EP4154371A1

EP4154371A1 - Method for stabilizing the dc voltage in a dc grid, and dc-to-dc converter for connecting a pv generator to a dc grid

Info

Publication number: EP4154371A1
Application number: EP21728049.4A
Authority: EP
Inventors: Andreas KNOBLOCH
Original assignee: SMA Solar Technology AG
Current assignee: SMA Solar Technology AG
Priority date: 2020-05-23
Filing date: 2021-05-21
Publication date: 2023-03-29
Also published as: US20230084081A1; WO2021239616A1; DE102020113871A1

Abstract

The invention relates to a method for stabilizing a DC voltage in a DC grid (1) that comprises a DC bus (10) which is connected to a higher-order grid (11, 12) and to which an energy generating system (18) and at least one load (13) are connected. A variable electric grid output is exchanged between the DC bus and the higher-order grid in order to keep the DC voltage in the DC bus at a nominal voltage. The energy generating system comprises a PV generator (18a) which is connected to the DC bus via a DC-to-DC converter (18b) and which exchanges an electric generator output with the DC bus. In a normal operating mode, the generator output is set to a normal operating output by the DC-to-DC converter on the basis of an MPP output of the PV generator. In a grid support mode, the generator output is set to a grid support output on the basis of the DC voltage in the DC bus in order to counteract a power imbalance between the electric power supplied in total to the DC bus and the power drawn in total from the DC bus.

Description

Method for stabilizing the direct voltage in a direct current network and direct voltage converter for connecting a PV generator to a direct current network

TECHNICAL FIELD OF THE INVENTION

The invention relates to the maintenance and stabilization of a direct current network (DC network) for supplying electrical consumers (loads). Direct current consumers (DC loads) can be connected to such a DC network via direct voltage converters (DC / DC converters) and / or alternating current consumers (AC loads) via inverters (DC / AC converters). In addition, converters for connecting the DC network to an alternating current network (AC network), electrical energy storage devices and energy generation systems with energy sources and other DC / DC converters can be connected to the DC network. The energy sources can in particular include photovoltaic generators (PV generators) or wind turbines.

Control of the electrical parameters of the DC network is necessary for stable operation of the DC network. In particular, it is necessary to stabilize the DC voltage to a value in a permissible range around a nominal voltage in order to operate the loads on the DC network reliably. Selected components of the DC network can autonomously contribute to the stabilization of the DC voltage or can be controlled accordingly by suitable control units.

An essential criterion for the stability of a DC network is the so-called transient DC voltage stability. A transient process such as a sudden switch-on or switch-off of a large load leads to a sudden power imbalance in the DC network. This power imbalance causes the direct voltage to deviate from the nominal value in particular and must be compensated for as quickly as possible in order to keep the direct voltage within the permissible range. For this purpose, what is known as an instantaneous reserve is necessary in particular, which reacts quickly and reliably to deviations in the direct voltage from the nominal voltage and provides a suitable instantaneous reserve power to compensate for the power imbalance. STATE OF THE ART

In DC networks with a DC network capacity C _DC , the direct voltage U _DC in particular is a characteristic network variable. The direct voltage U _DC depends on the energy and active power balance between inflow IN and outflow OUT at the DC network capacitance C _DC , according to:

In particular, if the resulting DC network capacity C _{DC is} cost-effective and therefore only a small amount of energy is temporarily stored in the DC bus, operating states in which energy flows into the DC bus without being able to drain off sufficiently cause rapid Increase in DC voltage in the DC bus. This can lead to the shutdown of electromechanical devices and thus, for example, in an industrial environment to a process stop or, in the worst case, to the destruction of the device.

With regard to the stabilization of the DC voltage and the equal distribution of power in DC networks, concepts are known that are based on a decentralized droop control. In this case, the direct voltage U _DC is in a DC network according to in their rate of change U _DC limited by means of an electrical energy storage device, which is represented by the capacitance C _DC ,

• Limited in their deviation from the nominal value AU _DC = U _DC - U _DC0 by adjusting the power P, _N _{flowing into the DC bus and the power P OUT flowing} out of the DC bus according to P (t / _Dc ) statics ,

• led back to its nominal value U _DC0 by providing the energy W = f (P, _N - P _Q UT) dt and power AP = P, _N - P _{0UT required for} this.

DC networks often have a direct voltage intermediate circuit (DC bus) to which a large number of individual electrical components can be connected. The components can be spatially spaced apart considerably and, during operation, exchange electrical power with the DC bus via corresponding electrical lines. The power exchanged by a component with the DC bus can depend on its respective operating status or can be specified in a targeted manner. Many components have converter units via which electrical power flows, for example, from the DC bus into a consumer or a memory or from a memory or an energy generation unit into the DC bus. A unidirectional or bidirectional connection of sources and sinks with the DC bus is characteristic of such a DC network, with the sources and sinks being connected to the DC bus via power converters. The DC bus has a limited energy storage capacity, which can be predetermined by a dedicated intermediate circuit capacity and / or capacities in the converter units themselves. So that the DC network can be operated stably at the nominal voltage or in a voltage range around the nominal voltage, suitable control and / or regulation methods are necessary to stabilize the DC voltage in the DC bus.

In such a DC network, operating states can occur in which energy flows into the DC bus without being able to flow off to a sufficient extent, or vice versa. This can lead to impermissible deviations of the DC voltage from the nominal voltage. This occurs in particular when an electric drive is electrically braked or when the dynamics of power converters supplying and withdrawing energy are very different. If a significant proportion of the power flowing out of the DC network is obtained from a higher-level AC network, network faults in the AC network can also have a significant impact on the DC voltage. For example, a converter arranged between the DC and AC networks can help stabilize the AC network in the event of a network fault in the AC network by providing AC control power and thus be used to capacity in such a way that the converter no longer makes a significant contribution to stabilizing the DC voltage can afford.

In the DC network, devices and methods for voltage maintenance are therefore necessary in order to keep the DC voltage in the DC bus within the permissible range around the nominal voltage. The units connected to the DC bus can participate in this control task to varying degrees, although not all units in the system can constructively contribute to maintaining the voltage. In particular, those units that contribute to industrial processes, for example motors or actuators, cannot or only to a limited extent contribute to voltage maintenance, since their operation and thus their power consumption are based on the industrial processes themselves. Rather, such largely self-sufficient units require a constant DC voltage in the DC bus, which enables interference-free power consumption for the operation of these units, whereby there are sometimes strict requirements for the stationary accuracy as well as for the control behavior and in particular for the disturbance behavior in the control of the process variables . Energy generation systems with renewable energy sources such as photovoltaics (PV) or wind can be connected to the DC bus and feed electrical power into the DC bus, which is usually based on the maximum available power of the respective energy source. As a result, the renewable energy source is used to the maximum; on the other hand, the volatile nature of the available power of the energy source can lead to power fluctuations that can negatively affect the DC voltage in the DC bus.

A chopper resistor or a controllable dump load can be arranged in the DC network and, if the DC voltage in the DC bus exceeds a threshold value, switched in parallel to the DC bus via power electronic switches before the DC voltage reaches an impermissible level. As soon as the DC voltage has decreased sufficiently, the chopper resistor is disconnected from the DC bus again. This process is repeated when the DC voltage in the DC bus rises above the threshold value again. The use of such a chopper resistance, however, has a number of disadvantages, in particular an additional space requirement and strong heating when implementing a high excess power, which is associated with an increased risk of fire in sensitive environments and / or a considerable amount of cooling. In addition, a specific design of the required chopper resistance for an extensive, especially a public DC network and for large excess power is time-consuming, complex and expensive.

In a DC network, for example, an overvoltage can occur in the following cases, the probability of an impermissible overvoltage in the DC bus increasing the higher the nominal voltage U _DC0 in the DC bus:

Electric braking of an electric drive

An electric drive connected to the DC bus can switch from a motor to a generator in certain operating states, e.g. when electrically braking a robot arm, an elevator in buildings, a centrifuge, a pump, a cable car when going downhill, or an electric vehicle. A part of the rotational energy of the drive is fed back into the DC circuit and thus leads to an excess of power in the DC bus. When braking an electric drive with constant braking torque, the braking time can be several seconds or even minutes, depending on the process.

