This document provides an overview of distribution transformers including their design, selection, operation, and efficiency. It discusses the need for large power stations and transformers to achieve economies of scale. It also covers transformer design components, equivalent circuits, vector groups, parallel operation, effects of nonlinear loads, and improvements in efficiency over time through better magnetic steel.

1. Design, selection and operation of distribution transformers Stefan Fassbinder Deutsches Kupferinstitut Am Bonneshof 5 D-40474 Düsseldorf Tel.: +49 211 4796-323 Fax: +49 211 4796-310 [email_address] www.kupferinstitut.de deutsch English

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3. 1. Basics: “Gigantomania” or a necessity of economics? Why build power stations in such huge units that you need such big transformers?

4. This one weighs 300t and does 600MVA... ...and this one here weighs 300g and should therefore do 600VA! But in fact it only does 6VA!

5. The coherence is given by an empirical formula It's a law of physics: Higher energy density in larger plant

6. Structure of the public mains 0.4kV 20kV 10kV 380kV 220kV 110kV 50 Hz 3~ 27kV, nuclear 21kV, e. g. coal 10kV, e. g. hydro 0.5kV, e. g. wind

7. 2. Design here of a distribution transformer Wooden spacers (not depicted in the following drawings) HV winding LV winding Current Current

8. Yoke spanning bar Winding spanning bolts Yoke lamination spanning Wooden winding spacers HV winding Yoke lamination spanning bolts LV winding Off-load tap changer

10. 3. Operating behaviour Equivalent circuit of a transformer: All values are referenced to one side, in this case the secondary. Note: Different loads influence the transformer differently! Load R Fe X 1  ‘ X 2  X m R Cu1 ‘ R Cu2

11. By the way, what really is short-circuit voltage? You apply a voltage magnitude to the input side which is just enough to drive the short-circuit current in the output winding when shorted. U SC I N X m R Fe R Cu1 ‘ R Cu2 X 1  ‘ X 2  A V

12. By the way, what really is short-circuit power? It doesn't really exist. You multiply the no-load voltage by the short-circuit current. X m R Cu1 ‘ R Cu2 X 1  ‘ X 2  V A R Fe

13. R load (rated load) L load Tricky: C load Voltage across the load ( ≈ 107%!) Equivalent input voltage U 1 ' (100%) Inductive drop u X Ohmic drop u R in the trans-former Total drop u SC in the transformer Voltage across the load ( ≈ 94%) Equivalent input voltage U 1 ' (100%) Inductive drop u X Ohmic drop u R in the trans-former Total drop u SC in the transformer Voltage across the load ( ≈ 99%) Equivalent input voltage U 1 ' (100%) Inductive drop u X Ohmic drop u R in the trans-former Total drop u SC in the transformer Inductive drop ( u X = 5.916%) Ohmic drop in the transformer (e. g. u R = 1%) Total drop inside the transformer (e. g. u SC = 6%)

14. Theoretical deduction of the load current Short circuit current I SC =16.7* I N  16.7*rated load magnitude ( Z SC = Z load : “half a short-circuit”)  E. g. non-detuned static Var com-pensator 1670kvar on a 100kVA transformer  Rated load

15. Excerpt with the actually occurring values  Nennlast  Rated load

16. No-load current of a 16kV / 420V, 630kVA transformer, here excitated from the low voltage side! R L ≈400m Ω R Fe ≈400 Ω X 1  ‘ ≈12m Ω X 2  ≈12m Ω X m >4k Ω R Cu1 ‘ ≈2m Ω R Cu2 ≈2m Ω

17. Vector groups Can the star point be loaded? 230V 400V

18. Vector groups Can the star point be loaded?  Yes Why? No  U V W U V W u v w n u v w n

19. Vector groups Can the star point be loaded? Approximate equivalent circuit of a Yyn vector group R L ≈400 m Ω R Fe ≈400 Ω X 1  ‘ ≈12m Ω X 2  ≈12m Ω X m >4k Ω R Cu1 ‘ ≈2m Ω R Cu2 ≈2m Ω R Fe ≈400 Ω X 1  ‘ ≈12m Ω X 2  ≈12m Ω X m >4k Ω R Cu1 ‘ ≈2m Ω R Cu2 ≈2m Ω L1 N L2 R L ≈ 400 Ω

20. Prerequisites for parallel operation: Equal voltages of windings to be paralleled, equal short-circuit voltage ratings, equal vector group figures, if input sides are not connected in parallel: Make sure the feeding grids are in phase with each other, if input sides are not connected in parallel: Make sure the feeding grids have approximately equal short-circuit powers, ratio of power ratings of units to be paralleled should be no greater than 3:1. u X = 3.91% u R = 0.9% u SC = 4% u X = 2.95% u R = 2.7% u SC = 4% 630 kVA transformer according to HD 428 list C 50 kVA transformer according to HD 428 list B

21. 4. Efficiency Development of magnetic steel

22. Division into classes according to the present HD 428

23. Division into classes according to the future HD 428

25. Comparing 8 designs of a 40 kVA transformer by Riedel transformer factory

27. Amorphous steel could cut the no-load losses down to a fraction However: bigger more expensive noisier

28. So the replacement of old transformers pays off for a variety of reasons 1958 1998 (Rauscher & Stoecklin) (ABB Sécheron SA)

29. No-load loss of a 400 kVA 16 kV / 400 V distribution transformer Improvement of efficiencies Load loss of a 400 kVA 16 kV / 400 V distribution transformer

30. 5. Too hot from “hot” loads Apparent power, TRMS voltage and TRMS current within limits – and yet it ran too hot? Mutual influences between the trans-former and its load

31. The power loss in a transformer is: The true power loss in a transformer is:

32. “ supplementary” additional losses in transformers can be calculated rapidly using the following two simple formulae: where: Oh well, perhaps a practical example is clearer: 1000 compact 11W (15VA) energy-saver lamps powered by a 15kVA transformer, u SC =4%, P ad =0.1 P Cu 81.4% etc. etc.

33. To some extent the transformer protects itself... Always remember: If the influence of the transformer upon the load did not exist, then the influence of the load upon the transformer would be nearly 9 times as high! 701.7% etc. etc.

34. A tool is available for this, too: The K Factor Calculator by www.cda.org.uk www.cda.org.uk/frontend/pubs.htm#ELECTRICAL/ENERGY%20EFFICIENCY

35. Deutsche Leonardo Schriften 1.1 Leitfaden Netzqualität – Einführung 1.2 Selbsthilfe-Leitfaden zur Beurteilung der Netzqualität 2.1 Kosten schlechter Netzqualität 3.1 Oberschwingungen – Ursachen und Auswirkungen 3.2.2 Echt effektiv – die einzig wahre Messung 3.3.1 Passive Filter 3.3.3 Aktive Filter 3.5.1 Auslegung des Neutralleiters in oberschwingungsreichen Anlagen 4.1 Ausfallsicherheit, Zuverlässigkeit und Redundanz 4.3.1 Verbesserung der Ausfallsicherheit durch Notstrom-Versorgung 4.5.1 Ausfallsichere und zuverlässige Stromversorgung eines modernen Bürogebäudes 5.1 Spannungseinbrüche – Einführung 5.1.3 Einführung in die Unsymmetrie 5.2.1 Vorbeugende Wartung – der Schlüssel zur Netzqualität 5.3.2 Maßnahmen gegen Spannungseinbrüche 5.5.1 Vom Umgang mit Spannungseinbrüchen – eine Fallstudie 6.1 Erdung mit System 6.3.1 »Erdungssysteme – Grundlagen der Berechnung und Auslegung

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