METHOD FOR PHASE-SEPARATED OVERCURRENT PROTECTION OF A THREE-PHASE BRIDGE CIRCUIT

Description

[0001] The invention relates to a method for operating a three-phase converter on a three-phase load, in particular an electrical network, wherein the three-phase converter has a DC intermediate circuit, at least one three-phase bridge circuit and at least one control unit for controlling the bridge circuit, wherein in the at least one bridge circuit at least two power breakers are provided per phase, which are connected in series to the DC intermediate circuit, wherein in the method the power breakers of each individual phase are controlled via the control unit depending on the predetermined target voltage values ​​of the three phases of the converter, so that a three-phase AC voltage is generated at the three-phase load via switching operations of the power breakers.In addition, the invention relates to a device for generating a three-phase alternating voltage at a three-phase load or an electrical network with at least one three-phase converter and a DC voltage intermediate circuit, wherein the converter has at least one three-phase bridge circuit and at least one control unit for controlling the bridge circuit, wherein in the at least one bridge circuit at least two power switches are provided per phase, which are connected in series to the DC voltage intermediate circuit, wherein the device is designed to control the power switches of each individual phase via the control unit depending on predetermined target voltage values ​​for all three phases of the converter, so that an AC voltage is generated via switching operations of the power switches.

[0002] Three-phase converters are used to generate a specific alternating voltage from a direct current, which is used to operate an electrical load, such as a motor, or to feed electrical energy into a three-phase electrical network. The bridge circuit used in the converter generally has two power switches per phase, so that six power switches are usually used to generate an alternating voltage with three phases. The power switches can be implemented using IGBT power switches, which optionally have an anti-parallel connected freewheeling diode. However, other power switches can also be used here. A corresponding bridge circuit known from the state of the art is shown in Fig. 1 shown.

[0003] The costs for three-phase converters, especially in higher power classes, are primarily determined by the necessary power electronics, i.e., the required IGBTs or IGBT modules. IGBTs are particularly fast power switches and exhibit excellent dynamic properties for use in converters. The power switches are dimensioned with respect to the maximum currents that can be switched without damaging the IGBT, i.e., the IGBTs are designed for the corresponding maximum currents. If the currents to be switched exceed the maximum currents, either permanently or briefly, the corresponding power switch or IGBT is switched off to prevent damage to or destruction of the IGBT due to overload.

[0004] The selection of the power breakers for a three-phase converter is not only based on the converter's continuous operation, but also on short-term power peaks, which must also be regularly provided by three-phase converters. For example, in the event of mains voltage faults, very large currents often occur in three-phase converters operated on electrical grids, which can lead to overloading of individual IGBTs. It is known from the prior art that in such a case, the IGBTs in the converter's bridge circuit can be completely blocked as soon as overload conditions occur in one of the power breakers of a phase, such as an overcurrent above an IGBT's shutdown limit. The current then flows only via the freewheeling diodes connected in anti-parallel to the IGBTs, which, in today's commonly used power breakers, can carry a briefly higher current than the IGBTs.This is part of a protection concept to protect all circuit breakers in the converter's bridge circuit from damage.

[0005] A converter whose bridge circuit is completely shut down in the event of a fault is known from European patent application EP 2 830 175 A2. European patent application EP 1 152 520 A2, on the other hand, deals with the control of a three-phase converter dependent on an input direct current.

[0006] For example, US 2007 / 0086226 A1 discloses a method for operating a converter in which an additional circuit breaker is switched on if a malfunction is detected in a circuit breaker of the three phases.

[0007] In the paper "Survey on Fault-Tolerant Techniques for Power Electronic Converters", IEEE Transaction on Power Electronics, Zhang Wenping et al., Vol. 29, No. 12, December 1, 2014, the use of additional power switches, for example in the form of TRIACs, is proposed for a fault-redundant topology.

[0008] However, both redundant topologies lead to a significant cost increase for circuit breakers with high switching capacity.

[0009] Concepts for riding through grid fault conditions are known from the paper "Fault Ride Through Control of Medium-Voltage Converters with LCL Filter in Distributed Generation Systems," by Robert Meyer et al., IEEE Energy Conversion Congress and Exposition, IEEEE, September 15, 2013, pages 1954–1961. The resulting overload conditions in individual phase arms of the converter are not addressed.

[0010] Three-phase converters are known from the German patent applications DE 10 2015 205 627 A1 and DE 10 2009 045 351 A1, which can continue to operate the three-phase bridge circuit with the remaining two phases in the event of a fault in one phase.

