US3215961A - Stabilized wye-wye transformers - Google Patents

Description

Nov. 2, 1965 1. K. DoRToRT 3,215,951

STABILIZED WYE-WYE TRANSFORMERS Filed March 6, 1962 l 23 ZINVENTOK J4 TE- 25- Amami Af Mercer 0.57.@ f/vx. 54eme. Cie-ee .SoFFr/v 22 23 24 53' l ,4

United States Patent O 3,215,961 STABILIZED WYE-WYE TRANSFORMERS Isadore K. Dortort, Philadelphia,Pa., assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Mar. 6, 1962, Ser. No. 177,801 6 Claims. (Cl. 336-12) l My invention relates to a novel stabilizing structure for Wye-Wye transformers, and more speciiically relates to a novel tertiary winding for wye-wye transformers wherein there is a high leakage reactance between the novel tertiary and the main windings of the transformer.

It is well known in the art that a wye-wye power transformer must have a tertiary winding to `stabilize the voltage neutral with respect to fundamental frequency and triple-n harmonic zero-sequence component voltages.

It is common practice to rate such tertiary windings at 35% of the transformer kva rating, even though this winding only carries the difference magnetizing current between the magnetizing currents of the various transformer legs. This has been necessary since the addition of a delta connected tertiary greatly reduces the zerosequence impedance of the transformer so that a large short circuit current can flow in the event of a phaseto-neutral short circuit on the transformer. The usual type of tertiary carries approximately 1/3 of the short circuit ampere turns.

The principle of the present invention is to provide a novel tertiary winding which has a high leakage reactance with respect to the main windings to thereby substantially decrease the current flowing in the tertiary winding during fault conditions, whereby a tertiary winding construction which is simple in nature and occupies little space is permissible.

Accordingly, a primary object of this invention is to reduce the size and cost of Wye-Wye transformers.

A further object of this invention is to provide a novel inexpensive delta connected tertiary winding for wye-wye transformers.

A further object of this invention is to provide a simple and inexpensive transformer construction for stabilizing the voltage neutral with respect to fundamental frequency and triple-n harmonic zero-sequence' component voltages.

These and other objects of my novel invention will become apparent from the following description when taken in connection with the drawings, in which:

FIGURE 1 schematically illustrates a three-phase transformer core which is to have windings connected on the legs thereof.

FIGURE 2 schematically illustrates the transformer of FIGURE l as comprising a wye-wye connected system.

FIGURE 3 is a vector diagram of the voltages and magnetizing currents on the transformer of FIGURE 2 under ideal ux conditions.

FIGURE 4 is a vector diagram of the magnetizing currents and voltages of the core of the transformer of FIGURES 1 and 2 for practical conditions of unbalance.

FIGURE 5 illustrates the connection of a delta connected tertiary winding for the transformer of FIGURES 1 and 2 to provide stabilization of the voltage neutral of the system.

FIGURE 6 shows a schematic illustration of my novel inventive structure.

FIGURE 7 shows a modification of FIGURE 6.

FIGURE 8 shows a schematicview of a three-phase transformer core with windings thereon which is provided with a delta connected tertiary winding in accordance with FIGURE 7.

FIGURE 9 is a top view of the transformer of FIG- URE 8.,

A ceive wye-wye connected windings.

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FIGURE 10 is a side cross-sectional view of FIGURE 8 taken across the lines 10-10 of FIGURE 8.

FIGURE l1 shows a modification of the tertiary Winding of FIGURE 6.

FIGURE 12 shows a further modication of the structure of FIGURE 1l.

FIGURE 13 is a perspective view of a part of the transformer core of FIGURE 1 when modified in accordance with a further embodiment of the invention.

FIGURE 14 shows a still further embodiment of the invention in the manner illustrated in FIGURE 13.

FIGURE 15 shows the manner in which la plurality of turns can be used for the tertiary.

