US-12620812-B2 - Island network detection by voltage source inverters, and inverter

US12620812B2US 12620812 B2US12620812 B2US 12620812B2US-12620812-B2

Abstract

The disclosure is directed to an island network detection method using a voltage-impressing converter, and includes determining the frequency and phase of a voltage curve of a sub-network connected to a network connection point of the converter, and generating a voltage curve using a bridge circuit of the converter with the determined frequency. The method also includes determining a correlation between a first temporal variation and a second temporal variation of an output exchanged by the converter with the sub-network, and detecting an island network if the determined correlation undershoots a specified correlation measurement.

Inventors

Björn Oliver Winter

Assignees

SMA SOLAR TECHNOLOGY AG

Dates

Publication Date: 20260505
Application Date: 20240606
Priority Date: 20220221

Claims (17)

1 . A method for detecting an island network using a voltage source inverter, comprising: determining a frequency and a phase of a voltage curve of a subnetwork connected to a network connection terminal of the voltage source inverter, generating, by a bridge circuit of the voltage source inverter, another voltage curve having the determined frequency, wherein a phase of the generated another voltage curve comprises a first temporal variation with respect to the determined phase, determining a correlation between the first temporal variation and a second temporal variation in a power exchanged between the inverter and the subnetwork, and detecting an island network when the determined correlation does not reach a predefined degree of correlation.
2 . The method according to claim 1 , wherein the method is repeated at time intervals.
3 . The method according to claim 1 , further comprising disconnecting the voltage source inverter from the subnetwork when an island network is detected.
4 . The method according to claim 3 , further comprising, when an island network is detected, operating the voltage source inverter so that it drives an amplitude and/or a frequency of the subnetwork to be outside of one or more permissible network parameters in order to cause disconnection of further network components connected to the subnetwork before the inverter disconnects from the subnetwork.
5 . The method according to claim 1 , further comprising providing a detection signal at a signal output terminal of the voltage source inverter when an island network is detected.
6 . The method according to claim 1 , further comprising modulating, via the voltage source inverter, a detection signal, as a PLC signal, onto a voltage at the network connection terminal when the island network is detected.
7 . The method according to claim 1 , wherein the first temporal variation comprises a periodic variation having a first component at a first variation frequency.
8 . The method according to claim 7 , wherein an island network is detected when a ratio between a component of an active power of the power exchanged between the voltage source inverter and the subnetwork at the first variation frequency and the first component of the first temporal variation defines the determined correlation and does not reach a predefined limit value.
9 . The method according to claim 7 , wherein the first temporal variation comprises a second component having a second variation frequency higher than the first variation frequency, and wherein an island network is detected when a ratio between a component of the power exchanged between the inverter and the subnetwork at the first variation frequency and a component of the power exchanged between the voltage source inverter and the subnetwork at the second variation frequency does not reach a predefined limit value.
10 . The method according to claim 1 , wherein generating the voltage curve comprises increasing an amplitude of the first temporal variation, and further comprising terminating the method without detecting an island network when the temperol variation in a power exchanged between the voltage source inverter and the subnetwork exceeds a predefined value.
11 . The method according to claim 10 , further comprising, when the method is terminated without detecting an island network, using the amplitude of the first temporal variation to determine a starting amplitude of the first temporal variation when subsequently carrying out the island network detection method.
12 . The method according to claim 1 , wherein determining the frequency and the phase of the voltage curve of the subnetwork connected to the network connection point of the voltage source inverter comprises determining a third temporal variation in the phase, wherein the another voltage curve is generated having a temporal variation synchronous with the determined third temporal variation in the phase.
13 . The method according to claim 1 , wherein the subnetwork is a network having a first phase conductor and a second phase conductor, wherein the method is performed on the first phase conductor and the second phase conductor concurrently or one after the other, and wherein an island network is detected when a phase island network is detected on at least one of the first and second phase conductors.
14 . The method according to claim 1 , wherein the subnetwork is a network having a first phase conductor and a second phase conductor, wherein the method is performed on the first phase conductor and the second phase conductor concurrently or one after the other, wherein an island network is detected only when a phase island network is detected on both the first phase conductor and the second phase conductor.
15 . The method according to claim 1 , wherein an active power is used for determining the correlation between the first temporal variation and the second temporal variation in the power exchanged between the voltage source inverter and the subnetwork.
16 . The method according to claim 1 , wherein an apparent power or a weighted root mean square of an active power and a reactive power is used for determining the correlation between the first temporal variation and the second temporal variation in the power exchanged between the inverter and the subnetwork.
17 . A voltage source inverter, comprising: a measuring circuit configured to determine a frequency and a phase of a voltage curve of a subnetwork connected to a network connection terminal of the voltage source inverter, and further configured to determine values of active power and reactive power exchanged with the subnetwork, a control circuit configured to generate switching commands for a bridge circuit of the voltage source inverter such that another voltage curve predefined by the control circuit and having the determined frequency is generated at a bridge output of the bridge circuit, wherein the control circuit is configured to provide the predefined another voltage curve comprising a first temporal variation in the phase with respect to the phase of the determined voltage curve, and a correlator device configured to determine a degree of correlation between the first temporal variation in the phase and a second temporal variation in a power exchanged between the voltage source inverter and the subnetwork, wherein the correlator device is configured to detect an island network when the determined degree of correlation does not reach a predefined limit value.

