US2449367A - Tapped transformer system - Google Patents

Description

Sept. 14',194s.- A. BOYAJIAN 2,449,361

TAPPED TRANSFORMER SYSTEM Filed Dec 12, 1945 Inventor: 'Aram Boyaj ian,

His Attorney.

Patented Sept. 14, 1948 TAPPED TRANSFORMER SYSTEM Aram Boyajian, Pittsfleld, Mass., assignor to General Electric Company, a corporation of New York Application December 12, 1945, Serial No. 634,571

6 Claims.

This invention relates to electric circuits and more particularly to alternating-voltage control systems based on tap-changing under load and involving an alternating-current transformer and auxiliary equipment including contactors adapted to control the output voltage of the transformer. while under load, by changing taps on the transformer without interruption of theload.

It has for its general object greater reliability and a substantial increase in the load which such auxiliaries can handle without a corresponding increase in' their'size.

A particular object of the invention is toreduce the recovery-voltage duty on the contactors so as to avoid their failure under short-circuit conditions and damage to other parts of the system.

A further object of the invention is to reduce fluctuations of load voltage due to the impedance drops of tap-changing equipment.

A still further object of the invention is to reduce circulating currents in loops formed by tapchanging equipment.

The invent-ion 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 accompanying drawing, Fig. l is the simplified diagram of a conventional tap-changing system, which is helpful in explaining the problems which the present invention solves, Figs. 2 and 3 show various operative positions of Fig. 1, Fig. 4 illustrates diagrammatically an embodiment of the present invention, and Figs. 5-8 inclusive illustrate various steps in a cycle of tapchanging by the equipment of Fig. 4.

Referring to Fig. 1. I is a two-winding trans-'- former with primary winding 2, fed from an alternating-current source 3, while 4 is a secondary winding of the transformer, with taps.

terminating at the fixed contacts 5 and B. A pair of movable fingers (contacts) are represented by i and 8; 9 and in are contactors in series with 1 and 8, respectively; and H is a mid-tapped impeder-usuall a reactor-autotransformer-with its two terminals connected to the circuits of I and 8, respectively, while the mid-point of II and a terminal I! of the secondary, are connected respectively to the two terminals of a load l3.

Fig. 1 shows both movable fingers 1 and 8 as engaging tap 5, so that the load is furnished by the voltage corresponding to this'trap. If it is desired to change to the adjoining tap 8, the circuit is made to pass through two successive representative positions illustrated by Figs. 2.and 3.

2 In the first position (Fig. 2). contactor iii is opened and the finger 8 is moved away from tap 5. In the second position (Fig. 3), the finger 8 is made to engage top 6, and the contactor ill is closed. In the first position (Fig. 2), the entire load current flows through the left-hand half of i i, to the mid-point, and out to the load. In this position, ii acts merely as a series reactor, producing a voltage drop. There are two objections to this: One is that the load voltage is impaired; and the other is that this voltage drop represents an important component of the recovery voltage of contactor I0, and under large currents such-.as load short circuit, it may cause failure of the contactor iii. Therefore, the lower the reactancc of i l, the better it is for this short circuit condition of the circuit. In the second position (Fig. 3), ii bridges across two taps like a shunt autotransformer, the load current dividing between its two halves and flowing through them non-inductively, except for the leakage reactance between the two halves of II. Now, however, a circulating current flows through ll (represented by the double-headed arrow in Fig. 3), this current being limited by the reactance of II, so that, for this position of the equipment, the higher the reactance of H the better. Consequently, there are two conflicting requirements, namely, to make the reactance of Ii low, so that in the position of Fig. 2 the load voltage will not be impaired and the contactor recovery voltage will be low; and to make it high, so that in the position of Fig. 3 there will not be much circulating current. In one common practice, these two requirements are compromised, designing H for a compromise value of reactance, in which case neither requirement is fully satisfied. In anothercommon practice, reactor Ii is short-circuited immediately after opening one of the contactors. of voltage and light-flicker, so far as the load is concerned; while so far as the opened contactor is concerned, if the contactor arc goes out before Ii is short-circuited, no help is afforded the contactor because its recovery voltage is not reduced' while it is recovering; and if the arc has not gone out before Ii is short-circuited, then a tap-to- But this does not help the initial drop- Fig. 4 by the addition of a contactor ll bridging across a portion of the reactor-autotransformer H. With the system operating on one tap (socalled full-cycle position), as shown in Fig. 4, contactor H is preferably closed, although it may also be left open in this position. This contactor ll is for special duty during the tap-changing operations as follows.