[0011] Instead of an IGBT with a freewheeling diode, other power switches consisting of an actively switchable power semiconductor and a reverse-conducting passive power semiconductor connected in parallel can also be used. The reverse-conducting state then refers to the current flow through the freewheeling diode in its forward direction, or the current flow in the forward direction of the passive power semiconductor.

[0012] However, it is problematic if high control dynamics are required during the overload condition. For example, grid operators regularly require in grid codes that in the event of a grid voltage drop, a maximum reactive current must be fed into the grid from the inverter within a very short time in order to support the grid voltage. Since the currents that then occur depend on the grid voltage drop itself, overload conditions can regularly occur in individual phases of the bridge circuit, causing the bridge circuit in the inverter to be switched off and protected. At the same time, however, high control dynamics are required from the inverter control system in order to comply with the grid codes. While the bridge circuit is switched off, the control system cannot influence the inverter, which leads to a deterioration in the dynamic control behavior of the inverter.To prevent shutdown, appropriately dimensioned circuit breakers must be provided, which, for example, do not require the bridge circuit to be shut down even in the event of a mains voltage drop. However, this increases the cost of the converters.

[0013] Fig. 3 shows, for example, simulated reactive power outputs of two converters during a simulated mains voltage drop with differently dimensioned

[0014] Circuit breakers. The 20-ms moving average values ​​of the reactive current are shown at a grid frequency of 50 Hz according to IEC 61400-21. The value 1.0 on the Y-axis represents the target reactive current output. A typically permitted range of 90% to 110% of the reactive current output is represented by two axes parallel to the X-axis. Time is shown on the X-axis, with the voltage dip simulated at t = 0.30 s.

[0015] Curve A shows the simulated behavior of a converter in which the power breakers were selected so that they do not exhibit overload conditions even in the event of a mains voltage drop. Curve B shows the behavior of a three-phase converter with smaller-sized power breakers. During the regulation of the reactive power output, two phases are briefly overloaded, and the converter's bridge circuit is briefly blocked. In Curve A, the permissible corridor for the reactive power output is already reached at t = 0.39 s. A disadvantage, however, is that the costs of the IGBT power breakers are relatively high, as they were dimensioned for the mains voltage drop.

[0016] Typically, the power switches or IGBTs are selected to allow the overcurrent protection measures to be triggered in such exceptional cases, thus reducing the cost of the IGBT power switches. However, if the overcurrent protection of individual IGBT power switches is triggered, the entire bridge circuit is blocked by the control unit, resulting in poorer dynamic control behavior of the bridge circuit. As described in Fig. 3 As can be seen, curve B with significant overshoot only reaches the permitted reactive current output corridor at approximately t = 0.41 s and thus later than curve A. With more cost-effective dimensioning of the IGBTs, cost advantages are achieved, but these result in an adverse dynamic behavior of the bridge circuit in the event of overcurrents.

[0017] The invention is based on this problem and has set itself the task of providing a method for operating a three-phase converter on a three-phase load, in particular an electrical network, with which very good dynamic control behavior can be achieved despite cost-effective dimensioning of the IGBT power switches of the three-phase bridge circuit. Furthermore, the present invention is based on the object of providing a corresponding, generic device for generating a three-phase alternating voltage on a three-phase load or an electrical network with at least one three-phase converter, which has significantly improved dynamic control behavior, particularly during grid voltage faults, at lower costs.

[0018] According to a first teaching of the present invention, the above-mentioned object is achieved by a method for operating a three-phase converter in that, using monitoring means, the individual phases are monitored for an overload condition, wherein in the event of the detection of an overload condition in one or two phases of the three-phase converter, the bridge circuit continues to operate with one or two non-overloaded phases, at least for the duration of the detected overload condition, wherein, at least for the duration of the overload condition, modified target voltage values ​​are determined instead of the predetermined target voltage values, which are used to control the bridge circuit, and to determine the modified target voltage values, the predetermined target voltage values ​​of the three phases are transformed into a predetermined target voltage rotation vector in the αβ coordinate system,the target voltage indicator is broken down into a differential voltage indicator and at least one modified target voltage indicator in the αβ coordinate system, wherein the at least one modified target voltage indicator takes into account the maximum phase voltage that occurs in the overloaded phases, and the modified target voltage values ​​can be determined from the modified target voltage indicator in the αβ coordinate system.

[0019] It has been shown that the continued operation of a three-phase bridge circuit despite overloading in one or two phases is possible without causing damage to the circuit breakers by only operating the IGBTs in the non-overloaded phases of the bridge circuit and allowing the current to flow in the overloaded phases, for example, via freewheeling diodes. This can improve the dynamic control behavior of the bridge circuit compared to completely shutting down the bridge circuit in the event of an overload.

