WO2001075914A1

WO2001075914A1 - High voltage induction device

Info

Publication number: WO2001075914A1
Application number: PCT/EP2001/004403
Authority: WO
Inventors: Tomas Jonsson; Mikael Dahlgren
Original assignee: Abb Ab
Priority date: 2000-04-03
Filing date: 2001-04-02
Publication date: 2001-10-11
Also published as: GB0008153D0; AU2001256304A1; GB2361109A

Abstract

A high voltage induction device (21; 30; 40) having core means (20; 31, 32; 41) comprising a solid closed magnetic core, an ac-winding wound (22; 33, 34; 42) on the closed magnetic core and magnetic means (23; 35, 36; 43) for increasing the H-field in the closed magnetic core. The magnetic means may comprise a permanent magnet (43) in the core or at least one auxiliary coil (23; 35, 36) connected to a dc-current source (24; 37).

Description

High Voltage Induction Device

Technical Field

This invention relates to a high voltage (HV) induction device of the kind having core means comprising a solid closed magnetic core and an ac-winding wound on the closed magnetic core. The invention also relates to a method of influencing the reluctance of the magnetic circuit of an induction device. In this specification the term "high voltage" is intended to mean in excess of 2 kV and preferably in excess of 10 kV.

Background of the Invention

HV induction devices such as reactors are used in power systems, for example, in order to compensate for the Ferranti effect from long overhead lines or extended cable systems causing high voltages in the open circuit or lightly loaded lines. Reactors are sometimes required to provide stability to long line systems. They may also be used for voltage control and switched into and out of the system during light load conditions. Similarly, transformers are used in power systems to step up and step down voltages to useful levels.

Such HV induction devices are manufactured from similar components. Typically, one or more coils are_. wound around a laminated core to form windings, which may be coupled to the line or load and switched in and out of the circuit in a desirable manner. The equivalent magnetic circuit of a static induction device comprises a source of magnetomotive force, which is a function of the number turns of the winding, in series with the reluctance of the core, which may be of iron and which may include an air gap.

While the air gap is not strictly speaking necessary, reactors and transformers without air gaps tend to saturate at high magnetic field densities. Thus, control is less precise and fault currents may produce catastrophic failures.

A known core may be visualized as a body having a closed magnetic circuit, for example, a pair of legs and interconnecting yokes. One of the legs may be cut through to form the air gap. The core may support the windings which, when energized by a current, produce a magnetic field Φ in the core which extends across the air gap. At high current densities the magnetic field is intense.

Although useful and desirable, the air gap represents a weak link in the structure of the core. The core tends to vibrate at a frequency twice that of the alternating input current. This is the source of vibrational noise and stress in such induction devices.

Another problem associated with an air gap is that the flux Φ fringes, spreads out and is less confined. Thus, field lines tend to enter and leave the core with a non-zero component transverse to the core laminations which can cause a concentration in unwanted eddy currents and hot spots in the core.

These problems can be somewhat alleviated by the use of one or more inserts in the gap designed to stabilize the structure and thereby reduce vibrations. In addition, the structure, or insert, is formed of materials which are designed to reduce the fringing effects in the gap.

However, these devices are difficult to manufacture and are expensive.

A typical insert comprises a cylindrical segment of radially laminated core steel plates arranged in a wedge shaped pattern. The laminated segments are moulded in an epoxy resin as a solid piece or module. Ceramic spacers are placed on the surface of the module to space it from the core, or when multiple modules are used, from an adjacent module. In the latter case, the modules, and ceramic spacers are accurately stacked and cemented together to make a solid core limb for the device.

The magnetic field in the core creates pulsating forces across all air gaps which, in the case of devices used in power systems, can amount to hundreds of kilo-newtons (k ) . The core must be stiff to eliminate these objectionable vibrations. The radial laminations in the modules reduce fringing flux entering flat surfaces of core steel which thereby reduce current overheating and hot spots.

These structures are difficult to build and require precise alignment of a number of specially designed laminated wedge shaped pieces to form the circular module. The machining must be precise and the ceramic spacers are likewise difficult to size and position accurately. As a result, such devices are relatively expensive.

Summary of the Invention

An aim of the present invention is to provide an induction device in which the reluctance of the magnetic circuit is increased by an alternative method to the use of air gaps in the core of the induction device.

According to one aspect of the present invention there is provided an induction device of the kind referred to characterised in that magnetic means are provided to increase the H-field in the closed core.

The principle of the invention is based on shifting the "working" or "operating" region of the magnetisation or hysteresis curve of the core means. Conventionally the working or operation region is around the origin on the B-H hysteresis curve. However, the provision of the magnetic means causes the effective permeability, μ , of the core means to be lowered by moving the operating region on the hysteresis curve to higher H-fields. Conveniently the magnetic means may comprise a permanent magnet arranged to form part of the closed magnetic core. In this case the magnet should be a strong permanent magnet with the "hard" axis of the magnet being in the direction of the core. The permanent magnet thus provides a constant or "dc" field H_dc.

