WO2013045072A2

WO2013045072A2 - Pv system having a safeguard against feeding into a public power distribution network

Info

Publication number: WO2013045072A2
Application number: PCT/EP2012/004002
Authority: WO
Inventors: Bernhard Beck
Original assignee: Adensis Gmbh
Priority date: 2011-09-28
Filing date: 2012-09-25
Publication date: 2013-04-04
Also published as: JP2014534794A; EP2761716A2; DE102011115189A1; WO2013045072A3; IL231553A0

Abstract

The invention relates to a photovoltaic system comprising a photovoltaic generator (1), which is connected to the DC input of an inverter (3), the AC voltage output of which is connected to a supply network and to at least one consumption point (V). A blocking device (5) prevents a power flow from the photovoltaic generator to the supply network. This measure prevents a power flow from the side of the photovoltaic generator into the supply network and therefore permits easier stabilization of the network voltage against overvoltages.

Description

description

PV system with protection against infeed into a public power grid

The invention relates to a photovoltaic system comprising a photovoltaic generator, which is connectable to the DC voltage input of an inverter whose AC output is connectable to a supply network and at least one consumption point.

Such photovoltaic systems are well known, especially as PV rooftops on homes, barns, industrial halls, etc. In operation, any feed of electrical power into the grid tends to increase voltage, and any removal of energy, ie any consumption, tends to reduce voltage. In particular, in regenerative energy producers, the fed power is very irregular. Depending on the temperature and position of the sun, photovoltaic systems sometimes produce more or less or no power and, if necessary, deliver it to the grid. The generation of power is therefore difficult to predict, which can lead to difficult network conditions with regard to its stability, when thousands of small and medium-sized PV systems feed independently of one another: the grid operator does not know when and at which point in the supply grid a voltage-increasing feed-in or a voltage-reducing consumption done so that he can always react only in hindsight. In the case of an impending mains breakdown due to overvoltage, the two parameters power supply and power consumption must therefore be taken into account.

The invention has for its object to facilitate the adjustment of the stability of the supply network. This object is achieved by a blocking device that prevents a power flow from the photovoltaic generator to the supply network. This measure has the consequence that the imponderables of the many voltage-increasing feeds are eliminated. There remains only one parameter, namely the power consumption,

CONFIRMATION COPY to be taken into account in the forthcoming stability measures. An optionally required voltage reduction can be achieved via a reactive power reference by means known per se. The reactive power reference and the determination of its height can be made in a simple manner, without having to take into account the disturbing influence of other power feeds consideration. The blocking device can be switched on, so that it is only active on request from the network operator. But it can also be permanently effective if the photovoltaic generator is to be used only for self-supply.

A practical embodiment of the blocking device provides that it has a controllable switching element which interrupts an electrical connection between the inverter output and the supply network in the presence of a higher inverter output voltage than the prevailing mains voltage. Since the power flow always takes place from a higher potential to a lower potential, a power supply to the supply network is avoided. Conveniently, the switching element is located immediately behind a power counter.

In order to achieve a maximum possible yield for self-consumption of photovoltaically generated power, it is advantageous if the blocking device generates a control signal at a threatening feed of photovoltaically generated power in the supply network, which is the maximum power point (MPP) by a targeted mismatch on the The DC side of the inverter shifts to a point of reduced power whose associated voltage level on the AC side of the inverter is below the prevailing mains voltage or equal to the prevailing mains voltage. This measure prevents unwanted feed into the supply network while optimally exploiting the generated solar energy. The impending power feed is detected for example by means of a comparison of the prevailing mains voltage with the current MPP voltage on the AC side of the inverter. In this case, line losses are disregarded with their associated voltage drops. If these are to be taken into account, the output voltage at the inverter must be are used, which leads to a correspondingly higher corrected output voltage. The corrected output voltage is dimensioned so that it is equal to the mains voltage at the power counter, with no consumption of the consumption point taking into account the line losses.

Alternatively or in addition to the voltage comparison described above, the blocking unit can be activated even when falling below a minimum current from the supply network to the point of consumption (V). The current to be compared with the minimum current is advantageously measured at the power counter. If the measured current falls below the minimum value of e.g. 0.5 amps, it is assumed that an imminent backfeed into the grid and the blocking unit is activated. Again, the activation of the blocking unit can be a disconnection of the photovoltaic generator from the public grid at any point in the network behind (from the public network view) of the power meter, as long as it is ensured that no photovoltaic electricity flows into the public grid. Such a separation point can be located both on the DC side, ie between the inverter and the photovoltaic generator, as well as on the AC side behind the inverter. It is advantageous, however, if the separation point is chosen so that the photovoltaic generator remains connected to the rest of the supply network for the consumption points.

