Stabilizing power in the electric grid

Abstract

A method for stabilizing power in an electrical grid (18), the method comprising: detecting a reduced power demand in the electrical grid (18), determining active and reactive power to be compensated in the electrical grid (18), and compensating at least a portion of the active and at least a portion of the reactive power by controlling a power compensation circuit (12) connected to the electrical grid (18). The power compensation circuit (12) comprises at least a resistive load (34) connectable to the electrical grid (18) via a semiconductor switch (32). The compensated active and reactive powers are adjusted by setting switching angles (α1, α2) of the semiconductor switch (32) relative to a phase angle of a grid voltage (24) in the electrical grid (18).

Description

[Technical Field]

[0001] The present invention relates to a method and system for stabilizing power in an electrical grid. [Background technology]

[0002] In weak or island grids, load shedding caused by grid faults or faults of large consumers such as electric arc furnaces or large drives can lead to an increase in frequency. This can lead to tripping of generators feeding the grid and / or tripping of consumers. In both cases, valuable production time can be lost and restarting the entire system can be time-consuming and costly.

[0003] Additionally, when harmonic filters are connected to the electrical grid, for example to compensate for reactive power from large drives, arc furnaces, or rectifier systems, and are switched back on after a grid fault, this can lead to overswings of the grid voltage due to reduced active and reactive power loads on the grid. This can also lead to tripping (switching off) of large electrical consumers if the voltage fluctuations are large enough. Drives, in particular, usually have low tolerance for overvoltages. Restarting generators is typically a time-consuming problem.

[0004] WO2020 / 113336A1 describes a method and system for stabilizing power for arc furnaces and their power supplies, the method comprising causing a load to absorb power in response to determining an arc extinction event of an arc furnace electrode. Summary of the Invention

[0005] An object of the present invention is to reduce the tripping of generators in an electrical grid and / or reduce the downtime of consumers in an electrical grid.

[0006] These objects are achieved by the subject matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

[0007] A first aspect of the present invention relates to a method for stabilizing power in an electric grid. The electric grid can be a low (up to 1 kV), medium (up to 50 kV), and high (above 50 kV) voltage grid. It can be a grid of production facilities or a large-scale grid for supplying large consumers and production facilities. The electric grid can be a three-phase grid.

[0008] According to an embodiment of the present invention, a method comprises detecting reduced power demand in an electrical grid and determining active and reactive power to be compensated in the electrical grid. Voltage and current in the electrical grid may be measured, and power demand may be determined therefrom. For example, power supplied by the electrical grid over time may be calculated, and if the power over time suddenly decreases, reduced power demand may be assumed. Also, current power demands of consumers and / or loads may be reported to control devices performing the method, and these control devices may sum the reported power demands to determine the power demand over time.

[0009] From the measured voltages and currents in the electrical grid, the active and reactive power in the electrical grid can also be determined. Considering voltages and currents as complex numbers, complex power is the product of the voltage and the complex conjugate of the current. Active power (or real power) is the real part of complex power. Reactive power is the imaginary part of complex power.

[0010] According to an embodiment of the present invention, the method further comprises compensating for at least a portion of the active power and at least a portion of the reactive power by controlling a power compensation circuit connected to the electric grid. The power compensation circuit can be used to generate at least a portion of the missing active power and reactive power. In this way, the reduced power can be balanced and the amplitude and frequency of the voltage on the grid can be stabilized.

[0011] The power compensation circuit comprises at least a resistive load connectable to an electrical grid via a semiconductor switch. The at least resistive load may provide resistance and, optionally, capacitance and / or impedance. Active and reactive power may be generated by connecting and disconnecting the at least resistive load to the electrical grid. This may be done at the frequency of the electrical grid.

[0012] The compensated active power and the compensated reactive power are adjusted by setting the switching angles of the semiconductor switches relative to the phase angle of the grid voltage in the electric grid. Thus, not only can reduced active power, which may be caused by tripped consumers and / or loads, be at least partially compensated for, but also changed reactive power, which may be caused, for example, by harmonic filters subject to frequency changes in the voltage, be at least partially compensated for.

[0013] According to an embodiment of the present invention, the semiconductor switch is a thyristor, and the switching angle is the firing angle of the thyristor. Such firing angle may have a value between 0° and 180°. For other actively controllable semiconductor switches, the turn-on switching angle and the turn-off switching angle may be determined.

