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Transformer Scr Arrangement For Vsd

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Greetings to Everybody!


I have an inquiry about the arrangement of Transformer for our 12 pulse Thyristor-DC Variable Speed Drive for Crane Application.


The Main Power line to our Rail Mounted Crane is from utility supply of 6.6 Kv 50 Hz.Transformer feeding the drive is 600 KVA,6.6 Kv 50Hz Delta Connection Primary, Secondary side got 2 winding arrangement, WYE and DELTA connection at 460 volts each, this transformer is feeding 2 individual DC drive, WYE for Drive A and Delta for drive B.The DC output of this 2 drives is in parallel connection controlling the 370KW DC Hoist motor.


I have a situation here, our crane was relocated wherein there is no utility power available and thereby needs to condemn the existing 600 KVA, 6.6 Kv Transformer and feeds the Crane from diesel generating set 1250 KVA 50Hz,460 Volts.Unfortunately we do not have a 6.6 Kv Generator.We use 1250 KVA capacity to feed other motors and auxiliary power.


The question arise is the original power arrangement of the Drives. Do we need to install a new 1:1 ratio Transformer 600KVA,460 volts with 2 individual-Wye and Delta winding arrangement to supply the drives ? or can we just directly supply both drives from our 460 V, WYE winding generator set???


I will appreciate advice and recommendation which may assist us technically, can anybody recommends a reference about Transformer?... Cheers for this great and valuable site!



Best regards,












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you can feed directly one drive and select a special trafo for only 300 kVA for the second drive.

I could recommend special trafos from www.mirusinternational.com






Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Hello Gerry


Welcome to the forum.

I am currently out of the office, (other side of the world at present) so replying is not always easy.


The current drawn by DC drives, is very distorted and so it has a very low distortion power factor. If two drives are fed from the same supply and operating at around the same time, the harmonics will add up and be bad for the supply.


With DC rectifiers fed from three phase, there are essentially current pulses at the crest of the waveform. The maximum conduction angles of the pulses is 120 degrees for a three phase rectifier supplying a resistive load.

If the rectifier is replaced with a 6 phase rectifier, the harmonic content on the supply, is greatly reduced. To get the six phases, a transformer is used with two secondaries, one star connected and the other delta connected. (There is a sixty degree phase shift between the two connections).


I am not convinced that the same theory will apply when using the two windings to supply to independent three phase rectifiers though. The two loads will not be identical and I doubt that there will be much if any cancellation. The current on the 6 KV side will have twice the number of current pulses and the peak value of the current will be lower as the peaks to not coincide.


For operation off a generator, you do not need the phase shifting transformer, but your generator does need to be oversized to allow for the poor distortion power factor and it does need to be designed to operate with a very distorted current waveform. I would strongly recommend that the AVR is a three phase averaging type and the excitation is independent rather than self excitation.


I expect that the DC drives are four quadrant and will rely on other AC loads of the system when Breaking to absorb the load. May be fun when you have four quadrant breaking and little other load on the generator!!


The addition of some form of line filter from the company quoted by Mario will also be of great benefit.


Have a good day,

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I am very pleased receiving your prompt reply.


Adding a single 300KVA for the remaining drive is an intelligent advice as well, a practical and economical solution.


Let me introduced this old drive model design, Each Drive composed of 6 High powered SCR, it is a Ward Leonard Static Drive Type which replaced the "older" design of Ward Leonard MG set or Motor-DC Generator type of Variable speed drive.The drive is 4 quadrant Mode operation, it acts as Converter during lifting motion and an Inverter during lowering motion.


A remote set of Resistor load banks are electrically interlocked to absorbs excess power during inversion mode to avoid Engine Overspeeding.High Power inductors are also connected in series for each AC Lines for filtering purposes.


Mark,you are absolutely correct in Generator requirement for Crane application, AVR is separately powered from PMG (Permanent Magnet Generator) and not shunt from the Main Generator lines.Generator is sized accordingly for this application to limit the voltage dip due to electrical disturbances caused by Semi conductor's characteristics


Please accept my apology for some ignorant questions in my mind.


What really matters if I feed both drives directly from the Generator?

Do we really need additional transformer? If I ignore the original arrangement of Delta to Drive A and Wye to Drive B instead I feed both drives directly from Generator set of WYE winding connections, what might be the consequences?For sure, I need to prepare my spare fuses before this trial, Generator set with complete protection and a fire extinguisher!...curiosity is not a solution, still needing advices from the expert.


Please bear with me, hope to hear from you all.


Best regards,











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Hi Gerry


I have just reread your first posting and my initial interpretation was of two separate units, but in reality what you were describing is a genuine 12 pulse input drive.

This will certainly give you better performance than 6 pulse.

The only way to get the six phases is to use a transformer. Using a single transformer in series with one rectifier only will not be as effective as a transformer with both rectifiers. To get the best results, you need to have equal impedances in the two supplies. If you use a single delta to star transformer in series with one rectifier and the second direct connected, the direct connected rectifier will draw more current and the phase shift will not be exactly 60 degrees due to leakage reactance etc. This will still be better than direct connection of both rectifiers though.

I suspect that the cost of the transformers may be higher than using a pair of good filters so that may still be a good way to go.


The disadvantage of connecting both inputs from the one supply, is that 1) one bridge may carry more than 50% of the load and 2) the harmonics will be high resulting in a very distorted current waveform and on a generator this can result in a quite distorted voltage waveform. If each rectifier has an AC reactor, this will help to balance the load and reduce the distortion. Adding further filtering in series with each unit willimprove things even further.


There should be no danger in trying the parallel connection to see what the problems are. The biggest issues could be the effect of voltage distortion on other equipment, particularly power factor correction, and the potential for voltage instability due to the effect of the distortion on the AVR system in the generator.


If the two units are connected in parallel, with equal line reactors fitted, I would recommend sizing everything upstream for 1.3 - 1.4 times the total current of the two units. The distorted current waveform will result in increased heating of all supply components and they should be oversized to accommodate this.


Best regards,

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Hi Gerry


Thank you for your kind comments.

There are a lot of good resources around if you look for them, but always bear in mind, these forums are voluntary and are the opinions of the posters only. Sometimes, the information can be erroneous due to misunderstandings, assumptions etc, so you do need to satisfy yourself that the information is valid. - we do try to keep the standard high on this forum, but things can be wrong at times.


Pleased to be of assistance, if we can help others to learn more and gain a better understanding, then perhaps they may do the same for us!!

Feel free to contribute, it is the contributions of others that make this forum of value.


Best regards,

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