In order to prevent damage to components due to an increase in voltage in the DC bus resulting from the feedback of the braking energy, countermeasures must be taken to keep the DC voltage in the DC bus below a maximum permissible value. Various approaches to a solution are known from the prior art for this purpose. A so-called AC crowbar can be used to brake electrical drives by short-circuiting the generator winding on the AC side via switchable resistors. This circuit is very robust and is often used for the emergency stop function of an industrial plant. The disadvantage of this, however, is that an additional resistance as well as its suitable control and cooling must be provided, the latter in particular if a high excess power has to be absorbed over a long period of time.

Further known solutions include discharging the excess energy from the DC bus or from the drive via a bypass past the DC bus into a (different) electrical network or into an energy store. This works to the extent that the current-carrying converters can actually convert the corresponding excess power and feed it into a corresponding sink and are not already fully utilized themselves.

It is not possible to dissipate the excess power for further use in a storage system and / or an AC network if no energy storage device is available or if only unidirectional AC / DC rectifiers are arranged between the DC bus and a higher-level AC network. As an alternative or in addition, controllable resistors (so-called chopper resistors or dump loads) can therefore be connected to the DC bus or can be connected to the DC network. As a rule, these resistors are activated and burn off excess power when the DC voltage exceeds a defined threshold value. Such chopper resistors can also be used if feeding back into another electrical network would jeopardize its stability, so that it cannot be fed back into this other network, or not sufficiently.

Provision of control power and grid support for the public AC grid by DC / AC converters between the DC grid and AC grid

A converter that is arranged between a DC network and a higher-level network and supplies the DC network with considerable electrical power from the higher-level network can be obliged to help stabilize the higher-level network. In particular, if the higher-level network is an AC network with low inertia and / or high electrical power in the megawatt range is regularly exchanged between the DC network and the higher-level network, it may be necessary for the DC network to display a virtual inertia of inertia provide instantaneous reserve power for the higher-level network by means of the DC / AC converter. The active power of the DC / AC converter can be varied depending on the deviation of the network frequency in the AC network from a nominal frequency and, alternatively or additionally, proportionally to the frequency gradient of the network frequency. For example, from converters that are connected to the European network or a high-voltage direct current transmission network (HVDC network), are required to be able to provide their full positive or negative nominal power for at least 500 milliseconds as an instantaneous reserve. A correspondingly large energy store is required for this, which can be arranged either in or on the DC / AC converter or otherwise in the DC network and can be controlled accordingly.

An actual provision of the instantaneous reserve for the AC network by a converter, which is intended to supply a DC network from a higher-level network, can cause a sudden significant power imbalance in the DC network. In particular, overvoltages in the DC bus can occur if a DC / AC converter suddenly increases the feed into the DC network due to an overfrequency in the higher-level AC network or suddenly has to reduce a supply from the DC network and thus a power surplus in the DC network effects. The overvoltage potentially resulting therefrom in the DC network can in turn be counteracted by heating up the excess power in a chopper resistor, in particular if the energy cannot be used or stored otherwise.

A similar situation arises in the event of a network fault in the higher-level network, a distinction being made between a network voltage dip and a network voltage increase. Temporary grid voltage drops as a result of short circuits in the higher-level grid are passed through by the grid-side converter in accordance with normative requirements until the fault is cleared, in that a reactive current is fed in proportionally to the voltage drop depth for a defined period of time (so-called fault ride through, FRT). The converter can no longer participate in the stabilization of the DC network, since the provision of the FRT reactive current is prioritized over the exchange of the necessary active power. This means that, in extreme cases, the exchange of active power between the DC network and the AC network can be completely interrupted for the entire duration of the FRT fault. Such behavior of a converter arranged between the DC network and the higher-level network endangers the voltage stability of the DC network if the conflict of objectives that emerges here is resolved in favor of the stabilization of the higher-level network. In addition, it is foreseeable that network protection mechanisms in AC networks with low inertia, i.e. in particular with a decreasing proportion of synchronous machines, will require an increasingly longer time to trigger and rectify faults, and consequently increasing fault durations can be expected. As a result, DC / AC converters do not contribute to stabilizing the DC voltage in the DC bus for a longer period of time, so that alternative control mechanisms are required to stabilize the DC bus voltage.

In addition, there may be a brief increase in feedback from the higher-level network into the DC bus and thus a rapid voltage increase in the DC bus if the AC mains voltage returns after a short-circuit fault has been resolved in the higher-level network and / or if there is a temporary overvoltage in the higher-level network. In addition, asymmetrical network faults in the higher-level network can lead to an asymmetrical exchange of power between the DC network and the higher-level network. This can cause oscillations in the DC voltage, which can also lead to overvoltages.

If such power imbalances in the DC network cannot be compensated for quickly enough by components that are already present in the DC network, such as controllable electrical drives, chopper resistors or an electronic load can be used according to the prior art.

AC-side limitation or nominal power operation of the grid-side converters with full grid feed-in from the DC sub-grid into a public energy grid

From an economic point of view, it can be desirable to supply all consumers in the DC network with energy from renewable sources that are connected to the DC network itself, and to feed excess energy into a higher-level network. Situations can arise in which the power converters, which dissipate power from the DC network into the higher-level network, are already working close to their maximum permissible power. As an alternative or in addition, the feed into the higher-level network can be limited at least temporarily, for example due to an excess of power in the higher-level network. As a result, there is hardly any leeway to participate in the stabilization of the DC voltage, for example to transfer additional excess power from the DC network to the higher-level network.

In the event of a sudden excess power in the DC bus, the excess power could either be used somewhere to maintain the DC voltage, which is not always possible for process engineering reasons, or it could be burned in a chopper resistor, especially if other components in the DC sub-network have theirs Not able to change performance appropriately or not quickly enough. Furthermore, the following measures for protection against overvoltages in a DC network are known from the prior art:

A larger design of the DC link capacitance, individually in each converter coupled on the DC side or in the DC network as a whole, is complex and expensive, in particular because of the hardware required.

Faster regulation of the converters between the DC network and a higher-level network is not always possible with conventional converters and requires more expensive components. From EP 1 929 604 A1 it is known to short-circuit bridge switches of a converter; In the event of a prolonged overvoltage, this can lead to a high level of heating of the switches and can therefore only be used for a short time.

OBJECT OF THE INVENTION

The invention is based on the object of providing a cost-effective and robust solution for voltage maintenance and for protection against transient overvoltages in DC networks.

SOLUTION

The object is achieved by a method with the features of claim 1 and by a DC voltage converter with the features of claim 15. Preferred embodiments are defined in the dependent claims.

DESCRIPTION OF THE INVENTION

In a method for stabilizing a direct voltage in a DC network, the DC network comprises a DC bus which has the direct voltage. A power generation system and at least one load are connected to the DC bus, and the DC bus is connected to a higher-level network. An electrical network power is exchanged between the DC bus and the higher-level network, which is varied in order to keep the DC voltage in the DC bus at a nominal voltage. The energy generation system comprises a PV generator that is connected to the DC bus via a DC voltage converter and exchanges electrical generator power with the DC bus. The generator power is set to a normal operating power in a normal operating mode by the DC / DC converter depending on an MPP power of the PV generator, the normal operating power being set variably in a predetermined relation to the MPP power of the PV generator or independently of the MPP power of the PV generator is permanently set. The direct voltage in the DC bus is monitored by the power generation system. In a grid support mode, the generator power is set to a grid support power as a function of the DC voltage in the DC bus in order to counteract a power imbalance between the total electrical power supplied to the DC bus and the total power withdrawn from the DC bus, with the grid support power in Depending on a deviation of the direct voltage from its nominal value and / or on a rate of change of the direct voltage is set. Such a DC network includes non-PV power converters, in particular those that exchange electrical network power between the DC bus and the energy supply network and take over the steady-state regulation of the DC bus voltage in normal operation. The aim here is to keep the DC bus voltage in a permissible voltage range around the nominal voltage U NOM by appropriately varying the electrical network power. The control dynamics of these non-PV power converters is, however, comparatively slow and characterized by a temporarily or permanently limited power consumption. For this reason, non-PV power converters can only keep the DC bus voltage within the permissible voltage range around the nominal voltage to a limited extent or not at all in grid support operation, which can occur in DC networks in particular due to the effects described above. By setting the generator power to a grid support power, the power generation system can make a decisive contribution to stabilizing the direct voltage in the DC bus in grid support mode. The DC / DC converter of the power generation system is characterized by a comparatively high level of control dynamics, so that the contribution of the power generation system also counteracts transient overvoltages. In particular, the avoidance of overvoltages due to the effects described at the beginning is considerably improved.