[0014] According to an embodiment of the present invention, reduced power demand is detected by measuring voltage and current on the electrical grid and calculating power from the measured voltage and current. A controller for determining the reduced power and controlling the power compensation circuit may receive signals from the voltage and current signals on the electrical grid. As previously mentioned, from these signals over time, active and reactive power may be calculated.

[0015] According to an embodiment of the present invention, a power compensation circuit comprises a pair of anti-parallel connected semiconductor switches for connecting and disconnecting two phases of an electric grid. At least a resistive load is connected in series with the pair of anti-parallel connected semiconductor switches. Every pair of phases of the electric grid can be connected by a pair of anti-parallel connected semiconductor switches and at least a resistive load. These pairs of anti-parallel connected semiconductor switches can be delta connected between the phases.

[0016] These pairs of anti-parallel connected semiconductor switches may also be star-connected between the phases, where each phase may be connected to a star-point via a pair of anti-parallel connected semiconductor switches.

[0017] According to an embodiment of the present invention, the power compensation circuit comprises an active rectifier with a half-bridge for each phase of the electrical grid, where at least a resistive load is connected in parallel to the half-bridge. A further possibility is to rectify the voltage in the electrical grid using an active rectifier, which may comprise a half-bridge for each phase. On the DC side, the rectified voltage may be applied to at least a resistive load.

[0018] Each half-bridge may include upper and lower semiconductor switches, where a phase of the grid is connected between the switches. At their other ends, the half-bridges are connected in parallel to provide the DC output of the rectifier. The switching angles of the upper and lower semiconductor switches may be selected so that the power compensation circuit provides the desired active and reactive power.

[0019] According to an embodiment of the present invention, a power compensation circuit includes a transformer connected between an electric grid and a semiconductor switch, the transformer having an adjustable transformation ratio. The transformer may include a tap changer that can change the number of turns of the transformer winding. The transformation ratio can be adjusted using the tap changer. In this way, the voltage applied to the at least resistive load can be adjusted, which can also be used to control the active and reactive power generated by the at least resistive load.

[0020] The compensating active power and the compensating reactive power are adjusted by setting an adjustable transformation ratio.

[0021] According to an embodiment of the present invention, a power compensation circuit includes a first rectifier and a second rectifier connected to an electric grid. Both rectifiers may be connected to the electric grid via a transformer or directly. The compensated active power and the compensated reactive power are adjusted by setting a first switching angle for the first rectifier and a corresponding, different, second switching angle for the second rectifier. The rectifiers may be designed identically, and the corresponding first and second switching angles may be associated with the same semiconductor switches of the first and second rectifiers, respectively.

[0022] According to an embodiment of the present invention, the switching angle of the upper semiconductor switches of the half-bridge of the first converter is different from the switching angle of the lower semiconductor switches of the half-bridge of the first converter. The switching angle of the upper semiconductor switches of the half-bridge of the second converter is equal to the switching angle of the lower semiconductor switches of the half-bridge of the second converter. The switching angle of the lower semiconductor switches of the half-bridge of the second converter is equal to the switching angle of the upper semiconductor switches of the half-bridge of the first converter. This switching scheme may be referred to as "split-alpha." Switching the two rectifiers in this manner results in somewhat lower high-order harmonics because the rectifiers are switched symmetrically with respect to canceling their respective phase voltages. Furthermore, using this switching scheme, active power and reactive power can be controlled independently of each other.

[0023] According to an embodiment of the present invention, the switching angle of the upper semiconductor switches of the half-bridge of the first converter is equal to the switching angle of the lower semiconductor switches of the half-bridge of the first converter. The switching angle of the upper semiconductor switches of the half-bridge of the second converter is equal to the switching angle of the lower semiconductor switches of the half-bridge of the second converter. The switching angles of the upper and lower semiconductor switches of the half-bridge of the first converter are different from the switching angles of the upper and lower semiconductor switches of the half-bridge of the second converter. In other words, the on-time of the semiconductor switches of the first rectifier can be different from the on-time of the semiconductor switches of the second rectifier (i.e., different closing angles and / or different opening angles). Furthermore, using this switching scheme, active power and reactive power can be controlled independently of each other.

[0024] Note that all of these switching angles are provided with respect to the negative to positive zero crossings of the respective phase voltages of the electrical grid.