[0020] According to the invention, modified target voltage values ​​are determined instead of the specified target voltage values, at least for the duration of the overload condition. These modified target voltage values ​​are used to control the bridge circuit. This makes it possible to maintain controllability of the three-phase bridge circuit and to set advantageous switching states despite overloading one or two phases of the bridge circuit. The modified target voltage values ​​can significantly improve the dynamic control behavior in the event of overload conditions of one or two phases compared to switching off the entire bridge circuit. This also results in the power rating of the circuit breakers being able to be reduced by 10% to 15% without significant loss of control behavior.

[0021] According to the invention, in order to determine the modified target voltage values, the predetermined target voltage values ​​of the three phases are transformed into a predetermined target voltage rotating vector in the αβ coordinate system, the target voltage rotating vector is broken down into a differential voltage rotating vector and a modified target voltage rotating vector in the αβ coordinate system, wherein the at least one modified target voltage rotating vector in the overloaded phases takes into account the maximum phase voltages that arise, and the modified target voltage values ​​for the phases can be determined from the resulting, modified target voltage rotating vector in the αβ coordinate system.

[0022] In the αβ coordinate system, a modified target voltage phasor can be determined by simply decomposing the specified target voltage phasor into a differential voltage phasor and a modified target voltage phasor. From this phasor, the modified target voltage values ​​of the individual phases of the bridge circuit can be determined. By specifying values ​​for the differential voltage phasor, the modified target voltage phasor and thus the control of the bridge circuit by the control unit can be easily varied and optimized in the event of an overload.

[0023] Alternatively, according to a further embodiment of the method according to the invention, the modified target voltage phasor in the αβ coordinate system can be used directly by the control unit of the bridge circuit without determining the modified target voltage values ​​for all three phases. For this purpose, the control unit only needs to be capable of using target voltage phasors in the αβ coordinate system to control the bridge circuit.

[0024] According to one embodiment of the method according to the invention, the monitoring means detects the overload condition of a phase by exceeding a predetermined current in the respective phase or a predetermined temperature of a component of the respective phase, preferably a power switch or a freewheeling diode. Manufacturers of power semiconductors typically specify values ​​for the maximum permissible current and the maximum permissible junction temperature of the power switches or freewheeling diodes. If these values ​​are exceeded, there is a risk of destruction of the component due to thermal overload. Therefore, the current and temperature are preferred parameters for detecting overload and taking preventative measures.

[0025] If, when monitoring the individual phases of the bridge circuit, the maximum phase voltage occurring during the respective overload, for example across the freewheeling diode, is determined and used as the modified target voltage value for the overloaded phase, the modified target voltage value for the overloaded phase can be easily calculated once the overload and the respective sign of the overload voltage have been determined. The maximum phase voltage is 2 / 3 of the DC voltage of the intermediate circuit or -2 / 3 of the DC voltage of the intermediate circuit, depending on the current direction during the overload. Once the modified target voltage values ​​for the overloaded phases have been determined, the modified target voltage values ​​for the non-overloaded phases can be used to optimize the control behavior.

[0026] According to a further embodiment of the method according to the invention, modified target voltage values ​​for the at least one non-overloaded phase are determined for at least one non-overloaded phase, taking into account the predetermined target voltage values ​​for three phases and the modified target voltage values ​​for at least one overloaded phase. By taking into account the predetermined target voltage value for three phases and the modified target voltage values ​​for at least one overloaded phase, the modified target voltage values ​​to be switched in the event of an overload can be optimized using the modified target voltage values ​​for the at least one non-overloaded phase, resulting in optimized control behavior.

[0027] According to a further embodiment of the method according to the invention, the magnitude of the differential voltage indicator has a predetermined value. This predetermined value does not necessarily have to be constant over time, but can, for example, depend on the respective position of the specified target voltage indicator. This can also influence the dynamic control behavior.

[0028] It is particularly advantageous if, according to a further embodiment, the magnitude of the differential voltage phasor is selected to be as small as possible. A minimal magnitude of a differential voltage phasor means that the selected modified target voltage phasor has a minimal difference from the originally specified target voltage phasor. This allows the transient response of the bridge circuit to be optimized.

[0029] According to a further embodiment of the method, the magnitude of the differential voltage phasor is at least temporarily zero. In this case, based on the remaining, non-overloaded phases, the specified target voltage phasor can be completely reconstructed, at least temporarily, using the remaining switching states of the non-overloaded phases, thus generating an identical three-phase alternating voltage at the converter at least temporarily despite the overload.

[0030] If IGBTs with at least one anti-parallel connected freewheeling diode are used as power switches in the bridge circuit, the bridge circuit of the converter can be designed to be particularly robust, since in the event of an overload the overload current can flow via the freewheeling diodes.

