TECHNICAL FIELD

The present application relates to the field of power electronics, and more particularly to inverters comprising nonlinear inductors.

BACKGROUND

Inverter is a device that changes direct current (DC) to alternating current (AC) . The inverters may need to meet many requirements when used, for example, as a part of a grid such as a substation. The inverters may also need to be able to handle rare fault situations that may occur in the grid. Taking multiple different needs and situations into account may lead to inefficiencies in the inverter design.

SUMMARY

It is an objective to provide an inverter comprising a nonlinear inductor. The objective is achieved by the features of the independent claims. Some embodiments are described in the dependent claims.

According to a first aspect, there is provided an inverter. The inverter comprises a DC source; a power conversion unit; and an output filter unit comprising at least one nonlinear inductor configured for two operating areas, wherein the at least one nonlinear inductor comprises a changing inductance value, the changing inductance value first configured for operation in a first operating area and configured to change to a different inductance value for operation in a second operating area before core saturation; wherein the power conversion unit is configured to receive a DC input from the DC source and to provide an AC output filtered by the output filter unit. An embodiment may enable reduction in total power losses of the inverter. Further, the embodiment may enable reducing total harmonic distortion values of the provided output to less than 5%, while achieving a low-cost and low-volume inverter design .

In an embodiment, the at least one nonlinear inductor comprises a magnetic core of a first magnetic material and gaps of a second magnetic material; wherein the first and the second magnetic materials have nonlinear properties such that inductance of the inductor is higher in the first operation area than in the second operation area. The embodiment may enable providing optimal operation points for the two operating areas. A high output short circuit current may be provided, and an inductor size and reactive power may be reduced.

In an embodiment, in addition or alternatively, a saturation flux level of the first magnetic material is higher than a saturation flux level of the second magnetic material. The embodiment may enable an impedance transition in the inductor from a relatively high inductance to a lower inductance between two operating areas.

In an embodiment, in addition or alternatively, the saturation flux level of the first magnetic material is at least three times higher than the saturation flux level of the second magnetic material. The embodiment may enable an impedance transition in the inductor from a relatively high inductance to a lower inductance between two operating areas.

In an embodiment, in addition or alternatively, the second magnetic material comprises ferrites. The embodiment may enable sharp saturation of the second magnetic material. In an embodiment, in addition or alternatively, the second magnetic material saturates in a short circuit condition. The embodiment may enable that the inductance of the gaps drops in the short circuit condition.

In an embodiment, in addition or alternatively, the output filter unit comprises at least two different types of inductors coupled in series. The embodiment may enable impedance transition from a relatively high inductance to a lower inductance in the output filter unit between two operating areas.

In an embodiment, in addition or alternatively, the at least two different types of inductors comprise a first inductor configured for the first operating area and a second inductor configured for the second operating area. The embodiment may enable impedance transition from a relatively high inductance to a lower inductance in the output filter unit between the two operating areas before core saturation of both of the inductors.

In an embodiment, in addition or alternatively, the output filter unit comprises at least one nonlinear inductor with widening air-gaps, each widening air-gap having a greater width at a first end than at a second end. The embodiment may enable impedance transition from a relatively high inductance to a lower inductance between two operating areas.

In an embodiment, in addition or alternatively, the two operating areas comprise a normal operating area and a short-circuit operating area. The embodiment may enable optimal operation in the two different operating ranges and taking into account differences of the two operating areas in dimensioning the at least one nonlinear inductor . In an embodiment, in addition or alternatively, the inverter comprises in a LVDC substation. The embodiment may enable reduction in total power losses of the LVDC substation. The embodiment may further enable reducing total harmonic distortion of injected voltage to customers to less than 5%, while providing a low-cost and low-volume LVDC substation.

According to a second aspect, there is provided a LVDC system comprising the inverter of the first aspect. An embodiment may enable reducing total power losses of the LVDC system. The embodiment may enable providing high- quality AC voltage to customers with less than a 5% total harmonic distortion.

According to a third aspect, there is provided a method for limiting total harmonics caused by a DC/AC inverter. The method comprises receiving an input DC power from a DC source; converting the input DC power, by a power conversion unit, into output AC power; and filtering the output AC current and AC voltage with an output filter unit comprising at least one nonlinear inductor configured for two different operating areas, wherein the at least one nonlinear inductor comprises a changing inductance value, the changing inductance value first configured for operation in a first operating area and configured to change to a different inductance value for operation in a second operating area before core saturation. An embodiment may enable reduction in total power losses of the inverter. Further, the embodiment may enable reducing total harmonic distortion of the provided output to less than 5%, while achieving a low- cost and low-volume inverter design. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

FIG. 1 illustrates a table of exemplary short circuit current requirements.

