Power conversion with stop conversion during low integrated power conditions

The present invention relates to a power converting apparatus, control method thereof, and generator and, more particularly, to a power converting apparatus such as an inverter used for a solar power generation apparatus and a control method thereof.

Power generated by a solar battery for generating power in accordance with sunlight is small in the morning/evening or in a cloudy/rainy day. If the generated power is small, an inverter in the solar power generation apparatus connected to a commercial electric power system (to be simply referred to as a “system” hereinafter) may be repeatedly activated and stopped. For this reason, an operation method for suppressing unnecessary stop of the inverter is required. Such operation methods are disclosed in, e.g., Japanese Patent No. 2509187 in which when the direct current input voltage of an inverter is equal to more than a threshold value, the inverter is activated after the elapse of a predetermined time, and Japanese Patent Laid-Open No. 2000-23367 in which when the direct current input voltage of an inverter is less than a threshold value, the inverter is stopped after the elapse of a predetermined time.

As shown in FIG. 1, when a direct current input voltage V of an inverter abruptly drops, input power P 1 also abruptly drops together. When power (to be sometimes referred to as “non-load power” hereinafter) necessary for, e.g., the control circuit of the inverter is supplied from the load side of the inverter, such as a system, a period from when the input voltage V becomes lower than a threshold voltage V 1 at which the inverter should be stopped, and the inverter is stopped after the elapse of a predetermined time may include a period when power consumed by the inverter become larger than power output from the inverter, i.e., power is wasted.

As shown in FIG. 2, if the input voltage V fluctuates near the threshold voltage V 1 , the operation of the inverter may not be stopped, and power may be wasted.

If the predetermined time is shortened to suppress wasteful power consumption, activation and stop of the inverter may be repeated at high probability.

The present invention has been made to solve the above-described problems individually or altogether, and has as its object to suppress wasteful power consumption of a power converting apparatus.

It is another object of the present invention to suppress unnecessary stop of a power converting apparatus.

In order to achieve the above objects, according to a preferred aspect of the present invention, a power converting apparatus comprising a power converter, arranged to convert direct current power to alternating current power, a measurement unit, arranged to measure input or output power of said power converter and calculate integrated power for every predetermined period on the basis of the measured power; and a controller, arranged to stop conversion operation of said power converter when the integrated power is less than a predetermined value is disclosed.

Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.

FIG. 1 is a view for explaining a situation in which the direct current input voltage and power of an inverter abruptly drop;

FIG. 2 is a view for explaining a case in which the direct current input voltage of an inverter fluctuates near a threshold voltage V 1 ;

FIG. 3 is a block diagram showing the arrangement of a system interconnection solar power generation apparatus;

FIG. 4 is a flow chart for explaining a basic inverter stopping method;

FIG. 5 is a block diagram showing the detailed arrangement of an inverter of the first example;

FIG. 6 shows timing charts showing the relationship between an input voltage V, input power P 1 , and integrated electric energy W 1 ;

FIG. 7 is a flow chart for explaining an inverter stopping method of the first example;

FIG. 8 is a block diagram showing the arrangement of a general inverter;

FIG. 9 is a flow chart showing a general inverter stopping method;

FIG. 10 is a block diagram showing the detailed arrangement of an inverter of the second example;

FIG. 11 shows timing charts showing the relationship between an input voltage V, output power P 2 , and integrated electric energy W 1 ;

FIG. 12 is a block diagram showing the detailed arrangement of a converter of the third example; and

FIG. 13 is a block diagram showing the arrangement of a general converter.

A generator according to an embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

[Arrangement]

FIG. 3 is a block diagram showing the arrangement of a system interconnection solar power generation apparatus including an inverter 2 . The inverter 2 has the circuit arrangement of a general system interconnection inverter.

A solar battery 1 uses amorphous silicon, crystallite silicon, polysilicon, single-crystal silicon, or a combination thereof, or a compound semiconductor. Normally, a solar battery array is formed by combining a plurality of solar battery modules in series and parallel such that a desired voltage and current can be obtained. The number of solar battery modules in the array is not limited.

The inverter 2 converts direct current power input from the solar battery 1 to alternating current power and outputs it to a system 3 . The system 3 is a general commercial electric power system but can take any form such as a non-utility power generation system in a factory.

Inverter

A power converting circuit 23 comprises a converter circuit, inverter circuit, interactive reactor and the like.

