Parallel transformers benefits

August 3rd, 2009 | Posted in Electrical distribution
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I wish to present here the benefits of utilization and operation of MV/LV in . Having such experience, i will be interested to get feedbacks from other engineers.

2 MV/LV parallel transformers distribution network

2 MV/LV parallel transformers distribution network

For all my experience, I’ve never met a working electrical installation, where operation of parallel transformers was used. Once, we used this solution in the design of welding shop, mainly to reduce the in the network and maintain a stable level of short-circuit current. But the project was stopped by the Employer, and the idea was gone. Below is the list of benefits of such solution, as it is seen. It would be great if interested engineers commented and shared experiences on this issue.

Generally, application of parallel transformers allows achieving the following benefits:

1) Reducing the total capacity of (as compared to separate their work). The decrease of total installed capacity is reached:

  • by lowering the overall  demand load to the diversity of loads connected to different transformers
  • by using a higher load rate of parallel transformers
  • less required backup in case of electrical transformer failure

2) Reduction of electricity losses in electrical transformers due to a possible disconnection of unloaded transformers

3) Improving the due to the stable level of short circuit current throughout the network

4) Increasing the reliability of operation of protective devices in the case of phase-to-earth short circuits in the network.

5) Possibility of placing electrical transformers in operation phase-by-phase

Parallel transformers are allowed, provided that none of the windings will be loaded by current exceeding allowable current for that winding.

Of course, there are limitations for using electrical transformers connected in parallel. For instance, Russian Federation “Rules for technical maintenance of electrical consumers” allows operation of parallel transformers under the following conditions:

  • windings connections of transformers have the same vector groups
  • the ratio of transformers capacities of less than 1.3
  • rates of transformation differ by no more than +/-0.5%
  • short circuit voltages do not vary more than +/-10%
  • transformers have same phase polarity

Artem Kropachev

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Comment(s) to “Parallel transformers benefits”

  1. This post is very useful. Parallel Distribution network information with diagrammatic representation is very helpful and nice.

  2. Reena Vishwakarma says:

    thanks for providing such information. it helps us to clear our queries.

  3. r says:

    This post is very useful.

  4. Abraham says:

    Good,Helpfull

  5. Parallel transformer is quite interesting. It will help to decrease transformer faults rate.

  6. M.Chandra Obul Reddy says:

    For parallel operation of the Transformers, they should have same short circuit current level.

  7. aubry pascal says:

    And what do you do with your short circuit current increase in your installations !!!

  8. syamdas says:

    Yes it’s a good Idea for the System reliability. I have seen it in refinaries and GasPlants (K.N.P.C-Kuwait) But only the problem is the short cirtcuit level will be higher and you have to go for a high sizing of Bus Bars and Switchgears ( economically burden is there).

  9. Leslie Eckard says:

    I have come across huge problems in the past with this arrangement.
    Whilst parallel transformers increase redundancy the prospective fault level doubles.
    Also if there is a fault on one incomer where the TFers are utilized more than 50% each the overall system will be lost due to overload. Capital cost / expenditure for clients is also increased.
    Would not recommend it.

  10. Rommert Boonstra says:

    Using two (or more) transformers in parallel might cause problems. The voltage level of the transformers need to be exactly the same, from no load to full load. If not, then there will be blind current running from one transformer to the other. Running with generators is about the same but then you can change the voltage charcteristics (voltage droop). With transformers you do not have any possibilities to change the voltage charcteristics.

    I would not recommend it.

  11. Andrea Quaranta says:

    In my field of application I do not apply such configuration. However some time I found it already implemented in some installation without the proper protection arrangment. It is extremely important to implement the directional overcurrent protections otherwise the configuration is useless and dangerous. Other important points are the short circuit level that shall be consdered.

  12. Ed Dhe says:

    Speaking on my behalf this type of distribution is quite common.
    Not just in refineries but also steel mills, glass production, pharmaceutical, ciment factories and even in some cases supplying tertiary installations.
    The comments made earlier regarding this type of distribution reflect the aspects with which this type of distribution has been evaluated.
    Cost, efficiency, redundancy, reliability, network quality, loss of production etc.