FIG. 2 illustrates a schematic representation of a diagram of a relationship between an inductance and losses of a power converter.

FIG. 3 illustrates a schematic representation of a block diagram of a LVDC substation structure according to an embodiment .

FIG. 4 illustrates a schematic representation of a circuit diagram of a DC/AC inverter according to an embodiment .

FIG. 5 illustrates a schematic representation of characteristics of a nonlinear inductor of a DC/AC inverter according to an embodiment.

FIG. 6 illustrates a schematic representation of a cross- sectional view of a nonlinear inductor according to an embodiment .

FIG. 7 illustrates a schematic representation of a nonlinear inductor according to an embodiment. DETAILED DESCRIPTION

An output of an inverter may cause harmonics because its output is not pure sine. Individual harmonics and total harmonics of the inverter output may be defined by standards for different systems. For example, an output filter may be used to meet given requirements. The output filter may be configured to reduce output ripple and to limit the total harmonics caused by the inverter. In case of inverters the choice of a sinusoidal output filter may be based on various limitations. For example, the resonance of the filter may be limited by total harmonic distortion (THD) . The inductors' current ripple may be limited to reduce inductor losses and a conduction loss of power switches. Further, reactive power introduced by an output filter capacitor may be limited. High capacitance causes higher reactive power and higher current flow through the filter inductor. Thus, a current stress and losses of power switches may be reduced by limiting the reactive power of the output filter capacitor. The losses of power switches may be also reduced by reducing the current ripple. Thus, the output filters may need to satisfy magnetic requirements, thermal limitations and restricted standard requirements.

For example, in low voltage direct current (LVDC) substation systems, a voltage THD should be equal to or less than 3% in case of a linear load, and equal to or less than 5% in case of a nonlinear load. The THD requirements may be fulfilled with proper output filter design. Also, it may be desired that reactive power caused by the output filter is low. Thus, an inductance of a filter inductor may be determined based on the reactance and harmonics limitations. Typically, the lower the THD value, the bigger inductor may be needed. Hence, aiming to the low THD value may also result in increased voltage drop on the filter inductor, as well as increased losses and filter element costs. In an embodiment, the output filter comprises at least one nonlinear inductor configured to two operating areas with different inductance value.

The size, cost and efficiency of the LVDC system may further depend on other features of the output inductor, such as its ability to store magnetic energy in the form of a magnetic field. Usually, higher current means higher magnetic energy. Therefore, bigger filter elements may be needed in high output systems. For example, dimensioning of the output inductors may be based on short circuit requirements of gG-fuses of the system. Some short circuit current requirements are provided in table 1 in FIG. 1 for illustrative purposes.

Table 1 comprises examples of nominal current values and corresponding short circuit current requirements for gG- fuses when the short circuit lasts for 0.4 and 5.0 seconds. As an example, the 5.0 second short circuit current should be at least 150A for 32A gG-fuses. Table 1 shows that a short circuit current of around five to nine times may be required to clear a fault in the system. Hence, in order to blow the 32A gG-fuse, a current between 150A and 270A may be required. Thus, also the filter inductor must be able to handle such energy during a short circuit without total saturation. Typically, the short circuit current may be scaled close to a nominal operating current of the inductor due to the output filter saturation. In the example above, the 32A gG-fuse may require at least a 62 kVA rating inverter .

Short circuit situations happen quite rarely. Thus, the filters and inductors should be dimensioned primarily for operation around a normal operation point, instead of making a compromise due to the possible need for short circuit operation. When the inductor is designed based on the short circuit, either the inductance decreases, or the size of the inductor increases, which may affect cost-efficiency of the inverter.

As another example, a conventional stepped filter with a gap inductor may be optimized only for a limited load range when considering the total harmonics to a customer grid and the power losses of the inductor. Usually, an inductor with a high inductance value may be selected for power converters to fulfill the THD and power loss requirements. The relation 2 between the inductance and the power losses of power switches of an inverter is illustrated in FIG. 2. However, due to the high inductance for a wide load range, the size and the voltage drop on the inductor, filter losses and filter element costs may increase.

According to an embodiment, a low-cost and low-volume DC/AC inverter with a reduced THD of injected output voltage may be provided. The embodiment may provide an inverter comprising an output filter with nonlinear properties to enable its dimensioning for two operating areas. Hence, there may be no need to make a compromise on filter design parameters at the expense of the normal operation due to requirements for short circuit situations. Thus, the filter inductor may operate well in both situations. In an embodiment, the inverter may comprise in a LVDC substation, and high-quality current may be provided to customers with increased cost- efficiency .