An input power detection section 211 detects input power P 1 of the inverter 2 and outputs the detection result to an integrated electric energy calculation section 25 . The integrated electric energy calculation section 25 integrates the input power P 1 during a predetermined time T 1 and outputs the integration result, i.e., an integrated electric energy W 1 to a comparator 28 .

A time count section 26 outputs a signal t representing a timing or time to the integrated electric energy calculation section 25 . A threshold value Y 1 of the integrated electric energy W 1 at which the operation of the inverter 2 should be stopped is stored in a set value memory 27 . The comparator 28 compares the integrated electric energy W 1 with the threshold value Y 1 , and when W 1 <Y 1 , outputs a stop signal to a control section 29 .

The control section 29 controls the entire inverter 2 and outputs a gate drive signal to switching elements in the converter circuit and inverter circuit of the power converting circuit 23 . Upon receiving a stop signal from the comparator 28 , the control section 29 outputs a gate block signal to the power converting circuit 23 . The control section 29 also has a connection protecting function and inverter protecting function as well as a boosting control function, output waveform control function, activation/stop control function, and MPPT (Maximum Power Point Tracking) function, which are generally provided in a system interconnection inverter. A detailed description of these functions will be omitted.

The control section 29 is formed from a CPU, DSP (Digital Signal Processor), one-chip microprocessor having a memory and I/O, A/D converter, analog circuit, and the like. When a CPU or DSP that is improving its performance and cost recently is used, various control operations can be implemented by software, and various advantages can be obtained in reducing the size and cost of the inverter 2 and improving the degree of freedom in its design.

The generator of the embodiment is not limited to the above arrangement. Any other arrangement capable of suppressing a power wasting period when the input power of the generator decreases, and power consumed by the inverter becomes larger than power output from the inverter can be employed. Any other arrangement capable of stopping the operation of the power converting apparatus on the basis of the integrated value or average value of input power or output power so as to suppress the period can be employed. Hence, the output power can take any form such as alternating current power or direct current power, and the system can also take any form such as a system interconnection type or stand-alone (isolated operation) type. In addition, the supply source of non-load power to the generator is not particularly limited unless the non-load power is wholly supplied from the input (solar battery 1 ) side.

[Control]

FIG. 4 is a flow chart for explaining a basic inverter stopping method.

During the predetermined time T 1 , the input power P 1 is integrated to calculate the integrated electric energy W 1 (S 1 ). The calculated integrated electric energy W 1 and threshold value Y 1 are compared (S 2 ). If W 1 <Y 1 , a gate block signal is output to stop the operation of the inverter 2 . When W 1 ≧Y 1 , the flow returns to step S 1 .

Under this control, when the integrated electric energy W 1 is smaller than the threshold value Y 1 , the operation of the inverter 2 is stopped. Hence, when the threshold value Y 1 corresponding to the product of the non-load power of the inverter 2 and the predetermined time T 1 is set, and the input power P 1 continuously becomes smaller than the non-load power, the operation of the inverter 2 can be stopped. The timing when the integrated electric energy W 1 becomes smaller than the threshold value Y 1 changes depending on the gradient of the decrease in input power P 1 assuming that the input power P 1 smoothly decreases.

Since control for stopping the operation of the inverter 2 is done on the basis of the integrated electric energy W 1 as the integrated value of the input power P 1 , unnecessary stop of the inverter 2 can be suppressed even when the power generated by the solar battery 1 is small or varies.

The method of stopping the inverter 2 of this embodiment does not depend on the activation method. Hence, a description of the activation method will be omitted.

Detailed methods of stopping the inverter 2 will be described below as examples.

FIG. 5 is a block diagram showing the detailed arrangement of an inverter 2 of the first example. The non-load power of the inverter 2 is 20 W. As a solar battery 1 , a solar battery module for outputting power of 2 kW under standard solar radiation (irradiance of 1 kW/m 2 ) is used. A system 3 is a single-phase three-wire system with an output of 200 V at 60 Hz.

FIG. 6 shows timing charts showing the relationship between an input voltage V [V], input power P 1 [W], and integrated electric energy W 1 [Ws] at time t [s]. Since solar radiation varies in units of seconds, the sampling frequency of an A/D converter (to be described later) can be several Hz. The sampling frequency of the A/D converter is set at 1 Hz. Hence, a time count section 26 outputs a signal t every second.