[0031] The method is particularly preferably carried out with three-phase converters of devices for generating and feeding electrical energy into an electrical grid, preferably with three-phase converters of wind turbines, photovoltaic systems with or without electrical storage, or components for storing electrical energy, in particular battery storage systems operated on the electrical grid. It has been shown that the method according to the invention makes it possible to dimension the circuit breakers smaller while still achieving very good control behavior.

[0032] For this reason, according to a further embodiment of the method, the method is preferably carried out during a grid voltage fault, preferably during "Fault Ride Through" (FRT) operation of the converter. It is precisely during this FRT operation that overloads frequently occur in individual phases of the bridge circuits used, which previously significantly impaired the control behavior of the bridge circuit. The method according to the invention can thus provide a control behavior with lower-power circuit breakers that was previously only achievable with larger circuit breakers. At the same time, the method according to the invention can meet the requirements of grid operators at lower cost, despite smaller circuit breaker dimensions.

[0033] The above-mentioned object is achieved by a generic device for generating a three-phase alternating voltage on a three-phase load or an electrical network with at least one three-phase converter in that means are provided for monitoring the individual phases for an overload condition, which means are designed to continue operating the bridge circuit with one or two non-overloaded phases in the event of an overload condition in one or two phases of the three-phase converter at least for the duration of the detected overload condition, and the means are designed to determine modified target voltage values ​​instead of the predetermined target voltage values, which are used to control the bridge circuit, and the means are further designedTo determine the modified target voltage values, the specified target voltage values ​​of the three phases are transformed into a specified target voltage vector in the αβ coordinate system, the target voltage vector is broken down into a differential voltage vector and at least one modified target voltage vector in the αβ coordinate system, whereby the at least one modified target voltage vector in the overloaded phases takes into account the maximum phase voltage that occurs, and the modified target voltage values ​​can be determined from the modified target voltage vector in the αβ coordinate system.

[0034] In contrast to devices that previously blocked the entire bridge circuit in the event of an overload, continued operation with non-overloaded phases results in improved control behavior of the bridge circuit in the event of an overload. Using the modified setpoint voltage indicator, the control unit's control of the device's bridge circuit can be easily varied and optimized in the event of an overload.

[0035] According to a first embodiment of the device, the means for monitoring the individual phases for an overload condition are preferably designed to determine modified target voltage values ​​instead of predetermined target voltage values ​​and to use the modified target voltage values ​​to control the bridge circuit. As already explained above, the bridge circuit can continue to operate despite an overload in one or two phases of the bridge circuit by providing modified target voltage values ​​to the control unit, thus providing improved dynamic behavior of the device in the event of an overload at lower costs for the circuit breakers. The use of modified target voltage values ​​allows a desired, for example, optimized, control behavior of the grid-side reactive current to be set.

[0036] Finally, the device is preferably a wind turbine operated on an electrical grid, a photovoltaic system with or without storage, or a grid-operated component for storing electrical energy, such as a battery storage system. The wind turbine can have a full-scale converter or a doubly fed asynchronous machine with a partial converter. These devices all have converters with bridge circuits and can provide improved dynamic control behavior, particularly in the event of grid voltage faults, at a reduced cost.

[0037] The invention will now be explained in more detail using exemplary embodiments in conjunction with the drawings. The drawings show in Fig. 1 the structure of a three-phase bridge circuit from the prior art, Fig. 2 in an αβ coordinate system the possible switching vectors of the bridge circuit from Fig. 1 Fig. 3 is a diagram of a simulated reactive power output of two different three-phase converters according to the prior art, Fig. 4 is a block diagram of an embodiment of the method according to the invention, Fig. 5 is an embodiment for the decomposition of the target voltage vector into a differential voltage vector and a modified target voltage vector in the event of an overload of one phase, Fig. 6 is an embodiment for the decomposition of the target voltage vector into a differential voltage vector and a modified target voltage vector in the event of an overload of two phases, Fig. 7 is a diagram of a simulated reactive current output of a three-phase converter according to an embodiment of the present invention compared to the reactive current output of a converter from the prior art with oversized circuit breakers, Fig.8 shows a diagram of a simulated reactive power output of a three-phase converter according to an embodiment of the present invention compared to the reactive power output of a converter with the IGBTs of the bridge circuit switched off according to the prior art.