Specifically, the generator output can be set in grid support mode if the DC voltage in the DC bus exceeds a specified limit value. In this way, the power generation system specifically counteracts overvoltages in the DC bus. In particular, the generator power can be set by clocking power semiconductors of the DC / DC converter. In the normal operating mode, a PV voltage applied to the PV generator is set to a normal operating voltage by means of a first clock rate, for example to the MPP voltage or a voltage that deviates from the MPP voltage. In contrast, in grid support mode, the generator power is reduced by operating the DC / DC converter using a second clock rate in such a way that the PV voltage changes in the direction of the DC voltage.

In one embodiment of the method, the grid support mode occurs and the DC / DC converter is operated at the second clock rate when the DC voltage in the DC bus exceeds a limit value or a value deviating upwards from the limit value by one hysteresis. If the DC voltage falls below the limit value or a value deviating from the limit value by one hysteresis, the normal operating mode re-enters and the DC / DC converter is operated at the first clock rate. By setting the grid support power based on a change in the clock rate of the DC voltage converter, which drives the generator voltage in the direction of the DC voltage in the DC bus, a particularly effective reaction to an overvoltage in the DC bus is possible. The DC / DC converter is preferred in the Grid support mode is not clocked at all, in that the second clock rate has the value zero or one, depending on the specific topology, so that the greatest possible voltage gradient between DC voltage in the DC bus and PV voltage is used, taking into account any capacitances and inductances of the DC / DC converter.

When using a second clock rate of zero or one, the PV voltage can be matched to the direct voltage in the DC bus. In one embodiment of the method, the DC voltage converter can also be operated in grid support mode in such a way that the PV voltage is increased further compared to the DC voltage when the PV voltage has adjusted to the DC voltage and the DC voltage in the DC bus continues to exceed the limit value . As a result, the difference between the normal operating power and the grid support power, i.e. the control power of the power generation plant, can be increased further, so that even higher power imbalances can be compensated for in the DC network. The sign of the generator power can change, so that the feed-in of generator power from the PV generator into the DC bus, which predominates in normal operation, is replaced by a return feed of generator power from the DC bus to the PV generator.

The generator output is therefore preferably changed by increasing the PV voltage (so-called right-hand derating, i.e. downward regulation in the direction of the open-circuit voltage of the PV generator). A corresponding change by lowering the PV voltage in the direction of a short circuit is basically also possible, with feedback in this case being effected by reversing the polarity of the voltage on the PV generator.

The grid support power is preferably lower than the normal operating power by a currently provided PV control power. The grid backup power can have an opposite sign to the normal operating power if the deviation of the direct voltage in the DC bus from its nominal value and / or if the rate of change of the direct voltage in the DC bus requires a PV control power amount that is greater than the current MPP power and therefore it is also greater than the normal operating power of the PV generator. In other words, the power generation system is able to provide a PV control power to stabilize the DC voltage in the DC bus, which significantly exceeds the MPP power of the PV generator. The PV control power can thus be at least the sum of the current MPP power and the nominal power of the PV generator.

In one embodiment of the method, the rate of change of the direct voltage is determined from a derivation of a voltage measurement or from a current measurement. In particular, the determination of the rate of change of the direct voltage from a current measurement makes considerably less demands on the necessary measurement technology and is also faster and more precise, since there is no need to create a derivation.

An energy storage device that exchanges storage power with the DC bus can be connected to the DC bus. The storage power can be zero in normal operating mode or it can include loading the storage. In the grid support mode, the storage capacity can include discharging the storage unit in order to counteract a reduction in the direct voltage in the DC bus compared to its nominal value. In this respect, the energy store makes a complementary contribution to stabilizing the DC voltage in the DC bus by counteracting an under voltage, while the energy generation system is set up to counteract an overvoltage.

Specifically, in the grid support operation, the storage capacity can be increased if necessary in order to counteract a power deficit in the DC bus, and the PV output can be reduced if necessary in order to counteract a power surplus in the DC bus. In this case, in the normal operating mode, a charge state of the energy store can be aimed for which is between 90% and 100% of the charge capacity of the energy store. This means that a large part of the capacity of the energy store is available for feeding in stabilizing control power in the event of a power deficit. This is only possible if the energy storage device is not responsible for stabilizing the DC voltage in the event of an excess power in the DC network, in that in such a case the energy generation system provides the necessary stabilizing control power.

In a further embodiment of the method, the DC bus is connected to the higher-level network via a bidirectional power converter. The bidirectional power converter is designed as a DC / DC converter if the higher-level network includes another DC network. Alternatively, the bidirectional power converter is designed as an inverter if the higher-level network comprises an alternating voltage network (AC network). The electrical network power, which is exchanged between the DC bus and the higher-level network via the bidirectional power converter, can be adjusted in particular as a function of the electrical properties of the higher-level network to stabilize the higher-level network. In certain situations, especially when control power has to be provided to stabilize the higher-level network, the network power does not contribute, or at least not preferably, to stabilize the DC voltage in the DC bus, but can, on the contrary, have a destabilizing effect. However, this effect is effectively overcompensated by the provision of control power by the power generation system, so that the bidirectional power converter can provide control power to stabilize the higher-level network without affecting the stabilization of the DC voltage in the DC bus. The DC bus can be connected to a supply network based on DC voltage via the DC voltage converter and at the same time via the inverter to an energy supply network based on AC voltage and can exchange both DC network power and AC network power. This creates a further degree of freedom for the method, which can be used for optimal utilization of the energy generated by the PV generator, in particular if one of the higher-level networks is not available or only available to a limited extent for the exchange of network power.

In addition, in the normal operating mode, network power can flow from the DC bus into the AC network and / or the further DC network. In the normal operating mode, the network power can include an excess power which corresponds to a difference between the generator power generated by the energy generation system and the power consumed by the consumer and / or the machine.

A DC / DC converter for connecting a PV generator to a DC bus of a DC network is set up to exchange electrical power between the PV generator and the DC bus and comprises a controller which is set up to carry out a method according to the preceding description .

In summary, a method for stabilizing the direct voltage in a DC bus has been disclosed that is particularly suitable for protecting against overvoltages in the DC bus due to transient power imbalances between generation and consumption in a DC network. The system in question comprises at least one PV generator coupled to the DC bus via a DC voltage converter, the PV DC voltage converter being set up to

- Take power or current from the PV generator and feed it into the DC bus and optionally take power or current from the DC bus and feed it into the PV generator;

- monitor the DC bus voltage,

- alternatively or additionally to monitor the current flow in the DC bus or in a partial capacity of the DC bus, in particular by means of a current sensor arranged in series with the DC bus (partial) capacity,

- to operate the PV generator in normal operating mode regardless of the DC bus voltage at or near the point of maximum power output (MPP),

- to modify its mode of operation deviating from the regulation objectives provided in the normal operating mode, in particular independently of the MPP of the PV generator, in order to counteract a change in the DC voltage in the DC bus in particular in a grid support mode, and - regulate the PV power in a controlled manner in grid support mode, ie change the current, power and operating point on the characteristic curve of the PV generator and, if necessary, feed power to the PV generator and convert it there into heat.