Referring now to the figures, FIGURE 1 illustrates a typical three-legged core-type transformer which will re- Thus, the core is comprised of an upper and lower yoke and 21, respectively, which are connected in any desired manner by core legs 22, 23 and 24. Each of the core legs 22 through 24 receive primary and secondary windings schematically illustrated by coil structures 25, 26 and 27 respectively.

The well-known schematic diagram for this system is illustrated in FIGURE 2 as comprising a wye connected primary winding 28 which is connected to a three-phase input line comprising lines 29, 30 and 31 and a Wye connected secondary 32 which is, for example, connected to output lines 33, 34 and 35.

From the construction of the three-legged core of FIG- URE l, the magnetizing currents required by the two outer legs 22 and 24 of the core will be considerably greater than that of the center leg 23. That is, the magnetic reluctance from the common flux point at the center of the bottom yoke 21 to the common linx point at the center of the top yoke 20 and through the outside core legs 22 and 24 will involve two lengths of iron and two joints which are not present in the path through the center leg 23. Other differences will exist between the three legs 22, 23 and 24 due to uncontrollable variations in manufacture.

FIGURE 3 shows a vector diagram of the phase voltages and magnetizing currents in the transformer of FIG- URES 1 and 2, assuming that the fluxes between the various legs are completely balanced. Similarly, FIG- URE 4 shows a vector diagram of the phase relationship of unbalanced magnetizing currents with respect to the resulting unbalanced phase voltages. Y

Because of the unbalanced flux paths in the transformer of FIGURE l as met in practice, the three magnetic paths will not have the same magnetic reluctance `so that the voltage vectors cannot remain balanced, as shown in FIGURE 3. Moreover, and since the system involved is a three-wire ungrounded system, the three magnetizing currents of FIGURE 4 must add up to zero vectorially, and are, therefore, restrained from assuming values that would permit balanced voltages. Therefore, the winding 26 on the center leg of the transformer of FIGURE 1 may have a greater voltage than the windings on the two outside legs 25 and 27 respectively, because the ux density in the center leg will be greater.

In consequence of this, and from the vector diagrams of FIGURE 4, both the voltage and flux systems will have zero sequence components. The zero sequence flux must return from the upper yoke 20 to the lower yoke 21 outside :of the core it is traversing through the insulation medium, structural members, and so on. If now a delta connected tertiary Winding is built into the transformer, a circulating current will flow in the tertiary which is equivalent to the zero-sequence magnetizing current required tlo balance the fluxes in the three legs of the core,

r 3 and restore the voltages to the balanced condition of FIGURE 3.

This is schematically illustrated in FIGURE 5 for the schematically illustrated core 40 which has Wye-connected primary windings 41, 42 and 43 wound thereon, which have magnetizing currents in the direction of the arrow to the right of the windings, and which has secondary windings 44, 45 and 46 wound thereon. The delta connected tertiary then includes windings 47, 48 and 49 which will conduct the magnetizing currents im, which is the zero-sequence magnetizing current required for flux balance.

From the above, it would be initially thought that the delta connected tertiary winding would present no problem in the manufacture of the transformer, since it only carries a dilference magnetizing current which is a -relatively small current. In practice, however, the zero-sequence impedance of a wye-wye transformer without a tertiary is very large, so that a phase-to-neutral short circuit on the secondary w-ould draw very little current. With a delta connected tertiary, however, the zero-sequence impedance of the transformer is greatly Ireduced so that a large short circuit current can now flow. However, since there can be no Zero-sequence current in the primary transformer winding, the equivalent zero-sequence ampere turns must appear in lthe tertiary, and would necessarily amount to 1/a of the ampere turns of the secondary short circuit.

Therefore, to prevent damage to the tertiary winding, it must have a far greater copper cross-section than would be required for i-ts normal operation. In practice, the tertiary winding is rated at 35% of the transformer kva. Therefore, the cost of the addition of these windings in material and labor is appreciable, and the transformer core size is greatly increased so that it can accommodate the extra winding and insulation to again increase the total cost of the transformer.