[0038] Fig. 1 shows first the bridge circuit 1 known from the prior art, which includes the power switches sw1, sw2, sw3, sw1', sw2' and sw3', which are assigned to the corresponding phases L1, L2 and L3. The power switches of a phase, for example sw1 and sw1', are arranged in series with the DC voltage intermediate circuit V DC. The same applies to the power switches sw2 and sw2' as well as sw3 and sw3' assigned to phases L2 and L3. With the help of the power switches mentioned, which here are implemented as IGBT power switches with an anti-parallel connected freewheeling diode, a defined, three-phase AC voltage can be generated across phases L1, L2 and L3.

[0039] Fig. 2 now shows the switching vectors that can be generated by the bridge circuit in the αβ coordinate system. Since the six power switches can only switch in binary mode, the bridge circuit 1 can only Fig. 2 A total of 8 switching pointers V0 to V7 can be switched. The states V0 and V7 are shown in the phasor diagram of the Fig. 2 not shown, however, as these result in zero vectors. Different voltage values ​​can be switched across the three phases L1, L2, L3 using the switching vectors V1 to V6. The control unit of the converter (not shown here) controls the switching of the different, discrete switching vectors of bridge circuit 1. For this purpose, the control unit of the converter requires, for example, predetermined target voltage values ​​of the individual phases v1, v2, v3 as an input variable. This input variable can, however, also be transferred to the control unit via a target voltage rotating vector in the αβ coordinate system. The control unit controls the bridge circuit, for example using known modulation methods, such as a pulse width modulation method.

[0040] In Fig. 3 A diagram is now shown showing a simulated reactive power output from two different, state-of-the-art three-phase converters.

[0041] While curve A exhibits very good transient response with low overshoot, curve B exhibits a deteriorated transient response to the demand for reactive power supply in response to a mains voltage drop. This is due to the fact that the IGBT power switches in curve A were dimensioned such that no overload conditions occur even in the event of a mains voltage drop. Curve B shows the control behavior of a more cost-effective bridge circuit, in which the IGBT power switches were dimensioned such that, in the event of a simulated mains voltage drop in "fault ride through" (FRT) operation, two phases are overloaded and the bridge circuit is briefly shut down due to overload conditions. The poorer transient response of the more cost-effective variant is clearly visible.

[0042] An embodiment of a method according to the invention for operating a three-phase converter, in which an improved control behavior in the event of overload conditions of individual phases is achieved even with cost-effectively dimensioned power switches of a bridge circuit, is described in Fig. 4 shown.

[0043] Fig. 4 First, the predetermined target voltage values ​​v1, v2, v3 are shown, which are transferred from a controller to circuit block 2. Additionally, means 3 are provided for monitoring the phases of bridge circuit 1, which can detect an overload of one or more phases. In the illustrated embodiment, these respond, for example, to transmitted temperature values ​​T or phase currents i1, i2, i3, which in the illustrated embodiment are compared with the maximum permissible values ​​T max and i max in order to detect an overload condition. The input variables of the monitoring means can be selected almost arbitrarily, provided they allow detection of an overload condition of a phase L1, L2, L3.

[0044] If no overload is detected, the specified target voltage values ​​v1, v2 and v3 are transferred without change from circuit block 2 to control unit 4 (in Fig. 4 not shown). The control unit 4 then controls the power switches sw1 to sw3' of the bridge circuit 1 based on the predetermined target voltage values ​​using known methods, for example a pulse width modulation (PWM) method.

[0045] In case of detection of an overload condition in one or two phases of the three-phase inverter, according to the embodiment in Fig. 4 The bridge circuit continues to operate with one or two non-overloaded phases, at least for the duration of the detected overload condition. This can already influence the control behavior of the bridge circuit. According to one embodiment of the method, if modified target voltage values ​​vmod1, vmod2, and vmod3 are determined in the event of an overload instead of the specified target voltage values ​​v1, v2, and v3 and used to control the bridge circuit, the control behavior can be further improved.

[0046] In the embodiment in Fig. 4 The voltage values ​​vmod1, vmod2, and vmod3 are transferred from circuit block 2 to control unit 4 as modified target voltage values ​​for the individual phases L1, L2, and L3. In contrast to the prior art, the transmission of modified target voltage values ​​makes it possible to continue operating bridge circuit 1 while taking into account additional specifications, such as optimized control behavior. The modified target voltage values ​​are, for example, modified target voltage values ​​compared to the target voltage values ​​v1, v2, and v3 specified by the controller with a view to improved control behavior.

[0047] To easily account for the overload condition of the individual phases L1, L2, and L3 of bridge circuit 1, the phase voltage that occurs during the respective overload is determined when monitoring the individual phases of the bridge circuit and used as a modified setpoint voltage value for the overloaded phases. The resulting phase voltage on an overloaded phase corresponds to the maximum phase voltage, which corresponds to 2 / 3 V DC or -2 / 3 V DC. With this simple modification of the setpoint voltage values, the circuit breakers can continue to operate with modified setpoint voltage values ​​vmod1, vmod2, and vmod3.