[0025] According to an embodiment of the present invention, the first rectifier and the second rectifier are connected in series at their DC outputs, and at least a resistive load is connected in parallel to the series-connected DC outputs. Also, the first rectifier, the second rectifier, and the at least a resistive load are connected in parallel to the DC outputs of the first rectifier and the second rectifier. It may also be possible to connect a separate at least a resistive load to each rectifier, i.e., to its DC output.

[0026] According to an embodiment of the present invention, the first rectifier and the second rectifier are connected to the electric grid via a transformer having a secondary winding for each rectifier. In this way, when the first and second rectifiers are switched symmetrically, high-order harmonics caused by the switching can be compensated in the transformer.

[0027] A further aspect of the invention relates to a system for stabilizing power in an electric grid, and it will be understood that features of the method as described above and below may also be features of the system as described above and below, and vice versa.

[0028] According to an embodiment of the present invention, a system comprises a power compensation circuit as described above and below, and a controller for controlling the power compensation circuit as described above and below, the system being adapted to perform a method as described herein.

[0029] According to an embodiment of the present invention, a harmonic filter is connected to the electric grid. The system may further comprise a harmonic filter, which may be a passive filter for filtering high-order harmonics caused by loads connected to the grid. In this way, reactive power generated by the harmonic filter when active power demand is reduced can be compensated for.

[0030] According to an embodiment of the present invention, at least one load is connected to an electric grid, which, when disconnected from the electric grid, causes a reduced power demand. The at least one load comprises at least one of an electric drive and an electric arc furnace. The electric drive may comprise a converter and an electric motor and / or a generator. It is noted that such a load may have an active power demand greater than 1 MW.

[0031] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0032] The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments shown in the accompanying drawings. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 shows a schematic diagram of a system according to an embodiment of the present invention. [Figure 2A] FIG. 2A illustrates a schematic diagram of a power compensation circuit used in an embodiment of the present invention. [Figure 2B] FIG. 2B shows a schematic diagram of a power compensation circuit used in a further embodiment of the present invention. [Figure 3] FIG. 3 shows a schematic diagram of a power compensation circuit used in a further embodiment of the present invention. [Figure 4] 4 shows schematically a power compensation circuit used in a further embodiment of the invention. [Figure 5] FIG. 5 shows a schematic diagram of a power compensation circuit used in a further embodiment of the present invention. [Figure 6] FIG. 6 shows a flow diagram for a method for stabilizing power in an electrical grid according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] The reference signs used in the drawings and their meanings are listed in summary form in the list of reference signs. As a rule, identical parts in the figures are given the same reference signs.

[0035] 1 shows a system 10 including a power compensation circuit 12, a harmonic filter 14, and several loads 16, such as an electric drive 16a, an electric DC or AC arc furnace 16b, and / or other large electrical loads 16c, all connected to an electrical grid 18. Large in this context may mean that the loads may have a maximum power consumption greater than 1 MW. The electrical grid 18 may be a three-phase AC grid, for example, having a voltage of 33 kV, i.e., it may be a medium-voltage grid. One or more generators 20 may supply power to the electrical grid.

[0036] 1 further shows a controller 22 for the power compensation circuit 12, which may be part of a controller for the overall system 10 or part of a controller for a particular component of the system, such as one or more of the loads 16. The controller 22 receives voltage measurements 24 and current measurements 26 for the electrical grid 18. These measurements 24, 26 may be performed at the input of the system 10 and / or at the input of the loads 16. Based on these measurements and / or additional data, the controller 22 controls the power compensation circuit 12. The controller 22 may also be communicatively connected to control devices and / or sensors of one or more of the loads 16 and / or the filter 14. The data received in this manner may also be used to control the power compensation circuit 12.

[0037] Generally, based on the received data (such as voltage measurements 24 and current measurements 26), controller 22 determines whether there is a power demand drop in electrical grid 18 and controls power compensation circuit 12 to balance or at least reduce this power demand drop. For example, active and reactive power may be compensated in the event of a load shedding of one of loads 16 or a grid fault in electrical grid 18, particularly when electrical grid 18 is weak or in islanded operation. This will prevent electrical grid 18 from becoming overvoltage and increasing in frequency.

[0038] The controller 22 may be adapted to detect disturbances and / or faults from one or more of the loads 16 or from the electrical grid 18, which may comprise overhead lines. Further faults and / or detected power demands may be determined from the extinction of the arc in the electric arc furnace 16b and its re-ignition within 100 ms to 1000 ms, tripping of the large electric drive 16, and generally tripping of the large electric load 16c.

