JPH04294411A - Power converter for solar power generation - Google Patents

Abstract

PURPOSE:To obtain a power converter for solar power generation which can attain a highly reliable system connection and surely prevent the generation of a reverse power flow without adding a dummy load or stopping a power conversion circuit. CONSTITUTION:The power converter is provided with a power detector 18 for measuring power supply from a power system 4 to a general load 17 and correcting means 19 to 21 for reducing the output power of the power converting circuit 3 when the power supply is less than a regulated value, and if the power supply from the power system 4 is reduced less than the regulated value causing the generation of the danger of a reverse power flow, the power supply to the power system 4 is increased by converging the output power of the circuit 3 to prevent the output power of the circuit from reversely flowing to the power system side 4.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to a power conversion device for solar power generation that converts DC power obtained from solar cells into AC power and connects it to an AC power system to supply power to a load. The present invention relates to a power conversion device for solar power generation that can reliably prevent reverse power flow toward the grid.

0002

2. Description of the Related Art FIG. 3 is a block diagram showing a conventional power converter for photovoltaic power generation disclosed in, for example, Japanese Patent Laid-Open No. 1-98008. In the figure, 1 is a photovoltaic array consisting of solar cells that receives solar energy and outputs DC power (corresponding to active power P), 2 is a backflow prevention diode that prevents power from flowing back to the photovoltaic array 1, and 3 4 is a power conversion circuit including an inverter that converts DC power from the photovoltaic array 1 into AC power; 4 is an AC power system that is connected to the output side of the power conversion circuit 3 and constitutes a power supply system together with the power conversion circuit 3; , 5 is a contactor for connecting the power conversion circuit 3 and the power system 4, and 6 is a interconnection device inserted between the power conversion circuit 3 and the power system 4 to generate a voltage phase difference δ between both ends. It is reactance.

Vd is the DC output voltage from the photovoltaic array 1, VI is the AC output voltage from the power conversion circuit 3, Vs is the AC voltage of the power supply system, PI is the power supplied from the power conversion circuit 3, and PG is the power This is the power supplied from grid 4.

7 is the grid current and grid voltage V on the power grid side.
Reactive power Q between the power conversion circuit 3 and the power system 4 from s
8 is a phase difference detector that detects the voltage phase difference δ before and after the interconnection reactance 6 and outputs a detected phase difference value δs, and 9 is an invalid power detector. A reactive power standard setting device 10 outputs a reference value Qref of power Q (usually 0), and 10 sets the deviation between the reactive power reference value Qref and the reactive power detected value Qs to 0 (reactive power detected value Qs is set as the reactive power standard). 11 is a control signal V for controlling the AC output voltage VI based on the manipulated variable k;
This is a voltage control circuit that outputs g.

12 is a DC voltage detector that detects the DC output voltage Vd from the photovoltaic array 1 and outputs a DC voltage detection value Vds, and 13 is a reference value Vdref of the DC output voltage Vd.
A DC voltage reference setting device 14 outputs a voltage value that maximizes the active power P, and sets the deviation between the DC voltage detection value Vds and the DC voltage reference value Vdref to 0 (DC voltage detection value V
ds to match the DC voltage reference value Vdref), an active power control circuit outputting a phase operation amount δref, 15
is the deviation between the phase operation amount δref and the detected phase difference value δs.
This is a phase control circuit that outputs a control signal δg for making the detected phase difference value δs match the phase operation amount δref.

Reference numeral 16 denotes a gate circuit that controls the switching elements of the inverter in the power conversion circuit 3 based on each control voltage Vg and δg, and performs PWM control on the AC output voltage VI based on the control signal Vg. The voltage phase difference δ is controlled based on δg. Further, the output signal of the gate circuit 16 is also input to the phase difference detector 8.

[0007] Reactive power detector 7 and reactive power control circuit 10
and the voltage control circuit 11 constitute a reactive power control loop, and the phase difference detector 8, the DC voltage detector 12, the active power control circuit 14, and the phase control circuit 15 constitute an active power control loop. . 17 is a general load to which the output power PI of the power conversion circuit 3 and the power PG of the power system 4 are supplied, and PL is load power supplied to the general load 17. The power conversion circuit 3 and the power system 4 are connected in parallel to each other and supply power to the general load 17.

Next, with reference to FIG. 4, the operation of the conventional power converter for solar power generation shown in FIG. 3 will be explained. At this time, in the inverter in the power conversion circuit 3, the AC output voltage VI of the general load 17 is subjected to PWM control by the reactive power control loop, and the voltage phase difference δ is controlled by the active power control loop, as described above. By controlling the voltage phase difference δ, the DC output voltage V
d is controlled.