Successive steps of a complete cycle of changing taps from 5 to 6, according to the present inventlon, are illustrated in Figs. 4-9 inclusive. As seen in Fig. 4, contactor H must first be closed, if it was open before. Contactor II is then opened, and finger 8 is disengaged from tap l, as shown in Fig. 5. This corresponds to the socalled quarter-cycle position of Fig. 2, but now the reactance of H is relatively low because a portion of it is short-cireuited. Finger I is now made to engage tap B, and contactor I is closed, making ll bridge across the two taps I and 6. as seen in Fig. 6. A circulating current now flows through the reactor-autotransformer bridging across the taps and 8, and the system delivers to the load a voltage intermediate to those of taps 5 and 6. If it is desired to operate at this voltage continuously, contactor H is opened so as to increase the reactance of I I and reduce the circulating current to a small value, as seen in Fig. 7. This is the so-called half-cycle position of the system. As the two reactance values of Ii-viith contactor H closed and open respectively-can be made to have any value independently of each other, both of the reactance requirements discussed above can be satisfied. For instance, a desirable value for the reactive drop of H in the position of Fig. 5 is one not to exceed a quarter of the tap-to-tap voltage. At the usual high power factors of load, this represents a negligible drop in the load voltage. A desirable value for the reactance of ii in Fig. '7,'that is, with I4 open, is such as to render the circulating current less than half of the full load current.

Considering the effect of this on the recovery voltages of the contactors: that of contactor l4 will be limited to the tap-to-tap voltage under all conditions. The maximum recovery voltage of contactors 9 and ID, at normal load, due to the reactive drop of II, will be limited to a quarter of the tap voltage; and under short circuit, this figure will be multiplied by the per-unit value of the short-circuit current. For instance, if the short-circuit current is 20 times normal, the short-circuit recovery voltage of contactors 9 and I0 due to the drop through H will be less than 5 times the tapvoltage. Still lower values of this short-circuit recovery voltage can be obtained by making the low-reactance value of reactor ll still lower within the desirable range of the circulating current in Fig. 6.

If it is desired to proceed further with the tapchanging so as to operate on tap 6 alone, the steps leading to it are to close contactor I4, to open contactor 9, to disengage finger I from tap 5 (Fig. 8) and make it engage tap 6, and to close contactor 9, arriving at Fig. 9 in which both fingers engage tap 6, all three contactors are closed, and the load voltage corresponds to the voltage of tap 6.

It will be evident from the foregoing that the double-valued reactor-autotransformer makes two different circulating-current values possible, a low circulating current for continuous mid-tap operation, and a higher circulating current in preparation for the breaking of this mid-tap connection, with a low reactive drop for the load and a low recovery voltage for the interrupting contactor. As the higher circulating current is only momentary, its heating effect is negligible.

It will be observed that the invention not only assures the following benefits, namely, the contactor recovery voltages are lowered, any tap-totap short circuits during tap-changing operations avoided, the circulating currents in continuous operation in bridgin positions are reduced, and the fluctuations of load voltage due to reactive drops during tap changing are reduced, but also all these benefits are accomplished by a single compact and economical reactor-autotransformer unit to provide two diflerent and independent impedance values with the aid of a contactor.

While there has been shown and described a particular embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true-spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an alternating-current voltage-control under load system, in combination, a transformer winding having a plurality of taps, mean for selectively connecting said taps into a circuit under load, said means including impedance means, and means. for causing said impedance means to offer two different and independent substantial impedance values, namely, a high impedance value to circulating currents when in continuous operation in shunt connection across a pair of said taps, and a low impedance value preparatory to interrupting said shunt connection.

2. In an alternating-current tap-changing system for voltage control, the method of changing taps under load with switching mean and impedance means, said method being characterized by the mid-tap operation of the system continuously with a high value of said impedance connected in shunt across a pair of said taps, and changing to a lower finite value of said impedance before changing said shunt connection.

3. In combination, a transformer winding having a plurality of taps, a terminal for said winding, means including a pair of contactors for selectively connecting said terminal to various ones of said taps including the connection of said terminal simultaneously to electrically adjacent taps. an impedance connected to limit the circulating current through said mean when said terminal is connected to electrically adjacent taps. and means for reducing the effective value of said impedance to a relatively low but finite value prior to the opening of either of said contactor so as to reduce its recovery voltage during short circuit conditions of said winding.

4. In combination, a transformer winding having a plurality of taps, a terminal for said winding, means including a pair of contactors for selectively connecting said terminal to various ones of said taps including the connection of said terminal simultaneously to electrically adjacent taps, an impedance connected to limit the circulating current through said means when said terminal is connected to electrically adjacent taps, and means for reducing the effective value of said impedance to a relatively low but finite value substantially simultaneously with the opening of either of said contaotors so as to reduce its recovery voltage during short circuit conditions of said winding.

tablishing a bridzinz connection between a pair of electrically adjacent taps, limiting the circulating current in said bridging connection by means or an impedance oi relatively high value, reducing the value of said impedance to a relatively low but finite value, and substantially coincidentally opening said bridging connection.

ARAM BOYAJIAN.

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