WO2013044918A2 - A harmonic filter arrangement - Google Patents

A harmonic filter arrangement Download PDF

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Publication number
WO2013044918A2
WO2013044918A2 PCT/DK2012/050316 DK2012050316W WO2013044918A2 WO 2013044918 A2 WO2013044918 A2 WO 2013044918A2 DK 2012050316 W DK2012050316 W DK 2012050316W WO 2013044918 A2 WO2013044918 A2 WO 2013044918A2
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WO
WIPO (PCT)
Prior art keywords
filter arrangement
inductor
winding
harmonic filter
harmonic
Prior art date
Application number
PCT/DK2012/050316
Other languages
French (fr)
Other versions
WO2013044918A3 (en
Inventor
Nicolae Popescu
Anshuman Tripathi
Amit Kumar Gupta
Original Assignee
Vestas Wind Systems A/S
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Filing date
Publication date
Application filed by Vestas Wind Systems A/S filed Critical Vestas Wind Systems A/S
Publication of WO2013044918A2 publication Critical patent/WO2013044918A2/en
Publication of WO2013044918A3 publication Critical patent/WO2013044918A3/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • H02M1/126—Arrangements for reducing harmonics from ac input or output using passive filters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics

Definitions

  • the present invention relates generally to a harmonic filter arrangement, and in particular, to a harmonic filter arrangement suitable for use in a wind turbine.
  • a wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for power grids.
  • wind incident on blades of the wind turbine generator (WTG) causes a rotor of the WTG to rotate.
  • the mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator.
  • the frequency of the AC (alternating current) voltage/current generated by the generator varies. Therefore, it is common for a wind turbine to have a power converter to convert the variable frequency AC voltage/current from the generator to a substantially fixed frequency AC voltage/current suitable to be connected/transmitted to AC distribution networks or power grids via a transformer.
  • a common type of harmonic filter is a LCL (inductor-capacitor-inductor) filter formed by two inductors LI, L2 and a capacitor C connected between a common node of the two inductors and a neutral node or ground as shown in Fig.l.
  • LCL inctor-capacitor-inductor
  • the inductor L2 is usually represented by the leakage inductance of the transformer, and the harmonic filter is simply implemented using a tuned LC circuit.
  • the current flowing through the winding of the inductor LI includes both the substantially fixed frequency current at a fundamental frequency and harmonics having high frequency.
  • the effective winding resistance becomes higher as the frequency of the current increases due to skin effects.
  • the impedance of a winding for high frequency current components is significantly higher (e.g. might be as high as 100 times) than the impedance of the winding for current components at the lower fundamental frequency. Accordingly, the power loss due to the harmonics (at high frequency) is significant even though the harmonic currents components are only 5 to 10 percent of the total current flowing through the winding, and hence cannot be treated as negligible.
  • a harmonic filter arrangement suitable for use in a wind turbine comprises a first filter arrangement, and a second filter arrangement.
  • the first filter arrangement comprises at least an inductor
  • the second filter arrangement comprises at least an inductor and a capacitor.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding.
  • a power generation system comprising an AC source and a harmonic filter arrangement.
  • the AC source is configured to generate AC power for an AC load
  • the harmonic filter arrangement is configured to remove undesirable AC current components from the AC power.
  • the harmonic filter arrangement comprises a first filter arrangement and a second filter arrangement.
  • the first filter arrangement comprises at least an inductor
  • the second filter arrangement comprises at least an inductor and a capacitor.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding.
  • a wind turbine comprising a generator, a power converter and a grid harmonic filter.
  • the power converter is configured to convert variable frequency AC power from the generator to substantially fixed frequency AC power
  • the grid harmonic filter system is configured to remove undesirable AC current components from the substantially fixed frequency AC power from the power converter.
  • the grid harmonic filter arrangement comprises a first filter arrangement, and a second filter arrangement.
  • the first filter arrangement comprises at least an inductor
  • the second filter arrangement comprises at least an inductor and a capacitor.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding.
  • Figure 1 shows a prior art LCL filter.
  • Figure 2 shows a general structure of a wind turbine.
  • Figure 3 shows an electrical system of the wind turbine.
  • Figure 4 shows the electrical system of the wind turbine of Fig.3 using prior art generator harmonic filter and grid harmonic filter.
  • Figure 5 shows a single phase circuit diagram of the prior art harmonic filter arrangement.
  • Figure 6 shows an electrical diagram of a harmonic filter arrangement according to an embodiment.
  • Figure 7 shows a single phase circuit of the harmonic filter arrangement of Fig.6 according to an embodiment.
  • Figure 8 shows the electrical system of the wind turbine of Fig.3 using the harmonic filter arrangement according to an embodiment.
  • Figure 9 shows a cross-sectional view of the harmonic filter arrangement according to an embodiment.
  • Figure 10a and 10b show a perspective view of the harmonic filter arrangement according to an embodiment.
  • Figure 11a and l ib show a perspective view of the harmonic filter arrangement according to an embodiment. Detailed Description of the Invention
  • the invention provides numerous advantages over the prior art.
  • embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention.
  • the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s).
  • reference to "the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).
  • a harmonic filter arrangement suitable for use in a wind turbine comprises a first filter arrangement, and a second filter arrangement.
  • the first filter arrangement comprises at least an inductor
  • the second filter arrangement comprises at least an inductor and a capacitor.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding.
  • This arrangement of the first winding and the second winding according to the first aspect of the invention uses minimal space, even though there are two inductor windings.
  • the harmonic filter arrangement according to the first aspect is not restricted to be used in the wind turbine. It may also be used in or in conjunction with other suitable applications such as machine-converter applications or solar plants.
  • the first filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency
  • the second filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
  • the harmonic filter arrangement has two inductors, one for each low impedance path. Accordingly, the AC current components substantially at the fundamental frequency pass through the inductor winding in the first filter arrangement and the AC current components (also known as harmonic current components) at the substantially higher frequency (referred herein as "high frequency”) pass through the inductor winding in the second filter arrangement.
  • the inductor of the first filter arrangement offers low impedance to the fundamental frequency components whereas the inductor of the second filter arrangement together with the capacitor offers low impedance to the high frequency harmonic components.
  • the problems associated with the prior art filter arrangement are avoided.
  • the inductor winding of the first filter arrangement only carries the substantially fundamental frequency AC current components and hence the core/winding losses are low.
  • the inductor winding of the second filter arrangement only carries the substantially higher frequency AC current components, and may be adapted to reduce the AC effects in order to reduce core/winding losses. This is not possible in the prior art filter arrangement where the common inductor winding carries both the fundamental frequency and high frequency AC current components. Therefore, the harmonic filter arrangement according to the first aspect does not requirement extensive cooling since the heat generated due to core losses is low compared to the prior art filter arrangement.
  • the inductance of the inductor winding of the second filter arrangement can be significantly higher and therefore there is no need for a large capacitor like in the case of the prior art filter arrangement.
  • the smaller capacitor may be placed near the inductors, leading to better modularity of the harmonic filter arrangement.
  • the fundamental frequency is substantially at 50 Hz or 60 Hz
  • the substantially higher frequency is a plurality of multiples of the fundamental frequency.
  • the first filter arrangement provides the low impedance path for AC current components at or around 50 Hz or 60 Hz, or at a range of 49 to 51 Hz or 59 to 61 Hz.
  • the second filter arrangement provides the low impedance path for the AC current components at multiples of 50 Hz or 60 Hz.
