EP2689462A2

EP2689462A2 - Photovoltaic system

Info

Publication number: EP2689462A2
Application number: EP12735085.8A
Authority: EP
Inventors: Bernhard Beck
Original assignee: Adensis GmbH
Current assignee: Belectric GmbH
Priority date: 2011-03-19
Filing date: 2012-03-16
Publication date: 2014-01-29
Also published as: EP2802014A1; WO2012126601A3; WO2012126601A2

Abstract

In a photovoltaic system (1) having a number of phases (7) arranged in parallel on the generator side, the invention provides a first switching element (17) between a phase-end positive terminal (11) and earth (18), a second switching element (19) between a phase-end negative terminal (13) and earth (18), a measuring apparatus (23, 25) for detecting the voltage between the terminals (11, 13) and earth (18), and a control apparatus (21) for receiving the measured voltage values from the measuring device (23, 25) and for triggering control signals (S1, S2) for the simultaneous or staggered closing of the switching elements (17, 19), when the voltage across the first open switching element (17) exceeds a first prescribed limit value (G1), or when the voltage across the second switching element (19) drops below a second prescribed limit value (G2).

Description

description

photovoltaic system

The invention relates to a photovoltaic system (PV system) with a photovoltaic generator having a plurality of parallel arranged strands of photovoltaic modules connected in series (PV modules), wherein the strands have a plus and a negative pole, between which one over the number of photovoltaic modules connected in series is applied to predetermined strand voltage, which is more than 1000 volts when the photovoltaic generator is idle, and to an inverter whose DC voltage input is connected to the two poles and whose output can be connected to a supply network.

When designing photovoltaic systems, it must always be considered that a maximum permissible voltage (U _z ) between the positive pole and the negative pole on the DC side of an inverter is not exceeded. Exceeding this would lead to the destruction of the inverter and of that part of the photovoltaic modules to which a voltage above the permissible voltage is applied.

For this reason, in the design of the photovoltaic system is usually ensured that even in the worst case of idling the open circuit voltage (U _L or U ₀ ) is always below the maximum permissible voltage (U _z ), which is why a plurality of strands is usually connected in parallel , The maximum number of strings depends on the power of the inverter to which the strings are connected. Modern inverters can be designed up to a DC input voltage of about 900 volts to 1000 volts.

Typically, each strand of the photovoltaic system is composed of eleven photovoltaic modules, each of which has 120 photovoltaic cells. In total, 1330 cells are connected in series with each other. Each cell has a voltage of 0.75 volts when idling, resulting in a string voltage of 990 volts below that of

CONFIRMATION COPY the manufacturers of the modules specified maximum voltage of 1000 volts. During operation of the photovoltaic system, the open-circuit voltage of the cells drops to an operating voltage of approximately 0.5 volts, so that a voltage of 660 volts is applied between the ends of the conventional strings. Should the grid operator, to which the photovoltaic system is connected, disconnect these from the grid, for example due to a short circuit in the supply cable, the voltage increases suddenly to the stated 990 volts, which is not critical for the modules and the system. If a higher voltage is applied, this can destroy part of the modules, the inverter and the entire system.

On the other hand, it is desirable, especially with regard to novel inverters with higher allowable operating and open circuit voltages, to operate the photovoltaic modules and also the inverter in normal operation at a voltage greater than 660 volts and preferably equal to the maximum allowable voltage of 100 volts. In order to make better use of the insulation resistance of the cabling, which is generally designed to be 10000 volts, it is also desirable to increase the number of modules per string in order to exploit the 1000V voltage during operation of the photovoltaic system. However, this is not readily possible because then in the event of a fault, a voltage of about 1500 volts to earth (ground potential) occur and could lead to the destruction of the photovoltaic modules and the lines.

To avoid these unduly high voltages, it is known from DE 30 41 078 AI and DE 10 2005 018 173 AI known to arrange a short-circuit switch between the positive pole and the negative pole, which short-circuits in the case of an inadmissibly high voltage between the poles.

From EP 2 086 020 A2 and DE 20 2006 008 936 Ul it is also known, the positive pole or the negative pole to a fixed allowable potential of z. B. to fix 1000 volts and to let the photovoltaic system float in operation from this potential down or up, which is also referred to as so-called float. However, this measure is not possible for plants with a floating potential. In such systems with a floating potential of the plus and the negative pole z. For example, potentials versus a virtual ground of plus (+) 600 volts to minus (-) 600 volts. Virtual earth means that the strands are not connected to earth at any point. However, if the center of the strand were set to earth, corresponding voltages of (+) 600 volts of the positive pole and (-) 600 volts of the negative pole would arise in relation to the grounded center of the line. For such systems, a switch could be provided between the center of the string and earth, which is closed in the event of a ground fault and the ground of the ground real. As a result, then fall only voltages up to 600 volts to the modules. However, this measure would be associated with a considerable amount of cabling, since the middle of each strand must be accessible via the switch. In addition, when using so-called TCO modules corrosion problems occur because due to cathode discharge of the module edge is eroded.

The invention has for its object to provide a photovoltaic system, in particular with free-floating or freely displaceable potential, indicate that for operation with a comparatively high operating voltage of z. B. 1500 volts is suitable. In particular, it should also be ensured that no inadmissible voltage exceedances occur at a PV module or at the input of the inverter of the PV system. Furthermore, a particularly suitable method for operating such a photovoltaic system should be specified.

With regard to the PV system, the object is achieved according to the invention by the features of claim 1. Advantageous embodiments, variants and developments are the subject of the dependent claims.

With regard to the method, the stated object is achieved according to the invention by the features of claim 6. In this case, the positive pole and the negative pole are connectable via a first or a second switching element to earth, wherein the switching elements are closed when the voltage across the first open switching elements exceeds a first predetermined limit or when the voltage across the second open switching elements a second predetermined Limit value falls below.

This measure ensures that at the components of the PV system, in particular at the PV modules and at the inverter no unacceptably high voltage (Uz) to earth occurs during idling. Due to the high operating voltage of z. B. lOOOVolt can be used with thinner cable cross-sections with the same power of the PV system cables, which is less expensive and allows larger systems. The inverter can be operated at its maximum voltage, allowing better utilization of the dimensioning of the inverter, i. H. the dielectric strength in particular of the installed capacitors, the electronic components and the wiring is achieved. In addition, for a given current, the inverter can deliver higher power to the utility or power grid. If the photovoltaic generator is operated with a freely floating potential, then the limit value should be at least half of the predefinable string voltage during idling.

It is advantageous if the first or the second limit value according to the amount is at least 3% smaller than the lowest permissible dielectric strength of all participating live components, such. As a terminal, a cable and the photovoltaic module. Thus, the short circuit depends on the weakest link in the chain, usually after the photovoltaic module or in older converted equipment also another component.

The second limit value should be at least 3% smaller in magnitude than the lowest operating point voltage laid down in the control algorithm of a MPP controller (Maximum Power Point Controller) typically assigned to a photovoltaic system. This measure particularly reliably prevents the use of so-called TCO photovoltaic modules address the problem of cathode erosion at negative potential, while TCO modules continue to work and self-destruct. With the embodiment according to the invention, further short circuit switching elements between the positive pole and the negative pole of the DC side of the inverter are not required.

In regenerative energy plants, each photovoltaic generator (PV generator) generates a direct current that is converted into an alternating current by means of the inverter. As inverters, both purely electronic devices and electromechanical converters can be used. Under inverter in this case are all devices to look at, which can generate an AC voltage from a DC voltage electronically. Although wind turbines generate directly an alternating current, but which is to be adapted via a frequency converter to the conditions of the public supply network. These frequency converters also include inverters with an internal DC link.

So far, measures to maintain the stability of a multi-phase three-phase supply network are known, which act evenly on the grid. Here it was recognized as a disadvantage that although a common voltage change of all phases may be necessary or at least useful for one of the phases in order to prevent the undershooting or overshooting of a voltage threshold, while for another phase of the three-phase network this may be undesirable. If z. B. causes a voltage boost to prevent impending undervoltage at a point in the network, it may be more undesirable at another point at the moment no major consumers, this increase in voltage.

It is therefore an inventive idea that the ability of electronic inverters to act separately on each phase can be used to contribute to voltage stabilization. These devices have due to their inherent components such. As IGBT ^' s, the ability to make a contribution to performance (VAr contribution) phase selective. Thus, an already existing capability of the devices is used for a further purpose. Advantageously, the stability in a supply network can be increased by equalizing the individual phase voltages by virtue of the inverter having a control input via which its operating mode is variable when a control signal is applied such that a supply to the supply network which is asymmetrical with respect to the three phases takes place. Alternatively, the inverter has a control and regulating unit, by means of which its operating mode enables a three-phase unbalanced feed into the supply network.

In an advantageous embodiment, the operating mode works towards a point of maximum power that can be generated, which is distributed asymmetrically to the three phases. It is also expedient if the operating mode works towards a point of maximum producible reactive power or to a point of apparent reactive power which is distributed asymmetrically to the three phases. It is also expedient if the operating mode works towards a point of maximum power that can be generated and, in addition, reactive power is fed into the three phases asymmetrically or is obtained asymmetrically from the three phases. It may also be advantageous for the operating mode to be exposed to the maximum power point and for a specifiable amount of reactive power to be fed into at least one of the three phases or to be drawn from one of the three phases as a function of the control signal.

Advantageously, the voltage of all three phases is measured at any network connection point in the supply network, in particular at the network connection point which connects the inverter to the supply network, the control signal being formed with the aid of the three voltage measurement values. To form the control signal, it is expedient that the voltage of three phases is measured at the connection point of a transformer, and / or that the current is measured at a network connection point, in particular at least one of the transformer outputs, and the current measurement value is included in the calculation of the control signal , In order to be able to obtain or supply a higher reference or a higher supply of reactive power to a selected phase, it is provided in an advantageous further development of the method that the output power P is increased at the relevant phase. It is also expedient for reactive power to be drawn at least in one of the three phases, while at the same time reactive power is supplied to another phase, that power is supplied to at least one of the three phases while at the same time power is supplied to another phase, at least at reactive power or power is supplied to one of the three phases, and / or that the power direction of at least one phase deviates from the power direction of at least one other phase, while reactive power is simultaneously applied to at least one phase and At least at another phase reactive power is delivered.

Such an inverter and such a method for its operation are considered as an independent invention.

Unbalanced is understood to mean that the generated power and / or reactive power of the photovoltaic generator is unevenly divided by the inverter on the three phases. This may also mean that one phase or two phases will only be considered with a small or no share of the generated power or will even take up power while feeding power over at least one other phase. The generation of reactive power is independent of the function of the PV generator and can also take place at night when there is no solar radiation and no power.

If all three phase voltages LI, L2 and L3 are determined, the values 230Volt, 225Volt and 228Volt result, for example. This can be due to the current behavior of consumers or even to a large number of photovoltaic roof systems feeding energy into the grid independently of each other. A precautionary network stabilization then serves to strengthen the weakest phase L2, which has only a phase voltage of 225 volts, by generating the phase generated in this phase Power is fed via a provided with the corresponding control input inverter.

In the case of large deviations, the inverter can also use free capacitances to feed reactive power into phase L2 for boosting the voltage. Free capacities of the inverter are present if the currently available power of the photovoltaic generator is below the nominal power of the inverter. For example, if the inverter is rated at 12kVA, but due to prevailing solar radiation, only 7KVA of solar energy is generated and converted to alternating current, then the considered inverter has a free capacity of 5KVA available for reactive power injection (VAr). This power of 5KVA corresponds to the difference between the currently supplied power and the rated power of the inverter. Thus, the reactive power amount is limited to the power remaining to reach the rated power of the inverter. As a result, the energy generated by the generator itself is not limited or reduced. The free power capacity of the inverter is used only to fulfill another function, namely that of the phase shifter or reactive power supplier. Typically, the VAr amount is less than the 5kVA VAr calculated above as the difference.

Via a corresponding control signal, which is applied to the control input, the request is implemented. It is irrelevant whether only the measured phase voltages are given to the control input, which are then converted by a computing unit in the inverter to a feed mode, or whether the appropriate direct feed mode is input to the control input, the inverter only still implements.

There are four main operating modes:

*

i) maintaining the standard mode of operation towards a point of maximum producible power and distributing photovoltaic power unbalanced to the three phases; ii) an operating mode is set towards a point of maximum reactive power which is distributed asymmetrically to the three phases;

iii) the standard operating mode towards a maximum producible power point is maintained as a priority and, in addition, free capacity of the inverter is used to inject reactive power asymmetrically into the three phases or to draw asymmetrically from the three phases;

iv) the standard operating mode towards the maximum power point is suspended and replaced by an operating mode in which, depending on the control signal, a specifiable amount of reactive power is fed into at least one of the three phases or obtained from one of the three phases.

The default mode of operation towards the point of maximum power is known per se and will not be discussed further here. It is important for the objective of the present inverter that it can convert a control signal into an asymmetrical distribution of its power, and / or that its electronic components are influenced via the control signal in such a way that reactive power can additionally be generated.

With regard to a suitable method for operating the inverter, it is advantageous that the voltage of all three phases is measured at any grid connection point in the supply network, in particular at that grid connection point which connects the inverter to the supply network, and that the control signal with the aid of the three Voltage measurements is formed.

The network connection point is understood to be the position of the counting point between the consumer and the network and between the feeder and the network. In the present case, this also includes any position within the public supply network and the network of consumers and the feeders, on which the voltage measurement is made understood. The choice of the measuring point in the immediate vicinity of the inverter means a spontaneous correction possibility of the three phase voltages at the location of the measure. In addition to this location, all network connection points are particularly suitable, where there is a high voltage sensitivity. This means that a particularly suitable location for the placement of the voltage measuring device at the end of a stub or in relation to a mains transformer feeding a loop transformer is in the middle of the loop or in the case of multiple feed transformers in the vicinity thereof. There are the consumers who have in the normal reference direction of the network, the lowest mains voltage with the largest difference of their phase voltages available, especially if one of the transformers fails, z. B. for maintenance. Because of the other consumers ahead of them, all of which cause a marginal voltage drop, the voltage available there is usually lowest, unless there is an energy supplier nearby.

At the end of a spur line and in relation to a power line feeding a loop transformer in the middle of the loop are to be seen relative. If, for example, a stub with 200 attachment points is given, the end is to be seen as a point of attachment of the last 10%, ie as one of the last 20 attachment points. Analogously, in the case of a loop of 200 connection points, the connection points lying on the left and right of the transformer connection points would be visible.

In general, the most voltage-sensitive network connection point, in particular under the hypothesis of a transformer failure, is selected. This can also be another place, for example, if there is a consumer who operates heavy machines with high starting current, which are frequently switched on and off during the day. Alternatively, the consumer with the highest variation in reactive power sourcing is determined, and the voltage value is measured at the grid connection point of that consumer. Generally, the most stress sensitive point is characterized by the highest voltage variation with respect to a feeding power P. Further, the voltage variation can be defined by a percentage change amount and not by the absolute voltage values considered. This is the case in the presence of a loop usually in the middle in relation to the transformer to the next higher network, that is, at the point where there is the same distance to the transformer in both directions. It follows that the voltage measurement of at least two phases at the connection point of a transformer is particularly suitable for the formation of the control signal. In addition, the current through at least one of the transformer outputs should be measured and used to enter into the calculation of the control signal.

Suitably, the operation of the inverter are maintained by its inherent control device to the maximum power MPP of the photovoltaic generator or wind turbine and fed from the inverter in addition such reactive power amount in the supply level or related to the maximum of the difference of the currently supplied power to the rated power of the inverter ,

With proper operation of the inverter, a measured phase voltage below a threshold will result in a reactive power feed into that phase from the inverter while an impending surge in phase, e.g. upon reaching an upper threshold, leads to a reference of reactive power from targeted this phase by the at least one inverter.

Some utilities may require that a fixed ratio of power to reactive power, ie a fixed cos phi value, be maintained. Optionally, it may be desirable to increase the delivered power P at the phase in question to obtain or deliver a higher reference or higher supply of reactive power to a selected phase. This is also about the Fixed cos phi value changes the reactive power accordingly to the selected phase.

In the course of the asymmetrical distribution, the direction of the reactive power generation can also vary from phase to phase. Thus, it may be appropriate that reactive power is obtained at least in one of the three phases, while reactive power is simultaneously supplied to another phase. This can be modified as desired, e.g. reactive power is supplied to two phases and reactive power is sourced from the third phase, or it is sourced from two phases reactive power and supplied to the third phase, etc. This means creating the possibilities: i) that at least one of the three phases reactive power or power and ii) that the power direction of at least one phase deviates from the power direction of at least one other phase while at the same time sourcing reactive power on at least one phase and providing reactive power at least at another phase becomes.

A particularly suitable method for optimizing the feed-in power and the stability in a public power grid with multiple energy suppliers of renewable energy, in which each energy supplier has an electronic components based on the basis of electronic components, according to the invention provides that each inverter its currently freely available reactive power capacity to a System control reports, which determines from the incoming data on the reactive power capacity a contribution for each energy supplier, which feeds it via its inverter as reactive power into the supply network or from the supply network. An expedient development of this method provides that each inverter reports the currently fed power as a percentage of its nominal power.

According to an alternative embodiment, it is provided according to the invention that each inverter reports its power currently fed into the supply network to a system controller which, from the data received for the power, determines a contribution for each energy supplier which the inverter uses as its dummy output via its inverter. feeds power into or from the supply network. An expedient development of this method provides that each inverter reports the currently fed-in power as a percentage of its rated power, that each inverter reports the currently fed or drawn reactive power as a percentage of its nominal power.

Suitably, the system controller is integrated in one of the inverters or is located in a control room. The rated power of the inverters of the energy suppliers involved is preferably stored in the system control. Suitably, in addition to the capacity data, the free reserve is reported to the system controller until the rated power of the inverter is reached. It is also advantageous that the inverters of the multiple energy suppliers are assigned to network areas and voltage levels. Suitably, the geographical location of the energy suppliers is included in the investigation. Particularly preferably, both the freely available capacity for the supply or purchase of reactive power and the geographic location of the energy suppliers and the allocation of the energy suppliers to a network area are included in the determination in order to achieve the highest efficiency in voltage stabilization through a defined feed / reference of reactive power to reach.

Although the goal of this method, considered as an independent invention, is to stabilize the supply network by means of the distribution and distribution of the reactive power contributions to the alternative energy suppliers involved, a notification of the instantaneous performance may also lead to the goal. This is z. As is the case when the control unit of the relevant energy supplier drives the inverter to a fixed cos phi value, which can be drawn from the knowledge of the performance of a conclusion on the instantaneous reactive power. If the nominal power of the individual energy suppliers is known, the freely available reactive power capacity can then be determined.

The advantages achieved are in particular to use the aforementioned property of inverters to stabilize the mains voltage without a reduction of To have to accept active power or, if necessary, to increase their feed-in.

Depending on the level of supply, in the practice of public electricity supply, the power supply companies set more or less strict limits for the purchase of reactive power or the reactive power supply of electricity suppliers and electricity consumers. In most countries, the allowable cos phi value for a penaltyless reactive power purchase or a reactive power feed-in is 0.95. This measure serves to stabilize the networks in order to avoid overvoltage, which can lead to the destruction of connected consumers, and an undervoltage, which can lead to a failure of consumers. The price serves as a regulator for compliance with the set cos phi values. Thus, a feed-in or a purchase of reactive power outside of the bandwidth prescribed by the utility or main network operator for the cos phi is substantiated with considerable additional payments.

In plants generating regenerative energy, each photovoltaic system generates in its PV generator a direct current, which is converted by means of an inverter into an alternating current. The electronic components of an inverter, as well as the combination of a DC machine with a synchronous generator as a mechanical inverter, allow setting a desired cos phi value in relation to the power. For most PV systems, this is done by a cos phi pointer, which can be used to set a fixed ratio of injected power to injected or referenced reactive power. So the control unit z. For example, it is required to set any power fed into the grid at a cos phi of 0.97.

For larger and modern systems, it is customary not rigidly specify the cos phi value as a pointer, but according to the diagram in accordance with FIG. 4 set. There, a cos phi value to be set is plotted on the output of the inverter via the mains voltage. The output voltage U _mains for feeding into the network may vary only within a minimum value U _min and a maximum value U _max . Except- The photovoltaic system should not be operated halfway through this area of supply to the grid, which has been approved by the energy supplier and has a cos phi of, for example, a maximum of 0.95. Within this permissible range is a narrower range between a minimum control voltage U _reg e min and a maximum control voltage U _rege i max, in which pure power can be delivered to the grid without a reactive power component. In the middle of this narrower range is the setpoint voltage Usoii.

As a rule, energy suppliers are obliged, above all with regard to their contractual relationship with the main network operator (eg nuclear power station, coal-fired power station etc. as energy supplier), not to exceed a reactive power reference value of cos phi 0.95 in order to ensure voltage stability in the supraregional main network. This means that in the numerical example, a cos phi value of 0.94 represents an overshoot of the reference, whereas a cos phi value of 0.96 represents a shortfall, that is, a non-exploitation of the maximum allowable reference. However, sourcing or supplying reactive power is often required in the lower networks to compensate for a voltage increase from the supply of solar and wind power, or to compensate for a voltage drop due to failed supply of alternatively generated energy or starting up of machinery.

Electronic inverters can produce ^'s of from reactive power that can be fed into the connected utility grid them by appropriate actuation thereof IGBT. Likewise, a control can be carried out such that reactive power is obtained from the network. The supply of reactive power has a voltage-reducing effect on the supply side and the supply of reactive power increases the voltage.

