Materials Science and Engineering, 99 (1988) 19-21 19

A New Design for Amorphous Core Distribution Transformer*

R. SCHULZ, N. CHRETIEN, N. ALEXANDROV, J. AUBIN and R. ROBERGE

Hydro-Qubbec, Vice presidence recherche (IREQ), 1800 Montbe Ste-Julie, Varennes, Qubbec, JOL 2PO (Canada)

Abstract

A new design and a pre-prototype 25kVA amor- phous-core dry distribution transformer under develop- ment at Hydro-Qubbec are described and compared with other designs. The power capacity of the trans- former can easily be doubled by internal cooling.

1. Introduction

Among all the possible core-coil configuration for distribution transformers only a few are adequate for amorphous-core transformers because of the particu- lar nature of amorphous metal ribbons. The small thickness of the ribbon, the lower saturation induc- tion, the hardness, the relative brittleness after heat treatment and the high magnetostriction (i.e. influ- ence of stress on core loss) are all important factors in determining an optimal design and the best manufac- turing procedure. The six most suitable configura- tions, listed under three assembly options, are shown in Fig. I [1]. In assembly options I and II, the coil is wound around a previously annealed wound core while assembly option III consists of winding pre- annealed amorphous metal into a previously formed coil.

Most of the transformer manufacturers have, so far, developed amorphous-core prototypes involving assembly options I and II. Table 1 shows a partial list of companies and utilities involved in the manufactur- ing of amorphous distribution transformers. Assem- bly option III was discarded by several transformer designers [2] because of the general belief that if the core is not annealed after forming, it would have un- acceptable losses. This paper reports the successful fabrication of a 25 kVA low-loss amorphous-core dry distribution transformer prototype using configura- tion IliA and gives preliminary test results.

2. Design considerations

Figure 2 illustrates a schematic of the transformer design. This configuration presents several advan-

*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987.

tages. First it is possible to use ribbons with very large width in contrast with configuration IA or IB which have cruciform-type core and therefore need either ribbons of varying width or at least much narrower ribbons in order to fill efficiently the circular coil opening. Second the core does not have to support the whole assembly as in core type configurations IA and liB and therefore simple and light core support can be used. There are no sharp corners or portions of the core with small radius of curvature as in IA, IB and liB where stress concentration can give rise to local plastic deformation during annealing. The magnetoe- lastic strain introduced during core annealing is known to contribute significantly to the total loss [3]. The core geometry in configuration I l i a allows straightforward application of the technique of flash heating [4] or dynamic annealing recently proposed in the literature [5]. This design offers great advantages in manufacturing techniques compared with other de- signs such as configuration IIA which require a toroidal winding technique and are therefore complex and expensive procedures. Finally, since the coil is formed first, this configuration is the most adequate for dry transformer design where the electrical circuit is molded in an epoxy resin.

3. Manufacturing details

The construction of the prototype begins with the coil assembly. The primary winding, located in the center, consists of 1084 turns of 2 mm x 2 mm copper strips. The secondary windings are stacked on each side of the primary winding and are made with copper strips of different cross-sections (average value 2 mm x 120 mm) in order to form a circular cross-sec- tion. The number of turns on the secondary is 19 and they are isolated from each other by Nomex paper. A cooling coil is soldered on the secondary winding at some strategic locations. The electrical circuit thus assembled is molded under vacuum in an epoxy resin (155 C class). The heat treatment of the Met- glas (2605-$2) amorphous ribbon is achieved in a spe- cially designed oven and heat treated at 360 C for 2 h under inert gas. The heat is applied in the axial core direction to ensure a homogeneous temperature

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TABLE 1 Partial list of participants in the development of distribution transformer

Company Capacity Core loss Copper loss (kVA) (W) (W)

Allied-Signal Inc. 15 14 166 (U.S.A.) 25 16 - -

50 28 422

General Electric 25 28 - - (U.S.A.) 25 18 330

50 30 455

Westinghouse 10 11 - - (U.S.A.) 25 29 - -

25 20 - - 500 - - - -

Osaka Transformers 10 8.6 173 (Japan) 30 30.0 390

McGraw-Edison 10 7.6 - - (U.S.A.) 25 15 - -

Tokaoka Electrical 20 18.9 348 (Japan)

Mitsubishi Electrical 35 49 98 (Japan)

Toshiba 100 85 1780 (Japan)

GEC 16 12.1 - - (U .K . )

Other participants in the field are ASEA (Sweden), RTE (U.S.A.), Kuhlman (U.S.A.), Sanyo Special Steel (Japan), Nip- pon Steel (Japan), Tokoku Metal (Japan), Central Moloney (U.S.A.), Kansai Electrical (Japan), Shanghai Transformers (China).