  • the second winding comprises at least one of a thin metal foil and a Litz wire.
  • the use of a thin metal foil or a Litz wire as the second winding reduces the AC effects caused by the harmonic current components, and hence reduces the core/winding losses of the second winding.
  • a Litz wire comprises many thin wire strands which are individually insulated and twisted together.
  • an insulation layer is comprised between the first winding and the second winding. The insulation layer isolates the second winding from the first winding, and further filters the flux of the second winding from the main flux in the core. By providing the insulation layer between the first winding and the second winding, a capacitance may be formed between the first winding and the second winding. Thus a physical external capacitor in the second filter arrangement may not be necessary.
  • a power generation system comprising an AC source and a harmonic filter arrangement.
  • the AC source is configured to generate AC power for an AC load
  • the harmonic filter arrangement is configured to remove undesirable AC current components from the AC power.
  • the harmonic filter arrangement comprises a first filter arrangement and a second filter arrangement.
  • the first filter arrangement comprises at least an inductor
  • the second filter arrangement comprises at least an inductor and a capacitor.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding.
  • the harmonic filter arrangement of the power generation system in this second aspect is similar to the harmonic filter arrangement described in the first aspect, and has all the advantages of the harmonic filter arrangement described with respect to the first aspect of the invention.
  • AC power is used herein as a collective term to refer to AC voltage and current, such as the AC voltage and current generated by the AC source or at the output of the power converter.
  • the AC source is a power converter and the AC load is a power grid.
  • AC power is generated or outputted by the power converter.
  • the substantially higher frequency AC current components of the AC power are filtered by the harmonic filter arrangement before being supplied to the power grid.
  • the power converter may comprise of power semiconductor switches operated by switching signals at a specified switching frequency.
  • a portion of the substantially higher frequency AC current components includes AC current components at the switching frequency and its multiples due to the switching of the semiconductor switches.
  • the AC source is a power generator and the AC load is a power converter.
  • AC power is generated by the power generator.
  • the substantially higher frequency AC current components from the power converter are filtered by the harmonic filter arrangement and hence not seen by the power generator.
  • the AC source is a power converter and the AC is an electric machine, such as an electric motor in other embodiments. It is also possible that the AC source is a power grid and the AC load is a power converter.
  • the first filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency
  • the second filter arrangement configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency
  • the fundamental frequency is substantially at 50 Hz or 60 Hz, and the substantially higher frequency is a plurality of multiples of the fundamental frequency.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding
  • the second winding comprises at least one of a thin metal foil and a Litz wire.
  • a wind turbine comprises a generator, a power converter and a grid harmonic filter.
  • the power converter is configured to convert variable frequency AC power from the generator to substantially fixed frequency AC power
  • the grid harmonic filter system is configured to remove undesirable AC current components from the substantially fixed frequency AC power from the power converter.
  • the grid harmonic filter arrangement comprises a first filter arrangement, and a second filter arrangement.
  • the first filter arrangement comprises at least an inductor
  • the second filter arrangement comprises at least an inductor and a capacitor.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding.
  • the grid harmonic filter arrangement of the power generation system described in the first aspect is used as the grid harmonic filter in this third aspect and has all the advantages of the harmonic filter arrangement described with respect to the first aspect of the invention.
  • the power converter may comprise of power semiconductor switches operated by switching signals at a switching frequency.
  • a portion of the substantially higher frequency AC current components in the processed power output includes AC current components at the switching frequency and its multiples due to the switching of the semiconductor switches in the power converter.
  • the substantially higher frequency AC current components in the substantially fixed frequency AC power are not desirable for transmission to a power grid and hence are to be removed by the grid harmonic filter arrangement.
  • AC power is used herein as a collective term to refer to AC voltage and current, such as the AC voltage and current generated by the AC source or at the output of the power converter.
  • the wind turbine further comprises a generator harmonic filter arrangement between the power converter and the generator.
  • the generator harmonic filter arrangement is configured to remove undesirable AC current components from the power converter and comprises a first filter arrangement, and a second filter arrangement.
  • the first filter arrangement of the generator harmonic filter arrangement comprises at least an inductor
  • the second filter arrangement comprises at least an inductor and a capacitor.
  • the inductor of the first filter arrangement comprises a first winding around a magnetic core
  • the inductor of the second filter arrangement comprises a second winding around the first winding.
  • the generator harmonic filter arrangement is substantially similar to the grid harmonic filter arrangement, and reduces high frequency AC current components from being injected into the generator windings. When high frequency AC current components in the generator windings are reduced, heat generated in the generator windings is also reduced. The advantages of having reduced generator heating include reduced cooling requirements and better power efficiency.
  • the generator harmonic filter arrangement also reduces any high rate of change of voltage from the power converter to the generator windings. Such sharp changes in voltage may affect the generator insulation. In addition, there is also lesser variation in turbine parameters which ensure a more stable turbine control and power curve.
  • the second winding of the grid harmonic filter arrangement comprises at least one of a thin metal foil and a Litz wire.
  • the second winding of the generator harmonic filter arrangement comprises at least one of a thin metal foil and a Litz wire.
  • the first filter arrangement of the grid harmonic filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency
  • the second filter arrangement of the grid harmonic filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency
  • the first filter arrangement of the generator harmonic filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency
  • the second filter arrangement of the generator harmonic filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency
  • Fig.2 illustrates an exemplary wind turbine 100 according to an embodiment.
  • the wind turbine 100 includes a tower 110, a nacelle 120, and a rotor 130.
  • the wind turbine 100 may be an onshore wind turbine.
  • embodiments of the invention are not limited only to onshore wind turbines.
  • the wind turbine 100 may be an offshore wind turbine located over a water body such as, for example, a lake, an ocean, or the like.
  • the tower 110 of such an offshore wind turbine is installed on either the sea floor or on platforms stabilized on or above the sea level.
  • the tower 110 of the wind turbine 100 may be configured to raise the nacelle 120 and the rotor 130 to a height where strong, less turbulent, and generally unobstructed flow of air may be received by the rotor 130.
  • the height of the tower 110 may be any reasonable height, and should consider the length of wind turbine blades extending from the rotor 130.
  • the tower 110 may be made from any type of material, for example, steel, concrete, or the like. In some embodiments the tower 110 may be made from a monolithic material. However, in alternative embodiments, the tower 110 may include a plurality of sections, for example, two or more tubular steel sections 111 and 112, as illustrated in Fig.2. In some embodiments of the invention, the tower 110 may be a lattice tower. Accordingly, the tower 110 may include welded steel profiles.
  • the rotor 130 may include a rotor hub (hereinafter referred to simply as the "hub") 132 and at least one blade 140 (three such blades 140 are shown in Fig.2).
  • the rotor hub 132 may be configured to couple the at least one blade 140 to a shaft (not shown).
  • the blades 140 may have an aerodynamic profile such that, at predefined wind speeds, the blades 140 experience lift, thereby causing the blades to radially rotate around the hub.
  • the hub 140 further comprises mechanisms (not shown) for adjusting the pitch of the blade 140 to increase or reduce the amount of wind energy captured by the blade 140. Pitching adjusts the angle at which the wind strikes the blade 140. It is also possible that the pitch of the blades 140 cannot be adjusted.
  • the aerodynamic profile of the blades 140 is designed in a manner that the lift experienced by the blades are lost when the wind speed exceeded a certain threshold, causing the turbine to stall.
