Method for operating an island network, and short-circuit current source

Abstract

The application relates to a method for operating an electrical island network (40) having at least one load (28) by means of an energy supply system (30), wherein the energy supply system (30) has a first energy source (22), a network-forming power-electronics inverter (20) and a short-circuit current source (10) having a bridge circuit (12), the method comprising: • operating the energy supply system (30) in an energy supply mode (EVM), wherein in the energy supply mode (EVM), the inverter (20) supplies the island network (40) with electrical power from the first energy source (22) at a supply voltage (U_n) and a nominal frequency (f_n), • operating the energy supply system (30) in a fault mode (FM), wherein in the fault mode (FM), the short-circuit current source (10) feeds a substantially square-wave short-circuit current (I_GTO) into the island network (40) by means of the bridge circuit (12). The application also relates to a short-circuit current source (10), to an energy supply system (30) and to an island network (40).

Description

[0001] 24-041 -P-WO - 1 - submitted version

[0002] METHOD FOR OPERATING AN ISLAND GRID AND SHORT-CIRCUIT SOURCE

[0003] TECHNICAL AREA

[0004] The application concerns a method for operating an island grid to supply consumers with electrical power. The application further concerns a short-circuit current source for the island grid.

[0005] STATE OF THE ART

[0006] Island grids are electrical distribution networks for supplying consumers with electrical power. Such island grids can be supplied by local power generators or power generation plants, which, depending on the demand for electrical power in the island grid, use a converter or inverter to generate an alternating voltage as the supply voltage of the island grid and feed an alternating current into the island grid.

[0007] In island grids, the short-circuit current—that is, the current fed in by the generators in the island grid in the event of a short circuit—may be too low to activate a fuse located between the generators and the short circuit. The fuse may be located at a load in the island grid and / or protect a section of the grid. The purpose of the fuse is to isolate a faulty component and / or a faulty section of the grid by triggering it in the event of a short-circuit current flowing to the fault. Failure of the fuse to trigger in the event of a fault can lead to damage to grid components or a complete grid outage.

[0008] This problem of fuses failing to trip in the event of a fault is particularly well-known in island grids where power is supplied by a distributed energy system (DES). Especially when an island grid is supplied by one or more grid-forming inverters with low overcurrent capability, the short-circuit current available from the distributed energy generation during a fault is only slightly more than the rated current of the respective distributed energy generators. Consequently, the necessary short-circuit current for the rapid tripping of the fuses typically used in distribution networks cannot be supplied in the event of a fault.

[0009] Decentralized energy systems are energy supply systems that produce energy on a small or medium scale in close proximity to the point of consumption, which can increase efficiency and reduce energy losses that would occur during long-distance transmission. Examples of decentralized energy systems include photovoltaic systems (24-041-P-WO - 2 - submitted version).

[0010] Photovoltaic (PV) systems, wind turbines, combined heat and power plants, biogas plants. In particular, PV systems and wind turbines used converters or inverters with power electronic circuits to exchange electrical power with the island grid.

[0011] From EP 3 308 438 A1 it is known to arrange an additional induction machine in a power electronics-dominated power network with limited fault current amplitude, with which an additional fault current can be generated in order to reliably ensure a fault current sufficient to trigger a fuse in the event of a fault.

[0012] From EP 2 306 607 A2 it is known that fuses in an island grid with predominantly power electronic energy supply may not trip reliably if the short-circuit current is too low. Therefore, EP 2 306 607 A2 discloses a network protection device which, in the event of a voltage dip in the island grid, feeds a portion of the electrical energy stored in an energy storage device into the distribution network, generating a discharge current that is at least as high as the activation threshold of the fuse with the highest activation threshold in the distribution network.

[0013] From EP 2290774 A1, a fault current source is known that comprises an energy storage device, a converter, a detector, and a controller. To increase the short-circuit current, EP 2290774 A1 discloses feeding energy from the energy storage device into the grid in the event of a fault. A fault is detected by the detector as a voltage dip in the grid, indicating a short circuit in the connected power grid, and the controller discharges at least a portion of the energy stored in the energy storage device into the island grid. A discharge current is generated that is at least high enough to activate the safety device with the highest activation threshold, i.e., to trip the "largest" fuse in the power grid.

[0014] TASK

[0015] The application is based on the task of providing a method and a short-circuit current source that enable the safe and efficient operation of an island grid.

[0016] SOLUTION

[0017] The problem is solved by a method having the features of independent claim 1 and a short-circuit current source having the features of independent claim 13. Embodiments of the application are specified in the dependent claims.

[0018] DESCRIPTION

[0019] An electrical island grid comprises at least one consumer and at least one power supply system. The power supply system comprises a first power source, a grid-forming power electronic inverter (24-041-P-WO-3, submitted version), and a short-circuit current source with a bridge circuit. A method for operating the electrical island grid comprises:

[0020] • Operating the power supply system in a power supply mode, wherein the inverter in power supply mode supplies the island grid with electrical power from the first power source at a supply voltage and a nominal frequency.

[0021] • Operating the power supply system in a fault mode, whereby the short-circuit current source feeds an essentially rectangular short-circuit current into the island grid by means of the bridge circuit in fault mode.