In grid support mode, PV power is provided that is dependent on the amplitude, the deviation from the nominal value and / or the rate of change of the direct voltage in the DC bus or on the amplitude of the current in the DC bus or in a partial capacity of the DC bus .

The method is characterized in that any power imbalances in the DC bus are counteracted by means of the PV generator, so that an undesired increase in the DC bus voltage due to the power imbalance can be slowed down or completely prevented. Electrical power can be fed back into the PV generator so that the PV generator is effectively involved in stabilizing the DC bus voltage under any irradiation conditions.

The versatile and technically advantageous use according to the invention of the power generation system with the PV generator, which acts both as a generator and as a flexibly controllable consumer, brings great advantages in supporting the stabilization of the DC voltage in the DC bus of a DC network. In particular in comparison with conventional means such as chopper resistors or elaborately extended connections to higher-level networks, there is much less effort, since the required components are usually already available and can be used synergistically as soon as a power generation system is connected to the DC network and operated according to the invention will. Particularly advantageous effects result for fast transient processes that are characterized by a lot of power but little energy, so that dedicated energy storage devices would be oversized and would not be amortized.

The disadvantage inherent in conventional chopper resistors, that energy is “burned up” and ultimately not used sensibly, also occurs in principle with the solution according to the invention. However, this is clearly outweighed by the advantages of using the power generation system to stabilize the grid voltage in grid support mode. In particular, a chopper resistor is in normal operating mode, i.e. largely quasi inactive, while the PV generator of the power generation system also produces electrical power from a renewable source in normal operating mode, which is sensibly used in the DC network and makes regeneratively generated energy available.

The invention represents a more cost-effective solution for improving voltage maintenance and for protecting against transient overvoltages in a DC network Using the PV system according to the invention, in particular chopper resistors including their peripherals such as cooling, control and measurement technology can be saved. Due to its flat construction and its constant contact with the surrounding air, the PV generator is well cooled and therefore offers a favorable surface for dissipating heat dissipation. The PV generator is therefore suitable for dissipating considerable excess power, so that ideally there is no need for any chopper resistors in the DC network. In addition, further existing system components can be used according to the invention, for example parts of the measurement technology of the PV-DC / DC converter that is already available for normal operation.

Alternatively or additionally, PV generators can be very easily connected to DC networks, possibly in the form of a retrofit solution. The control strategy of the other DC bus-coupled converters does not have to be changed, or only needs to be changed slightly.

The operation of the PV generator to stabilize the DC bus voltage can be activated depending on a threshold value for the DC bus voltage, with the PV generator working on the MPP in normal operation as long as the DC bus voltage is less than the Threshold deviates from a nominal value. The switch-on threshold can be smaller, equal to or larger than further switch-on thresholds of other actuators in the DC network that are also involved in stabilizing the DC voltage.

The contribution of the energy generation system to stabilizing the DC bus voltage can be quantified differently depending on the size ratio between the nominal output of the PV generator and the nominal output of the other parties involved in the DC network. Particularly in the event of an overvoltage in the DC network, the PV generator can provide control power that slows down the voltage increase, stops it or even reverses it and returns the DC voltage in the DC bus towards the nominal value. In each of these cases, the PV power can be returned to the normal operating power as soon as other actuators on the DC bus react to the overvoltage and, for their part, provide control power in order to return the DC voltage in the DC bus to a permitted range around its nominal value.

Various embodiments of methods for operating the PV converters involved in overvoltage protection in the DC network have thus been shown, the embodiments being basically combinable with one another.

BRIEF DESCRIPTION OF THE FIGURES In the following, the invention is further explained and described with reference to the exemplary embodiments shown in the figures.

Fig. 1 shows a DC network in a first embodiment,

Fig. 2 shows a DC network in a second embodiment,

3 shows a characteristic curve from an embodiment of a method for stabilizing a direct voltage in a DC network,

4 shows a characteristic curve from a further embodiment of a method for stabilizing a direct voltage in a DC network,

5 shows a characteristic curve from a further embodiment of a method for stabilizing a direct voltage in a DC network,

6 shows a control from an embodiment of a method for stabilizing a direct voltage in a DC network,

7 shows a DC voltage converter for the electrical connection between a PV generator and a DC network,

8a shows time curves of a direct voltage in a DC network and of power of a disturbance and a chopper resistor in the DC network, and

FIG. 8b shows time curves of a direct voltage in a DC network and of the power of a fault and a PV generator in the DC network.

FIGURE DESCRIPTION

1 shows a DC network 1 with a DC bus 10 for transferring energy between various electrical units 11-18. Electrical power flows between these units 11-18 within the DC network 1 via the DC bus 10. Such a DC network 1 can in particular be located in an industrial environment, for example a factory or a production plant, and have a considerable spatial extent , for example, dozens or hundreds of square meters. The DC bus 10 is connected to a superordinate AC network 11 and / or to a further DC network 12. The AC network 11 can in particular comprise a public energy supply network 11a, while the further DC network 12 comprises, for example, a higher-level or adjacent supply network 12a. Electrical power can be exchanged between the DC bus 10 and the AC network 11 or the DC network 12 by a bidirectional inverter (DC / AC converter) 11b between the DC bus 10 and the power supply network 11a or a bidirectional one Direct voltage converter (DC / DC converter) 12b is arranged between DC bus 10 and supply network 12a. An electrical AC network power is drawn from the energy supply network 11a via the DC / AC converter 11b and fed into the DC bus 10 or vice versa. An electrical DC network power is drawn from the supply network 12a via the DC / DC converter 12b and fed into the DC bus 10 or vice versa.

A load 13, which comprises a consumer 13a and a power supply unit 13b, is connected to the DC bus 10. The power supply unit 13b is set up to take electrical consumer power from the DC bus 10 and to make it available to the consumer 13a in a suitable form with regard to current and voltage. Depending on the type of consumer 13a (DC or AC), the power supply 13b can in particular comprise a DC / DC converter and / or an inverter, possibly an inverter with a variable output frequency. The actual consumer power flowing from the DC bus 10 into the load 13 can be set both by the consumer 13a and the power supply unit 13b, in particular also as a function of a higher-level control unit, for example a process controller (not shown).

In particular, if the consumer 13a is part of a larger system, for example a motor or a cooling system in an industrial or commercial system, the consumer power can be specified by a higher-level control unit in such a way that the system strives for safe and continuous operation. In this respect, the consumer power is generally to be taken as given in the context of the operation of the DC network 10 and can only be modified in exceptional cases for other reasons, for example to help stabilize the DC network. In return, the load 13 can have considerable repercussions on the DC bus 10 due to its operation, in particular if the consumer 13a carries out switching operations and / or has a rotating flywheel and thus an electromechanical inertia that generates considerable currents by accelerating and braking the flywheel.

The load 13 can alternatively or additionally have a consumer 13a, the consumer power of which can be modified within certain limits without fundamentally influencing the operation of the consumer 13a. For example, the load 13 may have a Include electrolyzer, which has a nominal power and can be operated in a working range around and below the nominal power. Such an electrolyzer can be connected to the DC bus 10 via a power supply unit 13b, the power supply unit 13b being able to change the consumer power of the electrolyzer on the basis of an external control signal or also independently as a function of electrical measured values.

In addition, a machine 14, which has a generator 14a and a bidirectional DC / AC converter 14b, can be connected to the DC bus. The machine 14 can exchange electrical machine power with the DC bus 10, the generator 14a being driven externally depending on the operating mode, for example by an internal combustion engine, so that the DC / AC converter 14b feeds electrical power into the DC bus, and / or develops driving force itself in that the DC / AC converter 14b takes electrical power from the DC bus 10 and feeds it into the generator 14a. The machine power actually exchanged with the machine 14 can be specified via a control signal and / or set independently by the machine 14.

