US1894827A

US1894827A - Polyphase system

Info

Publication number: US1894827A
Application number: US533751A
Authority: US
Inventors: Montville B Mallett
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1931-04-29
Filing date: 1931-04-29
Publication date: 1933-01-17
Anticipated expiration: 1950-01-17

Description

Jan. 17, 1933. M- B MALLET-r 1,894,827

POLYPHASE SYSTEM Filed April 29. 1931 Fig. I /l y g nventor: 1 Montviile B. MalleCC,

His Attorney.

Patented Jan. 17, 1933 UNITED STATES PATENT OFFICE MONTVILLE IB. MALLETT, OF PITTSFIELD, MASSACHUSETTS, ASSIGNOB TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK POLYPHASE SYSTEM Application led April 29,

My invention relates to polyphase'alternating current systems and particularly to improvements in the transformation of three phase power by two single phase transformers.

It is often desirable to be able to transform three phase power with two single phase transformers. For example, if one of a bank of three single phase transformers should fail, it would be very advantageous to be able to continue to transform three phase power in a satisfactory and substantially balanced manner with the remaining two transformers. Such transformation would, however, have to be at reduced capacity. Also, there will be many instances where the present power requirements of a transforming station are not up to its ultimate estimated capacity. In such cases it would be good practice to use only two transformers until such a time as it becomes necessary to add the third transformer.

I-Ieretofore, these results have been achieved to a limited extent by the use of two single phase transformers connected in open delta or V. However, as this connection presupposes an ultimate delta-delta connection of the three single phase transformers, its application has been limited because the delta-delta connection of transformers is seldom employed as none of the system voltages will be fixed with respect to ground. It is for this reason that most three phase transformer banks are connected with one of their windings wye connected and the other delta connected. One reason that wye-wye connections are not used is that such a connection does not provide a circulating path for third harmonic currents with the result that the wye-wye connection produces objectionable third harmonic voltages, which when the neutrals are grounded will cause third harmonic currents to flow in the circuit conductors. One alternative is to connect corresponding windings of the two transformers on one side in what may be termed open wye. The difficulty with this connection, however, is t-he fact that power will be transmitted at reduced voltage with respect to the voltage of full wye transmission and also, it requires 1931. Serial No. 533,751.

that the neutral be carried through between the open wye transformer connection and the apparatus which is connected thereto. Furthermore, the system voltages will be badly unbalanced. In addition, the transmission of power at reduced voltage results in increased losses. It is thus clea-r that the open delta or V-open wye connection of the two transformers does not result in very satisfactory operation.

In practicing my invention I utilize simple multi-windino' transformers which, when arranged according to my invention, will give substantially balanced three phase operation with two single phase transformers and the connections may be made in such a way that when athird transformer is added to the original two, the final connections may be either Wye-Wye, or delta-delta, or Wye-delta, or delta-Wye.

An object of my invention is to provide a new and improved method and system for producing substantially balanced three phase voltages and currents.

Another object of my invention is to provide a new and improved system of transformer connections.

A further object of my invention is to provide a new and improved system whereby substantially balanced three phase operation can be secured with two single phase transformers.

My invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. l illustrates three multi-winding transformers' connected in wye-delta between two three-phase circuits, Fig. 2 illustrates two of these transformers connected according to my invention in wyedelta to give substantially balanced threephase operation for the same three-phase circuits, while Figs. 3, 4 and 5 illustrate vectorially the voltage relations in the windings of Fig. 2.