An embodiment may enable a reduction in the inductor size while providing high output short circuit current, reduced reactive power, and reduced total power losses of a LVDC substation in a normal operation area. Further, the embodiment may enable a reduction of THD of the injected voltage to customers to less than 5% while providing a low-cost and low-volume system.

FIG. 3 illustrates a schematic representation of a block diagram of a LVDC substation structure according to an embodiment. The LVDC substation may comprise an inverter 100 configured to convert DC into high-quality AC.

The substation may comprise a DC source 102, a power conversion unit 104 and an output filter unit 106. The DC source 102 may be, for example, a DC/DC switching power supply. The power conversion unit 104 may comprise, for example, a single phase or a three-phase inverter bridge. The power conversion unit 104 may be coupled between the DC source 102 and the output filter unit 106. The output filter unit 106 may be, for example, a LC filter. The output filter unit 106 may comprise one or more nonlinear inductors depending on the application .

The one or more nonlinear inductors may be configured for operating in two different operating areas before total saturation. The at least one nonlinear inductor may comprise a changing inductance value, the changing inductance value first configured for operation in a first operating area and configured to change to a different inductance value for operation in a second operating area before core saturation. The first and the second operating areas may be, for example, a nominal operating area and a short circuit operating area. In the nominal operating area, the one or more nonlinear inductors may be configured to have a relatively high inductance value. In the short circuit operating area, the one or more nonlinear inductors may be configured to have a lower inductance value which may be optimized for operation in the short circuit condition. In the short- circuit condition the inductance may drop to a fraction of the inductance value in the nominal operating area. The inductance may drop, for example, to a quarter or a third of the higher inductance value. The inductance in the short-circuit operating area may be lower than in the nominal operating area but higher than in core saturation. The impedance transition from the higher inductance value to the lower inductance value may be provided, for example, by utilizing nonlinear properties of different magnetic materials in the construction of the filter inductors.

FIG. 4 illustrates a schematic representation of a circuit diagram of a DC/AC inverter 100 according to an embodiment. The DC/AC inverter 100 may convert DC current into high-quality AC current, for example, in a LVDC system.

The DC/AC inverter 100 comprises a DC source 102, a power conversion unit 104 and an output filter unit 106. The DC/AC inverter 100 may comprise in, for example, a LVDC substation. The DC source 102 may be, for example, a DC/DC switching power supply coupled to the power conversion unit 104. The power conversion unit 104 may comprise a plurality of power switches and a controller for inverting the DC supply 102 into a high-quality AC output. The power conversion unit 104 may be coupled to the output filter unit 106. The output filter unit 106 may comprise one or more inductors 108 and one or more capacitors 110 coupled in parallel. The one or more inductors 108 may be nonlinear inductors configured to operate in two different operating areas, wherein the at least one nonlinear inductor 108 comprises a changing inductance value, the changing inductance value first configured for operation in a first operating area and configured to change to a different inductance value for operation in a second operating area before core saturation. The inverter 100 comprising the nonlinear inductors 108 configured for two different operational ranges provides a cost-efficient solution for power conversion. Further, the inverter 100 may be a part of a LVDC system and increase cost-efficiency of the system.

FIG. 5 illustrates a schematic representation of characteristics of a nonlinear inductor 300 of a DC/AC inverter according to an embodiment. FIG. 5 underlines the difference between characteristics of the nonlinear inductor 300 and comparative filter inductors 302, 304.

In FIG. 5, the characteristics of the first comparative filter inductor 302 are illustrated with a solid line. The characteristics of the second comparative filter inductor 304 are illustrated with a dash-dot line. The comparative inductors 302, 304 may be optimized only for a limited load range when considering total harmonics to a customer grid and power losses of the inductor. As shown in FIG. 5, the inductance of the first comparative inductor 302 is approximately constant until it reaches a high load current at the end of a short-circuit operation area B and the inductor 302 saturates. The second comparative filter inductor 304 may have a constant inductance value which does not change between a normal operating area A and the short-circuit operating area B. The inductance value of the inductors 302, 304 may be optimized only for one operating area. Due to the high inductance for a wide load current range (e.g. for 0...10 pu as illustrated in FIG. 5) the filter inductors 302, 304 may be bigger with a greater voltage drop, and higher losses and filter element costs compared to the nonlinear inductor 300.