[0022] The island grid has at least one consumer that, in power supply mode, is supplied with electrical power from the first energy source via the inverter. The first energy source can be, for example, a photovoltaic generator and / or an energy storage device such as a battery. The power supply system can be designed, in particular, as a decentralized energy system with a renewable energy source.

[0023] The inverter converts a direct current (DC) voltage from the first power source into an alternating current (AC) voltage, which serves as the supply voltage for the island grid, and a direct current (DC) current from the first power source into an alternating current (AC). The corresponding power conversion from DC power from the first power source into grid-forming AC power for supplying the island grid takes place within the inverter, for example, by means of a bridge circuit with controllable power switches. The power conversion can be controlled, for example, by switching the power switches of the bridge circuit. The characteristics of the supply voltage, and in particular the frequency of the supply voltage, can be adjusted by controlling the power switches. The frequency of the supply voltage is also referred to as the nominal frequency.

[0024] In power supply mode, the inverter acts as the main grid-forming power source for the island grid by feeding in grid-forming AC power, while in fault mode, the short-circuit current source acts as a grid protection device by feeding in a short-circuit current. This short-circuit current is also inherently an alternating current. Therefore, in the event of a fault, the island grid operates in fault mode, with the short-circuit current being supplied by a different source than the supply voltage and thus the AC power for loads in power supply mode.

[0025] The short-circuit current differs significantly in at least one characteristic, such as its amplitude, from the alternating currents that flow as supply currents in the power supply mode (24-041-P-WO-4, submitted version) to supply consumers in the island grid. The short-circuit current is designed so that the protection mechanisms provided for the island grid, such as fuses, can be activated by the short-circuit current. In particular, the amplitude of the short-circuit current can be significantly larger than the possible amplitudes of the supply current in power supply mode, which are limited, in particular, by a rated current or rated power of the inverter of the power supply system. The frequency of the short-circuit current can correspond to the nominal frequency.

[0026] By providing a separate short-circuit current source, the inverter can be designed to efficiently create and supply the island grid by maintaining the supply voltage within its intended limits and providing sufficient supply current, at least as long as no fault, and in particular no short circuit, occurs in the island grid. The task of providing a short-circuit current sufficient to trip fuses can be transferred to the short-circuit current source. This, in turn, can be designed to efficiently provide the short-circuit current with the necessary amplitude for rapid fuse tripping.

[0027] The switch from power supply mode to fault mode can be triggered, in particular, by a voltage dip and / or a sharp increase in current in the island grid. After detecting the voltage dip and / or the increase in supply current, the system can switch from power supply mode to fault mode. Specifically, a measured value of the output current of the grid-forming inverter can be used to detect the fault. The inverter's specific output current can be used for this purpose. This simultaneously protects the inverter from uncontrolled overcurrents by generating an overcurrent, especially a short-circuit current, through the short-circuit current source.

[0028] In fault mode, the short-circuit current source preferably feeds a substantially rectangular alternating current into the island grid as the short-circuit current. The substantially rectangular shape of the short-circuit current offers the advantage that the short-circuit current source can be designed cost-effectively using a low-frequency switched bridge circuit without further control. This allows, in particular, the effort required to shape the short-circuit current to be minimized, and, for example, pulse-width modulation can be omitted.

[0029] In one embodiment, the rectangular short-circuit current fed into the island grid by the short-circuit current source in fault mode has an amplitude that is greater than a rated current of at least one melting point for at least a predefinable duration. 24-041 -P- WO - 5 - submitted version

[0030] The fuse in the island grid is a type of fuse, in particular one that is located in a current path to which at least one consumer is connected.

[0031] The short-circuit current is designed so that the fuses provided for safety in the island grid are activated by the short-circuit current. These fuses are designed to activate when a current exceeding their rated current for tripping is conducted through them. If the amplitude of the short-circuit current is significantly greater than the rated current, the fuse can trip, and the associated load or network section can be safely disconnected from the island grid. To increase safety, the duration for which the amplitude of the short-circuit current exceeds the rated current is predefined to ensure reliable fuse tripping. This predefined duration can therefore depend on the type of fuse.

[0032] In particular, the substantially rectangular short-circuit current fed into the island grid by the short-circuit current source in fault mode (FM) can have an amplitude that is at least twice, preferably at least five times, greater than the rated current of the grid-forming inverter for a predefinable duration. If several grid-forming inverters supply the island grid, the relevant rated current is the sum of the rated currents of the inverters. This ensures that the short-circuit current reliably reaches a sufficient amplitude to trip a fuse that is essentially designed for the rated current of the grid-forming inverter.

[0033] In one embodiment, the bridge circuit of the short-circuit current source is clocked at the nominal frequency of the island grid in fault mode. The short-circuit current source includes the bridge circuit, which, in fault mode, can provide the rectangular short-circuit current at its output. The bridge circuit comprises controllable, and in particular, clockable circuit breakers. The bridge circuit can convert a direct current at the input of the short-circuit current source into the alternating current for the short-circuit current. The characteristics of the short-circuit current can be adjusted by clocking the circuit breakers. A substantially rectangular current can be generated by simply clocking the circuit breakers of the bridge circuit at a low frequency, thus eliminating the need for further control or regulation within the short-circuit current source.The clocking can have essentially the same clock frequency as the supply voltage and a substantially constant duty cycle. 24-041 -P- WO - 6 - submitted version.

[0034] In one embodiment of the method, the bridge circuit of the short-circuit current source, in fault mode, converts a DC voltage and a DC current from a second power source into a square wave voltage and the square wave short-circuit current. The amplitude of the square wave voltage can be smaller than the amplitude of the supply voltage in fault mode. However, the DC voltage of the second power source, in the unloaded state, can be equal to or greater than the amplitude of the supply voltage.

[0035] The short-circuit current source can thus act as a network protection device, drawing electrical current from the second power source in fault mode and feeding it into the island grid. The electrical current fed into the island grid can be particularly large enough to, for example, reliably and quickly trip, i.e., activate, a (fuse) circuit breaker.

[0036] The second energy source can, for example, include an energy storage device connected to the short-circuit current source. This second energy source can be, for example, a capacitor, particularly a supercapacitor, and / or a battery. A battery offers high reliability and can be charged from the island grid using a dedicated charging circuit. A self-sufficient setup is easily achievable with a capacitor, especially by charging it to the island grid's peak voltage via a simple rectifier circuit.A supercapacitor, also known as an ultracapacitor or double-layer capacitor, is a special type of capacitor with a high energy density and is particularly suitable as an energy source for providing high short-circuit currents that occur relatively quickly in the event of a fault, namely until the fuse is activated and therefore require manageable amounts of energy.

[0037] In one embodiment, the grid-forming inverter, operating in power supply mode, converts a DC voltage from the first energy source into a substantially sinusoidal supply voltage and feeds a substantially sinusoidal supply current into the island grid. The inverter thus generates a nearly sinusoidal supply voltage and a substantially sinusoidal supply current from the DC voltage and DC current of the first energy source, e.g., a PV generator. The inverter is designed to form a grid and, for example, exhibits voltage source behavior to maintain a high-quality supply voltage and reliably provide the supply current, thereby enabling stable island grid operation.

[0038] In one embodiment of the method, the short-circuit current source is connected to the island grid via a switch arrangement when switching from power supply mode to fault mode, and when switching from fault mode to power supply mode via the version 24-041 -P- WO - 7 - submitted.

[0039] The switch arrangement is isolated from the island grid. Optionally, the fault detection in the island grid can be integrated into the circuitry of the switch arrangement, and the connection of the short-circuit current source can be implemented as a consequence of the detection.

[0040] The switching arrangement can be controlled, for example, by analog logic that uses a measure of the supply current delivered by the inverter, the rated current of the inverter, and / or a measure of the short-circuit current delivered by the short-circuit current source as input values. In particular, the switching arrangement can comprise two antiparallel thyristors and be controlled by a control signal. The control signal controls the disconnection and connection of the short-circuit current source to the island grid by means of the switching arrangement. The control signal can, in particular, depend on the amplitude of the sinusoidal supply current and be triggered when a high current occurs in the island grid, which in particular exceeds the rated current of the inverter and thus indicates a fault. The thyristors of the switching arrangement can, in particular, be GTO thyristors (gate turn-off thyristors).A GTO thyristor is a thyristor that, like a normal thyristor, can be switched on with a positive current pulse at the control input – the gate. Unlike a simple thyristor, it can also be switched off by means of a negative current pulse.

[0041] In one embodiment, the switch assembly can then be switched on by means of the control signal. Switching on the switch assembly connects the short-circuit current source to the island grid. Switching off the switch assembly disconnects the short-circuit current source from the island grid. Optionally, switching off the switch assembly can be linked to connecting the short-circuit current source to the island grid via a pre-charging device, in particular via a rectifier.

[0042] The system switches to fault mode when the amplitude of the sinusoidal supply current exceeds a first predefinable limit, where the predefinable limit is preferably between 10% and 50% greater than the inverter's rated current. If the supply current in the island grid becomes too high, a fault condition is assumed, the system switches to fault mode, and the short-circuit current source feeds the essentially rectangular short-circuit current with a significantly larger amplitude than the rated current into the island grid.

[0043] In one embodiment of the method, the switch arrangement remains switched on, i.e., the short-circuit current source remains connected to the island grid, as long as the amplitude of the substantially rectangular short-circuit current exceeds a second predefinable limit value, wherein the second limit value is preferably between 80% and 100% of the nominal value. 24-041-P-WO-8-filed version

[0044] The inverter's current is... The island grid therefore continues to operate in fault mode as long as the short-circuit current exceeds the second limit value and it is assumed that the fuse has not yet tripped and the short-circuit current is actually flowing through the short circuit. If the fuse trips, the short-circuit current is interrupted and the short-circuit current fed in by the short-circuit current source drops.

[0045] In one embodiment, the system switches from fault mode back to power supply mode when the short-circuit current falls below the second predefined limit. This recognizes that the fault condition no longer exists due to the fuse tripping. Upon switching from fault mode to power supply mode, the switch assembly is deactivated and the short-circuit current source is disconnected from the island grid.

