Method and three-phase inverter for feeding into a three-phase AC voltage network

Abstract

In order to feed three-phase electrical power from a direct current source (1) into a three-phase alternating current voltage network (3) by means of an inverter (2), wherein the inverter (2) has a control structure (10), wherein the control structure (10) includes a controller (11) and a multiplier (12), a method has the following steps: measuring a phase-specific network voltage (U_abc); determining a network frequency (f_Netz) from the measured network voltage (U_abc); generating a phase-specific sinusoidal voltage reference value (U_ref) by means of the controller (11) from the phase-specific measured network voltage (U_abc) and the determined network frequency (f_Netz), wherein the The voltage reference value (U_ref) has a phase-specific amplitude and a common frequency, which corresponds to the respective amplitude or frequency of the measured network voltage (U_abc) of the individual phases; a phase-specific current target value (I_abc_target) is generated by the multiplier (12, 12') by forming a product of a phase-specific predefined current amplitude target value (I_d_target) and a phase-specific voltage reference value (U_ref) and normalizing the product phase-specifically to the respective network voltage amplitude (U_d); the power switches of the inverter are actuated using the phase-specific current target value (I_abc_target). An inverter is provided for carrying out the method.

Description

Technical Field

[0001] The invention relates to a method for feeding electrical power from a direct current source into a three-phase alternating current voltage network by means of an inverter, and to an inverter which is provided for feeding electrical power from a direct current source into a three-phase alternating current voltage network. Background Art

[0002] An inverter connected to a three-phase AC voltage network and exchanging electrical power with the AC voltage network usually applies current The inverter works in this way, in which the AC current flowing through the AC current connection of the inverter and thus the power exchanged is in principle predetermined by the inverter itself. In the case of a photovoltaic generator as a DC current source, the power converted into AC current and regularly exchanged with the AC voltage network corresponds to the maximum possible power of the photovoltaic generator.

[0003] The inverter can also perform the function of supporting the network, in that the exchanged power can be indirectly dependent on characteristics of the AC voltage network, for example on the frequency and / or voltage. For this purpose, the exchanged power can vary within the scope of the available direct current power, for example when the network frequency deviates from the nominal frequency of the AC voltage network.

[0004] Inverters can define and maintain an AC voltage network, i.e., operate in a network-forming manner. In contrast to operation in which current is applied and the network is supported, inverters for network formation can in particular be operated in a voltage-applying manner in such a way that the power exchanged depends directly on the characteristics of the AC voltage network, i.e., in particular as a function of the frequency and / or voltage in the AC voltage network. In this case, the voltage-applying inverter can, for example, include a so-called droop control device or be operated as a so-called virtual synchronous machine. In this case, the operation of an energy generating device as an active network former requires a much higher dynamics of the connected DC current source than merely supporting the network and in terms of reactive operation.

[0005] EP 1 841 050 A1 discloses an energy generation system comprising three single-phase inverters, wherein the three inverters are connected to one of the three phases of the AC voltage network and are operated largely independently of one another in terms of the current applied. As a result, the energy generation system can form a so-called unbalanced load, in that the individual inverters exchange different powers with the AC voltage network.

[0006] DE 10 2012 220 582 A1 discloses a method for three-phase feeding of electrical power by means of an inverter, in which asymmetries in the AC voltage network are detected and in response to which asymmetric current components are fed in by the inverter. For this purpose, a comprehensive control structure is disclosed, which is provided for correspondingly controlling the inverter in direct response to the detected asymmetries.

[0007] A method is known from EP 2 348 597 A1, in which an asymmetry of the power flowing through the three phases of a network connection point of an electrical device is compensated for with the aid of an inverter, in that the inverter exchanges different powers with the AC voltage network via the three phases on the device side of the network connection point, wherein the difference in the exchanged powers precisely compensates for the asymmetry.

[0008] A similar method is known from DE 10 2011 085 676 A1, in which the feeding of electrical power into a three-phase AC voltage network by means of an inverter is controlled in a phase-selective manner taking into account the active power and reactive power ascertained at the feed-in point so that the unbalanced load at the feed-in point is minimized. Preferably, provision is made here for the inverter to have three individually controllable outputs for connection to each one phase of the three-phase AC current network.