    There are cases where this type of distribution is obligatory by law (Hospitals).

    The “rules” mentioned for the use of parallel transformers by the author are also used in Belgium although they are not officially included in the electrical regulations, they are considered however to be good practise.
    So proper -technically correct- design is mostly dependant on the way the design engineers conceive the whole distribution.

    Just as in any case if something is not done properly then it should not be expected to perform well.
    So yes, when dimensioning such a network all the afore mentioned factors (short-circuits levels, busbars cross-sections) should be taken into account to correct implement this type of distribution.

  13. Ahmad Abu Kharma-Jordan says:

    what about the circulating current between the two transformers in case of both are loaded or in case of now load moments ,we have noticed in one of the cases where we have energized two transformers in parallel that the measuring instruments and specially the KWHM was recording some values in one of the transformers although there was no load connected to the parallel busbar , does this mean we have losses due to this parallel operation or due to this circulating current between the two transformers specially when we are sure that there are no 100% matching impedance’s between the two transformers , this case happened when each Transformer was provided with its separate KWHM either on the LV or the MV side, do you have suggestions on how to overcome this problem

    • Mathieu Guillot says:

      The main parameter regarding circulating current is the transformers voltage ratio. They shall be equal as far as possible. For transformer with off load tap changer it’s important to check that the two transformers taps are in the same position.

    • Muhammad Ayaz says:

      Parallel operation of two or more than two transformers have the following conditions to operate and function effeciently.
      1)Primary and secondary Voltages are the same.
      2)Percentage impedance is the same otherwise circulating current causing losses and heating up of the Transformers.
      Rating in MVA/KVA is equal for loading sharing equally.
      3) TAPE POSITION particularly in”ON-Load Tape Changer Transformers” is the same otherwise one of them will be carrying high current and circulation of current cannot be avoided.

  14. Steve Alty says:

    I would recommend a bus section normally open switch connected as a 2 out of 3 autoselective scheme. Generally this is the approach in oil and gas electrical design. Switchgear is designed to include the high short circuit not for the two transformers operating in parrallel but for the contibution to the fault from the motors. In oil and gas installations there are generally duty and standby motors and with all A motors connected to the A bus and B connected to the B bus, the loss of any one of the 2 transformers would not effect items connected to the busbars; ie 1 transformer is sized for the total load. Note in normal operation there are 2 transformers carrying each 50% of the load. This is considered to be a very reliable solution and does not incur the increase of fault levels and circulating currents when parallelling transformers. To reduce the size of transformers forced cooling could be employed as one transformer carrying 100% of the load would not be a normal running condition; although the cost of forced cooling fans may be a similar cost of the difference in the kVA rating!
    As commented by other posts paralleling of transformers results in higher fault levels, circulating currents and if sized as 2×50% machines, possible overload of one transformer should one transformer be out of commission.
    Utility networks companies do not generally parallel transformers in the distribution of electrical energy; providing resillience of supply by alternative switching. I personally would not parallel transformers I think any benefits are outweighed by the disadvantages. Other apparent advantages mentioned in the text can be achieved in other ways.

    Hope this helps

    Kindest regards

    Stephen Alty

    • Raed Odeh says:

      In case of parralleling transfomers , the switchborad prospective fault rating will have to at least equal to the fault 2 * transfomer fault current. Even when the load is shed, when a fault occur at the main bus it will replicate fault situation on both transfomers. the fault current will realy on the short circuit volatge of each transformer minus the volatge drop at any impedance between the tranformers and the fault position. Hence it will be mandatory to sizs the switcborad ( combining both tranfomers secondaries) to 2 the fault capacity of s ingle transfomer.

      • Karen says:

        Which means the fault level would be twice as high, which would mean extra costs in buying switch gear to withstand a higher short circuit fault. All trip settings would also be less sensitive when it comes to grading of the complete network.

        The standard practice to do “paralleling transformers” is only worth while if you have a single load that runs on the limit of one transformer.

