WO2007035110A1

WO2007035110A1 - Autotransformer device with magnetic air gap

Info

Publication number: WO2007035110A1
Application number: PCT/NO2006/000327
Authority: WO
Inventors: Kjell Hystad
Original assignee: Magtech As
Priority date: 2005-09-23
Filing date: 2006-09-22
Publication date: 2007-03-29
Also published as: NO20054427L; WO2007035110A9; NO324270B1; NO20054427D0

Abstract

The invention relates to a system for voltage stabilization and reduction of total harmonic distortion in power transmission lines, for each phase comprising: an autotransformer A with a series winding 7 and a parallel winding 4, where the series winding 7 is connected between an input voltage Uin and an output voltage Uout; a magnetic controllable inductor B comprising a main winding 2 wound around a core and a control winding 3 wound around the core, where the control winding 3 is adapted for creating a magnetic field mainly orthogonal to the field created by the main winding 2, thereby controlling the inductance of the magnetic controllable inductor B; the parallel winding 4 of the autotransformer is connected between the input voltage Uin and the main winding 2 of the magnetic controllable inductor B, where the other terminal of the main winding 2 is connected to the neutral point. The system further comprises a magnetizing inductance, which provides an increase in the magnetizing current of the autotransformer. The magnetizing inductance can be provided by a magnetic air gap in the core of the autotransformer. Alternatively, the magnetizing inductance can be provided by separate inductor connected in parallel to the parallel winding 4 of the autotransformer.

Description

Autotransformer device with magnetic air gap

TECHNICAL FIELD

The present invention relates to methods and systems for voltage stabilization. More particularly, the invention relates to methods and systems that employ a variable inductance to compensate for voltage variations that may arise in power supply lines, and to reduce total harmonic distortion in the power supply.

BACKGROUND OF THE INVENTION

Undersized lines for electric power transmission, also referred to as "weak lines", have too small a conductor cross section in relation to the load requirements and a relatively high resistance. Excessive voltage drop will result from the losses caused by undersized conductors. The excessive voltage drop results in inadequate voltage levels for the electric power connected to the lines.

A transformer is a static unit which supplies an output voltage determined by the number of windings on the primary and secondary sides, i.e., the transformer ratio. A fixed transformer ratio may result in a voltage that is too low, (i.e., an under- voltage) when the load is high, and a voltage that is too high, (i.e., an over- voltage condition) when the load is low. Because the load is dependent at all times on the highly variable requirements of individual electric power consumers, fixed ratio transformers are often inadequate to serve a dynamic load in weak lines. The low voltage level can be compensated for by increasing the voltage in steps at the transformer that is supplying the line. In one prior art approach, the voltage level is controlled by means of a load tap changer on the transformer which is connected to the individual phase at the location where the voltage reaches an unacceptably low level. Another approach comprises replacing existing lines with new lines having a larger cross section and correspondingly lower resistive losses.

In another prior art approach to voltage regulation, a mechanically controlled variac (i.e., a transformer with variable transformer ratio) is used in connection with a transformer. However, mechanically controlled variacs, generally, are no longer used because the mechanical components required frequent service.

WO 2004/053615, the contents of which are hereby incorporated by reference, describes a system for voltage stabilization of power lines. One application of this system is illustrated in fig. 1, showing an autotransformer A (indicated with dashed lines) having a series winding 7, in series with the input voltage Uin and the output voltage Uout, and a parallel winding 4, parallel to the series winding 7. The system further comprises a variable inductor B (indicated with dashed lines) which is connected to the autotransformer, and a control system to control the inductance of the variable inductor B. The variable inductor B includes a magnetic core, a main winding 2 wound around a first axis, and a control winding 3 wound around a second axis orthogonal to the first axis. When the main winding and the control winding of the variable inductor are energized, orthogonal fluxes are generated in the magnetic core.

This voltage stabilization system automatically compensates for voltage variations in the power supply line to which it is connected. In the above publication the permeability control is performed using orthogonal fields and it is not performed by means of parallel fields which are added or subtracted.

In a three-phase system the above mentioned system can be used for dynamic control of each phase voltage individually to correct for voltage drop.

There have been several experiments with the above system. One has found that the system produces a large harmonic distortion to the output voltage. In some tests, the total harmonic distortion THD was found to be approximately 40%. The European standard EN50160 "Voltage characteristics of electricity supplied by public distribution systems" sets among other things a maximum level for THD which is far below this result (approximately 8%). Moreover, the experiments of the above system also showed a slightly reduced

Power Factor PF, which is a measure for the phase difference between the voltage and the current of the system. The PF is often referred to as cos φ, and, though not always achievable, a cos φ near 1 is desired in most electro technical equipment.