Alternatively, the magnetic means may comprise dc- coil means. For instance the coil means may comprise an auxiliary coil connected to a dc-current source so as to induce a constant field strength H_dc which can be adjusted as required. The ac- field, H_ac, created by the ac-current in said winding is superimposed on H_dc. The dc-current source should have a high resistance so as not to operate as the secondary coil or winding of a transformer. Compared to the case where the working or operating point is around the origin of the B-H hysteresis curve of the closed magnetic core, the change in the magnetic flux density, B, during the cycle of the induced ac-field is smaller and, hence, the effective magnetic permeability μ of the closed magnetic core appears lower. By using many dc-winding turns, the dc- current can be made low leading to low resistive energy losses .

A possible drawback of using a single winding for generating the ac-field is that the ac-response in the magnetic flux may be asymmetric due to the shape of the B-H hysteresis curve. A way of achieving a symmetric response is for the core means to comprise two identical magnetic closed cores, each having its own ac-winding and its own auxiliary dc-coil. In this case the two ac windings are connected together and wound in the same direction whereas the two dc-coils are connected together but wound in opposite directions.

A communications unit is preferably included in the induction device. The communications unit typically comprises at least one Input/Output (I/O) interface and a processor. Measured values for one or more sensors in the induction device may be received via the I/O interface and routed to the processor. An output channel of the I/O interface may be used to send a control signal to an actuator of any sort arranged in the induction device. The communications unit may also be used to send data out of the induction device by wire or wireless means, for supervision, data collection and/or control purposes. The communications unit may, for example, be mounted on the core.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with particular reference to the accompanying drawings, in which:

Fig. 1 illustrates schematically a known induction device, e.g. a power reactor or power transformer;

Fig. 2 is a perspective fragmentary view of a cable which may be used in the winding of a high power static induction device for a power system;

Fig. 3 is a cross-section through the cable shown in Fig. 1;

Fig. 4 is a schematic view of a B-H hysteresis curve for a magnetic core of an induction device according to the present invention;

Fig. 5 is one embodiment of a power induction device according to the invention having a closed magnetic core;

Fig. 6 is another embodiment of a power induction device according to the invention having a closed magnetic core; and Fig. 7 is a schematic view of a still further embodiment of a power induction device according to the invention having a closed magnetic core.

Description of Preferred Embodiments

Fig. 1 shows a known induction device 1, such as a power transformer or reactor, having at least one winding 2 and a core 3. Fig. 1 also shows a simplified view of the electric field distribution around the turns of the winding 2, with lines of equipotential designated E and indicating where the electric field has the same magnitude. The lower part of the winding is assumed to be at earth potential. The core 3 has an optional distributed air gap 4 and a window 5. The core is typically formed of laminated sheets of magnetically permeable material, e.g. silicon steel, but may, alternatively, be formed of magnetic wire, ribbon or powder metallurgy material. The direction of the magnetic flux 4 is shown by the arrow in Figs. 1 and 2 and, in general, is confined, or is at least nearly confined, within the core 3.

The potential distribution determines the composition of the insulation system, especially in high power systems, because it is necessary to have sufficient insulation between adjacent turns of the winding. In Fig. 1, the upper part of the winding is subjected to the highest dielectric stress. The design and location of a winding relative to the core 3 are in this way determined substantially by the electric field distribution in the core window 5. The windings^' 2 may be formed of a conventional multiturn insulated wire, as shown, or the windings 2 may be in the form of a high power transmission line cable discussed below. In the former case, the device may be operated at power levels typical for such devices in known power generating systems. In the latter case, the device may be operated at much high power levels not typical for such devices. Figs. 2 and 3 illustrate an exemplary cable 6 for manufacturing windings 2 useful in high voltage, high current and high power induction devices. Such cable 6 comprises at least one conductor 7 which may include a number of strands 8 with a cover 9 surrounding the conductor

7. In the exemplary embodiment shown, the cover 9 includes a semiconducting inner layer 10 disposed around the strands

8, a solid main electrically insulating layer 11 surrounding the semiconducting inner layer 10, and a semiconducting outer layer 12 surrounding the main electrically insulating layer 11 as shown. The inner and outer layers 10 and 12 have a similar coefficient of thermal expansion as the main electrically insulating layer 11. The cable 6 may be provided with additional layers (not shown) for special purposes. In a high power static conductor device, for example, the cable 6 may have a conductor area which is between about 30 and 3000 mm and the outer cable diameter may be between about 20 and 250 mm. Depending upon the application, the individual strands 8 may be individually insulated. A small number of the strands near the interface between the conductor 7 and the semiconducting inner layer 10 may be uninsulated for establishing good electrical contact therewith. As a result, no harmful potential differences arise in the boundary layer between the innermost part of the solid insulation and the surrounding inner semiconducting layer along the length of the conductor. The cable 7 may typically be as described in WO 97/45931 and such cable is incorporated herein by way of reference.

