US2312571A - Arrangement for reducing equalizing oscillations - Google Patents

Description

Mai-ch 2, 1943. H. MEYER 2,312,571

ARRANGEMENT FOR REDUCING EQUALIZING OSGILLATIONS Filed July 16, i941 Patented Mar. 2, 1943 ARRANGEMENT FOR REDUCING EQUALIZ- ING OSCILLATIONS Hans Meyer, Baden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie., Baden,

Switzerland,

Application July 16, 1941, Serial No. 402,713 In Switzerland December 24, 1940 11 Claims.-

In transformers where the star point is not grounded, either directly or indirectly it is known that equalizing oscillations are excited when multi-pole surges occur, Depending on the shape and duration of the surge the surge voltage at the star point can as a result of this oscillation process rise to twice its value at the ingoing terminals of the transformer. As a result of this phenomena the insulation of the Winding, particularly in the vicinity of the star point, is stressed to an undesirably high degree and any apparatus such as current transformers, are suppression coils and the like, which may be connected to the star point, must be provided with a stronger insulation to protect them against these equalizing voltages.

The prior attempts to limit the voltage rise at the ungrounded starpoint of a multiphase transformer have been based upon the idea of bleeding off the high frequency oscillating voltage to ground through a condenser, a seriesresonant circuit or a resistor. These expedients are not entirely satisfactory as the oscillation-limiting impedances were in shunt with are suppression coils, current transformers and other equipment that is normally connected between the transformer starpoint and ground. In general, it was not possible to design the oscillationlimiting impedance for optimum performance as such design would adversely affect the operating characteristics of the other equipment.

The present invention concerns an arrangement for reducing equalizing oscillations which occur with multi-pole surges, particularly star point oscillations in transformers, without the disadvantages which accompany the arrangements used hitherto. The invention is characterised by the feature that a composite impedanceis connected into a circuit of the of an embodiment of the invention as applied to a transformer having an ungrounded starconnected primary-winding; Fig. 2 is a similar circuit diagram of another embodiment in which the star point of the transformer primary is grounded; and Fig. 3 is a circuit diagram of an embodiment in which the surge reducing impedances are located in a measuring circuit.

Fig. 1 shows a transformer with the primary winding star connected. Multi-pole surges occur at the terminals u, '0, w of the primary windings and result in high frequency oscillatory Voltages at the neutral point that is not grounded but coupled to ground through the distributed capacity to ground, represented by the dotted line condenser C, of the transformer windings. The transformer secondary may be for instance a polygon-connected winding, and in the embodiment illustrated is a delta-connected winding into which the composite impedance Z, comprising an inductance or choke coil and resistance in parallel, is connected. This impedance is then according to the invention practically a short-circuit for currents of operating frequency. When multi-pole surges reach the primary winding, the induced high-frequency volt- 7 ages of the several phases of polygon arrangement add together and force the equalizing current over the impedance Z. This arrangement thus provides a very elfective damping of the equalizing oscillations due to the fact that the impedance is composed of partial impedances in such a manner that it has substantially the charactor of an ohmic resistance of suitable high magnitude for the equalizing oscillation frequency. This arrangement has the advantage, as compared with one where a corresponding number of damping impedances are associated with the phases, that the composite impedance Z has a negligible ohmic resistance at the frequency of the operating current, whereby the normal flow of power current established substantially no voltage drop across the terminals of the impedance. The impedance Z can of course also be divided up into a number of elements corresponding to the number of phases and connected alternately between adjacent. phase windings in order to obtain a symmetrical assembly.

Equalizing oscillations can be reduced very effectively or even completely suppressed when the equalizing voltages occurring at the impedance are utilized in the circuit between the star point and ground. According to the arrangement illustrated in Fig. 2 this is achieved by connecting both terminal points of the polygon, which is closed by the impedance Z directly or by means of additional impedances R, with the secondary winding of a transformer T located between the star point and ground in such a manner that when a voltage drop occurs at the impedance Z the voltage induced in the primary winding of the star-point transformer opposes the equalizing voltages due to the differential coupling. The impedance Z may be connected directly to the star-point transformer only if the secondary winding of the transformer can be short circuited in service. When the indirect connection is employed it is expedient to connect the damping part of the impedance Z in series with the secondary winding of the starpoint transformer, The impedance Z shown in Fig. 2 is then for instance a choke coil shunted by a condenser and resistance in series, and the secondary of the transformer T is connected across the composite impedance Z through the damping resistance R.