[0039] The power compensation circuit 12 and controller 22 are designed to simultaneously generate active and reactive power, and in particular to prevent voltage overswings on the electrical grid 18 and simultaneously prevent frequency increases. Voltage overswings and frequency increases can be caused by one or more generators 20 still producing large amounts of power while power demand drops. This prevents the loads 16 and generators 20 from tripping, thus allowing the system 10, such as a factory, mine, or remote industrial area, to continue its operation without any disturbance after the event. This may prevent the system 10 from restarting. Tripping and restarting can lead to lost production time.

[0040] In the event of a determined reduced power demand, such as load shedding or a grid disturbance, the power compensation circuit 12 can compensate not only for the active / real power but also for the changed reactive power. The reactive power can be changed by the harmonic filter 14, which can comprise a filter capacitance and a filter inductance, which can change the reactive power in the event of a changing voltage.

[0041] 2-5 below show embodiments of power compensation circuit 12 that may be used in system 10. Note that all of these embodiments may be used in combination with synchronous condensers connected to electrical grid 18, which may be used to compensate for remaining reactive power, if desired.

[0042] 2A and 2B show a power compensation circuit 12 having three semiconductor switch devices 30, each having a pair of anti-parallel semiconductor switches 32. Here, and in the following figures, the semiconductor switches 32 may be thyristors. However, other types of semiconductor switches 32, such as IGBTs, are also possible.

[0043] Each semiconductor switch device 30 includes a load 34, or, as shown in FIG. 2A, includes two loads 34 connected in series with a pair of anti-parallel semiconductor switches 32. The pair of anti-parallel semiconductor switches 32 is connected between the two loads 34. The loads 34 can also be arranged in different ways. Depending on the load arrangement, a series connection and / or a parallel connection of the semiconductor switches 32 can be implemented. This can also be applied to the following embodiments.

[0044] Each load 34 is at least a resistive (or ohmic) load and may include a reactive portion, i.e., a capacitive portion and / or an inductive portion. Each load 34 may be passive, i.e., comprised of resistors, capacitors, and / or inductors. For example, each load 34 may be adapted to dissipate at least 0.1 MW. The characteristics of the load 34 described with respect to FIG. 2 also apply to the following figures.

[0045] In FIG. 2A, each of the three semiconductor switch devices 30 is connected between a pair of phases of the electrical grid 18, ie, the semiconductor switch devices 30 are delta connected.

[0046] 2B, each of the three semiconductor switch devices 30 is connected between one of the phases of the electrical grid 18 and a neutral point 33. The semiconductor switch devices 30 are star-connected.

[0047] 2A, 2B and the following figures, the active and reactive power of the power compensation circuit 12 can be controlled by controlling the switching angle (or firing angle in the case of a thyristor) of the semiconductor switch 32. The larger the switching angle for switching the semiconductor switch 32 into conduction, the larger the reactive power. Note that reactive power is also generated when the load 34 is purely resistive.

[0048] The power compensation circuit 12 may also include a mechanical switch 35 for connecting the power compensation circuit 12 to the electrical grid 18 and for completely disconnecting it from the electrical grid 18 .

[0049] 3 shows the power compensation circuit 12 with a load 34 connected to the electrical grid 18 through a rectifier 36. The rectifier 36 includes three half-bridges 38, each with two semiconductor switches 32 connected in series across the DC outputs 40 of the half-bridges 38. A phase of the electrical grid is connected to the midpoint between the semiconductor switches 32. The DC outputs 40 are connected in parallel, and the load 34 is connected across the DC outputs 40.

[0050] As before, the active and reactive power of the power compensation circuit 12 can be controlled by controlling the switching angle of the semiconductor switch 32. In Figures 2 and 3, by simply changing the switching angle, the ratio between the active and reactive power supplied can be predefined and optimized for an expected operating point.

[0051] An optional transformer 42 may be connected between the electrical grid 18 and the rectifier 36. The transformer 42 may have an adjustable transformation ratio, for example, via a tap changer 44. By changing the transformation ratio, the ratio between the active and reactive power supplied by the power compensation circuit 12 may also be changed.

[0052] Using the tap changer 44, changes in the power load can be taken into account depending on the available taps. Depending on the transformer tap position of the tap changer 44 and the switching angles of the semiconductor switches 32, the active and reactive power drawn by the power compensation circuit 12 can be controlled more independently of each other. This allows for adapting two different operating points and / or is more flexible.