Stopping Method of Example

To appropriately stop the operation of the inverter 2 when the input power P 1 is smaller than the non-load power and to suppress unnecessary stop of the inverter 2 , an integration time T 1 for the input power P 1 is set at 9 sec. Hence, a threshold value Y 1 at which the inverter 2 should be stopped is set at 200 Ws (=20 W×10 s), i.e., the product of the predetermined time T 1 and non-load power.

FIG. 7 is a flow chart for explaining a method of stopping the inverter 2 of the first example.

A voltage detector 21 shown in FIG. 5 detects the input voltage V of the inverter 2 and supplies the detected value to an A/D converter 24 . A current detector 22 detects an input current I of the inverter 2 and supplies the detected value to the A/D converter 24 .

The A/D converter 24 converts each detected value to digital data every second and outputs the data to an integrated electric energy calculation section 25 (S 11 ). The integrated electric energy calculation section 25 calculates the input power P 1 =V×I from the received digital data (S 12 ) and adds 10 data of input power P 1 . That is, the input power P 1 is integrated (S 13 ). On the basis of the signal t (S 14 ), the integrated electric energy W 1 is output every second, and the next integration is started (S 15 ).

A comparator 28 compares the integrated electric energy W 1 with the threshold value Y 1 (S 16 ), and if W 1 <Y 1 , outputs a stop signal to a control section 29 . When W 1 ≧Y 1 , the oldest one of the above 10 data is discarded (S 18 ). In the next integrated electric energy calculation, the remaining nine data and one newly input data of the input power P 1 , i.e., a total of 10 data are added. Upon receiving the stop signal, the control section 29 outputs a gate block signal and turns off a switch 210 (S 17 ).

Stopping Method as Comparative Example

For the sake of comparison, a method of stopping a general inverter 2 will be described. The non-load power of the inverter 2 , the arrangement of a solar battery 1 , the form of a system 3 , and the sampling interval are the same as in the above-described example.

A threshold value V 1 is set at 198 V at which input power equals non-load power, as shown in FIG. 6 . To suppress unnecessary stop of the inverter as much as possible, a predetermined time T 2 is set at 20 sec, i.e., about twice the integration time T 1 .

FIG. 8 is a block diagram showing the arrangement of the general inverter 2 . FIG. 9 is a flow chart showing a general method of stopping the inverter 2 .

A voltage detector 21 shown in FIG. 8 detects an input voltage V of the inverter 2 and supplies the detected value to an A/D converter 24 .

The A/D converter 24 converts the detected value to digital data every second and outputs the data to a comparator 28 (S 21 ). The comparator 28 compares the input voltage V with the threshold value V 1 and outputs the comparison result to a control section 29 (S 22 ).

The control section 29 determines whether V<V 1 (S 23 ) and whether time is being counted (S 24 and S 25 ). If V<V 1 and time is not being counted, a time count section 26 is caused to start counting time (S 26 ). If V≧V 1 and time is being counted, the time count section 26 is caused to stop counting time and reset the count value (S 27 ).

In addition, when V<V 1 , the control section 29 determines whether the count value indicates the elapse of the predetermined time T 2 (S 28 ). If YES in step S 28 , the control section 29 outputs a gate block signal and turns off a switch 210 (S 29 ).

Hence, when a state wherein the input voltage V is less than the threshold value V 1 (=198 V) continues for the predetermined time T 2 (20 sec) or more, the operation of the inverter 2 is stopped.

Proof Experiment

A result of proof experiments for the stopping method of the first example and that of the comparative example will be described on the basis of FIG. 6 .

First, under such conditions shown in FIG. 6 that the input power P 1 abruptly decreased, an experiment was conducted with the stopping method of the comparative example. As a result, as shown in FIG. 6, the input voltage V became less than the threshold value V 1 (=198 V) at a timing of 27 sec, and the inverter 2 was stopped at a timing of 47 sec after the elapse of the predetermined time T 2 (=20 sec).

Next, under similar conditions, an experiment was conducted with the stopping method of the first example. As a result, as shown in FIG. 6, the integrated electric energy W 1 became less than the threshold value Y 1 (=200 Ws) and the inverter 2 was stopped at a timing of 30 sec. That is, the inverter 2 was stopped 3 sec after the input power P 1 became smaller than the non-load power of 20 W.

As described above, with the stopping method of the first example, the inverter 2 was stopped faster by 17 sec as compared to the comparative example, and wasteful power consumption could be decreased.