The DC network 10 can have a reserve load 15 which is intended to help stabilize the DC network. The reserve load can have a load resistor 15a, in particular a so-called chopper resistor, into which an electrical reserve power is fed by means of a DC / DC converter 15b. The reserve load 15 can in particular absorb excess power from the DC bus 10, which is then only converted into heat in the load resistor 15a and is therefore not available, or only available to a very limited extent, for further use. The actual reserve power can be specified via a control signal and / or set independently by the reserve load 15, in particular as a function of electrical parameters of the DC bus 10, for example the direct voltage U_DC in the DC bus 10.

An electrical energy store 16 can also be connected to the DC bus 10. The energy store 16 can in particular have a battery 16a which is connected to the DC bus 10 via a DC / DC converter 16b. Electrical storage power can be fed into the battery 16a or withdrawn from the battery 16a via the DC / DC converter 16b. The actual storage capacity can be specified via a control signal and / or set independently by the energy storage device 16.

The DC network can furthermore include energy generation systems, in particular a wind energy system (WEA) 17 and / or a photovoltaic system (PV system) 18. The wind energy system 17 can include a wind turbine 17a that generates electrical wind energy power and via an AC / DC Converter 17b feeds DC bus 10. The PV system 18 can include a PV generator 18a, which can generate electrical PV power via a DC / DC converter 18b is fed into the DC bus 10. The actual wind energy output and the actual PV output are based on the maximum possible output of the wind turbine 17a or the PV generator 18a in order to fully utilize the energy from these regenerative sources. In normal operation, the PV generator 18a can feed the maximum possible PV power into the DC network via the controllable DC / DC converter 18b, largely independently of the DC voltage in the DC bus 10. Depending on the voltage ratio between the direct voltage U_DC in the DC bus 10 and the PV voltage, a correspondingly suitable DC / DC converter 18b can be used, for example a step-up converter, a step-down converter or a two-quadrant converter. The DC / DC converter 18b generally comprises an arrangement of power semiconductors, ie in particular diodes and switches, as well as capacitances and / or inductances, the power semiconductors being able to be operated clocked to achieve a desired transformation ratio between the direct voltage in the DC bus 10 and to generate the PV voltage U_PV at the PV generator 18a.

A DC network 1 according to FIG. 1 can be operated in such a way that the renewable energy from the wind turbine 17 and the PV system 18 is preferably used to cover the power requirements of the loads 13 and possibly the machines 14. Any additional electrical power required can, if necessary, be drawn temporarily from the energy store 16 and / or in the longer term from the AC network 11 and / or the DC network 12. Conversely, if there is an excess of power in the DC network 1, electrical power can optionally be temporarily fed into the reserve load 15 and / or into the energy store 16 or fed into the AC network 11 and / or into the DC network 12 for a longer period of time. The economically optimal operation of such a DC network 1 usually results when the energy requirement within the DC network 1 is completely covered by a renewable energy source, ie by the wind turbine 17 and / or the PV system 18, the energy store 16 can compensate for shortfalls in the meantime if necessary. As a result, no current is drawn from the AC network 11 or the DC network 12, and instead as much current as possible, possibly excessively generated in the DC network 1, is fed into the AC network 11 and / or the DC network 12.

FIG. 2 shows a DC network 1 which is largely identical to the DC network 1 according to FIG. 1. In contrast to FIG. 1, the reserve load 15 in the DC network 1 according to FIG. 2 is only optional, ie it can be omitted. Any excess power in the DC bus 10, which was generated in particular by the energy generation systems 17, 18 but does not flow to the load 13 or the machine 14, can be fed into the AC network 11 and / or the further DC network 12 as well may be used to charge the energy store 16. The energy store 16 can be almost fully charged in the normal operating mode, ie have a state of charge between 90% and 100% of the charge capacity. In the event of a power deficit in the DC bus 10, storage power from the energy store 16 can be fed into the DC bus 10 in order to counteract an undervoltage in the DC bus 10. In the event of a power surplus, the PV power can be reduced, possibly also to a negative value, in that electrical power is fed back into the PV generator 18a. As a result of this distribution of work between the energy store 16 and the PV system 18, the energy store 16 in particular can be optimally designed and used, and the PV system 18 develops added value that goes well beyond the mere generation of electrical power.

Using the following FIGS. 3-8, specific embodiments of the method according to the invention for operating the DC network 1 are explained.

3 shows an embodiment of the method according to the invention in which the PV power P_PV is changed as a function of the direct voltage U_DC in the DC bus 10 along a Pp _V (U _DC ) characteristic. The specific dependence of the PV power P_PV on the direct voltage U_DC in the DC bus 10 can be specified by a characteristic curve which, according to FIG.

In a first range I with UDC <UDCI, the direct voltage U_DC in the DC bus 10 is in the range of the nominal voltage U_Nom. In this area I, the PV system 18 works in the normal operating mode and feeds the normal operating power P_0 into the DC bus 10. The normal operating power P_0 can correspond to the maximum possible MPP power of the PV generator 18a or be based on the MPP power, i.e. for example 90% of the MPP power, and in this respect be variable with fluctuating MPP power; alternatively, the normal operating power P_0 can have a fixed value that is smaller than the MPP power, independently of the MPP power. In area I, there is therefore no or only a slight limitation of the PV power compared to the MPP power.

In a second area II with UDCI <UDC <UDC2, the direct voltage U_DC in the DC bus 10 is increased compared to the nominal voltage U_Nom. In this area II, the PV system 18 changes to a grid support mode, and the PV power P_PV is curtailed according to a P _PV ( _. U _DC ) characteristic curve. For this purpose, in particular the voltage U_PV at the PV generator 18a can be changed in the direction of the open circuit voltage of the PV generator 18a. If the direct voltage U_DC in the DC bus 10 is higher than the open circuit voltage of the PV generator 18a, the voltage U_PV at the PV generator 18a thus approaches the direct voltage U_DC in the DC bus 10. The PV power P_PV can go back to zero with increasing DC voltage U_DC and become negative in the further course, ie it electrical power can be fed back into the PV generator 18a. The characteristic curve can run along the upper characteristic curve limit 40. As an alternative or in addition, a stronger, that is to say disproportionate, down regulation can take place, so that the P _PV ( _. U _DC ) characteristic curve has a non-linear profile in the characteristic curve range 42. This is particularly useful when additional measured values are used and indicate instabilities, for example when there is a high rate of change the direct voltage U_DC or a high direct current I_DC occurs in the DC bus 10 (compare FIG. 4 and FIG. 5).

In a range lla with UDC2 <UDC <UDC3, increased regulation of the PV generator 18a can take place, in particular by changing the voltage P_PV at the PV generator 18a further in the direction of the open circuit voltage of the PV generator 18a and possibly beyond, so that electrical power is fed back into the PV generator 18a. When the maximum possible regenerative power -P _Gen in the PV generator 18a is reached, the regenerative power can be kept constant with a further increase in the direct voltage U_DC in the DC bus 10 by means of regulation, as long as the direct voltage U_DC remains below the voltage threshold U _DC3 . Here, too, it is possible, as an alternative or in addition, to _{regulate along a non-linear P PV} (U _DC ) characteristic curve, so that the change in the PV power is amplified as the DC voltage U_DC increases.

Finally, in area III with UDC - UDC3, a protective shutdown of the PV system 18 can take place, as a result of which the DC / DC converter 18a is separated from the DC bus 10; If necessary, the entire DC network 1 can be switched off in such an extreme situation for safety reasons.

4 shows a further embodiment of the method according to the invention, in which the PV system 18 provides an instantaneous DC reserve; this corresponds to a virtual inertia or a virtual increase in the capacity of the DC bus 10, in that the PV system 18 adjusts the PV power exchanged with the DC bus 10 in accordance with FIG. 4.

