US3039041A - Load transfer switch with non-linear switching resistors - Google Patents

Description

June 12, 1962 B. JANSEN 3,039,041

LOAD TRANSFER SWITCH WITH NON-LINEAR SWITCHING RESISTORS Filed Sept. 24, 1957 2 Sheets-Sheet 1 NON LINEAR RES/STOPS A IVO/V 04 54 mas/s70)? 2 /N VEN 7'01 Bernhurd JANSE N United States Patent 1 3,039,041 LOAD TRANSFER SWITCH WITH NON-LINEAR SWITCHING RESISTORS Bernhard Jansen, Prufeningerstr. 20,

Regensburg, Germany Filed Sept. 24, 1957, Ser. No. 685,941 Claims priority, application Germany Sept. 28, 1956 11 Claims. (Cl. 323-435) The present invention relates to load transfer switches for power distribution transformers, the switching device including switching resistors each of which includes at least one resistor of constant resistance and one nonlinear resistor the resistance of which decreases with increasing current flow therethrough.

In most cases transfer switches for changing the taps of a transformer on load consist of a number of pairs of main and auxiliary contacts with interposed ohmic resistors (so-called switching resistors) which, due to the alternate closure and opening of the main and auxiliary contact pairs, are briefly connected into the external current circuit of the tap change transformer. During the transfer of the load from one transformer tap to another the switching resistors are designed to prevent an interruption in the working current and also to prevent a short-circuit between the transformer taps which may be simultaneously active for a brief period.

The resistors should be so dimensioned that during load transfer the working current should be fed to the consumer network with the minimum voltage drop, whilst simultaneously the balancing current prevailing between two adjacent taps on the transformer should be limited to a tolerable amount. For the first purpose the'switchi'ng resistors should have an ohmic resistance which is as low as possible, and in the second case as high as possible, Due to these conflicting requirements a compromise with regard to the magnitude of the resistance value is necessary in all cases. In addition, a third requirement is that when connecting the switching resistors in the path of the systern current and the balancing current the switching cur-. 4

rents and switching voltages (recovery voltages) prevailing, at the main and auxiliary contacts employed for this purpose should be as low as possible, because their magnitudeespecially in the case of extra high voltage trans-.. formers-to a large extent governs the dimensions of the transfer switch.

If a certain size of transfer switch and switching resistors has been decided upon then currents have to be interrupted during the transfer of load which in some switching positions are governed solely by the prevailing system current, and in other switching positions solely by the stage voltage between two transformer taps. In intermediate switching positions the switching currents are governod both by the system current and by the stage voltage. The previously mentioned recovery voltage which apart from the switching current governs arc quenching is, apart from the system current and stage voltage, also governed by the resistance value of the switching resistors.

If the transfer switch is limited solely to the normal operating conditions of tap changing taps at the transformer operating at normal voltages and currents, and if special protective gear is installed to cope with abnormal operating conditions such as the occurrence of surge waves and load transfer during a short-circuit, then in spite of the fact that they have conflicting influences on the required resistance value, the initially mentioned functions required of the transfer switch do lead to economically tolerable designs. If however it is required that a transfer switch should deal with high operating voltages and currents, surge waves and short-circuit currents, the principles previously employed for the construction of transfer switches result in disproportionately large dimensions and ,high costs.

This invention is intended to provide a better and more economical method of overcoming these difliculties. In accordance with the invention the ohmic switching resistances employed for changing the taps transformer during operation are so designed that their resistance varies with the cur-rent flowing and/or the applied voltage. By this means the switching currents and the switching voltages prevailing during normal transformer operation can be reduced to lower values than previously. Especially when carrying out tap changing under over-current conditions (in an extreme case during short-circuit) it is possible for the switching voltages prevailing at the switching contacts to be decisively reduced. In addition voltage surge waves can be reduced to a value which is harmless to the transfer switch.

Other objects and advantages will hereinafter appear.

In the drawing forming part of this specification:

FIG. 1 is a diagrammatic view of one form of generator tap transfer switch employing tap resistors which employ variable resistance elements;

FIG. 2. is similar to but less complete than FIG. 1, and particularly shows a tap resistor which combines fixed and variable resistance elements in both series and parallel relationships;

FIGS. 3 and 4 are diagrammatic views which show additional forms of tap resistors which involve both fixed and variable resistance elements in other combinations; and

FIGS. 5 and 6 are diagrammatic views similar to FIG. 1 but showing further modified forms of tap resistors in which elements of fixed and variable resistance are combined.

