EP0078985A1

EP0078985A1 - Internal voltage grading and transient voltage protection for power transformer windings

Info

Publication number: EP0078985A1
Application number: EP82109918A
Authority: EP
Inventors: William James Mcnutt; Eugene Clemens Sakshaug
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1981-11-09
Filing date: 1982-10-27
Publication date: 1983-05-18
Also published as: JPS5895932A; ES516519A1

Abstract

A series string (17, 18) of plural zinc oxide varistor elements (17A-17N 18A-18N) is electrically connected across each winding (11, 12) of a power transformer, with interior winding taps (20, 22) being electrically connected to the junctions between varistor elements. Each varistor string, disposed within the transformer casing, protects its associated winding from voltage surges in the same manner as externally mounted lightning arresters, provides highly effective voltage grading, and suppresses harmful transient voltage oscillations between the winding taps.

Description

BACKGROUND OF THE INVENTION

Power transformer windings are subjected to a variety of potentially damaging dielectric stresses in the 'field. The most catastrophic, variety is the extreme transient voltage surges induced by lightning strikes on power lines. To protect power transformers and other electrical distribution equipment, externally mounted surge arresters are utilized to shunt the currents associated with these-surges to ground and thus suppress the surge voltage peaks to an arrester protective level which the insulation system of the power transformer can safely handle. Of course, this protective level is greater than the normal operating voltage.
The fact that these voltage surges, although effectively prevented from exceeding the arrester protection level, typically have extremely fast rise and fall times creates additional problems for the power transformer dielectric system. That is, the inherent capacitance between the transformer winding and ground and between portions of the winding itself produces a vastly unequal or nonlinear voltage distribution (grading) across the complete winding of these voltage surges or impulses. As is well understood in the art, a disproportionately large percentage of the surge voltage is dropped across a disproportionately small percentage of the transformer winding which is adjacent the winding termination connected with the transmission line. Thus, the winding turns adjacent the line end of the transformer winding require more insulation than do the turns adjacent the ground or neutral end of the winding to withstand the greater voltage stress. To improve the distribution of voltage surges across the transformer windings, it has been common practice to resort to various interlaced winding patterns with or without myriad arrangements of internal and external electrostatic shields calculated to modify the winding capacitance such as to achieve improved voltage grading. Commonly assigned U.S. Patent No. 4,153,891 discloses one such approach. These capactive grading arrangements, although reasonably effective, fall short of affording ideal voltage grading, i.e., a uniform or linear distribution of aperiodic wave forms, i.e., single pulse voltage surges, across the multitnrns of a power transformer winding.
Yet another problem to be considered is the fact that oscillatory electrical disturbances appearing on a transmission line, which may be produced by sources other than lightning strikes, excite transient voltage oscillations within the transformer winding itself. If the frequency of the oscillatory electrical disturbance happens to match the natural frequency of the transformer winding, the voltage peaks of the oscillation occurring within portions of the winding can build up to well in excess of the proportionate share of the applied voltage as determined by percentage turns. The fact that these transient oscillations may be excited by disturbances having voltage peaks below the arrester protective level makes this problem even more insidious. Heretofore, this problem has not been fully addressed, and thus suppression of internal oscillations have been largely left to the inherent damping afforded by the insulation resistance.
It has been recognized that discontinuities created by the presence of winding taps and their connected leads can create high amplitude voltage oscillations in response to voltage surges. To suppress these localized oscillations, silicon carbide varisrors, accommodated within the transformer tank, have been connected across these taps. This protects only the shunted portion of the transformer winding and the tap leads which are often brought out to terminations in closely spaced relation, not the complete winding. Complete winding protection has traditionally been left to the externally mounted surge arrester, as supplemented by the transformer insulation system and the electrostatic shielding and/or interleaved winding arrangements generally noted above.
It is accordingly an object of the present invention to provide a system for fully protecting inductive devices,such as power transformers and shunt reactors, from extreme voltage stress.
An additional object is to provide a protection system of the above character wherein transformer winding voltage stress is relieved on an incremental basis along the entire winding length.
Yet another object is to provide a protection system of the above character wherein voltage surge arrestment is achieved internally of a power transformer tank, thus eliminating the need for externally installed lightning arresters.
Still another object is to provide a protection system of the above character wherein effective voltage surge arrestment and improved voltage grading is afforded throughout each power transformer winding while, at the same time, voltage oscillations occurring within the windings are suppressed by safely absorbing the energy of any extreme oscillating voltage which could unduly stress the insulation of adjacent turns located anywhere along the transformer winding length.
Another object is to provide a protection system of the above character which is eminently effective and reliable in operation, is capable of long service life, and affords significant reductions in the dimensions of power transformer parts and thus a more compact power transformer unit.
Other objects of the present invention will in part be obvious and in part appear hereinafter.

