Circulating currents in banks of transformers with star-delta connection

DiegoZ

Member
Location
honduras
Firts of all i want to apologize for the gramatical errors, i'm not a native english speaker, but i really need help with this topic.
I'm an intern in the electrical company of my country, at the present time the company is focusing its efforts to reduce tha technical and non technical loses in the distribution system, i was assigned the task of look into the behavior of star-delta connection transformers banks of high consumers, my job is basically use a amperimetric clamp and measure the currents of MV side, LV side and circulating currents, also power factor, impedancy of the bank and position of the tap's selector. The electrical system in my country has been left behind for decades making it a non-regulated system until now, i often find customers with circulating currents in the LV side, 8 times higher than the most high line current, note that almost all clients have unbalanced loads and power factor up to 0.9, what makes this to happen? if you see how much current enters in the primary and how much is out trought the secondary you can see heavy losses, what makes de banks to have this kind of behavior? the currents between phases shouldn't be line currents/√3 ? an unbalanced bank in delta will have phase currents higher than tha line currents in an effort to "balance" the loads? I know that for a real analysis we need the angles of the currents, but with the information avaliable, what conclusions can we obtain?

This is a short list of the costumers that I've inspected, the blue was de only one with a "theoretical" behavior, the others seems to have a erratical demeanor.

ClientPrimary VoltageSecundary VoltageTotal KVA of the bankImpedancy's %Primary Current APrimary Current BPrimary Current CSecundary Current ASecundary Current BSecundary Current CNeutral CurrentAB Current BC Current CA Current
FARAJ JESUS13,8120/2402252,320,411,762,128,147,215,928,85861,3
CANAHUATI MOURRA ISSA ABDALA13,82402252,390,24,15,594,918574,827,9104,3153,289,1
MINISTERIO EDUC CTHA13,82402251,71,10,11,158,965,28640,92,838,7
ZIPPERS DE HONDURAS S A13,8120/2402251,81,72,9211,215,90,310,784,9105,389,7
COMERCIAL MADERERA NORIEGA13,8240/4805012,82,75,85,448,77667,70,951,494,854
AGUA DE CHOLOMA13,8240/4803001,848,211,83,72052112087,3149,9181,369
CORDOBA RODOLFO13,8240/480225NV7,57,37,222,110,115,834,3230215225
EXA S A34,524055NV0,30,50,3669127,823,663,144,336,1
INVERSIONES MONSOL34,5240225NV1,250,30,624,220,6222,786,188,799,4
BABUN SIKAFFY ROBERTO RAMIRO34,5240250NV1,41,70,176,873,763,192,36574,1
PRODUCTOS MINERALES VENTURA34,52403002,30,512,61,6241233140,11123175,473
COMPAÑÍA AZUCARERA CHUMBAGUA34,54805013,11,42,41,43,30,21,6***53,650,955,2
COOPROLAVE34,52402252,180,30,40,319,62916,93,321,328,934,9
MOLINA ROSALES JOSE ANTONIO34,52403002,28 y2,233,43,93456436422***252286253
FUNDACION ESCUELA MHOTIVO13,8120/24050122,25,10,820823390,37,894,69,396,1
CHAVEZ M JUAN CARLOS13,8240/4805012,22,54,21,4131,8134,8128,33,325,158,371,6
BENEFICIO MENDEZ SA13,8120/2403001,790,20,10,17,87,67,80,32,55,32,4
INAMI13,8120/24122520,10,10,14,32,80,11,34,30,20,1


Captura de pantalla (986).jpg
 

ron

Senior Member
Diego,

Lots of theory you will need to review with your senior supervising engineers.

I'm not sure what circulating currents you are concerned about.

To determine losses, calculate the kVA on the primary and then kVA on the secondary and compare them (divide one by the other). For older transformers, 97% efficiency is pretty normal, especially if there are harmonics it will be likely lower.
 

DiegoZ

Member
Location
honduras
Star delta seems like an odd configuration. What are their applications and why are they used?

(I assume this is on topic enough)
There are many uses for star delta configuration in my country, like water pumps, fans for poultry farms , motors for coffee factories etc.
 

DiegoZ

Member
Location
honduras
Diego,

Lots of theory you will need to review with your senior supervising engineers.

I'm not sure what circulating currents you are concerned about.

To determine losses, calculate the kVA on the primary and then kVA on the secondary and compare them (divide one by the other). For older transformers, 97% efficiency is pretty normal, especially if there are harmonics it will be likely lower.

DSCN9529.jpg DSCN9530.jpg DSCN9519.jpg DSCN9520.jpg here you can see a good example of my problem, this is the "Cordoba Rodolfo" case, here we have a 3*75kva star-delta bank with a transformation relation of 13.8kV to 480/240 v , this bank is used to feed a poultry farm, if you see the images 3 and 4 you can see the currents in one secondary phase is just 22.1 A, meanwhile in the "bridges" that connects the invidivual transformers into a delta connection, (images 1 and 2) the currents are up to 215 A , it is important to remark that the other currents in rest of the bridges are similar (more or less 200A) and in the secondary you only have currents about 20A, i dont know the angles of the currents nor the voltajes, this is why i can calculate the real efficiency, if a use only apparent power the effiency of this case in specific is only 7.22% , and this is not the only case where the efficiency is below than 50%.
 

DiegoZ

Member
Location
honduras
Diego,

Lots of theory you will need to review with your senior supervising engineers.

I'm not sure what circulating currents you are concerned about.

To determine losses, calculate the kVA on the primary and then kVA on the secondary and compare them (divide one by the other). For older transformers, 97% efficiency is pretty normal, especially if there are harmonics it will be likely lower.
Diego...

Your presentation lacks consistency! Can you provide parameters obtained from Open-Circuit and Closed-Circuit tests?

Regards, Phil Corso
Sadly we cant cut the service of the clients, and my supervisor told me to measure the currents in normal operation conditions for every client, so i only have the information on the transformer data plate, the currents that i measured, and the voltages of the distribution net.
 
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