Description

Transformer

Technical field

[0001] This invention relates to a transformer design that is particularly applicable to use in downhole equipment such as is commonly used in oil and gas wells and the like. In particular, the invention relates to transformer designs suitable for use as three-phase transformers.

Background art

[0002] It is common to transmit electrical power over long distances in the form of three-phase alternating current at medium and high voltages (e.g. in the order of 10 ³ or 10 ⁴ V). However, most electrical devices operate at low voltages (of the order of 10 to 500V). Therefore it is necessary to provide a transformer to step the voltage down when it is to be used in a device. While this is common in domestic use, the same system is also used when providing electrical power to downhole equipment used in the oil and gas industry. One example of such equipment is an electric submersible pump (ESP) such as is used to provide artificial lift in oil wells that do not naturally produce to surface.

[0003] A typical three-phase transformer is shown in Figure 1 with three legs. Each leg consists of a core section which houses the primary and secondary transformer windings and is surrounded by a coil. The transformer windings housed within the core section may be wound concentrically or stacked in order to accommodate specific spatial requirements and the winding configuration is governed by requirements related to inter-winding insulation and leakage inductance. As shown in Figure 1 , each leg is then is interconnected by a star or delta oriented set of magnetic cross braces 12a, 12b, 12c. The delta oriented interconnection of core sections is more practical for downhole applications given the need to maintain a clearance path through the centre of the transformer. This path can be important for fluid flow passages, control lines, etc. Typical downhole applications may require use of the transformer with resonant DC/DC converters where a low voltage DC supply is needed.

[0004] The known transformers, which must operate at high frequencies, given their use with resonant DC/DC converters, are typically constructed from ferrite or iron-based amorphous alloys given the need to reduce core losses. With present technology the transformer must be assembled from a number of parts which leads to the need for glued joints which adversely affect the overall core losses. A key requirement of the resonant DC/DC converter is the ability to specify a leakage inductance value, that is substantially larger than what is normally achievable with the transformer topologies using known technologies. Furthermore the leakage inductance of the transformer must not be influenced by the housing in which the transformer is placed. With the traditional design, leakage flux paths may occur that can become part of the housing. Consequently the leakage inductance may be adversely affected by the presence of the housing. In free-standing applications, the housing can normally be chosen to be sufficiently large to ensure that the leakage paths cannot pose a problem. However in downhole applications, space is limited and hence the housing is usually close to the coil, which underlines the need for a transformer design with leakage paths that will not infringe on the housing.

[0005] It is an object of this invention to provide a transformer design that can avoid some or all of the problems indicated above.

Disclosure of the invention

[0006] One aspect of this invention provides a transformer, comprising:

- a circular ferrite structure including a plurality of inwardly directed spokes, adjacent spokes being connected by cross-legs which surround an opening in the centre of the structure;

- a primary winding around each cross-leg; and

- a secondary winding around each spoke.

[0007] The structure preferably further comprises wedges extending from the spokes so as to lie between adjacent parts of the primary and secondary windings. Also, inwardly directed projections can be provided, located between the spokes and separating adjacent secondary windings.

[0008] A non-magnetic slot liner may be provided for holding each secondary winding in place.

[0009] The ferrite structure may conveniently comprise a series of arc segments, each comprising a spoke having half of a cross-leg extending from either side thereof.

[0010] It is particularly preferred that the structure comprises six spokes, and that the winding ratio is 2:V3.

[0011] The transformer typically comprises two three-phase primary windings connected in series or parallel.

[0012] The ferrite structure can be formed from a low-loss, high-temperature manganese/zinc ferrite material. The ferrite structure can be cast in the desired geometry.

[0013] Further aspects of the invention will be apparent from the following description.

Brief description of the drawings

[0014] Figure 1 shows a known transformer design;

Figure 2 shows a plan view of a transformer according to an embodiment of the invention; and

Figure 3 shows one arc element of the embodiment of Figure 2.

Mode(s) for carrying out the invention

[0015] Referring now to Figure 2, the embodiment of the invention shown therein comprises a generally circular ferrite structure 14. The structure 14 is formed from six identical arc segments Ma-Mf (one of which is shown in isolation in Figure 3) which together form a circular outer rim 16 having six inwardly directed spokes 18a-18f projecting therefrom. The spokes do not extends all the way to the centre of the circle, but terminate about half way with half cross-legs 20a1 , 20a2, etc. extending laterally from either side of the end of each spoke 18. The half cross legs 20a1 , 20b2 on adjacent spokes extend towards each other to form a series of cross legs 21a-21f. The cross legs define a central opening in the structure. Inwardly directed projections 22a-22f are provided half-way between adjacent spokes (in this case formed by half projections defined at the edge of each arc segment), terminating part-way to the cross-legs 21. Wedges 24a1 , 24a2 etc. extend laterally from each spoke above the half cross legs 20a1 , 20a2 etc.

[0016] Primary transformer windings 26a-26f are provided around the cross legs 21a-21f, and secondary windings 28a-28f are provided around the spokes 18a-18f and are held in place by non-magnetic slot liners 30a1 , 30a2 etc extending between the ends of the wedges 24 and the adjacent projection 22.

[0017] The issue of combining a compact design with a relatively large leakage is addressed in the invention by providing the primary coil sections 26 wound on the cross legs 21 and the secondary coil sections 28 on the spokes 18 of the integral core structure 14. The winding ratio for this design is 2:V3 given that the flux in the secondary (spokes) is V3 times higher than in the primary (cross-legs) when a three-phase excitation is used, as is the case here. The cross-sectional area of the spokes may need to be widened to accommodate the larger flux, with the object of maintaining a uniform flux density level in the core. Furthermore the projections 22a-22f together with the ends of the wedges 24a1 , 24a2 etc provide an improved leakage flux path, that leads to a well definable leakage inductance for primary and secondary coils that is beyond that achievable with current transformer designs as discussed above. Furthermore the leakage flux is maintained within the structure which means that the housing in which the transformer is located is less likely to influence its magnetic parameters, notably the leakage inductance.

[0018] The three-phase transformer concept shown in Figure 2 consists of two three-phase primary windings which may be connected in series or parallel. The design meets the need for a centred clearance path for downhole applications, which is conveniently located in the proximity of the primary windings.

[0019] Construction techniques envisaged for this transformer can be based on the use of a low-loss high-temperature manganese/zinc ferrite material which may be cast to the desired geometry. In the embodiment of Figure 2 six identical core sections (see Figure 3) are used which together form the circular transformer concept. Each ferrite arc segment houses a secondary phase coil, which is kept in place by a non-magnetic slot liner as used in. This secondary phase winding may be wound via the slots between the

wedges and projections where the two liners are located. A primary phase winding may be wound separately on a bobbin and then placed on the inner cross-leg during assembly of the transformer. In another embodiment, the ferrite structure can be cast as a single piece, in which case the windings will be applied differently.

[0020] This invention provides a transformer design that is particularly suitable for downhole applications. The transformer can form the basic building block of a DC/DC converter required for ESP-type applications both in terms of compact design and required leakage inductance.

[0021] Further changes within the scope of the invention will be apparent.