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Title:
REDUCTION OF THIRD HARMONICS
Document Type and Number:
WIPO Patent Application WO/2006/068503
Kind Code:
A1
Abstract:
The invention comprises a device for reduction of harmonics in three-phase (supply) systems comprising nonlinear devices comprising, for each phase, a circuit element with controllable impedance, comprising a first magnetic core, a first main winding wound round the core to produce a main magnetic field, and a first control winding wound round the core to produce a control magnetic field mainly orthogonal to the main magnetic field, where the first main winding is arranged for connection to said phase and the impedance of the circuit element is controllable by the current flowing in the control winding. Said device is characterised in that it further comprises one second winding for each phase wound around said first magnetic core to create a field mainly parallel to the main magnetic field, and that said second windings are connected to one another in a delta configuration. The invention comprises also a method and a system for reduction of harmonics in three-phase supply systems.

Inventors:
JOHANSEN BJOERNAR SKAAR (NO)
Application Number:
PCT/NO2005/000483
Publication Date:
June 29, 2006
Filing Date:
December 23, 2005
Export Citation:
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Assignee:
MAGTECH AS (NO)
HARTMANN CHRISTIAN MAGNUS (NO)
JOHANSEN BJOERNAR SKAAR (NO)
International Classes:
H01F29/14; H02J3/01
Domestic Patent References:
WO2003044612A12003-05-30
Foreign References:
US4445082A1984-04-24
GB512789A1939-09-26
US4393157A1983-07-12
Attorney, Agent or Firm:
Onsagers AS. (Oslo, NO)
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Claims:
CLAIMS
1. Device for reducing harmonics in a threephased voltage supply with voltage stabilisation in the form of, for each phase: a circuit element (Cl, C2, C3) with controllable impedance; comprising a first magnetic core, a first main winding (MWl, MW2, MW3) wound round the core to produce a main magnetic field, and a first control winding (CWl, CW2, CW3) wound round the core to produce a control magnetic field mainly orthogonal to the main magnetic field, where the first main winding (MWl, MW2, MV3) is arranged for connection to said phase and the impedance of the circuit element is controllable by the current flowing in the control winding, one second winding (SWl, SW2, SW3) wound around said first magnetic core to create a field mainly parallel to the main magnetic field, where the second windings (SWl, SW2, SW3) are connected to one another in a delta configuration, characteriz ed in that the device comprises a variable decoupling inductor (LB) included in said delta configuration, whn Device according to claim 2, characterised in that the decoupling inductor is variable.
2. Device according to claim 3, characterised in that ere the decoupling inductor (LB) comprises a second magnetic core, a second main winding (MWB) wound around the core about a first axis and a second control winding (CWB) wound around the core about a second axis, where the second main and control windings are adapted to create orthogonal fields when energized, such that the inductance of the decoupling inductor is controllable by the current flowing in the second control winding. 5.2. Device according to any of claims 14, characteriz ed i n characterised in that the circuit element (Cl, C2, C3) is a controllable inductor.
3. 3. Device according to any of claim 1 or 2, claims 14, characterized in characterised in that the circuit element (Cl, C2, C3) is a transformer, comprising further a second main winding (MW4, MW5, MW6) wound around the first magnetic core to create a field mainly parallel to the main field.
4. 4 Method for reducing harmonics in a threephased voltage supply with voltage stabilisation in the form of, for each phase: a circuit element (Cl, C2, C3) with controllable impedance, comprising a first magnetic core, a first main winding (MWl, MW2, MW3) wound round the core to produce a main magnetic field, and a first control winding (CWl, CW2, CW3) wound round the core to produce a control magnetic field mainly orthogonal to the main magnetic field, where the first main winding (MWl, MW2, MV3) is arranged for connection to said phase and the impedance of the circuit element is controllable by the current flowing in the control winding, characterised in that the where the method further comprises, for each phase, providing for each phase a second winding (SWl, SW2, SW3) wound around the first magnetic core to create a field mainly parallel to the main field, and connecting the second windings (SWl, SW2, SW3) to one another in a delta configuration, characteriz ed in that the method comprises the following steps: connecting a . Method according to claim 7, characterised in that it comprises including variable a decoupling inductor (LB)in the delta configuration, where the .
5. 9 Method according to claim 8, comprising providing a decoupling inductor with comprises a second magnetic core, a second main winding (MWB) wound around the core and a second control winding (CWB) wound around the core, where the main and the control winding are adapted to create substantially orthogonal fields when energized, such that the inductance of the decoupling inductor (LB) is controllable by the current flowing in the second control winding. 10.5. Method according to claim 4, characterized in that the circuit element (Cl, C2, C3) is a controllable inductor.
6. 6. Method according to claim 4 or 5, characteriz ed in that the circuit element (Cl, C2, C3) is a transformer comprising further a second main winding (MW4, MW5, MW6) wound round the first magnetic core to create a field mainly parallel to the main field.
7. 7. Regulating system comprising a device according to any of claims 1 3, characteriz ed in that it further comprises measuring units (106, 107, 108, 109, 110, 111) for output voltage and current for each phase, input units (112, 113) for threshold values, and a processing unit (114) for, based on the measured output voltage and current for each phase and on the threshold values, varying the inductance of the decoupling inductor.
8. 8. System according to claim 7, characteriz ed in that the input unit is adapted to receive threshold values for third degree harmonics and the processing unit is adapted to based on measured values and on third degree harmonics thresholds, to vary the decoupling inductor's inductance.
9. 9. System according to claim 8, characterized in that the input unit is adapted to receive threshold values related to symmetry and the processing unit is adapted to, based on measured values, on third harmonics thresholds and on symmetry thresholds, to vary the decoupling inductor's inductance.
Description:
REDUCTION OF THIRD HARMONICS

