Method for operating inverter

Abstract

The invention relates to a control method for operating an inverter in an electrical network which is connected to an electrical supply network via an actuatable disconnect switch. The method comprises the following steps: operating the inverter in a current injection mode during the closing of the circuit breaker switch, continuously monitoring the power supply network with respect to a voltage drop, switching the operating mode of the inverter into a voltage setting mode in the event of detecting a voltage drop of the power supply network, a temporary voltage is set by the inverter, the temporary voltage being reduced relative to the normal voltage of the power grid. After a predefined duration has elapsed after the detection of the voltage drop, if a voltage drop of the supply network is continuously present, the method comprises opening the circuit breaker and increasing the voltage set by the inverter to the network normal voltage, after a predefined duration has elapsed after the detection of the voltage drop, the inverter is operated in the current injection mode if a fault of the supply network does not continuously exist.

Description

Technical Field

[0001] The invention relates to a method for operating an inverter and an energy system. Background Art

[0002] In recent decades, there has been a trend away from energy production from fossil energy sources. Instead, energy, in particular electrical energy, is preferably produced from renewable energy sources. Electrical energy from renewable energy sources is provided by energy sources in the form of direct current, which must first be converted into alternating current by an inverter in order to be used by the producer itself or also fed into a public power supply network, in particular an AC grid.

[0003] Various faults may occur in the power supply network, to which the inverter can react. For example, a short circuit may occur in the power supply network, in which case the maximum possible power of the inverter must be fed into the power supply network as specified by the standard. In another case, a fault may occur in the power supply network, making it impossible to feed electrical power into the power supply network or to draw electrical power from the power supply network.

[0004] Methods are known in the prior art that can be used to detect such faults. However, it is desirable that the subnetwork in which the energy source and the at least one inverter are arranged should continue to be supplied with electrical power. The prior art does not yet cover the transition from supplying the subnetwork via the power supply grid to supplying the subnetwork as an independent power supply grid after disconnection from the power supply grid.

[0005] DE 10 2019 116 254 A1 shows, for this purpose, the switching of an inverter between a current-setting operating mode and a voltage-setting operating mode.

[0006] Also known are so-called STATCOMs which are intended to stabilize the power supply network. Summary of the invention

[0007] The method according to the invention for operating an inverter in a subsystem connected to a power supply grid via a controllable disconnect switch comprises the following steps:

[0008] - During the closing period of the circuit breaker, the current injection Run the inverter in mode,

[0009] - continuous monitoring of the power supply network with regard to voltage drops,

[0010] - when a voltage drop in the power supply network is detected, the operating mode of the inverter is switched to a voltage setting mode, wherein a temporary voltage which is reduced compared to the normal voltage of the power supply network is set by the inverter,

[0011] - after a predetermined time period has elapsed since the detection of the grid fault, if the voltage drop in the power supply grid persists, the disconnect switch is opened and the set voltage is successively increased to the normal grid voltage, and

[0012] After a predefined time period has elapsed after the detection of the grid fault, if the fault in the power supply grid does not persist, the inverter is operated in the current injection mode.

[0013] The method described above regulates an inverter in a power grid (also called a subgrid) which can be disconnected from the power grid and in particular under certain conditions. These conditions include a failure of the power supply capacity of the power grid in the sense that the energy supply of the subgrid cannot be ensured by the power grid, or in other words, electrical power can no longer be drawn from the power grid.

[0014] In particular, in this case it is necessary that the inverter, which was initially operated in current injection mode, must construct a subnetwork when disconnected from the supply grid, i.e. must be switched to voltage injection mode. The voltage in the subnet is set in mode.

[0015] In the current injection mode, the inverter is synchronized with either the grid voltage of the power grid or the grid voltage of the subgrid and regulates the current fed in. Thus, as long as the power grid feeds energy into the subgrid, i.e. the disconnect switch between the subgrid and the power grid is closed, the inverter can synchronize with the power grid and feed into the subgrid in accordance with the frequency, voltage and phase of the power grid.

[0016] In the voltage injection mode, only insufficient grid voltage or no grid voltage is provided by the power supply network or is present in the subnetwork. Therefore, if it is determined in the method according to the invention that there is a voltage drop in the subnetwork during the current injection mode, i.e. during the closing of the disconnect switch for the power supply network, the inverter switches to the voltage injection mode. The inverter then independently sets the voltage in the subnetwork according to the standard pre-determinations regarding the voltage level and frequency. In this case, in Central Europe, the standard frequency is 50 Hz and the standard voltage is 230 volts. However, the standard pre-determinations may also differ depending on the local power supply operator.

[0017] The determination of the voltage drop can be achieved here by any means for measuring voltage. For example, a voltmeter can be connected to the line between the power supply network and the circuit breaker. In this way, the voltage provided by the power supply network directly upstream of the circuit breaker, i.e. the voltage at the connection point to the subnetwork, can be directly measured. Alternatively, the voltmeter can also be connected to the AC input of the inverter. If a voltage drop in the power supply network is now detected by the internal voltmeter, the inverter can switch its operation from the current injection mode to the voltage injection mode without delay, for example, without delay caused by the signal transmission of an external voltmeter, by the internal voltmeter.

