TOLERANCE-BAND FILTER FOR A FREQUENCY CONVERTER

Abstract

Provided is a method for controlling a current converter, in particular an inverter, preferably a frequency converter comprising an inverter, in particular of a wind power installation. He method includes specifying a tolerance band that has at least one band limit for the current converter, in particular for one or more switching devices of the current converter, specifying a delay that includes a dead time, in particular for the switching devices, sensing an actual current of the current converter, in particular an actual current of the switching devices, comparing the sensed actual current with the band limit in order to determine a departure from the tolerance band, switching the current converter, in particular the switching devices, in order to come within the tolerance band, and suppressing further, in particular non-system-relevant, switching operations of the current converter, in particular of the switching devices, for the specified dead time

Claims

1. A method for controlling a current converter of a wind power installation, comprising: specifying a tolerance band that has at least one band limit for one or more switching devices of the current converter; specifying a delay that includes a dead time for the one or more switching devices of the current converter; sensing a current of the one or more switching devices of the current converter, comparing the sensed current with the band limit to determine a departure from the tolerance band; switching the one or more switching devices of the current converter to bring the current within the tolerance band; and suppressing further switching operations of the one or more switching devices for the dead time.

2. The method as claimed in claim 1, wherein the current converter is an inverter.

3. The method as claimed in claim 1, wherein the current converter is a frequency converter including an inverter.

4. The method as claimed in claim 1, wherein suppressing the further switching operations includes suppressing non-system-relevant switching operations.

5. The method as claimed in claim 1, comprising: selecting the dead time to be greater than a time constant of a resonant circuit of the current converter on a generator side of the current converter or a network side of the current converter.

6. The method as claimed in claim 1, comprising: selecting the dead time to be less than a time constant that results in a virtual increase of the tolerance band by more than 10 percent.

7. The method as claimed in claim 6, comprising: selecting the dead time to be less than a time constant that results in a virtual increase of the tolerance band by more than 25 percent.

8. The method as claimed in claim 1, wherein the dead time is implemented by closed-loop control.

9. A current converter of a wind power installation, comprising: a plurality of switching devices; and a controller configured to: specify a tolerance band that has at least one band limit for the plurality of switching devices; specify a delay that includes a dead time for the plurality of switching devices; receive a sensed current of the plurality of switching devices of the current converter; compare the sensed current with the at least one band limit to determine a departure from the tolerance band; switch the plurality of switching devices to bring the sensed current within the tolerance band; and suppress further switching operations of the plurality of switching devices.

10. The current converter as claimed in claim 9, wherein the current converter is a frequency converter.

11. The current converter as claimed in claim 9, wherein the current converter is a power converter of a wind power installation.

12. The current converter as claimed in claim 9, wherein the plurality of switching devices include at least one insulated-gate bipolar transistor (IGBT).

13. A wind power installation, comprising: at least one current converter as claimed in claim 9, wherein the current converter is a power converter and is configured to feed an electrical power generated by the wind power installation into an electrical supply network.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0061] The present invention is now explained in more detail below on the basis of exemplary embodiments with reference to the accompanying figures, with the same references being used for assemblies that are the same or similar.

[0062] FIG. 1 shows a schematic view of a wind power installation according to an embodiment.

[0063] FIG. 2 shows a schematic structure of an electrical train of a wind power installation for feeding-in an electrical alternating current according to an embodiment.

[0064] FIG. 3 shows, in schematic form, the structure of a current converter for generating an electrical three-phase alternating current by means of a tolerance-band method according to an embodiment.

[0065] FIG. 4 shows the sequence of a common method for controlling a current converter by means of a tolerance-band method (prior art).

[0066] FIG. 5 shows the sequence of a method for controlling a current converter by means of a tolerance-band method.

DETAILED DESCRIPTION

[0067] FIG. 1 shows a schematic view of a wind power installation 100 according to an embodiment.

[0068] For this purpose, the wind power installation 100 has a tower 102 and a nacelle 104. Arranged on the nacelle 104 there is an aerodynamic rotor 106 that has three rotor blades 108 and a spinner 110. During operation, the wind causes the rotor 106 to rotate, thereby driving a generator in the nacelle 104.

[0069] In addition, the generator is connected to a current converter, described above or below, which feeds the electrical power generated by the generator into an electrical supply network, in particular an electrical wind farm network.

[0070] For the purpose of operating the wind power installation, and in particular the current converter, a control unit, described above or below, is also provided.

[0071] Shown in simplified form in FIG. 2 is an electrical train 200 of a wind power installation shown in FIG. 1.

