From the Energy Management Discussion Group (http://in.groups.yahoo.com/group/aipnpc/ )

A technical discussion on “Parallel Operation of Transformers”

1.	I need a technical advice from you or any of the experienced members of the group regarding parallel operation of transformers on LT side. I want two nos of 250 KVA transformers of 11 kV / 415 volts to run on parallel. BOTH THE TRANSFORMERS ARE CONNECTED TO A COMMON bUS BAR IN THE lt PANEL THROUGH INDIVIDUAL ACBs and Bus coupler. Kindly advice me the control equipments to be provided in the LT panel and wiring for parallel operation of these two transformers when the load exceeds 250 KVA. i will be much grateful if a suggestion can be given in this regard. with regards Narayanan
2.	Dear all, Very briefly the things to be considered are: 1. Check for short circuit currents during parallel operation to check if the fault level is within the limits of the switchgears selected. 2. Voltages for the side being paralleled should be same. 3. Impedance values should be almost same - it may be difficult to get exactly the same values. 4. Ideally have the same vector group for the transformers. 5. Phase sequence should be same. 6. Protection is another critical issue. It should be ensured that the overload relays are well set so that the full load current is not exceeded for any of the transformers. If one of the paralleled transformer trips (HT or LT side) the other transformer will take load of both the transformers. It will be ideal to have interlocks so that in case of one transformer failure the corresponding (predetermined) loads also get disconnected. Alternatively if one transformer fails ensure that the other also trips to avoid overloading condition. There may be some specific issues which may need consideration as well. Hope this will serve some purpose. Regards Arvind
3.	Dear Mr.Thukral, It is a good explanation. However, I would like to add two more additional points with you. 3) The purpose of impedance level matching is to withstand the circulating current effect under parallel operation. 6) Regarding the protection issue- separate CB should to provided to individual Tf and later connected to common bus bar in order to avoid the overloading of individual TF Kindly please give me feedback whether the two point are correct or not. Thanks & regards, Ananda Krishnan G M
4.	Dear Ananda, These points are valid. Thanks for the explanation. Regards Arvind
5.	Dear Anand / Arvind, I have some further clarifications: Item. # 3 The purpose of insisting equal % impedance value for the transformers for parallel operation is to minimize the circulating current (up to zero if the % impedances are exactly equal), not to withstand the circulating current, there by to achieve equal sharing of the total load by each transformers under parallel operation. Item # 4 If the vector groups of the transformers, (having almost same % impedance) are deferent, it should not be connected in parallel, unless the minimum requirement for parallel operation such as (1) Same voltage Ratio (2) Same phase angle deference between primary and secondary (3) Same Polarity and (4) same phase sequence can be achieved by suitably connecting the transformer terminals for paralleling. Item # 6 Tripping of pre-determined loads or tripping the healthy transformer in service, only because of tripping of one transformer, (without considering the total load) is not advisable. Pre-determined loads need be removed (by tripping) only if the total load exceeds the capacity of healthy transformer in service. This can be achieved by properly set over load relays, and by this tripping of healthy transformer also can be avoided. Please provide your valuables comments on these points. With Regards Johny.P.A
6.	Dear Mr.Pulikottil, Its a good explanation. However, i would like to excuse me for the mistake that you pointed .Actually i tried to mention Circulating current effect but i left the word effect hence, the whole meaning changed.i tried to convey the same as you did. Regarding the Item no 4: i think there is a contradiction in your statement.first you said if vector group is not matching parrelel connection should not be done and later you have explained if it satisfies the minimum requirements of 1)same voltage ratio 2)same phase angle diffrence between primary and secondary 3)same polarity4)phase sequence. If and only if it is of same vector group it satisfies same phase angle concept. I hope I am clear. Please feel free to correct the statement if i made mistake.
7.	Mr.Narayanan- The concept of phase sequence is the important issue. if the two TF are feed from different feeders from the distribution company make sure that under all condition the phase sequence should not be altered. For better aspect always try to get the supply from a single feeder. If you fail to maintain then you will face a serious impact. Thanks & regards, Ananda Krishnan G M
8.	The supply is only from single feeder. Thank you for all valuable advices. with regards narayanan
9.	Dear Ananda, First of all the point # 4 "ideally have the same vector group of the transformers" gives a possibility (not ideal) to connect two transformers having deferent vector groups. It is true, but should follow the requirement of parallel operation of the transformers. All vector groups of the transformers is grouped in 4 groups. Out of these 4 groups transformers grouped in two particular groups can be connected in parallel. For more details please refer to 'J&P Transformer Book". With Regards, Johny.P.A
10.	Dear jhony Thank you for the informations passed on regarding parallel operation of transformers. Narayanan