WO2009039888A1

WO2009039888A1 - Method and inverter for feeding ac-current into mains for part load conditions

Info

Publication number: WO2009039888A1
Application number: PCT/EP2007/060266
Authority: WO
Inventors: Peter Knaup; Karl Koch; Thomas Lenz
Original assignee: Conergy Ag
Priority date: 2007-09-27
Filing date: 2007-09-27
Publication date: 2009-04-02
Also published as: EP2206211A1

Abstract

The present invention is directed to a method for feeding electrical power into a grid by means of an inverter converting electrical energy of a DC-intermediate storage having a DC voltage or current into an AC output current, whereby the electrical power availably in the DC-intermediate storage is converted into an AC power by means of an AC output current decomposed into burst periods, whereby the output current comprises burst periods of a generated sinusoidal current having the frequency of the grid and pausing periods when no current is generated.

Description

Method and Inverter for Feeding AC-Current into Mains for Part Load Conditions

The present application is directed to a method for feeding electrical power into a grid by means of an inverter converting electrical energy of a DC link into an output current for feeding into the grid. The invention is further directed to an inverter for feeding electrical power into the grid.

It is generally known in the state of the art, to convert electrical energy of a solar generator or the like provided as DC-power into an AC-current for feeding into a grid, such as a mains or an island network. In such a solar system the inverter is dimensioned with respect to a nominal power of the solar generator. Commercially available grid converters continuously feed power into mains. The current wave form is predominant sinusoidal. In stationary operation there is a well balanced equilibrium between the energy supplied by the solar generator and the inverter output. If the solar generator can only supply part of the nominal power, the output power of the inverter will also decrease and the amplitude of the output current will decrease accordingly. In this situation, the system is in a condition of a part or light load. It is known, that the system efficiency is typically minimal at ultra light loads. It will be optimal around about half maximum load with respect to common systems.

It is therefore an object of the present invention to increase the system efficiency with respect to light and part load conditions.

Accordingly, the present invention proposes a method for feeding electrical power into a grid according to claim 1 . The inverter comprises an intermediate storage comprising electrical energy and generates a sinusoidal output current having the frequency of the grid. However, in case of light or part load this current is not continuously generated but comprises burst periods. One or a plurality of periods of the basically sinusoidal output current are only generated, when the interme- diate storage of the inverter comprises sufficient energy to generate an output current having a sufficient value, e. g. 50 % amplitude of the nominal value. If there is not enough energy in the intermediate storage to generate a sinusoidal output current having a sufficient value, the inverter will pause and not generate any sinusoidal output current.

According to the invention a burst period provides more energy than is delivered to the inverter with respect to the corresponding period of time. Accordingly, the additional energy needed for the burst period, that is not delivered by an energy source that is connected to the inverter must be stored in this intermediate storage. To be able to provide burst periods, even if there is only very few energy delivered to the inverter, the intermediate storage should be capable of storing at least sufficient energy to generate one burst period.

According to one embodiment a DC-link comprising a DC-link capacitor is used as the DC-intermediate storage. Such a method can easily be implemented into inverters, such as known inverters already comprising a DC-link having a DC-link capacitor. It might be advantageous to provide a sufficiently large DC-link capacitor or to provide one or a plurality of additional DC-link capacitors to be able to store sufficient energy in the DC-link. However, there are other possibilities to provide an intermediate storage such as an intermediate storage providing a DC current, e.g. as known in current inverters.

Accordingly, for light or part load conditions the inverter will not generate a current having a reduced amplitude but will involve an AC-current burst mode and therefore generate a current having periods of zero Amps (pause) and of at least one period of a sinusoidal current of a proper value (burst period). According to this principle of the invention for conditions of only few electrical energy being available an output current for feeding into a grid can be provided whereby a sinusoidal current having only a small amplitude can be avoided. For such part or light load conditions the efficiency of the system can be increased in comparison to the state of the art.

It is to be noted, that the amplitude of the output current refers to a peak amplitude of the sinusoidal fundamental wave of the current. However, other values can be used such as the RMS-value of each period and thus the RMS-value of one complete fundamental wave.