    • ahsen says:

      any 1 plz tell me wat is reverse circulating current? plz reply on email ahsen.arif@yahoo.com

      i shall be very thankful

  15. Very useful information – thanks – for my teaching – Electrical Power. Some books even don’t go into too much detail on paralleling T/Xs, only the usual sam clock number, same rating, etc. Thanks!

  16. Raed Odeh says:

    Very useful information.
    Correct me if Im wrong, but will the use of 2 transfomers in parallel result in higher recative losses as opposed to using a single transformer with combined KVA capacity. This may be significant when the transfomers are not fully loaded.

  17. According to statiistics, winding faults come to 50% and tap-changer faults come
    to 17% in power transformers. If one runs n power transformers of identical
    design in parallel, will there be n tap-changers? This could be reduced using
    magnetic shunts which give a much smoother regulation but this is not done
    in power transformers. I have no working experience with power transformers.
    So I don’t understand why these are not replaced by toroidal power transformers
    where magnetic shunts could be readily implemented. Could some experts
    comment on this?

  18. D Sunil says:

    Parallel operation of transformers results in increase of fault level on the downstreamside of transformers and resultant increase in cost of downstream side ciircuit breakers, panels etc may nullify the benefits of parallel operation. Perhaps this might be the reason why it is not practised widely.

    • Bhavani Prasad says:

      This is a very important consideration which affects the installation cost. It does not affect the MVA rating of the incoming breaker from the transformer; But the outgoing breakers from the bus with more than one transformer feeding it will have to have a MVA rating corresponding to the feed from all transformers in parallel. If outgoing are many, it will blow up the cost. Designers prefer to put normal operation scheme without paralleling and provide buscouplers to have flexibility and transfer of load without break and also to obtainin or realize most of the advantages of two transformers in paralled

  19. I have a friend who has been a very experienced designer of power transformers up to 1000 MVA. When I asked him why the core must be of laminated designs
    and not toroidal designs, his reply was that one cannot build large toroidal core
    transformers greater than 3KVA. I thought this may be a mindset since
    conventional designs had been around for more than a centuary. I believe that
    far more benefits could be accrued from large toroidal power transformers to
    the utilities industries. Toroidal transformers could be assembled by full
    automation … one cannot do that with laminated power transformers. Coreloss
    is far less in toroidal transformers. Do you realise that you household consumers of electricity are paying for the coreloss which you have no control over?

  20. Christian Collombet says:

    As far as I’m concerned, I have experience of parallel transformer in two types of installation.

    ***First in installation where high short circuit power is required for the well operation of the process. This is the case for example for welding process where the quality of the arc welding is directly related to the limitation of the voltage drop in the upstream installation. In this type of installation the two solutions to reach a high short circuit power are:
    - use of single low ukr transformer,
    - use of parallel transformers.
    In that case parallel transformers allow the use of standard transformer, and this point is interesting in terms of failure, it’s easier to replace a standard transformer.
    It has to be noted that sometimes:
    - the transformers are not coupled directly but via a busbar trunking system that can reach 100 meters (the busbar trunking system is used to distribute the energy on large areas)
    - up to 4 transformer can be coupled to reach the required power and short circuit power.
    The typical applications of these solutions are automotive industry or similar industry using welding process.

    ***Secondly, parallel transformers can also been used for standard application but with the following requirements:
    - high power to deliver, but necessity to use small transformer due to limitation of the access to the technical room,
    - redundant solution to limit the impact on the application in case of a transformer failure.
    The type of application in that case can be hospitals, big tertiary buildings, hypermarket in certain countries.

    also, you can read my own article in this Blog about parallel-transformer

  21. this great opportunity for the youth

  22. With the unusual bad weather in Europe, heavy snow and flooding, and failed undersea rail lines
    between UK and Europe, I guess that there must be a lot of breakdown of power transformers.
    I heard that Insurers would have to pay quite a sum for the downtime in electrical supply.
    This is also happening in Asia and South America. I feel that transformer technology must be
    upgraded. I agree that there is great opportunity for the youth who wants “change”. This is
    technological change.

  23. ededeh mgbowa marshall says:

    good topic.finding solutions to the disadvantages will be interesting.