The inventive method allows particularly advantageous to use the aforementioned property of inverters to stabilize the mains voltage, without having to accept a reduction of the active power or an injection of these may even increase. The advantage of the above measures will be explained by an example. The following terminology is used: A photovoltaic system consists of a PV generator and an inverter. There are three larger photovoltaic systems connected to the public grid. The The network is in an operating condition which requires measures to ensure stability. Possible reasons can be the failure of a transformer due to maintenance work, the start-up of induction furnaces, the feed-in of numerous photovoltaic systems on roofs with a respective small fluctuating voltage increases, etc.

The three photovoltaic systems are located at different locations, which are far apart, so that a given cloud pattern leads to different solar irradiation on the individual PV generators, which inevitably leads to a different feed of energy through the inverter into the power grid. The first photovoltaic system is e.g. operated under full load, that is, their associated inverter operates at its rated power to feed electrical energy into the grid. The other two PV systems generate at the same time only 70% or 80% of their rated power, which is fed by the assigned inverters in the supply network.

According to the invention, those inverters are used for sourcing or feeding in reactive power having free reactive power capacities, the rated power of which is currently not exhausted, in order to allow other energy suppliers or generators to feed additional active power into the supply grid. In the above example, the second and third photovoltaic systems are capable of providing voltage stabilizing reactive power contributions. Since the inverter of the first photovoltaic system operates at full load and can no longer process any further power without disregarding its reactive power obligation towards the network operator, its reactive power component can be reduced if the stabilization effect associated with the reactive power component is taken over by another photovoltaic system.

The contribution released by the withdrawal of reactive power in the case of the first photovoltaic system can be used to increase the injection of active power. According to the invention, therefore, an optimization of the reactive power feed-in or its train between different energy suppliers (power producers) with the aim of jointly feeding in the maximum power or the maximum number of kilowatt hours in the supply network, with a common view of all energy suppliers and not isolated from just one single energy supplier. In this case, it makes sense for devices with temporarily low available power and free VAr control capacity (power or reactive power capacity) to contribute more to grid support than energy suppliers with high power present at the same time and low reactive power control capacity. This is equivalent to assuming VAr control requirements from power producers who are currently able to do so without sacrificing their performance. Quasi free VAr capacities are shifted in order to balance a demand and an offer for the benefit of a maximum possible performance while maintaining the grid voltage within a predeterminable range.

The allocation of energy suppliers (energy producers) in network areas, voltage levels and the inclusion of their geographic location enable safe operation and optimizes the effect of reactive power supply or relation with respect to the voltage stabilization in the network while optimizing the total system of all energy suppliers, the maximum number of kilowatt hours ( kWh) into the grid.

Embodiments of the invention will be explained in more detail with reference to a drawing. Show:

Fig.l schematically photovoltaic system (PV system) with time-shifted short-circuiting of the plus and minus pole; and

FIG. 2 schematically simplified another embodiment of a PV system according to the invention with synchronous short-circuiting of the positive and negative poles, FIG.

3 is a schematic overview of a photovoltaic system with an inverter, 4 is a control diagram of an inverter of a solar system for setting a cos phi value on the output voltage, and

Fig. 5 is a schematic representation of a supply network.

A photovoltaic system 1, referred to below as a PV system, has as essential elements, according to FIGS. 1 and 2, a photovoltaic generator 3, referred to below as a PV generator, and an inverter 5. The connected to a three-phase (LI, L2, L3) supply network 4 PV generator 3 has a number of parallel-connected strands 7, each consisting of a series of sixteen (16) hereinafter referred to as PV modules photovoltaic modules 9. The ends of the strands 7 on the one hand form a positive pole 11 and on the other hand a minus pole 13.

Referring back to the introductory example, in which a PV module 9 was presented to each one hundred and twenty (120) cells, each having an operating voltage of 0.5V and an open circuit voltage of 0.75V, there is an open circuit voltage for each line 7 between the positive pole 11 and the negative pole 13 in the amount of 1440V. During operation of the PV generator 3, an operating voltage between the poles 11 and 13 in the amount of 960V sets. The operating voltage is harmless for the PV modules 9 and the inverter 5 and makes good use of the permissible voltage limit of 1000V. When switching off the PV generator 3 from the network 4 is avoided by appropriate measures that the no-load voltage of 1440V, which can lead to damage to the PV modules 9 and the DC voltage input 10 of the inverter 5 is applied. If the power output at the AC voltage side 8 of the inverter 5 unintentionally unplanned, so the input voltage at the DC voltage side (DC input 10) increases suddenly to the value of the open circuit voltage, which is to be avoided.

Another, unplanned, voltage increase may occur due to a ground fault or creeping ground fault 15 occurring on one of the connecting lines between the PV modules 9, between the strings 7 or to the inverter 5. Such a ground fault 15 is in the figures by a dashed mass tag symbolizes. For better understanding of the earth fault 15 is located at the bottom of the first PV module 9. At any other point, the earth fault 15 in principle - only comparatively slightly creeping - to the same effect.

The earth fault 15 is usually not a loadable short circuit, but causes a reduced contact resistance to ground, but sufficient to shift the potential at this point. The result of the shift is that the negative pole 13 is laid in the direction of ground when idling. For the sake of clarity, it is assumed that a short-circuited short to ground. If an idle state occurs, then an open-circuit voltage of 90V would build up at the first PV module 9 below, at the second PV module 9 above 180Vlt, at the next PV module 9 from three times 90V, ie 270V, etc. From PV Module 9 to PV module 9, the voltage at the PV module 9 increases by 90V, which leads to an undue voltage of 1080V from the twelfth module.