[0048] At the same time, taking the resulting phase voltage at the overloaded phases into account when determining the modified target voltage values ​​allows the remaining target voltage values ​​of the non-overloaded phases to be used to specify a modified target voltage value for the non-overloaded phases, which describes particularly good control behavior of the bridge circuit in an overloaded state. Overloads in only one or a maximum of two phases can be considered. If all three phases are overloaded, however, the IGBTs of the bridge circuit are completely switched off.

[0049] By specifying the modified target voltage values ​​vmod1, vmod2, vmod3 of at least one non-overloaded phase, an additional degree of freedom is created compared to the state of the art, with which the control behavior of the bridge circuit can be specifically improved.

[0050] In order to determine modified target voltage values ​​vmod1, vmod2, vmod3, which allow a better control behavior of the bridge circuit in the event of overloads, according to a preferred embodiment for determining the modified target voltage values, the predetermined target voltage values ​​of the three phases are transformed into a predetermined target voltage rotation vector V soll in the αβ coordinate system.

[0051] Fig. 5 und 6 show different possibilities for decomposing the nominal voltage phasor V soll into a differential voltage phasor V diff and a modified nominal voltage phasor V mod for different overload cases.

[0052] The Fig. 5 und 6 differ in that in Fig. 5 Phase L1 is overloaded with a positive current. Fig. 6 shows the case where phase L1 and phase L2 are overloaded with a positive current. A positive current corresponds to a current in the direction of the arrows on phases L1, L2 and L3 in Fig. 1 .

[0053] Furthermore, Fig. 5 that modified nominal voltage phasors V mod can be generated within the parallelogram P. By simple vector addition of the given nominal voltage phasor V soll with a differential voltage phasor V diff , a specific modified nominal voltage phasor V mod is obtained, as in Fig. 5 shown. V mod can now be generated depending on specifications for the differential voltage phasor V diff.

[0054] In case of overload of a phase, it is, as Fig. 5 shows, it is possible to completely reconstruct the voltage phasors lying within the parallelogram P via the non-overloaded phases L2 and L3, so that the differential voltage phasor V diff can assume the value 0 as long as the specified target voltage phasor V soll moves within the parallelogram P.

[0055] The same applies to Fig. 6 in the case that the specified target voltage indicator V soll lies exactly on the dashed line of the still possible, modified target voltage indicator V mod, whereby only one phase L3 can be activated. Due to the two overloaded phases L1 and L2 with positive overload current, in this case it is only possible to adjust the values ​​along the dashed line in Fig. 6 according to the length of the still possible, modified target voltage rotary pointer V mod.

[0056] As already explained above, the length of the differential voltage phasor V diff , i.e. the magnitude of the differential voltage phasor V diff , can assume a predetermined value. According to a further embodiment, it is thus possible to design the differential voltage phasor V diff to be as short as possible. This results in the bridge circuit 1 in the closest possible switching state to the switching state predetermined by the predetermined target voltage values ​​v1, v2, v3, taking into account the non-overloaded phases, so that the dynamic control behavior of the bridge circuit is further improved. For example, the differential voltage phasor V diff can have the shortest length not equal to zero when the modified target voltage phasor V mod is perpendicular to the differential voltage phasor V diff , i.e. both vectors form a right angle.

[0057] Mathematically, the modified nominal voltage phasor V mod can be compared with the differential voltage phasor V diff with the smallest value, for example, for the Fig. 5 specified target voltage indicator V soll with vmod 1 = vmax 1 vmod 2 = vmax 1 vmod 3 = vmax 1 − 3 valpha / 4 − 3 √ 3 vbeta / 4 Valpha and vbeta are the αβ-coordinates of the specified nominal voltage phasor V soll in the αβ-coordinate system. Vmax1 is the maximum value for the phase voltage, for the circuit according to Fig. 1 and a three-phase network with symmetrical impedances 2 / 3 V DC.

[0058] If the specified target voltage phasor V soll lies within the parallelogram P, it can be exactly reconstructed so that the differential voltage phasor V diff has a length of zero. The modified target voltage values ​​vmod1, vmod2, vmod3 of the modified target voltage phasor V mod with a differential voltage phasor V diff zero can be reconstructed within the parallelogram with vmod 1 = vmax 1 vmod 2 = √ 3 vbeta / 2 − 3 valpha / 2 + vmax 1 vmod 3 = − √ 3 vbeta / 2 − 3 valpha / 2 + vmax 1 be specified.