[0053] A transformer 42 may also be provided in FIG. 2, where it may be connected between the electrical grid 18 and the semiconductor switch device 30 .

[0054] Figure 4 shows the power compensation circuit 12 with two rectifiers 36a, 36b, each designed as in Figure 3. Each of the rectifiers 36a, 36b is connected to a secondary winding of a transformer 42', which is connected to the electric grid 18 via a primary winding. The transformer 42' may have an adjustable transformation ratio, for example, by using a tap changer 44, similar to the transformer 42 of Figure 3.

[0055] In FIG. 4, rectifiers 36a, 36b are connected in series with their DC outputs 40, and load 34 is connected in parallel with this series connection.

[0056] Figure 5 shows a power compensation circuit 12 with two rectifiers 36a, 36b similar to Figure 4, but connected in parallel with their DC outputs 40. A load 34 is connected in parallel to the rectifiers 36a, 36b.

[0057] 4 and 5, the switching angle of the semiconductor switch 32 can be controlled so that the reactive power can be adjusted within a certain range while the active power can be kept constant. This can be achieved by asymmetrically switching the rectifiers 36a, 36b, which can have the fastest response to transient disturbances.

[0058] 4, the semiconductor switches 32a in the upper half bridge of the rectifier 36a and the semiconductor switches 32b in the lower half bridge of the rectifier 36b may have the same switching angle α1, and the semiconductor switches 32b in the lower half bridge of the rectifier 36a and the semiconductor switches 32a in the upper half bridge of the rectifier 36b may have the same switching angle α2 (different from α1). This may be referred to as split alpha control.

[0059] As shown in FIG. 5, the semiconductor switches 32a and 32b of the upper and lower half bridges of the rectifier 36a may have the same switching angle α1, and the semiconductor switches 32a and 32b of the upper and lower half bridges of the rectifier 36b may have the same switching angle α2 (different from α1).

[0060] The switching scheme of FIG. 5 can also be applied to the power compensation circuit 12 of FIG. 4, and vice versa.

[0061] FIG. 6 shows a flow diagram of a method for stabilizing power in the electrical grid 18 that may be performed by the system 10 under the control of the controller 22.

[0062] In step S10, the controller 22 detects reduced power demand on the electrical grid 18. As described above, this may be done by measuring the voltage 24 and current 26 on the electrical grid 18 and calculating power from the voltage 24 and current 26. Additionally or alternatively, the controller 22 determines a grid fault or other disturbance by evaluating the measured data of the voltage 24 and current 26. Additionally or alternatively, the controller 22 receives data indicative of reduced power demand from the loads 16, 16a, 16b, 16c and / or from the harmonic filter 14. Such data may comprise information that one of the loads 16, 16a, 16b, 16c has a fault and / or has tripped.

[0063] In step S12, the controller 22 determines the active and reactive power to be compensated in the electrical grid 18. For example, the controller determines the active and reactive power drawn from the electrical grid 18 before the reduced power demand appears. The active power to be compensated can be the difference between the active power before the reduced power demand appears and the active power after it. Similarly, the reactive power to be compensated can be the difference between the reactive power before the reduced power demand appears and the reactive power after it. It is also possible that when one of the loads 16 trips, the reduced power demand in active and reactive power is already stored in the controller 22 and then used as the active and reactive power to be compensated. A further possibility is that the reactive power generated by the harmonic filter 14 for a particular voltage change in the grid can be stored and / or calculated by the controller 22 and then used as the reactive power to be compensated.

[0064] In step S14, the controller 22 controls the power compensation circuit 12 to compensate for at least a portion of the active power and at least a portion of the reactive power. The power compensation circuit 12 is controlled such that at least a resistive load 34 is connected to and disconnected from the electric grid 18 via the semiconductor switch 32, thereby generating the active and reactive power to be compensated.

[0065] In particular, the power compensation circuit 12 is controlled to not only compensate for the active power but also to compensate for the reactive power generated, for example, by the harmonic filter 14. When the voltage returns after a fault, this prevents the system 10 from overswinging the voltage due to the missing reactive and active power, and thus prevents the load 16 from tripping, which may save time for restarting the entire system 10.