According to the invention, a first switching element 17 is now provided both at the positive pole 11 and at the negative pole 13, of which, when a control or switching signal S1 is present at the first switching element 11, the positive pole 11 is connected to earth or ground 18. Analogously, when a control or switching signal S2 is applied to the second switching element 19, the negative pole 13 is connected to earth or ground 18.

The switching signals S1 and S2 are generated by a control device 21, which determines the voltage applied to the positive pole 11 as the input signal El of the first measuring device 23 connected to the positive pole 11 and the negative pole 13 which is determined by a second measuring device 25 connected to the negative pole 13 Voltage value is supplied as input signals E2.

The closing operation of the switching elements 17, 19 is controlled such that the first measuring device 23, first, a voltage value above a limit value Gl of z. B. 1000V determined. This is detected by the control device 21, which then emits a switching signal Sl to the first switching element 17, whereupon this closes. by virtue of the connection of the positive pole 11 to the ground 18, the voltage at the negative pole 13 is shifted downwards, since the applied PV generator voltage from ground 18 leads starting to a smaller potential. The downward shift leads to a second limit value G2 for triggering the switching signal S2 for closing the second switching element 19 being undershot. Then closes the second switching element 19 and the negative terminal 13 of the photovoltaic generator 3 is set to ground 18.

Thus, the photovoltaic generator 3 is short-circuited as a whole and none of the involved components of the PV generator 3, in particular mounting clamps, cables, cable branches, cable lugs or photovoltaic modules 9 continues to carry voltage. The two switching signals Sl and S2 are thus generated with a time delay, the short-circuiting of one of the two poles 11 or 13 automatically leads to a temporally subsequent short-circuiting of the respective other pole 13 and 11 respectively. Therefore closes first, the second switching element 19 due to a determined by the second measuring device 25 falls below the lower limit voltage G2 of z. B. minus (-) 850V, the loadable short to ground leads to a raising of the potential at the positive pole 11 of the PV generator 3 via the first limit value Gl associated therewith, wherein the exceeding of the first limit value Gl has the generation of the second switching signal S2 result , The switching elements 17, 19 are therefore preferably designed such that the time interval between the generation of the two switching signals Sl and S2 is between 10 ms and 100 ms, in particular between 20 ms and 50 ms.

In FIG. 2, the photovoltaic generator 3 is shown as a block for reasons of clarity. The photovoltaic generator 3 consists of the same components as the photovoltaic generator 3 according to FIG. 1. The difference between the photovoltaic system 1 according to FIG. 2 is that of the control device 21 instead of the two time-shifted generated switching signals Sl and S2 only a single control or switching signal S is generated, which leads to a synchronous triggering of the two switching elements 17, 19. There are switching elements 17, 19 available, which can also switch high DC currents of several hundred amps in two physically separate lines at the same time. The switching elements 17, 19 can work on a chemical, electrical or mechanical basis and in particular be varistors or IGBTs.

The first and the second limit values G1 and G2 can be set on the control device 21 via setting means (not shown). It is also possible to use predetermined limit values which are permanently preprogrammed in the control device 21 and are compared with the measured values of the first and second measuring devices 23, 25. In addition, instead of the two measuring devices 23, 25, a single measuring device can be provided.

In Fig. 3, a three-wire power supply system is shown with the conductors L1, L2, L3, which are connected to a measuring point 101, by means of which at least the voltage applied to the individual phases L1, L2, L3 is measured. It is also conceivable to additionally determine the current value through the conductors, in order to possibly detect an overload of a conductor, and then, e.g. to direct its own feed-in capacity to under-loaded conductors. Hereby, it may e.g. when tapping transformers or cables, it may be useful to measure only the current and not the voltage. The measured values determined are forwarded to a control and control unit 103, which among other things also includes the control unit for setting the point of maximum power MPP.

A photovoltaic generator 105 is provided as a regenerative power generator. The photovoltaic generator 105 generates a direct current, which is guided by means not shown electrical leads to a DC bus 107. The DC bus 107 is located in an inverter 109 whose AC side is connected to the three phases L, L2, L3. The actual conversion of the generated direct current is carried out separately for each phase by means of electronic components, currently preferably with IGBTs. This is indicated by three inverter symbols 110a, 110b, 110c within the inverter 109. From the control unit 103, three separate signal lines S10, S20, and S30 lead to the three IGBT blocks indicated by the inverter symbols 110a, 110b, and 110c. Via these signal lines S10, S20 and S30, the IGBTs are controlled such that they make the calculated by the control and control unit 103 settings in response to the voltage readings. For the sake of clarity, the inverter 109 and the control and regulation unit 103 are shown separated from each other. In reality, the control and control unit 103 is integrated with the MPP element in the inverter 109. Thus, there is an inverter 109 whose control and control unit 103 additionally assumes the described phase-selective reactive power management.

Regarding the operation of the inverter09, the three phase voltages LI, L2 and L3 are given with the values 230V, 235V and 227V. This can be due to the current behavior of consumers or even to a large number of photovoltaic roof systems feeding energy into the grid independently of each other. This situation is determined by the measuring point 102 and transmitted to the control and regulating device 103. This is programmed such that e.g. a maximum voltage difference between the phases LI, L2 and L3 of 4V is accepted, wherein the phase voltage should lie within a predefinable bandwidth, here 228V to 232V. The criterion 4V can be set arbitrarily according to the needs of the energy supplier and network operator. Other criteria such as the absolute voltage of the three phases LI, L2 and L3 may be used without taking into account the difference value of the voltage between the phases of consideration.