[0059] In Fig. 6 As already explained, phase 1 and phase 2 are overloaded with a positive current. For the given target voltage phasor V soll shown, for example, a minimum differential phasor V diff results with: vmod 1 = vmax 1 vmod 2 = vmax 1 vmod 3 = vmax 1 − 3 valpha / 4 − 3 √ 3 vbeta / 4

[0060] According to an alternative embodiment, the modified target voltage rotation indicator V mod can be used directly by the control unit 4 to control the bridge circuit even in the αβ coordinate system, provided that the control unit is suitable for this purpose.

[0061] The control behavior of bridge circuit 1 was then simulated using a vector optimization approach, in which modified target voltage phasors V mod were determined with a minimum value of the differential voltage phasor V diff , based on a required reactive power output in the event of a grid voltage drop. This is a common requirement defined in the grid codes of grid operators for grid voltage stabilization.

[0062] Fig. 7 shows the diagram of this simulated reactive power output of the three-phase converter with cost-optimized IGBT power switches and optimized target voltage rotation pointer specification according to an embodiment of the invention in curve C. In comparison, a reactive power output with IGBT power switches is shown, which are dimensioned in such a way that they do not show overload conditions in the event of a mains voltage drop (curve A).

[0063] How Fig. 7 As shown, the method according to the invention is particularly advantageous for the operation of bridge circuits in three-phase converters during mains voltage faults, as it enables particularly fast transient response to specified reactive current values ​​without having to oversize the IGBT power switches. As already explained, this allows the use of 10% to 15% smaller IGBT power switches without any loss of control performance.

[0064] A comparison of the embodiment according to the invention with cost-optimized IGBT power switches without application of the method according to the invention, curve B, shows Fig. 8 The method according to the invention leads to a significant improvement in the dynamic control behavior of the bridge circuit with the same hardware. Curve C of the exemplary embodiment according to the invention shows the permanent attainment of the tolerance band for the reactive current output after approximately 0.39 seconds, whereas, as stated above, the variant known from the prior art only reaches the tolerance band at 0.41 seconds.

[0065] It is therefore understandable that the method for operating the bridge circuit of a three-phase converter according to the present invention can be particularly advantageously implemented with devices for generating and feeding electrical energy into an electrical grid, preferably with three-phase converters of wind turbines. Furthermore, photovoltaic systems with or without storage, or even battery storage systems themselves, which are connected to the electrical grid, can be advantageously operated with the method according to the invention as devices for generating and feeding electrical energy and can benefit from the resulting cost advantages for the devices. Fig. 7 and 8 clearly show, the method according to the invention is particularly advantageous for the fault ride-through (F RT) operation of a converter on the grid.

Claims

1. Method for operating a three-phase inverter on a three-phase load, in particular on an electrical grid, wherein the three-phase inverter has a direct voltage intermediate circuit, at least one three-phase bridge circuit (1) and at least one control unit (4) for controlling the bridge circuit (1), wherein, in the at least one bridge circuit (1), at least two power switches (sw1, sw2, sw3, sw1', sw2', sw3') per phase (L1, L2, L3) are provided, which are connected in series parallel to the direct voltage intermediate circuit (VDC), wherein in the method, depending on predefined target voltage values (v1, v2, v3) of the three phases of the inverter, the power switches (sw1, sw2, sw3, sw1', sw2', sw3') of each individual phase (L1, L2, L3) are actuated via the control unit (4) such that a three-phase alternating voltage is generated on the three-phase load via switching operations of the power switches (sw1, sw2, sw3, sw1', sw2', sw3'), wherein a monitoring of the power switches in the individual phases (L1, L2, L3) for an overload state is carried out using monitoring means (3), wherein if an overload state is detected in one or two phases of the three-phase inverter, the bridge circuit (1) continues to be operated with one or two non-overloaded phases at least for the duration of the detected overload state, whereby instead of the predefined target voltage values (v1, v2, v3), modified target voltage values (vmod1, vmod2, vmod3) are determined at least for the duration of the overload state, which are used to control the bridge circuit, characterized in that, in order to determine the modified target voltage values (vmod1, vmod2, vmod3), the predefined target voltage values of the three phases are transformed into a predefined target voltage rotating phasor (Vtarget) in the αβ-coordinate system, the target voltage rotating phasor (Vtarget) is decomposed into a differential voltage rotating phasor (Vdiff) and at least one modified target voltage rotating phasor (Vmod) in the αβ-coordinate system, wherein the at least one modified target voltage rotating phasor (Vmod) in the overloaded phases takes into account in each case the resulting maximum phase voltage (vmax) and from the modified target voltage rotating phasor (Vmod) in the αβ-coordinate system, the modified target voltage values (vmod1, vmod2, vmod3) can be determined.

