Voltage-adjusting inverter, and energy-generating plant

Abstract

The invention relates to a voltage-adjusting inverter (16), which comprises an inner control loop, said inner control loop having a setpoint value input, for receiving a setpoint value signal (Id, soll) for an active power component that is to be provided by the inverter, and having an actual value input, for receiving a present actual value (Id, ist) of a provided active power component of the inverter, wherein the inner control loop is a controller (22) having a proportional component and having an integral component and is designed to determine, from the difference (ΔId) between the setpoint value signal (Id, soll) and the present actual value (Id, ist) as controller input variables, a frequency shift (Δf) as a controller output variable. The inverter is designed to provide an AC voltage that deviates from a specified base frequency (f0) by the determined frequency shift. An outer control loop of the inverter is designed to, in an isolated network mode, supply a setpoint value signal to the setpoint value input in accordance with the determined frequency shift. Also provided is an energy-generating plant, which may comprise two such inverters.

Description

[0001] Voltage-regulating inverter and power generation system

[0002] Description

[0003] The invention relates to a voltage-regulating inverter and a power generation system with such an inverter.

[0004] Voltage-regulating inverters are becoming increasingly important in the context of the energy transition. One application for such inverters is the provision of an AC grid without a connection to a higher-level AC grid, as these inverters, either alone or together with other voltage-regulating inverters, are capable of independently establishing such a grid and operating stably even under varying loads. If a group of voltage-regulating inverters is to jointly operate an isolated grid without the need for complex communication between the individual inverters or the generation of a high-precision reference frequency, the document DE101 40 783 A1 proposes generating a target voltage as a reference, the frequency of which is determined using a frequency droop from the current active power, and the amplitude of which is determined using a voltage droop from the current reactive power of the inverter.Additionally, phase pre-control is provided to effectively suppress power oscillations between the inverters. The distribution of reactive and active power between the inverters is defined by the relative position of the stored droops.

[0005] However, there is an increasing requirement for voltage-regulating inverters to also be operable on a higher-level AC grid. In this situation, these inverters are capable of spontaneously counteracting fluctuations in the grid parameters frequency and amplitude, thus contributing to grid stability. It is desirable that the inverter control in the presence of a higher-level grid differs as little as possible from the control of an isolated grid, to enable the smooth continued operation of a sub-grid when it is disconnected from the higher-level grid.Furthermore, it is desirable that, in the case of an island grid, a predefined power distribution between the inverters connected to the island grid is spontaneously established without communication, and that the required total power is spontaneously distributed among the available inverters even when individual inverters are switched on or off. If a higher-level grid is present, it should be possible to individually control the power of the individual inverters by specifying setpoints. Accordingly, the object of this invention is to provide a voltage-regulating inverter or a power generation system that meets these requirements.

[0006] This object is achieved by an inverter having the features of independent claim 1. Preferred embodiments of the inverter are the subject of the dependent claims. A power generation system comprises two such inverters and is described in claims 6 to 9.

[0007] A voltage-regulating inverter according to the invention comprises an internal control loop with a setpoint input for receiving a setpoint signal for an active power component to be provided by the inverter and an actual value input for receiving a current actual value of an active power component provided by the inverter. The internal control loop is a controller with a proportional component and an integral component and is configured to determine a frequency shift as the controller output variable from the difference between the setpoint signal and the current actual value as the controller input variable. The inverter is further configured to provide an alternating voltage that deviates from a predetermined fundamental frequency by the determined frequency shift at an output of the inverter, at which the inverter can be connected to a distribution grid.

[0008] The inverter according to the invention further comprises an outer control loop configured to supply a setpoint signal to the setpoint input in stand-alone operation depending on the determined frequency shift. Through the interaction of the inner control loop and the outer control loop in stand-alone operation, the inverter is capable of coordinating the active power to be provided with other inverters, if present, via the frequency or frequency shift, in order to meet the total active power demand of the connected grid at all times and without further communication.

[0009] Preferably, the inverter is configured to supply an external signal to the setpoint input during grid-commutated operation. This allows the active power provided by the inverter to be easily controlled via the external signal during grid-commutated operation, so that the desired active power is supplied to a distribution grid.