Description

REFERENCE TO RELATED APPLICATIONS This application is a Continuation of International Application number PCT/EP2023/052637, filed on Feb. 3, 2023, which claims the benefit of German Application number 10 2022 104 015.1, filed on Feb. 21, 2022. The contents of the above-referenced patent applications are hereby incorporated by reference in their entirety. FIELD The disclosure relates to a method for detecting an island network by a voltage source inverter and to an inverter of this kind. BACKGROUND Decentralized energy production by renewable energy sources, in particular by photovoltaics, is becoming increasingly important for the energy transition. In this respect, the component of electrical power generated in a decentralized manner in relation to power generated by power plants is constantly increasing. One concern here is that this increase in decentralized power generation raises the risk of power failures. Using voltage source or network-forming inverters instead of line-commutated inverters—the latter are also called current source inverters or current converters for short—can make a significant contribution to stabilizing the networks. Voltage source inverters are configured to provide their power by providing at their bridge output a voltage curve which is continually synchronized with a voltage curve at a network connection point of the inverter. Since a decoupling impedance is arranged between the bridge output and network connection terminal of the inverter, a defined active and reactive power exchanged with the network can be adjusted by a predefined, continually, that is to say continuously or repeatedly, updated phase and amplitude offset between the voltages at the bridge output and network connection point. For instance, EP 2 003 759 A1 discloses a method for island network detection for voltage source inverters, in which method a test voltage is superimposed on the network frequency, the test frequency of the test voltage being less than the network frequency. An island network is detected if a reactive power component assigned to the test voltage falls below a predefined minimum. For networks having a network impedance with a prevailing inductive component, the exchanged active power is adjusted or determined by a continually updated phase difference between the voltages at the bridge output and network connection point or terminal, and the reactive power is adjusted or determined by a continually updated amplitude difference between the two voltages. For networks having a network impedance with a prevailing resistive component, the role of reactive power and active power is typically reversed. Hybrid forms are likewise conceivable. As a result, voltage source inverters are capable of independently establishing an AC voltage network and maintaining it in a stable manner. Due to this property, however, there arises the risk that voltage source inverters also maintain the voltage in a subnetwork to which they are connected if this subnetwork is unexpectedly or forcedly disconnected from a superordinate network and for this case maintaining the voltage is undesired. In this case, the voltage source inverters readily form an island network provided that they are capable of providing the power required therefor. This can constitute a hazard if, for example, a subnetwork is disconnected from a superordinate network for maintenance purposes. It may therefore be necessary for voltage source inverters to be capable of independently detecting, within a predefined period of time, that they are connected to an island network and react to this appropriately. Such a requirement of island network detection is customary for line-commutated inverters and is described, for example, in the technical application standard VDE_AR-N 4105 for power generating plants in the low voltage network. Corresponding methods are known. These methods cannot be readily transferred to voltage source inverters, however. SUMMARY The present disclosure is directed to a method with which voltage source inverters can reliably detect whether they are connected to an island network. A method according to one embodiment of the disclosure for detecting an island network by a voltage source inverter comprises: determining a frequency and a phase of a voltage curve of a subnetwork connected to a network connection point of the inverter,generating, by a bridge circuit of the inverter, another voltage curve having the determined frequency, wherein the phase of the generated another voltage curve comprises a temporal variation with respect to the determined phase,determining a correlation between the temporal variation and a temporal variation in a power exchanged between the inverter and the subnetwork, anddetecting an island network when the determined correlation does not reach a predefined degree of correlation. In one embodiment, the power exchanged between the inverter and the subnetwork can be the active power, for example, for net