Method and control unit for reducing harmonic power flow, and sub-grid having control unit

Abstract

A method for reducing a harmonic power flow via a connection point (AP) at which a sub-grid (10) is connected to a superordinate AC supply grid (12) and via which a grid current (INetz) flows between the sub-grid (10) and the superordinate AC supply grid (12) is disclosed. Here, the sub-grid (10) has at least one load (14) which takes an electrical load current (ILast) from the sub-grid (10), the load current comprising an active power current (Id) at the grid frequency (f0) and a harmonic distortion current at one or more integer multiples of the grid frequency (f0). Furthermore, the sub-grid (10) has a power converter (16, 16. N) which exchanges electrical power between the capacitance (18, 18. N) connected to the DC side thereof and the sub-grid (10) connected to the AC side thereof by means of a bridge circuit. In the method, a network voltage (Uac) of the sub-network (10) is first detected. Subsequently, a compensation voltage and / or compensation current (Komp) is determined using the network voltage (Uac), the compensation voltage and / or compensation current (Komp) being provided for generation by the power converter (16, 16. N) and being adapted to reduce a harmonic distortion current in the network current (INetz) at a multiple of at least one network frequency (f0). The method also shows that the compensation voltage and / or compensation current (Komp) is generated by the power converter (16, 16. N) by means of appropriate clocking of the bridge circuit between the DC-side capacitance (18, 18. N) and the sub-grid (10).

Description

Technical Field

[0001] The present application relates to a method and a control unit for reducing harmonic power flows, as well as an alternating current subsystem (AC subsystem) having such a control unit, which is connected to a higher-level alternating current supply network (AC supply network). Existing technology

[0002] AC subgrids can be used in the industrial sector, for example, to supply power to industrial plants and / or industrial areas. An AC subgrid can consist of one or more AC line sections that can be connected together to a connection point and connected to electrical loads. The loads connected to the subgrid are supplied with electrical power via current components at the subgrid's nominal frequency (e.g., a 50 Hz grid frequency) and can also draw harmonic current components from the subgrid, i.e., currents with multiples of the grid frequency. These harmonic current components are typically caused by electrical consumers and operating devices with non-sinusoidal currents or currents that are periodically switched on and off, such as rectifiers, frequency converters, UPS systems, and / or similar equipment, in particular clock-controlled loads. In this case, multiples of up to 50 times the grid frequency can be observed. The resulting current distortion caused by these harmonics can distort the nominally sinusoidal grid voltage with frequency components comparable to the harmonic current components.

[0003] In this context, in subnetworks serving industrial plants and other large-scale installations with a high demand for electrical power (e.g., production plants with numerous machines or shopping malls), the voltage and / or current may contain distortions that exceed certain limit values ​​for THD (total harmonic distortion) and / or harmonics in the voltage and / or current. Consequently, if these limit values ​​are defined for such connection points, direct connection of such subnetworks and / or specific consumers supplied via individual sections of the subnetwork to the higher-level power supply network may not be possible in this case.

[0004] Passive filters using passive components such as capacitors and inductors are known. Such filters are designed as so-called absorption loops, for example, and are usually designed or parameterized for a specific load with given characteristics.

[0005] EP 2 436 092 describes the compensation of harmonics occurring in the current curve of a high-voltage network by means of a passive filter and a controllable voltage source.

[0006] A STATCOM is a converter that is configured to exchange inductive or capacitive reactive power with an AC grid. Here, a DC voltage capacitor acts as a DC voltage source, which is formed into an AC voltage source by a power converter for connection to the AC grid via a transformer.

[0007] EP2478610 describes a photovoltaic system that is connected to an AC power supply grid via a transformer and is designed to generate active power by means of current regulation and exchange the active power with the AC power supply grid, as well as to compensate for reactive power and harmonics in the AC power supply grid, so that the photovoltaic system operates as a STATCOM with current regulation.

[0008] DE 102 44 056 B3 discloses a method for generating a set of control signals for a converter of an active power filter to compensate for harmonics. In this method, a current including the harmonics to be compensated is measured, and the fundamental component is eliminated from the measured signal. A transformed control function of a PI controller is applied to the filtered measured values. The output variables of the PI controllers are then summed and transmitted as a target current value to the control device of the IGBT converter.

[0009] Purpose

[0010] The object of the present application is to provide a method and a control unit by means of which the quality of a subsystem with respect to electrical parameters within the subsystem and / or at the connection point of the subsystem to a higher-level power supply system can be improved.

[0011] Solution

[0012] This object is achieved by a method having the features of claim 1 and a control unit having the features of claim 11. Embodiments are given in the dependent claims.

[0013] Description of the Invention

[0014] A subgrid, particularly an AC subgrid, is connected to a higher-level AC power grid at a connection point. Grid current flows between the subgrid and the higher-level AC power grid via the connection point, particularly from the higher-level AC power grid into the subgrid. The AC power grid is the higher-level grid for the subgrid, for example, because the AC power grid can transmit electrical energy over significantly greater distances than the subgrid, the grid voltage in the higher-level AC power grid is higher than the grid voltage in the subgrid, and / or the AC power grid provides energy supply with defined grid parameters for a larger area, as provided by the grid operator.

[0015] The subgrid includes at least one electrical load that draws an electrical load current from the AC subgrid. The electrical load current includes active power current at the grid frequency and harmonic distortion current at one or more integer multiples of the grid frequency. For example, the at least one electrical load can be an AC load (e.g., a motor) or a DC load, the DC load being connected to the AC subgrid, for example, via a rectifier. Depending on the power demand of the load or the operating mode of the converter, the aforementioned components of the load current are formed, for example, through phase control, which can regulate the electrical power of the AC load.

[0016] The subgrid further comprises a power converter that exchanges electrical power between a DC unit (particularly a capacitor) connected on its DC side and a subgrid connected on its AC side using a bridge circuit. For this purpose, the bridge circuit can particularly comprise clocked semiconductor power switches. The power converter can operate bidirectionally, thus acting as an inverter for the flow of electrical power from the DC side to the AC side and as a rectifier for the flow of electrical power from the AC side to the DC side.

