Method for increasing the dielectric strength of isolated base integrated circuits used with variable frequency drives

[0002] The present invention is directed, in general, to heat dissipation for power electronics switching voltages above normal electrical isolation ratings and, more specifically, to providing a common heat sink for multiple power electronics integrated circuit devices connected in series and switching voltages for driving artificial lift equipment within a borehole.

[0003] Many power electronics semiconductor integrated circuit devices are currently employed in connection with power and control systems for production within a borehole. In particular, integrated circuit devices may be employed to step-up (i.e., increase) the voltage of power produced for transmission into the borehole from a power source such as a generator or a connection to a local fixed power grid. Power electronics integrated circuit devices are typically connected between the power source and the filter(s) and drive employed to generate the specific voltage waveforms transmitted into the borehole to power artificial lift equipment within the borehole.

[0004] Heat dissipation requirements of power electronics integrated circuit devices generally require that the integrated circuit packages be mounted on a heat sink (or, equivalently, a “heat spreader”), electrically isolated from the normally-metallic heat sink material by an interposed dielectric material. The rated electric isolation for an integrated circuit device capable of switching voltages up to 1200 volts is usually about 2500 volts.

[0005] When multiple integrated circuit devices are mounted to one or more heat sinks at a common voltage potential, generally no problems arise as long as the system operates only at voltages within the electric isolation ratings. However, when the integrated circuit devices are connected in series to increase the system voltage for use with “medium” voltage drives operating at 4-6 kilovolts (kV), the conventional base isolation becomes inadequate.

[0006] Moreover, in designing electrical power systems for use with production from a borehole, various considerations not germane to other applications of electrical power systems must be taken into account. Climatic conditions, in particular, must be considered since the equipment must be located proximate to the borehole, typically at an outdoor location subject to inclement weather conditions and, in the case of subsea wells, on a production platform. While the power electronics and drive may easily be contained in a weatherproof enclosure, the heat sinks employed for the power electronics preferably include at least one externally exposed surface for cooling purposes.

[0007] There is, therefore, a need in the art for improved design of heat sinks to be employed in connection with power electronics for medium voltage drives.

[0008] To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in an electric power system for borehole production, a heat sink structure increasing the dielectric, strength of isolation for power integrated circuits. The heat sink structure is formed from a common layer on which laterally spaced isolated layers for each individual integrated circuit are mounted. At least the isolated layers are formed of anodized aluminum coated with aluminum, oxide on exterior surfaces. The aluminum and aluminum oxide of the isolated layers provide good thermal conduction for heat dissipation while the electric isolation from the common electric potential of the common layer is improved, at least in part, by increasing the distance between the integrated circuits and the common layer. The heat sink structure may be mounted within a weatherproof enclosure with one externally exposed surface for heat dissipation.

[0009] The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.

[0010] Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives, thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.

[0011] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which:

[0012] FIG. 1 depicts a borehole production system employing improved electrical isolation and heat dissipation for power electronics relating to a drive transmitting power into the borehole according to one embodiment of the present invention; and

[0013] FIG. 2 depicts in greater detail improved electrical isolation and heat dissipation for power electronics according to one embodiment of the present invention.

[0014] FIGS. 1 and 2, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to, limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device.

[0015] FIG. 1 depicts a borehole production system employing improved electrical isolation and heat dissipation for power electronics relating to a drive transmitting power into the borehole according to one embodiment of the present invention. Production system 100 includes a power source 101, such as a generator or a connection to the local alternating current (A/C) power grid, coupled by power electronics 102 to an electrical drive 103, which in the exemplary embodiment is preferably a variable frequency drive (VFD) capable of operating in one or more of an n-step variable voltage inverter (VVI) mode and a pulse width modulation (PWM) mode.

[0016] Drive 103, under operational control of an associated controller 104, generates electrical power (typically three phase power) which is transmitted over power cable(s) 105 into a borehole 106. The transmitted power is received within the borehole 106 by artificial lift equipment 107 coupled to production tubing 108 and lowered within the borehole 106 in accordance with the known art. Those skilled in the art will recognize that artificial lift equipment 107, which in the exemplary embodiment preferably, comprises an induction motor and a submersible centrifugal pump forming an electrical submersible pump (ESP) system, operates in response to the received power to assist in production of oil, gas, and other hydrocarbon fluids from the borehole 106. A detailed description of the construction and operation of a suitable electrical submersible pump system is contained in U.S. Pat. No. 6,167,965, issued to the assignee of the present invention.

[0017] Those skilled in the art will recognize that the complete construction and operation of a borehole production system is not depicted or described herein. Instead, only so much of the borehole production system as is unique to the present invention or necessary for an understanding of the present invention is shown and described. However, borehole production system 100 includes, embodied chiefly within power electronics 102, improved electrical isolation and heat dissipation for power electronics according to the present invention as described in greater detail below.

