Island network detection, voltage-impressing converter, and converter

Abstract

The invention relates to an island network detection method using a voltage-impressing converter (14), having the steps of: - determining the frequency and the phase of a voltage curve of a sub-network (10) which is connected to a network connection point of the converter (14), - generating a voltage curve using a bridge circuit of the converter (14) with the specified frequency, the phase of the generated voltage curve having a first temporal variation with respect to the determined phase, - determining a correlation between the first temporal variation and a second temporal variation of an output exchanged by the converter (14) with the sub-network (10), and - detecting an island network if the determined correlation undershoots a specified correlation measurement. The invention likewise relates to a corresponding voltage-impressing converter (14).

Description

[0001] Island grid detection of voltage-impressing inverters and inverters

[0002] Description

[0003] The invention relates to a method for islanding detection by a voltage-impressing inverter, as well as to such an inverter.

[0004] Decentralized energy generation from renewable energy sources, particularly photovoltaics, is becoming increasingly important for the energy transition. The proportion of decentrally generated electrical power compared to power generation by power plants is steadily increasing. One concern here is that this increase in decentralized power generation increases the risk of power outages. The use of voltage-impressing or grid-forming inverters instead of grid-commutated inverters – the latter are also called current-impressing inverters or power converters for short – can make a significant contribution to stabilizing power grids. Voltage-impressing inverters are designed to generate power by providing a voltage waveform at their bridge output, which is continuously synchronized with a voltage waveform at a grid connection point of the inverter.Since a decoupling impedance is arranged between the bridge output and the grid connection point of the inverter, a defined active and reactive power exchanged with the grid can be regulated by a predefined, continuously updated (i.e., continuously or repeatedly) phase and amplitude offset between the voltages at the bridge output and the grid connection point. For example, EP 2 003 759 A1 discloses a method for islanding detection for voltage-controlled inverters, in which a test voltage whose test frequency is lower than the grid frequency is superimposed on the grid frequency. An islanding is detected when the reactive power component assigned to the test voltage falls below a predefined minimum.

[0005] In grids with a predominantly inductive impedance, the exchanged active power is regulated or determined by a continuously updated phase difference between the voltages at the bridge output and the grid connection point, and the reactive power is regulated or determined by a continuously updated amplitude difference between the two voltages. In grids with a predominantly resistive impedance, the roles of reactive power and active power are typically reversed. Mixed forms are also conceivable. This enables voltage-impressing inverters to independently establish or maintain a stable AC voltage grid.However, this property creates the risk that voltage-impressing inverters will maintain the voltage in a sub-grid to which they are connected, even if this sub-grid is unexpectedly or forcibly disconnected from a higher-level grid and maintaining the voltage is undesirable in this case. In this case, they form an island grid without further action as long as they are able to provide the required power. This can pose a risk if, for example, a sub-grid is disconnected from a higher-level grid for maintenance purposes. Therefore, voltage-impressing inverters may be required to be able to independently detect within a specified period of time that they are connected to an island grid and to react appropriately.

[0006] Such a requirement for islanding detection is common for grid-commutated inverters and is described, for example, in the technical application standard VDE_AR-N 4105 for power generation systems connected to low-voltage grids. Corresponding procedures are known, but these cannot simply be transferred to voltage-impressing inverters.

[0007] It is therefore an object of this invention to provide a method with which voltage-controlled inverters can reliably detect whether they are connected to an island grid.

[0008] This object is achieved by a method having the features of independent claim 1 or a voltage-impressing inverter having the features of independent claim 17. Preferred embodiments of the invention are described in the dependent claims.

[0009] A method according to the invention for islanding detection by a voltage-impressing inverter comprises the steps:

[0010] - Determining a frequency and phase of a voltage curve of a sub-grid connected to a grid connection point of the inverter,

[0011] - generating a voltage waveform by means of a bridge circuit of the inverter with the specific frequency, wherein the phase of the generated voltage waveform has a temporal variation compared to the specific phase,

[0012] - Determining a correlation between the temporal variation and a temporal variation of a power exchanged by the inverter with the sub-grid, and

[0013] - Detecting an island network when the determined correlation falls below a specified correlation level.