[0017] A method for reducing harmonic power flow via a connection point, comprising the steps of:

[0018] - Detect the grid voltage of the subgrid.

[0019] Using the grid voltage, a compensation voltage and / or a compensation current is determined, wherein the compensation voltage and / or the compensation current are provided for generation by a power converter and are suitable for reducing harmonic distortion currents in the grid current at at least one multiple of the grid frequency.

[0020] - The compensation voltage and / or compensation current is generated by the power converter by suitable clocking of the bridge circuit between the DC link capacitor and the subgrid.

[0021] A subgrid can be electrically isolated from a higher-level AC power grid. This electrical isolation can be achieved, in particular, by a transformer at the connection point. In addition to electrical isolation, the transformer can also achieve voltage conversion between the AC power grid and the subgrid. Electrical isolation at the connection point is an important safety feature for connecting to an AC power grid. Subgrids can be designed so that internal electrical isolation is not required. In particular, power converters can be connected to the subgrid without electrical isolation.

[0022] The described method can improve the grid voltage quality in a subgrid. This is particularly advantageous for industrial applications with loads that may negatively impact the grid voltage. The method improves the grid current quality, thereby improving the grid voltage via the grid impedance. Both of these improvements can specifically improve the power exchanged with the higher-level AC power supply via the connection point and reduce the negative impact of the power exchange via the connection point on the higher-level AC power supply.

[0023] Alternatively or additionally, the power converter can be connected on its DC side to a DC power source (e.g. a PV system (PV: Photovoltaic)). In inverter operation, the power converter can draw electrical power from the DC source and feed it into the subgrid. For example, the electrical power that can be drawn from the DC source can be used to meet the power demand in the subgrid. This has advantages in the industrial sector, for example, and can be achieved in particular by maximizing the self-consumption of locally generated power. In addition, by providing distorted reactive power to smooth the current at the connection point, a compensation function that supports the operation of the grid can be implemented. In this case, this compensation function that supports the operation of the grid can be mandatory in accordance with regulations and / or performed at the request of the AC power supply network and / or the grid operator. With the help of the claimed method, this requirement can be met while reducing the workload of any AC grid filters that are present, which can reduce costs.

[0024] Within the subgrid, a compensation voltage and / or compensation current is locally provided, which provides the required harmonic distortion current to at least one load, thereby reducing the harmonic distortion current provided by the AC supply network and thus reducing the harmonic power flow via the connection point.

[0025] In one embodiment of the method, different harmonics of the harmonic distortion current at different multiples of the grid frequency can be taken into account independently of one another when determining the compensation voltage and / or the compensation current.

[0026] In one embodiment of the method, a harmonic controller uses the grid voltage to determine the respective contributions of respective harmonics to the grid voltage. The respective harmonic contributions are oscillations of the grid voltage at frequencies that are respective multiples of the grid frequency, wherein these oscillations of the grid voltage are electrically coupled to the respective oscillations of the distortion current. In this case, the harmonic controller uses the respective harmonic contributions as manipulated variables to determine the respective harmonic compensation contributions to the compensation voltage. In particular, odd-numbered multiples of the grid frequency can be used.

[0027] It is further proposed to detect the DC voltage applied to the capacitor connected to the DC side of the power converter. The DC voltage is used by the DC regulator to determine the DC regulation contribution for the compensation voltage and / or compensation current, so that the DC voltage is regulated to a predefined level by appropriate active power exchange via the bridge circuit. This regulation balances any active power exchange due to other contributions to the compensation voltage or compensation current, in particular the active power generated by the harmonic compensation level, without interfering with the regulation of other contributions, allowing the capacitor of the power converter to continue to serve as the source of the compensation voltage or compensation current.

[0028] In one embodiment, the DC regulation includes an f(P)-PI regulator (PI regulator = proportional-integral regulator), which uses as input the difference between the active power current value and the actual active power current value of the active power exchanged via the bridge circuit. The active power current setpoint is determined based on the difference between the actual DC voltage value and a predetermined DC voltage level. In this case, the exchange of active power via the bridge circuit is regulated by varying the AC-side frequency of the power converter based on the output value of the f(P)-PI regulator. In this process, the AC-side frequency of the power converter corresponds to the output frequency of the electrical variable present on the AC side of the power converter. Specifically, the output frequency of the power converter can be varied, for example, by a variation consisting of a first component, generated by the P component of the f(P)-PI regulator and proportional to the difference between the active power current setpoint and the actual active power current value, and a second component, generated by the I component of the f(P) regulator and proportional to the time integral of the difference between the active power current setpoint and the actual active power current value. Implementations using f(P)-PI controllers enable grid-forming control of the power converter by varying the AC-side frequency of the power converter to adjust the phase angle difference between the power converter's output voltage and the grid voltage. This allows, on the one hand, precise setting of the active power current value, and, on the other hand, allows the control system to react, at least temporarily, to grid events such as sudden phase changes in the grid voltage or changes in the grid frequency with corresponding changes in active power.

[0029] In an alternative embodiment, the DC regulation includes a P-regulator (proportional regulator) that uses the difference between the actual DC voltage value and a predetermined DC voltage level as an input value. The active power exchange through the bridge circuit is regulated by specifying a current setpoint value based on the output value of the P-regulator. Specifically, for example, a current setpoint value can be generated that is proportional to the difference between the actual DC voltage value and the setpoint value. Embodiments employing a P-regulator enable efficient DC voltage maintenance within the characteristic linearization voltage range of the DC unit. Furthermore, DC regulation employing a P-regulator allows for rapid adjustment of the actual value to the DC voltage setpoint value.

[0030] The load current may include a reactive power current at the grid frequency. To reduce the reactive power exchange that typically occurs at the connection point of the subgrid to the higher-level AC power supply grid, the method may include reactive power regulation. The reactive power regulation determines a reactive power regulation contribution for compensating the voltage. To this end, the reactive power regulation includes a U(Q)-PI regulator, which uses as input the difference between a reactive power current setpoint and an actual reactive power current value of the reactive power exchanged via the bridge circuit. The reactive power current setpoint for the reactive power is determined as a function of the difference between a setpoint and an actual value of the grid voltage amplitude, thereby reducing the reactive power exchange at the connection point.

