SURGE ARRESTERS FOR POWER TRANSFORMER

Abstract

 The present disclosure relates to a surge arrester arrangement for a phase leg winding (2) of a transformer (1). The surge arrester arrangement comprises a plurality of series connected surge arresters (3) comprising at least a first surge arrester (3a) and a second surge arrester (3b) configured to be connected across the winding from a first end (2a) of the winding to a second end (2b) of the winding. The arrangement also comprises an intermediate connection (4b), between the first and second surge arresters and configured to connect to the winding such that the first surge arrester is connected across a first section of the winding formed between the first end and the intermediate connection and the second surge arrester is connected across a second section of the winding formed between the second end and the intermediate connection. The arrangement also comprises a plurality of bushings (6) configured to allow connections (4) to the series connected surge arresters to pass through a wall of a transformer tank (5) containing the phase leg winding.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a surge arrester arrangement for a phase leg winding of a transformer, as well as to a transformer arrangement comprising the surge arrester arrangement and the transformer.

BACKGROUND

[0002] There exist different ways of protecting transformers against over-voltages, for example using surge arresters and/or resistor-capacitor (RC) protection. Transformers are designed to withstand certain electrical stresses. In order to limit the voltages that a transformer is exposed to, surge arresters may be installed. These limit the voltage between their two connection points. Most commonly, surge arresters are installed phase-to-ground and thus limit the corresponding voltage. They may also be installed phase-to-phase (in case of a Delta connected winding) or phase-to-neutral (in case of a Y-connected winding) and will then also limit the voltage across the winding. Still however, the internal resonance frequency of the winding may be excited and potentially contribute to significant stress.

[0003] It has been proposed to improve damping of internal resonance in dry transformers by splitting surge arresters connected across a phase winding in two, with a connection to the midpoint of the transformer winding. This is explored in relation to transient over-voltages due to switching of vacuum circuit breakers in the Institute of Electrical and Electronics Engineers (IEEE) documents "Efficiency of surge arresters as protective devices against circuit breaker induced overvoltages" and "Effect of different types of over voltage productive devices against vacuum circuit breaker induced transients in cable systems" by E. Lindell and L. Liljestrand.

[0004] It is also known to use varistor surge arresters connected across sections of a winding to different taps of the winding in a liquid-filled transformer.

[0005] US 4,604,673 discloses a two-part varistor element mounted inside an oil-filled tank of a transformer. The midpoint of the varistor element is connected to the midpoint of the high-voltage winding of the transformer.

[0006] EP 0 078 985 discloses a string of varistor elements located within the transformer tank and connected across each transformer winding to interior winding taps.

SUMMARY

[0007] A protection which is based on surge arresters may limit the voltages phase-to-ground and phase-to-phase. The improved protection in accordance with the present invention also limits the internal resonance of the transformer winding by means of connecting the surge arresters also to the middle, or other point, of the phase winding. By the surge arresters being positioned outside of the transformer tank, they are e.g. not affected by hot transformer oil (or other insulation fluid) and are more easily accessible for maintenance etc.

[0008] According to an aspect of the present invention, there is provided a surge arrester arrangement for a phase leg winding of a transformer. The surge arrester arrangement comprises a plurality of series connected surge arresters comprising at least a first surge arrester and a second surge arrester configured to be connected across the winding from a first end of the winding to a second end of the winding. The arrangement also comprises an intermediate connection, between the first and second surge arresters and configured to connect to the winding such that a the first surge arrester is connected across a first section of the winding formed between the first end and the intermediate connection and the second surge arrester is connected across a second section of the winding formed between the second end and the intermediate connection. The arrangement also comprises a plurality of bushings configured to allow connections to the series connected surge arresters to pass through a wall of a transformer tank containing the phase leg winding.

[0009] According to another aspect of the present invention, there is provided a transformer arrangement comprising the surge arrester arrangement of any preceding claim, and the transformer comprising the phase leg winding.

[0010] A benefit of the novel surge arrester arrangement, which includes a connection point between at least two surge arresters connected in series with each other across (i.e. to and in parallel with) the transformer winding and the other connection point within the winding, e.g. at the midpoint of the winding, is that it limits the internal resonance of the winding. In accordance with the present invention, a modification of the transformer is done in order to allow the surge arresters to be located outside of the transformer tank. This modification includes installing a plurality of bushings which enable access to the windings, e.g. to the terminals of the transformer as well as to the midpoint or other point(s) within the windings, from outside the tank. For delta connected three-phase transformer windings, three bushings may be installed in addition to the three bushings for the terminals, for a total of six bushings. In case of wye (Y) connected three-phase windings, a further additional bushing may be installed to also enable access to the neutral point from outside the tank, for a total of seven bushings.

