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Transformer selection

Transformer selection

Transformer selection

(OP)
We are using diesel-electric trucks in our mine. (open-pit) When the trucks take some inclines, they use overhead lines, or pantograph lines, to supply them with electricity.
It is 1600V DC, and it is supplied through 1440kW 11kv/1200V 3ph transformers, and then rectified with a three phase bridge rectifier to 1600V. (six diodes) Some of the lines is almost a kilometer long, and have up to seven substations.
I have noticed that the transformers is in a certain order: Dd0 - Yd1 - Dd0 - Yd1 etc.

Firstly, is there any reason for this arrangement?
Secondly, does the phase shift (of different transformers) has any affect?

RE: Transformer selection

Check out Thread237-35734.  It's in the electric motors and controls engineering forum on this site and a nearly identical question is asked and answered.

RE: Transformer selection

hi

Actually i am not sure of the answer but would like to add something to u r question,
There are different types of transformer arrangments are used. Let me just brief the abbrevation that is used already and some of which i will be using it.

D = DELTA connection
Y = STAR Connection
P = Primany Winding
S = Secondary Winding
0 = 12o Clock position, means the phase difference between P & S is ZERO
1 = 1o Clock position, means the phase difference between P & S is 30 Degree (leading), because when u look at the Clock it is divided in 12 parts with 30 Degree each.
11 = 1o Clock position, means the phase difference between P & S is 30 Degree (lagging).

for u r case:
Dd0 = P is D connected, S is D connected with ZERO phase difference.
Rest is similar to other's all.

I would like to add one more thing, there are other types also like,
Dyn11 = P is D connected with NEUTRAL and 30 Degree lagging.
Dyn1  = P is D connected with NEUTRAL and 30 Degree leading.

Hope this might help u.
Incase if anybody would like to correct my answers or to add few more points please do add it.

Thanx

RE: Transformer selection

The arrangement described by redox if often used if a large number of rectifiers are used in an application.

The transformers used have different  phase shift. If the load on the Dd0 units and the Yd1 units is equal, in the total primary current of all transformers the typical 6-pulse harmonics (5,7,17,19,..) will cancel out.

If the loads are not equal the six-pulse harmonics will be reduced.

Choosing two different transformer connections is a way to reduce harmonics at nearly no cost, but it is not as favorable as using true twelve-pulse rectifiers.

If the rectifiers operate with thyristors it has to be considered that the phase shift caused by phase control also has an effect on harmonics cancelation.

E.g. one rectifier on a Dd0 transformer operating with 30° control angle and one rectifier with Yd1 transfomer operating at 0°control angle will draw a high amonut of six-pulse harmonics from the line since the six-pulse harmoncis of both rectifiers are in phase and will not cancel.

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