  24. I have a friend who acts as transformer consultant to insurers. He is kept so
    busy with investigating failed transformers especially in manufacturing plants
    which have been having a good run for tens of years but now suddenly one
    of the paralleled transformer failed. Imagine his surprise that even the original
    designers and manufacturers were no more found on planet Earth. Repairing
    means transporting from a remote factory to a nearby repair centre which will
    take up to 6 months. Meantime one line is idle and the workers … I don’t know
    what had happened. .. probably retrenched for the time being. If I am running
    such a plant I make sure that I have four transformers instead of two but that
    needs deep pocket. Insurance in this business must be very expensive.

  25. nagaraj says:

    ok….its good but more expenditure is required……….!

  26. james ireland says:

    i am only a student at the moment but i thought paralleling tranformers was only used for redundancy. as in that a large factory that cant afford a large amount of down time would have a medium voltage ring and if one transformer went down the other would be switched in.

    • Bhavani Prasad says:

      …if one tr went down, the other would be switched in.” —– This is not transformers in parallel operation. This is a case of a standby being brought on line following a failure.

    • Karen says:

      It would be far better and cheaper to use ring feeds to cut the amount of down time.

      “Paralleling transformers” would mean more down time if a fault do happen on a “paralleling transformers” arrangement so it is not a advantage but a disadvantage if you look carefully at it.

  27. Eric whadje says:

    am a student and i need a help to become electrical engineer so i can create a factor or contribute with my talent efficient run in electrical programs.Am pleading in the of God,if there is God sent one out there who can help me get to my destiny.Thanks

  28. I probably had not been explicit when citing that example of a factory
    going down on one production line because one paralleled transformer failed.
    That factory did have a spare transformer but of the wrong primary
    configuation. Furthermore, the control circuit was burnt out also leaving
    no option whatever on-site. Maybe a well managed factory would have
    seen to that but generally maintenance crews are not that well educated and
    experienced. I do think young bright electrical engineers should look into
    revolutionising the designs of power transformers. A hundred years of
    traditional power transformers design and manufactduring need some new thinking. Power transformers should be easily transportable and assembled
    with flexibility on sites. There must be a new way of designing such
    transformers but I cannot leak out due to patent infringement. In any case
    one would have to fight against mindsets in professions.

  29. javeedakhtar says:

    Very good and helpful information

  30. S.Khalid.Ali says:

    I work for Caterpillar Co; we usually connect our all generators in parallel those they output LV and then the Genset LV out put as each generator has its own MV transformer rating 13.8KV, then all transformers are connected to one buss-bar (we call it grid) through fs switch gear, mean each circuit has its own protection that is connected to grid. Furthermore, the advantage of raising the voltage is great as that decrease the diameter of supply conductor plus reduction of LV switch gears, as an example, we supply 10M Watts power at the distance of 200 meters through 4 500 mm2 cable that is almost 75% less copper conductor is used. Nevertheless, all that configuration required perfect design by professionals. As an engineer, that is one part of my activity as well. CAT trained their engineer and they have proper tool to train. I shall welcome any question if some body wants to explore it.

    • Murugan says:

      sir,
      i am an electrical engineer , work for saudi electricity company, i read your comment against parallel transformer benifits, it was very usefull to me , let me know the scematic diagram of the parallel connection of the transformer, then what is fs switchgear, i will be thankfull to you if you will reply to me

      • sou engenheiro eletrico em Brasilia Brasil e dono de uma firma do ramo.
        eu trabalho com baixa tensão e DATA CENTER e instalo muitos UPS em paralelo redundante com transformadores ligados em paralelo na saída dos UPS.
        se os transformadores não forem iguais como potencia, tensão, impedancia, variação de tensão e defasagem angular, esqueça.
        é problema na certa e cabeça a premio.
        se o casamento dos transformadores é perfeito a segurança do sistema é perfeita e a manutenção muito tranquila.

    • gerard ford says:

      would you know if it is ok for circuits from one transformer share a neutral from another if both neutrals are connected, will it hurt the transformers, to bond the neutrals?