[0031] The method may comprise the following further steps:

[0032] -Detect load current and grid current,

[0033] - Use the load current to determine the grid current setpoint,

[0034] - Determine the set output voltage of the power converter based on the difference between the actual grid current value and the grid current set value,

[0035] - generating a control signal based on the measured set output voltage and clock-controlling the bridge circuit based on the control signal to generate the compensation current.

[0036] In particular, the control signal may be a pulse width modulated signal which specifies the clocking of the semiconductor switches of the bridge circuit, for example by giving them appropriate opening and closing times, respectively.

[0037] Determining the grid current setpoint value can include bandpass filtering of the load current, wherein the center frequency of the bandpass depends on the grid frequency. In this case, the center frequency of the bandpass filter can essentially correspond to the grid frequency.

[0038] In embodiments of the method, the load current, the output current of the power converter and / or the grid voltage may be detected and used for pre-controlling the output voltage of the power converter.

[0039] A subgrid connected to a higher-level AC power grid via a connection point has at least one load that draws a load current from the subgrid. The load current includes active power current at the grid frequency and harmonic distortion current at one or more integer multiples of the grid frequency. The subgrid also has a power converter that exchanges electrical power between a capacitor connected to its DC side and the subgrid connected to its AC side using a bridge circuit.

[0040] A control unit for reducing harmonic power flows in the grid current, said harmonic power flows flowing between a subgrid and a higher-level AC power supply grid via a connection point, said control unit being designed to:

[0041] - receiving the grid voltage of the subgrid,

[0042] - using the grid voltage, determining a compensation voltage and / or a compensation current, wherein the compensation voltage and / or the compensation current are arranged to be generated by the power converter and are suitable for reducing harmonic distortion currents at at least one multiple of the grid frequency in the grid current, and

[0043] - outputting a control signal to the power converter, wherein the power converter can use the control signal to generate a compensation voltage and / or a compensation current by appropriately clocking a bridge circuit between the DC link capacitor and the subgrid.

[0044] In an embodiment, the control unit is designed to determine, for various harmonic contribution values ​​at different multiples of the grid frequency, corresponding harmonic compensation contribution values ​​for the compensation voltage independently of one another.

[0045] The control unit can be designed to execute a corresponding harmonic controller, by means of which corresponding harmonic contributions can be determined using the grid voltage, wherein corresponding harmonic compensation contributions for the compensation voltage can be determined using the corresponding harmonic contributions as manipulated variables.

[0046] In an embodiment, the control unit can be designed to receive a DC voltage (which is applied to a capacitor connected on the DC side of the power converter) and to perform DC regulation, by means of which a DC regulation contribution for the compensation voltage and / or compensation current can be determined using the DC voltage, so that the DC voltage is regulated to a predeterminable level by appropriate active power exchange via the bridge circuit.

[0047] The load current may also comprise a reactive power current at the grid frequency, and the control unit may be designed to perform reactive power regulation, by means of which a reactive power regulation contribution for compensating the voltage may be determined such that the reactive power exchange at the connection point is reduced.

[0048] A subgrid connected to a higher-level AC power supply grid at a connection point can have such a control unit. The subgrid has at least one line section to which at least one load is connected, the load being designed to draw a load current from the subgrid, the load current comprising an active power current at the grid frequency and a harmonic distortion current at one or more integer multiples of the grid frequency. In an embodiment, the load current can also comprise a reactive power current at the grid frequency. The subgrid further comprises a power converter, which is designed to exchange electrical power between a DC cell (in particular a capacitor) connected to the DC side of the power converter and the subgrid connected to its AC side by means of a bridge circuit, thereby reducing harmonic power flows via the connection point.

[0049] The subgrid may comprise a plurality of power converters, in particular on the same line section of the subgrid, wherein different power converters are configured to respectively generate compensation voltages and / or compensation currents to reduce different harmonic contributions at different multiples of the grid frequency.

[0050] Here, the power converter can be connected to a higher-level control unit. Alternatively or additionally, the corresponding control unit can be distributed on a single or all power converters in the subgrid. The higher-level control unit or the distributed control unit can output corresponding control signals to each of the multiple power converters, wherein the corresponding control signals are suitable for generating a compensation voltage and / or compensation current through the corresponding power converter to reduce harmonic distortion current at one or more multiples of the grid frequency. If the load current has a reactive power current at the grid frequency, the corresponding control signal can be suitable for generating a compensation voltage and / or compensation current through the corresponding power converter to reduce reactive power exchange at the connection point.

[0051] The subgrid can be electrically isolated from the superordinate AC supply network, in particular by a transformer at the connection point. The subgrid itself can be designed without electrical isolation, and in particular at least one power converter can be connected to the subgrid without electrical isolation. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The technical teaching according to the present invention will be further explained and illustrated below based on the exemplary embodiments shown in the drawings.

[0054] Figure 1 A method for reducing harmonic power flow is schematically illustrated.

[0055] Figure 2 An embodiment of a subgrid with a power converter is schematically shown.

[0056] Figure 3 An embodiment of a regulating system with different components is schematically shown.

[0057] Figure 4 A further embodiment of a subgrid with a power converter is schematically shown.

[0058] Figure 5 A further embodiment of a subgrid with a power converter is schematically shown.

[0059] Figure 6 A further embodiment of a regulating system with different components is schematically shown.

[0060] Figure 7 Exemplary current and voltage curves in a subsystem are shown schematically.

[0061] Figure 8 Another embodiment of a subgrid with a power converter and a DC source is schematically shown.

[0062] Figure 9 Another embodiment of a subgrid with a plurality of power converters is schematically shown.

[0063] In the drawings, like reference numerals are used for like or similar elements, and the illustrations in the drawings are not drawn to scale.

[0064] Description of the drawings

[0065] Figure 1 A method for reducing harmonic power flows via a connection point AP at which a subsystem 10 with power converters 16 , 16 .N is connected to a higher-level AC power supply network 12 is schematically shown.