The characteristics of the nonlinear inductor 300 are illustrated with a dash line in FIG. 5. The nonlinear inductor 300 may be designed for two different operating ranges A, B with different inductance values configured for operation in the different operating areas before core saturation occurs. The nonlinear inductor 300 may comprise a changing inductance value for a convenient operation in both operating areas. The inductance value of the nonlinear inductor 300 may be first configured for operation in a first operating area, which may be the normal operating area A, and configured to change to a different inductance value for operation in a second operating area, which may be the short circuit operation area B, before core saturation. For example, the nonlinear inductor 300 may comprise a magnetic core of a first magnetic material and gaps filled with a second magnetic material. The first and the second magnetic materials may have nonlinear properties such that the inductance of the inductor is higher in the normal operation area A and lower outside the operating area, which may be the short-circuit operating area B. The second magnetic material may have a lower flux density and it may saturate in the short-circuit condition. The second magnetic material may act as a supplementary air gap when the section comprising the second magnetic material has saturated, and the inductance of the inductor 300 may drop. Also, current durability may increase. The nonlinear inductor 300 in FIG. 5 may comprise a second magnetic material exhibiting soft saturation for changing the inductance value.

This enables the nonlinear inductor 300, which may be a filter inductor, to be designed for the higher inductance for normal operation range A around a normal operation point. If the inductor comprises an air-gap, the air gap may be kept small. By keeping the air gap small, also additional losses caused by a fringing effect may be decreased. The small air gap may further decrease winding and core losses. Also, the number of winding turns may be decreased. With the higher inductance, coil losses may be reduced because the resulting current ripple is smaller. In an embodiment, a saturation flux level of the first magnetic material may be higher than a saturation flux level of the second magnetic material. For example, the saturation flux level of the first magnetic material may be at least three times higher than the saturation flux level of the second magnetic material. The first magnetic material may comprise, for example, powdered iron. The second magnetic material may comprise, for example, ferrites.

In an embodiment, the filter may be designed for the two different operating areas by coupling two different types of inductors in series for implementing the changing inductance value. The filter may be like the filter unit 106 illustrated in FIG. 4, but the at least one nonlinear inductor 108 comprises two different types of inductors coupled in series. For example, the filter may comprise a first inductor designed for the normal operating area and a second inductor designed for the short-circuit operating area. The first inductor may have a higher inductance than the second inductor. When the first inductor saturates, the inductance may drop. The inductance of the second inductor may remain providing lower inductance in the filter after the saturation of the first inductor in the short-circuit operation. In another embodiment, the filter inductor may comprise a widening air-gap. The widening air-gap may have a greater width at a first end and a smaller width at a second end of the air-gap. A flux density may be greater at the second end of the air-gap. The embodiment may enable impedance transition from a relatively high inductance to a lower inductance between the normal operating area and the short-circuit operating area. The transition in inductance between the normal operating range (from a higher inductance) and outside the operating range (to a lower inductance) may provide multiple advantages as there is no need to implement the design based on a maximum short-circuit current. The saturation based on the magnetic materials may provide optimal operating points for the two operating areas. With the nonlinear inductor dimensioned for the two different operating areas, a high output short circuit current (e.g. 10 times the nominal current) may be provided, while reduction in inductor size and reactive power may be achieved. Thus, a low-cost and a low-volume DC/AC inverter comprising the nonlinear filter inductor may be provided. The DC/AC inverter may comprise in, for example, a LVDC substation. The total power losses of the LVDC substation may be reduced in the normal operation area. Further, THD of the injected voltage to customers may be reduced to <5% while providing a cost- efficient design.

FIG. 6 illustrates a schematic representation of a cross- sectional view of a nonlinear inductor 400 according to an embodiment. The nonlinear inductor 400 is illustrated also in FIG. 7, and the FIGS. 6, 7 are described together hereinafter. The exemplary structure of the nonlinear inductor 400 may enable dimensioning the inductor for two different operating areas before core saturation.

The nonlinear inductor 400 may comprise a magnetic core 402, bobbin 404, winding 406 and gaps material 408. The magnetic core 402 may comprise a first magnetic material. The gaps material 408 may comprise a second magnetic material. The first and the second magnetic material may have different saturation flux levels. The saturation flux level of the first magnetic material may be, for example, at least 3 times greater than the saturation flux level of the second magnetic material. The nonlinear inductor 400 may comprise in an output filter. The output filter may utilize the nonlinear properties of the two magnetic materials to cause an impedance transition from a relative high inductance to a lower inductance. The second magnetic material may exhibit saturation in a short circuit condition. In the short circuit condition, the inductance of the second magnetic material may drop and the gap 408 filled with the second material may correspond to properties of an air-gap. The second magnetic material may comprise, for example, ferrites. The first magnetic material may comprise, for example, powdered metal.

While there have been shown and described and pointed out fundamental novel features as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the disclosure. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the disclosure. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiments may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice .

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects/embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure.