DE102019116254B4 - Method for operating a power generation plant - Google Patents

Abstract

Method for operating a power generation plant (1) which has at least two inverters (4), in which electrical power is exchanged between direct current sources (5) and an alternating voltage network (3) by means of the inverters (4), the inverters (4) optionally injecting current or operated with voltage impressing, wherein at least one of the inverters (4) of the energy generation system (1) is connected to a memory as a direct current source (5), the inverter (4) connected to the memory being operated in voltage impressing mode when a charge state of the memory exceeds a first limit value, and is operated temporarily in current-impressing mode in order to charge the storage unit when the state of charge of the storage unit falls below a second limit value, with at least one of the inverters (4) of the energy generation system (1) having a PV generator as a direct current source ( 5) is connected, with the energy generation plant ge (1) exchanges a total electrical power (P_Netz) with the AC voltage network (3), the inverter (4) connected to the PV generator being operated in current-impressing mode and the exchanged total power (P_Netz) a maximum possible PV power (P_MPP ) of the PV generator and wherein the inverters (4) are switched individually or in groups between a current-impressing operation and a voltage-impressing operation, so that the energy generation system (1) is operated as a whole in a current-impressing overall operation, in a voltage-impressing overall operation or in a mixed operation .

Description

The invention relates to a method for network-beneficial operation of an energy generation plant, in which electrical power is exchanged with an alternating voltage network, as well as an energy generation plant which is set up for operation with a corresponding method.

An energy generation system can include an energy generation unit, in particular a renewable energy source, and an electrical storage device. The energy source and storage unit can each be connected to an AC voltage network via an inverter and serve as a direct current source or sink for the respective inverter. The inverters can exchange electrical power on the direct current side with the respective direct current source, convert it into an alternating voltage or an alternating current and exchange it on the alternating current side with an alternating voltage network or with one another. A direct current source can both output and absorb electrical power, i.e. act as a direct current sink, and the AC voltage network can also both absorb and output electrical power.

A normal power generation system with several direct current sources and inverters can process outputs between a few kilowatts and a few megawatts and exchange them with the alternating voltage network via a common network connection point. Alternatively or additionally, an energy generation system can be formed from several direct current sources and inverters, which are communicatively linked directly or indirectly via a common control and exchange a total adjustable total electrical power with the alternating voltage network via various network connection points.

With regard to the regulation of the behavior of a power generation plant on the AC voltage network, i.e. in particular with regard to the active power and reactive power to be set at the grid connection point of the power generation plant or a part thereof, a fundamental distinction can be made between grid-forming and grid-supporting power generation plants. Conventional synchronous generators usually work voltage-impressing and thus network-forming. Renewable energy generation systems work primarily in a manner that impresses the current, with the current being impressed in principle by the energy generation system itself and, in particular, based on the properties and the current state of the energy source used.

In a power generating system that impresses current, an inverter can itself specify the power exchanged on the DC and AC sides, whereby the limits of the performance of the connected DC source must be observed. In particular, the maximum available power is drawn from a photovoltaic generator as a renewable energy source and fed into an existing AC voltage network as the maximum possible alternating current by an inverter operated to inject current.

A current-collecting inverter is continuously synchronized to an existing grid voltage during operation. e.g. by means of a phase locked loop (PLL). It is set up to follow the mains voltage and to adapt its current output to the mains voltage. This adaptation requires a finite adaptation time in order to exchange the available or required electrical power with the AC voltage network.

The exchanged power of a current-injecting inverter can also be varied so that, for example, less than its maximum possible power is deliberately drawn from a photovoltaic generator and fed into the AC voltage network. In this respect, the electrical power to be exchanged can be set in a targeted manner on the basis of overriding considerations, e.g. to optimize self-consumption, to maximize feed-in tariffs, to keep reserve power available and / or to be able to contribute to network support depending on the electrical properties of the AC voltage network.

A current-injecting power generation system can also fulfill network-supporting functions in that the electrical power exchanged with the AC voltage network is indirectly dependent on properties of the AC voltage network, for example the frequency and / or the amplitude of the network voltage. For this purpose, the exchanged power can be varied within the scope of the available power of the energy source, for example in the event of a deviation of the network frequency from a nominal frequency or the network voltage from the sinusoidal nominal voltage of the AC voltage network. The AC voltage network can be stabilized in that the exchanged power depends on the electrical properties of the AC voltage network in such a way that such deviations are counteracted, for example using active power-frequency characteristics (P (f)) or reactive power-voltage characteristics (Q (U) ). An adaptation of the exchanged power to a changing target value is, however, due to the above-mentioned finite adaptation time compared to the Adaptation inducing deviation in the AC voltage network is inherently delayed.

As the proportion of power-controlled power generation systems in an AC voltage network increases, their influence on the electrical properties of the AC voltage network, in particular on the network voltage and the network frequency, increases. This influence can have a negative effect on the AC voltage network. For example, depending on the nature of the network, a high active power fed in by means of current-impressing inverters can cause a local increase in the network voltage. In a network section that on the one hand is weakly connected to an AC voltage network and on the other hand has a large number of power-generating power generation plants, flicker effects can be caused in the grid voltage if the power generation plants carry out active or escalating processes for normatively required islanding detection (AID). Furthermore, harmonics and harmonics can occur in the AC voltage network, which are not counteracted by current-injecting inverters, but are even conveyed by current-injecting inverters under unfavorable circumstances, for example if individual power generation systems or inverters supply a high proportion of the total power in the AC voltage network and thus significantly influence the network voltage .

In the conventional operation of power-injecting power generation systems, the power exchanged with the AC voltage network is based on the optimal use of the direct current source and its current and / or anticipated power or stored energy. For this reason, power generation systems operated in a current-impressing manner are generally not set up to adapt the power exchanged on the DC and AC sides instantaneously in response to rapid changes in the electrical properties of the AC voltage network. In addition, power-injecting power generation systems can additionally weaken the AC voltage network by attempting to maintain a nominal current to be impressed, largely regardless of whether this is beneficial for the stability of the AC voltage network or not. A power generation plant operated with an injection of current can additionally only include network-supporting functions, in that the power generation plant reacts to deviations in electrical properties of the AC voltage network from standard values and varies the setpoint current. The resulting change in the exchanged power has an inherent delay compared to the occurrence of the deviations due to measurement and evaluation periods and finite control reaction times.

A power generation system can also be operated with voltage impressing. Inverters operated with voltage impressions shape and stabilize the line voltage at their connection terminals by regulating the electrical power as a function of the line voltage. A voltage-impressing power generation plant is basically able to work in a grid-forming manner, i.e. in particular to set up and maintain a limited island grid. The power exchanged between a voltage-impressing inverter and the AC voltage network can depend directly on the properties of the AC voltage network, i.e. in particular it can be a function of the frequency and / or the voltage in the AC voltage network. As a result, a power generation system operated with a voltage impression reacts immediately to changes in the power balance in the AC voltage network. In addition, a voltage-impressing power generation system can contribute to smoothing the sinusoidal curve of the mains voltage by counteracting undesired frequency components with high dynamics.

An inverter operated in a voltage-impressing manner can, for example, include what is known as a droop control and / or be operated as a so-called virtual synchronous machine. With the aid of such a control system, which is basically known to the person skilled in the art, an island network can be set up, such an island network being able to be operated synchronously with a higher-level and possibly spatially significantly expanded AC voltage network. Electrical power can then be exchanged between the AC voltage network and the stand-alone network with the power generation system via a network connection point. Alternatively, the inverters of a voltage-impressing power generation system can also be connected directly to an AC voltage network via a common network connection point. As a result, the AC voltage network can be stabilized and the network voltage quality can be improved.

The voltage-impressing operation of a power generation plant as a proactive network builder demands a significantly higher dynamic of the connected direct current source than the current-impressing and optionally, at best, network-supporting operation. A network builder must react to a power imbalance in the AC voltage network that results in a change in network frequency with an immediate, highly dynamic power adjustment. Therefore, in particular, a so-called instantaneous reserve, which, among other things, instantaneous reaction of the power generation system to a gradient of the network frequency is characterized, preferably provided by voltage-impressively operated power generation systems.

Power generation systems operated with impressing voltage are based on the optimal stabilization of the network voltage using the resources available on the DC side, i.e. the available power of the DC source and its currently and / or likely stored energy. In this case, however, the power exchanged on the AC side results largely from the properties of the AC voltage network. In this respect, voltage-impressing power generation systems are usually not set up to use the resources available on the DC side due to specifications oriented on the DC side, for example to call up the available power of a photovoltaic generator as a DC source and to use it continuously. Therefore, voltage-impressing power generation systems can optimally stabilize the AC voltage network by attempting to exchange the electrical power required for this; However, this behavior is largely independent of whether the necessary electrical power is currently available cheaply on the DC side, for example from a photovoltaic generator, or can be exchanged with a battery if its state of charge allows this or is possibly not available at the moment.

STATE OF THE ART

From US 2017/0 353 127 A1 an inverter device is known in which several inverters are connected in parallel on the output side and autonomously, ie without connection to an AC voltage network, supply a load with a predetermined AC voltage by using one of the inverters as a voltage source and all other inverters as Power source are operated.

From the EP 2 182 626 A1 a method for operating a power converter is known, in which semiconductor switches are selectively or in combination controlled by means of a voltage-impressing and / or a current-impressing modulation. As a result, the in this EP 2 182 626 A1 Properties of the various types of modulation, which are described in detail and which result in a voltage-impressing or current-impressing behavior of the converter, are advantageously combined in a converter.

Embodiments of voltage-impressing operation of an inverter are, for example, from FIG DE 10 2006 047 792 A1 known, in which a so-called virtual synchronous machine (VISMA) is used for network support, the behavior of a synchronous machine being approximated by constantly solving differential equations. In the EP 3 376 627 A1 describes a voltage source inverter (VSI) whose regulation includes a structure for generating a virtual inertia so that the converter mimics the behavior of a synchronous generator.

From the EP 1 286 444 B1 a so-called droop control for a voltage-impressing inverter is known, the inverter being operated on the basis of a frequency droop f (P) and a voltage droop U (Q), so that the inverter reacts directly to frequency changes in the AC voltage network with a change in the active power and, in this respect, to the Parallel operation with other inverters and is particularly suitable for setting up an island grid.

The DE 10 2017 112 944 A1 and the DE 10 2017 112 936 A1 disclose wind farms with a large number of wind turbines, the operating mode of which can be selected from a current-defining operating mode and a voltage-defining operating mode, with a mixed operating mode that has or combines current-defining and voltage-defining properties at a respective network connection point.

WO 2017/097 354 A1 discloses a microgrid with a large number of distributed generators that can be connected to an energy grid via a switch, the operation of individual distributed generators being switched between a grid-following and a grid-shaping mode depending on the switching state of the switch.

OBJECT OF THE INVENTION

The invention is based on the object of showing a method for operating an energy generation system with several inverters, which enables efficient and profitable operation of the energy generation system by using the direct current sources in an energetically optimal manner, and with which, at the same time, the voltage and frequency quality and the stability of the alternating voltage network is improved.

SOLUTION

The object is achieved by a method with the features of independent claim 1 and with an energy generation plant according to claim 17. Preferred embodiments are defined in the dependent claims.

DESCRIPTION OF THE INVENTION

The object is achieved by a method for operating a power generation plant which has at least two inverters, in which electrical power is exchanged between direct current sources and an alternating voltage network by means of the inverters, the inverters being operated either with current impressing or voltage impressing. The inverters are switched individually or in groups between a current-impressing operation and a voltage-impressing operation, so that the energy generation system as a whole is operated in a current-impressing overall operation, in a voltage-impressing overall operation or in a mixed operation.

The power generation plant can thus be operated completely with current impressing or completely with voltage impressing, depending on whether a network-supporting or network-forming operation is advantageous for the stability or the structure of the AC voltage network. The method according to the invention is characterized in particular by the possibility of operating part of the power generation plant or the power generation plant as a whole, with either current impressing or voltage impressing, compared to conventional methods for operating an energy generation plant. In this way, the behavior of the power generation system can be tailored to the needs of the AC voltage network as required, in particular to help stabilize the AC voltage network, or to focus on the resources available on the DC side, in particular around the available power of a volatile renewable energy source and / or the storage capacity of a To use energy storage to the maximum. In addition, the weighting between these conflicting goals can be changed as required by switching over individual inverters of the power generation system.

Depending on the network quality or status as well as the operating status of inverters in a power generation plant, decisions can be made autonomously or externally or internally controlled via the specific operating mode of the individual inverters or inverter groups of the power generation plant. This determines, if necessary taking into account current internal boundary conditions, whether and to what extent a stress-absorbing operation of parts of the energy generation plant is necessary and possible.

Specifically, the inverters can, for example, be operated predominantly or completely in a current-impressing manner in normal operation within the scope of the method according to the invention, that is to say in particular at a network frequency that corresponds to a nominal frequency of the AC voltage network. Depending on requirements, in particular depending on the network quality, individual inverters can be switched to voltage-impressing mode. The worse the network quality, i.e. for example the greater the voltage or current distortion, flicker or the like, the more inverters can be switched to voltage-impressing mode in order to keep the network voltage and frequency stable and within the normative range. It proves to be advantageous to change the operating mode with as little interruption as possible and without significant compensation processes.

In the context of the method according to the invention, switching specifications can be transmitted to the inverters, the inverters changing between the current-impressing operation and the voltage-impressing operation as a function of the switching specifications. The switching specifications can in particular be transmitted to the inverters by a control unit of the power generation system. The switchover specifications can directly specify whether the inverters are operated in current-impressing or in voltage-impressing mode. As an alternative or in addition, the switching specifications can include limit values, the inverters determining electrical properties of the AC voltage network and, when the limit values are exceeded, autonomously switch between the current-impressing operation and the voltage-impressing operation. Through the switching specifications, individual inverters can be switched explicitly on the basis of a higher-level decision, for example by the control unit, or specific conditions can be established on the basis of which the inverters autonomously decide which operating mode is currently to be set. In addition, a preconfiguration of the inverters is conceivable, so that they automatically change the operating state independently depending on the grid status or the operating conditions of the power generation plant. Such a preconfiguration can be initiated initially via a central control unit and / or updated occasionally or regularly.

In one embodiment of the method, the energy generation plant is during the Switching over one or more inverters between the current-impressing operation and the voltage-impressing operation connected to the AC voltage network, ie any separating devices between the inverter or power generation system and the network are not operated. This ensures continuous operation of the energy generation system. In addition, the power generation plant can exchange active power and / or reactive power with the AC voltage network without interruption during the switchover of one or more inverters between the current-impressing operation and the voltage-impressing operation, in particular by the individual inverters of the energy generation plant exchanging active power and / or reactive power with the AC voltage network without interruption, if they can be switched between current-impressing operation and voltage-impressing operation. An uninterrupted switchover between the current-impressing operation and the voltage-impressing operation is particularly advantageous in sub-networks with a tight ratio between the power that can be generated and the power consumed, since in the event of an interruption in the exchange of power between the power generation system and the network, loads would be undersupplied and the frequency in the network could drop, which in turn could can result in an avalanche effect.

• - der Spannungsqualität, d.h. insbesondere der Amplitude der Netzspannung, dem Gradienten der Netzspannung, einem Maß für den Spannungsflicker und/oder für die harmonischen Verzerrungen der Netzspannung;
• - der Netzstabilität, d.h. insbesondere den Verzerrungen des zwischen Energieerzeugungsanlage und Wechselspannungsnetz ausgetauschten Wechselstroms;
• - der Frequenzstabilität, d.h. insbesondere der Abweichung der Netzfrequenz von einer Nennfrequenz und/oder dem Netzfrequenzgradienten, die einen jeweiligen Frequenzregelungs- bzw. Regelreservebedarf erzeugen;
• - der Netzimpedanz, wobei eine ggf. plötzliche Inselnetzbildung als Extremfall für die Netzimpedanz anzusehen ist.

In one embodiment of the method, the inverters of the energy generation system can be switched between current-impressing and voltage-impressing operation depending on the following parameters:

• the voltage quality, ie in particular the amplitude of the network voltage, the gradient of the network voltage, a measure for the voltage flicker and / or for the harmonic distortions of the network voltage;
• the network stability, ie in particular the distortion of the alternating current exchanged between the energy generation plant and the alternating voltage network;
• the frequency stability, ie in particular the deviation of the network frequency from a nominal frequency and / or the network frequency gradient, which generate a respective frequency control or control reserve requirement;
• - the network impedance, whereby a possibly sudden island network formation is to be regarded as an extreme case for the network impedance.

Suitable limit values can be established for these parameters within the scope of the method according to the invention, the exceeding of which can in turn be mapped in the switchover specifications to the inverters of the energy generation system.

In a further embodiment of the method, the inverters operated in current-impressing mode can exchange electrical power with the AC voltage network, the active power and / or reactive power of which are set depending on the voltage and / or the frequency of the AC voltage network. As a result, the inverters operated in current-impressing mode advantageously participate in grid support by being able to provide primary and / or secondary control power.

As an alternative or in addition, the inverters operated in voltage-impressing mode can exchange electrical power with the AC voltage network in such a way that they provide instantaneous reserve power. This is possible in a particularly advantageous manner in voltage-impressing operation and can make a decisive contribution to improving the stability of the AC voltage network. For this purpose, for example, a control structure can be used that simulates the behavior of a synchronous machine when the frequency changes, so that the network frequency is stabilized in a manner comparable to that of conventional power plants with rotating masses. Alternatively or additionally, a droop mode control can be used, which comprises a frequency-power characteristic. Such a control is also able to stabilize the network frequency and can additionally include a voltage-reactive power characteristic curve, by means of which, in addition to the network frequency, the network voltage can also be stabilized and, if necessary, an island network can be set up.

In a further embodiment of the method, individual or all inverters of the power generation system can be brought into current-impressing mode in the event of a network fault, i.e., if necessary, switch from voltage-impressing operation to current-impressing mode. In the event of a network fault, regularly defined requirements must be met, which can be specified in network guidelines in particular and which require the defined exchange of active and / or reactive power in order to drive through a network fault. By switching over to current impressing operation in the event of a network fault, these requirements can be optimally met. Optionally, after the error has been cleared, the inverters can return to the operating mode they were in before the error occurred, i.e. switch back to voltage-impressing mode if necessary. The inverters can recognize the presence of the grid fault themselves and, in the event of a grid fault, switch over to current-impressing operation themselves and optionally recognize the end of the presence of the grid fault themselves and independently switch back to the possibly different operating mode before the grid fault.

In the method in which at least one of the inverters of the energy generation system is connected to a storage unit as a direct current source, the inverter connected to the storage unit can preferably be used in voltage-impressing operation are operated when a charge state of the memory exceeds a first limit value. In this case, sufficient electrical energy is available to vary the power exchanged between this inverter and the AC voltage network in both directions, i.e. to increase or decrease and possibly even to reverse it, as part of the voltage-impressing operation to stabilize the AC voltage network. The relevant inverter can also be operated temporarily in current-impressing mode in order to charge the storage unit when the charge state of the storage unit falls below a second limit value.

In the method in which at least one of the inverters of the energy generation system is connected to a PV generator as a direct current source and the energy generation system exchanges a total electrical power with the AC voltage network, the inverter connected to the PV generator can preferably be operated in current-injecting mode, whereby the exchanged total power includes a maximum possible PV power of the PV generator. This ensures that the PV generator is used optimally in terms of energy and economy.

An energy generation system with direct current sources, which among other things include storage, can be operated with an embodiment of the method in which the at least one inverter connected to the storage is preferably switched to voltage impressing mode during a transition from the current-impressing overall operation to mixed operation of the energy-generating system. If the direct current sources include both storage systems and PV generators, all inverters connected to a storage system can be operated in the voltage-impressing mode in mixed operation of the energy generation system, provided that the respective state of charge allows this and there is no grid fault, and all inverters that are connected to a PV generator, operated in current-impressing mode. As a result, the storage systems are used optimally to stabilize the AC voltage network, while the energy that can be generated by the PV generators is fully retrieved and used.

In a specific embodiment of the method, the inverters comprise a control structure with a current control and voltage control, the current control and the voltage control each outputting control signals for controlling the power switches of the inverters and which of the output control signals for controlling the inverters is selected by means of a changeover switch in the respective inverter Circuit breakers of the respective inverter can be used. In a particularly advantageous embodiment of the method, the current regulation and the voltage regulation are coupled to one another within the framework of the regulation structure via mutual pilot controls. This means that switching between current-impressing operation and voltage-impressing operation can be carried out both without interruption and continuously, i.e. without jumps or distortions in the exchanged alternating current.

A power generation system according to the invention with a control unit and a plurality of inverters which are connected to the control unit and exchange electrical power with an AC voltage network is characterized in that it is set up to carry out the claimed method. In particular, the control unit can transmit switching specifications to the individual inverters, so that the inverters set a specific operating mode depending on the network quality, that is to say they are operated either with current impressing or voltage impressing.

Figure list

• 1 zeigt eine Energieerzeugungsanlage mit mehreren Wechselrichtern in einem stromeinprägenden Gesamtbetrieb;
• 2 zeigt eine Energieerzeugungsanlage mit mehreren Wechselrichtern in einem spannungseinprägenden Gesamtbetrieb;
• 3 zeigt eine Energieerzeugungsanlage in einem Mischbetrieb mit überwiegend stromeinprägenden Wechselrichtern;
• 4 zeigt eine Energieerzeugungsanlage in einem Mischbetrieb mit überwiegend spannungseinprägenden Wechselrichtern;
• 5 zeigt schematisch eine erste Ausführungsform einer Regelungsstruktur eines Wechselrichters einer Energieerzeugungsanlage;
• 6 zeigt schematisch eine weitere Ausführungsform einer Regelungsstruktur eines Wechselrichters einer Energieerzeugungsanlage;
• 7 zeigt eine erste Ausführungsform eines Ablaufdiagramms eines Verfahrens zum Betrieb einer Energieerzeugungsanlage;
• 8 zeigt eine weitere Ausführungsform eines Ablaufdiagramms eines Verfahrens zum Betrieb einer Energieerzeugungsanlage.

In the following, the invention is further explained and described with reference to the exemplary embodiments shown in the figures.

• 1 shows a power generation plant with several inverters in a current-impressing overall operation;
• 2 shows a power generation plant with several inverters in a voltage-impressing overall operation;
• 3rd shows a power generation system in mixed operation with predominantly current-injecting inverters;
• 4th shows a power generation system in mixed operation with predominantly voltage-impressing inverters;
• 5 shows schematically a first embodiment of a control structure of an inverter of a power generation plant;
• 6th shows schematically a further embodiment of a control structure of an inverter of a power generation plant;
• 7th shows a first embodiment of a flow chart of a method for operating an energy generation plant;
• 8th shows a further embodiment of a flow chart of a method for operating an energy generation plant.

FIGURE DESCRIPTION

1 shows a power generation plant 1 that have a network connection point 2 with an alternating voltage network 3rd connected is. The power generation plant 1 exchanges via the network connection point 2 electrical power in the form of an alternating current with the alternating voltage network 3rd out. The exchanged electrical power can comprise active power and / or reactive power.

The alternating voltage network 3rd can be a supra-regional network, whereby the energy generation plant 1 on one of the voltage levels of the interconnected network electrical power with the AC voltage network 3rd exchanges. The section of the network on which the power generation plant 1 with the AC voltage network 3rd connected, the lowest voltage level of the AC voltage network 3rd and comprise a spatially limited area, for example a distribution network of a property, or even have further subordinate voltage levels that cover a larger region, for example a medium-voltage level with several connected local networks.

The power generation plant 1 has multiple inverters 4th on. The inverters 4th are each directly or indirectly via connection systems (not shown), for example via transformers and isolating devices with the network connection point 2 connected.

In an alternative embodiment not shown here, the inverters 4th also arranged spatially and over several network connection points 2 with the AC voltage network 3rd be connected.

To the inverters 4th are each DC sources 5 connected. The direct current sources 5 can include generators, in particular photovoltaic generators, and / or energy stores, in particular batteries.

The power generation plant 1 can include further electrical components, not shown, for example further power electronic converters as well as alternating current and / or direct current loads.

The inverters 4th are set up to use the direct current sources 5 to take electrical power and as alternating current via the grid connection point 2 in the alternating voltage network 3rd feed and / or alternating current from the alternating voltage network 3rd and as direct current in the direct current sources 5 to feed. The inverters can do this 4th have a plurality of power switches, in particular semiconductor switches, which can be arranged in a bridge configuration known per se, so that the inverters 4th can transfer electrical power bidirectionally between their respective DC voltage and AC voltage side by means of suitable clocking of the circuit breakers.

The inverters 4th each include a controller 6th that are used to control the circuit breakers of the respective inverter 4th is set up. The controls 6th are with measuring devices 7th connected to the output lines of the inverter 4th are arranged. The measuring devices 7th record electrical properties that at least indirectly reflect the electrical properties of the AC voltage network 3rd correspond. In particular, the measuring devices 7th those from the respective inverter 4th Output alternating currents I or alternating voltages V and / or the voltages V of the alternating voltage network 3rd record by means of current sensors or voltage measuring devices.

The controls 6th the inverter 4th are set up to do this, the inverters 4th can be operated individually either in a current-impressing operating mode, hereinafter referred to as I-mode for short, or in a voltage-impressing operating mode, hereinafter referred to as V-mode for short.

An inverter is used in the current-impressing I-mode 4th controlled in such a way that an alternating current I is output with a predeterminable amplitude and thus a corresponding power, which is, for example, a desired or available power of the connected direct current source 5 oriented. In particular, it can be a maximum available power of a photovoltaic generator as a direct current source 5 or a charging or discharging capacity for charging or discharging a battery as a direct current source 5 act.

To set the exchange rate, in I-mode, the relevant control can 6th for example that of the measuring devices 7th Compare the detected actual value of the alternating current I with a setpoint value and clock the control of the circuit breakers in such a way that the alternating current I largely corresponds to the setpoint value. Clock methods suitable for this, for example pulse width modulation methods (PWM), are sufficiently known to the person skilled in the art.

In the voltage-impressing V-mode, an inverter is used 4th controlled in such a way that the alternating voltage V of the alternating voltage network 3rd by exchanging electrical power via the inverter 4th is influenced in the direction of a specifiable nominal value. Specifically, the controller in question 6th that of the measuring devices 7th recorded actual value of the alternating voltage V Compare with a nominal value which, for example, shows an ideal course of the alternating voltage V in the alternating voltage network 3rd maps. The control of the power switches can then be clocked in such a way that an alternating current I is generated, which counteracts a deviation of the alternating voltage V from the nominal value. Control methods suitable for this, for example droop controls or virtual synchronous machines or synchronous machine emulations, are sufficiently known to the person skilled in the art.

As an alternative or in addition to AC voltage V, the exchanged power, in V mode, can also depend on a variable derived from the detected AC voltage V, in particular on the network frequency, its deviation from a nominal frequency and / or its change gradient.

In the 1 and in the following figures with representations of the respective energy generation system 1 are the operating modes of the controls 6th indicated by boxes with the inscriptions “I” for the current-impressing I-mode and “V” for the voltage-impressing V-mode. The box shown with solid lines and bold symbolizes the currently used operating mode. Details of the controls 6th and the operating modes are in the 5 and 6th explained in more detail.

A higher-level control unit 8th can with the controls 6th the inverter 4th be communicatively connected. The control unit 8th can be part of the power generation system 1 be or outside of the power generation plant 1 be arranged. Especially if the inverter 4th not via a common network connection point 2 with the AC voltage network 3rd are connected, but spatially distributed in the AC voltage network 3rd are arranged, the control unit 8th be designed as part of a higher-level controller. The inverters 4th form the power generation plant 3rd then on the basis of a logical assignment via the control unit 8th .

The control unit 8th evaluates electrical properties of the AC voltage network 3rd from, in particular, the alternating voltage V and the alternating current I, which they, for example, from suitable at the network connection point 2 receives arranged measuring means.

The control unit 8th is set up on the basis of the recorded properties of the AC voltage network 3rd to decide in which operating modes the individual inverters 4th the power generation plant 1 are to be operated, and transmits these decisions to the controls as individual switching specifications 6th the inverter 4th . The switchover specifications can be a switchover of the operating mode of an addressed inverter 4th effect immediately. Alternatively or additionally, the control unit 8th Specify switching specifications that define the specific conditions under which an inverter 4th to switch the operating mode, with the conditions of the inverters 4th be checked by yourself. Individual inverters can also be used 4th under certain circumstances independently decide in which operating mode they are to be operated, for example in the event of a grid failure caused by the inverter 4th can be detected by yourself.

In the embodiment according to 1 become all inverters 4th operated in the current-impressing I-mode, ie the power generation plant 1 works in the current-impressing overall operation. The power generation plant 1 behaves as a whole in a current-impressing manner, ie between the energy generation plant 1 and AC voltage network 3rd exchanged electrical power is generated by the power generation plant 1 specified itself and possibly only indirectly depends on the properties of the AC voltage network 3rd from.

A target value for the exchanged electrical power when operating the power generation plant 1 according to 1 can depend on various internal and external parameters. In particular, in principle the maximum available power of the renewable energy sources used as direct current sources or a predetermined proportion thereof can be fed into the alternating voltage network 3rd be fed in. Alternatively or in addition, an operator of the energy generation plant can 1 dictate how the power generation system is 1 in the event of deviations in the electrical parameters of the AC voltage network 3rd should behave according to normal values. In particular, frequency-active power characteristics P (f) or voltage-reactive power characteristics Q (U) can be integrated into the current-impressing I-mode in a manner known per se.

Additionally or alternatively, a setpoint value for the electrical power of the energy generation system can be used 1 from the control unit 8th or another higher-level controller and / or, for example, can be specified in a variable manner within the framework of a primary or secondary control. In addition, the inverter can 4th be set up in the current-impressing I-mode to feed in suitable reactive and / or active currents for dynamic network support in the event of a network fault.

2 shows a power generation plant 1 , which are largely identical to the power generation plant 1 according to 1 is. In the power generation plant 1 according to 2 will the Inverter 4th operated in voltage-impressing V-mode. This operating mode can in particular come about in that in the control unit 8th it was decided the power generation plant 1 overall to operate voltage-impressing, and the control unit 8th then the corresponding identical individual specifications to the controls 6th the inverter 4th has transmitted. Alternatively, the inverter 4th have switched autonomously to the voltage-impressing V-mode after the inverters 4th have determined that the conditions for the switchover defined by the switchover specifications have occurred.

As all inverters 4th are operated in the voltage-impressing V-mode, the power generation system works 1 according to 2 in overall voltage-impressing operation and behaves in an overall voltage-impressing manner. This arises between the power generation plant 1 and AC voltage network 3rd exchanged electrical power directly dependent on the electrical properties of the AC voltage network 3rd a.

The inverters operated in voltage-impressing V-mode 4th can behave analogously to synchronous machines or synchronous generators, for example, in that the power fed in in each case is directly dependent on a deviation of an actual value of an electrical property of the AC voltage network 3rd from an associated nominal value and / or from a rate of change of an actual value of an electrical property of the AC voltage network 3rd depends. In particular, the electrical power of the power generation plant 1 in pure V-mode operation on the deviation of the current frequency and / or the current voltage from a nominal frequency or a nominal voltage of the AC voltage network 3rd as well as the rate of change of the frequency and / or the voltage of the AC voltage network 3rd depend.

In one embodiment, the voltage-impressing V-mode can be implemented in such a way that the inverter 4th each have a decoupling choke arranged on the AC voltage side. The voltage drop across the decoupling reactor can be used as a control variable in the V mode, which is applied to the voltage, so that the inverter 4th directly on gradients of the frequency of the AC voltage network 3rd responds.

Compared to the purely current-impressing operation according to 1 are in the voltage-impressing operation of the power generation plant 1 according to 2 increased demands on the performance of the direct current sources 5 posed. In particular, the power generation plant must 1 be able to work with the AC mains 3rd to change exchanged power in both directions, ie both to increase and to decrease, depending on the direction in which the electrical parameters of the AC voltage network change relative to the respective setpoint values. Provided that it is ensured that the energy generation system 1 meets these requirements, the power generation plant 1 can be used as a so-called instantaneous reserve, in particular in overall operation where the voltage is applied. The power generation plant 1 can also form and maintain a stand-alone network by using the electrical power required in the stand-alone network exclusively from the energy generation plant 1 is made available in the voltage-generating overall operation. In principle, such an island network can be created by the power generation plant 1 are temporarily supplied when an existing higher-level AC voltage network 3rd should not or cannot be used, for example in the event of a power failure.

3rd shows a power generation plant 1 , which are largely identical to the power generation plant 1 according to 1 or 2 is. In the power generation plant 1 according to 3rd become the inverters 4.1 and 4.2 operated in the current-impressing I-mode, for example by the control unit 8th corresponding specifications to the controls 6th the inverter 4.1 and 4.2 has transmitted. The inverter 4.3 is operated in the voltage-impressing V-mode. This operating mode can in particular from the embodiment according to FIG 1 arise as a result of that in the control unit 8th it was decided the power generation plant 1 to operate in a mixed operation, and the control unit 8th an explicit switchover specification for operation in the voltage-impressing V-mode on the inverter 4.3 has transmitted, or by the inverter 4.3 has found that conditions have occurred which, based on the switchover specifications that apply to him, cause a switchover to the voltage-impressing V-mode.

The power generation plant 1 can in particular from the current-impressing overall operation according to 1 in mixed operation according to 3rd pass over when a stabilization of the AC voltage network 3rd is indexed. This can be done in particular by the control unit 8th based on an evaluation of the electrical properties of the AC voltage network 3rd can be determined, in particular with regard to the voltage quality and the stability of the network frequency or the AC voltage network as a whole, or by a network failure was detected and a remaining island network by the power generation plant 1 is to be supplied with electrical power.

4th shows a power generation plant 1 , which are largely identical to the Power generation plant 1 according to 1 is. In the power generation plant 1 according to 4th become the inverters 4.1 and 4.2 operated in the voltage-impressing V-mode, in particular by the control unit 8th corresponding specifications to the controls 6th the inverter 4.1 and 4.2 has transmitted. The inverter 4.3 is operated in current-impressing I-mode. This operating mode can in particular from the embodiment according to FIG 2 arise as a result of that in the control unit 8th it was decided the power generation plant 1 to operate in a mixed operation, and the control unit 8th a switchover specification for operation in current-impressing I-mode on the inverter 4.3 has transmitted, or by the inverter 4.3 has found that conditions have occurred which, based on the switchover specifications that apply to him, specify a switchover to the current-impressing I-mode.