Devices for use in high power applications may have a power rating ranging from 10 kVA up to over 1000 MVA with a greater voltage ranging from about 3-4 kV and up to very high transmission voltages, such as 400 kV to 800 kV or higher.

The similar thermal properties of the various layers 10-12, results in a structure which may be integrated so that adjoining semiconducting and insulation layers exhibit good contact independently of variations and temperatures which arise in different parts of the cable. The insulating layer and the semiconducting layers form a monolithic structure and defects caused by different temperature expansion of the insulation and the surrounding layers do not arise.

Figure 4 illustrates the principle of the present invention and shows a B-H hysteresis curve for a closed or endless magnetic core 20 (see Figure 5) of a high voltage induction device 21 according to the present invention, where "B" represents the magnetic flux density and "H" represents the field strength. The core 20 has no air gap therein and is typically formed of laminated sheets of magnetically permeable material, e.g. electrical steel, but may, alternatively, be formed of magnetic wire, ribbon or powder metallurgy material.

Arranged on limbs of the core 20 are an ac-coil 22 and a dc-coil 23 connected to a dc-current source 24. The steady dc-current flowing through the dc-coil 23 induces a constant magnetic field strength H_dc in the core material. The ac-field, H_ac, created by the ac-current flowing through the ac-winding 22 superimposed on H_dc and shifts the operating point on the B-H hysteresis curve from around the origin to higher positive values of H. The change in the magnetic flux density, B, during the cycle of the induced ac-field is smaller and thus the effective permeability, μ, appears lower. The size of the dc- field, H_dc, can be adjusted by adjusting the current flowing through the dc- coil 23-. The dc-current source 24 should have a high resistance so as not to operate as the secondary coil or winding of a transformer.

A possible drawback of the arrangement shown in

Figure 5 is that the ac-response in the magnetic flux can be asymmetric due to the shape of the B-H hysteresis curve. Figure 6 illustrates a high voltage induction device 30 which provides a symmetric response. The induction device 30 has two at least substantially identical closed magnetic cores 31 and 32 having limbs on which are arranged respective substantially identical ac-coils 33 and 34 and respective substantially identical dc-coils 35 and 36 supplied from a dc-current source 37. The ac-coils 33 and 34 are wound in the same direction and the dc-coils 35 and 36 are wound in opposite directions.

An alternative high voltage induction device 40 according to the invention is shown in Figure 7. The induction device 40 is similar in many respects to the induction device 21 and has a closed solid core 41 on a limb of which is arranged an ac-coil 42. The induction device 40 differs from the induction device 21 shown in Figure 5 by the provision of a strong permanent magnet 43 as part of the magnetic core to provide the dc-field H_dc instead of a dc- coil. The hard axis of the magnet 43 is arranged in the direction of the core as indicated by the arrow in Figure 7. In an alternative embodiment the high voltage induction device of Figure 7 may additionally be provided with one or more d.c. coils to further increase the H- field in the closed core 41.

The embodiments of the invention described provide high voltage induction devices having magnetic cores with no air gaps. The induction devices can be relatively easily realised and with certain designs there is the facility to provide a variable magnetic permeability, μ .

Claims

1. A high voltage induction device (21; 30; 40) having core means comprising a solid closed magnetic core (20;31,32;41) and an ac-winding (22 ; 33 , 34; 42) wound on the closed magnetic core, characterised in that magnetic means (23;35,36;43) are provided to increase the H-field in the closed core.

2. A high voltage induction device (40) according to claim 1, characterised in that the magnetic means comprises a permanent magnet (43) arranged to form part of the closed magnetic core.

3. A high voltage induction device (21) according to claim 1, characterised in that the magnetic means comprises dc-coil means (23)

4. A high voltage induction device according to claim 1, characterised in that the magnetic means comprises d.c. coil means, e.g. one or more dc-coils, and a permanent magnet forming part of the closed magnetic core.

5. A high voltage induction device according to claim 3 or 4, characterised in that the dc-coil means (23) comprises an auxiliary coil connected to a dc-current source (24) so as to induce a constant field strength H_dc which can be adjusted as required.

6. A high voltage induction device according to claim 1, ' characterised in that said core means comprises two identical magnetic closed cores (31,32) , each having its own ac-winding (33,34) and its own auxiliary dc-coil (35,36), the two ac windings (33,34) being connected together and wound in the same direction and the two dc-coils (35,36) being connected together and wound in opposite directions.

7. A high voltage induction device according to claim 6, characterised in that said two dc-coils (35,36) are connected to a dc-current source (37) .

8. A high voltage induction device according to any one of the preceding claims, characterised in that it inlcudes a communications unit.

9. Use of a high voltage induction device according to any one of the preceding claims in order to influence the reluctance of a magnetic circuit.