The requirements for the network can also change, which is why an alternative is provided to provide the control and regulation unit 103 with a setting input E. The currently required criterion is transmitted to the inverter 109 from a control room (not shown) via the setting input E, which may be the currently required setpoint values for the three phase voltages LI, L2 and L3, for example. According to the aforementioned numerical example, the control unit 103 determines, based on the measured voltage values, that the phase LI should remain unchanged, that the phase L2 be reduced by at least 3V in its voltage by a reactive power reference, and that the phase L3 be replaced by a Reactive power supply to one volt (IV) is to strengthen. After setting the involved IGBTs, a voltage pattern results for the three phases of LI = 230V, L2 = 232V and L3 = 228V. In the event of serious deviations, in addition to the unbalanced distribution of the power generated by the PV generator 105, the inverter 109 can also use free capacitances to feed reactive power into the phase L3 for voltage boosting and to draw reactive power from the phase L2 to reduce the voltage.

In FIG. 5, a part of a public supply network 201 is shown, which includes a first and a second transformer 203 or 205, three photovoltaic systems 207, 209, 211, a city network 213 and a control room 215. The photovoltaic panels 207, 209, 211 are shown outside the city network 213 for clarity, but are operated by the utility of the city. The transformers 203, 205 are connected on their primary side with a 20kV cable, and their secondary side supplies the city network 213 on the 400V level. The primary-side cables of the transformers 203, 205 in turn lead to further, not shown transformers, which supply the transformers 203, 205 from a HOkV-level with voltage.

In addition to the transformers 203, 205, the first, second and third photovoltaic systems 207, 209, 211 feed power into the city network 213 at the 20 kV level. The three PV systems 207, 209, 211 consist in their essential elements in each case of a photovoltaic generator, which comprises a plurality of photovoltaic modules whose photovoltaic cells generate a direct current, and an inverter, which converts the generated direct current into an alternating current.

The inverter has a power adapted to the nominal power of the PV generator. The adjustment is such that from the maximum producible power a discount is made taking into account that for the fewest time of the year the sun is at the irradiates the maximum possible intensity on the PV modules, it seems uneconomical to design the inverter to this maximum power. Furthermore, in the dimensioning of the inverter, a reactive power component is taken into account, which the power generator must bring into the supply level for voltage stabilization. Such an obligation arises from the contracts of the local operator of the urban network with the operators of the higher level of care.

In a numerical example, for the first photovoltaic system 207, a maximum producible power of 1.55 MW is assumed, which can be achieved as active power at the highest possible position of the sun in summer and in clear skies. Since this constellation occurs only a few hours a year, a deduction is made for the design of the inverter, and a maximum power of 1.5 MW is assumed. The city grid operator with its three PV systems 207, 209, 211 is required by the higher-level energy supplier not to exceed a cos phi value of 0.95 at the transformers 203, 205 and at the PV systems 207, 209, 211.

The supply of the power of the PV systems 207, 209, 211 tends to increase the voltage, so that of the 1.5MW a stockpiling for the reactive power purchase of 5% is made, which at a power rating of the inverter 5 to 1.5MW to a maximum possible power supply of 1.5 x 0.95 = 1.425MW leads. The other two photovoltaic systems 209, 211 are to be designed in the numerical example to each 1 megawatt (1MW) rated power, of which 5% are to be provided for the reactive power purchase, so that they can provide 0.95MW power.

When the sky is cloudy, there is neither a cloud cover nor cloudless sky. In such weather conditions, it should be assumed that the first photovoltaic system 207 operates at full power, but nevertheless only feeds 1.425 MW of active power due to the five percent cos phi reduction, ie 1.5 MW - 1.425 MW = 0.075 MW below its nominal power. At the same time, the second photovoltaic system 209 is partially shaded and only feeds 0.7 MW into the city network 213, whereby the reactive power component should already be taken into account in the 0.7 MW. According to the invention, both PV systems 207, 209 now report their reactive power reserves to the control room 215. The first photovoltaic system 207 therefore reports zero available reactive power capacity and the second photovoltaic system 209 reports 0.3 W (1 MW rated power - 0.7 MW instantaneous power) available reactive power capacity , The control room 215 then sends a control signal to the second photovoltaic system 209, which changes the operation of the inverter so that it in addition to the 0.7MW power that generates the second photovoltaic system 209, still another 0.075MW reactive power relates.

The inverter of the second photovoltaic system 209 is then a total of

0.775MW, which is below its rated power of 1MW. The additionally requested by the control room 215 reactive power purchase of 0.075MW corresponds to the contractually required by the first photovoltaic system 207 reactive power purchase, which therefore no longer has to be provided by this, but is taken over by the second photovoltaic system 209.

Subsequently or simultaneously, the control room 215 sends to the first photovoltaic system 207 a control signal that their inverter no longer has to make any reactive power reference, which can be there regardless of a reactive power supply, the supply of sun-generated active power up to the rated power of 1.5MW can be made , The exchange of data between the PV systems and the control room can be wireless or wired. Likewise, it is freely selectable, which component of the photovoltaic system 207, 209, 211 emits the message to the control room.