I II "aT #Indlng lhe coil orcund on unj0inWd Winding lhe coil Oround on ungirded WindinQ l~lOnneelod omoqphow

I~ c~(e by ull~ Io~l Illllal ollkl 0 pl'ivlooliy li~md coil I~lnd ~e ~ (Gltlll Ihl ~1 WllldinQ nlll h~l$ ~ rGlotlni I l l ~

w~mO onMing w~m~ ~ omo(~mm mttm wimlin

(A)

(8)

GC -ostutA design

~ A-A Core-t~ (wound coil)

G( -(PRI Ue~ln o(e

Seclm A-A Shill-t)~e (l~und coil)

(A) ALLI~doS~

Seclion A-A Toroidol-ty~l

(B) Co~

(A) IRF.a ~s~ A Cote A

Shell -ty~l (wotund cole) (a)

Coil

Fig. I. Possible core-coil configurations and various assembly options for amorphous core distribution transformers.

A I 1

Fig. 2. Schematic diagram of the IREQ transformer design.

distr ibution. After heat treatment the r ibbon is wound

in its final posit ion around the legs of the electrical

coil. The technique used allows the relaxation of the

innermost port ion of wound cores which are known

to contr ibute significantly to the total core losses [3] at

TABLE 2 Major characteristics of Hydro-Quebec 25 kVA pre-prototype dry transformer

Power rating With no internal cooling 25 kVA With 4 cm 3 s- t cooling flow 50 kVA

Voltage rating 6.56 kV/115 V

Electrical circuit molded in epoxy resin ( 155 C class)

Number of primary turns 1084

Number of secondary turns 19

Internal cooling by copper tubes fixed on the secondary winding

Total weight 200 kg

Weight of amorphous metal 76.2 kg

Core loss at 1.3 T and 22 C 18.5 W

Copper loss at 25 kVA 300 W at 52 kVA 1230 W

High-voltage winding resistance 25 f2

Fig. 3. Pre-prototype 25 kVA dry transformer built at IREQ.

TABLE 3 Equilibrium temperature of the low- and high-voltage windings for various primary currents and cooling conditions

Cooling flow 27.3 k VA 52.5 k VA 65.6 k VA (cm 3 s- ')

Primary Primary Primary current current current 4.163 A 8.0 A 10 A

0 LV 109 HV 119

4 LV 72.9 HV 110

15 LV 26.3 LV 56.8 LV 82.7 HV 33.3 HV 92.7 HV 149

LV, low-voltage winding; HV, high-voltage winding. Tempera- ture in degrees Celsius above ambient.

21

the expense of a slight increase (no more than 5%) in the core space factor.

4. Test results

Table 2 gives the general characteristics of the 25 kVA pre-prototype under development at Hydro- Quebec and Fig. 3 shows a picture of the transformer. The equilibrium temperature of the primary and sec- ondary winding for various primary currents and cooling conditions are shown in Table 3. It should be noticed that with a cooling flow ar iow as 4 cm 3 s l, it is possible to double the nominal power capacity of the transformer keeping the raising temperature be- low 110 C. For a primary current of 10 A a large difference in the temperature between the primary and secondary windings is observed at 15 cm 3 s-~ arising from the fact that the cooling coil is not in direct contact with the high-voltage winding.

5. Conclusion

A low-loss amorphous-core dry distribution trans- former has been fabricated. The design does not re- quire core post-annealing and is therefore very easy to manufacture. The core losses are comparable with or less than other designs. The core space factor is ap- proximately 5% less than the value usually reported in the literature. With internal cooling we can easily double and almost triple the nominal power capacity of the transformer. The prototype can handle 100% overcharge with only a 4 cm 3 s-1 cooling flow.

Acknowledgment

The authors wish to acknowledge Gr~goire Par~ for his technical assistance.

References

1 R. Schulz, N. Alexandrov and R. Roberge, CIGRE Symp. on New and Improved Materials for Electrotechnology, Vienna, May, Cigr6 Symposium No. S 05-87, Paris, 1987, pp. 300-305.

2 E. L. Boyd and J. D. Borst, 1EEE Trans. Power Appar. Syst., 103 (1984) 3365.

3 D. M. Nathasingh and H. Liebermann, Transformer Applica- tions of Amorphous Alloys in Power Distribution Systems, 1EEE Transactions on Power Delivery, Vol. PWRD-2, No. 3, July 1987, pp. 843-850.

4 A. I. Taub, J. Appl. Phys., 55(1984) 1775. 5 A. I. Taub, IEEE Trans. Magn., 20(1984) 564.