Thus, if the burst periods comprise a constant amplitude of the output current, the power of the generated output current depends on the relation of burst periods to pausing periods. And thus the mean output power can be defined by the ratio of the generating time to the pausing time. Accordingly, the generating time to pausing time ratio can be set dependent on the electrical energy available in the DC link.

With respect to one embodiment the constant value of the generated current is about 20 to 40% of the nominal value and in particular 30 % of the nominal value. Such values are advantageously selected, if the system efficiency has its optimum at about half maximum load. The advantage of increasing the system efficiency can be explained with respect to the relation of the output power of the sinusoidal current P_Ac and the power received from the DC link:

P AC = PDC — Ptoss

Accordingly, the output power P_AC is smaller than the received DC power P_DC by the amount of the power loss P_/oss- However, if the AC power decreases in a system of an operation condition that is continuously generating AC current as known by the state of the art the power loss does not decrease by the same amount. On the other hand, if there is no AC power generated in the pauses according to the present invention, there is also no power loss with respect to the output current.

If current bursts have fixed amplitude, half maximum load according to one embodiment, the energy contained in a single period is fixed and exactly known. For the example, that the generated current (l_Acburst) comprises half the amplitude of the nominal or maximum output current {Uc_max) the energy of one period {E_{AC burst}) can be calculated assuming

Uc burst = 0.5 ^■ I AC max

The energy of one period is

E AC burst = 0.5 ^■ V Ac ^m I AC burst

whereby the voltage of the output signal is considered to be V_AC.

Since there is no AC current during pauses between burst periods, the output energy (E _Ac pause) is also zero during pauses:

-AC pause = 0 Thus, the mean output power is given by

EAC burst + EAC

P AC =

Attest + Δtpa∞. Tl ^■ Tgrύ + tpause

This relation results, if the burst duration equals multiple periods of the grid (nT_gπd). It can be seen, that it is possible to tune the mean output power by varia- tions of the pause length at a constant burst amplitude. Accordingly, the system's equilibrium state may be controlled by selecting an adequate burst-pause ratio which results to a continuous solar power generator load. Even further, the aim is to get a stable maximum power point operation (MPP operation).

As a result, the power is transmitted burst period by burst period and at least part of the power of a burst period has to be buffered by the grid inverter's DC link or other intermediate storage.

According to a further embodiment, generating of the sinusoidal current in the burst period is initiated, when the DC link voltage is above a first voltage threshold value and when a representing fundamental wave representing the current to be generated reaches zero. Accordingly, the voltage of the DC link indicates the available power, in particular the power stored in the DC link. A burst period of at least one period having a sufficient amplitude can only be generated, if there is sufficient DC power available. However, the output current to be generated must be adapted to the grid and should be started when the corresponding wavelet crosses zero. It is preferred to always generate burst periods of complete periods and in particular to generate the same kind of burst periods i.e. always generate burst periods starting with a rising slope or always generate burst periods starting with a falling slope. Accordingly, the burst periods will always end at a rising slope or a falling slope respectively.

According to a further aspect of the present invention the generating of the sinusoidal current in the burst period is stopped, when the DC link voltage is below the first or below a second voltage threshold value and when the sinusoidal cur- rent or a sinusoidal reference current representing the current to be generated reaches zero. One first voltage threshold value of the DC link voltage can be used to decide whether to initiate generating of the sinusoidal current in the burst period or to stop generating a current, for each instance, when the generated current or a reference current reaches zero. However, in some cases the threshold values for initiating and stopping might be different, to achieve a hysteresis. For deciding whether the current reaches zero and also for detecting a rising or falling slope the generated current can be used, if the system is in a burst period. If the system is in a pause period there is no output current and the reference current is used to detect zero and to detect falling or rising slopes. However, such a reference signal is often also used in general for paused periods and burst periods as well because an artificial reference signal is often easier to handle than a measured one.

Decomposing the AC output current is in particular advantageous, if only few power is delivered to the inverter. On the other hand, the benefit of decomposing the AC output current into burst periods decreases with increase of available power. According to one embodiment to avoid unnecessary switching operations in the inverter, and thus to avoid switching losses, the method switches over to a continuous mode in which the AC output current is not decomposed into burst periods. In this continuous mode the AC output current is continuously generated, as generally known in the art. If the available power delivered to the inverter decreases below the first predetermined power threshold value or if the power decreases below a second predetermined power threshold value, the method will switch back to a burst mode. In this burst mode the AC output current is decom- posed into burst periods. I.e. the burst modes are only used, if there is only few power available.