[0010] In a preferred embodiment, the inverter according to the invention additionally comprises a further internal control loop with a further setpoint input for receiving a further setpoint signal for a reactive power component to be provided by the inverter and a further actual value input for receiving a current actual value of the provided reactive power component, wherein the further internal control loop is a controller with a proportional component and an integral component and is configured to determine a voltage amplitude shift as a controller output variable from the difference between the further setpoint signal and the current actual value of the reactive power component to be provided as a controller input variable. The inverter is configured to provide an alternating voltage at the output that deviates from a predetermined base amplitude by the determined voltage amplitude shift.Furthermore, the inverter comprises an external control loop which is designed to supply a further setpoint signal to the further setpoint input in an island grid operation as a function of the determined voltage amplitude shift.

[0011] By means of this additional control structure, the inverter according to the invention can, in addition to the active power, also provide reactive power as required in the island grid operation, wherein the control structure determines a reactive power share of the inverter according to the invention in the total reactive power requirement of the connected grid via the switching variable of the voltage amplitude, also without further communication between any additional inverters that may be present.

[0012] Preferably, the outer control loop and / or the further outer control loop are designed as proportional controllers with a predefined proportional coefficient. This achieves a linear relationship between the respective controller input variables and controller output variables.

[0013] In one embodiment of the invention, the setpoint and the actual value for the active power component provided and the setpoint for the active power component to be provided are current values, so that the inner control loop comprises a current control.

[0014] A further aspect of the invention relates to an energy generation plant, comprising a first inverter with the properties described above and a second inverter with the properties described above, which is connected in parallel with the first inverter on the AC side, wherein a central controller supplies the inverters with individual setpoint signals at their setpoint inputs in grid-commutated operation and also supplies status signals in order to switch the inverters between grid-commutated operation and island operation. It is conceivable that the individual setpoint signals are only supplied in grid-commutated operation. In island operation, the setpoint signals can be omitted, so that the two inverters independently coordinate the required distribution of the power components to be provided via the switching variables frequency and voltage amplitude.However, it is also conceivable that at least one of the inverters continues to receive setpoint signals and adjusts its power delivery accordingly. In particular, the central controller can continue to supply individual frequency shift signals to the inverters, with the inverters being configured to adjust their base frequency according to the frequency shift signal supplied to them.

[0015] Alternatively or additionally, the central controller may further supply individual voltage amplitude shift signals to the inverters, wherein the inverters are configured to adjust their base amplitude in accordance with the voltage amplitude shift signal supplied to them.

[0016] In an advantageous embodiment, the central controller can further supply individual coefficient values ​​to the inverters, whereby the inverters are configured to adjust the proportional coefficients used in their external control loop according to the coefficient value supplied to them. In this way, the central controller can influence the distribution of the total power component to be provided among the participating inverters.

[0017] In the following, the invention is illustrated by means of figures, of which

[0018] Fig. 1 shows a schematic structure of an energy generation plant according to the invention,

[0019] Fig. 2 shows an embodiment of a control system for providing active power in an inverter according to the invention,

[0020] Fig. 3 shows an embodiment of a control system for the provision of reactive power in an inverter according to the invention,

[0021] Fig. 4 shows an exemplary characteristic curve stored in a proportional controller of the controlled system

[0022] Fig. 5 differently parameterized characteristic curves of two inverters in a power generation plant according to the invention, as well as

[0023] Fig. 6 shows a time course of power provided by two inverters according to the invention in an operating scenario.

[0024] Fig. 1 shows a structure of an energy generation plant 10 according to the invention, which has a plurality of voltage-regulating inverters 16 which are communicatively connected to a central controller 15, so that control signals can be transmitted from the controller 15 to the inverters 16, but optionally also signals in the opposite direction.

[0025] The inverters 16 are each connected to a common distribution grid via a disconnect switch 14 in order to exchange power with it, in particular to provide active power and reactive power. The distribution grid, to which additional loads 17 can be connected, is in turn connected to a higher-level grid 11, for example, a medium-voltage grid, via a grid disconnect switch 13 and a transformer 12.