The power generation plant 1 can in particular from the voltage-impressing overall operation according to 1 in mixed operation according to 4th pass over when the AC voltage network 3rd through the power generation plant 1 has been sufficiently stabilized in the overall voltage-impressing operation and / or for individual inverters 4th a specific active power exchange is desired or prioritized. For example, a setpoint for the power of the direct current source 5 on the inverter 4.3 are specified for drawing the maximum possible power of a photovoltaic generator as a direct current source 5 leads, or a charging power for a battery as a direct current source 5 with a low charge level, or which is otherwise directly or indirectly resulting from external requirements.

5 shows an exemplary embodiment of a control structure 50 of an inverter 4th . The measuring devices 7th of the inverter 4th record the current I_Netz and the voltage U_Netz at the output of the inverter 4th . From this, in a manner known per se, the between the inverter 4th and the alternating voltage network 3rd The exchanged active power P_Netz and the exchanged reactive power Q_Netz are determined.

The regulatory structure 50 includes current regulation 51 and a voltage regulation 52 . Both the current control 51 as well as the voltage regulation 52 according to 5 are basically known from the prior art.

In the current control 51 the detected current I_Netz is compared with a specified target current I_Soll. The target current I_Soll can be specified by a higher-level regulation or control system, for example by an algorithm for determining the maximum possible power of a photovoltaic generator as a direct current source 5 (so-called MPP tracking) or from a storage management of a battery as a direct current source 5 for recharging or targeted discharging.

Goal of the current control 51 it is to bring the recorded current I_Netz into line with the target current I_Soll as far as possible. For this purpose, a deviation between the recorded current I_Netz and the target current I_Soll is recorded in a current regulator 70 processed. The current regulator 70 can in particular be designed as a P controller, PI controller or PID controller, that is to say comprise a proportional component, an integral component and possibly a differential component.

The current regulator 70 calculates an output voltage that is expected at the output of the inverter 4th must be set in order to generate a current I_Netz, which corresponds to the target current I_Soll or at least approximates it.

In the voltage regulation 52 is the means of the measuring device 7th of the inverter concerned 4th detected grid voltage U_Netz compared with an inverter voltage U_WR. The inverter voltage U_WR can, for example, at the output of a bridge circuit of the inverter 4th are recorded. Between the measuring device 7th and a line filter, which in particular has an inductance, is usually arranged in the bridge circuit. This inductance of the line filter can in particular occur when the inverter is operating with the voltage applied 4th can be used as a so-called decoupling throttle. The voltage regulation 52 then regulates a voltage deviation between the line voltage U_Netz detected on the line side of the decoupling choke and the inverter voltage U_WR detected on the inverter side of the decoupling choke to a predetermined value, in particular to zero.

The voltage regulation 52 additionally determines a power deviation between an active power and / or reactive power setpoint value P_Soll or Q_Soll and that actually with the AC voltage network 3rd exchanged active power P_net or reactive power Q_net. The power setpoints P_Soll and Q_Soll are directly linked to the current setpoint I_Soll via the line voltage U_Netz, whereby the active power setpoint can correspond to the product of the current setpoint I_Soll and the line voltage U_Netz. The actual values of the active power P_Netz and the reactive power Q_Netz result in an analogous manner from those of the measuring device 7th of the inverter 4th recorded values of the network voltage U_Netz and the current I_Netz.

The power deviation between P_Soll or Q_Soll and P_Netz or Q_Netz is determined in a droop controller 71 processed. The droop control 71 can in particular have characteristics that the Power deviation due to the inverter 4th map the voltage to be set and / or a frequency to be set; in the prior art, these mappings are known as U (P) or f (Q) statistics.

The result of the droop control 71 is in the voltage regulation 52 added to the difference between the grid voltage U_Netz and the inverter voltage U_WR. The resulting sum is in a voltage regulator 72 processed. The voltage regulator 72 can in particular be designed as a P controller, PI controller or PID controller and accordingly comprise a proportional component, an integral component and possibly a differential component.

The voltage regulator 72 calculates an output voltage that is expected at the output of the inverter 4th must be set in order to generate a current I_Netz, which the electrical parameters of the AC voltage network 3rd influenced in such a way that deviations from normal values are counteracted.

In the embodiment according to 5 work the regulations 51 , 52 parallel and largely independent of each other. By means of a switch 81 one of the two becomes independent in the regulations 51 , 52 calculated setpoints for the inverter 4th selected output voltage to be set and a downstream modulator 82 fed. The switch 81 receives a specification as to whether the current control 51 calculated setpoints are used and the inverter 4th so that it is to be operated in the current-impressing I-mode, or alternatively the voltage regulation 52 calculated setpoints are used and the inverter 4th so that it is to be operated in the voltage-impressing V-mode.

The modulator 82 finally calculates a suitable control of the switches of the bridge circuit of the inverter 4th to generate the control structure 50 determined nominal voltage of the inverter 4th , for example on the basis of a pulse width modulation method known per se.

6th shows one opposite 5 further developed regulation structure 60 . The regulatory structure 60 includes current regulation 61 and a voltage regulation 62 . In contrast to the control structure 50 according to 5 are the regulations 61 , 62 coupled with each other via pilot controls. This is done by the current regulator 70 calculated output voltage for current-impressing operation to the voltage regulation 62 passed and there in the voltage regulator 72 further processed to calculate the output voltage for the voltage-impressing operation. In return, the voltage regulator 72 Calculated output voltage for the voltage-impressing operation to the current control 61 passed and there to that of the current regulator 70 calculated output voltage for current-impressing operation is added.

As a result, the regulations work 61 , 62 they are still in parallel, but they are synchronized by the cross-linking in the manner of a bidirectional pre-control. This prevents any jumps in the instantaneous value or the phase length of the exchanged alternating current when switching from the current-impressing I-mode to the voltage-impressing V-mode and vice versa.

7th shows a flow chart of a method for operating an energy generation plant 1 . In this exemplary embodiment of the method, the power generation plant is located 1 initially in the crotch S7.1 in the overall current-generating operation, compare 1 and related description. All inverters can 4th the power generation plant 1 are in current-impressing operation, in I-mode.

In step S7.2 it is determined that a first switchover criterion has been reached. For example, the control unit 8th determine that electrical properties of the alternating voltage network 3rd lie outside a permitted or desired range. In particular, the control unit 8th evaluate the electrical properties yourself, for example based on measured values of current and voltage, in particular at the network connection point 2 can be captured. The control unit 8th can also determine that the first switchover criterion has been reached on the basis of an external specification, for example a request for an instantaneous reserve by a network operator.

Alternatively or additionally, the inverter 4th even based on the measured values of the measuring devices 7th evaluate the respective current I and the voltage V and in step S7.2 determine that a first switchover criterion has been reached. For example, an inverter 4th determine that the voltage V at its output and / or the frequency of the AC voltage network 3rd lies outside a range for which characteristic curves for grid support are defined within the framework of the current-impressing I-mode. If such characteristics, for example Q (U) or P (f), are thus exhausted without having a lasting effect on the corresponding properties of the AC voltage network 3rd is exercised, this can be assessed as reaching the first switchover criterion.