[0011] By means of the bushings, the surge arrester arrangement may be installed to liquid filled transformers, e.g. distribution or high-voltage transformers (or any transformers enclosed in a tank), but remain outside of the tank.

[0012] It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

[0013] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of "first", "second" etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a schematic circuit diagram of an embodiment of a surge arrester arrangement connected to transformer windings in a delta configuration, in accordance with the present invention.

Fig 2 is a schematic circuit diagram of an embodiment of a surge arrester arrangement connected to transformer windings in a wye (Y) configuration, in accordance with the present invention.

Fig 3 is a schematic side view of an embodiment of a transformer arrangement with a surge arrester arrangement outside of a transformer tank, in accordance with the present invention.

DETAILED DESCRIPTION

[0015] Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description. Embodiments of the surge arrester arrangement, and the transformer arrangement comprising the surge arrester arrangement, of the present invention will now be discussed with reference to figures 1 and 2, where figure 1 illustrates an embodiment of a three-phase transformer 1 in delta configuration and figure 2 illustrates an embodiment of a three-phase transformer 1 in wye configuration. When connections are mentioned herein, electrical, e.g. galvanic, connections are intended.

[0016] The transformer 1 comprises three phase legs, each comprising a winding 2 having a first end 2a and a second end 2b. Across each phase leg winding 2, a respective series of surge arresters 3, here surge arresters 3a and 3b, are connected. Thus, the series of surge arresters is connected to the winding 2 at its first end 2a (by means of the first connection 4a) and its second end 2b (by means of the third connection 4c) such that at least a first surge arrester 3a and a second surge arrester 3b are connected in parallel with the winding 2. However, any number of at least two surge arresters 3 may be connected in series in parallel with the winding 2.

[0017] In accordance with the present invention, for each phase leg, there is an intermediate connection from between the first and second surge arrestors 3a and 3b to a point within the winding 2 e.g. to a winding tap of the winding. The connection within the winding 2 may be at the middle of the winding, to form two equally large sections (first and second sections) of the winding. Alternatively, the connection within the winding 2 may be such that the first and second winding sections are of different sizes. By means of the intermediate connection 4b, the first surge arrester 3a is connected across (to and in parallel with) the first section of the winding 2, and the second surge arrester 3b is connected across (to and in parallel with) the second section of the winding, for each phase leg winding respectively. In the embodiments of figures 1 and 2, each phase leg winding 2 is sectioned into two sections, each with a corresponding surge arrester 3 connected across it. However, each one of the phase leg windings 2 may be sectioned into any number of at least two sections by means of additional intermediate connections (not shown), where each section has a corresponding surge arrester of the series of surge arresters connected across it.

[0018] In addition to the series connected surge arresters 3a and 3b connected in parallel with the winding 2 of each phase leg, the plurality of series connected surge arresters may comprise a third surge arrester 3c which is connected between the winding and ground, as shown in the figures 1 and 2. The embodiments of the surge arrester arrangement shown in figures 1 and 2 thus each comprises nine surge arresters, three per phase of which two are connected in series with each other and in parallel with the winding 2 of the phase leg and one is connected between said winding 2 and ground. The surge arrester 3c connected between the winding and ground is typically connected between the terminal end (i.e. the second end 2b) of the winding and ground, but it is also conceivable to in some embodiments connect a surge arrester between the neutral point (i.e. the first end 2a) of the winding and ground.

[0019] Each surge arrester 3 may comprise one or several varistor(s).

[0020] Figure 3 schematically illustrates an embodiment of a transformer arrangement comprising a surge arrester arrangement connected to a transformer 1 in a transformer tank 5 via connections 4 in bushings 6 through a wall of the tank 5. For simplifying the figure, surge arresters 3 for only one phase leg winding 2 are shown. However, a corresponding set-up may be used for any number of phase leg windings 2, with corresponding connections 4 in additional bushings 6, or possibly via the same bushings shown in the figure for the one phase leg winding.

[0021] A first surge arrester 3a is connected across a first section of the winding 2 by means of a first connection 4a connected to the first end 2a of the winding and the intermediate connection 4b connected to (e.g. a tap) within the winding 2 e.g. at a midpoint thereof. Similarly, a second surge arrester 3b, connected in series with the first surge arrester 3a, is connected across a second section of the winding 2 by means of a third connection 4c connected to the second end 2b of the winding and the intermediate connection 4b connected to (e.g. a tap) within the winding 2 e.g. at a midpoint thereof. The first connection 4a passes through the wall of the tank 5 by means of a first bushing 6a, the intermediate connection 4b passes through the wall of the tank 5 by means of a second bushing 6b, and the third connection 4c passes through the wall of the tank 5 by means of a third bushing 6c. However, it should be noted that other bushing arrangements are also possible, in which a plurality of bushings through a wall of the tank 5 allows the surge arrester arrangement located outside of the tank to connect to a winding 2 of the transformer inside the tank. A third surge arrester 3c may also be used, connected between the third connection 4c and ground, thus being connected between the phase leg winding 2 and ground.

