Inverter circuit having a controller to supply a reactive current on a phase with a voltage drop

A three-phase inverter circuit includes an inverter incorporating a plurality of controllable power switches, and an electronic control device adapted to control the power switches. The control device in the event of a measured voltage drop on one phase is adapted to supply a reactive current on the phase with voltage drop and to supply and/or draw an active current on at least one phase without voltage drop.

The invention relates to a three-phase inverter circuit including an inverter comprising a plurality of controllable power switches, and an electronic control device adapted to control the power switches. The invention further relates to a method for operating a three-phase inverter circuit.

Such an inverter circuit is known for example from document WO 03 065567 A1. Three-phase inverter circuits are used for example in circuit arrangements for supplying current from renewable energy plants, in particular wind power plants, photovoltaic plants and fuel cells, to a three-phase network. Supplying reactive current on one phase as a result of an unbalanced voltage drop may lead to an undesired voltage increase up to over 110% of the standard voltage on the phases without voltage drop.

It is the object of the invention to provide a three-phase inverter circuit and a method, in which a voltage increase up to over 110% of the standard voltage on the phases without voltage drop can be prevented with simple means.

The invention solves this object with the features of the independent claims. Suitably supplying and/or drawing an active current on at least one phase without voltage drop can counteract a voltage increase on the phases without voltage drop and, in particular, can prevent its increase up to over 110%.

Preferably, an active current is supplied on one phase without voltage drop, whereas on the other phase without voltage drop an active current is drawn, in order to counteract the generation of additional active power. Ideally, the amounts of the active currents supplied and drawn on the phases without voltage drop are essentially equal so that the system shows an overall neutral behaviour with respect to active power.

In a particularly preferred embodiment of the invention, the set active currents on the phases without voltage drop are calculated in such a way that the sum of the currents on all phases totally amounts to zero. It is thus possible to comply with the requirements with respect to the maximum voltage increase on the intact phases without additional measures like for example a current compensation line between the DC voltage side and the AC voltage side. In this respect, the invention has realized that no individual control of the individual phases is required in order to comply with all afore-mentioned requirements.

As the voltage of the phase affected by a short circuit may drop to such a great extent that the position of the corresponding voltage vector can no longer be determined with sufficient precision, at least under such circumstances the voltage vector of the phase with voltage drop preferably is calculated with essentially higher precision from the measured voltage vectors of the remaining phases.

The inverter circuit10includes an intermediate dc circuit11, an inverter12, a filter14, a medium-voltage transformer15and an electronic control device13, for example a digital signal processor DSP. In a per se known manner, the inverter12for each phase16a,16b,16cof the AC voltage side17includes a respective cascade17a,17b,17ceach of which comprises two power switches18, in particular power transistors, for example IGBTs.

The three-phase current is smoothed on the AC voltage side17using the filter14and may then be transformed to a desired voltage using the medium-voltage transformer15. The filter14can be designed as an inductor or as a transformer. The medium-voltage transformer15may be suitably chosen depending on the application. For supplying current to a medium-voltage network, the transformer15may be designed for example as a DY medium-voltage transformer.

The power switches18are controlled using the electronic control device13in particular by means of pulse-width modulation control. The supply voltages are measured on the AC voltage side17of the inverter12, preferably between the filter14and the medium-voltage transformer15, and are supplied together with the ground potential via corresponding lines19to the control device13as measured voltage signals. The currents on the individual phases are measured on the AC voltage side17, preferably between the inverter12and the filter14, using corresponding current measuring devices20and are supplied via corresponding lines21to the control device13as measured current signals. The control device13calculates the set currents from the measured voltages and currents. On the basis of the set currents and the measured currents the control device13determines the control signals for the power switches18and controls them correspondingly.

FIG. 2ashows a phasor diagram for the D-side22of the transformer15fromFIG. 1in the event of a voltage drop, here for example of the voltage UL23, for example by 40%.FIG. 2bshows a corresponding phasor diagram for the Y-side23of the transformer15. In the present example the voltage UL2Ndrops extremely, namely by 50%, whereas each of the voltages UL1Nand UL3Nreduces only a little bit by approximately 9%.

In order to counteract the voltage drop shown in theFIGS. 2a,2b, a current control, which in the following will be explained in detail based onFIG. 3, is performed by the control device13. First, the dropped voltage vector UL2Nis calculated from the measured intact voltage vectors UL1Nand UL3N. This allows a significantly exacter determination of the dropped voltage vector UL2Ncompared to a direct measurement of UL2N, in particular in the event of a complete or essentially complete voltage drop.

The current control provides for a reactive current IL2to be supplied on the affected phase UL2Nonly. For this purpose, the control device13calculates a set reactive current IL2preferably amounting to at least 40% of the nominal current, which is to be supplied on the affected phase UL2Nonly. However, on the phases UL1Nand UL3N, which are not affected, active current is drawn and supplied, respectively. More precisely, on one of the intact phases, here on the phase UL1N, an active current IL1is supplied, and on the other intact phase, here on the phase UL3N, an active current IL3is drawn with the amounts of the active currents IL1and IL3preferably being equal so that the total active power balance is zero. The amount of the supplied and drawn active current IL1, IL3depends on the depth of the voltage drop on the affected phase UL2N, however, in any case, is less than the nominal active current.

When practically applying the above method, at first one of the set active currents IL1(IL3) is calculated in the control device13in such a way that the voltage on the corresponding phase UL1N(UL3N) does not increase up to more than 110% relative to normal conditions. The other set active current IL3(IL1) is then calculated using Kirchhoff's current rule leading to an active current having the same amount, so that also on the other intact phase UL3N(UL1N) the voltage does not increase up to more than 110% relative to normal conditions. The determined set active currents IL1, IL2, IL3are then supplied to the AC voltage network by controlling the power switches18correspondingly.

It is only possible to apply Kirchhoff's current rule in the described manner because the neutral point24of the transformer15is not connected to the intermediate circuit11, as needed for an individual phase current control in order to enable a current compensation between the DC voltage side25and the AC voltage side17of the inverter12. Compared to an individual phase current control the inverter circuit10thus is characterized by the neutral point24of the transformer15being held at a fixed potential, in particular ground potential; and the intermediate dc circuit11has no additional connection to the AC voltage side17of the inverter12so that a corresponding current compensation line can be disposed with.

The intermediate dc circuit11may be connected to a DC power source, for example a plant for generating renewable energy. Such an application is shown for example inFIG. 4in the form of a wind power plant26with variable speed, including a rotor27, a gearbox28, a synchronous generator29, a controlled rectifier30which is connected to the inverter circuit10according toFIG. 1so that the rectifier30and the inverter12form a frequency converter31. However, the invention is not limited to this application. Further preferred embodiments are photovoltaic plants, fuel cells or other DC power sources.

Furthermore, the inverter circuit10may also be operated in reverse, if, instead of a DC power source27-30, a DC power consumer is connected on the DC voltage side25of the inverter12. Finally, the inverter12according toFIG. 1may as well work at the intermediate circuit11without the connection of a DC power source or a DC power sink on the DC voltage side25, and thus may be connected to a three-phase network via the transformer15in an autarkic manner.

The current control of the inverter circuit10was described above for the case that the voltage drops significantly on one phase only. This description may be transferred accordingly to a current control of the inverter circuit10according to the invention for the case of a voltage drop on a plurality of phases.