The tertiary winding further serves to correct harmonic conditions within the transformer, due to 4the variations of the flux from a sinusoidal shape because of n-on-linear transformer steel. While this analysis can be rigorously given, it is well known to the art, and results in the existence of a flux in the three core legs of the transformer which will induce a peaked voltage conguration, whereby the difference between a sinusoidal voltage and the distored secondary voltage has the effect of producing a circular precession of the potential of the neutral point of the windings about the virtual voltage neutral in the vector diagram of FIGURE 3.

The tertiary winding can be shown to provide a path for the zero-sequence third harmonic currents land Sn harmonic currents, so that the winding neutral is stabilized with respect to both the fundamental frequency, as discussed fully above, and the triple-n harmonics.

It is, therefore, seen that a delta connected tertiary winding is a necessity in Wye-Wye transformers.

The principle of the present invention is to provide a simple inexpensive construction for the tertiary wherein a high leakage reactance exists between the tertiary and the main windings of the transformer to decrease cur: rent flow in the tertiary during fault conditions and, thus, permit the use of small cross-sectional copper for the 'tertiary windings, and to accomplish this without anyI significant increase in the dimensions and weight of the iron core and the main windings.

One manner in which the tertiary winding can be -produced in accordance with the invention is schematically illustrated in FIGURE 6 which shows core legs 22, 23 and 24 of FIGURE 1 in cross-section.

In FIGURE 6, two single-turn tertiaries are contemplated, one at the top transformer yoke within the Windows of the transformer and the other at the bottom of the transformer wi-thin the windows. Such a construction will be seen to only slightly increase the height of the core windows without any increase in width. Moreover, no winding space is required so that the tertiary can be assembled within the transformer for very little expendi- -ture in labor and material cost.

In FIGURE 6, the tertiary is formed of three windings 50, 51 and 52 of one turn each and connected in delta. Obviously, this would be the equivalent of a simple loop around the core legs, shown in FIGURE 7 as the single loop 53.

A transformer using the concept of FIGURES 6 and 7 may be manufactured as shown in FIGURES 8, 9 and 10 the core 60 having windows 61 and 62 receives three windings 63, 64 and 65 which are the corresponding primary and secondary windings of the transformer.

The tertiary is then made of flat conductive bands or sheets 66 and 67 which encircle the core in the manner shown in FIGURE 7. Each of sheets 66 and 67 are sandwiched between insulation sheets 68-69 and 70-71 respectively which insulates the delta connected tertiary windings 66 and 67 from the remaining structure.

When using the construction of FIGURES 8, 9 and l0, the reluctance of the core legs of the transformer between the tertiary windings 66 and 67 willbe relatively small,

as compared to the reluctance of the joints between the core legs and yoke external of the core legs. Therefore, most of the zero-sequence flux, both fundamental and third harmonic, will leave and re-enter the core in the areas of the yokes. Therefore, thelocation of single-turn tertiaries just inside the Windows will be an effective way of cancelling out these fluxes.

Moreover, and because of the high leakage reactance between tertiaries 66 and 67 and the primary wind-ows, the tertiary need be designed only for approximately 1/3 of the magnetizing ampere turns of the transformer, as contrasted to 1/3 of the fault current due to a short on the secondary winding.

In power transformers, the magnetizing current will generally be of the order of 1% of the rated ampere turns, which, therefore, permits construction of the tertiaries in the form of the at sheets or foils 66 and67 which can be between the two relatively thin layers of insulation. It will be noted that the insulation sheets 68, 69, 70 and 71 need not be greater than the turn-to-turn insulation in the main transformer windings.

In the transformer of FIGURES 8, 9 and 10, a straight loop concept, illustrated in FIGURE 7, was utilized. It may, however, be advantageous in certain applications to use the configuration of FIGURE 11 wherein a conductive sheet is stamped with two protrusions which enter between respective adjacent core legs. An equivalent of this type of connection is shown in FIGURE 12 where the conductive sheet 82 has the slot in the middle.

With this type of arrangement, it is possible to reduce the lluX leakages of the tertiary by magnetic flux which is not contained within the iron to thereby avoid the deviation of magnitude and wave shape of the current tlowing in the tertiary from that required for exact compensation.

The tertiaries of FIGURES 6 through 12 could, of course, be constructed of a single loop made of wire, bar or sheet material, and could also utilize multiple turns arranged in a flat or pancake configuration, as shown in FIGURE 15 for windings 83a, 83h and 83e. This reduces the effect of excessive impedance in the interconnecting straps by reducing the current in these interconnections.

As a further embodiment of the invention, the transformer core of FIGURE 1 is partially illustrated in schematic form in FIGURE 13 wherein a copper sheet band encircles the yoke 20. This type of arrangement will prevent the escape of flux from the sides of yoke 20.

Alternative to this, and as shown in FIGURE 13, the copper band 90 could be replaced by a conductive box 91 which completely encloses the yoke 20 to prevent the passage of zero-sequence flux in any direction.

In applications of wye-wye transformers of the type to which the invention is applied, such transformers are often used in rectifier systems requiring the double wye connection with an interphase transformer arrangement. The high zero-sequence impedance afforded by this systern will materially reduce arc-back type faults in mercury arc rectifers with their destructive effects on the rectifier transformers. The wye-wye power transformer is also required at times for matching phase angles of interconnected systems, or for impedance grounding of substation transformers. In any of these applications, the novel arrangement for stabilizing such wye-wye transformers as discussed above can be utilized.

When utilizing the present invention in an application using overload relays to protect the transformer against a phase-to-neutral fault on the secondary, some care should be taken in the connection of the relays. Thus, undervoltage relays connected from line to neutral on all three phases, or phase balance voltage relays should be used to provide adequate protection. These relays could be used on the primary or secondary side of the transformer, either with or without potential transformers, as required.

Moreover, and in accordance with the invention, an overcurrent relay can be connected in the tertiary loop where the relay and its connections have a very low impedance.

Although I have described preferred embodiments of my novel invention, many variations and modifications will now be obvious to those skilled in the art, and I prefer therefore to be limited not by the specific disclosure herein but only by the appended claims.

I claim:

1. In a three-phase transformer; a magnetic core having an upper and lower yoke connected by three core legs along respective coplanar junctions between said upper and lower yokes and the respective ends of each of said three core legs, a respective primary winding and secondary winding wound on each of said core legs, said primary windings being connected in wye, said secondary windings being connected in wye; a tertiary winding for each of said core legs; said tertiary windings being connected in delta; said tertiary windings being coupled to -said primary and secondary windings by a high leakage reactance; said tertiary windings being formed of a sheet of conductive material; said sheet of conductive material encircling said three said core legs in a plane slightly below the plane of said coplanar junction between said three core legs and one of said yokes.

2. In a three-phase transformer; a magnetic core having an upper and lower yoke connected by three core legs along respective coplanar junctions between said upper and lower yokes and the respective ends of each of said three core legs, a respective primary winding and -secondary winding wound on each of said core legs, said primary windings being connected in Wye, said secondary windings being connected in Wye; a tertiary winding for each of said core legs; said tertiary windings being connected in delta; said tertiary windings being coupled to said primary and secondary windings by a high leakage reactance; said tertiary windings being formed of a sheet of conductive material; said sheet of conductive material encircling said three said core legs in a plane slightly below the plane of said coplanar junction between said three core legs and one of said yokes; and a second set of tertiary windings identical to said last mentioned tertiary windings and positioned adjacent the other of said yokes.