In FIG. 1 we have shown the general application of variable switching resistors in conjunction with a transfer switch contact system having two main contacts Ha, Hb and two auxiliary contacts hm, hb, all spaced from each other. The contacts are connected with the transformer taps A, B directly via the switching resistors Ra, Rb. The current is tapped off at contact C through a sector contact K. The arrow passing obliquely through the switching resistors Ra, Rb indicates that the resistance varies with current change therethrough. The sector contact K has a curved side whose length exceeds the spacing of each two adjacent contacts Ha, ha, hb, and Hb. Contact C is located at the apex of contact K and serves as axis of rotation.

Main contact disconnection: In the first switch position shown in FIG. 1, in which the center radius of sector contact K extends in the direction of the dotted arrow at point 1, the current passes from top A through the main contact Ha and the sector contact K to the tap C and then on into the system. The leading radial edge of contact K is just past point 2. When the load is transferred from tap A to tap B the auxiliary contact ha is first connected up by turning sector contact K to the left so that at point 3 there is a second path for the current from A through Ra, ha, K to C (second switch position). The transformer loading only proceeds along this second current path when the main contact Ha which is directly connected with the tap A is opened. When the contact K moves further to the left (counterclockwise in the drawing) to point 4 an arc is drawn at the main contact Ha, whilst the auxiliary contact he remains closed. As the arc resistance increases at Ha an increasing proportion of the In the equation, and c are calculated constants, R is resistance, I is current, and U is voltage.

Inthe case exemplified there will, with an 8-fold inc-rease in system current In, not be an 8-fold increase in recovery voltage as with constant-value switching resistors, but only a 2-fold increase, because 1 is proportional to U. Since the dimensions of transfer switches depend more on the recovery voltage than on the magnitude of the switching current, the use of switching resistors which vary in accordance with the current flowing offers special advantages for this initial circuit breaking operation at the main contact Ha. The degree of reliability of this disconnection at Hot comes into account mainly however with overload switching (in extreme cases during shortcircuits).

Disconnection of auxiliary contact: when the sector contact K moves further from third switch position at point 4 to fourth switch position at point 5 the second auxiliary contact hb is closed, thus giving rise to a third current path from tap B through Rb, hb, K to C. At this moment the system current In will be divided between the two taps A and B in a proportion governed by the instantaneous values of the switching resistors Ra and Rb. However another factor affects the magnitude of these two divided currents, because simultaneously with the formation of the previously mentioned third current path, there is yeta fourth current path from A through Ra, ha, hb, Rb to B. The difference in voltage between A and B (stage voltage Uab') causes a balancing current I ab to flow along this fourth path which is governed by the sum of the instantaneous resistance values Ra-l-Rb. This balancing current as distinct from the previously mentioned divided currents, is independent of the system current In. This balancing current Iab' which is not desired flows even when tap changing is carried out with the transformer on noload. Where the switching resistors are normally of equal size and constant i.e. Ra=Rb=R, the magnitude of the balancing current is the same at both ta-ps A and B. This condition which prevails on no-load changes when the transformer is loaded with a system current which increases from 0 to In. Due to the equal size of the switching resistors the two taps A and B each receive half of In. As a result there is also an increase in the divisional current In coming from Ayfrom I ab to Jab+ whilst the divisional current I b which comes from B is reduced to reduction side (tap B). As a result the total resistance Ra-l-Rb increased and this greatly reduces the undesiredbalancing current I ab, which .is governed only by the constant stage voltage Uab on. the one hand and the total of the resistances Ra-l-Rb on the other. Consequently. there will also be a reduction in the switching current In at the auxiliary contact ha. In addition to this favourable effeet the recovery switching voltage at auxiliary contact ha is reduced when contact K reaches the fifth switch position at point 6. In this switching position the system current In flows completely from tap B through Rb, hb, K to C,

The recovery voltage which consequently prevails between the contact segment K and the auxiliary contact ha is ,now equal to the sum of the voltage drop a-t Rb and the stage voltage Ua'b. Whilst the latter is constant, the voltage drop at Rb is, by employing avariable resistor as described above, severely reduced in the same manner as already described for disconnection o-f the main contact. Thus the same considerations and conclusions concerning the suitability of the switching resistors in accordance with the invention also apply for the disconnectionof the auxiliary contacts described here. For the sake of completeness only it should be mentioned that the further rotation of contact K into sixth position at point 7 and beyond results in the direct connection of the main contact Hb, and the brief shunting of the switching resistor Rb. The current now flows from the new tap B through Hb, K to C. By this means the loa-d transfer process is concluded.