SUMMARY OF THE INVENTION

The protection system of the present invention comprises the utilization of a series connected string of discrete metal oxide varistor elements, such as zinc oxide varistors, electrically connected from termination to termination across the.entire multiturn winding of electromagnetic inductive-devices, such as power transformers and shunt reactors. A particularly significant application of the present invention is to power transformers, in which case a separate varistor string is connected across one or more windings on the high and low voltage sides, as well as each phase winding if a multiphase power transformer is involved. As an important feature of the invention, the varistor strings are located within the transformer tank and since they perform the same effective voltage surge suppression as traditional externally installed lightning arresters, need for the latter is eliminated. Thus the obvious advantages of self-containment are achieved by virtue of the invention.
To afford further voltage surge protection for the power transformer windings, taps, distributed along the length of each winding, are brought out for separate electrical connection to the junctions between the varistor elements of the associated string. Thus the varistor string provides surge protection for the entire winding across which it is connected in the manner of an external lightning arrester, while the individual varistor elements of the string provide surge protection for those portions or turns of the windings across which they are respectively connected. Under these circumstan_Qes, the individual varistor elements act both in concert and on an individual basis to improve the voltage grading of the associated winding and to suppress transient oscillations within the associated winding itself. All of these benefits are achieved without resort to electrostatic shielding and exotic winding patterns to modify the winding capacitance.
The invention accordingly comprises the features of construction and combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.
For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic diagram of a power transformer equipped with the protection system of the present invention;
FIGURE 2 is a graph illustrating the improvement in:the initial surge voltage gradient across the transformer winding achieved by the present invention;
FIGURE 3 is a graph illustrating the improved transient voltage suppression within a power transformer winding achieved by the present invention; and
FIGURE 4 is a fragmentary schematic diagram of a disc wound power transformer winding utilizing the present invention.

