Method for operating an inverter for a battery and inverter for carrying out the method

Abstract

The application describes a method for operating an inverter (10) for a battery (30) on the basis of a target power value (PSW) for a first power flow (P1) to be exchanged between the inverter (10) and an AC grid. The inverter (10) is connected to the AC grid via an AC connection (11) and to the battery (30) via a first DC connection (12). It additionally has a bidirectional DC / AC converter (16) arranged between the AC connection (11) and the first DC connection (12). The method comprises the following steps: i) providing the target power value (PSW), ii) operating the inverter (10) in a normal operating mode in which the a first power flow (P1) between the inverter (10) and the AC grid is set to the target power value (PSW) if an absolute value of the target power value (PSW) is greater than or equal to a positive first threshold value (PTH,1), where PTH,1 > 0, and iii) operating the inverter (10) in a standby mode in which the first power flow (P1) between the inverter (10) and the AC grid is suppressed if the absolute target power value (PSW) falls below the first threshold value (PTH,1). The invention further relates to an inverter (10) suitable for carrying out the method.

Description

[0001] Method for operating an inverter for a battery and inverter for carrying out the method

[0002] Technical field of the invention

[0003] The invention relates to a method for operating an inverter for a battery. The invention further relates to an inverter for implementing the method.

[0004] State of the art

[0005] The use of batteries as energy storage devices, coupled to an alternating current (AC) grid via an inverter, is widespread in the field of renewable energy generation, for example, in conjunction with photovoltaic (PV) systems or wind turbines. Their use serves to temporarily store renewably generated energy during periods of high energy production but low consumption, in order to then release the temporarily stored energy during periods of low energy production but high consumption. Such systems are particularly advantageous when there is a desire to increase the self-consumption of a household's electrical installation, especially if it features a renewable energy generation system.Specifically, when using batteries, it is not necessary to curtail the power output of the renewable energy generation system—and thus forgo the excess amount of renewable energy generated. Nor is it necessary to feed the excess energy into a higher-level energy grid at a relatively low feed-in tariff simply because it cannot be consumed locally at the time of generation. An inverter connecting the batteries to an AC grid can operate in this case to provide power.

[0006] Another use for batteries has become established in connection with an emergency power backup system. Specifically, a battery can ensure that loads within a local AC distribution grid are supplied even in the event of a failure of a higher-level power supply grid connected to the local AC distribution grid. In this case, an inverter, via which the battery is connected to the AC distribution grid, usually operates as a voltage source, i.e., it regulates the voltage. It is desirable for an inverter, via which the battery is connected to an AC grid, to be as efficient as possible. Every charging and discharging process of a battery leads to battery aging. If the energy originally stored in the battery can only be reused to a comparatively small extent during discharging due to the low efficiency of the inverter, this is disadvantageous.When storing energy in the battery, the highest possible efficiency of the inverter connected to the battery is also desirable. Ultimately, the goal here too is to lose as little of the electrical power drawn through conversion losses within the inverter as possible, regardless of whether it is drawn from a higher-level energy supply grid or a local renewable energy generation plant. Typically, however, the efficiency of the inverter depends on the currently converted power of the inverter, as described in more detail in connection with Fig. 3. Common inverters usually have a lower efficiency at a low ratio of converted power to a nominal power of the inverter, and sometimes even at a high ratio, than is the case in a medium power range.Depending on the power at which the inverter is operated and for how long, the inverter achieves an effective average efficiency over a given operating period. For an inverter user, it is desirable not only for the inverter's efficiency curve to be as high as possible, but also for the inverter to operate with the highest possible average efficiency over a given period of time (hereinafter also referred to as the effective efficiency).

[0007] From the publication CN 112803741 A, an automatic start-stop control method and a circuit for a photovoltaic system inverter are known. In the automatic start-stop control method, the voltage of a battery of a photovoltaic system is monitored at night. If the battery voltage is lower than a first voltage, the inverter is shut down. Since the photovoltaic system cannot charge the battery at night, the inverter is switched off in a timely manner to suppress its energy extraction from the battery. This prevents the battery's electrical capacity from being depleted and extends the battery's service life. The publication EP 3 945 656 A1 discloses a method for feeding electrical power into an electrical grid by means of an inverter-controlled feed-in unit comprising a plurality of inverters.Each inverter is assigned a circuit breaker to galvanically isolate the respective inverter from the electrical supply grid. Each inverter has a circuit arrangement of semiconductor switches to generate an output current that is as sinusoidal as possible via pulsed control of the circuit arrangement. Each inverter is also designed to generate a variable partial power, with the output power being the sum of all partial power generated. Depending on the output power level, one or more of the inverters are operated as active inverters, each generating a partial power, while the remaining inverters are operated as passive inverters, each generating no partial power.

[0008] The document DE 10 2018 130 453 A1 discloses a method for supplying an inverter with an alternating voltage. The inverter comprises an AC output for connection to an AC grid, a DC input for connection to a DC source, a DC / AC converter, and a control unit. The control unit is connected to a switching unit such that, in a first switching state, it is supplied via the AC grid and, in a second switching state, via an auxiliary power source providing an alternating voltage. The inverter further comprises a grid monitoring unit for detecting a property of an alternating voltage present in the AC grid. In the method, the switching unit is operated in the first switching state when a detected alternating voltage in the AC grid meets predetermined criteria.On the other hand, the switching unit is operated in the second switching state if the alternating voltage in the AC network is not detected or the detected alternating voltage in the AC network does not meet the specified criteria.

[0009] The document DE 10 2015 204 561 A1 discloses a method for operating an inverter device with a plurality of inverters connected in parallel, which differ in at least one of their technical properties and are operable by means of a switching device at a DC voltage input of the inverter device connected to a DC voltage source. The method comprises the steps: - detecting at least one selection variable that describes the operating state of the inverter device and / or a connected further device;

[0010] - Selection of an inverter configuration comprising at least one inverter to be operated at the DC voltage input depending on at least one selection criterion; and

[0011] - Commissioning of at least one inverter described by the selected inverter configuration at the DC voltage input by means of the switching device.