[0009] DE 10 2010 029 951 A1 discloses a method for feeding an asymmetrical three-phase current into a three-phase AC voltage network, in which the asymmetrical current is generated by superimposing a positive sequence system and a negative sequence system, wherein one inverter module is provided for generating the positive sequence system or the negative sequence system.

[0010] According to the prior art, active asymmetrical feeding of electrical power into a three-phase AC voltage network by means of an energy generating device that applies current could thus only be achieved until now by using three largely independently fed single-phase inverters or by using two independently fed three-phase inverters. Alternatively, an asymmetrical feeding can be achieved by means of a control structure, the result of which depends essentially on the electrical characteristics of the AC voltage network; in particular, an asymmetrical feeding can be achieved using an inverter that applies voltage, wherein, however, the feed-in power to the individual phases cannot be controlled in a targeted manner but depends directly on the characteristics of the connected AC voltage network. Summary of the invention

[0011] Mission of the present invention

[0012] The invention is based on the object of specifying a method for asymmetrically feeding electrical power from a direct current source into a three-phase alternating current voltage network via a three-phase current-imparting inverter, in which method the current or power fed into the alternating current voltage network via the individual phases can be regulated independently of one another in a simple and efficient manner.

[0013] Solution

[0014] This object is achieved by having a method according to the invention and an inverter according to the invention. Preferred embodiments are defined in optional embodiments.

[0015] The method according to the invention for feeding electrical power from a direct current source into a three-phase alternating voltage network by means of an inverter, wherein the inverter has a control structure with a regulator and a multiplier, comprises the following steps:

[0016] - measurement of the phase-specific network voltage;

[0017] - Determine the network frequency from the measured network voltage;

[0018] - generating phase-specific sinusoidal voltage reference values ​​by means of a controller from the phase-specific measured network voltage and the ascertained network frequency, wherein the voltage reference values ​​have phase-specific amplitudes and a common frequency, which correspond to the respective amplitudes of the measured network voltages of the individual phases or the frequency of the network voltage;

[0019] - generating a phase-specific current target value by means of a multiplier by forming a first product of a phase-specifically predefined current amplitude target value and a phase-specific voltage reference value and normalizing this first product phase-specifically to the corresponding network voltage amplitude;

[0020] - Use phase-specific current target values ​​to control the power switches of the inverter.

[0021] Therefore, in the method, a predefined current amplitude target value is multiplied phase-specifically according to the invention by a voltage reference value, wherein the phase-specific voltage reference value is generated (directly) from the measured network voltage and has a mathematically sinusoidal curve, whose amplitude and frequency run synchronously with the network voltage of the individual phases. After phase-specific normalization to the determined network voltage amplitude according to the invention, a simple mapping of the current amplitude target value to a matching current desired value is available, in particular without having to perform a relatively complex coordinate transformation. The resulting current desired value is again phase-specific and has a sinusoidal time curve. The current desired value is therefore suitable for use as a desired value for the known activation of the power switches of the inverter, for example within the scope of conventional pulse width modulation (PWM).

[0022] In one embodiment of the method, the regulator for generating the voltage reference value comprises a filter, in particular a bandpass filter, or a generalized integrator. In particular, a generalized integrator (English: "Second Order Generalized Integrator", second-order generalized integrator) is advantageously suitable for determining a phase-specific, sinusoidal voltage reference value, because the output value of the generalized integrator can accurately simulate the basic network frequency curve of the sinusoidal network voltage without being affected by higher-frequency interference.

[0023] The current amplitude target value can be predefined by feeding an individually predefined active power into the individual phases. In conventional methods, only the electrical power to be exchanged overall, thus the total power over all phases, is predefined and must be converted into a desired value by complex transformations depending on the coordinate system used. In contrast, in the method according to the invention, the active power for the individual phases can be easily predefined individually by means of the current amplitude target value, since the conversion from the current amplitude target value in conjunction with the voltage reference value to the phase-specific current desired value takes place directly.

[0024] In particular, the current amplitude target value can advantageously be predefined in such a way that the active power exchanged has essentially the same magnitude in all phases. This ensures a completely symmetrical operation of the inverter.

[0025] Alternatively, the current amplitude target value can advantageously be predetermined by exchanging active power of a given sign with the AC voltage network at at least one of the phases and exchanging active power of an opposite sign at at least one of the other phases. The method thus makes it possible in a simple and advantageous manner to generate such asymmetric power that power flows from the inverter into the AC voltage network at one phase while power flows from the AC voltage network into the inverter at the same time at another phase. This feature can be used particularly advantageously if there is already a significant asymmetry in the power flow in the individual phases in the AC voltage network.

[0026] In an advantageous embodiment of the method, a second product consisting of a phase-specific predefined reactive current amplitude target value and a phase-specific voltage reference value, each phase-shifted by 90 degrees, can be formed by means of a further multiplier, and these second products are added to the first product consisting of a phase-specific predefined current amplitude target value and a phase-specific voltage reference value. The resulting sum value can then be normalized phase-specifically to the corresponding network voltage amplitude. This results in a current desired value, which contains both an active current component and a reactive current component and can be used as a desired value for controlling the power switches of the inverter, so that the output current of the inverter phase-specifically includes both active power and reactive power. In this case, the reactive current amplitude target value can be predefined by feeding in individually predefined reactive power on these phases.

[0027] The three-phase inverter according to the invention for feeding electrical power from a direct current source into a three-phase alternating voltage network has a control structure which is provided for carrying out the method according to the above description. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the following, the invention is further explained and described based on the exemplary embodiments shown in the drawings.

[0029] Figure 1 An energy generating device connected to an alternating voltage network is shown;

[0030] Figure 2 An embodiment of a regulation structure of an inverter is shown; and

[0031] Figure 3 Another embodiment of a control structure of an inverter is shown. DETAILED DESCRIPTION

[0032] Figure 1 A simplified diagram of an energy generation device connected to an alternating voltage network 3 is shown. The energy generation device comprises an inverter 2 which receives a direct current source 1 (at Figure 1The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3. The inverter 2 is a power supply unit 200, which is used to supply electrical power to the AC voltage network 3 ...

[0033] The inverter 2 has a control device 4 which is responsible, in particular, for the operation management of the inverter 2. For this purpose, the control device 4 includes, for example, functions such as starting and stopping the inverter 2, regulating and optimizing the voltage at the DC current input of the inverter, fault monitoring, etc. In particular, the control device 4 can cooperate with the inverter bridge 2a in that measured values ​​are transmitted from the inverter bridge 2a to the control device 4 and / or the control device 4 predetermines a basic operating state of the inverter bridge 2a, such as normal operation or emergency shutdown.

[0034] Furthermore, the control device 4 can be connected to and exchange data with a further external measuring or control unit 20. The external measuring or control unit 20 can be, for example, a power measuring device at the network connection point of the inverter 2 to the AC voltage network 3. Alternatively or additionally, the external measuring or control unit 20 can be a control unit of an energy generation system, which in addition to the inverter 2 also includes further electrical devices such as measuring locations, generators, memories and / or loads, wherein the control unit 20 of such a system is connected to a plurality of the other electrical devices of the system and can influence or control their electrical properties, if necessary.

[0035] The inverter 2 further comprises a regulating structure 10. The regulating structure 10 is connected to a voltage measuring device 5, which is respectively assigned to one of the phase conductors L1, L2 or L3 and the neutral conductor N. The voltage measuring device 5 detects the network voltage U_abc of the individual phases. The regulating structure 10 receives the measured network voltage U_abc from the voltage measuring device 5. For the construction of a suitable voltage measuring device 5, different embodiments are known to those skilled in the art from the prior art, so that according to Figure 1 The specific embodiment of the voltage measuring device 5 is to be understood as an example only.

[0036] The control structure 10 is also connected to the control device 4 of the inverter 2. The control device 4 transmits parameters to the control structure 10. The control structure 10 is configured to output phase-specific current target values ​​I_abc_target to the inverter bridge 2a, wherein the current target values ​​I_abc_target are determined by the control structure 10 as a function of the parameters of the control device 4 and as a function of the measured network voltage U_abc. The inverter bridge 2a receives the current target values ​​I_abc_target and, based on the internal control devices of the power semiconductors of the inverter bridge 2a, generates clocked output voltages suitable for stimulating output currents that follow the current target values ​​I_abc_target and, in particular, correspond to the current target values ​​to a large extent in normal operation of the inverter 2.

[0037] Figure 2 The schematic diagram of the control structure 10 of the inverter 2 is shown. The control structure 10 includes a controller 11. The controller 11 receives various input variables. These input variables include general parameters that can be predefined by the control device 4 of the inverter 2 and the measured network voltage U_abc. In addition, the network frequency f_Netz belongs to the input variables of the controller 11, wherein the network frequency f_Netz can be determined from the measured network voltage U_abc, for example by means of a phase-locked loop PLL or other common methods.

[0038] The regulator 11 determines and outputs instantaneous voltage reference values ​​U_ref for the three phases from the instantaneous values ​​of the network voltage U_abc and the network frequency f_Netz of the three phases of the AC voltage network 2. In addition, a feedback of the output voltage reference value U_ref to the input of the regulator 11 can be provided.

[0039] The regulator 11 is designed so that, on the one hand, the voltage reference value U_ref deviates from the actual measured network voltage U_abc with an ideal sinusoidal curve, i.e. follows three mutually phase-shifted sinusoidal functions. On the other hand, not only the amplitude of the voltage reference value U_ref corresponds to the amplitude of the measured network voltage U_abc, but also the frequency of the voltage reference value U_ref corresponds to the network frequency f_Netz.

[0040] The voltage reference value U_ref thus simulates the curve of the network voltage in an idealized form in that the voltage reference value U_ref has a sinusoidal curve in frequency and amplitude that is synchronous with the network voltage. This is achieved in particular in that the regulator 11 includes a frequency-selective regulation. For example, a bandpass filter is suitable for this. Since the voltage reference value U_ref is fed back to the regulator 11, if necessary after scaling and / or delay, a so-called generalized integrator can be used in particular as the regulator 11 in order to further improve the determination of the voltage reference value U_ref.

[0041] The control structure 1 further comprises a multiplier 12. The multiplier 12 receives from the controller 11 a phase-specific voltage reference value U_ref and a phase-specific network voltage amplitude U_d, which can be determined from the measured network voltage U_abc, for example, by a phase-locked loop PLL.

[0042] Phase-specific current amplitude target values ​​I_d_target are provided at another input of the multiplier 12. The current amplitude target values ​​I_d_target can consist in particular of three values ​​which predetermine the desired amplitudes of the output currents of the inverter 2 for the three phases of the AC voltage network 3. The current amplitude target values ​​I_d_target which apply to the three phases simultaneously can in principle be selected to a large extent freely, at least with regard to the performance of the DC current source 1 connected to the inverter 2. In particular, for the individual phases, not only the same current amplitude target value I_d_target but also significantly different current amplitude target values ​​I_d_target can be predefined, wherein the current amplitude target value I_d_target can also have different signs, i.e. represent different flow directions of active power from the inverter 2 into the AC voltage network 3 or vice versa.

[0043] The current amplitude target value I_d_soll may correspond to Figure 2 The current amplitude target value I_d_target is generated in the control structure 10, in particular as a function of parameters which are transmitted to the control structure 10 from the control device 4 of the inverter 2. Alternatively, the current amplitude target value I_d_target can also be transmitted directly from the control device 4 to the multiplier 12.

[0044] In the multiplier 12, the current amplitude target value I_d_soll is phase-individuell multiplied by the voltage reference value U_ref of the network frequency and divided by the phase-individual network voltage amplitude U_d. As a result, the multiplier 12 outputs the phase-individual current desired value I_abc_soll which is normalized to the actual amplitude of the network voltage of the corresponding phase.

[0045] Figure 3An extended embodiment of the method according to the invention is shown. Figure 2 In addition to the implementation methods, Figure 3 In the embodiment of the invention, the predetermined reactive current amplitude target value I_q_soll is processed. The current amplitude target value I_d_soll predetermines the desired amplitude of the output current of the inverter 2, which is in phase with the network voltage on each phase, so that the current amplitude target value I_d_soll indirectly predetermines the phase-specific active power. Conversely, the reactive current amplitude target value I_q_soll predetermines the desired phase-specific amplitude of the output current of the inverter 2, which is phase-shifted by 90 degrees relative to the network voltage on each phase, so that the reactive current amplitude target value I_q_soll indirectly predetermines the phase-specific reactive power. It is understood that this phase shift can optionally correspond to underexcitation or overexcitation, that is, it can have a positive sign or a negative sign.

[0046] Specifically, the voltage reference value U_ref can be phase-shifted by 90 degrees by means of the phase shifter 13. For this purpose, the phase shifter 13 can include, for example, a transformation to the α-β-0 coordinate system and back, wherein a phase shift of 90 degrees can be performed particularly easily in the α-β-0 coordinate system. In a second multiplier 14, it is multiplied by a predetermined reactive current amplitude target value I_q_target. The multiplication in the multiplier 14 produces reactive current target values, which are added in an adder 15 to the active current target values, which correspond to the active current target values ​​by means of the multiplier 12'. Figure 2 Generated by the voltage reference value U_ref and the current amplitude target value I_d_soll.

[0047] The sum of the active and reactive current target values ​​calculated by the adder 15 is divided by the phase-individual network voltage amplitude U_d in a further multiplier 16. As a result, the multiplier 16 outputs phase-individual current target values ​​I_abc_target, which include both active and reactive current components and are normalized to the actual amplitude of the network voltage of the respective phase.

[0048] will be so, according to Figure 2 or Figure 3 The calculated current expected value I_abc_soll is based on Figure 1The inverter bridge 2a can be used to control the power semiconductor switches of the inverter bridge 2a. For this purpose, the inverter bridge 2a can include an internal control device known from the prior art, which ensures that the power semiconductors of the inverter bridge 2a are clocked in such a way that the current fed into the AC voltage network 3 by the inverter 2 corresponds to the current target value I_abc_target specified by the control structure 10. Such a control loop is known to a person skilled in the art, for example as an AC current regulator with pulse width modulation (PWM), and is therefore not described in detail here.

[0049] In a specific embodiment of the present invention, the current amplitude target value may be selected so as to achieve at least one of the following objectives.

[0050] Characterization of components of inverter 2:

[0051] In order to characterize the components of the inverter 2, a check step can be provided, which can be carried out in time separately from the normal operation of the inverter 2 if necessary. Specifically, the capacitance of the normally divided DC voltage intermediate circuit of the neutral point clamped (NPC) inverter 2 can be determined in that a positive current amplitude target value I_d_set is respectively predefined for two of the three phases, and a negative current amplitude target value I_d_set is predefined for the third of the three phases, wherein the sum of the current amplitude target values ​​I_d_set is equal to zero. The inverter bridge 2a is caused to generate the corresponding current in the phases L1, L2, L3 by means of the control structure 10 according to the invention. Due to this operation of the inverter 2, the currents on the phases with the positive current amplitude target value I_d_set are in phase with the curve of the network voltage U_abc on these phases, while the currents in the phases with the negative current amplitude target value I_d_set are in phase with the curve of the network voltage U_abc of this phase. Since the sum of the current amplitude target values ​​I_d_target has been selected to be equal to zero, overall no active power is exchanged with the AC voltage network 3. However, due to the sinusoidal course of the AC current, there is a dynamic redistribution of energy between the two halves of the DC voltage intermediate circuit. This dynamic redistribution can be detected and evaluated by measurement technology in order to determine the partial capacity of the DC voltage intermediate circuit.

[0052] True Zero Feedback:

[0053] In an installation comprising a plurality of electrical units connected to an AC voltage network 3 via a common network connection point, in particular a single-phase connection, an asymmetrical power flow may occur at the network connection point, wherein power flows from the AC voltage network 2 into the installation at individual phases, while power flows from the installation into the AC voltage network 2 at other phases. This can be counteracted by measuring the power at the network connection point in a phase-specific manner and modifying the current amplitude target value I_d_target, for example by a central system control device 20 or a control device 4 of the inverter 2 itself, in such a way that the power at the network connection point is individually adjusted to a predefined value, in particular to zero, for each phase.