Alternatively, a reduction of the inverter output current may occur via an intervention in the operation of the electronic components (IGBTs) of the inverter: the current generated by the inverter should be as high as possible on the premise that it is smaller than the current taken from the point of consumption or consumption. This always leaves a small supply current from the public supply network. If no reserve is to be used, ie if no power is to flow from the supply network to the point of consumption, the current at the AC output side of the inverter must be set equal to the current consumed by the point of consumption or consumption. A particularly advantageous embodiment when using an inverter with a three-phase AC output provides that the operating points of the three phases of the inverter can be controlled separately, so that depending on the load of the three phases by the at least one consumption point, the power generated by the photovoltaic generator is divided phase-selectively before the power flow to the supply network is prevented. If the at least one consumption point is, for example, a detached house with a rotary power connection, then it is the normal case that the phases L1, L2 and L3 of the three-phase supply network are not utilized equally. If at one phase L1 the dishwasher and a hotplate are in operation, while another phase L2 is only loaded with some lighting, this situation is taken into account by drawing the first phase L1 more current from the DC bus than the other phase L2 , This procedure has the advantage that an interruption between the photovoltaic generator and the supply network is only made when all phases of the consumption point are sufficiently supplied with solar energy according to their respective loads. By reducing the photovoltaic power by shifting the operating point of the inverter away from the MPP, the operating point will only be mismatched as much as necessary to generate a reduced power sufficient to photovoltaically power all the phases.

A practical implementation for this provides that the inverter has a DC voltage rail from which the electronic parts of the inverter, in particular its IGBT's, have phase-selective access to feed as needed a current in the DC busbar or to obtain from her. In this case, all three phases L1, L2, L3 can draw a current, or a phase L1 can feed one current, while two other phases L2, L3 receive a current.

A further, advantageous embodiment of the invention is characterized by a measuring point, which measures the power consumption from the supply network. From the measured value, a signal is generated which corresponds to the value of the measured payment of benefits. This signal is provided to the inverter for setting an operating point at which the associated generated power on the AC side of the inverter is set to be smaller or, in particular, equal to the power consumed by the at least one point of use or, in the case of several points of consumption compared to the sum of all the consumed at the points of consumption. As a result, it is achieved that an exact adaptation of the required by the point of consumption and supplied by the photovoltaic power is performed. To avoid even a low power input into the supply network, the operating point should be set so that always slightly less power is generated photovoltaic than consumed by the point of consumption. Thus, there is always a low power consumption from the grid and the photovoltaic system can contribute to any increase in voltage in the supply network.

The signal, which is preferably generated by the measuring point, is modulated onto an electrical connecting line between the measuring point and the inverter and used by a receiving unit for the modulated signal, which is accommodated in the inverter or at least assigned to it, for controlling the same.

As already stated, shifting the operating point of the inverter away from the MPP results in a mismatch in which less power is produced photovoltaically than it could be under given weather conditions. This undesirable effect can be counteracted by providing an energy store, in particular a battery, as one of the consumption points. Then, the energy storage is first loaded before the blocking device takes effect by the mismatch is made.

Further embodiments and advantages of the invention will become apparent from the description of an embodiment with reference to FIGS. Show it:

A photovoltaic system with inventive blocking device; Figure 2 shows three single-phase connected to a common DC rail inverter.

3 shows a power redistribution between the phases by means of inverters.

In the figure 1, 1 denotes a photovoltaic generator, which is arranged on the roof of a building. The photovoltaic generator 1 is connected to the DC side of an inverter 3 whose AC output is connected via a blocking device 5 to a counter 7 for electric power P. The blocking device 5 is provided immediately behind (from the point of view of an energy supplier) the counting device 7 whose input side is connected to a supply network. Behind the AC output of the inverter 3 is still a switching device 8, with the aid of the inverter 3 and thus the photovoltaic generator 1 can be separated from the rest of the power supply system to prevent a return current in the photovoltaic generator from the supply network. The same effect can be achieved with a correspondingly polarized diode whose reverse direction to the inverter 3.

The output side of the inverter 3 is connected via a switching device 9 to the AC side of a further inverter 11, on whose DC side a battery 13 is connected. Further, at least one consumption point V is connected to the output side of the inverter 3, wherein usually a plurality of consumption points (not shown), such as connected household appliances, air conditioners, TV, etc. are connected.

The blocking device includes a control and regulating unit 15 and a switching unit 17, by means of which a flow of current through the counting device 7 into the supply network can be prevented. For this purpose, the closed and open position of the switching unit 17 via a signal line S1 of the control unit 15 can be predetermined. To the control unit 15, a signal line S2 is also connected to the inverter 3 to adjust its operating point, and a signal line S3, which leads to the switching device 9 to connect or disconnect the other inverter 11 with the AC side of the inverter 3 , A fourth signal line S4 leads from the counter 7 to the control unit 15 to inform them about the current power reference. Finally, a signal line S5 is still provided for

Switching device 8 leads, so that the solar generator 1 can be decoupled from the mains supply when needed.