The instantaneous DC reserve is provided by the PV system 18 by increasing the PV power P_PV as a function of the rate of change of the direct voltage U _DC is set. The PV power P_PV is in a normal operating mode, ie when the rate of change is below a first limit value, set to the normal operating power P_0 in the order of magnitude of the MPP power. When the rate of change the first

Exceeds the limit value, the PV power is reduced compared to the normal operating power P_0. The PV system 18 thus reacts to a rapid change in Direct voltage U_DC in the DC bus 10 with a proportional or disproportionate change in the PV power P_PV. The change in PV power or PV current counteracts the deviation and the rate of change of the direct voltage U_DC and, if necessary, includes a feed back into the PV generator 18a, ie a negative PV power according to FIG. 4

P PV. When the rate of change exceeds the first limit value ^dUoc · ² , the dt dt maximum possible feedback power -Pc _{en is} fed back into the PV generator 18a. As a result of the method, the deviation of the direct voltage U_DC from its nominal value U_Nom is limited, reduced and, ideally, completely avoided.

The specific dependency of the PV power P_PV on the rate of change of the direct voltage (J _DC = can be specified by a characteristic curve that runs in a characteristic curve range that, analogous to FIG In particular, a family of characteristics can be specified from which, during operation, a characteristic curve to be used is selected on the basis of further parameters, for example on the basis of the current direct voltage U_DC in the DC bus 10.

FIG. 5 shows a further embodiment of the method according to the invention, in which the PV system 18 provides an instantaneous DC reserve. In contrast to the embodiment according to FIG. 4, here the PV power P_PV is set as a function of an amplitude of a direct current I_DC in the DC bus 10. The direct current I_DC can be determined from a measurement of a partial current I_DCX in a partial capacity C_DCX of the DC bus 10 with the total capacity C_DC.

The instantaneous DC reserve according to FIG. 5 is provided by the PV system 18 in that the PV power P_PV is set as a function of the partial current I_DCX. The partial current I_DCX can be measured on a partial capacity C_DCX, for example on an output capacity of the PV-DC / DC converter 18b. The partial current l_DCX is in the same ratio to the direct current l_DC in the DC bus 10 as the partial capacity C_DCX to the total capacity C_DC of the D-bus 10.A corresponding scaling factor for determining the total current l_DC from the partial current l_DCX can be used when the DC network 1 or be determined continuously. In contrast to the method according to FIG. 4, only a current measurement is necessary here, and in particular a derivation of the direct voltage U_DC can be dispensed with.

Specifically, the PV power P_PV is set to the normal operating power P_0 in a normal operating mode, ie when the partial current I_DCX is below a first limit value I_DCX, 1. If the partial current I_DCX exceeds the first limit value I_DCX, 1, the PV power reduced compared to normal operating power P_0. The PV power P_PV follows a characteristic curve which, analogously to FIGS. 3 and 4, runs within a characteristic curve range which is spanned by an upper and a lower characteristic curve limit and which can run non-linearly within the characteristic curve range.

Due to the proportional or disproportionate change in the PV power P_PV according to FIG. 5 depending on the amplitude of the DC bus current I_DC or the partial current I_DCX, the PV system 18 affects the change and the rate of change of the direct voltage U_DC in the DC bus 10 against. The change in the PV power P_PV can include feeding back into the PV generator 18a. If the partial current I_DCX exceeds a second limit value I_DCX, 2, the maximum possible feedback power -Pc _en can be fed back into the PV generator 18a. As a result of the method, the deviation of the direct voltage U_DC in the DC bus 10 from its nominal value U_Nom is limited, reduced and, in the ideal case, completely avoided.

6 shows an embodiment of a specific control 60 and its effect on the direct voltage U_DC in the DC bus 10 within the scope of the method according to the invention. The embodiments explained with reference to FIGS. 3 to 5 can be combined by means of the control 60. The mechanisms of this embodiment of the method according to the invention and the resulting reaction of the direct voltage U_DC to sudden power imbalances in the DC bus 10, that is, in the event of a sudden, strongly unbalanced power _balance _{AP = P, N} - P 0UT in the DC, are shown on the basis of FIG. 6 and the following explanations -Bus 10 explained.

The total capacity C_DC of the DC bus 10 is virtually increased by C _{v T} by a differential control component of the control system 60, in that the PV system 18 converts the PV power P_PV in

Dependence on the rate of change adjusts. The differential control component limits the

Gradients of the direct voltage U_DC in the DC bus 10 by providing an instantaneous DC reserve (compare FIG. 4 and the associated description with deriving the direct voltage U_DC or FIG. 5 as an embodiment with determination of the direct current I_DC). This behavior corresponds to a virtual increase in the total capacity C_DC of the DC bus 10 and, in the event of a power imbalance in the DC bus 10, leads to a slower drift of the direct voltage U_DC, but not to a stop, since the power balance in the DC bus 10 is reduced by the higher capacity alone is not offset.

The aim is to balance the power balance in the DC bus 10 by means of an additional proportional control component of the control 60, in that the PV system 18, possibly supported by the energy store 16, adjusts its output as a function of the direct voltage U_DC itself or of the deviation of the direct voltage U_DC from its nominal value (compare FIG. 3 and the associated description). The proportional control component has a voltage-holding function in that a power imbalance in the DC bus 10 is at least reduced and a drifting away of the direct voltage U_DC is slowed down. _{The smaller a virtual parallel resistance} l / R vmT used as a scaling factor in this proportional control component of the control system 60, the greater the change in the PV power P_PV (represented here by the current l_Rvirt) due to the proportional control component of the control 60 with otherwise the same Voltage change, the closer the direct voltage U_DC is to its nominal value U_Nom and the faster the drifting is stopped.

In addition to the PV system 18, further converters can be connected to the DC bus 10, which converters can also be set up to return the direct voltage U_DC in the DC bus 10 to the nominal value. These further converters, in particular a bidirectional inverter (DC / AC converter) 11b between DC bus 10 and power supply network 11a or a bidirectional DC voltage converter (DC / DC converter) 12b between DC bus 10 and supply network 12a (compare FIGS and 2) in the event of a persistent deviation of the direct voltage U_DC from its nominal value, the power exchanged with the DC bus 10 can likewise suitably change in order to compensate for the power imbalance; this corresponds to an additional integrating control component which can be implemented in the control 60 or as part of a higher-level control of the DC network 1. This integrating control component contributes to the return of the direct voltage U_DC to the nominal value U_Nom and can be implemented with reduced dynamics. The transient overvoltages are already effectively countered by the PV system 18 by means of the proportional and differential control components of the control 60, in that these control components limit the gradient of the direct voltage U_DC or contribute to voltage maintenance.

By adapting the power of the (further) converters involved in returning the direct voltage U_DC to the nominal value U_Nom, the direct voltage U_DC in the DC bus 10 is returned to a voltage band around its nominal value; In return, the change in the PV power P_PV can be successively withdrawn within the framework of the proportional or integral control component of the control 60, so that the PV power P_PV is returned to the normal operating power. In the following, in the event of any further (transient) power imbalance, in particular starting from a direct voltage U_DC close to the nominal voltage U_Nom, the full DC instantaneous reserve of the PV system 18 is available. This also means that the effect of the solutions described is particularly advantageous when either the deviation or the rate of change of the direct voltage U_DC or the deviation of the Direct current I_DC in the DC bus 10 each exceed defined dead band values; The dynamic behavior in the DC bus 10 is not influenced within the respective dead band.

7 shows an embodiment of a power generation system 18 with a bidirectional DC / DC converter 18b for connecting the PV generator 18a to the DC bus 10. The DC / DC converter 18b can be located between the PV generator 18a and the DC bus 10 be arranged and master the voltage translation types boost and buck with different energy flow directions. Specifically, the DC / DC converter 18b can be designed as a two-quadrant converter. For a transfer of electrical power from the PV generator 18a to the DC bus 10, the DC / DC converter 18b includes a step-up converter 71 with a first switch S_L and a first diode D_H, and vice versa for feeding back from the DC bus 10 into the PV -Generator 18a has a buck converter 72 with a second switch S_H and a second diode D_L. The DC / DC converter 18b thus in particular has a half-bridge with the power semiconductors S_L, S_H, D_L and D_H and, through suitable timing of the switches S_L and S_H, enables a needs-based exchange of power between the PV generator 18a and DC bus 10. If both switches S_L, S_H are open and are not clocked, the PV voltage U_PV adjusts to the direct voltage U_DC in the DC bus 10. In addition, electrical power can be fed back from the DC bus 10 into the PV generator 18a by bridging the first diode D_H by closing the switch S_H.