  31. Murugan says:

    hi,
    this is murugan , iam an elctrical enginner , i have been working as a project enginneer in transmission and distribution field , you gave some good needs, i want to know the parallel conection of transformer with single circuit line . i will be thankfull to you if you will give information about the above.

  32. I am ending my participation on this blog. This is because
    I shall be shortly going for major surgery in a local hospital.
    Not being an expert, my remarks are basically armaturish.
    Best regards to all of you. Please do not contact me by
    email. Thanks.

  33. Bhavani Prasad says:

    Operating identical (or nearly identical or equivalent) transformers does not pose problems. That is the transformers should have the same (1)voltage ratio, (2)same percentage impedance, so that load sharing is even at different loads, (3) protection to avoid flow of current from one to the other either as a fault current or a circulating current due to mismatch of internal voltage (terminal voltage will have to be the same as they are connected). Protection scheme will have to have reverse power relay.

    Generally designers prefer to avoid parallel operation as operating personnel and maintenance personnel can create problems itf not adequately equiped. If parallel operation is provided to realize some of the advantages, then specific operating & maintenance instructions should also be dispalyed.

    Transformer losses partly come down with parallel operation (Iron loss is not affected; But copper loss in comes down as it is proportional to the square of the current.)

    • Bhavani Prasad says:

      In addition to the three points mentioned, there are two more obvious points – (1) same phase sequence and (2) same vector group.

  34. i always prefer to use brushless cooling fans because they last longer and needs less maintennance::-

  35. zaali100 says:

    Hi, what is the advantages of using two 500 kva transformers over one 1000 KVA. thanks

    • Gabby says:

      This depends merely on your requirement. At first, it is not ecenomical to employ two-500kva transformers as compared to a single 1000kva unit. In the economic point of view, it is not advisable as you will be doubling the cost for: switchgears, controls, maintenance.However, if money is not a problem, two 500kva transformer will be giving you more advantages like: flexibility in terms of more maintenance door, load shifting and would avoid total equipment breakdown as it is very rare to experience both transformers failed at the same time.

  36. Mathieu GUILLOT says:

    Hello,

    There are no big differencies from electrical standpoint for supplied loads.

    Total Inrush current from MV side will be higher for 2 x500kVA, but with a lower time constant. No load losses could be higher too.

    Benefit of such solution are:
    - smaller / lighter transformer for electrical room with handling issue

    - Possibility to partially supply the site in case of one transformer failure (after load shedding). In that case the operation of overcurrent protection with half the short-circuit current shall be checked.

    - Some customer operating many MV/LV transformers (such as airport, underground transportation, but also hotel chain, retail chain…) have some benefit to use only one rated power of tranformer to limit spare parts and standardize installation. They adjust the number of transformers according to power budget of each site.

    You have to remember that transformers don’t fail often but when they do the down time could be long because of delivery term, handling issues… So the possibility of power transformer replacement shall be checked for critical electrical distribution.

    Regards

  37. Delmain says:

    What procedure would one follow when connecting two transformers(Two transformers which meets all the requierments) in parallel keeping all safety aspects in mind?

    • Gabby says:

      The manufacturer will tell you the proper procedure to connect and install the transformers. Unless you are thinking of a special application that requires extensive planning and preparation.

  38. Ken Pitt says:

    While this setup does have it’s advantages like the use of parallell transformers to improve redundancy, it does raise some concerns as well. How about increasing of the fault level? OR Overload issues? While precautions can be taken to minimize these risks, the greatest downfall at least in my opinion is the cost/value ratio. The increase in capital expendature to clients in my opinion does not justify the proposed benefits. Just my 2 cents.

  39. eshwar says:

    Simple,economical,practical,cost effective solution-at the stage of design paralleling is possible.

    For addition and alteration purpose -paralleling is too difficult and will not be cost effective.

    So try in designing stage not in operation.

  40. Gabby says:

    For me, paralleling has more disadvantages than advantages like:
    1. higher first cost at it will need higher primary disconnect, redundant transformer, extra switchgear and controls.
    2. when one of the transformer fails, there is a tendency to overload the surviving unit unless the connected load is limited to below 50% of the designed capacity.
    3. when on of the transformer fails, it will take time to order and procure for a replacement as the replacement unit must be customized according to the characteristics and specifications of the existing unit.
    4. it will also need a bigger space to accommodate two transformers.