[0012] The document DE 102005 008 809 A1 describes an electrical inverter with an intermediate circuit and several inverter bridge branches that can be individually switched on and off by a control device.

[0013] The publication DE 10 2013 113 786 B3 discloses a photovoltaic system with at least two photovoltaic generators and an inverter. Electrical energy generated by the photovoltaic system can be supplied to local consumers as well as fed into a public power grid. The inverter can be connected to the first or second photovoltaic generator on the DC side via a DC switching device. The local consumers have at least two separate consumer circuits. An AC switching device can connect the inverter to the first or second consumer circuit on the AC side.A control device controls the direct current switching element and the alternating current switching element in a coordinated manner such that a connection of the inverter to the first consumer circuit is accompanied by a connection of the inverter to the first photovoltaic generator, and a connection of the inverter to the second consumer circuit is accompanied by a connection of the inverter to the second photovoltaic generator.

[0014] Object of the invention

[0015] The invention is based on the object of providing a method for operating an inverter designed for connection to a battery that ensures the highest possible efficiency during operation of the inverter. In particular, the effective efficiency of the inverter, which results from normal operation of the inverter over a specified period of time and possibly at different power levels, is to be maximized. Furthermore, the object of the invention is to provide an inverter suitable for implementing the method.

[0016] Solution

[0017] The object of providing a method for operating an inverter that ensures the highest possible effective efficiency of the inverter during its normal operation is achieved according to the invention with the features of independent patent claim 1. The object of providing an inverter designed to implement the method is achieved according to the invention with the features of independent claim 13. Advantageous embodiments of the method are recited in claims 2 to 12. Claims 14 to 17 specify advantageous embodiments of the inverter.

[0018] Description of the invention

[0019] The method according to the invention aims at operating an inverter for a battery, in which the inverter is operated as a function of a power setpoint Psw for a first power flow P1 to be exchanged between the inverter and an AC grid. The inverter is connected to the AC grid via an AC connection and to the battery via a first DC connection. The inverter has a bidirectional DC / AC converter arranged between the AC connection and the first DC connection. The method comprises the following steps: i) providing the power setpoint Psw, ii) operating the inverter in a normal operating mode, in which a first power flow P1 between the inverter and the AC grid is set to the power setpoint Psw if an absolute value of the power setpoint Psw is greater than or equal to a positive first threshold value PTH.I , with PTH > 0, and iii) operating the inverter in a standby mode in which the first power flow P1 between the inverter and the AC grid is suppressed when the absolute value of the power setpoint Psw falls below the first threshold value PTH.I. An inverter according to the invention for a battery comprises: an AC connection for connecting the inverter to an AC grid, a first DC connection for connecting the inverter to the battery, a bidirectional DC / AC converter arranged between the AC connection and the first DC connection, and a control unit connectable to an energy management device and / or an energy meter and / or a battery management system for controlling the inverter.The inverter is characterized in that the control unit connected to the energy management device and / or the energy meter and / or the battery management system is configured to operate the inverter in a state connected to the AC grid and the battery according to the method according to the invention.

[0020] The power setpoint Psw specifies the electrical power that the inverter must exchange with the AC grid. The power setpoint Psw can, for example, be constant over a limited period of time, but it usually changes and is therefore time-dependent. The power setpoint Psw can characterize active power, reactive power, or a combination of active and reactive power. To allow both active and reactive power to be specified, the power setpoint Psw can be a complex-valued quantity in which the active power is described by its real part and the reactive power by its imaginary part. However, it is also within the scope of the invention to specify both quantities as separate but temporally correlated values. The power setpoint Psw can be generated by the inverter's control unit.Alternatively, it can also be generated by another device, for example a separate energy management device, a battery management system assigned to the battery, or an energy meter, and transmitted to the control unit. It is also possible to generate and / or provide the power setpoint Psw with the cooperation of several of the aforementioned components: control unit, energy management device, energy meter, and battery management system. The power setpoint Psw can be generated as part of a higher-level or subordinate operating process. For example, the power setpoint Psw can be used to define a battery charging cycle in order to charge it as gently as possible and / or with the lowest possible energy costs.Alternatively, the power setpoint Psw can also define which power is currently required to supply selected loads connected to the AC grid, especially if the purchase of electrical power from a higher-level energy supply grid is to be suppressed or at least reduced.

[0021] The inverter can be designed as a single-phase inverter. In this case, its AC connection can include a phase conductor connection and a neutral conductor connection. Alternatively, the inverter can also be designed as a multi-phase inverter with multiple phase conductor connections and optionally a neutral conductor connection. The AC grid to which the inverter is connected can, in particular, be an AC distribution grid that is linked to a higher-level energy supply grid via a grid connection point. In this case, the inverter, which is connected to an AC grid designed as an AC distribution grid, can be used to support the supply – or possibly even the sole supply – of a house or a sub-area within the house. The house / sub-area of ​​the house can also be supplied with electrical power drawn from the higher-level energy supply grid.Such inverters are typically selected so that their nominal power covers the intended application. A DC / AC converter within the inverter is typically designed for the inverter's nominal power and may, in particular, be the inverter's only DC / AC converter. In other words, the inverter can contain a maximum of one DC / AC converter.

[0022] In this method, the inverter has two operating modes: a normal operating mode and a standby mode. The respective operating mode in which the inverter is operated depends on the level of the power setpoint Psw. For the purposes of the following explanation, it is assumed that the power setpoint Psw specifies pure active power. The power setpoint Psw, as well as the first power flow P1, is negative if it characterizes active power that flows from the AC grid into the inverter, for example to charge the battery connected to the inverter. A positive power setpoint Psw and a positive first power flow P1 with P1 > 0 characterize active power that flows from the inverter into the AC grid, for example to supply loads connected there.In normal operating mode, the inverter, which is connected to the AC grid on the one hand and to the battery on the other, is operated such that the electrical power exchanged with the AC grid, i.e., the first power flow P1, is set or regulated to the power setpoint Psw. To this end, the control unit controls the inverter's DC / AC converter, in particular its semiconductor switch, such that the first power flow P1 assumes the power setpoint Psw. Normal operating mode is entered when an absolute value of the power setpoint Psw reaches or exceeds the positive first threshold value PTH,I, with PTH,I > 0.