Depending on the design of the PV generator 18a, the PV voltage U_PV to be set in normal operating mode, which is based on the MPP voltage of the PV generator, can be less than or equal to the direct voltage U_DC in the DC bus 10. In particular if the maximum PV voltage U_PV, max and the maximum DC voltage U_DC, max are approximately the same (e.g. U_PV, max = U_DC, max = 1000V), the PV generator 18a can be designed in such a way that the system voltage is also used in the PV Energy recovery mode is not exceeded. In this case, the PV voltage is always lower than or equal to the maximum direct voltage U_DC in the DC bus 10 and can be set by means of a step-up converter. If, on the other hand, the PV generator design is such that the PV voltage U_PV can assume higher values than the direct voltage U_DC in the DC bus 10, a multi-quadrant controller can be used for feedback.

Figures 8a and 8b show examples of time curves of the direct voltage U_DC and the power P in the DC bus 10 of a DC network 1 for two different cases. In the upper part of each of FIGS. In the lower part of each of FIGS. 8a and 8b, power curves 83, 85 of the powers P in the DC bus 10 over the time t, the dotted line 81 representing the power balance in the DC bus 10.

In FIG. 8a, the direct voltage U_DC corresponds to the nominal voltage U_0 at a point in time t <t1, the sum of the powers fed into the DC bus 10 and the sum of the powers drawn from the DC bus 10 being approximately the same; in other words, there is no imbalance of power and the dotted line 81 is at zero.

At time t1, a linearly increasing disturbance of the power equilibrium begins, which is caused, for example, by a consumer 13a with decreasing consumer power, a braking generator 14a or a wind turbine 17 with increasing wind power output, and the dotted line 81 rises. At the same time, the direct voltage U_DC also rises in the DC bus 10 and, at time t2, exceeds a threshold value U_max or a value U_max + ÄU which deviates from the threshold value U_max by a hysteresis. A load resistor 15a is then activated and takes an electrical reserve power from the DC bus 10, represented by the power curve 83, as a result of which the direct voltage U_DC drops again. When the direct voltage U_DC is again below the threshold value U_max or below a value U_max-ÄU which differs by one hysteresis from the threshold value U_max, the load resistance is deactivated and the direct voltage U_DC rises again above the threshold value U_max, and so on.

The result is a clocked reserve power with the power curve 83 and the voltage curve 82. If the disturbance rises further, represented by the rise of the dotted line 81 between the times t2 and t3, the duty cycle of the clocked reserve power, ie the load resistance 15a, also increases activated longer and longer per unit of time and consequently warms up more and more. At time t3, the disturbance of the power equilibrium has reached a value which roughly corresponds to the instantaneous reserve power of the load resistor 15a, so that the load resistor 15a remains activated for relatively long periods of time in order to establish the power equilibrium and to stabilize the direct voltage U_DC. If the disturbance continues to rise, the reserve power would no longer be sufficient to stabilize the direct voltage U_DC in the DC bus 10 and the direct voltage U_DC would continue to rise. However, between times t3 and t4, the disturbance falls back to zero, for example when a consumer 13a (again) increases its consumer power, or when other control reserves connected to the DC bus 10 but reacting more slowly take excess power from the DC bus 10 , in particular due to an integrating control component and / or on request of a higher-level control of the DC network. The pulse duty factor of the power curve 83 is correspondingly reduced. That is at time t4 The power balance in the DC bus 10 is restored, the direct voltage U_DC i DC bus 10 corresponds to the nominal voltage U_0, and the load resistor 15a is deactivated.

In Fig. 8b, the direct voltage U_DC at a point in time t <t1 again corresponds to the nominal voltage U_0, i.e. there is no disturbance of the power equilibrium and the dotted line 81 is at zero. At the same time, a PV system 18 feeds MPP power P_MPP from a PV generator 18a into the DC bus 10 via a DC / DC converter 18b.

At time t1, a linearly increasing disturbance of the power equilibrium begins, i.e. both the dotted line 81 and the direct voltage U_DC increase. At the time t2, the direct voltage U_DC in the DC bus 10 exceeds the threshold value U_max or a value U_max + ÄU which deviates from the threshold value U_max by a hysteresis. The DC / DC converter 18b then changes its operating mode in such a way that the PV voltage moves away from the MPP in the direction of the direct voltage U_DC. In particular, the DC / DC converter 18b can interrupt a pulsing of its power semiconductors so that the direct voltage U_DC in the DC bus 10 is applied to the PV generator 18a via any capacitances and inductances of the DC / DC converter 18b. As a result of this change in the PV voltage, the PV power fed into the DC bus, represented by the power curve 85, decreases. This counteracts the excess power in the DC bus 10, so that the direct voltage U_DC drops again. When the direct voltage U_DC is again below the threshold value U_max or below a value U_max-ÄU which deviates by one hysteresis from the threshold value U_max, the DC / DC converter 18b is operated again in such a way that the PV voltage is moved again in the direction of MPP , in particular by resuming the clocking of the power semiconductors of the DC / DC converter 18b, and the PV power increases. The direct voltage U_DC then rises again above the threshold value U_max or above U_max + ÄU, and so on.

This results in a modulated PV power with the power curve 85 and the voltage curve 84. If the disturbance continues to rise, represented by the rise of the dotted line 81 between times t2 and t3, the mean value of the PV power continues to decrease, while the period of the modulation can remain largely the same. At time t3, the disruption of the power equilibrium has reached a value that is greater in magnitude than the MPP power of the PV generator 18a, so that the PV power has changed sign and a modulated feedback into the PV generator 18a takes place. Depending on the design of the PV voltages in relation to the direct voltage U_DC in the DC bus 10, it may be necessary to activate the DC / DC converter in a different operating mode in order to transfer the PV voltage to the direct voltage in the DC bus to change away, in particular to generate a PV voltage above the direct voltage U_DC in the DC bus 10. In principle, the PV generator 18a is able to receive feedback power in the order of magnitude of its nominal power. With a suitable design and control of the DC / DC converter 18b, the PV system 18 can thus provide a reserve power that always comprises at least the nominal power of the PV generator 18a and more than twice the current MPP power of the PV generator 18a in order to at least limit a power imbalance due to a correspondingly large disturbance and to stabilize the direct voltage U_DC.

Between times t3 and t4, the disturbance falls back to zero, and the mean value of the PV power rises in accordance with the power curve 85 Nominal voltage U_0, and the PV system 18 feeds the MPP power of the PV generator 18a into the DC bus 10.

The comparison of the dynamic aspects when using reserve loads 15 (Fig. 8a) or PV systems 18 (Fig. 8b) to stabilize the direct voltage U_DC in the DC bus 10 shows that a PV generator 18a, which is powered by a suitable DC / DC converter 18b is connected to the DC bus 10, similar to how a load resistor 15a can be operated in order to contribute to the sufficiently rapid stabilization of the direct voltage in the DC bus 10. A load resistor 15a has a linear, time-invariant, invariable U / I characteristic. In contrast to this, a PV generator has a non-linear, time-variant U / I characteristic curve, the equivalent internal resistance of which decreases with increasing voltage. This non-linearity of the PV characteristic curve can advantageously be used to stabilize the direct voltage in the DC bus 10 and in particular to avoid overvoltages in the DC bus 10. By deactivating the DC / DC converter 18b of the PV system 18, the PV voltage rises above the MPP voltage, the PV current decreasing exponentially with increasing PV voltage and becoming negative at PV voltages above the PV open circuit voltage . Due to the steep slope of the PV characteristic curve at voltages above the MPP, even small changes in the PV voltage cause relatively large changes in the PV current.