[0046] Optionally, the bridge circuit of the short-circuit current source can continue to be clocked at the nominal frequency of the island grid even when disconnected from the island grid, even in power supply mode. The switching of the circuit breakers in the bridge circuit of the short-circuit current source can continue continuously in both fault mode and power supply mode. In power supply mode, the clocking of the short-circuit current source can therefore run synchronously with the island grid, even without the short-circuit current source feeding electrical power into the island grid. This enables efficient operation of the short-circuit current source, and the bridge circuit of the short-circuit current source can also receive operating parameters from the inverter. Furthermore, the bridge circuit of the short-circuit current source can quickly deliver the short-circuit current when needed, as the clocking does not have to be initiated only when switching to fault mode.

[0047] In one embodiment of the method, the supply of electrical power by the grid-forming inverter is stopped when switching from power supply mode to fault mode. In particular, the conversion of electrical power by the inverter can remain stopped during operation in fault mode. The power conversion can be stopped, and remain stopped, particularly by stopping the switching of the inverter's bridge circuit, while the island grid is operating in fault mode.

[0048] The short-circuit current source for operation in the electrical island grid is designed to feed the essentially rectangular short-circuit current into the island grid as needed. The short-circuit current is generally an alternating current. The need to feed in the short-circuit current can arise, for example, when a fault occurs in the island grid. Such a fault can be detected, for example, by comparing the amplitude of the supply current (24-041 -P- WO - 9 - submitted version).

[0049] The current of the inverter and / or the amplitude of the supply voltage can be detected with a respective limit value.

[0050] In one embodiment of the short-circuit current source, the substantially rectangular short-circuit current fed into the island grid in the event of a fault has an amplitude that is higher than a predefinable rated current for at least a predefinable duration. The predefinable rated current can, in particular, depend on the rated current of at least one fuse in the island grid, which is, for example, arranged in a current path leading to the at least one load of the island grid.

[0051] In one embodiment, the short-circuit current source comprises a bridge circuit. The short-circuit current source can provide the substantially rectangular short-circuit current at its output via the bridge circuit. The bridge circuit includes controllable bridge switches, which are particularly capable of being clocked.

[0052] In one embodiment, the bridge circuit of the short-circuit current source is configured to convert a DC voltage from a second energy source into a square wave voltage when the short-circuit current is injected as needed. The amplitude of the square wave voltage can be smaller than the DC voltage of the second energy source in fault mode due to the flowing short-circuit currents. Thus, the essentially rectangular short-circuit current can be generated using the square wave voltage in the event of a fault.

[0053] In one embodiment, the short-circuit current source comprises the second energy source. The second energy source can, for example, be a capacitor and / or a battery. In this embodiment, the DC voltage of the second energy source in the unloaded state can be greater than the AC voltage amplitude in the island grid.

[0054] The bridge circuit of the short-circuit current source features controllable circuit breakers. The bridge circuit can be clocked, for example, at the nominal frequency of the island grid's supply voltage. The frequency of the short-circuit current can then correspond to the island grid's nominal frequency. The bridge circuit can, for example, convert direct current at the input of the short-circuit current source into the rectangular short-circuit current. The characteristics of the short-circuit current can be adjusted by controlling the switching of the circuit breakers. An essentially rectangular current can be generated simply by switching the circuit breakers of the bridge circuit, eliminating the need for further control within the short-circuit current source and, in particular, for current shaping, e.g., by pulse-width modulation. The bridge circuit can, for example,The bridge circuit of the short-circuit current source can be clocked at the nominal frequency of the island grid's supply voltage and with a fixed duty cycle as specified in the 24-041-P-WO-10 version. The clocking of the bridge circuit can therefore be synchronized with the island grid.

[0055] The bridge circuit of the short-circuit current source can be unregulated. No feedback loop is needed to ensure the quality of the output voltage, and it is sufficient to clock the bridge circuit at the nominal frequency, e.g., 50 Hz, so that the short-circuit current source can be generated with relatively simple means.

[0056] In one embodiment of the short-circuit current source, the bridge circuit incorporates thyristors as bridge switches. The bridge circuit is clocked at the nominal frequency of the island grid to generate the square wave voltage and the substantially rectangular short-circuit current. The thyristors in the bridge circuit can, in particular, be GTO thyristors (gate turn-off thyristors).

[0057] The power supply system for supplying the island grid with electrical power includes the first power source, the grid-forming inverter, the described short-circuit current source, and the switch arrangement for connecting and disconnecting the short-circuit current source from the island grid as required.

[0058] In one embodiment of the power supply system, the grid-forming inverter is configured to output a supply voltage with a nominal frequency to supply the island grid with electrical power in power supply mode. The short-circuit current source, on the other hand, is configured to feed the essentially rectangular short-circuit current into the island grid in fault mode.

[0059] In one embodiment of the power supply system, the amplitude of the short-circuit current is greater than the rated current of at least one fuse assigned to a consumer in the island grid for at least a predefinable duration. Additionally, the amplitude of the short-circuit current can be greater than the rated current of the grid-forming inverter by at least a factor of 2, preferably at least a factor of 5, for at least a predefinable duration.

[0060] In one embodiment of the power supply system, the supply voltage is essentially sinusoidal and the grid-forming inverter is configured to feed the essentially sinusoidal supply current into the island grid.