  41. Towana says:

    But the short circuit will be increase, and in case they have OLTC they should have master and follow.

  42. Dear Sir,

    I studythe above drawing. My questionis that
    1. Tranformer take 1100 volts and out put 410 voltss and other transformer also same.
    2. If one of them transformer’s effecincy may be down or there will be any broblem or any varriation in any transformer, (Spose transformer in put 11000 volts and out put 410 volts and other transformer in put 11000 volts and out put 400 volts) and both transformer ‘s togher on one bus bar, what will be happen. I think this will creat problem.

    Please send me its solution.

    Thanks

    Zahid Mahmoo
    zahidm4@yahoo.com

    • Gabby says:

      The requirements for paralleling are: transformers should have the same voltage, same frequency , same phase sequence and also they should have the same size. If there is voltage difference between the two transformers, arcing would take place and could damage the transformers depending on the extent of voltage difference.

      • Sinisa Spremic says:

        If two transformers are on the same high voltage busbar and on the same low voltage busbar, difference in transformers ratios will produce circulating currents (there will be no arcing). Transformer with lower value of ratio (which means higher voltage on low voltage side of transformer) will increase reactive current (to obtain higher voltage drop in transformer imedance) due to circulating current and transformer with higher value of ratio (which means lower voltage on low voltage side of transformer) will decrease reactive current (to obtain lower voltage drop in transformer impedance) due to circulating current – this is physical explanation of circulating currents and why both transformers have the same voltage at low voltage busbar.
        Any changing in position of transformer will cause arcing, and it is allowed only in transformers with On Load Tap Changers (OLTC), where arcing happens in special chamber.

  43. Towana says:

    In case the secondary voltages are not same (but other requirements for parallel operation); during closing circuit breaks at low voltage may be will cause some sparks inside circuit breaker chamber but for continuance operations will cause circulation of current between two transforms which cause increase or transformer’s temperature.

  44. Nelson says:

    thank for the big benefit for me,, although i am not electrical engineer..

  45. VSAR says:

    Connecting single phase transformers having the same specifications will enable the use of a physically smaller packaged, while increasing the total capacity or amperage rating thru the parallel connections of the low voltage secondary windings. Typical example of such approach are used in low to medium sized spot welding, or electroplating application machines. Where a very low voltage is required, say 50V, that is enough as well as the capacity to carry and provide current range to up several hundred amperes. The cost of manufacture is another advantage achieve through the use of a much smaller gauge or size of magnet wires in the transformer fabrications.

  46. Muhammad Ayaz says:

    Can we run two transformers of different rating(MVA/OR KVA) in parallel for equal load sharing.Voltage ratio,frequency and phase sequence are the same?

    • Sinisa Spremic says:

      You can run two desribed transformers of different nominal power rating S1 and S2 (MVA or kVA) in parallel with same voltage ratio, frequency and phase sequence. Loads of transformers with different nominal power ratings will be equal only if their short circuit voltages are in this relation Uk1/Uk2=S1/S2. Transformer with higher nominal power should have higher value of short circuit voltage. This means that impedances Z1 and Z2 are equal. Impedances can be calculated from Uk (short circuit voltage) as Zk=(Uk/100)*(Un**2/Sn) where Uk (%), Un (kV) and Sn (MVA).
      Transformers used in distribution ordinarily have approximate equal Uk (short circuit voltages) and according to that transformers with higher nominal power has lower value of impedance. It means that in parallel transfomer with higer nominal power will have higer load, and transformer with lower nominal power will have lower load. For example two transformers with equal short circuit voltages with load of 15 MVA – transformer of S1=20 MVA will be loaded with 10 MVA and transformer of S2=10 MVA will have load of 5 MVA.