  • the hub 132 typically rotates about a substantially horizontal axis along a drive shaft (not shown) extending from the hub 132 to the nacelle 120.
  • the drive shaft is usually coupled to one or more components in the nacelle 120, which are configured to convert the rotational energy of the shaft into electrical energy.
  • the wind turbine 100 shown in Fig.2 has three blades 140, it should be noted that a wind turbine may have different number of blades. It is common to find wind turbines having two to four blades.
  • the wind turbine 100 shown in Fig. l is a Horizontal Axis Wind Turbine (HAWT) as the rotor 130 rotates about a horizontal axis. It should be noted that the rotor 130 may rotate about a vertical axis. Such a wind turbine having its rotor rotates about the vertical axis is known as a Vertical Axis Wind Turbine (VAWT).
  • VAWT Vertical Axis Wind Turbine
  • the embodiments described henceforth are not limited to HAWT having 3 blades. They may be implemented in both HAWT and VAWT, and having any number of blades 140 in the rotor 130.
  • Fig.3 shows an electrical system of the wind turbine.
  • the electrical system includes a generator 201, a power converter 202, a generator harmonic filter 203, a grid harmonic filter 204 and a main transformer 205.
  • the electrical system is connected to a power grid 207.
  • the power converter 202 includes a generator-side converter 210 and a grid-side converter 211 connected via a direct current (DC) link 212.
  • the DC-link 212 includes a DC-link capacitor 213.
  • the electrical system also include generator- side breaker 215 between the generator 201 and the generator harmonic filter 203, and grid- side breaker 216 between the grid harmonic filter 204 and the transformer 205.
  • the generator 201 converts mechanical energy or power to electrical energy or power having AC voltage and current (collectively referred to as "AC signals"), and provides the generated AC signals to the generator- side converter 210.
  • the AC signals from the generator have a variable frequency, due to varying wind speed.
  • the generator- side converter 210 converts or rectifies the AC signals to DC voltage and current (collectively know as "DC signals").
  • the grid-side converter 211 converts the DC signals from the DC-link 212 into fixed frequency AC signals for the power grid 207.
  • the voltage of the fixed frequency AC signals at the output of the grid- side converter 211 is stepped up by the main transformer 205.
  • the generator- side converter 210 and the grid-side converter 211 comprises a plurality of power semiconductor switches such as IGBTs (Insulated Gate Bipolar Transistors) and their operations are controlled by switching signals (also known as PWM signals) at a specific switching frequency.
  • the switching frequency is usually much higher than the frequency of the AC signals generated by the generator 201 and outputted from the grid-side converter 211.
  • the frequency of the AC signal from the grid-side converter 211 is 50 Hz and the switching frequency could be in the range of 0.5 to 20 kHz.
  • high frequency AC current components corresponding to the switching frequency may be introduced into the AC current at the output of the converter 202.
  • switching harmonics are undesirable and may be injected into the generator windings of the generator 201 or supplied to the power grid 207.
  • the generator harmonic filter 203 is used to filter such undesirable AC current components or switching harmonics and prevent them from being injected into the generator windings.
  • the grid harmonic filter 204 is used to filter the undesirable AC current components from the output of the grid- side converter 211 and prevent them from being injected into the power grid 207.
  • Fig.3 is only an illustration of an electrical system in a wind turbine where only common components are shown.
  • the electrical system may include other components such as dump loads, sensors, DC-link voltage pre-charge arrangement, resonant filter, etc.
  • the harmonic filter is usually a LCL filter formed by two inductors LI, L2 and the capacitor C connected between the common node of the two inductors and a neutral node as shown in Fig. l.
  • the inductor L2 is provided by the leakage inductance of the transformer 205 windings.
  • the grid harmonic filter 204 is usually a LC circuit.
  • the inductor L2 is provided by the inductance of the generator windings.
  • the generator harmonic filter 203 is also usually a LC circuit, but may also include other circuitries such as a RC circuit.
  • Fig.4 shows the electrical system of the wind turbine of Fig.3 having using the LC circuit as the generator harmonic filter 203 and the grid harmonic filter 204.
  • Fig.5 shows a single phase circuit diagram of the prior art harmonic filter arrangement.
  • ii is an AC current at the substantially fundamental frequency
  • i h is AC current at high frequency.
  • the AC current source 501 represents the power converter which supplies both the substantially fundamental frequency and high frequency AC current components. Both the substantially fundamental frequency and high frequency currents ii, i h pass through the inductor LI .
  • the filter arrangement is properly tuned, the high frequency current passes through the capacitor C and the fundamental frequency current passes through the inductor L2 and is supplied to the load 502.
  • the inductor LI carries both the fundamental frequency and high frequency currents.
  • the power loss due to resistive heating through the winding of the inductor LI is the sum of the power loss caused by each of the AC current com onents, and can be represented as:
  • Ri and 3 ⁇ 4 are the effective winding resistances at the fundamental frequency and the high frequency respectively.
  • the effective winding resistance increases with higher frequency and thicker conductor windings. This is due to AC effects such as skin, proximity and eddy, resulting in spatial non-uniformities of the AC flow in the windings.
  • the effective resistance of the inductor LI is high due to the high frequency current, resulting in high power loss.
  • Fig.6 shows the harmonic filter arrangement according to an embodiment.
  • the harmonic filter arrangement according to the embodiment includes a first filter arrangement having an inductor L3 and a second filter arrangement having an inductor L4 and a capacitor C2.
  • the first and second filter arrangements are connected at a common node A.
  • the capacitor C2 is connected to the inductor L4 at one end, and to ground or a neutral node at the other end.
  • the operation of the harmonic filter arrangement according to the embodiment is explained with reference to Fig.7.
  • Fig.7 shows a single phase circuit diagram of the harmonic filter arrangement of Fig.6 according to an embodiment.
  • ii is the AC current at the substantially fundamental frequency
  • 3 ⁇ 4 is the AC current at the high frequency.
  • the AC current source 601 supplies both the fundamental frequency and high frequency AC current.
  • the inductor L4 and capacitor C2 are tuned so that it provides a low impedance path for the high frequency current i h . Accordingly, the high frequency AC current passes through the low impedance path of the inductor L4 and capacitor C2 in the second filter arrangement.
  • the inductor L3 is designed so that it provides a low impedance path for fundamental frequency current ii. Accordingly, the fundamental frequency AC current passes through the inductor L3 of the first filter arrangement, and subsequently through the inductor L2 and supplied to the load 602.
  • the inductor L3 now only carries the fundamental frequency AC current.
  • the windings of the inductor L4 carrying the high frequency AC current can be structured in such way that the AC effects are minimized.
  • the inductor L4 may be made of very thin wire windings such as thin foils or Litz wires. Accordingly, the power loss due to AC effects contributed by the high frequency AC current is eliminated or minimized.
  • Fig.8 shows the electrical system of the wind turbine of Fig.3 using the harmonic filter arrangement of Fig.6 as the generator harmonic filter 203 and the grid harmonic filter 204 according to an embodiment.
  • a 3-phase system is shown and hence there are three harmonic filter arrangements for each of the generator harmonic filter 203 and the grid harmonic filter 204, each harmonic filter arrangement corresponds to each phase.
  • the power converter 202 generates AC signals having a fundamental frequency.
  • the fundamental frequency may be 50 Hz or 60 Hz.
  • Harmonic components having frequency components at least some multiples of the fundamental frequency (high frequency) are also injected into the AC signals. These harmonic components may be due to the switching of the semiconductor switches and may be about 5 kHz and its multiples.