[0014] The power exchanged by the inverter with the sub-grid can be the active power, particularly in networks with a grid impedance with a dominant inductive component. However, the apparent power can also be used, which is the easier quantity to measure in certain applications, or, in general terms, a weighted root mean square of active power and reactive power, with the averaging weights determined as a function of the resistive and inductive components of the grid impedance. This can enable better detection accuracy of the method. For the sake of simplicity, the invention will be described below partly with reference to active power as the quantity used for the correlation, although this does not exclude the other two possibilities.

[0015] The temporal variation of the phase is a deliberately induced deviation of the phase difference between the voltage curve at the bridge output and the curve of the grid connection point voltage extrapolated from the previously determined frequency and phase of the voltage curve at the grid connection point from a temporally constant phase difference Acp. Thus, the extrapolated voltage curve at the grid connection point

[0016] UNAp(t) = ÜNAP * sin (tot), results for the voltage curve set at the bridge output

[0017] Ußr(t) = Ußr * Sin (COt+A(p+A(pAID(t)).

[0018] The phase variation A <p AiD(t) can be generated as a periodic or non-periodic signal and results in a time-correlated power component that is proportional to the phase variation if the voltage curve at the grid connection point exactly corresponds to the extrapolated curve, i.e., the grid voltage curve is not influenced by the power provided by the inverter. In this case, the grid is referred to as a rigid grid, since, in other words, the grid remains constant in its phase. The power component of the rigid grid can be calculated in a known manner from the phase variation using the known inductance of the line filter arranged between the bridge output and the grid connection point.

[0019] If an island grid is present, i.e. there is no connection to a rigid grid, the power provided by the inverter influences the phase curve of the voltage at the grid connection point in such a way that this follows the temporal variation of the phase generated by the inverter to a certain extent, so that the phase difference between the voltage curves at the grid connection point and the bridge output is smaller than the variation generated and thus smaller than the phase difference in the case of a rigid grid. It is conceivable here, particularly if the inverter forms the only voltage-impressing component of a sub-grid, that the phase at the grid connection point completely follows the generated phase variation, so that despite the generated phase variation, there is a temporally constant phase difference between the grid connection point and the bridge output.In this case, the power provided by the inverter does not exhibit a power component that correlates temporally with the generated phase variation. This makes it easy to identify an islanded grid in this case.

[0020] However, a situation can also arise in which a certain phase inertia exists in the subgrid, meaning that the phase at the grid connection point only partially follows the phase variation. The phase difference then only weakly replicates the generated phase variation, and a power component that correlates temporally with the generated phase variation is smaller than the power component that occurs in the case of the rigid grid described above. An islanded grid can be identified by a suitably selected correlation measure, for example, a ratio between a power component determined from the correlation with the phase variation and the power component of the rigid grid. The lower this ratio, the more likely it is to be an islanded grid.

[0021] One way to determine a correlation measure K is through the relationship

[0022] Given and particularly suitable for periodic phase variations. This method assumes that f (p A / D (t) dt = 0. In practice, the correlation measure is often determined by summing a time series of measured power values ​​and the corresponding generated phase variation. Other methods of correlation calculation are also conceivable. For example, using a lock-in amplifier, the frequency component of the power variation can be determined at the frequency of the phase variation and related to the amplitude of the phase variation to determine a suitable correlation measure.