To avoid an often change between continuous mode and burst mode, the first predetermined power threshold value is larger than the second predetermined power threshold value. In this way, a hysteresis behavior is achieved. I.e. if the available power falls below the second predetermined power threshold value, the method will switch to burst mode. But if the available power rises again above the second predetermined power threshold value, the method will remain in burst mode until the available power also exceeds the first predetermined power threshold value, which is according to this embodiment larger than the second predetermined power threshold value. According to the same principle the method will also not change back to burst mode, if the available power falls again below the first predetermined power threshold value, unless it also falls below the second predetermined power threshold value.

In general, it is advantageous to change between continuous mode and burst mode when the available power is approximately 30 % of the nominal value of the inverter. I.e. according to one example the first predetermined power threshold value can be 40 % of the nominal value of the inverter and the second predetermined power threshold value can be 20 % of the nominal value of the inverter. To determine the available power, it is usually sufficient to measure the currently delivered power and a prediction of available power is avoided.

With respect to a further embodiment it is proposed to use at least two inverters, each having a DC link or other intermediate storage and each inverter generating an AC output current decomposed into burst periods, whereby at least one output current is generated by assembling burst periods of the AC output currents of at least two inverters. If a solar system comprising at least two inverters feeds elec- trical current into the grid, each inverter would - according to the invention - provide an AC output current decomposed into burst periods. To avoid feeding too many currents decomposed into burst periods into the grid, such currents can be combined. According to one explanatory example, there are two inverters and each inverter generating a decomposed AC output current having one burst period followed by two paused periods. These two signals are then assembled to get one current signal comprising two burst periods followed by one paused period.

Advantageously, the at least one assembled output current is a continuously sinusoidal output current. To adapt the example given above, three inverters could be provided each generating a decomposed AC output current having one burst period followed by two paused periods. These three decomposed signals could be assembled to one signal comprising only burst periods and no paused periods and thus the assembled signal comprises one continuously sinusoidal output current.

This aspect is in particular useful for a system providing a plurality of inverters each having an intermediate storage such as a DC -link. To give a further example, ten inverters could each generate a decomposed signal comprising one burst period followed by nine paused periods, if there is only very few energy or power available. These ten decomposed signals can be assembled to one continuously sinusoidal current. If more energy is available and accordingly each inverter generates two burst periods followed by eight paused periods, the ten decomposed signals can be assembled to one continuously sinusoidal output currents having twice the amplitude of the example given above. Accordingly, the burst periods of all inverters involved are assembled to one continuously sinusoidal current, having an amplitude corresponding to the amount of the available power of all inverters.

It is to be noted, that a single inverter according to aspects of the invention can generate sinusoidal burst periods of almost a constant amplitude. However, when combining two or more inverters to receive a continuously sinusoidal current, according to an aspect of the invention, the amplitude of the burst periods of each inverter might slightly vary, e.g. by 20% of an average value. This variation also depends on the number of inverters involved.

The proposed invention including the proposed aspects is in particular useful in combination with at least one solar generator, which supplies electrical energy to an intermediate storage such as a DC -link of at least one inverter. Thus, the method for feeding electrical power into a grid allows for the situation, that solar generators supply energy of a varying amount depending on the weather and the time of the day. The invention further proposes an inverter for feeding electrical power into the grid according to claim 15. According to one aspect the inverter comprises an intermediate storage in particular a DC -link adapted to store electrical energy sufficient to generate one burst period of a current having an amplitude in the range of 20 to 40 %, in particular 30 % of the nominal value. Accordingly, when a burst period comprising at least one wavelet is started, the size of the DC -link ensures that at least one complete wavelet can be generated without having a decrease with respect to the amplitude of the wavelet. Furthermore, the inverter can generate an output current of at least one burst period of a current having such an amplitude, even when there is only few DC power available for supplying into the DC -link. In such a case, when only few DC power is available, it just has to be waited until the DC -link is loaded and comprises sufficient energy to generate at least one wavelet.