[0061] In one embodiment of the power supply system, the second energy source is included in the first energy source. In this embodiment, the energy for supplying the at least one consumer with electrical power in power supply mode and the energy for the short-circuit current in fault mode can therefore be drawn from the same source (24-041-P-WO-11).

[0062] Energy source is drawn, whereby the inverter, due to its design, can feed a maximum nominal current into the island grid, but the short-circuit current source can supply a short-circuit current with a much higher amplitude.

[0063] In one embodiment, the short-circuit current source can be integrated into the inverter. This integration can be physical and / or electrical. In particular, in this embodiment, the first energy source can encompass the second energy source, or vice versa. Specifically, in this embodiment, the first energy source can be identical to the second energy source.

[0064] The island grid comprises the described power supply system and at least one electrical consumer. The consumers of the island grid can be connected to the power supply system individually or collectively via at least one fuse.

[0065] BRIEF DESCRIPTION OF THE FIGURES

[0066] The registration process is further explained and described below using the examples shown in the figures.

[0067] Fig. 1 schematically shows a method for operating an island network.

[0068] Fig. 2 schematically shows an embodiment of an island network.

[0069] Fig. 3 schematically shows an embodiment of a switch arrangement.

[0070] Fig. 4 schematically shows an embodiment of a short-circuit current source.

[0071] Fig. 5 schematically shows current and voltage waveforms in the island grid.

[0072] The same reference symbols are used in the figures for identical or similar elements. The representations in the figures cannot be to scale.

[0073] FIGURE DESCRIPTION

[0074] Fig. 1 schematically shows a method for operating an electrical island grid 40 according to Fig. 2. The island grid 40 has consumers 28 and a power supply system 30. The power supply system 30 has a first power source 22, a grid-forming power electronic inverter 20, and a short-circuit current source 10.

[0075] After the start of the procedure according to Fig. 1, the power supply system 30 of the island grid 30 is operated in a power supply mode EVM. In the power supply mode EVM, the inverter 20 supplies the island grid 40 with electrical power from the first energy source 22. The inverter establishes the electrical network of the island grid 40 by setting a supply voltage U_n with a nominal frequency f_n and feeding a supply current l_WR into the island grid 40. The nominal frequency f_n can be, for example,

[0076] B. 50 Hz and correspond to the frequency of public power grids. The short-circuit current source 10 is disconnected from the island grid 40 in power supply mode EVM.

[0077] In energy supply mode EVM, the supply current l_WR, which the inverter 20 feeds into the island grid 40, is monitored. If the supply current l_WR exceeds a first predefined limit – path “+” – the system switches to fault mode FM via S10 and S12. The first predefined limit for the supply current l_WR can be, for example, between 110% and 200%, and in particular between 110% and 150%, of the rated current l_n of the inverter 20.

[0078] In S10, the supply of electrical power to the island grid 40 by the inverter 20 is stopped. For example, the switching of a bridge circuit of the inverter 20 can be stopped in S10.

[0079] In S12, the short-circuit current source 10 is connected to the island grid 40. The connection can be made, for example, via a switch arrangement 16.

[0080] In fault mode FM, the power supply system 30 of the island grid 40 is operated such that the short-circuit current source 10 feeds a substantially rectangular short-circuit current l_GTO into the island grid 40. The inverter 20 may be inactive in fault mode FM.

[0081] In fault mode FM, the short-circuit current l_GTO, which the short-circuit current source 10 feeds into the island grid 10, is monitored. As long as the short-circuit current l_GTO exceeds the second predefined limit value (path "-"), the power supply system 30 remains in fault mode FM via S10 and S12. In S10, the supply of electrical power to the island grid 40 by the inverter 20 remains stopped, in particular by stopping the switching of the bridge circuit of the inverter 20. The short-circuit current source 10 remains connected to the island grid 40 in S12, e.g., via the switch arrangement 16.

[0082] If the short-circuit current l_GTO falls below a second predefined limit (path "+"), the system switches back to energy supply mode EVM via S20 and S22. The second predefined limit is, for example, between 80% and 100% of the rated current l_n of inverter 20, specifically at approximately 90% of the rated current l_n of inverter 20. 24-041 -P- WO - 13 - submitted version

[0083] Fig. 2 schematically shows an embodiment of an island grid 40. The island grid 40 comprises consumers 28, also called loads, and a power supply system 30, wherein the consumers 28 are connected to the power supply system 30 via various current paths. The power supply system 30 comprises a first power source 22, a grid-forming power electronic inverter 20, and a short-circuit current source 10. The island grid 40 is an AC voltage network, and the consumers 28 are accordingly AC loads, in particular those that can also be operated on public supply networks.

[0084] The energy supply system 30 can, for example, be designed as a decentralized energy system that processes, in particular, renewable energy. The energy supply system 30 can have one or more first energy sources 22. The first energy source 22 can, for example, include a PV generator and / or an electrical storage system. The island grid 40 can be single-phase or multi-phase, in particular three-phase with the three phases a, b, c, and can be generated and maintained by one or more grid-forming inverters 20 of the energy supply system 30.