DETAILED DESCRIPTION

The surge protection system of the present invention is illustrated in FIGURE 1 in its application to a power transformer, generally indicated at 10, comprising at least one multiturn high voltage winding 11, and at least one multiturn low voltage winding 12 arranged about a magnetic core 14. These windings may be of various configurations, e.g., layer windings, disc windings, etc. The line and ground or neutral terminals of the high and low voltage windings are indicated at Hl, H2, Ll, and L2, respectively. Located within the transformer tank (not shown) and electrically connected between the high voltage winding terminals H1, H2 is a series connected string, generally indicated at 17, of individual varistor elements 17A - 17N. Similarly, a series string; generally indicated at 18, of individual varistor elements 18A - 18N is connected between the low voltage winding terminals Ll, L2. At selected intervals along the high voltage winding length from its line end to its ground end, winding taps 20 are brought out for respective electrical connection to the junctions between varistor elements 17A - 17N. It is thus seen that varistor element 17A is individually connected across a portion or section 11A of the high voltage winding between its line end and the first tap 20 thereon. Varistor element 17B is individually electrically connected with the first and second taps across the next high voltage winding section 11_B, and so on. Similarly, the low voltage winding is provided with spaced taps 22 which are respectively connected to the junctions between the varistor elements 18A - 18N, and thus these varistor elements are seen to be individually connected across respective low voltage winding sections 12A - 12N.
Preferably, the varistors 17A - 17N, 18A- 18N are zinc oxide varistors having a highly nonlinear resistance characteristic and proven voltage surge suppression capabilities. A suitable zinc oxide varistor composition is disclosed in U.S. Patent No. 3,928,245. If transformer 10 utilizes an oil or gas dielectric coolant, the internally mounted zinc oxide varistor elements should include protective coatings, such as disclosed in commonly assigned U.S. patent application Serial No. 161,935, filed June 23, 1980.
From FIGURE 1, it will be appreciated that the varistor strings 17 and 18 serve to shunt to ground the excessive currents associated with lightning- induced, high-magnitude voltage surges arriving at winding terminals H1, Ll. Consequently, externally installed lightning arresters to protect power transformer windings equipped with the internal varistor strings of the present invention become redundant and therefore can be eliminated. The additional benefits derived from the present invention can be appreciated from the fact that the varistor elements provide individualized surge suppression for the specific winding portions or sections across which they are respectively electrically connected. This means that suppression is made available for the severe voltage stresses normally experienced by the winding sections near the line end of the transformer .winding due to the fact that a disproportionately large percentage of the initial voltage surge, albeit clipped to the arrester protection level established by the varistor strings 17, 18, is dropped across a disproportionately small percentage of the total winding adjacent the line end. By virtue of the present invention, the varistor elements protecting those winding sections adjacent the winding line terminals H1, LI act to individually suppress the magnitude of the voltage oscillations appearing thereacross to readily manageable, safe levels. Consequently, distribution of the voltage across the total winding is made more uniform, i.e., vastly improved voltage grading is achieved. It will be noted that resistive grading is achieved by the present invention, as contrasted to the capacitive grading approaches of the prior art.
Yet another benefit achieved by the present invention is the ability of the individual,varistor elements to suppress any extreme transient voltage oscillations that are excited in the transformer windings by aperiodic voltage surges or by high frequency oscillatory electrical disturbance on the transmission lines. Regardless of which winding section 11A - llN, 12A - 12N happens to be subjected to an extreme voltage peak, there is a discrete varistor element standing by to instantly suppress the voltage oscillation to a safe level.
The benefits of the present invention discussed above can also be appreciated by reference to the graphical presentations of FIGURE 2 which plot the initial voltage distribution along a power transformer winding 11 or 12 of a voltage surge arriving at the line terminals thereof. Curve 26 depicts a typical voltage gradient for a transformer winding lacking any voltage grading improvement means. It is seen that fifty per cent of a typical voltage surge is dropped across only the first fifteen per cent of the total winding measured from the line end. Curve 28 is a straight line representing an idealized linear surge voltage distribution, i.e., perfect voltage grading. Curve 30 represents a typical surge voltage distribution achieved by the resistive grading approach of the present invention, which, although not perfect, is a vast improvement over the voltage distribution represented by curve 26.
FIGURE 3 depicts a series of profile curves depicting the maximum voltage differential, in terms of percentage of winding terminal voltage which can typically appear across consecutive winding section pairs progressing along the winding length starting from the line end for a worst-case transient voltage oscillation excited within the winding. The data for plotting these curves was obtained from a 500 kV transformer high voltage winding of a plain disc configuration depicted schematically in FIGURE 4, having sixty disc winding sections indicated at 11 - 1 through 11 - 60. Curve 32 is a profile of the consecutive section pair voltages of the disc winding without transient suppression, while curve 34 is a-section pair voltage profile of the same winding equipped with internal shielding. The humps in these curves occurring adjacent the ground end of the winding are produced by voltage reflections. Curve 36 of FIGURE 3 is a profile of the section pair voltage differential achieved.with the individual zinc oxide varistor elements 17 of the string tapped across every six disc winding sections, i.e., shunting sections 11 - 1 through 11 - 6, 11 - 7 through 11 - 12, etc. as indicated in solid line. The jaggedness of this curve indicates that oscillations in the shunted winding sections do occur to some extent; however, the magnitudes of the voltage differentials all along the winding are consistently suppressed to significantly lower levels than those illustrated by curves 32 and 34. If varistor elements, indicated in phantom at 17a, are connected across every two disc winding sections, i.e., every section pair, the profile curve 38 is obtained, which is seen to closely approach the ideal turns ratio voltage differential profile represented by the dashed, straight-line curve 40. There is no significant advantage in connecting the varistor elements across each individual section, since there is considerable spacing and thus relatively low dielectric stress between the innermost and outermost turns of a section. In contrast, due to the contiguous relationship of, for example, the outermost turns of sections 11 - 1 and 11 - 2 and the voltage therebetween being the drop across these two sections in their entirety, the dielectric stress on the insulation of these turns can be extremely severe under transient voltage conditions. By virtue of the present invention, this dielectric stress is dramatically relieved.
It will be appreciated that the increments or portions of the transformer winding shunted by each varistor element of the string need not be uniform. In fact, since, as mentioned above and shown by curve 32 of FIGURE 3, the inherent capacitive grading of a transformer winding causes a disproportionately large percentage of a voltage surge to be dropped across those winding sections nearest the line end, it would be appropriate-to connect the varistor element at the line end of the string across smaller increments of winding than the varistor elements further down in the string. Thus, the number of varistor elements in a string, their shunt connection tap locations along the winding, and the ratings of the varisto= elements themselves are determined in part by the transient section-to-section voltage differential profile, e.g., curve 32 of FIGURE 3, of the particular transformer winding involved, taking into consideration the requisite degree of winding insulation and spacings to accommodate normal operation conditions. Also, if the power transformer winding is equipped with voltage regulating taps, they should be shunted by individual varistor elements of the string.
The fact that the present invention provides such complete voltage surge and transient suppression along the full winding length, the amount of winding insulation, particularly for the turns adjacent the line end can be significantly reduced from that necessary heretofore. Moreover, the spacings between winding turns of adjacent sections and between the windings and other parts of the transformer can be reduced, thus providing for a more compact power transformer than has heretofore been possible.
While the foregoing description has dealt with the protection of windings whose terminations are connected from line to neutral or ground, e.g., a single phase power transformer, wye-connected power transformer windings, shunt reactor, etc., it should be understood that the present invention may be applied to windings terminated line to line, as in the case of delta-connected power transformers. A varistor string is connected across the winding terminations and into the winding taps as described above to provide internal voltage oscillation suppression. Additional varistors are then connected from the winding line terminations to ground to divert surge energy safely to ground. These additional varistors may be of the externally mounted, lightning or surge arrester type. This approach would also be applied to windings of transformers incorporated in ungrounded distribution systems.
Moreover, the present invention contemplates that the single varistor string herein shown shunting a transformer winding may be paralleled with additional varistor strings for increased energy absorbing capacity. In this case, the various winding taps are jointly connected to corresponding junctions in each string. Also, rather than a single varistor element connected between consecutive winding taps, the various winding portions or sections may be shunted by plural, series-connected varistor elements, again for increased energy absorbing capacity.
It will thus be seen that the objects set forth above and those made apparent from the foregoing description are efficiently attained, and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