Advantageously, the DC power is supplied by at least one solar generator. It is to be noted, that a solar generator may comprise one or a plurality of photovoltaic modules.

The invention is now described with respect to one embodiment with reference to the accompanying figures, wherein

Fig. 1 shows a decomposed output current according to the invention in comparison to an output current having a small amplitude according to the state of the art and

Fig. 2 is an illustration of one method for feeding electrical power into the grid according to the invention.

Figure 1 illustrates the general design of an AC output current decomposed into burst periods 1 in comparison to a continuously sinusoidal current S1 according to the state of the art. The diagram according to figure 1 shows the time on the x- axis and the current on the y-axis. The diagram just shows the principle design of the two signals 1 and S1. However, the decomposed current signal 1 has an amplitude of 16A in this example and accordingly the y-axis starts at -16A and goes to +16A. The signal S1 according to the state of the art shows a continuously sinusoidal form having a fairly small amplitude. The amplitude of this signal S1 depends on the available power in the corresponding DC link. I. e. if the avail- able power would further decrease, the amplitude of the signal S1 would also further decrease and on the other hand if the available power would increase, the amplitude of the signal S1 would also increase. This would also take place continuously.

The decomposed current 1 according to the invention comprises a burst period 2 having a constant amplitude with respect to further burst periods, which are not shown in figure 1 . Before and after the period 6 of the burst period 2 the decomposed current 1 comprises periods 8 without any generated current 4. Accordingly, the output current 1 is divided in burst periods 6 and pausing periods 8. If the available power of the corresponding DC link will decrease, the amplitude of the burst period 2 will remain unchanged but the ratio of the burst periods 6 to the pausing periods 8 will decrease. I. e. there will be more pausing periods 8 than burst periods 6. On the other hand, if the available power increases the ratio of burst periods 6 to pausing periods 8 will increase while the amplitude of the burst periods 2 will still remain unchanged.

Of course, the amplitude of the burst period 2 can also be changed but it is preferred to stay with one amplitude that results in a high conversion efficiency.

Figure 2 illustrates an operation mode of an embodiment of the invention. Starting with the diagram D1 the system is in a condition providing a pause of the output current. Accordingly, since at least some energy is supplied to a corre- sponding DC link, the voltage at the DC link V_Dcι_ increases. At T₁ the voltage at the DC link V_Dcι_ reaches the nominal voltage V_NOM and is ready to generate a burst period 2. However, as a second criteria the reference current I_REF is monitored. The reference current, which is in fact a reference signal, corresponds to the burst period to be generated with respect to frequency and phase. At the instance of T₁ the reference current is not zero and thus the inverter will not start to generate a burst period. At T₂ the reference current decreased to zero, whereby a value of zero is indicated by the horizontal slashed line. But at T₂ the reference current I_REF comprises a falling slope and thus a burst period is still not generated.

At T₃ the reference current I_REF is zero and has a rising slope. Accordingly, since the DC voltage V_Dcι_ is still above the nominal voltage V_NOM the inverter will generate a burst period 2.

The further operation is then illustrated in the second diagram D2. As a result of generating a burst period 2 and feeding this into the grid, the DC link voltage V_Dcι_ falls. At T₄ the DC link voltage V_Dα_ reaches the nominal voltage V_NOM- However, starting from T₄ the first time to have a rising slope reaching zero of the reference current I_REF is at T₅. Accordingly, at T₅ the generator will stop generating a burst period but will generate no current and the output current 1 has a pause then.

The operation then continues as described with reference to diagram D1 and because of no output current being generated the voltage of the DC link V_Dcι_ will rise again, if any electrical power is supplied to the DC link.

Involving the described criteria of comparing the DC voltage V_DCL to a voltage threshold value and monitoring a reference current I_REF with respect to its rising slope and crossing the zero line, the principle of the invention is easily imple- mented. This way the ratio of burst periods 6 to periods of pauses 8 will also automatically decrease or increase if the available power at the DC link decreases or increases respectively.