[0085] In the energy supply mode EVM, the consumers 28 of the island grid 40 are supplied with electrical power via the inverter 20. For this purpose, the inverter 20 sets and maintains a supply voltage U_n with a nominal frequency f_n. The inverter 20 feeds a supply current l_WR into the part of the island grid 40 that contains the current paths to the consumers 28 via an optional transformer 24. The supply current l_WR and the supply voltage U_n are essentially sinusoidal with the nominal frequency f_n.

[0086] The current paths to the respective consumers 28 are protected by individual fuses 26. If a fault current l_ERR, which flows through one of the fuses into a fault in the island grid 40, exceeds a rated current of the fuse 26, the respective affected fuse 26 trips and disconnects the current path with the fault, here with the faulty consumer 28, from the island grid 40.

[0087] The power supply system 30 further comprises the short-circuit current source 10, which includes a second power source and a bridge circuit 12. The second power source is a DC voltage source. In the illustrated embodiment, the second power source includes a capacitor 14, in particular a supercapacitor. The bridge circuit 12 is configured to convert, when required, a DC voltage or a DC current from the second power source 14 into a nearly rectangular AC voltage or, as per the submitted version 24-041 -P- WO - 14 -, a nearly rectangular AC current, so that a substantially rectangular short-circuit current l_GTO is available at its output.

[0088] The power supply system 30 further includes the switch assembly 16, via which the short-circuit current source 10 can be connected to or disconnected from the island grid 40 as needed. The connection is established when the switch assembly 16 is switched on, and the connection is broken when the switch assembly 16 is switched off.

[0089] The island network 40 can be operated in the power supply mode EVM and, if necessary, in the fault mode, as described with reference to Figure 1.

[0090] Fig. 3 schematically shows an embodiment of a switch arrangement 16. The switch arrangement 16 is arranged between the short-circuit current source 10 and the part of the island grid 40 to which the loads 28 are connected. The short-circuit current source 10 and the inverter 20 can be connected to the same grid connection or to different grid connection points of the island grid 40.

[0091] The switch arrangement 16 comprises two antiparallel thyristors, which are controlled by a control signal EN (EN: Enable). The two antiparallel thyristors can, in particular, be configured as GTO thyristors. When switched on, the two antiparallel thyristors connect one of the three phases a, b, c of a three-phase input of the switch arrangement 16 to one of the three phases a, b, c of a three-phase output of the switch arrangement 16. When switched off, the two antiparallel thyristors disconnect the three-phase AC input of the switch arrangement 16 from the corresponding phase of the three-phase AC output of the switch arrangement 16 on one of the phases a, b, c. Corresponding circuit arrangements 16 can be assigned to the two other phases of the three-phase short-circuit current source 10.

[0092] The switch arrangement 16 can be configured, in particular, as an analog circuit. The control signal EN, which controls the antiparallel thyristors, can be generated by an analog circuit that takes measured values ​​of various currents of the island grid 40 as input values. The embodiment shown in Figure 3 has an S / R analog circuit (S: Set, R: Reset) that generates the control signal EN.

[0093] A transition from power supply mode EVM to fault mode FM can be triggered, in particular, by the switch arrangement 16 if an overcurrent flows from the inverter 20 into the island grid 40. Therefore, in the switch arrangement, the supply current l_WR delivered by the inverter 20 is compared with a first predefinable limit l_n. Here, the first predefinable limit l_n corresponds to the nominal current l_n of the inverter 20 (see version 24-041-P-WO-15), but can alternatively be set somewhat higher, for example, 1.1 to 1.5 times the nominal current l_n. If the supply current l_WR delivered by the inverter 20 exceeds this first predefinable limit, which depends on the nominal current l_n, the control signal EN is activated, the antiparallel thyristors are switched on, and the short-circuit current source 10 is connected to the island grid 40 via the switch arrangement 16.The control signal EN is kept active by the S / R analog circuit, even if the supply current l_WR falls below the first limit value again, for example because the clocking of the inverter 20 is stopped in fault mode FM.

[0094] Furthermore, in the switch arrangement 16, the short-circuit current l_GTO from the short-circuit current source 10 is compared with a second limit value. In fault mode FM, if the short-circuit current l_GTO drops below the second predefinable limit value, the switch arrangement 16 triggers the transition from fault mode FM to power supply mode EVM. In the embodiment shown in Fig. 3, the second predefinable limit value also corresponds to the rated current l_n of the inverter 20. Alternatively, the second limit value can be specified separately and, in particular, can be between 80% and 100% of the rated current l_n.If the short-circuit current l_GTO supplied by the short-circuit current source 10 falls below the second predefinable limit value dependent on the nominal current l_n, the control signal EN is reset by the S / R analog circuit, the antiparallel thyristors are switched off and the short-circuit current source 10 is disconnected from the island grid 40 via the switch arrangement 16.

[0095] Fig. 4 schematically shows an embodiment of a short-circuit current source 10. The short-circuit current source 10 has a second energy source, which in the illustrated embodiment is designed as a capacitor 14.