Due to the dependence of the PV characteristic on various external parameters, in particular on irradiation and temperature, the specifically desired operating point on the PV characteristic must be permanently monitored and set by means of suitable control of the DC / DC converter.

In addition, a PV generator 18a can absorb the simple nominal PV power as feedback power even without irradiation, ie in the dark, with a somewhat higher current compared to the load resistance being the same due to lower PV voltages Nominal power flows. With full irradiation on the PV generator 18a, ie in normal operation close to the PV nominal power, almost twice the PV nominal power is available as reserve power by reducing the PV power from the MPP power to zero and also a power in the order of magnitude of the nominal PV power is fed back into the PV generator 18a. A PV system 18 thus provides at least as much reserve power as a reserve load 15 with a comparable nominal power, the available reserve power of the PV system 18 increasing to twice the PV nominal power with increasing irradiation.

When the direct voltage U_DC is applied to the load resistor 15a or when the voltage is changed at the PV generator 18a, a corresponding operating point is usually set without delay, provided that parasitic effects are negligible. However, since the PV generator 18a is operated via a power electronic DC / DC converter 18b, which can include, for example, smoothing filters with inductive and / or capacitive filter energy stores, the operating point of the PV generator 18a is not shifted arbitrarily quickly, but with a time delay that depends on the size of the filter energy store and the maximum permissible electrical values. This delay can be minimized by using advanced power semiconductors and / or by increasing the switching frequency and / or by reducing the filter energy storage, if necessary. The rate of change in current on the filter energy storage device must be greater than the rate of change in voltage in the DC bus 10. In addition, suitable precautions must be taken to prevent excessive compensating currents, overvoltages and / or excessive electrical power and damage to components of the DC / DC converter 18b when applying the direct voltage U_DC in the DC bus 10 to the PV generator 18a. For this purpose, the determination and monitoring of the currents flowing in the DC / DC converter 18b and the voltages that occur in any case within the scope of the usual regulation of the DC / DC converter 18b can be used. Alternatively or additionally, the power semiconductors of the DC / DC converter 18b responsible for the voltage adjustment can be bridged, for example with an additional switched line. REFERENCE LIST

DC network

DC bus

AC network a power supply network b DC / AC converter DC network a supply network b DC / DC converter load a consumer b power supply

Machine a generator b DC / AC converter

Reserve load a Load resistance b DC / DC converter

Energy storage a battery b DC / DC converter

Wind turbine a wind turbine b AC / DC converter

PV system a PV generator b DC / DC converter, 41 characteristic curve limitation

Characteristic range

regulation

Boost converter

Buck converter

nominal voltage

Fault .84 Voltage curve .85 Power curve

Claims

PATENT CLAIMS

1. A method for stabilizing a direct voltage in a DC network (1), the DC network (1) comprising a DC bus (10) which has the direct voltage, a power generation plant (10) being connected to the DC bus (10). 18) and at least one load (13) are connected, the DC bus (10) being connected to a higher-level network (11, 12), wherein between the DC bus (10) and the higher-level network (11, 12) an electrical network power is exchanged, which is varied in order to keep a direct voltage in the DC bus (10) at a nominal voltage, wherein the energy generation system (18) comprises a PV generator (18a) which is connected to the DC bus (10) is connected and with the DC bus (10) exchanges an electrical generator power, the generator power in a normal operating mode through the DC voltage converter (18b) depending on an MPP power of the PV generator (18a) to a Normal operating power is set, the normal Drive power is set in a predetermined relation to the MPP power of the PV generator (18a) variably or independently of the MPP power of the PV generator (18a), whereby the direct voltage in the DC bus (10) is provided by the energy generation system (18) is monitored, the generator power in a grid support mode depending on the DC voltage in the DC bus (10) being set to a grid support power in order to avoid a power imbalance between the total electrical power supplied to the DC bus (10) and that of the DC bus ( 10) to counteract the total drawn power, the grid support power being adjusted depending on a deviation of the DC voltage from its nominal value and / or on a rate of change of the DC voltage.

2. The method according to claim 1, wherein the setting of the generator power in the network support mode takes place when the DC voltage in the DC bus (10) exceeds a predetermined limit value.

3. The method according to claim 1 or 2, wherein the generator power is set by clocking power semiconductors of the DC voltage converter (18b), a PV voltage applied to the PV generator (18a) being set to a normal operating voltage in normal operating mode by means of a first clock rate, and wherein the generator power is reduced in grid support mode in that the DC voltage converter (18b) is operated using a second clock rate in such a way that the PV voltage changes in the direction of the DC voltage.

4. The method according to claim 3, wherein the grid support mode occurs and the DC-DC converter (18b) is operated at the second clock rate when the DC voltage exceeds a limit value or a value deviating from the limit value by a hysteresis upwards, and wherein the normal operating mode occurs and the DC-DC converter (18b) is operated at the first clock rate when the DC voltage falls below the limit value or a value that deviates downwards from the limit value by one hysteresis.

5. The method according to claim 3 or 4, wherein the second clock rate has the value zero or one, so that the DC voltage converter (18b) is not clocked in the grid support mode.

6. The method according to any one of claims 3 to 5, wherein the DC-DC converter (18b) is operated in grid support mode such that the PV voltage is increased compared to the DC voltage when the PV voltage has adjusted to the DC voltage and / or the DC voltage continues to exceed the limit value.

7. The method according to any one of the preceding claims, wherein the grid support power is one PV control power less than the normal operating power and wherein the grid support power has in particular an opposite sign to the normal operating power when the deviation of the direct voltage in the DC bus (10) from its nominal value and / or the rate of change of the direct voltage in the DC bus (10) requires an amount of PV control power that is greater than the MPP power.

8. The method according to any one of the preceding claims, wherein the rate of change of the direct voltage is determined from a derivation of a voltage measurement or from a current measurement.

9. The method according to any one of the preceding claims, wherein an energy store is connected to the DC bus, wherein the energy store exchanges a storage power with the DC bus, wherein the storage power in normal operating mode is equal to zero or includes charging the memory, wherein the storage power comprises discharging the memory in the grid support mode in order to counteract a reduction in the direct voltage in the DC bus (10) compared to its nominal value.

10. The method according to claim 9, wherein in the grid support operation the storage capacity is increased in order to counteract a power deficit in the DC bus (10), and the PV power is reduced in order to counteract an excess power in the DC bus (10) works.

11. The method according to claim 9 or 10, wherein in the normal operating mode a charge state of the energy store (16) is sought which is between 90% and 100% of the charge capacity of the energy store (16).

12. The method according to any one of the preceding claims, wherein the DC bus (10) via a bidirectional power converter (11b, 12b) is connected to the higher-level network (11, 12), wherein the bidirectional power converter (11b, 12b) as

DC / DC converter (12b) is designed if the higher-level network (11, 12) comprises a further DC network (12), or is designed as an inverter (11b) if the higher-level network (11, 12) is an AC network (11 ), and wherein the electrical network power exchanged via the bidirectional power converter (11b, 12b) between the DC bus (10) and the higher-level network (11, 12) as a function of the electrical properties of the higher-level network (11, 12) for stabilization of the higher-level network (11, 12) is set.

13. The method of claim 12, wherein the DC bus (10) both via the

DC voltage converter (12b) is connected to a supply network (12a) and via the inverter (11b) to an energy supply network (11a) and both an electrical DC network power to the further DC network (12) and an AC network power to the AC grid (11) replaced.

14. The method according to claim 12, wherein in the normal operating mode a network power from the DC bus (10) flows into the AC network (11) and / or the further DC network (12), the network power in the normal operating mode comprising an excess power which corresponds to a difference between the generator power generated by the power generation system (18) and the power consumed by the consumer (13a) and / or the machine (14a).

15. DC voltage converter (18b) for connecting a PV generator (18a) to a DC bus (10) of a DC network (1), the DC voltage converter (18b) for exchanging electrical power between the PV generator (18a) and the DC bus (10) is set up and comprises a controller, wherein the controller is set up to carry out a method according to one of the preceding claims.