[0023] The choice of a correlation measure threshold below which an island grid is detected can be adapted to the inverter installation situation. For example, if it is known that additional voltage-impressing inverters are installed in a grid section, the sub-grid will exhibit a certain phase inertia, even when disconnected from a higher-level grid, in response to the phase variation generated by the inventive method, which is caused by the additional voltage-impressing inverters. During normal operation, these inverters are synchronized continuously or at certain intervals, hereinafter also referred to as the synchronization frequency, to the phase at their grid connection point.It is therefore preferable to generate the phase variation with a variation frequency that is lower than the synchronization frequency or lower than the typical value range of the synchronization frequency of the further inverters, and that is particularly preferably a fraction, for example half, a third or a quarter of the synchronization frequency, since the phase inertia with respect to such a slow phase variation is reduced due to the synchronization taking place during normal operation.

[0024] It is also advantageous to generate the phase variation as a periodic variation with a first and a second frequency that is higher than the first frequency, of which the first variation frequency is preferably lower than the synchronization frequency and the second variation frequency is preferably higher than the latter. By comparing, in particular a ratio of the power component at the first variation frequency relative to the power component at the second variation frequency, it is possible to distinguish whether phase inertia is caused by a higher-level grid or other voltage-impressing inverters. If the phase inertia is caused by other voltage-impressing inverters, the power component at the second variation frequency will be lower than at the first, lower

[0025] The variation frequency, i.e., the ratio of the power components mentioned above, can be greater than in a still existing higher-level grid. This behavior can be used additionally or alternatively when detecting an island grid.

[0026] In an advantageous embodiment of the invention, the method is repeated at intervals. As soon as an island grid is detected, the inverter preferably disconnects from the sub-grid. It is conceivable that, after detecting an island grid but before disconnecting from the sub-grid, the inverter is operated in such a way that it targets an amplitude and / or frequency of the AC grid that lies outside the permissible grid parameters in order to cause the disconnection of other grid components connected to the sub-grid. This increases the reliability of reliably de-energizing a detected island grid.

[0027] In addition to or instead of disconnecting from the subgrid, the inverter can provide a detection signal if an island grid is detected. A dedicated signal output can be provided for this purpose, or the inverter can use a digital communication interface to provide the detection signal. The detection signal can also be provided and transmitted as a signal modulated onto the generated AC voltage in the form of a power line communication (PLC) signal.

[0028] The detection signal thus provided can be transmitted to a network operator and / or used to cause another device connected to the sub-network, in particular a synchronous generator, to shift the amplitude and / or frequency of the AC network to values ​​outside of permissible network parameters in order to cause a disconnection of other network components connected to the network.

[0029] In one embodiment of the invention, the temporal variation of the phase comprises a periodic variation with a first component at a first variation frequency. Preferably, an islanded grid is detected when a ratio between a component of the active power at the first variation frequency and the first component of the periodic variation as a correlation measure falls below a predetermined limit (first detection criterion).

[0030] In a further embodiment of the invention, the temporal phase variation comprises, in addition to the first component at the first variation frequency, a second component with a second variation frequency that is higher than the first variation frequency. An island grid is detected, for example, if the ratio between a component of the active power at the first variation frequency and a component of the active power at the second variation frequency falls below a predetermined limit. This second detection criterion can be used as a standalone detection criterion or in logical combination with the first detection criterion.

[0031] To minimize the impact of the detection method on grid quality, the generation of the voltage waveform can involve a gradual increase in the amplitude of the phase variation. The method is terminated without detecting an islanded grid if the variation in the active power exchanged by the inverter with the AC grid exceeds a predefined termination value. The termination value can be specified as a fixed value or determined based on a voltage component at the grid connection point determined before the method is implemented, at the applied variation frequency.The latter allows for potential interaction with other voltage-impressing inverters that also perform the inventive method for islanding detection at the same or only slightly different variation frequency to be taken into account, so that the detection methods influence each other less and thus become more reliable. It is also conceivable to use the value of the amplitude of the phase variation when the method is aborted to define a starting amplitude of the phase variation for subsequent islanding detection methods. This can reduce the impact of the inventive method on grid quality.

[0032] To further account for such interactions, determining the frequency and phase of a voltage waveform of the AC grid connected to the grid connection point of the inverter can include determining a third temporal phase variation. The generation of the voltage waveform during islanding detection occurs with a temporal phase variation that is synchronous with the previously determined third temporal phase variation. In this way, multiple voltage-impressing inverters can interact constructively during islanding detection and increase detection sensitivity.

[0033] The method according to the invention can be implemented using either a single-phase inverter or a multi-phase inverter with a first and a second phase conductor. In the latter case, the method can be executed simultaneously or sequentially on the first and second phase conductors, with an island network being detected if a multi-phase island network is detected on at least one of the phase conductors. Alternatively, an island network can only be detected if a multi-phase island network is detected on both the first and second phase conductors.

[0034] According to these options, the procedure can also be extended to other phase conductors with which the inverter is connected to the grid.

[0035] In a further aspect of the invention, a voltage-impressing inverter comprises:

[0036] - a measuring unit for determining the frequency and phase of a voltage curve of a sub-grid connected to a grid connection point of the inverter, as well as for determining the values ​​of active power and reactive power exchanged with the sub-grid,

[0037] - a control unit for generating switching commands for a bridge circuit of the inverter, so that a voltage curve predetermined by the control unit is generated at an output point of the bridge circuit with the determined frequency, wherein the control unit is configured to provide the predetermined voltage curve with a temporal variation of the phase compared to the phase of the determined voltage curve, and

[0038] - a correlator for determining a correlation between the temporal variation of the phase and a temporal variation of an active power exchanged by the inverter with the sub-grid, wherein the correlator is configured to detect an island network if a correlation measure of the determined correlation falls below a predetermined limit value.

[0039] In the following, the invention is illustrated by means of figures, of which

[0040] Fig. 1 an installation example for a sub-grid with several inverters,

[0041] Fig. 2 shows a controller structure for an inverter according to the invention,

[0042] Fig. 3 is a schematic representation of an inverter according to the invention, Fig. 4 shows time profiles of phases and exchanged power when executing the method according to the invention in the case of a connection to a higher-level network,

[0043] Fig. 5 Time profiles of phases and exchanged power when carrying out the method according to the invention in a first embodiment in the case of an island network and

[0044] Fig. 6 shows time courses of phases and exchanged power when carrying out the method according to the invention in a second embodiment in the case of an island network.

[0045] Fig. 1 shows an exemplary structure of a sub-grid 10, which is connected to a higher-level grid 11 via a disconnector 13 and medium-voltage transformer 12. The sub-grid 10 has a voltage-impressing inverter 14, a further inverter 15, and a number of loads 16, which are at least partially supplied by the inverters 14, 15 and, if applicable, from the grid 11. The further inverter 15 can also be voltage-impressing or, alternatively, grid-following. The sub-grid 10 can have further power-feeding components. The voltage-impressing inverter 14 can be connected on the DC side to a battery and / or to generators, for example, to photovoltaic generators. It is also conceivable to connect a load via the voltage-impressing inverter instead of a feeder.The inverters 14, 15 can each be designed as purely feeding inverters or as bidirectional inverters.

[0046] A controller structure of a voltage-impressing inverter according to the invention is shown in Fig. 2. Measured values ​​of a voltage UNAP at the grid connection point of the inverter, optionally supplemented by measured values ​​of a current I NAP flowing through the grid connection point, are evaluated in a phase evaluation unit 20 to determine an amplitude Üo, a frequency αo, and a phase αp0 of the voltage UNAP at the grid connection point. The phase evaluation unit can be designed, for example, as a phase-locked loop (PLL). The determined values ​​are fed to a pulse-width modulation (PWM) generator as input variables for generating a pulse pattern with a duty cycle d, with which a bridge circuit 22 is operated to provide a voltage Ußr at its bridge output.The bridge circuit 22 can be a single-phase or multi-phase bridge circuit, whereby a separate pulse pattern can be generated for each phase conductor. Different topologies of the bridge circuit 22 with a varying number of bridge switches, in particular multi-level topologies, are conceivable, whereby the pulse pattern can have individual switching commands for the respective bridge switches.

[0047] The measured values ​​of the voltage UNAP at the grid connection point, as well as the measured values ​​of the current INAP at the grid connection point are also fed to a power determiner 23, which calculates the current values ​​of the active power Rist and reactive power Qj flowing through the grid connection point from the measured values. S t. The phase evaluation unit 20 and the power determiner 23 are combined into a measuring unit 28.

[0048] The values ​​of active power P flowing through the grid connection point ist and reactive power Q istare fed as input variables to a characteristic unit 24, which determines characteristic variables from the additional input variables of the amplitude Üo and frequency co determined by the phase evaluation unit 20, which determines characteristic variables that represent setpoints for reactive power and active power. When determining the setpoints, other variables can of course also be taken into account, for example the intermediate circuit voltage or the state of charge of a battery connected on the DC side. In the case shown in Fig. 2, these characteristic variables are generated as amplitude difference AU and phase difference Acp, which are fed as further input variables to the PWM generator 21 in order to generate a pulse pattern with which the voltage curve Ußr generated by the bridge circuit 22 has a corresponding amplitude difference or phase difference to the voltage curve UNAP at the grid connection point.The amplitude difference AU and phase difference Acp are determined as a function of the inductance of a decoupling choke of the voltage-impressing inverter located between the bridge output and the grid connection point. To determine the setpoints for active power and / or reactive power, 24 characteristic curves are stored in the characteristic curve unit. These curves, for example, determine a setpoint for active power from the frequency co and a setpoint for reactive power from the amplitude Üo. However, the characteristic curves can also contain additional parameters that can be fixed or variable, such as the aforementioned intermediate circuit voltage or the battery charge level.

[0049] During those periods in which islanding detection is performed by the voltage-impressing inverter, an additional phase difference Acp, which is generated as a time-variable variable by a generator 26, is added to the phase difference Acp determined by the characteristic unit 24. The time-variable variable can be a periodic signal having one or more frequency components. However, it is also conceivable to use a non-periodic signal, for example, a step function. The non-periodic signal can preferably be parameterized based on communicated information, particularly with regard to form or time, or in another way.

[0050] During the periods in which no islanding detection is carried out, the additional phase difference A <p A iD zero or it is not added to the phase difference Acp.

[0051] A correlator 25 determines a temporal correlation between values ​​of the active power P supplied as input variables ist and the phase difference ACPAID generated by the generator. From this, it calculates a correlation measure between these quantities, which is compared with a predefined threshold value to generate a signal SAID. This signal is provided as an internal or external signal by the voltage-impressing inverter and indicates whether the inverter detects an existing island grid. If the calculated correlation measure falls below the predefined threshold value, an island grid is detected. The inverter itself, as well as other devices that receive the provided signal SAID, can react to this signal accordingly.

[0052] The PWM generator 21, the generator 26 and the characteristic unit 24 are part of a control unit 27 of the voltage-impressing inverter.

[0053] Fig. 3 shows a schematic structure of an inverter according to the invention. DC-side terminals DC+, DC- are connected via an intermediate circuit 29 to a bridge circuit 22, at whose bridge output a desired bridge output voltage Uß can be generated by appropriately controlling the bridge switches with a duty cycle d. ris generated. The bridge output is connected to a grid connection point 32 of the inverter via a decoupling choke 31, which is part of a grid filter. Other components that can be arranged in the connection between the bridge output and the grid connection point, for example an isolating relay, are not shown for the sake of simplicity. Suitable measuring sensors continuously or periodically record the current and voltage at the grid connection point 32, and the measured values ​​are fed to a measuring unit 28, where they are evaluated to determine grid parameters 30 such as frequency, phase, amplitude, reactive power, and active power. These grid parameters 30 are transmitted to a control unit 27, which uses them to determine the duty cycle d required to generate the desired bridge output voltage Ußr and control the bridge switches.

[0054] The control unit 27 transmits the periodic or non-periodic phase difference AcpAiD generated to implement the islanding detection method according to the invention to a correlator 25, which determines a temporal correlation of the phase difference Aq iD with a power quantity determined by the measuring unit 28, here an active power Rist, and calculates a corresponding correlation measure between the two quantities. From the correlation measure thus calculated, a detection signal SAID for an islanding grid is generated, for example, by comparison with a predetermined limit value and provided for transmission within the inverter or at an output or a communication interface.

[0055] Fig. 4 shows the time profiles of variables relevant for understanding the invention when implementing the method according to the invention in the case of a connection of the voltage-impressing inverter executing the method to a higher-level network. The upper part of the diagram shows the time profile of the phase <PNAP am Netzanschlusspunkt und der Zeitverlauf der entsprechenden Phase <p Br shown at the bridge output. The phase <p N In the example shown in Fig. 4, AP at the grid connection point is strictly linear with a slope that depends on the frequency co of the grid. The phase cp Br At the bridge output, the phase difference Acp is offset by a constant phase difference Acp relative to the phase CPNAP at the grid connection point (dashed line), but deviates periodically, here in the form of a triangular oscillation with an amplitude AcpAiD, from this offset. Due to the fact that the periodic deviation of the phase cpBr at the bridge output is not transferred to the course of the phase CPNAP at the grid connection point, so the phase CPNAP at the grid connection point remains unchanged, the difference between the phase cp Br at the bridge output and the phase cp N AP at the grid connection point exhibits a periodic curve with an amplitude AcpAiD.

[0056] The lower part of the diagram shows a time curve of an active power P at the grid connection point corresponding to the phase curve of the upper part of the diagram. The active power has a value that is proportional to the difference between the phase <p Br at the bridge output and the phase < NAP at the grid connection point corresponding periodic curve with an amplitude APAID around a mean value Po, which in turn corresponds to the constant phase difference Acp. The coupling between the curve of the active power P at the grid connection point and the curve of the phase <p BrThe voltage at the bridge output is achieved by the inverter's decoupling reactor located between the bridge output and the grid connection point. A ratio of K = APAID / AC AID corresponds to a correlation measure that can be used to detect an island grid. In the present case of a connection of the voltage-impressing inverter to a higher-level grid, this ratio is greater than a threshold value specified for detecting an island grid, so that this connection to the higher-level grid is recognized. However, it is also conceivable to determine the correlation measure based on the amplitude of APAID and the amplitude of ACPAID using a different relationship.

[0057] Fig. 5 shows the time profiles of variables relevant for understanding the invention when implementing the method according to the invention in the case of a missing or only very weak connection between the voltage-impressing inverter executing the method and a higher-level network. The upper part of the diagram again shows the time profile of the phase <PNAP am Netzanschlusspunkt und der Zeitverlauf der entsprechenden Phase (pß r shown at the bridge output. The phase (pß r at the bridge output deviates again periodically, here in the form of a sawtooth curve with an amplitude A <PAID von einem linearen Verlauf (gestrichelte Linie) ab. Anders als im in Fig. 3 gezeigten Fall verursacht diese Abweichung auch eine Abweichung der Phase <p NAP at the grid connection point follows a linear curve corresponding to the grid frequency co. In other words, the phase CPNAP at the grid connection point does not adhere to the strictly linear curve, but follows to a certain extent the phase cpßr at the bridge output.

[0058] This following can be complete, meaning that the amplitude of the deviation of the phase cpNAP at the grid connection point from a purely linear curve would then correspond to the amplitude AcpAiD of the deviation of the phase cpßr at the bridge output from its linear curve, and the difference between the phase cpßr at the bridge output and the phase CPNAP at the grid connection point would be constant over time. This can be the case, for example, if there are no other voltage-impressing inverters in the island grid that is completely separated from the higher-level grid.