In order to increase the favourable effect of the differential coupling it is an advantage to select the ratio of the number of phase coils of the primary and polygon winding so as to be larger than that of the number of coils of the primary and secondary windings of the star-point transformer T used for the purpose of compensation.

The frequency dependent impedance Z can, however, also be employed directly to effect a reduction of the equalizing oscillations in the circuit between the star point and ground of the transformer arrangement. The impedance can be switched into this circuit or as shown in Fig. 3 connected to the terminals of the secondary winding of a star-point transformer and should be so dimensioned that its value is as high as possible for the operating frequency, but at the same time assumes the most favourable value as a damping resistance for the equalizing frequencies. The star-point transformer T can with advantage consist of an existing arc suppression coil or a voltage transformer used for measuring the voltage. In the case of the latter impedances a: or similar elements should be located in the measuring circuit as shown in Fig. 3 in order to render the circuit impassable for equalizing currents.

In order to achieve the best results the frequency dependent impedance Z should consist of resonance circuits and damping elements. Tests have shown, however, that particularly when the arrangement according to Fig. 1 is employed it is generally sufficientif the impedances are formed by a choke coil and a resistance connected in parallel. The choke coil should then be so dimensioned that it offers a negligible resistance to the operating currents, but at the same time such a large reactance for the higher frequency equalizing currents that these are forced to flow over the damping resistance connected in parallel. This difference in effect, which forms the basis of the present invention, can be still further improved, without any appreciable additional cost, if a condenser of suitable size is connected in series with the damping resistance.

The damping of the equalizing process determined mainly by the size of the ohmic resistance employed, should be so selected that the course of the equalizing currents for all loads between no-load and full load, that is at equalizing frequencies which differ slightly from each other, approaches the most favourable case of aperiodicity. The equalizing process is then maintained within controlled limits independent of the shape and duration of the surges which occur. By this means it is possible to dispense with a strong insulation for the primary winding, particularly in the vicinity of the star point or of any apparatus which may be connected to this point.

I claim:

1. A multiphase power transformer comprising a star connected primary winding having an ungrounded star point, secondary. windings, and means reducing the magnitude of high frequency oscillations arising from multiphase surges; said means comprising a composite frequency-variant impedance connected in circuit with one of said windings in the paths of power current and of the oscillating current, said impedance being of negligible magnitude at the frequency of the power current and of high magnitude at oscillation frequencies.

2. A multiphase power transformer as recited in claim 1, wherein said secondary windings are polygon connected and closed through said composite impedance.

3. A multiphase power transformer as recited in claim 1, wherein the secondary windings are three-phase and delta connected in series with said composite impedance. 1 4. A multiphase power transformer as claimed in claim 1, in combination with a circuit connected between said star point'and ground, and means coupling said circuit to said composite impedance to establish across the same an oscillatory voltage opposingthe equalizing oscillatory current flow through said composite impedance.

5. A multiphase power transformer comprising a star connected primary winding, a polygon connected secondary winding closed through a frequency-variant composite impedance, said impedance being of negligible magnitude at the power current frequency and of high magnitude at the frequency of equalizing oscillations arising from multiphase surges, and a transformer hav-" ing a primary winding connected between the star point of said first primary winding and ground, the secondary winding of said second transformer being connected to said impedance to establish across the same an oscillatory volt-'- age opposing the equalizing oscillatory current flow through said impedance.

6. A multiphase power transformer as recited in claim 5, wherein saidcomposite impedance comprises a choke coil in parallel with a resistin claim 5, wherein said second primary winding is an arc suppression coil.

10. A multiphase power transformer as recited in claim 5, wherein said second transformeris avoltage transformer.

11. A multiphase power transformer as recited in claim 5, wherein said second transformer is a voltage transformer, and the measuring circuit connected across the secondar winding thereof includes impedances suppressing the flow of oscillatory current in said measuring circuit.

HANS MEYER.

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