[0066] The compensating active power and the compensating reactive power are regulated and / or generated by correspondingly setting the switching angles α1, α2 of the semiconductor switches 32 of the power compensation circuit 12. The switching angles α1, α2 are set relative to the phase angle of the grid voltage 24 in the electrical grid 18. For example, the switching angles for a particular semiconductor switch may be set to a particular angle after the zero crossing of the respective phase voltage of the electrical grid.

[0067] Additionally, when the power compensation circuit 12 includes transformers 42, 42' with adjustable transformation ratios, the compensation active power and the compensation reactive power can be adjusted by correspondingly setting the adjustable transformation ratios.

[0068] When the power compensation circuit 12 includes the rectifiers 36, 36a, 36b, different switching strategies can be used to generate the desired active and reactive power. For example, the rectifier 36 of FIG. 3 can be switched with different switching angles for the upper and lower semiconductor switches 32a, 32b of the half-bridge 38 of the rectifier 36. However, this can generate high-order harmonics in the electrical grid 18.

[0069] When two rectifiers 36a, 36b are used, harmonic generation can be balanced by symmetrically switching the rectifiers 36a, 36b, either with each rectifier alone (as shown in FIG. 5) or with respect to each other (as shown with respect to FIG. 4). Generally, in both cases, compensating active and reactive powers are adjusted and / or generated by setting a first switching angle α1, α2 for the first rectifier 36a and a corresponding different second switching angle α1, α2 for the second rectifier 36b.

[0070] The first possibility is to switch the rectifier with a control scheme known as "split alpha." The switching angle α1 of the upper semiconductor switch 32a of the half-bridge 38 of the first converter 36a is selected to be different from the switching angle α2 of the lower semiconductor switch 32b of the half-bridge 38 of the first converter 36a. The switching angle α2 of the upper semiconductor switch 32a of the half-bridge 38 of the second converter 36b is set equal to the switching angle α2 of the lower semiconductor switch 32b of the half-bridge 38 of the second converter 36b. The switching angle α1 of the lower semiconductor switch 32b of the half-bridge 38 of the second converter 36b is set equal to the switching angle α1 of the upper semiconductor switch 32a of the half-bridge 38 of the first converter 36a.

[0071] A second possibility is to use the same switching angles α1, α2 for the upper and lower semiconductor switches 32a, 32b for each rectifier 36a, 36b, but different switching angles α1, α2 for the rectifiers 36a, 36b. The switching angle α1 of the upper semiconductor switch 32a of the half bridge 38 of the first converter 36a is selected to be equal to the switching angle α1 of the lower semiconductor switch 32b of the half bridge 38 of the first converter 36a. The switching angle α2 of the upper semiconductor switch 32a of the half bridge 38 of the second converter 36b is selected to be equal to the switching angle α2 of the lower semiconductor switch 32b of the half bridge 38 of the second converter 36b. The switching angle α1 of the upper and lower semiconductor switches 32a, 32b of the half bridge 38 of the first converter 36a is selected to be different from the switching angle α2 of the upper and lower semiconductor switches 32a, 32b of the half bridge 38 of the second converter 36b.

[0072] While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered exemplary or illustrative and not restrictive, and the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain means are recited in mutually different dependent claims does not indicate that a combination of these means cannot be used to advantage. Any reference signs in the claims are not to be construed as limiting the scope.