[0059] However, the following of the phase CPNAP at the grid connection point can also be only partial, i.e. the amplitude of the deviation of the phase cp N AP at the grid connection point from a purely linear course would then be smaller than the amplitude AcpAiD of the deviation of the phase cpß r at the bridge output of their linear course and the difference between the phase cpß r at the bridge output and the phase < NAP at the grid connection point would vary periodically with an amplitude smaller than the amplitude AcpAiD of the phase deviation (p Brat the bridge output. This case would occur, for example, if there is still a weak connection to the higher-level grid or if additional voltage-impressing inverters are in operation in an island grid. This case is the basis for Fig. 4. As a consequence of this situation, the curve of the active power P shown in the lower part of the diagram in Fig. 4 has an amplitude APAID around a mean value Po, which is smaller than the amplitude of the active power P shown in Fig. 3 for the case of an existing low-impedance connection to a higher-level grid. Accordingly, a voltage-impedance factor (APAID) can also be calculated, for example, by the relation K= APAID / Aq> A iD a certain correlation measure is reduced compared to the case of Fig. 3 and a limit value below which an island network is to be recognized can easily be defined by the person skilled in the art.

[0060] Fig. 6 shows exemplary time courses of variables relevant for understanding the invention when carrying out the method according to the invention in the case of an island network, wherein a non-periodic variation of the phase (pBr) at the bridge output, here a step function, is used. The phase <PNAP am Netzanschlusspunkt folgt dieser Stufe nicht instantan, sondern nähert sich nur allmählich der vor dem Zeitpunkt der Stufe herrschenden Phasendifferenz Acp wieder an. Entsprechend weist der Verlauf der Wirkleistung P einen zu der Höhe Aq> AiD of the stage corresponding to the jump AP, with the curve then approaching the mean value Po of the active power before the jump. In this case, the correlation measure with which the presence of an islanded grid can be detected can be determined, for example, as the time period in which the deviation APAID from the mean value Po of the active power has fallen below a specified percentage of the jump height AP. Other methods of determining the correlation measure are also conceivable.

[0061] In the case of an existing low-impedance connection of the voltage-impressing inverter to a higher-level network, the phase <PNAP am Netzanschlusspunkt auf dem linearen Verlauf verharren, so dass der Sprung AP in der Wirkleistung, der durch die Stufe in der Phase (pBr am Brückenausgang verursacht wird, nicht oder nur sehr langsam abgebaut wird.

[0062] Of course, other periodic or non-periodic variations of the phase (p Br at the bridge output and adapted determination methods for a correlation measure or adapted limit values ​​for distinguishing between an island grid and a sub-grid connected to a higher-level grid with sufficiently low impedance by the voltage-impressing inverter are applied without departing from the scope of the claims.

[0063] 10 subnetwork

[0064] 11 Network

[0065] 12 T ransformer

[0066] 13 disconnect switches

[0067] 14 inverters

[0068] 15 inverters

[0069] 16 Last

[0070] 20 Phase evaluation unit

[0071] 21 PWM generator

[0072] 22 bridge circuit

[0073] 23 performance determiners

[0074] 24 characteristic curve unit

[0075] 25 Correlator

[0076] 26 Generator

[0077] 27 Control unit

[0078] 28 measuring units

[0079] 29 intermediate circuit

[0080] 30 network parameters

[0081] 31 Decoupling choke

Claims

Patent claims:

1. Method for islanding detection by a voltage-imprinting inverter (14), comprising the steps: - Determining the frequency and phase of a voltage waveform of a subnetwork connected to a grid connection point of the inverter (10), - Generating a voltage waveform by means of a bridge circuit of the inverter (14) with the specified frequency, wherein the phase of the generated voltage waveform exhibits a first temporal variation relative to the specified phase, - Determining a correlation between the first temporal variation and a second temporal variation of power exchanged by the inverter (14) with the subnetwork (10), and - Detecting an island network when the specific correlation falls below a predetermined correlation measure.