Alternatively or additionally, the inverter can 4th reaching the first shift Criterion based on the current properties of the connected direct current source 5 determine. In particular, an inverter 4th to which a battery is used as a DC power source 5 is connected, switch to the current-impressing mode when the state of charge of the battery falls below a first limit value, so that the battery can no longer reliably contribute to voltage stabilization and is recharged in the current-impressing mode; as soon as the battery has reached a sufficient charge level, the relevant inverter can 4th can be switched back to the possibly preferred voltage-impressing mode.

In step S7.3 become individual inverters 4th the power generation plant 1 switched from current-impressing operation, from I-mode, to voltage-impressing operation, to V-mode. This can be done on the explicit instruction of the control unit 8th to the inverters concerned 4th , in particular by transmitting an explicit switchover specification or based on autonomous decisions by the inverters concerned 4th take place, in particular due to the detection of the achievement of a for the inverter 4th specified switching default.

The power generation plant 1 is then in step S7.4 operated in mixed operation, ie some of the inverters 4th is operated with current injection and another part of the inverter 4th is operated with voltage impressing; compare the 3rd and 4th as well as the corresponding description. During mixed operation, ie in the context of step S7.4 can add more inverters 4th be switched, from I-mode to V-mode and / or vice versa, for example to raise more voltage-generating resources for improving the network quality or to recharge batteries in current-generating mode. In particular, a control loop can be provided that controls the number of inverters 4th increased in the voltage-impressing V-mode for as long as required by a stability criterion and, if necessary, decreased again if the stability criterion continues to be met; In terms of control technology, this procedure essentially corresponds to control with a proportional and an integral component (so-called PI controller).

In step S7.5 A second switchover criterion can be achieved, for example, in that a measure for the voltage quality or for the network stability is beyond the corresponding limit values.

This will be done in step S7.6 all inverters 4th the power generation plant 1 switched to voltage impressing mode, either directly by the control unit 8th or indirectly by switching specifications from the control unit 8th or autonomously by the inverter 4th caused.

In step S7.7 works the power generation plant 1 then in the voltage-impressing overall operation, compare 2 and the associated description, and can be optimally used to improve the voltage quality and / or to stabilize the AC voltage network 3rd contribute. In particular, the power generation plant 1 the alternating voltage network 3rd in step S7.7 form completely, even if no other power generation systems are connected to the AC voltage network 3rd are connected.

Throughout the course of the procedure in accordance with 7th can the power generation plant 1 with the AC voltage network 3rd remain connected, ie an interim separation of the energy generation system 1 from the AC voltage network 3rd is not necessary. Especially when switching over individual inverters 4th in step S7.3 or in step S7.6 can the power generation plant 1 uninterrupted active and / or reactive power with the AC voltage network 3rd change. Unless the inverter 4th comprise a suitable control structure, in particular a control structure according to 6th , the inverter can 4th even uninterrupted active power and / or reactive power with the AC voltage network 3rd replace when they are switched between current-impressing operation and voltage-impressing operation.

8th shows a flow chart of another embodiment of the method for operating the power generation plant 1 . In this exemplary embodiment of the method, the power generation plant is located 1 initially in the crotch S8.1 in the overall stress-generating operation, compare 2 and related description. In particular, all inverters can be 4th the power generation plant 1 are in voltage impressing mode, in V-mode.

Such a power generation plant 1 In overall operation with impressing voltage, a stand-alone network can be completely formed and the loads in it can be supplied within the scope of the capacity of the connected direct current sources. Here is the power generation plant 1 in step S8.1 particularly suitable for starting up the entire AC voltage network 3rd support, for example after a breakdown of a national or continental network. For such a black start of an interconnected network, it is first necessary to provide a stable alternating voltage to which the existing energy generation systems, which are currently not feeding in due to the breakdown, can synchronize. The power generation plant according to the invention 1 is particularly suitable for providing the initially necessary to contribute stable alternating voltage.

In step S8.2 it is determined that a third switchover criterion has been reached. For example, the control unit 8th determine that electrical properties of the alternating voltage network 3rd lie in a range of values that represent the presence of a stable alternating voltage. In particular, the control unit 8th evaluate the electrical properties yourself, for example based on measured values of current and voltage, in particular at the network connection point 2 can be captured. The control unit 8th can also determine that the third switchover criterion has been reached on the basis of an external specification that is sent, for example, as part of a coordination of the restart of the interconnected network by a network operator.

Alternatively or additionally, the inverter 4th even based on the measured values of the measuring devices 7th evaluate the respective current I and the voltage V and in step S8.2 determine that a third switchover criterion has been reached. For example, an inverter 4th determine that the voltage V at its output and / or the frequency of the AC voltage network 3rd and / or the gradient of voltage V or frequency is stable within a range in which the inverter 4th only little electrical power with the AC voltage network 3rd exchanges. The inverter 4th can then be switched to the current-impressing mode autonomously, without the stability of the AC voltage network 3rd to question. In addition, the inverter can 4th the DC power source connected to it 5 optimal use, in the case of a photovoltaic generator as a direct current source 5 For example, take the maximum available power from the photovoltaic generator.

In step S8.3 specifically become individual inverters 4th the power generation plant 1 switched from voltage-impressing operation, from V-mode, to current-impressing operation, to I-mode. This can be done on the explicit instruction of the control unit 8th to the inverters concerned 4th in particular by transmitting an explicit switchover specification or based on autonomous decisions by the inverters concerned 4th take place, in particular due to the detection of the achievement of a for the inverter 4th specified switching default.

The power generation plant 1 is then in step S8.4 operated in mixed operation, ie some of the inverters 4th is operated with current injection and another part of the inverter 4th is operated with voltage impressing; compare the 3rd and 4th as well as the corresponding description. During mixed operation, ie in the context of step S8.4 can add more inverters 4th be switched, from I-mode to V-mode and / or vice versa, for example to in the further course of a restart of the AC voltage network 3rd to work increasingly in a current-impressing manner and thus to have a grid-supporting effect, provided that the (re) achieved stability of the AC voltage network 3rd this allows. In particular, a control loop can be provided that controls the number of inverters 4th reduced in the voltage-impressing V-mode as soon as a stability criterion is met, and possibly increased again if the stability criterion is no longer met and network-forming resources are required. The inverters operated in current-impressing mode can 4th Preferably make active power available, in particular from to the inverter 4th connected photovoltaic generators. This means that the inverters, which are operated simultaneously in voltage-impressing mode, are 4th the power generation plant 1 relieved and can concentrate on the provision of reactive power, which is particularly important when connecting batteries as direct current sources to this inverter 4th is particularly advantageous.

In step S8.5 A fourth switchover criterion can be achieved, for example, if a measure for the voltage quality or for the network stability is within corresponding tolerance bands, or if a network fault is detected that requires a specified feed of reactive and / or active power. This will be done in step S8.6 all inverters 4th the power generation plant 1 switched to the current-impressing mode, either by the control unit 8th or initiated autonomously.