Claims

1 . Method for feeding electrical power into a grid by means of an inverter converting electrical energy of a DC- intermediate storage having a DC voltage or current into an AC output current, whereby the electrical power availably in the DC- intermediate storage is converted into an AC power by means of an AC output current decomposed into burst periods, whereby the output current comprises burst periods of a generated sinusoidal current having the frequency of the grid and pausing periods when no current is generated.

2. Method according to claim 1 , whereby a DC-link comprising a DC-link capacitor is used as the DC-intermediate storage.

3. Method according to claim 1 or 2, whereby the output current of the burst periods has an amplitude of a constant value.

4. Method according to claim 3, whereby the generated amplitude is in the range of 20 to 40%, in particular 30% of the nominal value.

5. Method according to any of claims 1 to 4, whereby generating of the sinusoidal current in the burst period is initiated, when the DC voltage or current respectively is above a first threshold value and when a sinusoidal reference signal representing the current to be generated reaches zero.

6. Method according to any of claims 1 to 5, whereby generating of the sinusoidal current in the burst period is stopped, when the DC voltage or current respectively is below the first or below a second threshold value and when the sinusoidal current or a sinusoidal reference signal representing the current to be generated reaches zero.

7. Method according to claim 5 or 6, whereby the generating of the current is only initiated or stopped, when the slope of the reference signal or of the generated current respectively is positive.

8. Method according to claim 5 or 6, whereby the generating of the current is only initiated or stopped, when the slope of the reference signal or of the generated current respectively is negative.

9. Method according to any of the preceding claims, further comprising the step of estimating the available electrical power for feeding the AC output current into the grid, whereby the method switches to a continuous mode, according to which the AC output current is continuously fed into the grid without decomposing the output current into burst periods, if the available power rises above a first predetermined power threshold value, and whereby the method switches into a burst mode in which the output current is decomposed into burst periods, if the available power falls below the first predetermined power threshold value or below a second predetermined power threshold value.

10. Method according to claim 9, whereby the first power threshold value is above the second power threshold value.

1 1. Method according to any of the preceding claims, whereby at least two inverters are used, each having a DC- intermediate storage and each inverter generating an AC output current decomposed into burst periods, whereby at least one output current is generated by assembling burst periods of the AC output currents of at least two inverters.

12. Method according to claim 1 1 , whereby at least one continuously sinusoidal output current is generated.

13. Method according to claim 11 or 12, whereby three or more inverters generate two or more continuously sinusoidal output currents, depending on the overall power availably in the DC- intermediate storage of all involved inverters.

14. Method according to any of the preceding claims, whereby electrical energy is supplied by at least one solar generator.

15. Inverter for feeding electrical power into the grid, comprising: a DC- intermediate storage providing electrical energy, a conversion circuit for converting electrical energy of the DC- intermediate storage into a sinusoidal current for feeding into the grid, a control unit for controlling the generating of the sinusoidal current and the feed- ing of the current into the grid, whereby the control unit is adapted to control the inverter such, that the inverter generates an AC output current decomposed into burst periods, whereby the output current comprises burst periods of generated sinusoidal current having the frequency of the grid and pausing periods when no current is generated.

16. Inverter according to claim 15, comprising a DC- intermediate storage adapted to store electrical energy sufficient to generate one burst period of a current having an amplitude in the range of 20 to 40%, in particular 30% of the nominal value.

17. Inverter according to claim 16, whereby the intermediate storage comprises a DC-link having at least one capacitor for storing electrical energy.

18. Inverter according to any of claims 15 to 17, whereby the control unit is adapted to apply a method according to any of claims 1 to 10.

19. Energy system comprising at least one energy source, in particular a solar generator, and one or more inverter each having a DC- intermediate storage in particular a DC-link connected to at least one of the energy sources, whereby inverter according to any of claims 15 to 18 are used.

20. Energy system comprising at least one energy source, in particular a solar generator, and two or more inverter each having a DC- intermediate storage in particular a DC-link connected to at least one of the energy source, further comprising a control unit being adapted to control the energy system such, that in case of only few energy being available, at least two inverter generate an assembled current.

21. Energy system according to claim 20, whereby the control unit is adapted to apply a method according to any of claims 1 1 to 14.