[0026] By using voltage-regulating inverters 16 with sufficient power capacity and corresponding connected sources, such as PV generators or batteries, it is possible to reliably supply the loads 17 with power via the distribution grid even when the grid disconnector 13 is opened, thus separating the distribution grid from the higher-level grid 11. In this case, the inverters 16 jointly ensure stable electrical parameters, in particular voltage and frequency, within the permissible value range. The grid disconnector 13 can be operated manually or by the controller 15. The controller 15 can also transmit individual setpoint specifications as well as status signals, such as a signal indicating whether a connection between the distribution grid and the higher-level grid 11 exists, to the inverters 16.For this purpose, the controller 15 monitors the network state with a suitable sensor 18 or the controller 15 can also actively influence the network state by controlling the network disconnector 13 or passively determine it by monitoring the switching state of the network disconnector 13.

[0027] Fig. 2 shows a controller structure for providing active power in an inverter according to the invention. The description of the mode of operation of the controller structure begins under the assumption that a setting unit 20, which can be part of the controller 15 shown in Fig. 1, inputs an individual setpoint Id.soii into the controller structure and controls a status switch 23 such that this setpoint Id.soii is compared with an actual value Id, ist and a difference Aid between the two variables is transmitted as an input variable to an internal controller 22. The setpoint Id, son and the actual value Id, ist are in this case values ​​of a positive-sequence system component of the current amplitude provided by the inverter, but can alternatively also be other electrical variables characterizing the active power to be provided or provided.

[0028] The inner controller 22 is preferably designed as a proportional-integral controller and provides as an output variable a frequency deviation Af by which a frequency f set by the inverter at its output S0 n deviates from a fundamental frequency fo. The fundamental frequency f0 is provided here by the setting unit 20, but can also be a predetermined fixed value. Since the frequency f provided by the inverter so n in turn has a direct impact on the actual value l d st, the internal controller 22 ensures that the set frequency deviation Af leads to an actual value Id, ist, which corresponds to the setpoint l d ,soii corresponds. With the above-assumed control of the state switch 23, the internal controller 22 therefore ensures compliance with the setpoint l specified by the setting unit 20 d , S0 n by the inverter.

[0029] In the event that the setting unit 20 controls the status switch 23 so that the setpoint l d , is not specified by the setting unit 20, but by an output variable of an external controller 21, the inverter behaves differently. In this case, the external controller 21 determines a setpoint for the variable l based on a deviation Af of a frequency currently set by the inverter from a specified base frequency f0. d ,soii, which instead of a quantity generated by the preset unit 20 with the actual value l d ,i st and their difference is transmitted as an input variable to the inner controller 22. The outer controller 21 is preferably designed as a pure proportional controller, so that the setpoint depends linearly on the frequency deviation Af with a proportionality coefficient a, within permissible controller limits. This coefficient a is transmitted in Fig. 2 by the setting unit 20 in a variable manner, but can also be a predetermined fixed value. A variation of the coefficient a enables the setting unit 20 to specify different values ​​for the coefficient a, as well as different values ​​for the fundamental frequency fo, for different inverters connected to the setting unit 20 and thus to influence a proportional distribution of the total active power to be provided by the inverters. This will be discussed in more detail below in connection with Fig. 5.

[0030] The controller structure with the inner controller 22, the outer controller 21 and the state switch 23 are components of an inverter.

[0031] Fig. 3 shows another, comparable controller structure for providing reactive power in an inverter according to the invention. The description of the controller structure's operation also begins here under the assumption that a setting unit 20, which may be part of the controller 15 shown in Fig. 1, provides an individual setpoint value l q , so ii into the controller structure and controls a status switch 33 so that this setpoint specification l q , S0 n with an actual value l q , is t compared and a difference Al q between both variables is transferred as an input variable to an internal controller 32. The setpoint value l q , S0 n and the actual value l q , isIn this case, t are values ​​of a negative system component of the current amplitude provided by the inverter, but can alternatively also be other electrical quantities characterizing the active power to be provided or provided.