  • the fundamental frequency AC signal passes through the first filter arrangement and is supplied to the grid.
  • the high frequency harmonic components pass through the second filter arrangement, and hence filtered from the AC signal.
  • both the fundamental frequency AC signal and the harmonic components do not pass through the same inductor winding, but they are segregated and pass through different inductor windings.
  • the inductor winding carrying the harmonic components may be further optimized to reduce the power loss due to AC effects, without affecting the fundamental frequency AC current components. With this reduction of power loss, the grid harmonic filter get significantly less hot as in the case of prior art arrangements, and cooling requirements can be reduced.
  • the capacitance need not be as high as before, and hence can be placed closer to the inductors as a module. The less strict requirement for capacitors also provides a wider selection of capacitors to be used in grid harmonic filter arrangement.
  • harmonic filter arrangement may also be used as the generator harmonic filter, with the advantages associated therewith.
  • the reduced heating of the generator core & windings reduces cooling requirements of the generator and also results in more stable control of the generator and better efficiency of the power production.
  • the generator harmonic filter arrangement 203 also includes a resistor 702 and a capacitor 703. This RC circuit 702,703 is connected in parallel with the inductor L3 of the first filter arrangement. As mentioned earlier, the generator harmonic filter arrangement 203 filters off high switching frequency voltages from the generator windings.
  • the harmonic filter arrangement according to the embodiment may be used only as the generator harmonic filter or the grid harmonic filter, and need not be used for both.
  • the harmonic filter arrangement according to the embodiment may only be used as the grid harmonic filter, and a prior art LCL filter may be used for the generator harmonic filter.
  • the inductor L3 of the first filter arrangement of the harmonic filter arrangement is formed by a first winding around a magnetic core
  • the inductor L4 of the second filter arrangement is formed by a second winding around the first winding.
  • Fig.9 shows a cross-sectional view of the harmonic filter arrangement according to an embodiment.
  • the first winding 801 surrounds the magnetic core 802
  • the second winding 803 surrounds the first winding 801 and the magnetic core 802.
  • An insulation layer 804 may be formed between the first winding 801 and the second winding 801 as shown in Fig.9. This arrangement provides a simple and space efficient implementation of the harmonic filter arrangement according to the embodiment.
  • An external capacitor (not shown) is connected to the second winding 803, forming the capacitor of the second filter arrangement. It is possible that the capacitance is formed by the insulation layer 804 between the first and second windings 801, 802. In this case, the external capacitor may not be needed. The need to have a separate capacitor is reduced in this case because (a) there is an alternative path through the second windings for high switching harmonics and (b) there is an inherent capacitance between the two sets of windings. The information on the switching frequency and winding material is used to design this inherent capacitance.
  • the arrangement shown in Fig.9 is used to implement LCL harmonic filter arrangement of Fig.l and Fig.5.
  • the inductor LI of the first filter arrangement is formed by the first winding around the magnetic core
  • the inductor L2 is formed by the second winding around the first winding.
  • the first winding 801 surrounds the magnetic core 802
  • the second winding 803 surrounds the first winding 801 and the magnetic core 802.
  • the insulation layer 804 may be formed between the first winding 801 and the second winding 801. This arrangement also provides a simple and space efficient implementation of the LCL harmonic filter arrangement.
  • An external capacitor (not shown) may be connected to the second winding 803, forming the capacitor of the second filter arrangement. It is also possible that the capacitance is formed by the insulation layer 804 between the first and second windings 801, 802. In this case, the external capacitor may not be needed.
  • Fig.10a shows a perspective view of a single-phase harmonic filter arrangement according to an embodiment.
  • the first winding 801 which is the inductor L3 (or inductor LI) of the first filter arrangement, is wounded around the magnetic core 802.
  • the second winding 803 is made of a thin foil.
  • the thin foil has a very small cross-sectional area, and hence minimizes the AC effects due to high frequency AC current components flowing therethrough.
  • Fig.10b shows a perspective view of a three-phase harmonic filter arrangement of the single-phase harmonic filter arrangement shown in Fig.10a.
  • Fig.11a shows a perspective view of a single-phase harmonic filter arrangement according to an embodiment.
  • the first winding 801 which is the inductor L3 (inductor LI) of the first filter arrangement, is wounded around the magnetic core 802.
  • the second winding 803 is made of a Litz wire.
  • the Litz wire is made of many tiny individually insulated wires twisted together, and hence minimizes the AC effects due to high frequency AC current components flowing there through.
  • Fig. l ib shows a perspective view of a three-phase harmonic filter arrangement of the single-phase harmonic filter arrangement shown in Fig.11a.

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Abstract

A harmonic filter arrangement suitable for use in a wind turbine is provided. The harmonic filter arrangement comprises a first filter arrangement, and a second filter arrangement. The first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.

Description

A HARMONIC FILTER ARRANGEMENT
Field of the Invention
The present invention relates generally to a harmonic filter arrangement, and in particular, to a harmonic filter arrangement suitable for use in a wind turbine.
Background of the Invention
A wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for power grids. Specifically, wind incident on blades of the wind turbine generator (WTG) causes a rotor of the WTG to rotate. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. As wind speed fluctuates, the frequency of the AC (alternating current) voltage/current generated by the generator varies. Therefore, it is common for a wind turbine to have a power converter to convert the variable frequency AC voltage/current from the generator to a substantially fixed frequency AC voltage/current suitable to be connected/transmitted to AC distribution networks or power grids via a transformer.
Power converters of wind turbine generate voltage or current harmonics which are undesirable for transmission to the power grid due to the harmonic losses and compliance issues. It is common to have a harmonic filter arrangement between the power converter and the transformer/grid to remove such harmonics from the line currents before transmitting the power to the power grid. A common type of harmonic filter is a LCL (inductor-capacitor-inductor) filter formed by two inductors LI, L2 and a capacitor C connected between a common node of the two inductors and a neutral node or ground as shown in Fig.l. In the wind turbine, the inductor L2 is usually represented by the leakage inductance of the transformer, and the harmonic filter is simply implemented using a tuned LC circuit.
The current flowing through the winding of the inductor LI (also known as grid choke) includes both the substantially fixed frequency current at a fundamental frequency and harmonics having high frequency. The effective winding resistance becomes higher as the frequency of the current increases due to skin effects. In practical applications, the impedance of a winding for high frequency current components is significantly higher (e.g. might be as high as 100 times) than the impedance of the winding for current components at the lower fundamental frequency. Accordingly, the power loss due to the harmonics (at high frequency) is significant even though the harmonic currents components are only 5 to 10 percent of the total current flowing through the winding, and hence cannot be treated as negligible.
Hence it is desirable to provide a harmonic filter arrangement which does not have or minimizes the problems described above.
Summary of the Invention
According to a first aspect of the invention, a harmonic filter arrangement suitable for use in a wind turbine is provided. The harmonic filter arrangement comprises a first filter arrangement, and a second filter arrangement. The first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
According to a second aspect of the invention, a power generation system is provided. The power generation system comprises an AC source and a harmonic filter arrangement. The AC source is configured to generate AC power for an AC load, and the harmonic filter arrangement is configured to remove undesirable AC current components from the AC power.