[0022] The transformer tank 5 is typically filled with an electrically insulating fluid, e.g. a gas or a liquid such as an oil or ester-based liquid. Thus, the transformer may be better insulated, and also cooled, by means of the insulation fluid, allowing higher voltages than if the transformer was dry, in air, together with the surge arrester arrangement.

[0023] Thus, in some embodiments of the present invention, the plurality of bushings comprises a first bushing 6a for a first connection 4a between the first end 2a of the winding and the first surge arrester 3a, a second bushing 6b for the intermediate connection 4b, and a third bushing 6c for a third connection 4c between the second end 2b of the winding and the second surge arrester 3b.

[0024] In some embodiments of the present invention, the surge arrester arrangement comprises a third surge arrester (3c) connected between the phase leg winding (2) and ground.

[0025] In some embodiments of the present invention, the transformer 1 is a fluid-filled transformer wherein the tank 5 is filled with an electrically insulating fluid, e.g. a liquid such as an oil or ester-based liquid.

[0026] In some embodiments of the present invention, the transformer comprises windings 2 of a plurality of phase legs, e.g. of a three-phase system, each connected to the surge arrester arrangement.

[0027] In some embodiments of the present invention, the phase leg windings 2 are connected in delta configuration. For a three-phase delta-configuration, six bushings 6 may be needed, one for each intermediate connection 4b, and one for each terminal wherein a bushing for a first connection 4a of one phase leg winding 2 is also for a third connection 4c of an adjacent phase leg winding 2 of the three-phase transformer 1.

[0028] In some embodiments of the present invention, the phase leg windings 2 are connected in wye configuration. In this case, the first connection 4a of each phase leg winding 2 are interconnected, forming a neutral point of the transformer. Typically, the neutral point in the wye configuration may be accommodated in the same first bushing 6a,. Thus, a total of seven bushings 6 may be needed for a three-phase wye-configuration; one for the neutral point (the first bushing 6a), three for the terminals (one third bushing 6c for a third connection 4c of each phase leg winding 2), and one second bushing 6b for the intermediate connection 4b of each phase leg winding. However, it may in some embodiments be convenient to use a separate first bushing 6a for each phase leg to connect the neutral point, in which case there may be a total of nine bushings 6 for connecting the surge arrester arrangement.

[0029] The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

Claims

1. A surge arrester arrangement for a phase leg winding (2) of a transformer (1), the surge arrester arrangement comprising:

a plurality of series connected surge arresters (3) comprising at least a first surge arrester (3a) and a second surge arrester (3b) configured to be connected across the winding (2) from a first end (2a) of the winding to a second end (2b) of the winding;

an intermediate connection (4b), between the first and second surge arresters (3a, 3b), configured to connect to the winding (2) such that the first surge arrester (3a) is connected across a first section of the winding formed between the first end (2a) and the intermediate connection (4b) and the second surge arrester (3b) is connected across a second section of the winding formed between the second end (2b) and the intermediate connection (4b); and

a plurality of bushings (6) configured to allow connections (4) to the series connected surge arresters (3) to pass through a wall of a transformer tank (5) containing the phase leg winding (2).

2. The arrangement of claim 1, wherein the plurality of bushings comprises a first bushing (6a) for a first connection (4a) between the first end (2a) of the winding and the first surge arrester (3a), a second bushing (6b) for the intermediate connection (4b), and a third bushing (6c) for a third connection (4c) between the second end (2b) of the winding and the second surge arrester (3b).

3. The arrangement of claim 1 or 2, wherein the surge arrester arrangement comprises a third surge arrester (3c) connected between the winding (2) and ground.

4. A transformer arrangement comprising:

the surge arrester arrangement of any preceding claim, and

the transformer (1) comprising the phase leg winding (2).

5. The arrangement of claim 4, wherein the transformer (1) is a fluid-filled transformer wherein the tank (5) is filled with an electrically insulating fluid, e.g. a liquid such as an oil or ester-based liquid.

6. The arrangement of claim 4 or 5, wherein the transformer comprises windings (2) of a plurality of phase legs, each connected to the surge arrester arrangement.

7. The arrangement of claim 6, wherein the phase leg windings (2) are connected in delta configuration.

8. The arrangement of claim 6, wherein the phase leg windings (2) are connected in wye configuration.

9. The arrangement of claim 8, wherein the plurality of bushings comprises a first bushing (6a) for a first connection (4a) connecting the surge arrester arrangement to a neutral point of the transformer (1).

Drawing