[0023] If, however, the power setpoint Psw is less than the positively defined first threshold value PTH.I, i.e., Psw < PTH.I, the inverter operates in standby mode. In standby mode, power exchange between the inverter and the AC grid is suppressed, so the first power flow P1 is 0. This can be achieved, for example, by not controlling the semiconductor switches of the DC / AC converter, thus deactivating the DC / AC converter.

[0024] If an inverter that has a maximum of one DC / AC converter is put into standby mode, the power exchange between a DC source connected to the inverter (for example, the battery) and the AC grid is changed as a whole, in particular suppressed. A load that is connected in parallel with the inverter to the same AC grid can therefore no longer receive electrical power from the DC source connected to the inverter. It must either obtain the electrical power from another source, have its consumption throttled, or be deactivated. This is deliberately tolerated within the scope of the invention in favor of the highest possible efficiency of the inverter. It is therefore different from a case in which the inverter has several DC / AC converters, of which only one DC / AC converter or individual DC / AC converters, but not all of the inverter's DC / AC converters, are deactivated.In this case, the power exchange between the inverter, specifically the DC source connected to it, and the AC grid remains unchanged. However, a number of actively operated DC / AC converters used to convert the specified and unchanged power exchange between the DC source connected to the inverter and the AC grid is changed. The goal is to ensure that the specified and unchanged power exchange occurs with the highest possible efficiency using the majority of DC / AC converters, and thus using the inverter comprising the multiple DC / AC converters.

[0025] Standby mode can be used to avoid operating situations in which only a minimal amount of power is exchanged between the inverter and the AC grid, and in which the inverter operates particularly inefficiently and, in particular, with low efficiency. If such situations occur more frequently and / or for a longer period of time, they would lead to a decrease in the effective efficiency of the inverter. By specifically avoiding these situations during inverter operation, or at least reducing them, the effective efficiency achieved by the inverter during its operation over the specified period can be maximized.The inverter is essentially only operated in normal mode when its efficiency justifies it, namely when a minimum required efficiency is reached or exceeded during operation. This also reduces the aging of the battery connected to the inverter, as at least some of the otherwise occurring charging and / or discharging cycles are suppressed. For the inverter designed to implement the process, the corresponding advantages of the process arise.

[0026] Advantageous embodiments of the invention are specified in the following description and the subclaims, the features of which can be used individually and in any combination with one another.

[0027] In an advantageous variant of the method, the first power flow P1 can be suppressed in standby mode by avoiding activation of the semiconductor switches of the DC / AC converter, i.e., by deactivating the DC / AC converter. This also prevents power loss associated with the switching operations of the semiconductor switches. The control unit can also be operated with lower energy consumption in the inverter's standby mode, if necessary, since the computing effort otherwise required in normal operating mode is only required to a limited extent in standby mode. Alternatively or additionally, it is possible for an AC relay of the inverter arranged between the DC / AC converter and the AC connection to be open.Since the AC relay is usually a normally open switch whose closed connection is maintained by energy consumption, the electrical power required for this can be eliminated in standby mode.

[0028] In an advantageous variant of the method, method steps i) to iii) can be performed repeatedly, particularly sequentially. This ensures that in the event of an unforeseen change in the power setpoint Psw, in which the power setpoint Psw deviates from its initially generated profile, a check is always made again and almost instantly with the provision of the new, current power setpoint Psw to determine whether the inverter should be operated in normal operating mode or in standby mode. Such an unforeseen change in the power setpoint Psw can occur, for example, due to changed framework conditions, such as a changed consumption profile and / or the switching on or off of individual loads connected to the AC grid.

[0029] In a variant of the method, it is possible for the inverter to only be operated in standby mode when it is ensured that the absolute value of the power setpoint Psw falls below the first threshold value PTH for a predefined first time period Ati. This can be ensured, for example, by it being known from another source before or upon the onset of standby mode that the power setpoint Psw is below the first threshold value PTH within the first time period Ati and, if applicable, also beyond that. This can be the case when the power consumption of loads connected to the AC grid is at least largely known in advance. Alternatively, it is also possible for the ensuring to include the inverter only being put into standby mode and operated in it after the power setpoint Psw has fallen below the first threshold value PTH for the first time period Ati.This is the case, for example, when a load's consumption profile is unknown in advance, but it can be assumed that current consumption will change only slightly over time. Overall, this variant of the method can prevent, or at least reduce, rapid successive changes in the inverter's operating modes and the associated negative impacts on the inverter's components.

[0030] In a further variant of the method, the power setpoint Psw for the first power flow P1 to be exchanged between the inverter and the AC grid can also be monitored and / or provided in standby mode. The transition from standby mode to normal operating mode can occur when an absolute value of the power setpoint Psw exceeds a second threshold PTH,2 with PTH,2 > PTH, which is greater than or equal to the first threshold PTH. By also providing the power setpoint Psw in standby mode, the inverter can be woken up from standby mode and returned to normal operating mode. The second threshold PTH,2 is positively defined, similar to the first threshold PTH. If it is selected to be greater than the first threshold PTH, a hysteresis behavior can be induced when the operating states of the inverter change.Optionally, the inverter can also be switched from standby mode to normal operating mode only when it is ensured that the power setpoint Psw exceeds the second threshold PTH,2 for a predefined second period Δt2. Regarding the options for ensuring this, reference is made to the options already mentioned in connection with the first period Δt2, which also apply here. Specifically, for example, it may already be known from another source before the inverter is switched from standby mode to normal operating mode that the power setpoint Psw exceeds the second threshold PTH,2 for the predefined second period Δt2.In this case, the inverter can be switched from standby mode to normal operating mode immediately upon reaching or exceeding the second threshold value PTH,2 and in particular before exceeding the second time period Δt2, since it can already be assumed at this point that the exceedance will continue for the second time period Δt2. Alternatively, it is also possible for the inverter to be switched from standby mode to normal operating mode only after the expiry of the second time period Δt2 from the expiry of the second time period Δt2 when the second threshold value PTH,2 is reached or exceeded. In this case, the expiry of the second time period Δt2 corresponds to the safeguarding and it can be assumed, for example due to sluggishly reacting load profiles, that the exceedance of the second threshold value PTH,2 will continue.In both cases, an otherwise excessively frequent change of operating modes is prevented or at least reduced.