[0032] The inner controller 32 is also preferably designed as a proportional-integral controller and provides as an output variable a deviation AU of a voltage amplitude by which a voltage U provided by the inverter at its output S0 n deviates from a base voltage Uo. The base voltage Uo is provided here by the setting unit 20, but can also be a predefined fixed value. Since the voltage amplitude U provided by the inverter S0 n in turn has a direct effect on the actual value Iq st, the inner controller 32 ensures that the set deviation AU of the voltage amplitude leads to an actual value l q , is t, which corresponds to the setpoint lq , S0 n. With the above-assumed activation of the state switch 33, the internal controller 32 therefore ensures compliance with the setpoint l specified by the setting unit 20 q , so ii by the inverter.

[0033] In the event that the setting unit 20 controls the status switch 33 so that the setpoint l q ,soii is not specified by the setting unit 20, but by an output variable of an external controller 31, the inverter behaves differently. In this case, the external controller 31 determines a setpoint for the variable l based on a deviation AU of a voltage amplitude currently provided by the inverter from a specified base voltage Uo. q , S0 n, which instead of a quantity generated by the preset unit 20 is set to the actual value l q , ist and the difference between them is transmitted from the input variable to the inner controller 32. The outer controller 31 is preferably designed as a pure proportional controller, so that the setpoint depends linearly on the deviation AU of the voltage amplitude with a proportionality coefficient b, within permissible controller limits. This coefficient b is transmitted in Fig. 3 by the setting unit 20 in a variable manner, but can also be a predetermined fixed value. A variation of the coefficient b enables the setting unit 20 to specify different values ​​for the coefficient b, as well as different values ​​for the basic voltage Uo, for different inverters connected to the setting unit 20 and thus to influence a proportional distribution of the total reactive power to be provided by the inverters.

[0034] Fig. 4 shows a characteristic curve 41 of the external controller 21 from Fig. 2. A frequency deviation Af is plotted on the x-axis as the input variable of the external controller 21. The resulting setpoint, which can be a current amplitude Id in the positive sequence system or another controlled variable P linked to an active power to be provided by the inverter comprising the external controller 21, varies linearly with the coefficient a as a function of the frequency deviation Af. Accordingly, the external controller 31 from Fig. 3 can be designed in such a way that the output variable of the external controller 31 is a controlled variable Q linked to a reactive power to be provided by the inverter comprising the external controller 31, for example a negative sequence system component l q the current amplitude, linear with the coefficient b as a function of a deviation AU of the voltage amplitude from a fundamental voltage Uo.

[0035] In order to explain the influence of the values ​​of the coefficient a and the fundamental frequency fo, which can be variably specified by the setting unit 20, on a distribution of a total active power to be provided between two inverters, Fig. 5 shows, by way of example, a first characteristic curve 51 of a current amplitude Id in the positive sequence system or of another controlled variable P of a first inverter linked to an active power to be provided, to which the values ​​a1 as coefficient of the external controller and the fundamental frequency f Oi are specified. Accordingly, a second characteristic curve 52 of a current amplitude l d in the positive sequence system or another controlled variable P of a second inverter linked to an active power to be provided, to which the values ​​a2 as coefficient of the external controller and the fundamental frequency f 02are specified. In this way, it is possible to define frequency ranges in which the active power is predominantly provided by the first inverter, and frequency ranges in which the active power is predominantly provided by the second inverter. This distribution can of course also be designed to vary over time and, in particular, can be made dependent on current operating conditions such as a bridge temperature or the charge state of a storage device connected to the respective inverter. In a similar way, the distribution of reactive power to be provided between inverters can also be flexibly designed by variable specifications of coefficients b1,2,..n and fundamental voltage Uoi,o2,..on between two or more inverters.

[0036] In Fig. 6, the mode of operation of an embodiment according to the invention is illustrated using a first time profile 61 (dashed line) of an active power P provided by a first inverter according to the invention and a second time profile 62 (solid line) of an active power P provided by a second inverter according to the invention. In a first time period I, an island grid is provided solely by the first inverter, with excess energy in the island grid being used to charge a storage device connected to the first inverter. In the second time period II, a load in the island grid requires an operating power P , which is provided solely by the first inverter. At the beginning of the third time period III, the second inverter is switched on if the load requirement remains unchanged.After a brief transition, the load demand is distributed between the two inverters according to the coefficients a1, a2 of the external controller and the stored fundamental frequencies foi, fo2. At the beginning of the fourth time period IV, the first inverter is shut down if the load demand remains unchanged. The second inverter immediately assumes the full operating load P, ensuring continuous operation of the stand-alone grid even when individual inverters are switched on or off without the need for communication.