[0066] The method comprises the following steps:

[0067] S1: Detecting the grid voltage U of the subgrid 10 ac .

[0068] S2: Use grid voltage U ac Determine the compensation voltage and / or compensation current Komp.

[0069] S3 : Generate a compensation voltage and / or a compensation current Komp via the power converter 16 , 16 .N.

[0070] In particular, the method is carried out repeatedly so that distortions caused by harmonics in the electrical power exchanged via the connection point can be reduced by the continuous generation and adjustment of the compensation current and / or the compensation voltage Komp.

[0071] For example, the method is executed by a control unit 20, 20.N, which is designed as a computing unit with a memory and a processor. The method can be executed on the control unit 20, 20.N, for example, as software. For detecting the measured values, corresponding measuring devices can be provided.

[0072] By means of this method, the compensation voltage and / or the compensation current Komp is determined, for example, by the control unit 20 , so that the compensation voltage and / or the compensation current Komp can be generated by the power converter 16 , 16 .N.

[0073] Figure 2 An exemplary embodiment of a subgrid 10 with a power converter 16 is schematically shown. In the example shown, subgrid 10 is designed as an AC subgrid with line sections. In the example shown, subgrid 10 has three loads 14. A greater or fewer loads are also conceivable. Loads 14 can be designed, in particular, as AC or DC loads, which are connected to subgrid 10 directly or via suitable converters.

[0074] Subgrid 10 is connected to AC power grid 12 via connection point AP. Grid current I_Netz flows between subgrid 10 and higher-level AC power grid 12 via connection point AP. Subgrid 10 can, for example, be designed such that no electrical isolation is required within subgrid 10. In particular, power converter 16 can be directly connected to subgrid 10 without a transformer. The connection of subgrid 10 to AC power grid 12 via connection point AP is implemented with electrical isolation, in particular, via transformer T.

[0075] The load 14 receives a load current I_Last from the subgrid 10. The load current I_Last includes an active power current I_d at the grid frequency f0 and harmonic distortion currents at one or more integer multiples of the grid frequency f0. The load current I_Last may also include a reactive power current I_q at the grid frequency f0.

[0076] Power converter 16 has an AC side and a DC side and can operate bidirectionally, i.e., as an inverter and / or a rectifier. Power converter 16 is configured to exchange electrical power between a capacitor 18 connected to its DC side and subgrid 10 connected to its AC side using a bridge circuit. Electrical power can be stored in capacitor 18 by power converter 16 and retrieved from capacitor 18 again. Power converter 16 can be connected to the subgrid, for example, via a filter inductor 22.

[0077] The control unit 20 detects the grid voltage U by means of a suitable measuring device. ac, and determining a compensation voltage and / or compensation current Komp such that the compensation voltage and / or compensation current Komp can be generated by appropriate clock control of the bridge circuit of the power converter 16. For example, appropriate clock control of the bridge circuit can be generated via a control signal PWMS (e.g., by pulse width modulation). The compensation voltage and / or compensation current Komp generated by the power converter 16 are suitable for reducing harmonic distortion currents at at least one multiple of the grid frequency f0 in the grid current I_Netz.

[0078] In order to start the power converter 16, the DC voltage UDC can first be raised by means of a precharging circuit (not shown) fed on the AC side. After the DC voltage UDC is sufficiently precharged, the actual operation of the power converter 16 is started to reduce the harmonic power flow and the DC voltage UDC is set by means of the DC regulation 26. Thereafter, during the operation of the power converter 16, the compensation voltage and / or the compensation current Komp can be generated as described above to promote the grid voltage UDC. ac Harmonic processing.

[0079] Figure 3 An embodiment of a control system in which the described method can be implemented is schematically shown. The control system can be run, for example, in a control unit 20 and comprises different components. Figure 3 In the embodiment shown, the regulation comprises three sub-aspects: harmonic regulation 24, DC regulation 26, and reactive power regulation 28, which can be active simultaneously.

[0080] The DC regulation 26 and reactive power regulation 28 described below operate in the so-called dq coordinate system. This coordinate system is derived by transforming the abc coordinate system (containing the time variations of the individual phase voltages or currents) into the dq coordinate system (containing symmetrical components) and is widely used in multiphase AC. The dq coordinate system typically has symmetrical components: a positive sequence system, a negative sequence system, and a zero sequence system.

[0081] The DC regulator 26 can ensure a constant DC voltage UDC. In this case, the DC voltage on the DC side (in particular the DC voltage of one or more capacitors 18 of the DC voltage intermediate circuit) is stabilized by suitable power exchange with the subgrid 10 connected to the AC side via the power converter 16, in such a way that a positive sequence system current, i.e., an active current I_d, is generated, which is a function of the actual value of the DC voltage. and DC voltage setting value The deviation is controlled by the active current regulator I d (ΔU dc ) adds energy to or takes energy from the intermediate circuit capacitance (eg, capacitor 18).

[0082] Using the actual value of the DC voltage UDC with a predetermined level of DC voltage UDC The difference between the active power and the current setting value is used to determine the active power. The DC regulation 26 comprises an f(P)-PI regulator which sets the active power current setpoint of the active power exchanged via the bridge circuit of the power converter 16 to and actual value of active power and current The difference between ΔI d By changing the frequency f on the AC side of the power converter 16 according to the output value of the f(P)-PI regulator soll , regulating the active power exchange via the bridge circuit of the power converter 16 .

[0083] Reactive power regulation 28 pairs of AC voltage amplitude |U ac |(i.e., grid voltage U ac When the voltage amplitude |U ac The actual value of | When it increases, the reactive power current regulator I q (ΔU ac ) generates reactive power current I q Absorb reactive power. When the voltage amplitude |U ac The actual value of | When it is too low, the reactive power current regulator I q (ΔU ac ) generates reactive power current I q Provides reactive power.

