these solutions. And way too often there is the presumption that one just must get power back on permanently. This presumption continues until the customer realizes how long it will actually take. Meanwhile many hours, even days, are wasted by chasing the wrong solution.

So the most critical event is to decide how fast one wants power restored.  MIDWEST has transformers, temporary high voltage cables and low voltage cables, and switches in stock.  But, more important, is the knowledge that temporary power installations can be a life saver for a facility that is suddenly down.

The best test to predict the reliable operating condition of an oil filled transformer is the combustible gas-in-oil analysis. Using a syringe, 40 cc of oil is taken from the transformer and analyzed. When threshold levels of specific gases in the oil are reached, protocol demands appropriate action be taken.  This is an extreme oversimplification of a combustible gas-in-oil analysis.

One such case involved a 3000 kva, 25 kv primary, 600 volt secondary, foundry furnace transformer.  The oil test results indicated the transformer began generating high levels of acetylene.  Additional tests were performed and the transformer was shut down and taken out of service. A temporary transformer was put in its place to continue plant production. The defective transformer was sent out to be rebuilt.

Furnace transformers are special breeds of cats, designed to withstand the stresses of heavy shifting loads. Often times these transformers are pushed to the max in terms of load limits. Based on secondary ampere readings, this transformer was pushed to over 100% of rated load capacity, creating high thresholds of heat in the windings. Transformers don’t much like that too well. Heat puts all kinds of funny stresses on the internals of transformers.

In two months time the rebuilt transformer was put back in service and life continued on.