[0072] The electrical train 200 has a 6-phase ring generator 210 which is rotated by the wind, via a mechanical drive train of the wind power installation, in order to generate a 6-phase alternating electrical current.

[0073] The 6-phase electrical alternating current is transmitted by the generator 210 to the rectifier 220, which is connected to the 3-phase inverter 240 via a DC link 230. The 6-phase ring generator 210, which is realized as a synchronous generator, is in this case electrically excited via the excitation 250 from the DC link 230.

[0074] The electrical train 200 thus has a full frequency converter concept, in which feed-in to the network 270 is effected by means of the 3-phase inverter 240, via the wind power installation transformer 260.

[0075] Such frequency converters, which feed the electrical power generated by the generator of the wind power installation into an electrical supply network, are usually also referred to as power converters.

[0076] The network 270 is usually an electrical wind farm network that feeds into an electrical supply network via a wind farm transformer. However, a direct in-feed, directly into the electrical supply network, may also be a possibility.

[0077] For the purpose of generating the three-phase current I1, I2, I3 for each of the phases U, V, W, the inverter 240 is controlled by means of a tolerance-band method.

[0078] The control in this case is effected via the control unit (e.g., controller) 242, which, by means of a current sensing device (e.g., ammeter or multimeter) 244, senses each of the three currents I1, I2, I3 generated by the inverter 240.

[0079] The control unit 242 is thus configured, in particular, to control each phase of the inverter individually by means of the current sensing device 244. For this purpose, the control unit 242 may receive, for example, a current setpoint I.sub.setpoint from the wind power installation control, in dependence on which the respective currents I1, I2, I3 are set.

[0080] Shown schematically in FIG. 3 is the structure of a current converter 300, in particular the inverter 240 shown in FIG. 2, for generating an electrical three-phase alternating current by means of a tolerance-band method.

[0081] The current converter 300 is realized as an inverter, and is connected to a DC link 330 that is connected to the generator of a wind power installation via a rectifier.

[0082] The DC link 330 has a first potential +V.sub.dc and a second potential −V.sub.dc with a center tap M that is configured to be connected to a filter, for example in order to feed back a filter, connected to the output 346 of the inverter, to the DC link 330.

[0083] In addition, arranged between the center tap M and the two potentials +V.sub.dc, −V.sub.dc there is a respective capacitor comprising the capacitor device C1, C2, in order to store energy in the DC link 330 and to smooth the DC voltage 2V.sub.dc accordingly.

[0084] The current converter 300, which is connected to the DC link 330, generates a separate current I1, I2, I3 for each of the three phases U, V, W at the output 346 of the current converter 340. For this purpose, the current converter 340 has two switching devices for each of the three phases U, V, W, namely an upper switch T1, T3, T5 and a lower switch T2, T4, T6, the upper and lower switches T1, T2, T3, T4, T5, T6 being controlled, in particular, via the control unit by means of a tolerance-band method.

[0085] The control unit 342 itself operates with a current-guided tolerance-band method. For this purpose, the control unit 342 senses the currents I1, I2, I3 generated by the inverter 340 at the output 346 of the current converter 340 by means of a current sensing device 344. The currents I1, I2, I3 sensed thus are compared with a setpoint value I.sub.setpoint in order to determine the band limits UB12, LB12, UB34, LB34, UB56, LB56 for upper and lower switches T1, T2, T3, T4, T5, T6.

[0086] The switching devices T1, T2, T3, T4, T5, T6 are thus controlled by the control unit 342 by means of band limits UB.sub.i, LB.sub.i, switching signals S.sub.i and dead times t.sub.D, in particular in order to generate a three-phase current I1, I2, I3.

[0087] FIG. 4 shows the sequence 400 of a common method for controlling a current converter by means of a tolerance-band method (prior art).

[0088] In the upper part 410 of FIG. 4, the current I.sub.i of a switching device is plotted over time t, and in the lower part 420 the corresponding switching operations S.sub.i.

[0089] At instant t1, the current I.sub.i exceeds the upper band limit UB.sub.i, whereupon the switching device switches from +1 to −1 by means of switching operation S1.

[0090] This causes a spike, which at instant t2 results in the lower band limit LB.sub.i being undershot.

[0091] This causes the switching operation S2 to be effected, which in turn results in a spike, which in turn results in the upper band limit UB.sub.i being exceeded at the instant t3.

[0092] The switching operation S1 therefore triggers two further switching operations S2 and S3.

[0093] At a later instant t4, the current I.sub.i falls below the lower band limit LB.sub.i, whereupon the switching device switches from −1 to +1 by means of the switching operation S4.