[0018] In one embodiment, the voltage drop of the power supply grid is detected in that the voltage of the power supply grid drops by a threshold value, wherein the threshold value is preferably 5%.

[0019] In other words, a voltage drop is detected when the voltage of the power supply network drops by at least the threshold value, preferably 5%, relative to the normal network voltage. In this case, the current voltage of the power supply network is compared with the normal network voltage. The normal network voltage is here the nominally determined voltage in the local or regional power supply network (230 V in Europe).

[0020] This embodiment provides a number of advantages when a voltage drop is detected. Small voltage drops in the voltage in the supply network can already be detected. Thus, a gradual voltage drop can already be detected at an early stage, so that the delay in switching between the current injection mode and the voltage injection mode of the inverter is low. The generation of an independent supply network in a subnetwork and the supply of electrical power to the subnetwork by the inverter is low-latency. Thus, users can be supplied with electrical power almost seamlessly.

[0021] If the voltage drop is merely a fluctuation in the voltage of the power grid and the power grid again supplies the normal grid voltage after the fluctuation has ended, the method is terminated. However, if, as described above, there is no recovery and the voltage of the power grid is gradually decreasing, the inverter is already in voltage injection mode and can take over the energy supply of the subgrid.

[0022] In one embodiment, the control method can be designed in such a way that the voltage set by the inverter is selected to have the same frequency and phase as the voltage before the voltage drop of the power supply network was detected. This embodiment particularly, but not exclusively, relates to the case where the power supply network is operating normally again after the voltage drop. This means that the voltage drop is a temporary effect that is subsequently eliminated by the voltage of the power supply network returning to the nominal value. This embodiment facilitates the transition to normal operation, which is described in more detail below.

[0023] If a voltage drop occurs in the power supply network, the inverter of the subnetwork switches from the current injection mode to the voltage injection mode as described above. The voltage injected into the subnetwork by the inverter in the voltage injection mode has the same frequency and phase as the voltage before the voltage drop of the power supply network was detected. If the power supply network is now available again, the phase of the voltage in the inverter does not need to be re-adapted, because the power supply network and the subnetwork are in phase. Therefore, the switch to the normal operation of the power supply network supplying electrical power to the subnetwork can be carried out quickly and without problems.

[0024] In one embodiment, the regulation method may be designed such that the voltage set by the inverter is reduced by 10% to 30% relative to the normal voltage of the power grid.

[0025] In one embodiment, the control method can be designed in such a way that if, when monitoring the subnetwork with respect to a voltage drop in the power supply network, it is detected that the grid fault is caused by an irreparable short-circuit fault, the temporary voltage set by the inverter is adjusted to the voltage last detected in the subnetwork before the voltage drop was detected and the circuit breaker is immediately opened.

[0026] This embodiment relates in particular to irreversible short-circuit faults in the power supply network. This can lead to a short circuit in the line in the surroundings of the subnetwork. In order to avoid a breakdown of the power supply network in the surroundings, there is a standard pre-determination that the inverter continues to feed electrical power into the power supply network for a certain duration in the event of a short-circuit fault - as long as possible and necessary. In this case, the inverter recognizes a short-circuit fault in the power supply network in the following way: the voltage of the power supply network suddenly drops to a lower value, wherein the residual voltage in the power supply network can still be measured. For example, the voltage of the power supply network drops by 50%. If the short-circuit fault is only temporary, the power supply network can be supported so that it does not completely fail due to a short-circuit fault that is only temporary.

[0027] In the event of an irreversible short-circuit fault, the power supply network cannot be restored after a certain time. This means that even if further electric power is fed in through the inverter of the subnetwork, the power supply network cannot be restored. In order to avoid failures in the subnetwork, the regulation method according to the present invention is arranged so that the inverter switches to the voltage injection mode and the subnetwork is immediately disconnected from the power supply network. Disconnection from the power supply network is achieved here, for example, by a circuit breaker arranged between the power supply network and the subnetwork. In this way, when a short-circuit fault occurs in the power supply network, a distinction can be made between reversible short-circuit faults and irreversible short-circuit faults and therefore can be handled in the sense of the subnetwork while satisfying the pre-determined provisions of the standard. The reversible short-circuit fault is no longer relevant due to the restored power supply network, and the irreversible short-circuit fault cannot be eliminated within the time specified in the standard.

[0028] Furthermore, the inverter can also distinguish between a short-circuit fault and a grid fault. In the case of a grid fault, the measurable voltage of the power supply grid drops to 0 V, ie the power supply grid has no measurable voltage.

[0029] In one embodiment, the control method can be designed in such a way that if, when monitoring the subnetwork with respect to a voltage drop in the power supply network, it is detected that the voltage drop in the power supply network is caused by a fault in the power supply network, the temporary voltage set by the inverter is set to 90% of the voltage last detected before the fault in the power supply network was detected and the circuit breaker is opened immediately.