[0096] The bridge circuit 12 of the short-circuit current source 10 is, for example, designed as a GTO-B6C bridge. This B6C bridge type uses, for example, antiparallel GTO thyristors as bridge switches and can be built cost-effectively with high current-carrying capacity. The bridge circuit 12 is clocked at the nominal frequency f_n. The control of the bridge circuit 12 to generate the rectangular short-circuit current l_GTO can be achieved by a simple controller. In particular, pulse-width modulation and regulation can be omitted, further simplifying the design. No high demands need to be placed on the short-circuit current l_GTO, since it is only generated to trip the fuse 26. As a fault current l_ERR, it essentially travels the current path to the fault location until the fuse 26 trips.The other consumers 28 are not reached by the short-circuit current l_GTO and therefore cannot be damaged by it. 24-041 -P- WO - 16 - submitted version.

[0097] Fig. 5 schematically shows current and voltage waveforms in the island grid 40.

[0098] The first line shows the supply current l_WR, which is fed into the island grid 40 by the inverter 20. In the energy supply mode EVM, which is active until time t1, a sinusoidal supply current is fed in. In the event of a fault, i.e., in particular if a short circuit occurs at time t2, the supply current l_WR initially becomes very large and exceeds the first limit, so that the system switches to fault mode FM. From time t1 onwards, the rectangular short-circuit current l_GTO, shown in the second line, is fed into the island grid 40 by the short-circuit current source 10.

[0099] The third line of Fig. 5 shows the load voltage U_Last across the loads 28. It can be seen that in the island grid 40, the load voltage U_Last is sinusoidal in power supply mode EVM, as specified by the grid-forming inverter 20. In fault mode FM, the load voltage U_Last is rectangular because the DC voltage of the power source for the short-circuit current source 10 is converted into a pulsed square wave by means of the bridge circuit 12, causing a rectangular short-circuit current l_GTO. The amplitude of the load voltage U_Last is greater in power supply mode EVM than in fault mode FM.

[0100] The fourth line shows the fault current l_ERR, which flows through the current path to the fault location in the island grid 40. It can be seen that in fault mode FM, the fault current l_ERR corresponds to the short-circuit current l_GTO. It is evident that the short-circuit current l_GTO flows entirely as the fault current l_ERR through the fault location.

[0101] The fifth line shows the RMS value of the short-circuit current l_GTO_eff. The short-circuit current l_GTO flows with a high amplitude for as long as the short circuit persists. At time t2, the RMS value l_GTO_eff has decreased sufficiently to indicate that the short circuit has been resolved, in particular by the tripping of the fuse 26 located in the current path to the short circuit. Therefore, at time t2, the system can switch back to power supply mode EVM.

[0102] The sixth line shows the control signal EN as it is generated in the switch assembly 16. During the power supply mode EVM, the control signal EN is zero, i.e., the switch assembly 16 is open and the short-circuit current source 10 is disconnected from the island grid 40. As long as a fault mode FM exists, in particular as long as the short-circuit current l_GTO exceeds the second limit, the control signal EN is active, i.e., the switch assembly 16 is switched on and the short-circuit current source 10 is connected to the island grid 40. 4-041 -P- WO submitted version

[0103] REFERENCE MARK LIST

[0104] 10 Short-circuit current source 12 Bridge circuit

[0105] 14 second energy source

[0106] 16 Switch arrangement 18 Control of the switch arrangement

[0107] 20 inverters 22 first energy source

[0108] 24 T transformer

[0109] 26 fuse 28 consumer

[0110] 30 Energy supply system 40 Island grid

[0111] EVM power supply mode

[0112] FM error mode

[0113] S10, S12, S20, S22 Process steps f_n Nominal frequency U_n Supply voltage l_n Nominal current l_WR Supply current l_GTO Short-circuit current l_GTO_eff RMS value of the short-circuit current l_ERR Fault current U_Last Load voltage EN Control signal

Claims

24-041 -P- WO - 18 - submitted version PATENT CLAIMS 1. Method for operating an electrical island grid (40), wherein the island grid (40) comprises at least one consumer (28) and a power supply system (30), wherein the power supply system (30) comprises a first power source (22), a grid-forming power electronic inverter (20) and a short-circuit current source (10) with a bridge circuit (12), wherein the method comprises: • Operating the power supply system (30) in a power supply mode (EVM), wherein the inverter (20) in power supply mode (EVM) supplies the island grid (40) with electrical power from the first power source (22) at a supply voltage (U_n) and a nominal frequency (f_n), • Operating the power supply system (30) in a fault mode (FM), wherein the short-circuit current source (10) feeds a substantially rectangular short-circuit current (l_GTO) into the island grid (40) by means of the bridge circuit (12) in fault mode (FM).

2. Method according to claim 1, wherein the substantially rectangular short-circuit current (l_GTO) fed into the island network (40) by the short-circuit current source (10) in fault mode (FM) has an amplitude that is greater than a rated current of at least one fuse (26) in the island network (40) for at least a predefinable duration, wherein the fuse (26) is in particular arranged in a current path to the at least one consumer (28).

3. Method according to claim 1 or 2, wherein the substantially rectangular short-circuit current (l_GTO) fed into the island grid (40) by the short-circuit current source (10) in fault mode (FM) has an amplitude that is greater than a rated current of the grid-forming inverter (20) by at least a factor of 2, preferably at least a factor of 5, for at least a predefinable duration.

4. Method according to one of claims 1 to 3, wherein the bridge circuit (12) of the short-circuit current source (10) is clocked in fault mode (FM) with the nominal frequency (f_n).