[0031] Monitoring the power setpoint Psw in the inverter's standby mode can be performed particularly when the AC grid is an AC distribution grid connected to a higher-level power supply grid via a grid connection point, as is the case, for example, with a residential electrical distribution system. The power setpoint Psw for the first power flow P1 to be exchanged between the inverter and the AC distribution grid can be determined using an energy meter located at the grid connection point. The energy meter detects and monitors the consumption of the loads connected to the AC distribution grid so that their energy consumption can be billed to the energy supplier.By transmitting the energy consumption from the energy meter to the inverter, specifically to its control unit, the inverter can interpret this consumption as the power setpoint Psw for the first power flow P1 to be exchanged with the AC distribution grid, which is required to supply the loads with electricity. The control unit can thus track and monitor, even in standby mode, the power exchange with the AC distribution grid required to supply the loads connected there with electricity. It is understood that the power setpoint Psw can be provided and monitored by the energy meter not only in standby mode, but also in the inverter's normal operating mode.

[0032] In one variant of the method, the power setpoint Psw can be generated using an energy management device. This is particularly advantageous when the inverter, along with other loads, is connected to an AC distribution grid that is connected to a higher-level energy supply grid via a grid connection point. The power setpoint Psw can be selected such that one or more of the following objectives are pursued during normal operation of the inverter: minimizing a power flow PNAP from the higher-level energy supply grid via the grid connection point towards the AC distribution grid, minimizing energy costs for an energy quantity AE that is drawn from the higher-level energy supply grid via the grid connection point within one billing period or within a combination of several billing periods.

[0033] In one variant of the method, the AC network can comprise several phase conductors L1, L2, L3 and the inverter can be designed as an inverter with unbalanced load capability. The power setpoint Psw can comprise several different power setpoints Psw for the respective phase conductors L1, L2, L3 of the AC network. In this way, even loads which individually or as a whole cause asymmetrical power consumption in the respective phase conductors L1, L2, L3 of the AC network can be supplied via the inverter. If the power setpoint Psw comprises several different power setpoints Pswj for the respective phase conductors L1, L2, L3 of the AC network, a sum of the power setpoints Psw across all phase conductors L1, L2, L3 can be used to compare the power setpoint Psw with the first threshold value PTH.

[0034] The inverter can be designed as a single-stage inverter and comprise a DC / AC converter arranged between the AC connection and the first DC connection, but not a DC / DC converter. Alternatively, however, it is also possible for the inverter to be designed as a multi-stage inverter and have a first DC / DC converter arranged between the DC / AC converter and the first DC connection. The first DC / DC converter is usually a bidirectional DC / DC converter. In such a multi-stage inverter, the first DC / DC converter can be deactivated in standby mode in addition to the DC / AC converter. In this way, the energy consumption associated with controlling the first DC / DC converter can be minimized, which in turn results in increased efficiency of the inverter.

[0035] In one embodiment, the inverter can be designed as a hybrid inverter which, in addition to the first DC connection, has a second DC connection for connecting a further DC source. The second DC connection can be connected to the DC / AC converter of the inverter either directly or via a second DC / DC converter. In one variant of the method, an exchange of power between the at least one further DC source and the battery can be enabled at least temporarily in standby mode, for example to charge the battery from the further DC source. This is particularly advantageous when the battery has a low state of charge (SOC) and / or sufficient electrical power is available from the at least one further DC source.Therefore, to at least temporarily enable the power exchange between the at least one further DC source and the battery, one or more of the following conditions can be checked: a) a state of charge of the battery falls below a SOC limit value SOCTH, b) a maximum possible third power flow P3max from the at least one further DC source exceeds a third threshold value PTH,3.

[0036] In a variant of the method, criterion a) can be regarded as a necessary criterion for enabling power exchange in standby mode. If the battery already has a high state of charge, it is not sensible, perhaps even impossible, to charge it further. Criterion b) checks whether the maximum available third power flow P3 of the at least one additional DC source ensures a power exchange with a sufficiently high degree of efficiency between the first DC / DC converter and the at least one second DC / DC converter. However, the third threshold value PTH,3 for criterion b) can be dependent on the state of charge SOC of the battery. If both criteria a) and b) are not met, it is typically not advantageous to enable power exchange between the battery and the at least one additional DC source in standby mode.In this case, the first DC / DC converter and the at least one second DC / DC converter of the hybrid inverter can also be deactivated in standby mode. The control unit of the hybrid inverter can also be put into an energy-saving mode, since only some, but not all, of its functionalities need to be guaranteed in the hybrid inverter's standby mode. It is understood that the inverter can have not just one, but several second DC connections, each designed to connect a further DC source. In this case, each of the second DC connections can be connected to the DC / AC converter of the inverter via a separate second DC / DC converter. This can enable a power exchange between one of the further DC sources and the battery, and if necessary also a power exchange between several of the further DC sources and the battery in the inverter's standby mode.

[0037] The additional DC source could be, for example, a PV generator or a fuel cell. If the additional DC source can be operated exclusively as a DC source and not as a DC load, it is sufficient for the second DC / DC converter to be designed for unidirectional power flow. If the DC source can also be operated as a DC load, as is the case with certain fuel cells, for example, the second DC / DC converter can also be designed for bidirectional power flow.

[0038] It is possible for the inverter itself to have an energy management device for controlling one or more loads that are connected to the AC grid together with the inverter. In an alternative embodiment, however, the energy management device can be a device separate from the inverter. In this case, the inverter can have an interface for connecting to the separately designed energy management device. Alternatively or cumulatively, the inverter can also have an interface to the energy meter, which is usually a separate device relative to the inverter. Furthermore, the inverter can have an interface for a battery management system that is separate from the inverter. The interfaces can be a plurality of interfaces, each of which is separate from one another.Alternatively, however, it can also be one and the same interface, designed to connect to the separate battery management system, the separate energy management device, and the separate energy meter. The interface(s) can each operate wired or based on radio signals.

[0039] Under certain conditions, it may be desirable to deliberately prevent the inverter from operating in standby mode. These conditions can occur, for example, in a house connection, in other words an AC distribution grid that is connected to a higher-level AC power supply grid via a grid connection point. Specifically, in the event of a failure of the higher-level power supply grid, it may be desirable for the AC distribution grid to be deliberately disconnected from the higher-level power supply grid and for the loads connected to the AC distribution grid to be supplied via the inverter and the battery connected to it. If the inverter were to operate in standby mode in this case, there is a risk, depending on the consumption of the connected loads, that at least some of the loads will not be supplied at all because their consumption is insufficient to operate the inverter in normal operating mode.For this reason, in one embodiment, the inverter can be designed to suppress a change in operating modes between normal operating mode and standby mode and / or operation in standby mode under predetermined framework conditions. The predetermined framework conditions can, in particular, be a failure of an otherwise properly operating power grid, in which the inverter operates as part of an emergency power supply for loads and, in particular, to regulate the voltage. It is possible for such functionality to be activated or deactivated on the inverter. Activation or deactivation can, for example, be achieved via parameterization in an operating program of the inverter or via a switch attached to the inverter.

[0040] In one embodiment of the method, the first threshold value PTH can be determined taking into account an efficiency curve as a function of the converted power of the inverter. This is described in more detail again in connection with Fig. 3. The determination can optionally be carried out by specifying a minimum tolerable efficiency r|o. In particular, the minimum tolerable efficiency r|o can be selected to be smaller than the quotient of electricity generation costs to electricity purchase costs. In addition, storage wear costs and a so-called round-trip efficiency r|roundtn Pof the system must be taken into account. The round-trip efficiency T| roundtrip indicates how efficiently an energy storage system stores and releases an amount of energy. The efficiency curve as a function of the converted power is a parameter that characterizes a particular inverter type and is usually determined in laboratory tests for this purpose. In this way, the existing efficiency curve can be used in addition to determining the first threshold value PTH.I , and if necessary also the second threshold value PTH,2 and / or the third threshold value PTH,3, in order to suppress operation of the inverter with poor efficiency, i.e. below the minimum tolerable efficiency r|o. The first threshold value PTH can be selected such that it does not exceed a value of 25%, preferably a value of 15%, particularly preferably a value of 10% of the nominal power Pnom of the inverter.

[0041] Short description of the characters

[0042] The invention is illustrated below with the aid of figures, of which

[0043] Fig. 1 shows an inverter according to the invention connected to an AC network in a first embodiment;

[0044] Fig. 2 shows an inverter according to the invention connected to an AC network in a second embodiment;

[0045] Fig. 3 is a diagram showing efficiency as a function of AC power for an inverter;

[0046] Fig. 4 is a flowchart of the method according to the invention in one embodiment.

[0047] Figure description

[0048] Fig. 1 shows an inverter 10 according to the invention connected to an AC grid in a first embodiment. The AC grid is, by way of example, an AC distribution grid 54 which is connected to a higher-level energy supply grid 55 via a grid connection point 51. Further loads (not explicitly shown in Fig. 1) are connected to the AC distribution grid 54, which loads have a total power consumption Pi_ast. The inverter 10 comprises an AC connection 11 with several phase conductor connections - here: by way of example, three phase conductor connections - which are each connected to one of the phase conductors L1, L2, L3 of the AC distribution grid 54. A neutral conductor connection of the AC connection 11 is connected to a neutral conductor N of the AC distribution grid 54. The inverter 10 further comprises a first DC connection 12, to which a battery 30 is connected.Within the inverter 10, the AC terminal 11 is connected to the first DC terminal 12 via an AC relay 18, a DC / AC converter 16, and optionally a first DC / DC converter 14. To symbolize the optional nature of the first DC / DC converter 14, it is shown in dashed lines. If the inverter 10 includes the first DC / DC converter 14, a DC intermediate circuit is formed between the DC / AC converter 16 and the first DC / DC converter 14, which intermediate circuit may include one or more intermediate circuit capacitors 19 for buffering DC energy (also shown in dashed lines in Fig. 1 due to their optional nature). The inverter 10 further comprises a control unit 17 for controlling the inverter 10, in particular the AC relay 18, the DC / AC converter 16 and - if present - the first DC / DC converter 14.The inverter 10 can optionally further comprise an energy management device 20 that is connected to the control unit 17 for control purposes (shown in dashed lines in Fig. 1 due to its optional existence). In Fig. 1, the control connections of the control unit 17 and its components are symbolized by dashed lines.