[0018] FIG. 2 depicts in greater detail improved electrical isolation and heat dissipation for power electronics according to one embodiment of the present invention. A heat sink or spreader structure 200 according to the present invention is built is electrically isolated layers. Heat sink 200 includes a common heat sink layer 201 (which may be electrically connected to ground) and n isolated layers 202a-202nfor each of n power electronics integrated circuit devices 203a-203nemployed within power electronics 102 of the borehole production system 100. Integrated circuit devices 203a-203nare connected in series in the exemplary embodiment and switch voltages of 4-6 KV.

[0019] Isolated heat sink layers 202a-202nare electrically isolated from each other as well as from common heat sink layer 201 and the integrated circuit devices 203a-203nmounted thereon. Each isolated heat sink layer 202a-202nadds to the dielectric strength of the integrated circuit devices 202a-202nmounted thereon, as well as to the overall dielectric strength of the system.

[0020] The specific method of insulating isolated heat sink layers 202a-202nis not critical, although the dielectric employed should be both a good thermal conductor and a good electrical insulator. In the exemplary embodiment, aluminum is employed for both common heat sink layer 201 and isolated heat sink layers 202a-202n, and at least isolated heat sink layers 202a-202nare anodized to form a layer of aluminum oxide on the exterior surfaces of isolated heat sink layers 202a-202n. Common heat sink layer 201 may optionally also be anodized. Aluminum oxide is a good electrical insulator while providing a good conductor for heat transfer. Where mounted screws are claw employed to secure packaged integrated circuit devices 203a-203nto the isolated heat sink layers 202a-202n, screw holes within isolated heat sink layers 202a-202nare preferably drilled and threaded prior to anodizing the isolated layers 202a-202n, so that the interior of the screw holes are also coated with insulation.

[0021] The dielectric strength of heat sink structure is increased, at least in part, by the increased distance between integrated circuit devices 203a-203nand a common potential (common layer 201) provided by isolated layers 202a-202n. A dielectric layer, only, having an equivalent thickness to isolated.layers 202a-202nis unlikely to have acceptable thermal conduction properties between integrated circuit devices 203a-203nand common layer 201. Use of a heat sink material coated on opposing sides with a dielectric increases the thermal conductivity of the isolated layers 202a-202nwhile providing acceptable dielectric properties, including distance from the common potential.

[0022] Many suitable variations of the basic structure shown for heat sink structure 200 may be utilized. Each isolated layer 202a-202nneed not be only a single layer as depicted, but may be formed of a number of stacked layers each coated on all sides by an insulating material, or otherwise separated by insulating material. Moreover, the number of stacked layers with intervening dielectric material forming each isolated layer 202a-202nneed not be uniform, with each isolated layer 202a-202nemploying a number of levels within the stack related to the voltage developed by the integrated circuit device 203a-203nmounted on the corresponding isolated layer 202a-202n.

[0023] Additionally, while lateral isolation of isolated layers 202a-202nis preferred, a staggered “stair-step” configuration of layers may be alternatively employed, in which, for example, isolated layer 202awould extend on common layer 201 beneath all of integrated circuit devices 203a-203nas well as isolated layers 202b-202n; isolated layer 202bwould extend, on isolated layer 202a, beneath all of integrated circuit devices 203b-203nand isolated layer 202n; and isolated layer 202nwould extend, on the surface of isolated layer 202bor an intervening isolated layer, beneath only integrated circuit device 203n. Such a staggered configuration of layers would permit different levels of electric potential to exist within the conductive body or core of each “level” or isolated layer, such that the highest voltage developed is insulated by multiple levels (each at different electric potentials) from a common or ground potential.

[0024] For use in connection with borehole production, heat sink structure 200 may be mounted within a weatherproof cabinet 204 with an externally exposed surface 205 for heat dissipation. The externally exposed surface 205, which in the exemplary embodiment is preferably at least a portion of the major surface of common layer 201 opposite the surface on which isolated layers 202a-202nand integrated circuit devices 203a-203nare mounted, may be either directly exposed or covered by optional vented cover 206. Mounted of heat sink structure 200 within cabinet 204 with surface 205 externally exposed should, however, preferably be dust-tight and sealed against moisture.

[0025] The present invention improves the dielectric strength of isolated base semiconductor integrated circuit devices employed in connection with variable frequency drives for borehole production and connected in series to switch voltages in excess of the electric isolation rating for conventional mounting of such devices on a heat sink. Good thermal conduction for the heat sink is maintained while increasing the distance of separation between the integrated circuit devices and a common electric potential such as a ground potential.

[0026] Although the present invention has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, enhancements, nuances, gradations, lesser forms, alterations, revisions, improvements and knock-offs of the invention disclosed herein may be made without departing from the spirit and scope of the invention in its broadest form.