The harmonic filter arrangement comprises a first filter arrangement and a second filter arrangement. The first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
According to a third aspect of the invention, a wind turbine is provided. The wind turbine comprises a generator, a power converter and a grid harmonic filter. The power converter is configured to convert variable frequency AC power from the generator to substantially fixed frequency AC power, and the grid harmonic filter system is configured to remove undesirable AC current components from the substantially fixed frequency AC power from the power converter. The grid harmonic filter arrangement comprises a first filter arrangement, and a second filter arrangement. The first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
Brief Description of the Drawings
The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings.
Figure 1 shows a prior art LCL filter.
Figure 2 shows a general structure of a wind turbine.
Figure 3 shows an electrical system of the wind turbine.
Figure 4 shows the electrical system of the wind turbine of Fig.3 using prior art generator harmonic filter and grid harmonic filter.
Figure 5 shows a single phase circuit diagram of the prior art harmonic filter arrangement.
Figure 6 shows an electrical diagram of a harmonic filter arrangement according to an embodiment.
Figure 7 shows a single phase circuit of the harmonic filter arrangement of Fig.6 according to an embodiment.
Figure 8 shows the electrical system of the wind turbine of Fig.3 using the harmonic filter arrangement according to an embodiment.
Figure 9 shows a cross-sectional view of the harmonic filter arrangement according to an embodiment.
Figure 10a and 10b show a perspective view of the harmonic filter arrangement according to an embodiment.
Figure 11a and l ib show a perspective view of the harmonic filter arrangement according to an embodiment. Detailed Description of the Invention
In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention.
Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to "the invention" shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).
In the first aspect, a harmonic filter arrangement suitable for use in a wind turbine is provided. The harmonic filter arrangement comprises a first filter arrangement, and a second filter arrangement. The first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
This arrangement of the first winding and the second winding according to the first aspect of the invention uses minimal space, even though there are two inductor windings.
It should be noted that the harmonic filter arrangement according to the first aspect is not restricted to be used in the wind turbine. It may also be used in or in conjunction with other suitable applications such as machine-converter applications or solar plants. According to an embodiment, the first filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency, and the second filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
The harmonic filter arrangement according to this embodiment has two inductors, one for each low impedance path. Accordingly, the AC current components substantially at the fundamental frequency pass through the inductor winding in the first filter arrangement and the AC current components (also known as harmonic current components) at the substantially higher frequency (referred herein as "high frequency") pass through the inductor winding in the second filter arrangement. This is because, the inductor of the first filter arrangement offers low impedance to the fundamental frequency components whereas the inductor of the second filter arrangement together with the capacitor offers low impedance to the high frequency harmonic components.
This is in contrast to the prior art filter arrangement where both the fundamental frequency and high frequency AC current components pass through a common inductor winding. The effective winding resistance of the common inductor winding for the high frequency AC current components is greater than the winding resistance for the fundamental frequency AC current components due to AC effects such as the skin effect. This results in high core/winding losses in the common inductor. As a result of the high losses, the inductor windings become significantly hot and require extensive cooling. Such a need for thermal management or part degradation leads to complexity especially for wind turbines where day to day accessibility is not possible. Therefore the common inductor windings or grid chokes are often over-rated to provide for the proper cooling, adding to the cost of the component. Large capacitors are also required in the filter arrangement, and these large capacitors have to be place at some distance from the inductor due to space constraint in the wind turbines. This requires long cables to be connected from the inductor to the capacitors located at a distance away.
By using the harmonic filter arrangement having separate inductors for fundamental frequency and high frequency AC current components according to the first aspect, the problems associated with the prior art filter arrangement are avoided. Specifically, the inductor winding of the first filter arrangement only carries the substantially fundamental frequency AC current components and hence the core/winding losses are low. The inductor winding of the second filter arrangement only carries the substantially higher frequency AC current components, and may be adapted to reduce the AC effects in order to reduce core/winding losses. This is not possible in the prior art filter arrangement where the common inductor winding carries both the fundamental frequency and high frequency AC current components. Therefore, the harmonic filter arrangement according to the first aspect does not requirement extensive cooling since the heat generated due to core losses is low compared to the prior art filter arrangement.
In addition, the inductance of the inductor winding of the second filter arrangement can be significantly higher and therefore there is no need for a large capacitor like in the case of the prior art filter arrangement. This allows a smaller capacitor to be used in the second filter arrangement, therefore both cost and space requirements are reduced. The smaller capacitor may be placed near the inductors, leading to better modularity of the harmonic filter arrangement.
According to an embodiment, the fundamental frequency is substantially at 50 Hz or 60 Hz, and the substantially higher frequency is a plurality of multiples of the fundamental frequency. According to the embodiment, the first filter arrangement provides the low impedance path for AC current components at or around 50 Hz or 60 Hz, or at a range of 49 to 51 Hz or 59 to 61 Hz. The second filter arrangement provides the low impedance path for the AC current components at multiples of 50 Hz or 60 Hz. For example, the substantially higher frequency may be at 50/60*n where n = 1,5,7,11,13 and so on. It is also possible that n is an even number or a non-integer number.
According to an embodiment, the second winding comprises at least one of a thin metal foil and a Litz wire. The use of a thin metal foil or a Litz wire as the second winding reduces the AC effects caused by the harmonic current components, and hence reduces the core/winding losses of the second winding. A Litz wire comprises many thin wire strands which are individually insulated and twisted together. According to an embodiment, an insulation layer is comprised between the first winding and the second winding. The insulation layer isolates the second winding from the first winding, and further filters the flux of the second winding from the main flux in the core. By providing the insulation layer between the first winding and the second winding, a capacitance may be formed between the first winding and the second winding. Thus a physical external capacitor in the second filter arrangement may not be necessary.
In the second aspect of the invention, a power generation system is provided. The power generation system comprises an AC source and a harmonic filter arrangement. The AC source is configured to generate AC power for an AC load, and the harmonic filter arrangement is configured to remove undesirable AC current components from the AC power. The harmonic filter arrangement comprises a first filter arrangement and a second filter arrangement. The first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
The harmonic filter arrangement of the power generation system in this second aspect is similar to the harmonic filter arrangement described in the first aspect, and has all the advantages of the harmonic filter arrangement described with respect to the first aspect of the invention. It should be noted that AC power is used herein as a collective term to refer to AC voltage and current, such as the AC voltage and current generated by the AC source or at the output of the power converter.
According to an embodiment, the AC source is a power converter and the AC load is a power grid. In this embodiment, AC power is generated or outputted by the power converter. The substantially higher frequency AC current components of the AC power are filtered by the harmonic filter arrangement before being supplied to the power grid.
The power converter may comprise of power semiconductor switches operated by switching signals at a specified switching frequency. A portion of the substantially higher frequency AC current components includes AC current components at the switching frequency and its multiples due to the switching of the semiconductor switches.
According to another embodiment, the AC source is a power generator and the AC load is a power converter. In this embodiment, AC power is generated by the power generator. The substantially higher frequency AC current components from the power converter are filtered by the harmonic filter arrangement and hence not seen by the power generator.
It should be noted that it is also possible that the AC source is a power converter and the AC is an electric machine, such as an electric motor in other embodiments. It is also possible that the AC source is a power grid and the AC load is a power converter.
According to an embodiment, the first filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency, and the second filter arrangement configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
According to an embodiment, the fundamental frequency is substantially at 50 Hz or 60 Hz, and the substantially higher frequency is a plurality of multiples of the fundamental frequency.