The object of the present invention is to provide a method and a system for voltage stabilization, employing a variable inductance to compensate for voltage variations that may arise in power supply lines, and where the above disadvantages are avoided.

Further, it is an object of the present invention to provide a system has few and reliable components. Another object is to reduce the total weigh of the components in the total system.

SUMMARY OF THE INVENTION

The present invention relates to a system for voltage stabilization and reduction of total harmonic distortion in power transmission lines, for each phase comprising: an autotransformer with a series winding and a parallel winding, where the series winding is connected between an input voltage Uin and an output voltage Uout; a magnetic controllable inductor comprising a main winding wound around a core, and a control winding wound around the core, where the control winding is adapted for creating a magnetic field mainly orthogonal to the field created by the main winding, thereby controlling the inductance of the magnetic controllable inductor, and wherein: the parallel winding of the autotransformer is connected between the input voltage Uin and the main winding of the magnetic controllable inductor, and the main winding of the magnetic controllable inductor is connected between a parallel winding and a neutral point, where the other terminal of the main winding is connected to the neutral point, characterized in that: the system comprises a magnetizing inductance, which provides an increase in the magnetizing current of the autotransformer.

The magnetizing inductance can be provided by a magnetic air gap in the core of the autotransformer and/or by separate inductor connected in parallel to the parallel winding of the autotransformer.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be explained by means of an example illustrated in the drawings, where:

Fig. 1 illustrates a prior art system for voltage stabilization; Fig. 2 illustrates a first embodiment of the invention; Fig. 3 illustrates a second embodiment of the invention;

Fig. 4a - 4d illustrates the autotransformer with its physical representation and the corresponding "ideal transformer" equivalent;

Fig. 5a and 5b illustrates phase coordination diagrams of voltages and currents in fig. 1 - 3, and

Fig. 6 and 7 shows the physical construction of the autotransformer with or without the magnetic air gap. Fig. 1 is briefly described in the introduction above, and shows one phase of a voltage stabilisation system comprising an autotransformer A with a series winding 7 and a parallel winding 4. The system further comprises a magnetic controllable inductor B (MCI) comprising a main winding 2 and a control winding 3. The series winding 7 is connected between an input voltage Uin and an output voltage Uout. The parallel winding 4 is connected between the input voltage Uin and the main winding 2 of the magnetic controllable inductor B. The other terminal of the main winding 2 is connected to the neutral point. The control winding 3 of the controllable inductor B is adapted for creating a magnetic field mainly orthogonal to the field created by the main winding 2, thereby controlling the inductance of the magnetic controllable inductor B. The construction and the operation of the controllable inductor B is described in prior art and will not be described in detail here. It should be noted that in fig. 1, the current I_MCI through the main winding 2 of the controllable inductor B in fig. 2 is equal to the current Ip through the parallel winding 4 of the autotransformer A. The voltages and currents of the autotransformer are shown as a physical representation and its ideal equivalent in fig. 4a and 4b respectively. As shown, the parallel current Ip is equal to the current Is of the series winding 7 divided to the transformer ratio n.

We will now refer to fig. 2, which shows a first embodiment of the invention which solves the problems and disadvantages with the system shown in fig. 1, as described in the introduction above. Equal or similar elements in fig. 2 have the same reference numbers as in fig. 1, and will not be described here in detail. In fig. 2, an additional inductor C is connected in parallel to the parallel winding 4 of the autotransformer A. Consequently, the current I_MCI through the main winding 2 of the controllable inductor B in fig. 2 is equal to the current Ip through the parallel winding 4 plus the current Ic through the additional inductor C.

We will now refer to fig. 3, where second embodiment of the invention is shown, comprising an autotransformer A' and a magnetic controllable inductor B, which is as described above with the same reference numbers. The main difference between fig. 1 and fig. 3 is that the autotransformer A' comprises a magnetic air gap, as will be described in detail below.

In the system according to the invention, the current I_MCI through the main winding 2 of the controllable inductor B, is equal to the current Ip' through the parallel winding 4 of the autotransformer A'.