2. The method according to claim 1, wherein the method is repeated at time intervals.

3. Method according to claim 1 or 2, wherein in the event that an island grid is detected, the inverter (14) disconnects from the subnetwork (10).

4. Method according to claim 3, wherein, in the event that an island grid is detected, the inverter (14) is operated in such a way that it aims for an amplitude and / or a frequency of the subnetwork (10) that is outside of permissible grid parameters in order to effect a separation of further grid components connected to the subnetwork (10) before the inverter (14) disconnects from the subnetwork (10).

5. Method according to one of the preceding claims, wherein a detection signal is provided at a signal output of the inverter (14) in the event of detection of an island grid.

6. Method according to one of the preceding claims, wherein the inverter (14) modulates a detection signal as a PLC signal onto the voltage at the grid connection point in the event of detection of an island grid.

7. Method according to one of the preceding claims, wherein the first temporal variation has a periodic variation with a first component at a first variation frequency.

8. Method according to claim 7, wherein an island grid is detected when a ratio between a component of the active power at the first variation frequency and The first component of the first temporal variation, as a correlation measure, falls below a predetermined limit. A method according to claim 7, wherein the first temporal variation includes a second component with a second variation frequency higher than the first variation frequency, wherein an island grid is detected if the ratio between a component of the power at the first variation frequency and a component of the power at the second variation frequency falls below a predetermined limit. A method according to any of the preceding claims, wherein generating the voltage waveform includes an increase in the amplitude of the first temporal variation, wherein the method is terminated without detecting an island grid if the variation of the power exchanged by the inverter with the subnetwork (10) exceeds a predetermined limit.A method according to claim 10, wherein the amplitude of the first temporal variation upon termination of the method without detection of an island grid is used to determine a starting amplitude of the first temporal variation in a subsequent execution of the island grid detection method. A method according to any of the preceding claims, wherein determining the frequency and phase of a voltage waveform of the subnetwork (10) connected to the grid connection point of the inverter (14) comprises determining a third temporal variation of the phase, wherein the generation of the voltage waveform is performed with a temporal variation that is synchronous with the determined third temporal variation of the phase.A method according to any one of the preceding claims, wherein the subnetwork (10) is a network with a first and a second phase conductor, wherein the method is executed simultaneously or sequentially on the first and the second phase conductor, wherein an island network is detected if a phase island network is detected on at least one of the phase conductors. A method according to any one of the preceding claims, wherein the subnetwork (10) is a network with a first and a second phase conductor, wherein the method is executed simultaneously or sequentially on the first and the second phase conductor, wherein an island network is detected only if a phase island network is detected on both the first and the second phase conductors. A method according to any one of the preceding claims, wherein active power is used to determine the correlation between the first and second time variations of the power exchanged by the inverter (14) with the subnetwork (10). A method according to any one of claims 1 to 13, wherein apparent power or a weighted square mean of active and reactive power is used to determine the correlation between the first and second time variations of the power exchanged by the inverter (14) with the subnetwork (10). A voltage-imping inverter (14) comprising: - a measuring unit (20) for determining a frequency and phase of a voltage profile of a subnetwork (10) connected to a grid connection point of the inverter (14), as well as active power and reactive power values ​​exchanged with the subnetwork (10), - a control unit (21) for generating switching commands for a bridge circuit (22) of the inverter (14), such that a voltage profile specified by the control unit (21) is generated at a bridge output of the bridge circuit (22) with the specified frequency, wherein the control unit (21) is configured to provide the specified voltage profile with a first temporal variation of the phase relative to the phase of the specified voltage profile, and - a correlator (24) for determining a correlation measure between the first temporal variation of the phase and a second temporal variation of a power exchanged by the inverter (14) with the subnetwork (10), wherein the correlator (24) is configured to detect an island grid when the determined correlation measure falls below a predetermined limit value.

Images