A first voltage measuring device 19 measures the mains voltage U net present at the counter 7 and a second voltage measuring device 21 measures the voltage U _WR on the inverter output side which prevails behind the blocking device when the photovoltaic generator 1 is switched on, which is also the consumer side when the switching device 8 is closed.

The system operates as follows, starting from a situation in the morning in which the photovoltaic generator 1 can not provide any power yet and the point of consumption V is supplied solely from the supply network via the counter with the switching unit 17 closed. The switching device 8 was opened overnight, so that no return current from the supply network could flow into the photovoltaic generator 1. Depending on the season and cloud conditions, the photovoltaic generator 1 is switched on during the morning by the switching device 8 is transferred to the closed state. The power point at the MPP controller (Maximum Power Point) inherent in the inverter is controlled to the maximum possible power at the prevailing mains voltage U _Ne tz which is also applied to the inverter output in order to deliver as much photovoltaically generated energy to the consumption points V. The remaining demand is further obtained from the supply network via the counter 7. This remaining demand, assuming the same consumption power, is decreasing more and more, since the incident light of the sun's rays on the photovoltaic generator 1 becomes steeper and more intensive. This operating state is maintained until the photovoltaically generated power reaches the value of the power obtained at the point of consumption V, which can be detected with the aid of the two voltage measuring devices 19, 21 or communicated from the power counter 7 via the signal line S4 of the control and regulation unit 15 , This is where the invention comes in, in a first variant, in which the control and regulation unit 15 closes the switching unit 17 via the signal line S1 and thus disconnects the photovoltaic generator 1 and the point of consumption V from the supply network. This separation is possible because the power generated by the photovoltaic generator 1 exceeds the power required at the point of consumption or equal to this power. When there is an excess, the further switching device 9 is closed by the control and regulation unit 15 via the signal line S3, and the photovoltaically generated energy exceeding the current demand is used to charge the battery 13. If the battery 13 is fully charged, the inverter 3 is controlled by the control and regulation unit 15 via the signal line S2 in such a way that there is a mismatch with the MPP. The amount of mismatch is such that the photovoltaic power produced corresponds to the power consumed by point of sale V.

If no battery 13 is provided with further inverter 11, then in a second variant, the optionally provided switching unit 17 can be omitted and the mismatch at the inverter 3 is made continuously in order to prevent a flow of photovoltaically generated power in the supply network. Thus, a balance between photovoltaically generated energy and consumed energy is permanently sought, as long as the solar energy generated allows this. For this purpose, in the control unit 15, the voltage measured at the first measuring point 19 is compared with the voltage detected at the second voltage measuring device 21. The operating point on the MPP controller of the inverter 3 is always misadjusted so that the voltage at the second voltage measuring point 21 is always slightly lower, for example 1 volt to 5 volts less than the voltage measured by the first voltmeter 19. This may possibly occur under the input discussed consideration of voltage drops due to line losses. Remains the photovoltaic power below the power consumed by point of sale V does not require any mismatching. The photovoltaic generator 1 can always be operated on its MPP and the remaining energy to increase the currently required energy is obtained from the public grid.

Instead of the comparative voltage measurement by means of the first and the second voltage measuring device 19, 21, the control and regulating unit 15 can also be supplied via a signal line S4 from the power counter 7 with the information as to whether there is a risk of backflow of power into the supply network. The control unit 15 then determines the right time to make the separation of the photovoltaic generator from the supply network. The right time is when the photovoltaic energy is sufficient to supply the consumer or V stable, but not so high that a feed can occur in the supply network.

FIG. 2 shows three single-phase inverters 3 connected to a common DC bus 23, the output side of which feeds one of the phases L1, L2 and L3 respectively with solar energy. The three inverters 3 and the DC bus 23 are basic components of a corresponding three-phase inverter 3, which are located in a common housing (not shown). At the three phases L1, L2 and L3 are correspondingly connected three consumption points V1, V2 and V3, which should have one or more consumers with different sizes or in the sum of different sizes nominal powers. It should therefore be discussed a condition in which the three phases L1, L2 and L3 are charged differently.

The arrangement according to the figure 2 still has a three-phase switching device 8 in order to separate the photovoltaic generator 1 from the supply network and the consumer side, so that a feedback feed of energy from the supply network is prevented in the photovoltaic generator 1. A power counter 7 is in turn immediately behind the supply lines of the network operator. Between the counter 7 and the consumption points V1, V2 and V3 a switching unit 17 with three switching elements 17a, 17b and 17c is arranged, which can selectively separate the three phases L1, L2 and L3 from the counter 7. From a control and control unit 15 leads a signal lines S1 to the switching unit 17, a signal line S2 to the inverters 3, a signal line S4 to the counter and a signal line S5 to the three-phase switching device 8. All signal lines S1 to S5 are shown in dashed lines.