Since power conversion by a power converting circuit 23 suffers a loss, the output power of an inverter 2 is smaller than input power. That is, whether the output power of the inverter 2 is smaller than non-load power can be more accurately determined on the basis of the output power. Hence, stop of the inverter 2 is more preferably controlled on the basis of the output power than input power P 1 .

The inverter 2 of the second example is stopped on the basis of not the input power P 1 but output power P 2 , as shown in FIG. 10 . The arrangement and operation of the inverter 2 are the same as in the first example except that a voltage detector 21 and current detector 22 are inserted between the power converting circuit 23 and a switch 210 , and an integrated electric energy calculation section 25 calculates alternating current power.

FIG. 11 shows timing charts showing the relationship between an input voltage V [V], output power P 2 [W], and integrated electric energy W 1 {Ws] at time [t]. In this example, the output power P 2 fluctuates near non-load power of 20 W.

Under such conditions, an experiment was conducted with the stopping method of the comparative example. As a result, as shown in FIG. 11, the input voltage V became less than the threshold value V 1 (=198 V) many times but not continuously for a predetermined time T 2 (=20 sec), and the inverter 2 was continuously operated. For this reason, during a period when the output power P 2 was smaller than the non-load power, power corresponding to the shortage was supplied from a system 3 . A similar experiment was conducted with the stopping method of the comparative example, for which the predetermined time T 2 was shortened to 3 sec corresponding to a delay time T 3 . The inverter 2 was stopped at a timing of 6 sec immediately after the start of operation.

Under the same conditions, an experiment was conducted with the stopping method of the second example. As a result, as shown in FIG. 11, the integrated electric energy W 1 became less than a threshold value Y 1 (=200 Ws) at a timing of 45 sec, and the inverter 2 was stopped. Hence, from a timing of 50 sec, no power corresponding to the shortage of the non-load power was supplied from the system 3 .

In the third example, a DC/DC converter 4 is used as an inverter such that direct current power supplied from a solar battery 1 is converted to direct current power having a desired voltage and supplied to a load 6 and battery 7 , as shown in FIGS. 12 and 13.

The arrangement of the main circuit of the DC/DC converter 4 is the same as that of a general converter. When a lead battery of 12 V is used as the battery 7 , the output voltage of a power converting circuit 23 is about 13 V. The non-load power for the DC/DC converter 4 is supplied from the output (battery 7 ) side.

As the load 6 , a heating lamp of 12 V and 100 W is used as a general electric load. As the battery 7 , a lead battery of 12 V and 100 Ah is used as a general secondary battery.

Note that the DC/DC converter 4 shown in FIG. 12 executes the stopping method of the third example, and the DC/DC converter 4 shown in FIG. 13 executes the stopping method of the comparative example.

Under almost the same conditions as in FIG. 6, almost the same proof experiments as in the first example were conducted. As in the first example, in the stopping method of the comparative example, the operation of the DC/DC converter 4 was continued, and the power of the battery 7 was consumed during a predetermined time T 2 from when input power P 1 became smaller than the non-load power (=20 W). On the other hand, in the stopping method of the third example, when the input power P 1 became smaller than the non-load power, the operation of the DC/DC converter 4 was stopped at a timing of 30 sec so that wasteful power consumption could be suppressed.

Under almost the same conditions as in FIG. 11, almost the same proof experiments in the second example were conducted. Instead of integrating output power P 2 shown in FIG. 11, the input power P 1 was integrated. As a result, as in the second example, in the stopping method of the comparative example, the operation of the DC/DC converter 4 was continued, and the power of the battery 7 was continuously consumed. When the predetermined time T 2 was shortened to 3 sec corresponding to a delay time T 3 , the operation of the DC/DC converter 4 was stopped at a timing corresponding to 6 sec in FIG. 11 . On the other hand, in the stopping method of the third example, the operation of the DC/DC converter 4 was stopped at a timing corresponding to 45 sec in FIG. 11 when an integrated electric energy W 1 became less than a predetermined value Y 1 (=200 Ws).

As described above, according to this embodiment, independently of the form such as alternating current output or direct current output or the form such as a system interconnection type or stand-alone (isolated operation) type, operation when the output power of the generator is smaller than the non-load power can be avoided, and unnecessary stop can be suppressed. When the generator of this embodiment is connected to a system or the like, an efficient generator can be implemented in consideration of a decrease in generated power in the morning/evening or in a cloudy/rainy day.

As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.