[0049] To implement the method, the control unit 17 is connected via an interface 21 of the inverter 10 to an energy meter 52, which detects, in a phase-sensitive manner, the electrical power PNAP exchanged between the energy supply network 55 and the AC distribution network 54 via the grid connection 51. The electrical power PNAP detected by the energy meter 52 can be communicated to the control unit 17 and—if present—also to the energy management device 20 via the interface 21. The control unit 17 and / or the energy management device 20 can, taking these values ​​into account, determine and provide a power setpoint Psw for a first power flow P1 to be exchanged between the inverter 10 and the AC distribution network 54. The control unit 17 then checks whether an absolute value of the power setpoint Psw is greater than or equal to a predefined first threshold value PTH with PTH > 0.If the absolute value of the power setpoint Psw reaches or exceeds the first threshold value PTH, the method assumes that operation of the inverter 10 with a tolerable efficiency r| is guaranteed. In this case, the inverter 10 is operated in the normal operating mode, in which the first power flow P1 is set according to the power setpoint Psw. To this end, the control unit 17 clocks the semiconductor switches of the DC / AC converter—and, if present, of the first DC / DC converter 16—accordingly, whereby the first power flow P1 is set at the AC terminal 11 of the inverter 10. The electrical power required to set and maintain the first power flow P1 is taken from the battery 30 as the second power flow P2.If the absolute value of the power setpoint Psw is below the first threshold value PTH, the method assumes that operation of the inverter 10 with sufficient efficiency r| is not guaranteed and then places or operates the inverter 10 in standby mode, in which the first power flow P1 between the inverter 10 and the AC distribution grid 54 is suppressed. The first power flow P1 can be suppressed by deactivating the DC / AC converter 16 and—if present—also the first DC / DC converter 14 by not controlling its semiconductor switch(es). Furthermore, the connection between the inverter 10 and the AC distribution grid 54 can be severed by opening the AC relay 18.

[0050] However, even in standby mode, the electrical power PNAP flowing through the grid connection point 51 can continue to be monitored by the energy meter 52 and transferred to the inverter 10 via the interface 21. Thus, even in standby mode, the control unit 17 can determine when operation with sufficient efficiency r| is again guaranteed and then return the inverter 10 to normal operating mode or operate it in normal operating mode.

[0051] Fig. 2 shows a second embodiment of an inverter 10 according to the invention connected to an AC grid. Only the differences from the first embodiment shown in Fig. 1 are explained below. For the features of the second embodiment that correspond to those shown in Fig. 1, reference is made to the description in Fig. 1.

[0052] The inverter 10 according to the second embodiment is designed as a hybrid inverter which, in addition to the first DC connection 12, has a second DC connection 13 for connecting an additional DC source. While a battery 30 is connected to the first DC connection 12, analogous to Fig. 1, the second DC connection 13 in Fig. 2 is exemplarily connected to a photovoltaic (PV) generator 40 as an additional DC source. Both the first DC connection 12 and the second DC connection 13 are each connected to the DC / AC converter 16 via a separate DC / DC converter 14, 15 and the common DC intermediate circuit, on which one or more intermediate circuit capacitors 19 can be arranged to buffer DC energy.

[0053] In contrast to Fig. 1, the energy management device 20 is no longer an integral component of the inverter 10, but is designed as a separate energy management device 20, which is connected on the one hand to the energy meter and on the other hand via the interface 21 to the control unit 17 of the inverter 10. It is thus designed to receive measured values ​​of an electrical power PNAP ZU flowing via the grid connection 51, as detected by the energy meter 52, to process the measured values, to determine a power setpoint Psw for the first power flow P1 to be exchanged between the inverter 10 and the AC distribution network 54, and to provide the power setpoint Psw to the control unit 17 of the inverter 10 via the interface 21. Similar to the first embodiment according to Fig.1, the control unit 17 then checks whether the absolute value of the provided power setpoint Psw reaches or exceeds a predefined first threshold value PTH. If this is the case, it then operates the inverter 10 in a normal operating mode, in which the first power flow P1 is set to the power setpoint Psw by appropriately controlling the DC / AC converter 16 at the AC connection 11. The electrical energy required for this is taken from the battery 30 by controlling the first DC / DC converter 14 and from the PV generator 40 by controlling the second DC / DC converter 15. If the absolute value of the provided power setpoint Psw falls below the predefined first threshold value PTH, it is concluded that normal operation with sufficient efficiency r| of the inverter 10 is not possible and the inverter 10 is put into standby mode or operated in standby mode.

[0054] In the second embodiment of the inverter 10, the DC / AC converter 16 is also deactivated in standby mode and, if necessary, the AC relay 18 is opened, thereby suppressing the first power flow P1 between the inverter 10 and the AC distribution grid 54. However, under certain conditions, a power exchange between the additional DC source (here: the PV generator 40) and the battery 30 may also be permitted in standby mode. This may be advantageous, for example, if the battery's state of charge falls below a limit value SOCTH and the PV generator 40 delivers a sufficiently high maximum possible power P3max to ensure efficient operation of the DC / DC converters 14, 15. It is therefore possible, but not absolutely necessary, for the first DC / DC converter 14 and the second DC / DC converter 15 to also be deactivated in the standby mode of the hybrid inverter.

[0055] 1 and 2, the AC grid is shown as a three-phase AC grid and the inverter 10 is shown as a three-phase inverter. Within the scope of the invention, however, it is also possible for the AC grid to have a different number of phase conductors and / or for the inverter 10 to have a different number of phase connections. For example, the AC grid can also be designed as a so-called split-phase grid, which has only two phase conductors and one neutral conductor. The number of phase connections of the inverter 10 can be the same or different to the number of phase conductors of the AC grid. For example, despite a multi-phase AC grid, the inverter 10 can also be a single-phase inverter, which in this case can be connected to one of the phase conductors and a neutral conductor.