In step S8.7 works the power generation plant 1 then in the overall operation that impresses the current, compare 1 and the associated description, so that the power exchanged with the AC voltage network can be adjusted based on the needs and possibilities of the DC current sources, taking into account the statics for network support. In particular, the power generation plant 1 possibly as direct current sources 5 The connected photovoltaic generators draw the maximum possible power and transfer it to the AC voltage network 3rd feed in as well as with the as direct current sources 5 Replace connected batteries in such a way that a desired state of charge of the batteries is aimed for.

The switching criteria mentioned in connection with the described embodiments can be based on the various electrical properties. In particular, the quality of the mains voltage can be quantified on the basis of parameters that are fundamentally known to the person skilled in the art. These quality parameters include, in particular, measured values of the network voltage amplitude and gradient, of the network frequency and frequency gradient, of the voltage flicker, and / or the non-linear distortion of the voltage (English: Total Harmonic Distortion, THD). At least indirectly and in particular in current-impressing operation, measured values of the current flicker or the current harmonics are also suitable as quality parameters.

Global and / or individual limit values can be defined for the measured values which are mentioned as examples and which are suitable as quality parameters. The control unit 8th can compare the measured values with the limit values and, if the limit values are exceeded or fallen below, corresponding explicit switchover specifications are sent to the inverter 4th to transfer. Alternatively, the control unit 8th Define individual limit values for the quality parameters and corresponding implicit switchover specifications for the individual inverters 4th transmit so that each inverter 4th autonomously based on a comparison of the sent to the inverter 4th The measured values determined in each case with the switchover specifications decide whether the inverter 4th can be operated in the voltage-impressing V-mode or in the current-impressing I-mode.

List of reference symbols

1

Power generation plant

2

Grid connection point

3

AC voltage network

4th

Inverter

4.1, 4.2, 4.3

Inverter

5

DC power source

6th

control

7th

Measuring devices

8th

Control unit

50, 60

Regulatory structure

51, 61

Current regulation

52, 62

Voltage regulation

70

Current regulator

71

Droop control

72

Voltage regulator

81

Toggle switch

82

modulator

S7.1 - S7.7

steps

S8.1 - S8.7

steps

Claims (17)

Method for operating a power generation plant (1) which has at least two inverters (4), in which electrical power is exchanged between direct current sources (5) and an alternating voltage network (3) by means of the inverters (4), the inverters (4) optionally injecting current or operated with voltage impressing, wherein at least one of the inverters (4) of the energy generation system (1) is connected to a memory as a direct current source (5), the inverter (4) connected to the memory being operated in voltage impressing mode when a charge state of the memory exceeds a first limit value, and is operated temporarily in current-impressing mode in order to charge the storage unit when the state of charge of the storage unit falls below a second limit value, with at least one of the inverters (4) of the energy generation system (1) having a PV generator as a direct current source ( 5) is connected, with the energy generation plant ge (1) exchanges a total electrical power (P_Netz) with the AC voltage network (3), the inverter (4) connected to the PV generator being operated in current-impressing mode and the exchanged total power (P_Netz) a maximum possible PV power (P_MPP ) of the PV generator and wherein the inverters (4) are switched individually or in groups between a current-impressing operation and a voltage-impressing operation, so that the energy generation system (1) is operated as a whole in a current-impressing overall operation, in a voltage-impressing overall operation or in a mixed operation . Procedure according to Claim 1 , the inverters (4) being transmitted switchover specifications, in particular from a control unit (8) of the power generation plant (1), the inverters (4) changing between the current-impressing operation and the voltage-impressing operation depending on the switchover specifications. Procedure according to Claim 2 , the switching specifications directly stipulating whether the inverters (4) are operated in current-impressing mode or in voltage-impressing mode. Procedure according to Claim 2 , wherein the switching specifications include limit values, the inverters (4) determining electrical properties of the AC voltage network (3) and, if the limit values are exceeded, autonomously switch between the current-impressing operation and the voltage-impressing operation. Method according to one of the preceding claims, wherein the energy generation system (1) is connected to the AC voltage network (3) during the switching of one or more inverters (4) between the current-impressing operation and the voltage-impressing operation. Procedure according to Claim 5 , the power generation system (1) exchanging active power and / or reactive power with the AC voltage network (3) without interruption during the switching of one or more inverters (4) between the current-impressing operation and the voltage-impressing operation. Procedure according to Claim 5 or 6th , the individual inverters (4) of the power generation system (1) exchanging active power and / or reactive power with the AC voltage network (3) without interruption when they are switched between the current-impressing operation and the voltage-impressing operation. Method according to one of the preceding claims, wherein the switching of the inverters (4) of the energy generation plant (1) between the current-impressing operation and the voltage-impressing operation is carried out as a function of at least one of the parameters voltage quality, network stability, frequency stability and / or network impedance. Method according to one of the preceding claims, wherein the inverters (4) operated in current-impressing mode exchange electrical power with the AC voltage network (3), the active power and / or reactive power of which are set depending on the voltage and / or the frequency of the AC voltage network (3) so that the inverters (4) operated in current-impressing mode provide primary and / or secondary control power. Method according to one of the preceding claims, wherein the inverters (4) operated in voltage-impressing mode exchange electrical power with the AC voltage network (3) in such a way that the inverters (4) operated in voltage-impressing mode provide instantaneous reserve power. Method according to one of the preceding claims, wherein individual or all inverters (4) of the energy generation system (1) switch from voltage-impressing mode to current-impressing mode in the event of a network fault and optionally switch back to voltage-impressing mode after the error has been cleared. Procedure according to Claim 11 One of the preceding claims, wherein the inverters (4) recognize the presence of the grid fault themselves and, in the event of a grid fault, automatically switch to current-impressing operation and optionally recognize the end of the presence of the grid fault themselves and independently switch to the operating mode before the grid fault. Procedure according to Claim 1 , with the at least one inverter (4) connected to the storage device preferably being switched to voltage-impressing operation when there is a transition from overall current-impressing operation to mixed operation of the energy generation system (1). Procedure according to Claim 13 , with all inverters (4) connected to a storage system being operated in the voltage-impressing mode in mixed operation of the energy generation system (1), provided that the respective state of charge permits this and there is no grid fault, and all inverters that are connected to a PV generator are connected, are operated in current-impressing mode. Method according to one of the preceding claims, wherein the inverter (4) comprises a control structure (50, 60) with a current control (51, 61) and voltage control (52, 62), the current control (51, 61) and the voltage control (52 , 62) each output control signals for controlling the power switches of the inverters (4) and a changeover switch (81) is used to select which of the output control signals are used to control the power switches of the inverters (4). Procedure according to Claim 15 , the current regulation (61) and the voltage regulation (62) being coupled to one another within the framework of the regulation structure (60) via mutual pilot controls. Energy generation system (1) with at least two inverters (4), in which electrical power is exchanged between direct current sources (5) and an AC voltage network (3) by means of the inverter (4), characterized in that the energy generation system (1) is set up to have a To carry out a method according to any one of the preceding claims.

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