According to an embodiment, the inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
According to an embodiment, the second winding comprises at least one of a thin metal foil and a Litz wire. In the third aspect of the invention, a wind turbine is provided. The wind turbine comprises a generator, a power converter and a grid harmonic filter. The power converter is configured to convert variable frequency AC power from the generator to substantially fixed frequency AC power, and the grid harmonic filter system is configured to remove undesirable AC current components from the substantially fixed frequency AC power from the power converter. The grid harmonic filter arrangement comprises a first filter arrangement, and a second filter arrangement. The first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
The grid harmonic filter arrangement of the power generation system described in the first aspect is used as the grid harmonic filter in this third aspect and has all the advantages of the harmonic filter arrangement described with respect to the first aspect of the invention. The power converter may comprise of power semiconductor switches operated by switching signals at a switching frequency. A portion of the substantially higher frequency AC current components in the processed power output includes AC current components at the switching frequency and its multiples due to the switching of the semiconductor switches in the power converter. The substantially higher frequency AC current components in the substantially fixed frequency AC power are not desirable for transmission to a power grid and hence are to be removed by the grid harmonic filter arrangement.
Similarly, AC power is used herein as a collective term to refer to AC voltage and current, such as the AC voltage and current generated by the AC source or at the output of the power converter.
According to an embodiment, the wind turbine further comprises a generator harmonic filter arrangement between the power converter and the generator. The generator harmonic filter arrangement is configured to remove undesirable AC current components from the power converter and comprises a first filter arrangement, and a second filter arrangement. The first filter arrangement of the generator harmonic filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor. The inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
The generator harmonic filter arrangement is substantially similar to the grid harmonic filter arrangement, and reduces high frequency AC current components from being injected into the generator windings. When high frequency AC current components in the generator windings are reduced, heat generated in the generator windings is also reduced. The advantages of having reduced generator heating include reduced cooling requirements and better power efficiency. The generator harmonic filter arrangement also reduces any high rate of change of voltage from the power converter to the generator windings. Such sharp changes in voltage may affect the generator insulation. In addition, there is also lesser variation in turbine parameters which ensure a more stable turbine control and power curve.
According to an embodiment, the second winding of the grid harmonic filter arrangement comprises at least one of a thin metal foil and a Litz wire.
According to an embodiment, the second winding of the generator harmonic filter arrangement comprises at least one of a thin metal foil and a Litz wire.
According to an embodiment, the first filter arrangement of the grid harmonic filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency, and the second filter arrangement of the grid harmonic filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
According to an embodiment, the first filter arrangement of the generator harmonic filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency, and the second filter arrangement of the generator harmonic filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
The following is a detailed description of embodiments of the invention depicted in the accompanying drawings. The embodiments are examples and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
Fig.2 illustrates an exemplary wind turbine 100 according to an embodiment. As illustrated in Fig.2, the wind turbine 100 includes a tower 110, a nacelle 120, and a rotor 130. In one embodiment, the wind turbine 100 may be an onshore wind turbine. However, embodiments of the invention are not limited only to onshore wind turbines. In alternative embodiments, the wind turbine 100 may be an offshore wind turbine located over a water body such as, for example, a lake, an ocean, or the like. The tower 110 of such an offshore wind turbine is installed on either the sea floor or on platforms stabilized on or above the sea level.
The tower 110 of the wind turbine 100 may be configured to raise the nacelle 120 and the rotor 130 to a height where strong, less turbulent, and generally unobstructed flow of air may be received by the rotor 130. The height of the tower 110 may be any reasonable height, and should consider the length of wind turbine blades extending from the rotor 130. The tower 110 may be made from any type of material, for example, steel, concrete, or the like. In some embodiments the tower 110 may be made from a monolithic material. However, in alternative embodiments, the tower 110 may include a plurality of sections, for example, two or more tubular steel sections 111 and 112, as illustrated in Fig.2. In some embodiments of the invention, the tower 110 may be a lattice tower. Accordingly, the tower 110 may include welded steel profiles.
The rotor 130 may include a rotor hub (hereinafter referred to simply as the "hub") 132 and at least one blade 140 (three such blades 140 are shown in Fig.2). The rotor hub 132 may be configured to couple the at least one blade 140 to a shaft (not shown). In one embodiment, the blades 140 may have an aerodynamic profile such that, at predefined wind speeds, the blades 140 experience lift, thereby causing the blades to radially rotate around the hub. The hub 140 further comprises mechanisms (not shown) for adjusting the pitch of the blade 140 to increase or reduce the amount of wind energy captured by the blade 140. Pitching adjusts the angle at which the wind strikes the blade 140. It is also possible that the pitch of the blades 140 cannot be adjusted. In this case, the aerodynamic profile of the blades 140 is designed in a manner that the lift experienced by the blades are lost when the wind speed exceeded a certain threshold, causing the turbine to stall.
The hub 132 typically rotates about a substantially horizontal axis along a drive shaft (not shown) extending from the hub 132 to the nacelle 120. The drive shaft is usually coupled to one or more components in the nacelle 120, which are configured to convert the rotational energy of the shaft into electrical energy.
Although the wind turbine 100 shown in Fig.2 has three blades 140, it should be noted that a wind turbine may have different number of blades. It is common to find wind turbines having two to four blades. The wind turbine 100 shown in Fig. l is a Horizontal Axis Wind Turbine (HAWT) as the rotor 130 rotates about a horizontal axis. It should be noted that the rotor 130 may rotate about a vertical axis. Such a wind turbine having its rotor rotates about the vertical axis is known as a Vertical Axis Wind Turbine (VAWT). The embodiments described henceforth are not limited to HAWT having 3 blades. They may be implemented in both HAWT and VAWT, and having any number of blades 140 in the rotor 130.
Fig.3 shows an electrical system of the wind turbine. The electrical system includes a generator 201, a power converter 202, a generator harmonic filter 203, a grid harmonic filter 204 and a main transformer 205. The electrical system is connected to a power grid 207. The power converter 202 includes a generator-side converter 210 and a grid-side converter 211 connected via a direct current (DC) link 212. The DC-link 212 includes a DC-link capacitor 213. The electrical system also include generator- side breaker 215 between the generator 201 and the generator harmonic filter 203, and grid- side breaker 216 between the grid harmonic filter 204 and the transformer 205.
The generator 201 converts mechanical energy or power to electrical energy or power having AC voltage and current (collectively referred to as "AC signals"), and provides the generated AC signals to the generator- side converter 210. The AC signals from the generator have a variable frequency, due to varying wind speed. The generator- side converter 210 converts or rectifies the AC signals to DC voltage and current (collectively know as "DC signals"). The grid-side converter 211 converts the DC signals from the DC-link 212 into fixed frequency AC signals for the power grid 207. The voltage of the fixed frequency AC signals at the output of the grid- side converter 211 is stepped up by the main transformer 205.
The generator- side converter 210 and the grid-side converter 211 comprises a plurality of power semiconductor switches such as IGBTs (Insulated Gate Bipolar Transistors) and their operations are controlled by switching signals (also known as PWM signals) at a specific switching frequency. The switching frequency is usually much higher than the frequency of the AC signals generated by the generator 201 and outputted from the grid-side converter 211. As an example, the frequency of the AC signal from the grid-side converter 211 is 50 Hz and the switching frequency could be in the range of 0.5 to 20 kHz.
Due to the switching of the switches in the converters 210, 211, high frequency AC current components corresponding to the switching frequency (also known as "switching harmonics") may be introduced into the AC current at the output of the converter 202. Such switching harmonics are undesirable and may be injected into the generator windings of the generator 201 or supplied to the power grid 207. The generator harmonic filter 203 is used to filter such undesirable AC current components or switching harmonics and prevent them from being injected into the generator windings. The grid harmonic filter 204 is used to filter the undesirable AC current components from the output of the grid- side converter 211 and prevent them from being injected into the power grid 207.