5. Method according to one of the preceding claims, wherein the bridge circuit (12) of the short-circuit current source (10) in fault mode (FM) converts a DC voltage of a second energy source (14) into a square wave voltage, wherein the amplitude of the square wave voltage in fault mode is less than the amplitude of the supply voltage (U_n) and / or wherein the DC voltage of the second energy source in the unloaded state is equal to or greater than the amplitude of the supply voltage. 24-041 -P- WO - 19 - submitted version 6. Method according to one of the preceding claims, wherein the grid-forming inverter (20) in power supply mode (EVM) converts a DC voltage of the first power source (22) into the substantially sinusoidal supply voltage (U_n) and feeds a substantially sinusoidal supply current (l_WR) into the island grid (40).

7. Method according to one of the preceding claims, wherein the short-circuit current source (10) is connected to the island grid (40) via a switch arrangement (16) when switching from the power supply mode (EVM) to the fault mode (FM) (S12) and is disconnected from the island grid via the switch arrangement (16) when switching from the fault mode (FM) to the power supply mode (EVM) (S20).

8. Method according to claim 6, wherein the switch arrangement (16) comprises two antiparallel thyristors and is controlled by a control signal (EN) which depends on the amplitude of the sinusoidal supply current (l_WR).

9. Method according to claim 8, wherein the switch arrangement (16) is switched on by means of the control signal (EN) and switches to fault mode (FM) when the amplitude of the sinusoidal supply current (l_WR) exceeds a first predefinable limit, wherein the predefinable limit is preferably between 10% and 50% greater than a nominal current (l_n) of the inverter (20).

10. Method according to claim 9, wherein the switch arrangement (16) remains switched on as long as the amplitude or the RMS value of the substantially rectangular short-circuit current (l_GTO) exceeds a second predefinable limit, wherein the second limit is preferably between 80% and 100% of the rated current (l_n) of the inverter (20).

11. Method according to claim 10, wherein the system switches from fault mode (FM) back to power supply mode (EVM) when the substantially rectangular short-circuit current (l_GTO) falls below the second predefinable limit value.

12. Method according to one of the preceding claims, wherein the bridge circuit (12) of the short-circuit current source (10) is clocked at the nominal frequency (f_n) in power supply mode (EVM).

13. Method according to any one of claims 6 to 12, wherein the supply of electrical power by the grid-forming inverter (20) is stopped when switching from the power supply mode (EVM) to the fault mode (FM) (S10). 24-041 -P- WO - 20 - submitted version 14. Short-circuit current source (10) for operation in an electrical island grid (40), wherein the short-circuit current source (10) is configured to supply a substantially rectangular short-circuit current (l_GTO) into the island grid (40) on demand, wherein the short-circuit current (l_GTO) has an amplitude that is higher than a predefinable rated current for at least a predefinable duration.

15. Short-circuit current source according to claim 14, wherein the short-circuit current source (10) has a bridge circuit (12), wherein the bridge circuit (12) is configured to convert a DC voltage of an energy source (14) into a square wave voltage when the short-circuit current (l_GTO) is supplied on demand.

16. Short-circuit current source according to claim 15, wherein the short-circuit current source (10) comprises the energy source (14).

17. Short-circuit current source according to claim 15 or 16, wherein the bridge circuit (12) in particular comprises thyristors as bridge switches and is configured to be clocked with a nominal frequency (f_n) of the island grid (40) in order to generate the square wave voltage and the substantially rectangular short-circuit current (l_GTO).

18. Power supply system (30) for supplying an island grid (40) with electrical power, wherein the power supply system (30) comprises a first power source (22), a grid-forming inverter (20), the short-circuit current source (10) according to one of claims 14 to 17 and a switch arrangement (16) for connecting and disconnecting the short-circuit current source (10) from the island grid (40) as required.

19. Power supply system (30) according to claim 18, wherein the grid-forming inverter (20) is configured to output a supply voltage (U_n) with a nominal frequency (f_n) for supplying the island grid (40) with electrical power in a power supply mode (EVM), and wherein the short-circuit current source (10) is configured to feed the substantially rectangular short-circuit current (l_GTO) into the island grid (40) in a fault mode (FM).

20. Energy supply system (30) according to claim 18 or 19, wherein the amplitude of the short-circuit current (l_GTO) is greater than a rated current of at least one fuse (26) assigned to a consumer (28) in the island network (40) for at least a predefinable duration.

21. Energy supply system (30) according to one of claims 18 to 20, wherein the amplitude of the short-circuit current (l_GTO) is reduced by at least for at least a predefinable duration. 24-041 -P- WO - 21 - submitted version a factor of 2, preferably at least a factor of 5 greater than a rated current of the grid-forming inverter (20).

22. Energy supply system according to one of claims 18 to 21, wherein the supply voltage (U_n) is substantially sinusoidal and wherein the grid-forming inverter (20) is configured to feed a substantially sinusoidal supply current (l_WR) into the island grid (40).

23. Energy supply system according to one of claims 18 to 22, wherein the first energy source (22) comprises the second energy source (24).

24. Island grid (40) comprising the power supply system (30) according to one of claims 18 to 22 and electrical consumers (28), wherein the consumers (28) are connected individually or jointly to the power supply system (30) via the at least one fuse (26).

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