First, the term "magnetic air gap" should be described. A "magnetic air gap" is a part of a core in a transformer comprising a non-magnetizable material, or a material with a high magnetic "resistance". Consequently, introducing a magnetic air gap in a transformer increases the reluctance (magnetic "resistance") in the flux path, hence increasing the current that is drawn from the power source. The voltages and currents of the autotransformer in fig. 3 are shown as a physical representation in fig. 4a and 4b, and its ideal equivalent in fig. 4c and 4d respectively. As shown, the magnetic air gap introduces an increased magnetizing current Im. Consequently, the primary current Ip' is equal to a magnetizing current Im plus the secondary current Is divided to the transformer ratio n. When comparing fig. 4b and 4d, it can be seen that Ip' is larger than Ip.

A comparison of the systems in fig. 1, 2 and 3 can also be made using phase coordination diagrams.

Fig. 5a shows phason diagram of MVB without additional reactor or magnetic air gap in the auto transformer. The figure illustrates a typical diagram of the system in fig. 1 , while fig. 5b, showing a phason diagram of MVB with additional reactor or magnetic air gap in auto transformer, illustrates the same operation situation when using the system according to fig. 2 or 3. As can be seen, the length of the vector U_MCI is reduced in fig. 5b, which indicates that there are lower voltages over the controllable inductor B.

Fig. 6 is an electromechanical sketch of an autotransformer without a magnetic air gap in MVB, and fig. 7 shows the same, but with a magnetic air gap in MVB.

Fig. 6 is a prior art transformer comprising a toroid magnetic core 10 and a primary winding 12 and a secondary winding 14 wound around the core 10. The autotransformer in the preferred embodiment of the system according to the invention can be made by dividing the core 10 into two halves and then insert a non-magnetizable material between the two halves, as shown in fig. 7 with the magnetic air gap.

Consequently, the system according to the invention comprises a magnetizing inductance, which provides an increase in the magnetizing current of the autotransformer. Consequently, the total harmonic distortion THD is reduced. Moreover, it can be seen that the angle α between the output voltage Uout and the input voltage Uin is reduced, consequently the power factor PF is improved.

The magnetizing inductance can be provided by a magnetic air gap in the core of the autotransformer or can be provided by separate inductor connected in parallel to the parallel winding 4 of the autotransformer.

The system further comprises a control circuit, which based on measured voltages and/or currents, controls the control current of the control winding 3, thereby controlling the inductance of the magnetic controllable inductor B.

Claims

1. System for voltage stabilization and reduction of total harmonic distortion in power transmission lines with at least one phase, where each phase comprises: an autotransformer (A) with a series winding (7) and a parallel winding (4), where the series winding (7) is connected between an input voltage Uin and an output voltage Uout; a magnetic controllable inductor (B) comprising a main winding (2) wound around a core, and a control winding (3) wound around the core, where the control winding (3) is adapted for creating a magnetic field mainly orthogonal to the field created by the main winding (2), thereby controlling the inductance of the magnetic controllable inductor (B), and wherein: the parallel winding (4) of the autotransformer is connected between the input voltage Uin and the main winding (2) of the magnetic controllable inductor (B), and the main winding (2) of the magnetic controllable inductor (B) is connected between a parallel winding and a neutral point, where the other terminal of the main winding (2) is connected to the neutral point, characterized in that: the system comprises a magnetizing inductance (C), which provides an increase in the magnetizing current of the autotransformer (A).

2. System according to claim 1, where the magnetizing inductance (C) is provided by a magnetic air gap in the core of the autotransformer (A).

3. System according to claim 1, where the magnetizing inductance (C) is provided by a separate inductor (C) connected in parallel to the parallel winding (4) of the autotransformer (A).

4. System according to claim 2, characterized in that the magnetic air gap is provided as a part of a non-magnetizable material.

5. Method for stabilization and reduction of third harmonic distortion in a power transmission system with at least one phase, where the system for each phase comprises: - an autotransformer (A) with a series winding (7) and a parallel winding (4), where the series winding (7) is connected between an input voltage Uin and an output voltage Uout; a magnetic controllable inductor (B) comprising a main winding )2 wound around a core and a control winding (3) wound around the core, where the control winding (3) is adapted for creating a magnetic field mainly orthogonal to the field created by the main winding (2), thereby controlling the inductance of the magnetic controllable inductor (B); the parallel winding (4) of the autotransformer is connected between the input voltage Uin and the main winding (2) of the magnetic controllable inductor (B), where the other terminal of the main winding (2) is connected to the neutral point, characterized in that the method comprises the following step: controlling the inductance of a magnetizing inductance, which provides an increase in the magnetizing current of the autotransformer.