[0054] Fuse protection.

[0055] Analogously to zero feed-in, the power flowing through the individual phases at the network connection point can be limited by presetting a correspondingly opposite change in the current amplitude target value I_d_target for the phase in question. As a result, overcurrents on the individual phases can be effectively prevented as long as the sum of the powers on all phases is less than the total permissible power at the network connection point.

[0056] Reference numerals list

[0057] 1 DC current source

[0058] 2 Inverter

[0059] 3 AC voltage network

[0060] 4 Control device

[0061] 5 Voltage measurement device

[0062] 10 Adjustment structure

[0063] 11 Regulator

[0064] 12, 12' Multiplier

[0065] 13 Phase Shifter

[0066] 14 Multiplier

[0067] 15 Adder

[0068] 16 Multipliers

[0069] 20 Measuring or control unit

[0070] L1, L2, L3 phase conductors

[0071] N Neutral conductor

[0072] U_abc Network voltage

[0073] I_abc_soll Current expected value

[0074] f_Netz network frequency

[0075] U_ref, Voltage reference value

[0076] U_d Network voltage amplitude

[0077] I_d_soll Current amplitude target value

[0078] I_q_soll Reactive current amplitude target value

Claims

1. A method for feeding electrical power from a direct current source (1) into a three-phase alternating voltage network (3) by means of a three-phase inverter (2), wherein: The three-phase inverter (2) has a regulating structure (10), wherein the regulating structure (10) comprises a regulator (11) and a multiplier (12), wherein the method comprises the following steps: - measurement of the phase-specific network voltage U_abc; - Determine the network frequency f_Netz from the measured network voltage U_abc; - generating a phase-specific sinusoidal voltage reference value U_ref by means of the regulator (11) from the phase-specific measured network voltage U_abc and the ascertained network frequency f_Netz, wherein the voltage reference value U_ref has a phase-specific amplitude and a common frequency, the amplitude and the common frequency corresponding to the respective amplitude of the measured network voltage U_abc of the individual phases and the frequency of the measured network voltage U_abc of the individual phases, - generating a phase-specific current target value I_abc_target by means of the multiplier (12, 12') by forming a first product of a phase-specifically predefined current amplitude target value I_d_target and a phase-specific voltage reference value U_ref and normalizing the first product phase-specifically to the corresponding network voltage amplitude U_d, - using the phase-specific desired current values ​​I_abc_soll to control the power switches of the three-phase inverter.

2. The method according to claim 1, wherein: The regulator (11) for generating the voltage reference value U_ref comprises a bandpass filter or a generalized integrator.

3. The method according to claim 1 or 2, wherein: The current amplitude target value I_d_target is predetermined in the following manner: An individually predefined active power is fed into the individual phases.

4. The method according to claim 1 or 2, wherein: The current amplitude target value I_d_target is predetermined in the following manner: The same active power is fed into all phases.

5. The method according to claim 1 or 2, wherein: The current amplitude target value I_d_target is predetermined in the following way: An active power is fed into at least one of all phases, which has an opposite sign to the active power of at least one other phase.

6. The method according to claim 1 or 2, wherein: The voltage reference values ​​U_ref are phase-shifted by 90 degrees to form phase-shifted phase-specific voltage reference values The reactive current amplitude target value I_q_target and the phase-shifted phase-specific voltage reference value I_q_target are formed by means of a further multiplier (16). A second product is formed and added to a first product of the phase-specifically predefined current amplitude target value I_d_set and the phase-specific voltage reference value U_ref, wherein the resulting sum value is normalized phase-specifically to the corresponding network voltage amplitude U_d.

7. The method according to claim 6, wherein: The reactive current amplitude target value I_q_target is predetermined in the following manner: An individually predefined reactive power is fed into the individual phases.

8. A three-phase inverter (2) for feeding electrical power from a direct current source (1) into an alternating voltage network (3), wherein: The three-phase inverter (2) has a control structure (10), wherein the control structure (10) is provided for carrying out the method according to any one of claims 1 to 7.

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