The present invention relates to a device, a method and a system for reduction of harmonics in three-phase (supply) systems comprising nonlinear devices.

The publication US 4,393,157 describes a controllable inductor comprising a first and second magnetic circuit of anisotropic material. The magnetic fields provided by the respective circuits are orthogonal with respect to each other.

WO 03044612 "Device with controllable impedance" describes among other things use of a controllable inductor for series/parallel compensation of transmission lines. Said controllable inductor comprises an iron core, a main winding for series/parallel connection to the transmission line and a control winding for creation of a field mainly orthogonal to the field created by the main winding.

Use of such controllable inductors and other nonlinear devices may lead to a significant level of third degree harmonics as a consequence of the non- linear characteristic of magnetic materials. In "insulated terra" (IT) networks, that is in networks isolated from ground, the most important harmonic components caused by the non-linearity of the e.g. saturable elements (the zero sequence harmonics) will cancel each other out. This is because they are in phase and represent no difference in potential between the three phases. Since in an IT system all loads are connected in a delta configuration directly between the three phases, no resulting current will be caused by these harmonics. In TN ("terra neutral") networks, however, said non-linearity will lead to significant third harmonic load voltages that will not cancel each other out. Instead, the zero sequence harmonics will add up relative to ground and cause significant currents to ground through the loads, and a corresponding high level of total harmonic distortion (THD).

The European standard EN50160, "Voltage characteristics of electricity supplied by public distribution systems" sets, among other things, a maximum level for THD and harmonics for voltage supplied to consumers.

It is thus an object of the present invention to provide a device, a method and a system for reduction of harmonics in a three phase system (so called four wire systems, with a ground conductor) with voltage stabilisation/control in the form of a circuit element with controllable impedance.

In particular it is an object of the invention to provide a device, a method and a system for reduction of harmonics in a system where harmonics are injected into the system by a circuit element with non-linear characteristics.

To achieve this object the invention comprises a device for reducing harmonics in a three-phased voltage supply with voltage stabilisation in the form of, for each phase: a circuit element with controllable impedance, comprising a first magnetic core, a first main winding wound round the core to produce a main magnetic field, and a first control winding wound round the core to produce a control magnetic field mainly orthogonal to the main magnetic field, where the first main winding is arranged for connection to said phase and the impedance of the circuit element is controllable by the current flowing in the control winding, - one second winding wound around said first magnetic core to create a field mainly parallel to the main magnetic field, where the second windings are connected to one another in a delta configuration, characterized in that the device comprises a variable decoupling inductor included in said delta configuration, whn.

3. Device according to claim 2, characterised in that the decoupling inductor is variable.

4.Device according to claim 3, characterised in that ere the decoupling inductor comprises a second magnetic core, a second main winding wound around the core about a first axis and a second control winding wound around the core about a second axis, where the second main and control windings are adapted to create orthogonal fields when energized, such that the inductance of the decoupling inductor is controllable by the current flowing in the second control winding.

The term "included" means in this context that the decoupling inductor is connected in series with all third windings and thus is part of the delta configuration.

In one embodiment of the invention, each of said circuit elements is an inductor (with a secondary winding). In another embodiment, each of said circuit elements is a transformer, comprising further a second main winding wound round the first magnetic core to create a field mainly parallel to the main field.

The invention comprises also a method for reducing harmonics in a three-phased voltage supply with voltage stabilisation in the form of, for each phase: - a circuit element with controllable impedance, comprising a first magnetic core, a first main winding wound round the core to produce a main magnetic field, and a first control winding wound round the core to produce a control magnetic field mainly orthogonal to the main magnetic field, where the first main winding is arranged for connection to said phase and the impedance of the circuit element is controllable by the current flowing in the control winding, characterised in that the where the method further comprises, for each phase, providing for each phase a second winding wound around the first magnetic core to create a field mainly parallel to the main field, and

connecting the second windings to one another in a delta configuration, characterized in that the method comprises the following steps: connecting a .