2. Method according to claim 1, characterised in that the overload state of a phase is detected by the monitoring means (3) when a predetermined current (imax) in the relevant phase or a predetermined temperature (Tmax) of a component, preferably of a power switch or of a freewheeling diode, in the relevant phase is exceeded.

3. Method according to any one of claims 1 or 2, characterised in that during the monitoring of the individual phases (L1, L2, L3) of the bridge circuit (1), the phase voltage resulting during the respective overload is determined and used as a modified target voltage value (vmod1, vmod2, vmod3) for the overloaded phase (L1, L2, L3).

4. Method according to any one of claims 1 to 3, characterised in that for at least one non-overloaded phase (L1, L2, L3), modified target voltage values (vmod1, vmod2, vmod3) for the at least one non-overloaded phase are determined taking into account the predefined target voltage value (v1, v2, v3) for three phases and the modified target voltage values for at least one overloaded phase.

5. Method according to any one of claims 1 to 4, characterised in that alternatively the modified target voltage rotating phasor (Vmod) in the αβ-coordinate system is used by the control unit (4) to actuate the bridge circuit (1).

6. Method according to any one of claims 1 to 5, characterised in that the absolute value of the differential voltage rotating phasor (Vdiff) has a predetermined value.

7. Method according to any one of claims 1 to 6, characterised in that the absolute value of the differential voltage rotating phasor (Vdiff) is selected to be as small as possible.

8. Method according to any one of claims 1 to 7, characterised in that the absolute value of the differential voltage rotating phasor (Vdiff) at least temporarily has the value zero.

9. Method according to any one of claims 1 to 8, characterised in that IGBTs with at least one freewheeling diode connected in anti-parallel thereto are used as power switches (sw1, sw2, sw3m, sw1', sw2', sw3') in the bridge circuit (1).

10. Method according to any one of claims 1 to 9, characterised in that the method is carried out with three-phase inverters of devices for generating and feeding electrical energy into an electrical grid, with three-phase inverters of wind power systems, of photovoltaic systems with or without electrical storage devices or of components for storing electrical energy, in particular of battery storage systems, which are operated on the electrical grid.

11. Method according to any one of claims 1 to 10, characterised in that the method is carried out during a grid voltage fault, preferably in the fault ride through operation of the inverter.

12. Device for generating a three-phase alternating voltage on a three-phase load or on an electrical grid with at least one three-phase inverter and a direct voltage intermediate circuit (VDC), wherein the inverter has at least one three-phase bridge circuit (1) and at least one control unit (4) for controlling the bridge circuit (1), wherein, in the at least one bridge circuit (1), at least two power switches (sw1, sw2, sw3, sw1', sw2', sw3') per phase (L1, L2, L3) are provided, which are connected in series parallel to the direct voltage intermediate circuit (VDC), wherein the device is configured to actuate the power switches (sw1, sw2, sw3, sw1', sw2', sw3') of each individual phase via the control unit (11) depending on predefined target voltage values (v1, v2, v3) for all three phases (L1, L2, L3) of the inverter such that a three-phase alternating voltage is generated via switching operations of the power switches (sw1, sw2, sw3, sw1', sw2', sw3'), in particular for carrying out a method according to any one of claims 1 to 11, wherein means (2, 3) for monitoring the power switches in the individual phases for an overload state are provided, which are configured to continue to operate the bridge circuit (1), if an overload state is detected in one or two phases of the three-phase inverter, with one or two non-overloaded phases (L1, L2, L3) at least for the duration of the detected overload state, whereby the means are configured to determine, instead of the predefined target voltage values (v1, v2, v3), modified target voltage values (vmod1, vmod2, vmod3) at least for the duration of the overload state, which are used to control the bridge circuit, characterized in that, the means are also configured, in order to determine the modified target voltage values (vmod1, vmod2, vmod3), to transform the predefined target voltage values of the three phases into a predefined target voltage rotating phasor (Vtarget) in the αβ-coordinate system, to decompose the target voltage rotating phasor (Vtarget) into a differential voltage rotating phasor (Vdiff) and at least one modified target voltage rotating phasor (Vmod) in the αβ-coordinate system, wherein the at least one modified target voltage rotating phasor (Vmod) in the overloaded phases takes into account in each case the resulting maximum phase voltage (vmax) and from the modified target voltage rotating phasor (Vmod) in the αβ-coordinate system, the modified target voltage values (vmod1, vmod2, vmod3) can be determined.

13. Device according to claim 12, characterised in that the device is a wind power system operated on a grid, a photovoltaic system with or without a storage device or a component operated on the grid for storing electrical energy, in particular a battery storage system.