In the division of the PV systems 207, 209, 211 into a PV generator and an inverter, it is the inverter in the broadest sense that undertakes the notification of the available capacity. Thus, the inverter can include a communication interface, which reports the operating status of the control of the IGBTs in the inverter, which sends a message in plain text, etc. In the above exemplified situation, intervention in the operation of the inverter of the third photovoltaic system 211 is unnecessary. Rather, the free reactive power capacities of several, preferably all, cooperating on the considered supply level alternative energy generator is reported to the control room 215, which then possibly taking into account the geographical location of the power generator in the electrical network, the reactive power and / or the reactive power delivery of that or Power generator modified so that a maximum power output in the overall view of all participating power generator is present.

The invention is not limited to the embodiments described above. Rather, other variants of the invention can be derived therefrom by the person skilled in the art without departing from the subject matter of the invention. In particular, all individual features described in connection with the exemplary embodiments are also combinable with one another in other ways, without leaving the subject matter of the invention

LIST OF REFERENCE NUMBERS

1 photovoltaic system 101 measuring point

3 P hotovo Ita g i n d e rt o r 103 Control unit

4 supply network 105 PV generator

5 Inverter 107 DC bus

7 string 109 inverters

9 photovoltaic module 110a-110c inverter symbol

10 DC input

11 positive pole 201 supply network

13 negative pole 203, 205 transformer

15 earth fault 207 first photovoltaic system

17 first switching element 209 second photovoltaic system

18 earth / mass 211 third photovoltaic system

19 second switching element 213 city network

21 Control Device 215 Le te rs / syste m s

23 first measuring device ung

25 second measuring device

E setting input

El, 2 input signal

Gl, 2 limit

L1, L2, L3 phase

S control / switching signal

Sl, 2 control / switching signal

S2 control / switching signal

S10, S20, S30 signal input

Claims

P120177P-RF / EE March 16, 2012 claims

Photovoltaic plant (1) having a photovoltaic generator (3) with a number of parallel arranged strands (7) of photovoltaic modules (9) connected in series, the strands (7) having a positive pole (11) and a negative pole (13) and between them Poland (11, 13) based on the number of photovoltaic modules (9) predetermined strand voltage is applied, which is greater than lOOOVolt in idle case of the photovoltaic generator (3), and with an inverter (5), the DC input (10) with the two Poland (11, 13) is connected, and the output side with a supply network (4) is connectable,

marked by

a first switching element (17) between the positive pole (11) and earth (18) and a second switching element (19) between the negative pole (13) and earth (18), a measuring device (23, 25) for detecting the voltage between the poles (11, 13) and earth (18), and

a control device (21) for receiving the measured voltage values from the measuring device (23, 25) and for triggering control signals (S1, S2) for simultaneous or time-delayed closing of the switching elements (17, 19) when the voltage across the first open switching element ( 17) exceeds a first predetermined limit value (G1), or when the voltage across the second open switching element (19) falls below a second predetermined limit value (G2).

2. Photovoltaic system (1) according to claim 1,

characterized, the time offset is between 10ms and 100ms, especially between 20ms and 50ms.

3. Photovoltaikanfage (1) according to claim 1 or 2,

characterized,

that the predetermined limit value (G1, G2) is at least half of the predefinable string voltage in the case of idling, when the photovoltaic generator (3) is operated with a free-floating potential.

4. Photovoltaic system (1) according to one of claims 1 to 3,

characterized,

that the first and the second limit value (G1, G2) are unequal in magnitude.

5. Photovoltaic system (1) according to one of claims 1 to 4,

characterized,

that the first limit value (G1) or the second limit value (G2) is at least 3% smaller than the lowest permissible dielectric strength of the participating live components, in particular a terminal, a cable and / or the photovoltaic module.

6. Photovoltaic system according to one of claims 1 to 5, with an MPP controller, characterized

that the second limit value (G2) is at least 3% smaller than the lowest in the MPP controller, in particular in its control algorithm, stored operating point voltage.

7. A method for operating a photovoltaic system (1), the

a photovoltaic generator (3) having a number of parallel strands (7) of photovoltaic modules (9) connected in series, the strands (7) having a positive pole (11) and a negative pole (13) and between the poles (11, 13) one based on the number of photovoltaic modules (9) predetermined strand voltage is applied, which is greater than 1000 volts in idle case of the photovoltaic generator (3), and

an output side connected to a supply network (LI, L2, L3) or connectable inverter (5) whose DC voltage input (10) is connected to the two poles (11, 13),

characterized,

in that the voltages of the positive pole (11) and of the negative pole (13) are measured with respect to ground (18) and from the measured values a first control signal (S1) for a first switching element (17) arranged between the positive pole (11) and ground (18) and a second control signal (S2) for a between the negative terminal (13) and earth (18) arranged second switching element (19) are generated, and

if the first switching element (17) is closed by a first limit value (G1) when a voltage value at the first switching element (17) is exceeded, and / or

if the switching element (19) is closed when a voltage value at the second switching element (19) falls below a second limit value (G2).

8. Inverter (9), in particular a photovoltaic system according to one of claims 1 to 5, which is connectable to a three-phase three-phase supply network for feeding renewable energy,

marked by

Means (103, E), in particular a control and / or control unit or a control input, for changing the operating mode when applying a control signal such that with respect to the three phases (LI, L2, L3) unbalanced supply or an unbalanced reference into the supply network.

9. A method for controlling the feed-in power of an electrical energy supply network (213) with a plurality of regenerative energy generators (207, 209, 211), each having an inverter,

characterized, in that each inverter reports its currently available reactive power capacity and / or power fed into the supply network to a system controller (215) which determines a contribution from the available reactive power capacity or power for each power generator (207, 209, 211) the respective energy generator (207, 209, 211) feeds via the inverter as reactive power into the supply network or from this.