[0056] Fig. 3 shows a typical curve of an efficiency r| of an inverter 10, for example, an inverter 10 as shown in Fig. 1 or Fig. 2. The diagram shows the efficiency r| of the inverter 10 (on the ordinate) as a function of an AC power P1 / Pnom normalized to a nominal power Pnom of the inverter (on the abscissa). The converted AC power according to Fig. 1 and Fig. 2 corresponds to the first power flow P1. The curve shows a maximum efficiency r|max at a certain ratio of converted AC power P1 to nominal power Pnom (in Fig. 3, for example, at P1 / Pnom =0, 38). Starting from this maximum efficiency r|max, there is a sharp drop in efficiency with decreasing AC power towards lower AC power levels – i.e., to the left of the maximum – as the AC power decreases. Towards higher AC power levels – i.e., to the right of the maximum efficiency r|max – there may be a slight drop in efficiency r| with increasing AC power, which, however, is significantly weaker compared to a curve to the left of the maximum efficiency r|max. For some inverters 10, it is also possible that no noticeable decrease in efficiency can be observed to the left of the maximum efficiency r|max, i.e., that the efficiency there remains at least approximately constant. The respective curve of the efficiency r| typically depends on various variables, for example, the topology of the inverter 10, the respective components, such as the semiconductor switches, as well as their control during operation.Environmental influences, e.g., ambient temperature, can also play a role here. However, for a specific type of inverter (i.e., same topology, same components, and same design), the efficiency curve is largely identical. Furthermore, the efficiency curve is also a parameter that is determined in laboratory tests to characterize the performance of a specific inverter type. Therefore, the efficiency curve r| characterizing the inverter type can advantageously be used to determine the threshold values ​​PTH and PTH,2. Specifically, for example, a minimum tolerable efficiency r|o can be defined or specified for the inverter 10 in question.Based on the efficiency curve r| (P1 / Pnom) measured in laboratory tests, a minimum tolerable power ratio between converted AC power and nominal power, and thus a minimum tolerable AC power of the inverter, can be derived. This can then be specified or stored for the inverter 10, in particular its control unit 17, as the first threshold value PTH. For a power setpoint Psw, and thus a first power flow P1, that reaches or exceeds the first threshold value PTH, it can be assumed that the efficiency r| also reaches or exceeds the minimum tolerable efficiency r|o in the normal operating mode of the inverter 10, and the inverter can be operated in the normal operating mode.On the other hand, if the power setpoint Psw, and thus the first power flow P1, falls below the first threshold value PTH, it can be assumed that the efficiency r| would also fall below the minimum tolerable efficiency r|o in the normal operating mode of the inverter 10. Accordingly, the inverter 10 can then be put into standby mode, in which the first power flow P1 is suppressed, since operation of the inverter 10 in the normal operating mode with an efficiency that reaches or exceeds the minimum tolerable efficiency r|o is not guaranteed.

[0057] Fig. 4 shows a flow diagram of a method according to the invention in one embodiment, as can be carried out, for example, with the inverter 10 shown in Fig. 1 or Fig. 2. To make it easier to compare the method with the components shown in Fig. 1 and / or 2, the components concerned, where mentioned, are also listed below with their respective reference numerals. The method begins with a first step S1, in which the inverter 10 is operated in standby mode as an example according to Fig. 4. In standby mode, the first power flow P1 between the inverter 10 and the AC distribution network 54 is suppressed and, optionally, the AC relay 18 of the inverter 10 is opened. In a second method step S2, a power setpoint Psw is provided.The determination and provision of the power setpoint Psw can be carried out with the assistance of the energy meter 52, the energy management system 20, the control unit 17, and / or a battery management system. Furthermore, a higher-level operating procedure of the inverter 10 can be taken into account, which, for example, aims to limit the amount of energy AE drawn from an AC distribution grid 54 via a grid connection point 51 from a higher-level energy supply grid 55—and thus the electrical power PNAP flowing into the AC distribution grid 54—during a billing period. Specifically, it can therefore be provided that the loads connected to the AC distribution grid 54 are supplied predominantly or at least partially from electrical power drawn from the battery 30 and, if applicable, the PV generator 40.

[0058] In a third step S3, it is checked whether an absolute value of the power setpoint Psw exceeds a second threshold value PTH,2. In particular, it can be checked whether it is guaranteed that the power setpoint Psw exceeds the second threshold value PTH,2 for a second time period Δt2. If this is not the case ("no" at S3), it is concluded that normal operation of the inverter 10 with a sufficiently high efficiency r| is not guaranteed. In this case, the method returns to step S1, and the inverter 10 remains in standby mode.If, however, during the check in step S3, the absolute value of the power setpoint Psw is greater than the second threshold value PTH,2, and it is optionally ensured that the absolute value of the power setpoint Psw is greater than the second threshold value PTH,2 for a second time period Δt2, then, in response, the inverter 10 is operated in the normal operating mode in a fourth step S4. In the normal operating mode, the AC relay 18 of the inverter 10 is closed, and the first power flow P1 is set to the power setpoint Psw. In a fifth step S5, a power setpoint Psw is again provided, optionally with the cooperation of the energy meter 52, the energy management device 20, the control unit 17, and / or a battery management system.In a sixth step S6, a check is performed to determine whether the absolute value of the newly provided power setpoint Psw reaches or exceeds a first threshold value PTH. In particular, a check can be performed to determine whether it is ensured that the absolute value of the power setpoint Psw reaches or exceeds the first threshold value PTH for a first time period Ati. If this is the case ("yes" at S6), the method returns to the fourth step S4, and the inverter 10 remains in normal operating mode, in which the first power flow P1 is set to the power setpoint Psw. If, however, the check in the sixth step S6 is negative ("no" at S6), it is concluded that normal operation of the inverter 10 with a sufficiently high efficiency r| is not guaranteed, and the method returns to the first step S1, in which the inverter 10 is operated in standby mode with the first power flow P1 suppressed.

[0059] The method was described using a start (first step S1) in standby mode. Alternatively, however, it is also possible for the method to begin with the normal operating mode, i.e., starting from the fourth step S4.