It should be noted that Fig.3 is only an illustration of an electrical system in a wind turbine where only common components are shown. The electrical system may include other components such as dump loads, sensors, DC-link voltage pre-charge arrangement, resonant filter, etc.
As described with reference to Fig.l, the harmonic filter is usually a LCL filter formed by two inductors LI, L2 and the capacitor C connected between the common node of the two inductors and a neutral node as shown in Fig. l. In the grid harmonic filter 204, the inductor L2 is provided by the leakage inductance of the transformer 205 windings. Hence the grid harmonic filter 204 is usually a LC circuit. Similarly, in the generator harmonic filter 203, the inductor L2 is provided by the inductance of the generator windings. Hence the generator harmonic filter 203 is also usually a LC circuit, but may also include other circuitries such as a RC circuit. Fig.4 shows the electrical system of the wind turbine of Fig.3 having using the LC circuit as the generator harmonic filter 203 and the grid harmonic filter 204.
Fig.5 shows a single phase circuit diagram of the prior art harmonic filter arrangement. In the circuit, ii is an AC current at the substantially fundamental frequency and ih is AC current at high frequency. The AC current source 501 represents the power converter which supplies both the substantially fundamental frequency and high frequency AC current components. Both the substantially fundamental frequency and high frequency currents ii, ih pass through the inductor LI . When the filter arrangement is properly tuned, the high frequency current passes through the capacitor C and the fundamental frequency current passes through the inductor L2 and is supplied to the load 502.
As can be seen from the above, the inductor LI carries both the fundamental frequency and high frequency currents. The power loss due to resistive heating through the winding of the inductor LI is the sum of the power loss caused by each of the AC current com onents, and can be represented as:
Figure imgf000015_0001
where Ri and ¾ are the effective winding resistances at the fundamental frequency and the high frequency respectively. The effective winding resistance increases with higher frequency and thicker conductor windings. This is due to AC effects such as skin, proximity and eddy, resulting in spatial non-uniformities of the AC flow in the windings. Thus the effective resistance of the inductor LI is high due to the high frequency current, resulting in high power loss.
Fig.6 shows the harmonic filter arrangement according to an embodiment. The harmonic filter arrangement according to the embodiment includes a first filter arrangement having an inductor L3 and a second filter arrangement having an inductor L4 and a capacitor C2. The first and second filter arrangements are connected at a common node A. The capacitor C2 is connected to the inductor L4 at one end, and to ground or a neutral node at the other end. The operation of the harmonic filter arrangement according to the embodiment is explained with reference to Fig.7.
Fig.7 shows a single phase circuit diagram of the harmonic filter arrangement of Fig.6 according to an embodiment. In the circuit, ii is the AC current at the substantially fundamental frequency and ¾ is the AC current at the high frequency. The AC current source 601 supplies both the fundamental frequency and high frequency AC current. The inductor L4 and capacitor C2 are tuned so that it provides a low impedance path for the high frequency current ih. Accordingly, the high frequency AC current passes through the low impedance path of the inductor L4 and capacitor C2 in the second filter arrangement. Similarly, the inductor L3 is designed so that it provides a low impedance path for fundamental frequency current ii. Accordingly, the fundamental frequency AC current passes through the inductor L3 of the first filter arrangement, and subsequently through the inductor L2 and supplied to the load 602.
As can be seen from the Fig.7, the inductor L3 now only carries the fundamental frequency AC current. The windings of the inductor L4 carrying the high frequency AC current can be structured in such way that the AC effects are minimized. For example, the inductor L4 may be made of very thin wire windings such as thin foils or Litz wires. Accordingly, the power loss due to AC effects contributed by the high frequency AC current is eliminated or minimized.
Fig.8 shows the electrical system of the wind turbine of Fig.3 using the harmonic filter arrangement of Fig.6 as the generator harmonic filter 203 and the grid harmonic filter 204 according to an embodiment. In Fig.8, a 3-phase system is shown and hence there are three harmonic filter arrangements for each of the generator harmonic filter 203 and the grid harmonic filter 204, each harmonic filter arrangement corresponds to each phase. The power converter 202 generates AC signals having a fundamental frequency. The fundamental frequency may be 50 Hz or 60 Hz. Harmonic components having frequency components at least some multiples of the fundamental frequency (high frequency) are also injected into the AC signals. These harmonic components may be due to the switching of the semiconductor switches and may be about 5 kHz and its multiples.
As explained above, the fundamental frequency AC signal passes through the first filter arrangement and is supplied to the grid. The high frequency harmonic components pass through the second filter arrangement, and hence filtered from the AC signal. According to the embodiment, both the fundamental frequency AC signal and the harmonic components do not pass through the same inductor winding, but they are segregated and pass through different inductor windings. The inductor winding carrying the harmonic components may be further optimized to reduce the power loss due to AC effects, without affecting the fundamental frequency AC current components. With this reduction of power loss, the grid harmonic filter get significantly less hot as in the case of prior art arrangements, and cooling requirements can be reduced. Also in this arrangement according to the embodiment, the capacitance need not be as high as before, and hence can be placed closer to the inductors as a module. The less strict requirement for capacitors also provides a wider selection of capacitors to be used in grid harmonic filter arrangement.
Similarly the harmonic filter arrangement may also be used as the generator harmonic filter, with the advantages associated therewith. The reduced heating of the generator core & windings reduces cooling requirements of the generator and also results in more stable control of the generator and better efficiency of the power production.
The generator harmonic filter arrangement 203 also includes a resistor 702 and a capacitor 703. This RC circuit 702,703 is connected in parallel with the inductor L3 of the first filter arrangement. As mentioned earlier, the generator harmonic filter arrangement 203 filters off high switching frequency voltages from the generator windings.
It should be noted that the harmonic filter arrangement according to the embodiment may be used only as the generator harmonic filter or the grid harmonic filter, and need not be used for both. For example, the harmonic filter arrangement according to the embodiment may only be used as the grid harmonic filter, and a prior art LCL filter may be used for the generator harmonic filter.
It is also possible to have multiple string of power converters connected in parallel in an alternative embodiment. In this alternative embodiment, the generator harmonic filter 203, the power converter 202 and the grid harmonic filter 204 shown in Fig.8 are duplicated in the multiple strings of power converters.
In an embodiment, the inductor L3 of the first filter arrangement of the harmonic filter arrangement is formed by a first winding around a magnetic core, and the inductor L4 of the second filter arrangement is formed by a second winding around the first winding. Fig.9 shows a cross-sectional view of the harmonic filter arrangement according to an embodiment. The first winding 801 surrounds the magnetic core 802, and the second winding 803 surrounds the first winding 801 and the magnetic core 802. An insulation layer 804 may be formed between the first winding 801 and the second winding 801 as shown in Fig.9. This arrangement provides a simple and space efficient implementation of the harmonic filter arrangement according to the embodiment.
An external capacitor (not shown) is connected to the second winding 803, forming the capacitor of the second filter arrangement. It is possible that the capacitance is formed by the insulation layer 804 between the first and second windings 801, 802. In this case, the external capacitor may not be needed. The need to have a separate capacitor is reduced in this case because (a) there is an alternative path through the second windings for high switching harmonics and (b) there is an inherent capacitance between the two sets of windings. The information on the switching frequency and winding material is used to design this inherent capacitance.