[0073] [List of references] 10 Systems 12 Power compensation circuit 14 Harmonic Filter 16 Load 16a Electric drive unit 16b Electric Arc Furnace 16c Large electrical load 18 Electrical Grid 20. Generator 22 Control device 24 Voltage Measurements 26 Current measurement value 30 Semiconductor switch device 32 Semiconductor Switch 33 Neutral point 34 Load 35 Mechanical Switch 36 Rectifier 36a rectifier 36b rectifier 38 Half Bridge 40 DC output 42 Transformer 42' transformer 44 Tap changer α1 switching angle α2 switching angle The following is a summary of the claims as originally filed: [C1] A method for stabilizing power in an electrical grid (18), the method comprising: detecting reduced power demand in the electrical grid (18); determining the active and reactive power to be compensated in the electrical grid (18); and compensating at least a portion of the active power and at least a portion of the reactive power by controlling a power compensation circuit (12) connected to the electrical grid (18); The power compensation circuit (12) comprises at least a resistive load (34) connectable to the electrical grid (18) via a semiconductor switch (32); The compensated active power and the compensated reactive power are determined by the switching angle (α) of the semiconductor switch (32) relative to the phase angle of the grid voltage (24) in the electrical grid (18). 1 、α 2 ) is adjusted by setting the [C2] The semiconductor switch (32) is a thyristor, and the switching angle (α 1 、α 2 ) is the firing angle of the thyristor. [C3] The method of C1 or C2, wherein the reduced power demand is detected by measuring voltage (24) and current (26) in the electrical grid (18) and calculating power from the voltage (24) and current (26). [C4] the power compensation circuit (12) comprises a pair of anti-parallel connected semiconductor switches (32) for connecting and disconnecting two phases of the electric grid (18); The method according to any one of C1 to C3, wherein the at least resistive load (34) is connected in series with the pair of anti-parallel connected semiconductor switches (32). [C5] The power compensation circuit (12) comprises an active rectifier (36, 36a, 36b) having a half bridge (38) for each phase of the electrical grid (18); The method according to any one of C1 to C4, wherein the at least resistive load (34) is connected in parallel with the half bridge (38). [C6] The power compensation circuit (12) comprises a transformer (42, 42') connected between the electric grid (18) and the semiconductor switch (32), the transformer (42, 42') having an adjustable transformation ratio; The method according to any one of C1 to C5, wherein the compensation active power and the compensation reactive power are adjusted by setting the adjustable transformation ratio. [C7] The method of C6, wherein the transformer (42, 42') comprises a tap changer (44). [C8] the power compensation circuit (12) comprises a first rectifier (36a) and a second rectifier (36b) connected to the electrical grid (18); The compensated active power and the compensated reactive power are calculated by the first switching angle (α 1 、α 2 ) and a corresponding different second switching angle (α 1 、α 2 8. The method according to any one of C1 to C7, wherein the adjustment is performed by setting [C9] The switching angle (α 1 ) is the switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the first converter (36a). 2 ), unlike The switching angle (α 2 ) is the switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the second converter (36b). 2 ) and The switching angle (α 1 ) is the switching angle (α) of the upper semiconductor switch (32a) of the half bridge (38) of the first converter (36a). 1 ) the method described in C8. [C10] The switching angle (α 1 ) is the switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the first converter (36a). 1 ) and The switching angle (α 2 ) is the switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the second converter (36b). 2 ) and The switching angle (α) of the upper and lower semiconductor switches (32a, 32b) of the half bridge (38) of the first converter (36a) 1 ) is the switching angle (α) of the upper and lower semiconductor switches (32a, 32b) of the half bridge (38) of the second converter (36b). 2 ) A method according to C8, which is different from the method according to C8. [C11] the first rectifier (36a) and the second rectifier (36b) are connected in series at their DC outputs (40), and the at least resistive load (34) is connected in parallel to the series-connected DC outputs (40); or The method according to any one of C8 to C10, wherein the first rectifier (36a), the second rectifier (36b) and the at least resistive load (34) are connected in parallel via the DC outputs (40) of the first rectifier (36a) and the second rectifier (36b). [C12] The method according to any one of C8 to C11, wherein the first rectifier (36a) and the second rectifier (36b) are connected to the electric grid (18) via a transformer (42') having a secondary winding for each rectifier (36a, 36b). [C13] A system (10) for stabilizing power in an electric grid (18), said system comprising: the electrical grid (18); a power compensation circuit (12) connected to the electrical grid (18); a control device (22) for controlling the power compensation circuit (12); The system (10) is adapted to carry out the method according to any one of C1 to C12. [C14] The system (10) of C13, further comprising a harmonic filter (14) connected to the electrical grid (18). [C15] further comprising at least one load (16) connected to the electrical grid (18), the at least one load (16) causing the reduced electrical demand when disconnected from the electrical grid (18); The system (10) of C13, wherein the at least one load (16) comprises at least one of an electric drive (16a) and an electric arc furnace (16b).

Claims

1. 1. A method for stabilizing power in an electrical grid (18), the method comprising: Detecting reduced power demand on the electrical grid (18); determining the active and reactive power to be compensated in the electrical grid (18); and compensating at least a portion of the active power and at least a portion of the reactive power by controlling a power compensation circuit (12) connected to the electrical grid (18), The power compensation circuit (12) comprises at least a resistive load (34) connectable to the electrical grid (18) via a semiconductor switch (32); The compensated active power and the compensated reactive power are determined by the switching angle (α) of the semiconductor switch (32) relative to the phase angle of the grid voltage (24) in the electrical grid (18). １ , α ２ ) is adjusted by setting

2. The semiconductor switch (32) is a thyristor, and the switching angle (α １ , α ２ 2. The method of claim 1, wherein ≅ 1 / 2 is the firing angle of the thyristor.