Referring now to Fig. 1 of the accompanying drawing, it will be seen that I have shown three transformers, l, 2 and 3, which are used in practicing my invention, connected for transforming three-phase power. These transformers have

cores

4, 5 and 6, respectively, on each of which are wound four separate windings. It will be assumed that the

upper windings

7, 8, 9, 10, 11 and 12 are the primary windings and that the

lower windings

18, 14, 15, 16, 17 and 18 are the secondary windings, although it will lof course be obvious to those skilled in the art that this might be reversed if desired. As will be seen, one terminal of each of the upper windings is connected to a common conductor, while the remaining terminals of the two windings of each transformer are connected to the

conductors

19, 2O and 21 respectively, of a three phase circuit. The primary windings of the transformers are thus wye connected and each transformer primary consists of two similar windings connected in parallel. rlhe secondary windings on the other handare'connected in parallel with each other in each transformer .and each parallel group of secondary windings is connectedto the other group to form a delta or mesh connection. The corners of this delta connection are connected to

conductors

22, 23 and 24 of another three phase power circuit. It will thus be seen that the transformer connections of Fig. 1 are Wyedelta. skilled in the art, however, that they might be wye-wye or delta-delta or delta-Wye just as well. The connections shown in Fig. 1 are ordinary standard transformer connections, each of the transformer windings comprising two similar windings connected in parallel.

Assume now that transformer 2 has failed or that it is desired to secure substantially balanced three phase transformation by the 4Q use of transformers 1 and 3 only. Under these circumstances these two transformers are connected as shown in Fig. 2. For purposes of illustration, I have shown the upper windings, as viewed in the drawing, to be equivalent wye connected, while the lower windings are equivalent delta connected. However, it will of course be obvious to those skilled in the art, that these connections could be reversed ifdesired, or they vcould both be equivalent Wye or equivalent delta without departing from my invention in its broader aspects. As shown in this figure, windings 7 and 12 are connected as in Fig. 1, but windings 8 and 11 have been connected in series between the common point of windings 7 and 12 and conductor 20. Fig. 8 shows how balanced three phase voltages are attained by this connection. Thus vectors V7 .and V12 indicate the voltages in windings 7 and 12, which are 120 degrees out of phase as they would'be in two of the legs of the wye connected three phase circuit. Vectors V8 and V11 represent the voltages in the windings 8 and 11, which due to the connections of these'windings will be reversed with respect to the voltages of wind- It will of course be obvious to thoseV ings 7 and 12, while the vector sum of these two voltages is indicated by the vector V8 plus V11 which will be the voltage of the leg connected to conductor 20. It will be seen that this latter vector is equal in magnitude to` vectors V7 Vand V12 and is 120 degrees out of phase with both. This therefore gives balanced three phase wye voltages.

In the equivalent delta connection of the lower winding,

windings

13 and 18 are connected as in Fig. 1, but windings 14 and 17 are connected in series to complete the loop or mesh circuit of all four windings. The relationoftheequivalent delta voltages produced by this connection is shown in Fig. 4 wherein V13 and V18 represent the voltages in the

windings

13 and 18 and the dotted vector V14 plus V17 represents the voltage of windings 14 and 17 combined. Fig. 5 shows how this latter voltage is derived from the addition of voltages V14 and V17 which correspond to the voltages of windings 14 and 17 respectively.

It is seen therefore that with the connections illustrated in Fig. 2 three phase power maybe transformed by means of two single phase transformers which are so connected that the iluXes in their cores are 120 degrees out of phase and that these fluxes produce directly two ofthe voltages of the three phase system and indirectly produce the third voltage by combining another pair of voltages which correspond to the first mentioned pair o-f voltages of the three phase system.

The power capacity of the arrangement shown in Fig. 2 is one-half, or 50%, of the power capacity of the arrangement shown in Fig. 1. A The connections illustrated by the upper four windings of Fig. 2 will not generate appreciable third harmonic voltage, even thoughV the Y neutrals are isolated and no delta winding is present.

Vhile I have shown and described a particular embodiment of my invention, it will bek obvious to those skilled in the art that changes and modifications may be made without departing from my invention and l, therefore, aim in the appended claims to cove er all such changes and modifications as fall within the true spirit and scope of'my invention.

What IV claim asnew and desire to secure by Letters Patent of the United States, is:

1. The method ofproducingbalancedthree phase voltageswhich consists of producing two alternating fluxes which are 120 degrees out of phase,'generating therewith two of said three phase voltages, also generating there with a similar pair of voltages and combining said last mentioned voltages to produce the third of said three phase voltages.

2. The method of transforming a three phase supply at one voltage to a three phase system at another voltageV which consists of producing with said supply two fluxes which are 120 kdegrees out of phase, generating two voltages of said three phase system therewith, generating a similar pair of said voltages therewith and combining the latter to form the third voltage of said three phase systems.

3. The method of connecting together two three phase circuits by two single phase transformers, each having two identical primary and secondary windings, so as to secure substantially balanced three phase operation which consists of connecting one of the primary and secondary windings of each of the transformers in two of the phases of their respective circuits and connecting the remaining primary and secondary windings in series in the remaining phase of their respective circuits.

4. The method of securing substantially balanced wye operation between two single phase transformers, having two similar windings respectively, and a three phase three wire ungrounded neutral circuit which consists of connecting two of the windings of different transformers between a common point and two of the conductors of the circuit respectively and connecting the remaining two windings in series between said common point and the remaining conductor of said circuit.

5. The method of securing substantially balanced delta operation between two single phase transformers, having two similar windings respectively, and a three phase circuit which consists of connecting all of said windings in series, and connecting the conductors of said circuit to said windings in such a way that two of the windings of different transformers are between one pair of conductors of said circuit and one each of the remaining windings are between each of the other two pairs of conductors of said circuit.

6. In combination, a three phase three conductor ungrounded neutral circuit, and two single phase transformers connected thereto to give substantially balanced three phase operation, said transformers being equivalent wye connected to said circuit.

7. In combination, a three phase circuit, a pair of transformers, said transformers each having two windings for connection to said circuit, one winding on each transformer being connected between a common point and a different one of the conductors of said circuit, the remaining windings being connected in series between said common point and the remaining conductor of said circuit.

8. In combination, a three phase circuit., a pair of transformers, said transformers each having two windings for connection to said circuit, all of said windings being connected to form a closed loop, two of the conductors of said circuit being connected to said loop at points between the two windings of each transformer respectively, the other conductor of said circuit being connected to said loop at a point between two windings of different transformers.

9. In combination, a three phase supply circuit, a three phase load circuit, two single phase transformers for connecting said circuits, said transformers each having two identical primary windings and two identical secondary windings, one primary winding of each transformer being connected in a different phase of said supply circuit, one secondary winding of each transformer being connected in a different phase of said load circuit, the remaining two primary windings being connected in series in the remaining phase of said supply circuit, and the remaining two secondary windings .being connected in series in the remaining phase of said load circuit.

10. In combination, a pair of three phase circuits, a pair of single phase transformers for connecting said circuits, said transformers each having two windings of which all four are similar, said transformers each having two additional windings of which all four are similar, two of said first mentioned windings on different transformers being connected respectively between a common point and two of the conductors of one of said circuits, the remaining two of said first mentioned windings being connected in series between said common point and the remaining conductor of said circuit, two of said additional windings on different transformers being connected respectively between different pairs of conductors of the other circuit, and the remaining two additional. windings being connected in series between the remaining pair of conductors of said other circuit.

1l. In combination, a three phase supply circuit, a three phase load circuit, a pair of single phase transformers for connecting said circuits, said transformers each having a pair of similar primary windings and a pair of similar secondary windings, a primary winding of each transformer being connected between a common point and a different conductor of said supply circuit, the remaining two primary windings being connected in series between said common point and the remaining conductor of said supply circuit, a secondary winding of each of said transformers being connected between diiferent pairs of conductors of said load circuit, and the remaining two secondary windings being connected in series between the remaining pairs of conductors of said load circuit.

In witness whereof I have hereunto set my hand.

MONTVILLE B. MALLETT.