In the previous description of FIG. 1 the variable nature of the switching values of the two switching resistances Ra and Rb was indicated merely by way of an example, without going into detail concerning the characteristics of such variability. In a practical design attempts will be made to match the resistance characteristic to conditions prevailing. This can be achieved simply by combining resistance elements having constant and variable resistance values whichcan be selected independently of each other. The various elements can also be given different variation characteristics and either connected in series or parallel. As an example a constant-value resistor which is connected in series with a variablevalue resistor will result in a minimum resistance limit; 'A parallel-connected constant value resistor which is connected in parallel with a variable value resistor will resultin a maximum resistance limit. Similarly the parallel-connection of two variable resistors with diiferent characteristic curves will result in an increase in the low resistance value and a reduction in the high resistance value. FIG. -2 illustrates a general example for such a combination. For the sake of simplicity this shows only the upper half of the transfer switch device. The rest can be readily visualized by analogy to FIG. 1, the tap B connections forming a mirror image of the tap A connections. Here the variable resistor Rwl is first'connected in parallel with a constant resistor R112. This partial co-mbination first results in a maximum limit on the variable resistance value. Apart from this however these is also a further variable value resistor with a different variable resistance characteristic, R004 which is connected in series with a constant value resistor R03 which in turn imposes a minimum limit on'the variable resistance values. Resistors Ra3, Ra4 are additionally connected in parallel as a third unit with the two above-mentioned parallel resistors. With this general arrangement it is possible for all characteristic or absolute values of the resistors Ral Ra l to be so arranged that the resistance combination most suitable for the particular system involved is obtained.

In practice it will usually suffice to employ variable sisters to secure more favourable conditions concerning spare part stocks and the spatial layout of the switching resistors in the transfer switch. FIGS. 3 and 4 show embodiments which permit standard-unit design.

FIG. 3 shows tapping A connections only, the arrangement comprising a fixed resistance Ra2 in parallel with a plurality of resistance units, each comprising a fixed resistance Ra3 and a variable resistance Ro4 in series with one another. This arrangement would be duplicated for tapping B.

FIG. 4 also shows tapping A connections only, the arrangement comprising a fixed resistance Ra3 in series with a succession of circuit loops, each including a variable resistance Rail in parallel with a fixed resistance Ra2.

All the combinations shown in FIGS. 2-4 of various resistors to form a single variable switching resistor with a predetermined characteristic envisage that the various elements are constantly interconnected. With difficult cases involving high powers it is however desirable during load transfer not merely to have two similar, or in some cases combined switching resistors, as shown in FIGS. 1 and 2, but to have recourse to more complex arrangements like those of FIGS. 5 and 6. The description of the stresses during disconnection on the main contact Ho and on the auxiliary contact ha in FIG. 1 has already indicated that in the various witching positions diiferent problems have to be overcome concerning the reduction in switching current and the recovery switching voltage. This applies to an even higher degree if transfer switches with more than two disconnections per load trans-fer operation are to be equipped with such variable switching resistors. The combination in accordance with FIG. 5 will serve as a simple example. Here in the second, fourth and sixth switch positions, the resistance combinations Ral-l-RaZ with Ra3+Ra4, Ra3-l-Ra4 with Rb-3 -i-Rb4, Rb3 +Rb4 with Rb1+Rb2 are made during one load transfer operation one after the other by the auxiliary contacts ha l, ha2, 11b2, hbrl at points .1, 3, 5, 7. Due to the interim connection of three auxiliary contacts ha1+ha2+hb2 at point 5 in third switch position and ha2+hb2+hb1 at point 7 in fifth switch position, this being possible in the transfer switch shown owing to the contact overlap of wide angle sector contact K, we obtain two further combinations Ral Ra4 with Rb3+Rb4 and Ra3+Ra4 with Rb=1 R124. If we compare the resistance combination Ral Ra4 in FIG. 5 with the resistance combination in 'FIG. 2, we see that the only difference lies in the separation of the parallel-connected group Rm1+Ra 2 and the series-connected group Ra3+Ra4 by connection at two auxiliary contacts hal and ha2 (FIG. 5) instead of at only one auxiliary contact ha ('FIG. 2.) Thus by doubling the number of auxiliary contacts it has become possible, for the same expenditure on resistors, to obtain different parallel and series combinations during the load transfer process by means of the intermittent interconnection of several auxiliary contacts. It is obvious that by this means it is possible to adapt the characteristics and the absolute values of the resistance combinations to the appropriate switching purposes at the individual main or auxiliary contact.