In this embodiment, with three differently loaded phases L1, L2 and L3 each inverter 3, analogously as described for the figure 1, operated. Each phase L1, L2 and L3 is thus supplied with photovoltaically generated energy according to their current consumption situation. A feed of photovoltaically generated energy into the supply network is correspondingly also prevented phase-selectively, in which only that switching element 17a, 17b, 17c of the switching device 17 is driven, threatens in its associated phase L1, L2 and L3, a feed into the supply network.

Instead of the phase-selective blocking of power flow into the supply network described with the aid of FIG. 1, the power exchange shown in FIG. 3 can also be carried out among the phases L1, L2 and L3 in order to optimally divide the photovoltaically generated energy into the differently loaded phases L1 To achieve L2 and L3. This procedure is particularly useful in single-phase inverters 3, whose output side is connected to only one phase, in the case of Figure 3 with the phase L1.

For this purpose, two inverters 25a, 25b are provided, of which the inverter 25a is provided between the phases L1 and L2, and the inverter 25b between the phases L1 and L3. The converter 25a transforms the AC voltage of the phase L1 first into a DC voltage and then from the DC voltage back into an AC voltage with the phase position and the voltage level of the phase L2. Similarly, voltage of the phase L1 and the phase compensation current between the phases L1 and L3 are first formed into a DC voltage and a DC current, respectively, before they are adapted by the converter 25b to the ratios of the phase L3. Finally, it should be pointed out that the photovoltaic generator can have any shape and size, ie can also be formed by a single photovoltaic module, which is assigned a correspondingly small inverter.

LIST OF REFERENCE NUMBERS

1 photovoltaic generator

3 inverters

5 blocking device

7 power counter

8 switching device

9 more inverters

11 switching device

13 battery

15 control unit

17,17a-c switching unit

19 1. Voltage measuring device

21 2. Voltage measuring device

23 DC bus

25a, 25b inverter

S1 - S5 control signal

V1 - V3 consumption points

Claims

claims

1. Photovoltaic system comprising a photovoltaic generator (1) which is connectable to the DC voltage input of an inverter (3) whose AC output is connectable to a public utility grid and at least one point of consumption (V), characterized by a blocking device (5) prevents a power flow from the photovoltaic generator to the supply network.

2. Photovoltaic generator according to claim 1,

characterized,

in that the blocking device (5) comprises a controllable switching unit (17) which interrupts an electrical connection between the inverter output and the supply network in the presence of a higher inverter output voltage (U _WR ) than the prevailing mains voltage (U _NETZ ).

3. Photovoltaic generator according to claim 2, characterized in that the switching unit is arranged directly behind a power counter (7).

4. Photovoltaic generator according to one of claims 1 to 3,

characterized,

that the blocking device (5) generates a control signal (S2) in the event of an impending supply of photovoltaically generated power into the supply network, which shifts the maximum power point (MPP) to a point of reduced power through a targeted mismatch on the DC side of the inverter whose assigned voltage value on the AC side of the inverter is below the seeing mains voltage or equal to the prevailing mains voltage.

5. Photovoltaic generator according to claim 4,

characterized,

the impending power supply is detected by means of a comparison of the prevailing mains voltage (UN _E TZ) with the current MPP voltage on the AC side of the inverter (3).

6. Photovoltaic generator according to one of claims 1 to 5 with a three-phase inverter output,

characterized,

the operating points of the three phases (L1, L2, L3) of the inverter (3) can be controlled separately, so that, depending on the load of the three phases by the at least one consumer point (V), that of

Photovoltaic generator (1) power is divided in a phase-selective manner, before the power flow to the supply network is blocked (Fig. 2).

7. Photovoltaic generator according to one of claims 1 to 6,

marked by

a measuring point (7) for the supply of power from the supply network, wherein a signal (S4), which corresponds to the value of the measured power reference, the inverter (3) is provided for setting an operating point whose associated power point on the AC side of the inverter smaller or in particular is equal to the power consumed by the at least one point of consumption (V).

8. Photovoltaic generator according to one of claims 6 or 7,

characterized,

in that the inverter (3) has a DC voltage rail (23) to which the electronic parts of the inverter, in particular its IGBT's, phase-selectively have access to feed or receive a current in the DC bus as needed.

9. Photovoltaic generator according to one of claims 1 to 9,

characterized,

that the blocking unit (5) is activated when falling below a minimum flow from the supply network to the point of consumption (V).

10. Photovoltaic generator according to claim 9,

characterized,

that the activation of the blocking unit is a separation of the

Photovoltaic generator from the supply network or a reduction of the inverter output current includes.