HARMONICS; HOW THEY AFFECT THE DEVELOPMENT AND DESIGN OF "K" FACTOR

TRANSFORMERS by JEROME M. FRANK P.E. IEEE FELLOW

OLSUN ELECTRICS CORPORATION

The design and development of solid state switching devices has made a very important contribution to our ability to handle and process data as well as our ability to more efficiently control electrical equipment. New items must now be taken into consideration in the design, development and application of electrical equipment.

When 60 hertz current flows through a conductor, i t is relatively uniformly distributed thruout the cross section of the conductor and i ts heating effect on the conductor can be readily calculated.

As the frequency increases, "skin effect" causes the current to flow towards the outer edges of the conductor and the current distribution in the conductor is no longer uniform. T o reduce this heating effect , smaller insulated conductors in parallel were used rather than one larger conductor to provide as much surface skin as possible resulting in a lower resistance to the higher frequency currents as the higher frequency currents tended to flow towards the skin rather than to the center of the conductor. This principle was known and used in the design of 400 hertz conductors in airport lighting design. These 400 hertz cables consisted of multiple insulated conductors rather than one larger conductor.

A transformer has a core loss which is on 24 hours of the day as long as the transformer is energized. It has a conductor loss at 60 hertz that varies with the transformer loading. In addition, it has eddy losses in the conductor as well as stray losses that are caused by the flux linking the frame parts. These losses and the cooling ducts in the windings determine the temperature rise of the transformer windings. If the transformer is designed for a temperature rise of 150 Degrees Centigrade by resistance , with a 40 Degree C ambient and a 30 Degree C differential between the average rise by resistance of the winding and its hot spot, the ultimate temperature of the winding will

be 220 Degrees C. Ventilated Dry type t r ans fo rmers h a v e a UL recognized insulation system that is rated 220 Increased harmonics will increase the eddy current loss of the windings and this will appreciably decrease the life expectancy of the transformer. As pointed out in one of the papers in the Bibliography, the Arhenius Rule of Thumb reduces the life in half for every 8 to 10 Degrees C that the temperature is increased.

That is one of the main reasons that lower temperature rises of 115C and 80C were suggested in the Energy Consideration paper in the Bibliography. Overloading as well as Harmonics are factors that must be taken i n t o cons ide ra t ion in a t r ans fo rmer app l i ca t ion .

Changes in design have greatly increased the harmonic content of present day loads. In fact, there are very few loads today that do not produce harmonics.

T h e c o m p u t e r m a n u f a c t u r e r s formerly built what was known as a "Linear Power Supply." This device had a heavy 60 Hertz input transformer, and was large and expensive to manufacture. The various computer manufacturers device to eliminate the 60 Transformer, made it lighter and less expensive and added a high frequency output transformer. This, they called a Mode Switching Power Supply. The ~ i a l y and very important disadvantage was that t harmonics produced by the rapid mo s w i t c h i n g p o w e r supp ly were v e r appreciably increased , pr harmonic, which has c a m transformer and neutral problems. ~ ~ ~ ~ r t s a r e now u n d e r way by compute r manufacturers to reduce this harmonic content in the future generations of computers but a large number of computers have been built in the meantime with these higher harmonic currents.

The new generation of Solid State Ballasts for lighting applications have been described as being greatly improved with appreciable increase in efficiency. slight drawback, an increase in harmonic content. The almost negligible arm^^^^ content of incandescent lighting has been largely supplanted by the improvement in the better and more economical lighting of

0-7803-5148-6-5196 $4.00 0 9996 IEEE 204

arc also des

They give a procedure for derating a ~ ~ ~ ~ ~ ~ ~ r ~ e ~ exposed to non-linear loads. However. when you derate for use with a

you are violating the old os& listing and in doing so,

y s a also violate the National Electric Code, which ~ e q - e a i ~ s Ui listed devices to be used

Mlben transformers are derated, the namep'aale 1s usualEy not changed and the ncxi pcrson coming along does not know that the t r ans fo rmer was derated due to

e is likely to consider that the is l ightly loaded and will

increase the load up towards the KVA on the n ameplare. 'The derated transformer still has

core loss of the original VA rating and it is a core loss,

necessary, for the reduced e core loss is on continuously, it ecessary expense. .