[0084] If the loads 14 connected to the subgrid 10 are mainly inductive, such as motors, the subgrid 10 provides reactive power for the operation of these loads. This reactive power interacts particularly with the inductance in the subgrid 10 and results in an AC voltage amplitude |U in the subgrid 10. ac | increases. If, on the other hand, the load 14 is predominantly capacitive, reactive power is fed into the subgrid 10. This results in an AC voltage amplitude |U ac | Reduction. By providing corresponding reverse reactive power, the power converter 16 thus relieves the subgrid 10 of the reactive power exchange with the AC power supply network via the connection point AP.

[0085] exist Figure 3 In the embodiment shown, reactive power is provided by the following means: the power converter 16 generates a negative sequence system current I q, which is then detected by measurement at the output of the power converter 16. The reactive power control 28 determines the reactive power control contribution for the compensation voltage Komp. For this purpose, the reactive power control 28 comprises a U(Q)-PI controller, which sets the reactive power current setting value for the reactive power exchanged via the bridge circuit of the power converter 16. and actual value of reactive power and current The difference between ΔI q Used as input value. Reactive power current setpoint for reactive power It is based on the amplitude of the grid voltage |U ac |Set value and actual value The U(Q)-PI regulator generates a voltage difference Δu as the output value, which is used together with the given voltage U0 to generate the set active voltage Combined with the set reactive power voltage of zero This generates a set voltage in the dq coordinate system

[0086] The output values ​​of DC regulation 26 and reactive power regulation 28 are transformed into the abc coordinate system and generate the set voltage of the grid frequency

[0087] There may be a load 14 which draws certain higher harmonics or harmonics from the subgrid 10 via the load current I_Last. This causes harmonic distortion currents at one or more multiples of the grid frequency f0. The harmonic control 24 has corresponding harmonic controllers 24.3, 24.5, 24.n for the harmonics at one or more multiples of the grid frequency f0. Figure 3 In the example shown, the harmonic control 24 has harmonic controllers for harmonics of three times the grid frequency f0, five times the grid frequency f0, and n times the grid frequency f0. Preferably, n is selected as an odd natural number.

[0088] The corresponding harmonic regulators 24.3, 24.5, 24.n are based on the grid voltage U ac The actual value of Filter out the corresponding harmonic contribution value This harmonic contribution is used as the manipulated variable and the setpoints for the harmonics are set to zero accordingly. is adjusted to zero.

[0089] Harmonic regulation 24 generates harmonic contribution values ​​for compensating voltage These harmonic contributions are related to the set voltage of the grid frequency in the abc coordinate system from DC regulation 26 and reactive power regulation 28 Combined, and the set voltage in the abc coordinate system is obtained With the help of the set voltage in the abc coordinate system The power converter 16 generates the compensation voltage Komp. To this end, a control signal PWMS is generated by generating a control signal PWM. With the help of the control signal PWMS, the semiconductor switches of the power converter 16 are appropriately controlled, and the AC output voltage of the power converter 16 is appropriately adjusted by the compensation voltage Komp. Figure 3 In the embodiment shown, the compensation voltage Komp subsequently generates the compensation current Komp.

[0090] exist Figure 4 The upper portion of FIGURE 1 graphically illustrates exemplary time curves of grid current I_Netz and load current I_Last. Compensating current Komp, generated by power converter 16 and symbolically illustrated at the output of power converter 16, improves the quality of grid current I_Netz exchanged via connection point AP. In particular, the harmonic power flows contained as harmonics in load current I_Last are not supplied from higher-level AC supply grid 12 via connection point AP, but rather by power converter 16. Consequently, grid current I_Last essentially has a sinusoidal shape at the grid frequency, and compensating current Komp corresponds to the harmonic distortion current in load current I_Last.

[0091] Figure 5 A further embodiment of a subsystem 10 is schematically shown, which has a line section and a power converter 16 connected to the line section. A calculation unit 20 detects the grid voltage U ac , grid current I_Netz, filter current I_filter, DC voltage UDC and load current I_Last.

[0092] The control unit 20 determines the compensation voltage and / or compensation current Komp so that the compensation voltage and / or compensation current Komp can be generated by appropriate clock control of the bridge circuit of the power converter 16. For example, an appropriate clock of the bridge circuit can be generated via a control signal PWMS (e.g., by pulse width modulation).

[0093] In this embodiment, the harmonic distortion current of load current I_Last can also be provided by power converter 16, without necessarily or entirely being provided by AC power grid 12. Therefore, the impact of the harmonic distortion current on AC power grid 12 and the electric power exchanged via connection point AP can be mitigated, thereby reducing the flow of harmonic power via connection point AP. To this end, grid current I_Netz is measured "before" power converter 16, and load current I_Last is measured "after" power converter 16. In this case, power converter 16 functions as a filter, "filtering out" harmonics that appear without higher-level control from grid current I_Netz.

[0094] Figure 6 An exemplary embodiment of a control system is schematically shown, which can implement the method according to the present application and which can be run, for example, in a control unit 20 .

[0095] The DC regulation 30 comprises a proportional regulator (P regulator) P which uses the actual value of the DC voltage UDC with a predetermined level of DC voltage UDC The difference between the two is used as the input value. The P regulator P outputs a DC current As an output value, it is suitable for stabilizing and / or reducing the difference between the actual value and the set value of the DC voltage UDC, so that the DC voltage UDC is regulated to a settable level.

[0096] Determine the grid current setpoint in the time domain using the load current I_Last And transform it from the abc coordinate system to the dq coordinate system. Get the grid current setting value in the dq coordinate system Determine the grid current setting value This may include a bandpass filtering of the load current I_Last in the time domain, wherein the center frequency of the bandpass depends on the grid frequency f0 and may correspond to the grid frequency f0. Thus, the grid current setpoint is obtained It essentially corresponds to the expected sinusoidal curve of the grid current I_Netz at the grid frequency f, ie the load current I_Last minus any harmonic distortion current.

[0097] The detected grid current I_Netz is also transformed from the abc coordinate system to the dq coordinate system, and the grid current in the dq coordinate system is obtained

[0098] According to the actual value of the grid current and grid current setting value and the output value of the DC regulator 30 (ie, the DC current ), first determine the current setting value in the dq coordinate system Then the proportional integral derivative regulator (PID regulator) PID is used to determine the set output voltage of the power converter 16 in the dq coordinate system. Specifically, the grid current setpoint can be determined and actual value of grid current and compare this difference with the DC current Then, the sum is used as the current setting value in the dq coordinate system Provided to the PID regulator PID, which outputs the set output voltage at its output It is suitable for generating a compensation current Komp, which includes both any harmonic distortion currents present and any active current present for regulating the DC voltage UDC.