Claims

1. A system for providing voltage surge protection and voltage oscillation suppression for an electromagnetic device including a multiturn winding (11) arranged about a magnetic core (14); said system including:

A: a series string (17) of individual varistor elements (11A - 11N) connected across the terminations (H1, H2) of the winding (11) in shunt relation with the complete winding;

B: a plurality of taps (20) provided on the winding at locations spaced along the length thereof; and

C: separate electrical conductors respectively connecting said taps (20) to different junctions between said varistor elements (11A - 11N) of said string.

2. The system defined in claim 1, wherein said varistor elements (11A - 11N) are zinc oxide varistors.

3. The system defined in claim 1, wherein the electromagnetic device is a transformer (10) having at least one high voltage winding (11) and at least on low voltage winding (12) arranged about the magnetic core, said system further including a separate series string (17, 18) of individual varistor elements (17A - 17N; 18A - 18N) connected across the terminations of each of the high and low voltage windings, said plurality of taps (20, 22) being provided on both windings (17, 18) at locations spaced along the lengths thereof, and said conductors respectively connecting said taps (20, 22) of each winding (17, 18) to different junctions between said varistor elements of the one of said strings shunting the associated winding.

4. The system defined in claim 3, wherein said varistor elements (17A - 17N; 18A - 18N) of said strings are zinc oxide varistors.

5. The system defined in claim 1, 2 or 3, wherein the spacings between said winding taps (20, 22) are varied in accordance with a predetermined voltage differential profile of the associated winding (11, 12).