In another embodiment, the arrangement shown in Fig.9 is used to implement LCL harmonic filter arrangement of Fig.l and Fig.5. The inductor LI of the first filter arrangement is formed by the first winding around the magnetic core, and the inductor L2 is formed by the second winding around the first winding. The first winding 801 surrounds the magnetic core 802, and the second winding 803 surrounds the first winding 801 and the magnetic core 802. The insulation layer 804 may be formed between the first winding 801 and the second winding 801. This arrangement also provides a simple and space efficient implementation of the LCL harmonic filter arrangement.
An external capacitor (not shown) may be connected to the second winding 803, forming the capacitor of the second filter arrangement. It is also possible that the capacitance is formed by the insulation layer 804 between the first and second windings 801, 802. In this case, the external capacitor may not be needed.
Fig.10a shows a perspective view of a single-phase harmonic filter arrangement according to an embodiment. The first winding 801, which is the inductor L3 (or inductor LI) of the first filter arrangement, is wounded around the magnetic core 802. The second winding 803, which is the inductor L4 (or inductor L2) of the second filter arrangement, is wounded around the first winding 803. In this embodiment, the second winding 803 is made of a thin foil. The thin foil has a very small cross-sectional area, and hence minimizes the AC effects due to high frequency AC current components flowing therethrough. Fig.10b shows a perspective view of a three-phase harmonic filter arrangement of the single-phase harmonic filter arrangement shown in Fig.10a. Fig.11a shows a perspective view of a single-phase harmonic filter arrangement according to an embodiment. The first winding 801, which is the inductor L3 (inductor LI) of the first filter arrangement, is wounded around the magnetic core 802. The second winding 803, which is the inductor L4 (or inductor L2) of the second filter arrangement, is wounded around the first winding 803. In this embodiment, the second winding 803 is made of a Litz wire. The Litz wire is made of many tiny individually insulated wires twisted together, and hence minimizes the AC effects due to high frequency AC current components flowing there through. Fig. l ib shows a perspective view of a three-phase harmonic filter arrangement of the single-phase harmonic filter arrangement shown in Fig.11a.
It should be emphasized that the embodiments described above are possible examples of implementations which are merely set forth for a clear understanding of the principles of the invention. The person skilled in the art may make many variations and modifications to the embodiment(s) described above, said variations and modifications are intended to be included herein within the scope of the following claims.

Claims

Claim:
1. A harmonic filter arrangement suitable for use in a wind turbine, comprising: a first filter arrangement; and
a second filter arrangement,
wherein the first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor, and
wherein the inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
2. The harmonic filter arrangement according to claim 1, wherein
the first filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency; and
the second filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
3. The harmonic filter arrangement according to claim 2, wherein the fundamental frequency is substantially at 50 Hz or 60 Hz, and the substantially higher frequency is a plurality of multiples of the fundamental frequency.
4. The harmonic filter arrangement according to any of the preceding claims, wherein the second winding comprises at least one of a thin metal foil and a Litz wire.
5. The harmonic filter arrangement according to any of the preceding claims, further comprises an insulation layer between the first winding and the second winding.
6. A power generation system comprising:
an AC source configured to generate AC power for an AC load; and
a harmonic filter arrangement configured to remove undesirable AC current components from the AC power,
wherein the harmonic filter arrangement comprises: a first filter arrangement; and
a second filter arrangement,
wherein the first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor, and
wherein the inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
7. The power generation system according to claim 6, wherein the AC source is a power converter and the AC load is a power grid.
8. The power generation system according to claim 6, wherein the AC source is a power generator and the AC load is a power converter.
9. The power generation system according to claim 6, wherein
the first filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency; and
the second filter arrangement configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
10. The harmonic filter arrangement according to claim 9, wherein the fundamental frequency is substantially at 50 Hz or 60 Hz, and the substantially higher frequency is a plurality of multiples of the fundamental frequency.
11. The harmonic filter arrangement according to claim 9 or 10, wherein the second winding comprises at least one of a thin metal foil and a Litz wire.
A wind turbine comprising:
a generator; a power converter configured to convert variable frequency AC power from the generator to substantially fixed frequency AC power; and
a grid harmonic filter system configured to remove undesirable AC current components from the substantially fixed frequency AC power from the power converter, wherein the grid harmonic filter arrangement comprises:
a first filter arrangement; and
a second filter arrangement,
wherein the first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor, and wherein the inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
13. The wind turbine according to claim 12, further comprising a generator harmonic filter arrangement between the power converter and the generator, the generator harmonic filter arrangement is configured to remove undesirable AC current components from the power converter, the generator harmonic filter arrangement comprises:
a first filter arrangement; and
a second filter arrangement,
wherein the first filter arrangement comprises at least an inductor, and the second filter arrangement comprises at least an inductor and a capacitor, and wherein the inductor of the first filter arrangement comprises a first winding around a magnetic core, and the inductor of the second filter arrangement comprises a second winding around the first winding.
14. The wind turbine according to claim 12 or 13, wherein the second winding of the grid harmonic filter arrangement comprises at least one of a thin metal foil and a Litz wire.
15. The wind turbine according to claim 13 or 14, wherein the second winding of the generator harmonic filter arrangement comprises at least one of a thin metal foil and a Litz wire.
16. The wind turbine according to any of claims 12 to 15, wherein
the first filter arrangement of the grid harmonic filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency; and
the second filter arrangement of the grid harmonic filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
17. The wind turbine according to any of claims 13 to 16, wherein
the first filter arrangement of the generator harmonic filter arrangement is configured to provide a low impedance path for AC current components substantially at a fundamental frequency; and
the second filter arrangement of the generator harmonic filter arrangement is configured to provide a low impedance path for AC current components at substantially higher frequency than the fundamental frequency.
PCT/DK2012/050316 2011-09-30 2012-08-29 A harmonic filter arrangement WO2013044918A2 (en)

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US201161541088P 2011-09-30 2011-09-30
DKPA201170537 2011-09-30
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EP3054465A1 (en) * 2015-02-05 2016-08-10 ABB Technology AG Passive low-pass filter and current limiter with a passive low-pass filter
US10164598B2 (en) 2016-08-12 2018-12-25 Hamilton Sundstrand Corporation Power filter arrangement having a toroidal inductor package
WO2020076654A1 (en) * 2018-04-23 2020-04-16 Sella Robert Carmine Mario Alternating current neutral and ground inductive electromagnetic rectification apparatus

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US6937115B2 (en) * 2002-02-25 2005-08-30 Massachusetts Institute Of Technology Filter having parasitic inductance cancellation
EP1908163A1 (en) * 2005-07-01 2008-04-09 Vestas Wind Systems A/S A variable rotor speed wind turbine, wind park, method of transmitting electric power and method of servicing or inspecting a variable rotor speed wind turbine
US8212416B2 (en) * 2008-12-24 2012-07-03 Synergy Energy Inc. Device for filtering harmonics

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3054465A1 (en) * 2015-02-05 2016-08-10 ABB Technology AG Passive low-pass filter and current limiter with a passive low-pass filter
US10164598B2 (en) 2016-08-12 2018-12-25 Hamilton Sundstrand Corporation Power filter arrangement having a toroidal inductor package
WO2020076654A1 (en) * 2018-04-23 2020-04-16 Sella Robert Carmine Mario Alternating current neutral and ground inductive electromagnetic rectification apparatus

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