[0099] The regulation system also has a pre-circuit, which can be used to pre-control the output voltage U_dq of the power converter 16. The input value of the pre-circuit is the set output voltage of the power converter 16. Other input variables include the load current I_Last transformed from the abc coordinate system to the dq coordinate system, the grid voltage U transformed from the abc coordinate system to the dq coordinate system ac , grid voltage U ac The frequency and the output current I_filter of the power converter 16 are pre-set to set the output voltage. Changes in various input variables of the control system can be taken into account before they have a concrete effect on the set output voltage along the control path. In particular, the output current I_filter as well as the load current I_Last and the grid voltage U ac The pre-control enables the regulation system to respond more quickly to, for example, externally induced changes in the output current I_filter of the power converter 16. Furthermore, the pre-circuit compensates for the influence of periodically fluctuating AC variables on the DC regulation, making the P-regulator sufficient for DC regulation. The dynamic characteristics of the DC voltage can be further improved by means of a high proportionality factor.

[0100] The output value of the pre-circuit is the output voltage U_dq of the power converter 16. Subsequently, the control signal generator PWM uses the output voltage U_dq of the power converter 16 transformed from the dq coordinate system to the abc coordinate system to generate a control signal PWMS for the bridge circuit of the power converter 16. The bridge circuit is clock-controlled according to the control signal PWMS to generate the compensation current Komp. Figure 5 and Figure 6 In the embodiment shown, the compensation current Komp is associated with the compensation voltage Komp, and the compensation voltage Komp is Figure 6It is generated in the regulation system such that the filter current I_filter fed by the power converter 16 includes both the possibly present harmonic distortion current and the possibly present active current for regulating the DC voltage UDC.

[0101] Figure 7 Schematic diagram示例性示出了电网电流I_Netz、负载电流I_Last、功率转换器的输出电流I_filter以及DC电压UDC的示意图。

[0102] 可以看出,当功率转换器16未激活时,负载电流I_Last对应于经由连接点AP的电网电流I_Netz。在 Figure 7 的示例性示图中,这对应于t<t1的时段,其中负载电流I_Last和电网电流I_Netz除了电网频率的基波之外还包括谐波畸变电流,而滤波器电流I_filter等于零。

[0103] 在时刻t1,功率转换器16被激活,并执行根据本申请的用于降低电网电流I_Netz中的谐波功率流的方法。在短暂的过渡阶段中,DC电压UDC短暂下降,并通过所使用的DC调节再次升高到正常水平。在t1和t2之间的时段的稳态中,负载电流I_Last中的谐波畸变电流由功率转换器16接管,并作为滤波器电流I_filter馈入,使得电网电流I_Netz基本上仅包括正弦电网频率的基波。

[0104] 在时刻t2,负载电流I_Last发生改变,随后是另一个短暂的过渡阶段,伴随着短暂下降的DC电压UDC的调节。在这种情况下,过渡阶段可以特别地借助于根据 Figure 6 的预控制来缩短。在下文中,通过负载电流I_Last中的谐波畸变电流由功率转换器16接管并作为滤波器电流I_filter馈入,再次降低AP连接点处的谐波功率流。

[0105] Figure 8, an embodiment is shown in which the power converter 16 has a connected DC source G. Here, the above-described method and regulation can be supplemented by feeding in active electrical power from the DC source G. For example, the power converter 16 can perform AC current-regulated feeding from the PV system as DC source G and have a selective grid-forming function. The power converter 16 implemented as a PV inverter 16 can feed the PV power into the subgrid 10 in the form of AC active power in a voltage-controlled manner, for example in the form of grid following / current injection / current regulation, and, within its capabilities, additionally exert a targeted influence on specific harmonics, for example by means of a control according to Figure 3 Voltage injection harmonic regulation or according to Figure 6 Current injection filtering.

[0106] In this case, the active power of the PV generator can be fed into the AC power grid at grid frequency f0 via AC current regulation. Furthermore, power converter 16 can feed a compensation voltage and / or compensation current Komp into subgrid 10, which compensates for one or more AC harmonics (e.g., the 3rd, 5th, 7th, 9th, 11th, 13th, etc.) in terms of their frequency, with the goal of minimizing these AC harmonics at the connection point AP with AC power grid 12. This allows power converter 16 (e.g., PV inverter 16) to be used as an active grid filter. Furthermore, it can be determined whether the permissible apparent power of power converter 16 should be used to feed the entire power of DC source G into the AC grid as active power, or whether a portion of the permissible apparent power of power converter 16 should be used to reduce distortion currents in grid current I_Netz. Such distortion currents can be generated, for example, by load 14, which can be, for example, a large consumer among electrical consumers 14. Thus, the power converter 16 can help maintain limit values ​​with respect to THD (total harmonic distortion) and / or harmonics (particularly at the connection point AP), which limit values ​​might otherwise be exceeded due to the operation of the load 14 in the form of a large electrical consumer. This characteristic can be appropriately compensated, for example, by the large electrical consumer and / or the grid operator of the AC power supply network.

[0107] Figure 9An embodiment of a subsystem 10 is shown having a plurality of power converters 16, 16.N. Power converters 16, 16.N may have capacitors 18, 18.N connected to their respective DC sides, with DC voltages UDC, UDC.N applied to capacitors 18, 18.N. Power converters 16, 16.N may be connected to subsystem 10 on the AC side via filter inductors 22. Each power converter 16, 16.N may have a control unit 20, 20.N. A superordinate control unit is also conceivable that may alternatively or additionally control two and possibly further power converters 16, 16.N, for example by specifying individual parameters of the control system in power converters 16, 16.N and / or performing the above-described method and control.