[0037] List of reference symbols

[0038] 10 power generation plant

[0039] 11 Network

[0040] 12 T ransformer

[0041] 13 Mains disconnect switch

[0042] 14 disconnectors

[0043] 15 Control

[0044] 16 inverters

[0045] 17 Last

[0046] 18 Sensor

[0047] 20 default unit

[0048] 21 outer regulator

[0049] 22 inner regulator

[0050] 31 outer regulator

[0051] 32 inner regulator

[0052] 41 Characteristic curve

[0053] 51, 52 characteristic curve

[0054] 61, 62 Performance history

Claims

Patent claims:

1. Voltage-regulating inverter (16) comprising: - an inner control loop with a setpoint input for receiving a setpoint signal (Id.soii) for an active power component to be provided by the inverter, and an actual value input for receiving a current actual value (Id, ist) of a provided active power component of the inverter, wherein the inner control loop is a controller (22) with a proportional component and an integral component and is designed to determine a frequency shift (Af) as a controller output variable from the difference (Aid) between the setpoint signal (Id, son) and the current actual value (Id, ist) as a controller input variable, wherein the inverter is designed to provide an alternating voltage deviating from a predetermined fundamental frequency (fo) by the determined frequency shift, - an external control loop which is designed to supply a setpoint signal to the setpoint input in island grid operation as a function of the determined frequency shift.

2. Inverter (16) according to claim 1, wherein the inverter is configured in a grid-controlled operation to supply an external signal to the setpoint input.

3. Inverter (16) according to claim 1 or 2, wherein the inverter (16) additionally comprises: - another inner control loop with another setpoint input for receiving another setpoint signal (l q , S0 n) for a reactive power component to be provided by the inverter (16), and a further actual value input for receiving a current actual value (l q , ist) of the reactive power component provided, wherein the further inner control loop is a controller (32) with a proportional component and an integral component and is designed to calculate the reactive power component from the difference (Al q ) between the further setpoint signal (l q , S0 n) and the current actual value (l q , ist ) of the reactive power component to be provided, to determine a voltage amplitude shift (AU) as a controller output variable, wherein the inverter (16) is designed to provide an alternating voltage deviating from a predetermined base amplitude (Uo) by the determined voltage amplitude shift, - an external control loop which is designed to supply a further setpoint signal to the further setpoint input in island grid operation as a function of the determined voltage amplitude shift. Inverter (16) according to one of the preceding claims, wherein the outer control loop and / or the further outer control loop are designed as proportional controllers with a predetermined proportional coefficient. Inverter (16) according to one of the preceding claims, wherein the setpoint and the actual value for the provided active power component and the setpoint for the active power component to be provided are current values, so that the inner control loop comprises current control.A power generation plant (10) comprising a first inverter (16) according to one of the preceding claims and a second inverter (16) according to one of the preceding claims, which is connected in parallel with the first inverter (16) on the AC side, wherein a central controller (15) supplies individual setpoint signals to the inverters (16) at their setpoint inputs in grid-commutated operation, and further supplies status signals in order to switch the inverters (16) between grid-commutated operation and island operation. A power generation plant (10) according to claim 6, wherein the central controller (15) further supplies individual frequency shift signals to the inverters (16), wherein the inverters (16) are configured to adapt their fundamental frequency in accordance with the frequency shift signal supplied to them.The power generation plant according to claim 6 or 7, wherein the central controller (15) further supplies individual voltage amplitude shift signals to the inverters (16), the inverters (16) being configured to adjust their base amplitude according to the voltage amplitude shift signal supplied to them. The power generation plant according to claim 6, 7 or 8, wherein the central controller (15) further supplies individual coefficient values ​​to the inverters (16), the inverters (16) being configured to adjust their proportional coefficients used in the external control loop according to the coefficient value supplied to them.

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