Another important fact is that a properly designed K rated transformer is

wed at a lower flux density than a non K transfDrmer, due to the fact that the

~ ~ ~ r ~ ~ ~ n ~ ~ in the secondary winding causes a 3rd harmonic current to circulate in

Ita winding, increasing the the core. Unless additional d to the core, the core loss e to the third harmonic flux he delta primary winding. .

As a consequence, there is the possibility er line voltage which occurs at drive the core into saturation, a esirable effect. The additional

col-e material in a K rated transformer will make the transformer quieter. A good grade

t ransformer has an that is grounded between secondary windings that barrier between 480 and

ion, the shield gives some c ~ m r n o n and t r ansve r se m o d e no i se a t t e n u a t i o n , a l t h o u g h t h e r e a r e n o

IEEE standards to my knowledge, for ring the degree of high frequency

second problem with C57-110, and e for revision, is the example on

which should include a third 2s i t is one of the naost frequently

withdin their l:sting.

6, is also desirable.

~",ojse attenuation.

encountered and most troublesome. of the h a r m o n i c s .

A t ransformer has a core loss (consisting of an eddy current and hysterisis loss in the iron) , an 1 squared R loss in the windings and an eddy current and stray loss in the winding conductors. A DC current flows uniformly through the entire cross section of a conductor. As the f r e ~ u ~ n c y increases, due to skin effect, the eddy current component increases and it is very difficult to separate the eddy from the stray in the conductor. The M factor of the load, determined from the C57.110 calculation, multiplies the eddy current loss of the windings. When it is used to multiply the combined eddy and stray loss, since they are difficult to separate, you have increase factor of safety of the application.

Eddy current losses in transformers can be reduced by using multiple insulated conductors rather than a single larger conductor. Strip windings often are used. The purpose is to get as much skin as poss ib l e . Q the r t echn iques include interleaving windings and transposing conduc to r s .

The windings of all transformers have resistances, inductances and capacitances. While the voltage received from the utility has less than 1% voltage distortion, the output voltage of the transformer becomes increasingly distorted especially as non- linear currents flow through the windings to the load. The greater the impedance, the greater t he voltage distortion. Voltage distortion, measured with a non-linear load bank, was found to be appreciably dependent o n t h e t r ans fo rmer design and the transformer impedance.

T h e transformer impedances, thru which all currents f low, will cause secondary voltage distortion. This is especially true, if the currents are non- s inusoida l .

As mentioned earlier, a distorted voltage to an induction motor will cause non- l inear currents to flow in the motor windings , adding to the motor's heating .

otors fed by a balanced sinusoidal voltage drawn a reasonably sinusoidal current.

It is desirable to keep the impedance of the transformer, in my opinion, as low as possible, but still large enough that the

primary protection will take the unit off the line in the event of a fault. Coordination must be maintained with the protection devices. The lower the impedance, the higher the short circuit current and vice versa. It is good practice to specify the minimum impedance the application can s tand .

Since on a new design, the load profile may not be known, it is my recommendation that K-13, UL1561 listed transformers be u s e d o n L o w V o l t a g e t r a n s f o r m e r applications and K-4 on UL 1562 listed Medium Voltage Units. If on Low Voltage Transformers, the cost of down time is very high or dependability very important, than K-30 would be my recommendation for low voltage and K-13 for medium vol tage t r a n s f o r m e r s .

Medium Voltage Transformers have a greater load diversity and that is the reason for the K-4 recommendation.

In addition to Harmonics there are several other factors to be taken into c o n s i d e r a t i o n o n M e d i u m V o l t a g e T r a n s f o r m e r s .

It used to be that only lightning was the primary cause of transformer failures. Today, there are devices that replace the load break switches previously used, and these devices are known as vacuum switches and even more t roublesome are vacuum contactors, which operate more frequently.

When a load break switch was opened the L Di/Dt in the transformer winding was partially dissipated in the resulting arc in the switch.