[0108] Power converters 16, 16.N can be connected in parallel on the AC side to the subgrid 10. Different subfunctions (e.g., subaspects related to harmonic regulation 24, DC regulation 26, 30, and reactive power regulation 28) can be implemented by different power converters 16, 16.N. It is also conceivable that multiple power converters 16, 16.N perform the same subfunction. In particular, a first group of power converters 16, 16.N can be responsible for reducing the harmonics in the grid current I_Netz, wherein each power converter 16, 16.N can reduce a specific harmonic; at the same time, a second group of power converters 16, 16.N can perform reactive power compensation. This reduces the burden on the power converters 16, 16.N in each case and protects them from damage, increasing their service life.

[0109] Reference Mark List

[0110] 10 Subgrid

[0111] 12 AC power supply network

[0112] 14 Load

[0113] 16,16.N Power Converter

[0114] 18,18.N capacitor

[0115] 20,20.N Control Unit

[0116] 22,22.N filter inductor

[0117] 24 Harmonic Regulation

[0118] 24.3, 24.5, 24.n Harmonic Regulator

[0119] 26 DC Regulation

[0120] 28 Reactive power regulation

[0121] 30 DC regulation

[0122] Komp compensation voltage / compensation current

[0123] f0 grid frequency f soll Set frequency Δu voltage difference U0 given voltage Setting voltage of dq coordinate system Setting voltage of abc coordinate system

[0124] AP connection point

[0125] GDC Source

[0126] P ratio regulator

[0127] PI proportional integral regulator

[0128] PID Proportional Integral Derivative Regulator

[0129] PWM control signal

[0130] PWMS, PWMS.N control signal generation

[0131] S1, S2, S3 method steps

[0132] T Transformer

[0133] UDC, UDC.N DC voltage Predetermined level of DC voltage

[0134] DC voltage (actual value)

[0135] I d (ΔU dc ) Active current regulator Active power current setting value Actual value of active power current ΔI d The difference in active power and current values Δf frequency difference Set base voltage U ac Grid voltage |U ac | Grid voltage amplitude / amplitude Set the amplitude of the grid voltage Grid voltage (actual value)

[0136] Amplitude of grid voltage (actual value) Harmonic contribution I_q Reactive power current I q (ΔU ac ) Reactive current regulator Set reactive power current Iq ist Actual reactive power current ΔI q Reactive power current difference Δu voltage difference Set active voltage Set reactive power voltage Set voltage in dq coordinate system Harmonic setting voltage I_Netz grid current Ifilter filter current I_Last load current Grid current in dq coordinate system Grid current setting value Grid current setting value in dq coordinate system Current setting value in dq coordinate system U_dq Output voltage of the power converter in the dq coordinate system Set output voltage of dq coordinate system DC current.

Claims

1. A method for reducing harmonic power flows via a connection point (AP), at which a subgrid (10) is connected to a higher-level AC power supply network (12) and via which a grid current (I_Netz) flows between the subgrid (10) and the higher-level AC power supply network (12), in, The subgrid (10) has at least one load (14) which draws an electrical load current (I_Last) from the subgrid (10), the electrical load current comprising an active power current (I_d) at a grid frequency (f0) and a harmonic distortion current at one or more integer multiples of the grid frequency (f0), wherein the subgrid (10) further has a power converter (16, 16.N) which exchanges electrical power between a capacitor (18, 18.N) connected to the DC side of the power converter and the subgrid (10) connected to the AC side of the power converter by means of a bridge circuit. Furthermore, the method comprises the following steps: Detect the grid voltage (U ac ), Use grid voltage (U ac ), determining a compensation voltage and / or a compensation current (Komp), wherein the compensation voltage and / or the compensation current (Komp) are suitable for reducing harmonic distortion currents at at least one multiple of the grid frequency (f0) in the grid current (I_Netz), and The compensation voltage and / or the compensation current (Komp) is generated by the power converter (16, 16.N) by suitable clocking of the bridge circuit between the DC link capacitor (18, 18.N) and the subsystem (10).

2. The method according to claim 1, wherein When determining the compensation voltage and / or the compensation current (Komp), different harmonics of the harmonic distortion current at different multiples of the grid frequency (f0) can be taken into account independently of one another.

3. The method according to claim 2, wherein: The corresponding harmonic regulator (24.3, 24.5, 24.n) uses the grid voltage (U ac ) Determine the corresponding harmonic contribution value and use the corresponding harmonic contribution values The corresponding harmonic compensation contribution to the compensation voltage (Komp) is determined as a manipulated variable.

4. A method according to any one of the preceding claims, wherein A DC voltage (UDC, UDC.N) is detected, which is applied to a capacitor (18, 18.N) connected to the DC side of the power converter (16, 16.N), and a DC regulator (26, 30) uses the DC voltage (UDC, UDC.N) to determine a DC regulation contribution for the compensation voltage and / or the compensation current (Komp), so that the DC voltage (UDC, UDC.N) is regulated to a predeterminable level by suitable active power exchange via the bridge circuit.

5. The method according to claim 4, wherein the DC regulation (26) comprises an f(P)-PI regulator which sets the active power current setting value of the active power exchanged via the bridge circuit to and the actual value of active power and current The difference between (ΔI d ) is used as input value; where, The active power current setting value According to the actual value of the DC voltage (UDC, UDC.N) with a predetermined level of the DC voltage (UDC, UDC.N) and wherein the AC side frequency (f(P)) of the power converter (16, 16.N) is changed according to the output value of the f(P)-PI regulator. soll ), regulating the active power exchange via the bridge circuit.

6. The method according to claim 4, wherein: The DC regulation (30) includes a P regulator (P) which converts the actual value of the DC voltage (UDC, UDC.N) and the predetermined level of the DC voltage (UDC, UDC.N) The difference between the two is used as the input value, and the current setting value is given by the output value of the P regulator Active power exchange via the bridge circuit is regulated.

7. The method according to claim 4 or 5, wherein: The electrical load current (I_Last) includes a reactive power current (I_q) at the grid frequency (f0), and a reactive power control (28) determines a reactive power control contribution for the compensation voltage (Komp), wherein the reactive power control (28) includes a U(Q)-PI controller which sets a reactive power current setting value for the reactive power exchanged via the bridge circuit. and the actual value of reactive power current (Iq ist ) between the difference (ΔI q ) is used as input value, where the grid voltage amplitude The difference between the set value and the actual value is used to give the reactive power current set value of the reactive power This reduces reactive power exchange at the connection point (AP).