Method according to claim 7, characterised in that it comprises including variable a decoupling inductor in the delta configuration, where the .

9. Method according to claim 8, comprising providing a decoupling inductor with comprises a third magnetic core, a second main winding wound around the core and a second control winding wound around the core, where the main and the control winding are adapted to create substantially orthogonal fields when energized, such that the inductance of the decoupling inductor is controllable by the current flowing in the second control winding.

In one embodiment of the method the circuit element is a controllable inductor with an extra winding. According to another embodiment of the invention, the circuit element is a transformer comprising further a second main winding wound round the first magnetic core to create a field mainly parallel to the main field.

The invention comprises also a regulating system comprising a device according to the invention, measuring units for output voltage and current for each phase, an input unit and a processing unit for, based on the measured output voltage and current for each phase and on the threshold values, varying the inductance of the decoupling inductor.

The invention will now be explained by means of an example illustrated in the attached drawings, where: Figure 1 shows a first embodiment of the invention.

Figure 2 shows a second embodiment of the invention.

Figure 3 shows an element for use in a device according to the invention.

Figures 4 and 5 show a regulating system according to the invention.

The device 1 shown in figure 1 comprises, for each phase, a circuit element Cl, C2, C3 which in this case is a controllable inductor, comprising a first magnetic core, a first main winding MWl, MW2, MW3 respectively wound round the core to produce a main magnetic field and a first control winding CWl, CW2, CW3 respectively wound round the core to produce a control magnetic field mainly orthogonal to the main magnetic field, where each first main winding MWl, MW2,

MW3 respectively is arranged for connection to one phase in the three-phase system and each first control winding CWl, CW2, CW3 respectively is arranged for connection to a control unit for controlling the impedance in said phase. The device further comprises, for each phase, a second winding SWl, SW2, SW3 wound around the first magnetic core to create a field mainly parallel to the main field.

According to the invention the second windings corresponding to each phase SWl, SW2 and SW3 respectively are connected to one another in delta configuration.

Figure 1 shows also a decoupling inductor LB included in the delta configuration. This decoupling inductor comprises a second magnetic core, a second main winding MWB wound around the core about a first axis and a second control winding CWB wound around the core about a second axis, where the second main and control windings are adapted to create orthogonal fields when energized. The decoupling inductor LB is adapted to vary the decoupling inductor's inductance by means of a second control current. In the embodiment shown in figure 1 the circuit element is a variable inductor, but it is also possible to implement the invention by means of a transformer as e.g. described in WO 03044612. This is shown in figure 2. Elements with the same or similar function as those shown in figure 1 are given the same reference number.

Figure 2 shows circuit elements Cl, C2 and C3, which in this case are transformers, comprising a first magnetic core, a first main winding MWl, MW2, MW3 respectively wound round the core to produce a main magnetic field and a first control winding CWl, CW2, CW3 respectively wound round the core to produce a control magnetic field mainly orthogonal to the main magnetic field, where each first main winding MWl , MW2, MW3 respectively is arranged for connection to one phase in the three-phase system and each first control winding CWl, CW2, CW3 respectively is arranged for connection to a control unit for controlling the impedance in said phase. The device further comprises, for each phase, a second winding SWl, SW2, SW3 wound around the first magnetic core to create a field mainly parallel to the main field. According to the invention the second windings corresponding to each phase SWl, SW2 and SW3 respectively are connected to one another in delta configuration. As in any transformer, the device comprises also a second main winding MW4, MW5, MW6 wound round the first magnetic core to create a field mainly parallel to the main field. The first magnetic core as shown in the figure comprises preferably one leg for SWl (SW2, SW3) and MWl (MW2, MW3) and one leg for MWl (MW2, MW3).

Figure 3 shows an element for use in a device according to the invention. The figure shows the core and windings MW, CW and SW.

Figures 4 and 5 illustrate a regulating system according to the invention.

Said system 105 for voltage stabilisation systems for three-phase power supply comprises a device according to the invention. It also comprises measuring units 106, 107, 108, 109, 110, 111 for output voltage and current for each phase, input units 112, 113, for threshold values and a processing unit 114 for, based on the measured output voltage and current for each phase and on the threshold values, providing a control signal for varying the inductance of the decoupling inductor.

In an embodiment of the invention where the decoupling inductor comprises a magnetic core, a main winding wound around the core and a control winding wound around the core, where the main and the control windings are adapted to create substantially orthogonal fields when energized, the processing unit 114 comprises means 116 for regulating the current supplied to the control winding to vary the decoupling inductor's inductance.

In figure 5 is shown a system where the input units 112 and 113 are adapted to receive threshold values related to symmetry and THD the processing unit 114 is adapted to based on measured values, on third harmonics thresholds and on symmetry thresholds, to provide a control signal for varying the inductance of the decoupling inductor.