3. In a three-phase transformer; a magnetic core having an upper and lower yoke connected by three core legs along respective coplanar junctions between said upper and lower yokes and the respective ends of each of said three core legs, a respective primary winding and secondary winding wound on each of said core legs, said primary windings being -connected in wye, said secondary windings being connected in wye; a tertiary winding for each of said core legs; said tertiary windings being connected in delta; said tertiary windings being coupled to said primary and secondary windings by a high leakage reactance; said tertiary windings being formed of a sheet of conductive material; said sheet of conductive material encircling said three said core legs in a plane slightly below the plane of said coplanar junction between said three core legs and one of said yokes; said sheet of conductive material having a substantially negligible height, said tertiary winding having a predetermined magnetizing current and a predetermined number of ampere turns under short circuit conditions; the cross-sectional current carrying area of said tertiary winding being insuflicient to carry more than the magnetizing current of said transformer and smaller than the cross-sectional current carrying area required to carry 1/3 the short circuit ampere turns of said transformer.

4. In a three-phase transformer; a magnetic core having an upper and lower yoke connected by three core legs, a respective primary winding and secondary winding wound on each of said core legs, said primary windings being connected in wye, said secondary windings being connected in wye; a tertiary winding for each of said core legs; said tertiary windings being connected in delta; said tertiary windings being coupled to said primary and secondary windings by a high leakage reactance; said tertiary windings comprising a band of conductive material encircling the external sides of at least one of said yokes.

5. In a three-phase transformer; a magnetic core having an upper and lower yoke connected by three core legs, a respective primary winding and secondary winding wound on each of said core legs, said primary windings being connected in wye, said secondary windings being connected 'in wye; a tertiary winding for each of said core legs; said tertiary windings being connected in delta; said tertiary windings being coupled to said primary andsecondary windings by a high leakage reactance; said tertiary windings comprising a conductive box enclosing the outer sides of at least one of said yokes.

6. In a three-phase transformer; a magnetic core having an upper and lower yoke connected by three core legs along respective coplanar junctions between said upper and lower yokes and the respective ends of each of said three core legs, a respective primary winding and secondary winding wound on each of said core legs, said primary windings being connected in wye, said secondary windings being connected in wye; a tertiary winding for each of said core legs; said tertiary windings being connected in delta; said tertiary windings being coupled to said primary and secondary windings by a high leakage reactance; said tertiary windings being formed of ilat coplanar windings connected in series with one another; said flat coplanar windings encircling said three said core legs in a plane slightly below the plane of said coplanar function between said three core legs and one of said yokes.

References Cited by the Examiner UNITED STATES PATENTS 1,412,782 4/22 Dwyer 336-5 2,779,926 l/57 Johnson et al 336-5 JOHN F. BURNS, Primary Examiner. E. JAMES SAX, Examiner.

Claims (1)

1. 6. IN A THREE-PHASE TRANSFORMER; A MAGNETIC CORE HAVING AN UPPER AND LOWER YOKE CONNECTED BY THREE CORE LEGS ALONG RESPECTIVE COPLANAR JUNCTIONS BETWEEN SAID UPPER AND LOWER YOKES AND TH ERESPECTIVE ENDS OF EACH OF SAID THREE CORE LEGS, A RESPECTIVE PRIMARY WINDING AND SECONDARY WINDING WOUND ON EACH OF SAID CORE LEGS, SAID PRIMARY WINDINGS BEING CONNECTED IN WYE, SAID SECONDARY WINDINGS BEING CONNECTED IN WYE; A TERTIARY WINDING FOR EACH OF SAID CORE LEGS; SAID TERTIARY WINDINGS BEING CONNECTED IN DELTA; SAID TERTIARY WINDINGS BEING COUPLED TO SAID PRIMARY AND SECONDARY WINDINGS BY A HIGH LEAKAGE REACTANCE; SAID TERTIARY WINDINGS BEING FORMED OF FLAT COPLANAR WINDINGS CONNECTED IN SERIES WITH ONE ANOTHER; SAID FLAT COPLANAR WINDINGS ENCIRCLING SAID THREE SAID CORE LEGS IN A PLANE SLITHTLY BELOW THE PLANE OF SAID COPLANAR FUNCTION BETWEEN SAID THREE CORE LEGS AND ONE OF SAID YOKES.

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