The vacuum breaker and/or contactor opens the circuit in a few microseconds and the L Di/Dt does not have the opportunity to dissipate , As a result there is a transient rise in voltage in the t ransformer primary winding that can cause a dielectric failure unless the winding is properly designed to withstand it.

UL1562 requires a Transient Voltage Analysis Test to determine the maximum voltage rise when a transient hits the primary winding.

Dry Type Transformers have a BIL of 60 KV at 15000 volts per IEEE and ANSI Standards. Liquid transformers have a 95 KV Basic Impulse Rating. Since Oil and Dry type Transformers are connected to the same

utility system , there is no reason why Dry Type Transformers should not be specified to have a 95 KV Basic Insulation Level, the same as the Oil Filled units. They should also have, in addition, solid state arrestors mounted at the terminals of the transformer. If the arrestors are more than 5 feet from the terminals, the arrestor does very little good as far as helping to protect the transformer's windings from the transients caused by l ightning, your vacuum devices or your neighbors vacuum devices. As pointed out in one of the papers in the Bibliography, transformer designers must also take into consideration the rise in the primary winding voltage for a few micro-seconds due to the distributed reactance and capacitance of the winding. The iimpressed voltage upon energization can double in magnitude for a few micro-seconds although it is usually in the order of 30 to 35 % increase in the primary applied voltage. On 5 KV units specify 60 BIL the sarne as for liquid filled u n i t s .

In summary, dry type and liquid filled transformers can be designed to give many years of satisfactory operation even when exposed to the ever increasing non-linear load environment. They can have all of the desirable features, like copper or aluminum windings, lower than normal full load temperature rises and indoor or outdoor enclosures . As more instal la t ions are completed and harmonic measurements taken, we will be in a better position to specify appropriate K factor transformers for non-linear load applications.

R e f e r e n c e s

Unde rs tan d i n g Trans f ID rm e r by Jerome M. Frank, Electrical Construction and Maintenance, April, 1970.

In su 1 at i on 'I

"Specifying Transformer Insulation" by Jerome M. Frank, Midwest Electrical News, July, 1970.

"Understanding Electrical Insulation Life" by Jerome M. Frank, Machine Des ign October, 1970.

207

"Increasing Insulation Life 'I by Jerome M. Frank, Electrical Consultant, March, 1971.

"Watch Out f o r Ene rgy L o s s e s in Transformers" by Jerome M. Frank Electrical Construct ion and Maintenance August 1975

"Transients and Harmonics" by C.R. Luebke and Jerry Frank Square D company November 1985

"Transients and Harmonics" by Jerome M. Frank and C.R.Luebke IEEE CN2272-3 1986

"Transformer Listings" by Jerome M. Frank Electrical Construct ion and Maintenance January 1988

"Maximum Insu la t ion S t r e s s e s Under Transient Voltages in HV Barre l -Type Wind ing of D i s t r ibu t ion and P o w e r T r a n s f o rm e rs " by Alexander Mazur, Isodoro Kerszenbaum and Jerome Frank IEEE Transactions IAS May/June 1988 IEEE Prize Paper

" S p e c i f y i n g D r y D i s t r i b u t i on Transformers for Solid State Applications" by I. Kerszenbaum, A. Mazur, M. Mistry and J. Frank IEEE Prize Paper

T y p e

"Nonlinear Loads and Transformers ... there is another solution by Jerome M. Frank Electrical Construct ion and Maintenance May 1990

I' H arm on i c T r an s f o rm e r Loading" by Joseph F. McPartland, Warren Lewis and Jerome Frank Electrical Design and Installation NOV/DEC 1990

C u r r en t s an d

"New Transformer Technology" by Jerome F r a n k Winter issue NETA World 1992-1993

Methods to Determine K Factor" by Jerome M. F r a n k IEEE PSE 04-346 October 1994

Underwriters Laboratory UL 1561

Underwriters Laboratory UL 1562 "Transformers, Distribution, Dry Type- Over 600 Volts December 1988

ANSI/IEEE C57-110 1986 Recommended P rac t i ce f o r Es t ab l i sh ing Transformer Capability when Supplying Non-Sinusoidal Load Currents

"The How and Why of K-Factor Transformers by Jerry Frank Intertec Publishing April 1993