[0060] List of reference symbols

[0061] 10 inverters

[0062] 11 AC connection

[0063] 12 DC connection

[0064] 13 DC connection

[0065] 14 DC / DC converters

[0066] 15 DC / DC converters

[0067] 16 DC / AC converters

[0068] 17 Control unit

[0069] 18 AC relays

[0070] 19 DC link

[0071] 20 Energy management facility

[0072] 30 Battery

[0073] 40 PV generator

[0074] 51 connection point

[0075] 52 energy meters

[0076] 54 AC distribution network

[0077] 55 Energy supply network

[0078] P1, P2, P3 power flow

[0079] Ati, At2, At3Duration

[0080] S1 -S6 process step

Claims

Patent claims 1. A method for operating an inverter (10) for a battery (30) as a function of a power setpoint Psw for a first power flow P1 to be exchanged between the inverter (10) and an AC grid, wherein the inverter (10) is connected to the AC grid via an AC connection (11) and to the battery (30) via a first DC connection (12), and wherein the inverter (10) has a bidirectional DC / AC converter (16) arranged between the AC connection (11) and the first DC connection (12), comprising the steps of: i) providing the power setpoint Psw, ii) operating the inverter (10) in a normal operating mode, in which the first power flow P1 between the inverter (10) and the AC grid is set to the power setpoint Psw when an absolute value of the power setpoint Psw is greater than or equal to a positive first PTH threshold.I , with PTH > 0, and iii) operating the inverter (10) in a standby mode in which the first power flow P1 between the inverter (10) and the AC grid is suppressed when the absolute value of the power setpoint Psw falls below the first threshold PTH.I.

2. The method according to claim 1, wherein in the standby mode the DC / AC converter (16) is deactivated, and optionally an AC relay (18) of the inverter (10) arranged between the DC / AC converter (16) and the AC connection (11) is open.

3. The method according to claim 1 or 2, wherein the method steps i) to iii) are repeated.

4. Method according to one of the preceding claims, wherein operation in the standby mode only occurs when it is ensured that the absolute value of the power setpoint Psw falls below the first threshold value PTH.I for a predefined first time period Ati.

5. Method according to one of claims 2 to 4, wherein the inverter (10) is designed as a multi-stage inverter and comprises a first DC / DC converter arranged between the DC / AC converter (16) and the first DC terminal (12). converter (14), and wherein in the standby mode, in addition to the DC / AC converter (18), the first DC / DC converter (14) is also deactivated.

6. Method according to one of the preceding claims, wherein the first threshold value PTH is determined taking into account an efficiency curve as a function of the converted power, and wherein the determination is optionally carried out under specification of a minimum tolerable efficiency r|o.

7. Method according to one of the preceding claims, wherein the power setpoint Psw is also provided in the standby mode for the first power flow P1 to be exchanged between the inverter (10) and the AC grid, and wherein the transition from the standby mode to the normal operating mode occurs when an absolute value of the power setpoint Psw exceeds a second threshold value PTH,2 with PTH,2 > PTH, optionally when it is ensured that the power setpoint Psw exceeds the second threshold value PTH,2 for a predefined second time period Δt2.

8. The method according to claim 7, wherein the AC network is an AC distribution network (54) which is connected to a higher-level energy supply network (55) via a network connection point (51), and wherein the power setpoint Psw for the first power flow P1 to be exchanged between the inverter (10) and the AC distribution network (54) is determined by means of an energy meter (52) which is arranged at the network connection point (51).

9. The method according to one of claims 5 to 8, wherein the inverter (10) is designed as a hybrid inverter which, in addition to the first DC connection (12), has at least one second DC connection (13) for connecting at least one further DC source (40), wherein the at least one second DC connection (13) is connected to the DC / AC converter (16) via a second DC / DC converter (15), and wherein, in the standby mode, a power exchange between the at least one further DC source (40) and the battery (30) is made possible at least temporarily.

10. The method according to claim 9, wherein for at least temporarily enabling the power exchange between the at least one further DC source (40) and the battery (30) one or more of the following conditions is / are checked: a) a state of charge SOC of the battery (30) falls below a SOC limit value SOCTH, b) a maximum possible third power flow P3max from the at least one further DC source (40) exceeds a third threshold value PTH,3.

11. Method according to one of claims 8 to 10, wherein the power setpoint Psw is generated by means of an energy management device (20) and is selected such that in the normal operation of the inverter (10) one or more of the following objectives are pursued: Minimizing a power flow PNAP from the higher-level energy supply network (55) via the network connection point (51) towards the AC distribution network (54), Minimizing energy costs for an amount of energy AE that is drawn from the higher-level energy supply network (55) via the network connection point (51) within one billing period or within a combination of several billing periods.

12. Method according to one of the preceding claims, wherein the AC network comprises a plurality of phase conductors L1, L2, L3 and the inverter (10) is designed as an inverter capable of unbalanced loads, and wherein the power setpoint Psw comprises a plurality of different power setpoints Psw for the respective phase conductors L1, L2, L3 of the AC network (50).

13. Inverter (10) for a battery (30) comprising: - an AC connection (11) for connecting the inverter (10) to an AC network, - a first DC connection (12) for connecting the inverter (10) to the battery (30), - a bidirectional DC / AC converter (16) arranged between the AC terminal (11) and the first DC terminal (12), and - a control unit (17) connectable to an energy management device (20) and / or an energy meter (52) and / or a battery management system for controlling the inverter (10), characterized in that the control unit (17) connected to the energy management device (20) and / or the energy meter (52) and / or the battery management system is designed to control the inverter (10) in a state connected to the AC mains and the battery (30) according to the method of any one of the preceding claims.

14. Inverter (10) according to claim 13, characterized in that the inverter (10) is designed as a multi-stage inverter and additionally has a first DC / DC converter (14) which is arranged between the DC / AC converter (16) and the first DC connection (12).

15. Inverter (10) according to claim 14, characterized in that the inverter (10) is designed as a hybrid inverter which, in addition to the first DC connection (12), has a second DC connection (13) for connection to a further DC source (40), and wherein the second DC connection (13) is optionally connected to the DC / AC converter (16) via a second DC / DC converter (15).

16. Inverter (10) according to one of claims 13 to 15, characterized in that the inverter (10) has an interface (21) for connection to a separately designed energy management device (20) and / or the energy meter (52) and / or the battery management system.

17. Inverter (10) according to one of claims 13 to 16, characterized in that the inverter (10) is designed to prevent a change of the operating modes between normal operating mode and standby mode and / or operation in standby mode under given framework conditions.

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