8. The method according to any one of the preceding claims, wherein the method comprises the further steps of: detecting the load current (I_Last) and the grid current (I_Netz), Use the load current (I_Last) to determine the grid current setpoint According to the actual value of the grid current and the grid current set value The difference between the set output voltage of the power converter is measured According to the measured set output voltage A control signal (PWMS, PWMS.N) is generated, and the bridge circuit is clock-controlled according to the control signal (PWMS, PWMS.N) to generate the compensation current (Komp).

9. The method according to claim 8, wherein Determine the grid current setting value A bandpass filtering of the load current (I_Last) is included, wherein the center frequency of the bandpass depends on the grid frequency (f0).

10. A method according to any one of the preceding claims, wherein The load current (I_Last), the output current (I_filter) of the power converter (16, 16.N) and / or the grid voltage (U_ac) are detected and used for pre-controlling the output voltage (U_dq) of the power converter (16, 16.N).

11. A control unit (20, 20.N) for reducing harmonic power flows via a connection point (AP) of a subgrid (10) to a superordinate AC power supply network (12), wherein: A grid current (I_Netz) flows between the subgrid (10) and the upper AC power supply grid (12) via the connection point (AP), wherein the subgrid (10) has at least one load (14) which draws a load current (I_Last) from the subgrid (10), wherein the load current includes an active power current (I_d) at a grid frequency (f0) and a harmonic distortion current (I_d) at one or more integer multiples of the grid frequency (f0), wherein the subgrid (10) further has a power converter (16, 16.N) which exchanges electric power between a capacitor (18, 18.N) connected to the DC side of the power converter and the subgrid (12) connected to the AC side of the power converter by means of a bridge circuit. Wherein, the control unit (20, 20.N) is designed as follows: Receive the grid voltage (U ac ), Using the grid voltage (U ac ), determining a compensation voltage and / or a compensation current (Komp), wherein the compensation voltage and / or the compensation current (Komp) are provided for being generated by the power converter (16, 16.N) and are suitable for reducing harmonic distortion currents at at least one multiple of a grid frequency (f0) in the grid current (I_Netz), and A control signal (PWMS, PWMS.N) is output to the power converter (16, 16.N), wherein the power converter (16, 16.N) can use the control signal (PWMS, PWMS.N) to generate the compensation voltage and / or the compensation current (Komp) by appropriately clocking the bridge circuit between the DC link capacitor (18, 18.N) and the subgrid (10).

12. The control unit according to claim 11, wherein: The control unit (20, 20.N) is designed to calculate the corresponding harmonic contribution values ​​at different multiples of the grid frequency (f0). The respective harmonic compensation contributions for the compensation voltage (Komp) are determined independently of one another.

13. The control unit according to claim 12, wherein: The control unit (20, 20.N) comprises a corresponding harmonic regulator (24.3, 24.5, 24.n), by means of which the grid voltage (U ac ) can determine the corresponding harmonic contribution value Wherein, the corresponding harmonic contribution value is used The corresponding harmonic compensation contribution for the compensation voltage (Komp) can be determined as a manipulated variable.

14. The control unit according to any one of claims 11 to 13, wherein: The control unit (20, 20.N) is designed to receive a DC voltage (UDC, UDC.N) which is applied to a capacitor (18, 18.N) connected to the DC side of the power converter (16, 16.N), and wherein the control unit comprises a DC regulation (26, 30) by means of which a DC regulation contribution for the compensation voltage and / or the compensation current (Komp) can be determined using the DC voltage (UDC, UDC.N), so that the DC voltage (UDC, UDC.N) is regulated to a predefined level by suitable active power exchange via the bridge circuit.

15. The control unit according to any one of claims 11 to 14, wherein: The load current (I_Last) has a reactive power current (I_q) at the grid frequency (f0), and the control unit (20, 20.N) is designed to perform reactive power regulation (28), by means of which a reactive power regulation contribution for the compensation voltage (Komp) can be determined, so that the reactive power exchange at the connection point (AP) is reduced.

16. A subgrid comprising a control unit (20, 20.N) according to any one of claims 11 to 15, in, The subgrid (10) is connected to the superordinate AC power grid at the connection point (AP), The subgrid has at least one load (14), which is designed to obtain the load current (I_Last) from the subgrid (10), wherein the load current includes an active power current (I_d) at a grid frequency (f0) and a harmonic distortion current at one or more integer multiples of the grid frequency (f0). The subgrid further comprises the power converter (16, 16.N), which is designed to exchange electric power between the capacitor (18, 18.N) connected to the DC side of the power converter and the subgrid (10) connected to the AC side of the power converter by means of the bridge circuit, so that the harmonic power flow via the connection point (AP) is reduced.

17. The subgrid according to claim 16, wherein: The subgrid (10) comprises a plurality of power converters (16, 16.N), wherein different power converters (16, 16.N) are configured to respectively generate a compensation voltage and / or a compensation current (Komp) to reduce different harmonic contribution values ​​at different multiples of the grid frequency (f0) 18. The subgrid according to claim 17, wherein: The power converter (16, 16.N) is connected to a superior control unit, and the superior control unit outputs a corresponding control signal (PWMS, PWMS.N) to each power converter in the plurality of power converters (16, 16.N). The corresponding control signals (PWMS, PWMS.N) are configured to: generate the compensation voltage and / or the compensation current (Komp) through the corresponding power converter (PWMS, PWMS.N), reduce the harmonic distortion current at one or more multiples of the grid frequency (f0), and / or reduce the reactive power exchange at the connection point (AP).

19. The subgrid according to any one of claims 16 to 18, wherein: The subgrid is electrically isolated from the superordinate AC power supply network, in particular by a transformer at the connection point.

20. The subgrid according to any one of claims 16 to 19, wherein: The power converter is connected to the subgrid without electrical isolation.

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