[0094] This in turn results in a spike, which results in the switching operations S5 and S6 to the instants t5 and t6.

[0095] Two further switching operations S5 and S6 are therefore also triggered by the switching operation S4.

[0096] In the case of common tolerance-band methods, therefore, resonant circuits upstream or downstream of the current converter can cause dissipative, and thus unnecessary, switching operations.

[0097] FIG. 5 shows the sequence of a method for controlling a current converter by means of a tolerance-band method, in particular for avoiding unnecessary switching operations.

[0098] In the upper part 510 of FIG. 5, the current I.sub.i of a switching device is plotted over time t, and in the lower part 520 the corresponding switching operations Si.

[0099] At instant t1, the current I.sub.i exceeds the upper band limit UB.sub.i, whereupon the switching device switches from +1 to ˜1 by means of switching operation S1.

[0100] This causes a spike, which at instant t2 results in the lower band limit LB.sub.i being undershot.

[0101] The under-shooting of the lower band limit LB.sub.i would normally result in a further switching operation, as shown for example in FIG. 4.

[0102] However, this switching operation is suppressed by the dead time t.sub.D.

[0103] The spike disappears again at instant t3, such that the current continues to move within the tolerance band UB.sub.i, LB.sub.i.

[0104] At a later instant t4, the current I.sub.i falls below the lower band limit LB.sub.i, whereupon the switching device switches from −1 to +1 by means of the switching operation S4.

[0105] This in turn results in a spike, which would normally result in further switching operations at the instants t5 and t6, as shown for example in FIG. 4.

[0106] However, these further switching operations are likewise suppressed by the dead time t.sub.D.

[0107] Unnecessary further switching operations are avoided by means of an additional dead time t.sub.D, as a comparison with FIG. 4 shows.

[0108] It is therefore proposed, in particular, that the closed-loop control waits for a short time to see what the characteristic of the current will be. Only after the dead time has elapsed are further switching operations effected, if necessary.

[0109] In this respect it is also proposed, in particular, that, if the current is outside of the tolerance band UB.sub.i, LB.sub.i after the dead time t.sub.D has elapsed, further switching operations are effected, which bring the current back into the tolerance band UB.sub.i, LB.sub.i.

LIST OF REFERENCES

[0110] 100 wind power installation [0111] 102 tower, in particular of the wind power installation [0112] 104 nacelle, in particular of the wind power installation [0113] 106 aerodynamic rotor, in particular of the wind power installation [0114] 108 rotor blade, in particular of the wind power installation [0115] 110 spinner, in particular of the wind power installation [0116] 200 electrical train, in particular of the wind power installation [0117] 210 generator, in particular of the wind power installation [0118] 220 rectifier, in particular of the wind power installation [0119] 230 DC link, in particular of the wind power installation [0120] 240 inverter, in particular of the wind power installation [0121] 242 control unit, in particular of the inverter [0122] 244 current sensing, in particular of the inverter [0123] 250 excitation, in particular of the generator [0124] 260 transformer, in particular of the wind power installation [0125] 270 electrical network [0126] 300 current converter, in particular inverter [0127] 330 DC link, in particular for the current converter [0128] 342 control unit, in particular of the current converter [0129] 344 current sensing, in particular of the current converter [0130] 346 output, in particular of the current converter [0131] 400 sequence of a common method (prior art) [0132] 500 sequence of a method [0133] I.sub.i current [0134] t time [0135] t.sub.D dead time [0136] S.sub.i switching operations [0137] I1, I2, I3 three-phase alternating current, in particular of the wind power installation [0138] T1, . . . , T6 switching devices, in particular of the current converter [0139] T1, T3, T5 upper switches, in particular of the current converter [0140] T2, T4, T6 lower switches, in particular of the current converter [0141] UB.sub.i upper band limit [0142] UB12 upper band limit, in particular for the first and second switching devices [0143] UB34 upper band limit, in particular for the third and fourth switching devices [0144] UB56 upper band limit, in particular for the fifth and sixth switching devices [0145] LB.sub.i lower band limit [0146] LB12 lower band limit, in particular for the first and second switching devices [0147] LB34 lower band limit, in particular for the third and fourth switching devices [0148] LB56 lower band limit, in particular for the fifth and sixth switching devices [0149] U, V, W phases of the electrical network [0150] V.sub.dc intermediate-circuit voltage, in particular of the DC link [0151] +V.sub.ac first potential, in particular of the DC link [0152] −V.sub.dc second potential, in particular of the DC link

[0153] The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.