BS IEC 61378-3-2015 pdf free download – Converter transformers Part 3: Application guide.
Industrial units with tertiary windings loaded with filter and compensation are covered in IEC 61 378-1 :201 1 , 5.7. It should be noted that the connection of the filter to the tertiary winding will lead to a higher than rated voltage. Therefore the fundamental current flowing through the filter impedance will be higher than the rated current as defined in the clause just mentioned. So it is important that the system engineer takes into account this phenomenon when specifying maximum tertiary winding fundamental current.
6.2 Industrial applications
6.2.1 Rectifier In general, converter transformers for industrial application will be designed for a fairly high current on the valve side. The valve windings shall therefore have simple connections such as star or delta. For some industrial applications with high DC currents and low DC voltage, it is often of advantage to use a double-star arrangement instead of bridge configuration.
The line side is either connected in star or delta and the valve side has two star systems with180° phase shift. The valve windings should be physically interleaved to avoid Dcmagnetization of the core and to minimize magnetic leakage fluxes (eddy losses) related toeven current harmonics. An interphase transformer (IPT) between the two star points isneeded together with a 3 limbs core to ensure 120° valves conduction and to minimize effectsof unbalance between the 2 neutral currents of the valve windings. When these 2 neutralcurrents are balanced, a 5 limbs core may be used instead of the lPT.
The transformer for a six-pulse bridge converter is a three phase unit with the valve windingsconnected in star or delta.
Twelve-pulse reactions can be obtained by using two six-pulse units with different connectiongroups or by using a transformer with two valve windings,connected in star and delta.Thislast solution requires a normal three phase core.
More complex cases require a different arrangement with star and delta on the primary sideand valve windings connected in star or delta. These particular arrangements may alsorequire a special core design. For 12 pulse double-star rectifiers, phase displacement of 30needs to be achieved on the line side windings (star – delta, zig-zag, etc.).
Converter plants with more than twelve-pulses require the use of different shifting anglesamong the various units.This is obtained in most cases by using a zigzag or extended deltaconnection or more seldom with polygon connection. Detailed description and examples canbe found in lEC61378-1:2011, Annex l.
The physical arrangement of the transformer is preferably done with the valve winding as theouter winding to cater for the simple arrangement of the valve-side exits. The valve-side exitarrangement depends upon the converter structure.
lf the converter requires the presence of transductors,they are generally located in the sametank as the transformer and in this case, it is necessary to provide two lead exits by phase.Transductors, if used, may be connected according to Figure’6.
The physical layout (both inside and outside the tank) of high current busbars may significantly affect the transformer load losses, short circuit impedance and tank hot spots. IEC 61 378-1 :201 1 , Annex F gives guidance on how to estimate these effects.BS IEC 61378-3 pdf download.