Power Transformers Blog – Expert Safety and Usage Info

A recent series of blogs by MIDWEST’s Engineering Department at Circuit Breakers Blog and Power Transformers Blog have described the failure of some surge suppressor strips, and the author’s custom transformer isolated power conditioner and three stage surge suppression system.

The common thread in these blogs is surge suppression of voltage spikes utilizing metal oxide varistors.    

Voltage spikes, or transients, can be produced by a wide variety of electrical devices, such as electric motor starters, circuit breakers opening or closing, electrical switchgear, motor control centers, soft starters, vacuum circuit breakers, industrial transformers switched on or off, motor contactors, and of course lightening strikes.

There is a little known effect that metal oxide varistor surge suppressors employ in order to attenuate voltage spikes.  This was first researched by a highly respected PhD. on our staff in the process of writing a White Paper on varistor protection of industrial power electronics.  The effect is the series impedance provided by the power company’s distribution system, the building wiring, the surge suppressor strip’s line cord, substation transformers, pole transformers, and isolation transformers.  This aspect is never really mentioned in the literature.    

The line impedance consists of a small resistance, and a small but important inductance.   Fourier analysis tells us that a spike is mostly high frequency.  Thus, the inductive reactance at spike frequencies can be considerable. This reactance, and to a lesser extent, the resistance, is the vital ingredient that makes a metal oxide varistor suppression scheme work in the first place.  Inductive reactance equals 2 times pi times frequency in hertz times inductance in Henries X_L=2*π*f*L.

Normally, everybody thinks that when a large spike hits a varistor, the varistor conducts huge currents and “eats” the spike.  But, what is never mentioned, is that the huge current also passes through the equivalent series impedance of the line.  Kirchhoff’s voltage law says the huge voltage has to be impressed somewhere.  If we assume that a 10 KV spike is applied, the varistor might conduct a very large current, hundreds to a few thousand amperes.  According to the voltage-current characteristic VI curves of a Max Allowable Voltage of 130 VAC RMS varistor, the varistor might have about 300 Volts across it.  What happens to the other 9.7 KV?  As the huge current passes through the line reactance, the majority of the spike voltage is impressed across the line reactance.  The huge spike voltage is not impressed across the varistor.  But, if the 10 KV voltage would instead be impressed across the varistor, such as might happen from a very nearby lightening strike, the varistor would conduct tens of thousands of amps, and instantly vaporize, along with a lot of the wiring in the house.  But if instead the lightening strike is 1 mile away, there is 1 mile of resistance and reactance to act as a  current limit by having most of the 10 KV dropped across it.

Thus, what really happens during a high voltage spike is that the varistor starts conducting appreciably around 300 Volts, and conducts hundreds or thousands of Amperes.  The applied voltage spike is then impressed across the line reactance, according to Ohm’s law.  Thus it is actually the line reactance that “eats” the spike voltage.  The varistor is only the current path for the spike current.  Of course, a large power dissipation also occurs in the varistor too.  It is usually only this dissipation that is ever mentioned or considered.  But the line impedance is truly the vital element, as it limits the varistor current and dissipations to reasonable values. 

Now, with this said, the real value of the isolation transformer in the line conditioner can be seen.  Its leakage reactance provides a large series impedance for any spikes that get through to the secondary.  The secondary’s spike elimination network starts to conduct large currents, and the spike’s voltage is impressed across the transformer’s leakage reactance.

Isolation Transformers Isolated Power Conditioner Block Diagram 1

Isolation Transformers Isolated Power Conditioner Schematic

Over the years MIDWEST has had to do some creative and sometimes risky work. These things typically occurred when there were very unusual or strange circumstances.  And they were usually performed against our advice to customers that had no easy or inexpensive alternative. When we say risky, we are not talking about risk to life safety. This following example was a very simple task, unless something went wrong. We just needed to replace the lower main valve of a 2000 kva oil filled power transformer without draining the transformer first.  The owner could de-energized the transformer for a short time, but could not tolerate an outage long enough to drain the oil, change the lower main valve, and replace the oil.  They were willing to take on the risk of changing the value with the transformer full, and de-energized and grounded, of course. Actually, we have changed valves on smaller and larger power transformers, but the circumstances were always different.  We knew in this case we would end up with possibly two gallons of messy waste oil, even if things went well. Our procedure was as follows. Loosen the existing lower valve from the 2000 kva power transformer lower valve stem. Prepare the replacement valve such that it could be immediately installed after the existing valve was removed. Secure the area. A large spill pan was placed under the existing transformer valve and stem. All the needed tools and the new replacement valve were placed inside a sturdy large clear plastic bag, including a pipe cap, just in case. A vacuum was pulled on the transformer. The plastic bag was secured about the pipe stem at the bottom of the old transformer. The bag was big enough to hold and manipulate the valves.  Then, like two lab rats, one technician removed the old valve while a second technician held the new valve adjacent the existing valve. The old valve was removed and slide to the side just as the new valve was slide into place and screwing onto the pipe stem. Works well if you catch the threads on the first try.  Either way, one ends up with a blast of old oil all over the inside of the bag.  If the valve quick exchange fails, the cap is put on the stem. It is amazing what happens inside the clear plastic bag in the seconds it takes, hopefully, to change the valve.  This task may sound like a good idea, but we do not recommend it. And we have a hundred years of experience working on old and new power transformers. We’re good at it and we like it. But we know old oil filled electrical power transformers are good at hiding defects until you try to work on them.  Sometimes the messy work is the fun work on transformers.  

Recent circuit breaker blogs by MIDWEST have dealt with circuit breakers and surge suppressors.   For expensive computer equipment, superb surge suppression is vital.    Surge suppression is exceedingly cheap insurance for such applications.  The initial / replacement equipment costs can be expensive.  But even more importantly, downtime and data loss can be catastrophic.

In fact, when putting together my own home computer system, I considered surge suppression so extremely important, that I put together my own 3 stage surge suppression system.

The 3 stage system consists of:

1. a 3000 Joule commercial surge suppressor strip, with integral circuit breaker.
2. a custom designed and built, transformers isolated power conditioner.
3. a 2000 Joule commercial surge suppressor strip.

The 3000 and 2000 Joule commercial surge suppressor strips were the highest quality (i.e. expensive) that I could find.   But surge protection is so important, that I considered these insufficient.

Therefore, I additionally designed and built a custom transformer isolated power conditioner.   It contains a 2 KVA dry transformer and six metal oxide varistors.  It also contains four RC snubber networks for high frequency EMI attenuation.  The main purpose of the transformer is to provide additional series impedance.  The design of the power conditioner is covered in a separate blog.

The theory behind the entire scheme is that each stage of the surge suppression system helps dissipate the spike’s energy.   An applied voltage spike first is partially dissipated by the building’s line impedance and the first 3000 Joule suppressor strip.   The portion of the spike that gets through is next attenuated by the custom transformer isolated power conditioner.  In addition, this helps slow down steep wavefronts and reduce EMI.    The final 2000 Joule surge suppressor strip attenuates whatever spike remains.

A block diagram of the system is shown in the Visio diagram below.

3 Stage Surge Suppression System