[0030] One aspect of the invention relates to an energy system having at least one inverter which is designed for regulation according to the method as claimed in claim 1 and a disconnect switch which can disconnect the energy system from a power supply grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention is described below with reference to the accompanying drawings, in which:

[0032] Figure 1 Flow chart of the regulation method according to the invention. DETAILED DESCRIPTION

[0033] This figure is explained in detail below.

[0034] exist Figure 1 . The method starts with step S100. Step S100 means the start of the method. In most cases, this is achieved by starting the inverter on which the regulation method runs. Subsequently, the method continues with step S110.

[0035] In step S110, the inverter is operated in a current injection mode. In the current injection mode, the inverter is synchronized with the voltage of the power supply network and feeds the electric power of the connected DC power source into the subnetwork to which the inverter is connected. Here, the power supply network is connected to the subnetwork to which the inverter is connected via a disconnect switch. For example, a voltmeter that continuously measures the voltage of the power supply network can be connected to the disconnect switch that is permanently connected to the power supply network. The method then and continuously implements step S120.

[0036] In step S120, the power supply network is continuously monitored for voltage drops. For this purpose, the voltage of the power supply network is continuously measured by a voltage measuring device, such as, for example, a voltmeter. If a voltage drop is now detected in step S130, the inverter switches to a voltage injection mode. The injected voltage is reduced by 10% to 30% of the normal grid voltage of the power supply network.

[0037] There is then a situation in which the voltage drop persists. If the voltage drop persists and there is a residual voltage in the power supply network, after a standard predefined time, it is determined that an irreparable short-circuit fault exists. In this case, a fault in the power supply network is assumed. In the case of a network fault, i.e. if the voltage can no longer be measured in the power supply network, the persistence of the voltage drop can be assumed immediately upon identification of a network fault. The method continues with decision Y in step S130 with step S140.

[0038] In step S140, the disconnect switch between the power supply grid and the subgrid is opened, that is, the power transmission connection between the power supply grid and the subgrid is disconnected. The power of the inverter now operating in the voltage injection mode is now fed into the subgrid, wherein the loads in the subgrid are operated by the fed-in power. The inverter whose voltage is reduced relative to the grid normal voltage of the power supply grid is now increased to the grid normal voltage of the power supply grid.

[0039] In another case, the voltage drop of the voltage of the power supply network decreases. In other words, the voltage of the power supply network rises again from the temporarily reduced value due to the temporary fault to the normal voltage of the grid, so that the voltage of the power supply network no longer drops by a value greater than the threshold value. In this case, the method continues with S130:N and continues with step S110, i.e. the inverter is operated in the current injection mode again. After this switching of the mode of the inverter, the process continues to the continuous monitoring of the power supply network in step S120.

[0040] Reference numerals list

[0041] S100-S150 steps

Claims

1. A control method for operating an inverter in a power grid, wherein the power grid is connected to a power supply grid via a controllable disconnect switch, the method comprising the following steps: - operating (S110) the inverter in a current injection mode during the closing of the circuit breaker, - continuously monitoring ( S120 ) the power supply network for voltage drops, - upon identifying a voltage drop in the power supply network, switching the operating mode of the inverter to a voltage setting mode (S130), wherein a temporary voltage is set by the inverter, the temporary voltage being reduced relative to a normal voltage of the power grid, - after a predetermined time period has elapsed after the voltage drop has been detected, if the voltage drop in the power supply network persists (S130: Y), opening (S140) the disconnect switch and increasing the voltage set by the inverter to the above-mentioned normal network voltage, After a predefined time period has elapsed after the voltage drop has been detected, if the fault in the power supply network is not still present ( S130 : N), operating ( S110 ) the inverter in the current injection mode.

2. The adjustment method according to claim 1, wherein: A voltage drop in the power supply grid is detected in that the voltage of the power supply grid drops by a threshold value.

3. The adjustment method according to claim 2, wherein: The threshold value is at least 5% of the normal voltage of the power grid.

4. The method of regulation according to any one of the preceding claims, wherein: The voltage set by the inverter is selected to have the same frequency and to be in phase with the voltage before the voltage drop of the power supply grid was detected.

5. The adjustment method according to claim 1, 2 or 3, wherein: The temporary voltage set by the inverter is reduced by 10% to 30% relative to the normal voltage of the power supply network.

6. A method of regulation according to any one of the preceding claims, wherein: If, when monitoring a subsystem for a voltage drop in the power supply network, an irreparable short-circuit fault is detected, the temporary voltage set by the inverter is set to the voltage last detected before the fault in the power supply network was detected and the disconnect switch is immediately opened.

7. The adjustment method according to any one of claims 1 to 5, wherein: If a fault in the power supply network is detected when monitoring a subnetwork for a voltage drop in the power supply network, the temporary voltage set by the inverter is set to 90% of the voltage last detected before the fault in the power supply network was detected and the disconnect switch is immediately opened. 8 . An energy system having at least one inverter which is designed for regulation according to the method as claimed in claim 1 and a disconnect switch which can disconnect the energy system from a power supply grid.

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