  47. Amang says:

    Dear Mr. Artem Kropachev,

    could you explain operation procedure for this system..? Is it possible to parallel of those transformer where one of those transformer already loading 80% with the other one with load =0 (Zero), or transformers must be parallel in condition load =0 for both of transformer..? are we need an interlock between MV breaker and LV breaker of those Transformer..?
    Thanks for your respond.

    Best Regards,

    Amangkurat

  48. Sinisa Spremic says:

    Yes, it is possible to put in parallel transformers in case that before paralleling one have 80 % of rated load and other one 0 %. Before paralleling transformers the best is if transformers are at the same positions, but there will be no problem to paralleling them if there is a litlle difference in positions among transformers (one, two and possible morethan that), but in this case you will have circulating currents.
    I did not know what did you mean with your second question about interlocking.

    Best regards,
    Sinisa Spremic

  49. trishla goyal says:

    as m an final yr. B.E. student i have done my recent training from a 220kv g.s.s. where i practically saw this
    benefit of using two transformers in parallel it really increases the efficiency of the system but obviously this system is expensive. we can connect a 80% loaded unit in parallel with a 0% loaded unit. but the conditions of polarities ,voltage ratings i.e. turns ratio,inductance to resistance ratios phase displacement between primary and secondary line voltages and phase sequence in case of three phase transformers should be met.
    and for your second question according to my knowledge a split second bus with a tie breaker interlocked to prevent parallel operation is necessary in this case.

  50. mathan says:

    hi sir .. as am a final year student. i need clear abstrat of partial breakdown of power transformer

  51. Thomas Rooth says:

    Question regarding possible elimination/reduction of the disadvantiges of circulatig currents, load sharing etc.:
    Will the adding of inductors on the secondary side of each transformer help. It’s obvious this cause higher cost but in spite this disadvantage, will it help?

    Kind regards,
    Thomas Rooth

    • Sinisa Spremic says:

      Disadvantage of circulating currents is increasing of losses. Load sharing is connected with different short circuit impedance (mainly reactance because R<<X) characteristics of transformers. In most cases transformer impedance has the biggest value of short circuit impedance at lowest position and the smallest at highest postiton. Characteristic of short circuit impedance is not straight line but curve which is mostly close to straight line. It should be noted that in some OLTC and winding construction there could be large steps between two position while other has smooth step.
      Parallel operation methods for parallel operation of transformers imply circulating currents minimization so circulating currents (which could not be fully removed due to different factors) are at low values and thus circulating currents increase losses at very low level.
      Of course increasing overal inductance (reactance) using inductors (transformer reactance+inductor reactance) will effect in circulating current reduction. Load sharing could be equalized if added inductors equalize overall impedance (reactance) among transformers.
      Inductors built in secondary side of transformers could help to decrease short circuit currents in places where it is necessary.
      My opinion is that there is no need to add inductors on secondary side of transformers regarding to parallel operation of similar transformers (circulating currents and load sharing) because of inductors costs, increasing maintenance cost, decreasing reliability (more devices), need for new parametrisation of protection devices and automatic voltage regulators, etc).

      Best regards, Sinisa Spremic

  52. murali says:

    sir i want to know complete detail with protective equipment and single line diagram of the same for parallel operation of 1mva transformer 33kv/6.6kv with switch gear details and ratings .thanking you please help me

    • Sinisa Spremic says:

      Mr. Murali,
      Switch gear details and ratings depend of some factors. One of them are rules and techinical rules in countries and technical recommendations also. Rules are different in different countries.
      Best regards, Sinisa Spremic!

  53. insan says:

    Is parallel operation of 10MVA and 5MVA possible?

    • Sinisa Spremic says:

      Yes, it is possible. Transformers must have same vector group, similar nominal turn ratio. It is desirable that short circuit voltages are not differ more than 10 %, because difference in short circuit voltages cause difference in percent loading of transformers (transformer with higher short circuit voltage will have lower percent load). If this two transformers have the same short circuit voltages transformer with 10 MVA will have (depends on ratings) impedance 2 Ohms and transformer with 5 MVA will have 4 Ohms, so load of transformer 1 will be 10 MVA while transformer 2 will have load of 5 MVA.
      Best regard, Sinisa Spremic!

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