In the arrangement shown in FIG. 5 these different combinations were obtained by the positive sequence of main and auxiliary contact interconnections with the aid of the sector contact K. However in load transfer switch practice there are also cases in which it is desirable to make such resistance combinations only in special operating cases, such as overcurrents due to shortcircuit and overvoltage due to lightning stroke, which do not occur in normal operation. This can be obtained by installing additional automatic-acting switching elements. In FIG. 6 we have shown one embodiment in which, in the event of a surge voltage wave, the spark gaps fab, fw, fb are actuated and cause the parallel or series connection of the switching resistors Ral or Rbl with the constant value resistors RaZ or Rb2. When the surge has died away the 6 spark gaps are again quenched and the normal load transfer process makes use in the normal way solely of the switching resistors Ra2 and Rb2. These spar-k gaps can also respond in the event of an overcurrent, because the overcurrent produces a voltage drop in the constant value resistors Ra l or Rb2 which supplies the corresponding striking voltage at the spark gaps. As a result the variable resistors Ral or Rbl are connected in parallel and this results in the extremely marked reduction in the resistance of the combination, already discussed in connection with FIG. 1, which also reduces the recovery switching voltage at contacts Ha or ha to a value which can more easily be dealth with by the transfer switch.

What I claim is:

l. A switching circuit for selective connecting one of two taps of a power supply device to an external circuit, comprising a plurality of fixed contacts disposed in a spaced arcuate array, said array including two main end contacts and a plurality of auxiliary contacts disposed between the two end contacts, one of the auxiliary contacts being connected via a purely ohmic resistive variable resistance circuit to one of the main end contacts, another of the auxiliary contacts being connected via another purely ohmic resistive variable resistance circuit to the other main end contact, each of said variable resistance circuits including a resistor of constant ohmic resistance, and a none-resistive rotatable contact having a curved end contact portion movable in contact with said fixed contacts, said contact portion being longer than the spacing between each two adjacent fixed contacts so that said contact portion contacts at least two of said fixed contacts in all switching positions thereof between the end contacts, there being additional purely ohmic resistive variable resistance circuits respectively connected between further ones of said auxiliary contacts and said end contacts, said additional circuits being respectively connected in parallel to respective ones of the first named variable resistance circuits when said contact portion simultaneously contacts one of the firs-t named auxiliary contacts and one of said further auxiliary contacts.

2. A switching circuit according to claim 1 wherein each of said variable resistance circuits comprises a resistor the resistance of which decreases with increasing current flow therethrough.

3. A switching circuit according to claim 2, further comprising spark gap means connecting each resistor of constant ohmic resistance into the variable resistance circuit in which it is included.

4. A switching device for changing transformer taps under load, comprising first and second main contacts connected, respectively, to two transformer taps, a movable switching member separately engageable with either of said main contacts, first and second intermediate contacts successively engageable by said switching member during its movement from engagement with said first main contact into engagement with said second main contact, said switching member, during said movement, establishing contact with said first main contact, said first intermediate contact, said second intermediate contact and said second main contact with make before-break sequence in the order named and in the inverse order during return movement, a firs-t resistance means connected between said first main contact and said'first intermediate contact and a second resistance means connected between said second main contact and said second intermediate contact, both of said resistance means comprising a first resistor the resistance of which decreases with increasing current flow therethrough and a second resistor of constant resistance.

5. A device according to claim 4 wherein in each resistance means, said resistors are connected in series.

6. A device according to claim 4, wherein in each resistance means, said resistors are connected in parallel.

7. A device according to claim 4, comprising, within each resistance means, a second resistor and at least one 7 further first resistor and one further second resistor, said further resistors being connected in parallel.

8. A device according to claim 4, comprising, within each resistance means, asecond resistor, at least one further first resistor and at least one further second resistor, said further resistors being connected in series.

9.- A device according to claim 4, further comprising spark gap means interconnecting said first and second resistors.

10. A device for changing transformer taps under load, comprising, a first main contact connected to a first transformer tap, a second main contact connected to a second trans-former tap, a movable switching member selectively engageable only separately with either of said main contacts, at least four intermediate contacts arranged for sequential engagement by said switching member during its movement from engagement with one of said main contacts into engagement with'the other main contact, afirst resistance means connected from a first intermediate contact to said first main contact, a second resistance means connected between a second one of said intermediate contacts and said first main contact, a third resistance means connected between a third one of said intermediate contacts and said second main contact, and a fourth resistance meansconnected between a fourth one of said intermediate contacts and said second main contact, said switching member during movement between engagement with said first main contact and said second main contact engaging successively said first, second, third and fourth intermediate cont-acts in the order named with make-beforebreak sequence and in the reverse order during return movement, each of said resistance means including a resistor of constant resistance and a resistor the resistance of which decreases with increasing current flow thcrethrough.

11. A device according to claim 10, wherein said switching member at all times simultaneously engages at least two of said contacts.

References Cited in the file of this patent UNITED STATES PATENTS 745,379 Pearson etal. Dec. 1, 190 3 20 2,063,693 McCarty Dec. 8, 1936 2,138,652 Biermanns Nov. 29, 1938 2,276,855 Meador Mar. 17, 1942 2,435,438' Fowler Feb. 3, 1948 2,680,790 Jansen June 8, 1954

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