3. 3. The method of claim 1, wherein the reduced power demand is detected by measuring voltage and current in the electrical grid and calculating power from the voltage and current.

4. the power compensation circuit (12) comprising a pair of anti-parallel connected semiconductor switches (32) for connecting and disconnecting two phases of the electrical grid (18); The method according to any one of claims 1 to 3, wherein the at least resistive load (34) is connected in series with the pair of anti-parallel connected semiconductor switches (32).

5. the power compensation circuit (12) comprises an active rectifier (36, 36a, 36b) having a half bridge (38) for each phase of the electrical grid (18); The method according to any one of claims 1 to 4, wherein the at least resistive load (34) is connected in parallel with the half-bridge (38).

6. the power compensation circuit (12) comprises a transformer (42, 42') connected between the electrical grid (18) and the semiconductor switch (32), the transformer (42, 42') having an adjustable transformation ratio; The method according to any one of claims 1 to 5, wherein the compensating active power and the compensating reactive power are adjusted by setting the adjustable transformation ratio.

7. The method of claim 6, wherein the transformer (42, 42') comprises a tap changer (44).

8. the power compensation circuit (12) comprises a first rectifier (36a) and a second rectifier (36b) connected to the electrical grid (18); The compensated active power and the compensated reactive power are calculated by the first switching angle (α １ , α ２ ) and a corresponding different second switching angle (α １ , α ２ 8. The method according to claim 1, wherein the saturation is adjusted by setting

9. The switching angle (α) of the upper semiconductor switch (32a) of the half bridge (38) of the first rectifier (36a) １ ) is the switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the first rectifier (36a). ２ ) unlike The switching angle (α) of the upper semiconductor switch (32a) of the half bridge (38) of the second rectifier (36b) ２ ) is different from the switching angle (α 1 ) of the lower semiconductor switch (32b) of the half bridge (38) of the second rectifier (36b); The switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the second rectifier (36b) １ ) is the switching angle (α) of the upper semiconductor switch (32a) of the half bridge (38) of the first rectifier (36a). １ 9. The method of claim 8, wherein the .times. ...

10. The switching angle (α) of the upper semiconductor switch (32a) of the half bridge (38) of the first rectifier (36a) １ ) is the switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the first rectifier (36a). １ ) and The switching angle (α) of the upper semiconductor switch (32a) of the half bridge (38) of the second rectifier (36b) ２ ) is the switching angle (α) of the lower semiconductor switch (32b) of the half bridge (38) of the second rectifier (36b). ２ ) and The switching angle (α) of the upper and lower semiconductor switches (32a, 32b) of the half bridge (38) of the first rectifier (36a) １ ) is the switching angle (α) of the upper and lower semiconductor switches (32a, 32b) of the half bridge (38) of the second rectifier (36b). ２ 9. The method of claim 8, wherein the .alpha.-methyl-.beta ...

11. the first rectifier (36a) and the second rectifier (36b) are connected in series at their DC outputs (40), and the at least resistive load (34) is connected in parallel to the series-connected DC outputs (40); or 11. The method according to claim 8, wherein the first rectifier (36a), the second rectifier (36b) and the at least resistive load (34) are connected in parallel via the DC outputs (40) of the first rectifier (36a) and the second rectifier (36b).

12. 12. The method of any one of claims 8 to 11, wherein the first rectifier (36a) and the second rectifier (36b) are connected to the electric grid (18) via a transformer (42') having a secondary winding for each rectifier (36a, 36b).

13. A system (10) for stabilizing power in an electric grid (18), the system comprising: the electrical grid (18); a power compensation circuit (12) connected to the electrical grid (18); a control device (22) for controlling the power compensation circuit (12); A system (10), wherein the system (10) is adapted to carry out the method according to any one of claims 1 to 12.

14. The system (10) of claim 13, further comprising a harmonic filter (14) connected to the electrical grid (18).

15. and at least one load (16) connected to the electrical grid (18), the at least one load (16) causing the reduced power demand when disconnected from the electrical grid (18); The system (10) of claim 13, wherein the at least one load (16) comprises at least one of an electric drive (16a) and an electric arc furnace (16b).

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