[0009] Here, the AC output voltage VI and the grid voltage V
The voltage phase difference of s is equal to the voltage phase difference δ (sufficiently small) between both ends of the grid interconnection reactance 6, and if the reactance value of the grid interconnection reactance 6 is X, then the active power P and the reactive power output from the power conversion circuit 3 are The electric power Q is expressed by the following formulas (1) and (2), respectively.

0010

[Math 1]

[0011]

[Math 2]

As is clear from equations (1) and (2), the active power P changes by changing the phase difference δ, and the reactive power Q changes by changing the AC output voltage VI of the power conversion circuit 3. It can be seen that there is a correlation between the two.

Furthermore, from equation (2), even if the system voltage Vs increases, the reactive power Q increases and the utilization rate of the power conversion circuit 3 deteriorates, so normally Q=0 (that is, Vs=VI). The AC output voltage VI is changed as follows. At this time, considering the output characteristics of the photovoltaic array 1, the DC output voltage Vd of the photovoltaic array 1 is set to a reference value (a value that maximizes the active power P).
The AC output voltage VI is subjected to PWM control by changing the conduction rate (pulse width) of the inverter so that.

FIG. 4 is a characteristic diagram showing the output power characteristics and output current characteristics of the photovoltaic array 1, where the horizontal axis represents the DC output voltage Vd, and the vertical axis represents the output power (that is, the active power P) and the DC output current I ( (corresponding to solar radiation energy ε). The output power P and the current I change as shown by a dashed line, a solid line, and a broken line due to differences in solar radiation energy εO, εR, and εU, and due to a difference in DC output voltage Vd. That is, the active power P
is maximum when the DC output voltage Vd is Vop, and V
It decreases as the voltage Vx becomes higher than op, and becomes 0 in the no-load state of the open circuit voltage Voc.

Therefore, in order to extract the active power P output from the photovoltaic array 1 most effectively, it is necessary to control the DC output voltage Vd always near the voltage Vop at which the active power P becomes maximum. This control method is applied to the DC output voltage V
This is called a constant control method of d.

Next, the operations of the reactive power control loop and the active power control loop will be explained. First, the reactive power control loop is controlled. When the reactive power detection value Qs is obtained from the reactive power detector 7 and the reactive power reference value Qref (=0) is given from the reactive power standard setting device 9, these deviations (Qref-Qs) are calculated by the reactive power control circuit 10. is input.

As a result, the reactive power control circuit 10 allows the reactive power detection value Qs as a feedback signal to be equal to Qre.
The voltage control circuit 11 outputs the manipulated variable k so that f (=0), and the voltage control circuit 11 changes the AC output voltage VI to P based on the manipulated variable k.
A control signal Vg for WM control is output to the gate circuit 16. At this time, since voltage control by PWM is harmonic control, a sine wave modulation method is usually adopted. Further, the relationship between the DC output voltage Vd and the AC output voltage VI is expressed in units by the following equation (2).

[0018]

[Math 3]

The gate circuit 16 controls on/off of the inverter in the power conversion circuit 3 based on equation (2) so that the AC output voltage VI becomes equal to the system voltage Vs.

Next, the active power control loop will be explained. When the DC voltage detection value Vds is obtained by the DC voltage detector 12 and the DC voltage reference value Vdref is given by the DC voltage reference setting device 13, the deviation (Vdref
-Vds) is input to the active power control circuit 14. Normally, the DC voltage reference value Vdref is the voltage Vo shown in FIG.
p.

Thereby, the active power control circuit 14 outputs the phase operation amount δref so that the DC voltage detection value Vds as a feedback signal becomes the DC voltage reference value Vdref (=Vop). Moreover, the phase control circuit 15 is
The deviation amount (δ
ref-δs), outputs a control signal δg for matching the detected phase difference value δs as a feedback signal with the phase manipulation amount δref.

The gate circuit 16 controls on/off of the inverter in the power conversion circuit 3 based on the control signal δg so that δs=δref and Vds=Vdref are satisfied. For example, when the DC output voltage Vd exceeds the DC voltage reference value Vdref (=Vop), if you want to lower the DC output voltage Vd, advance the phase of the AC output voltage VI from the phase of the system voltage Vs, The power PI from the power conversion circuit 3 may be moved to the general load 17 side and consumed.

In this way, the output power PI from the power converter circuit 3 is supplied to the general load 17, and the shortage of the load power PL is supplied by the power PG from the power system 4. That is, the load power PL is the sum of the power PI from the power conversion circuit 3 and the power PG from the power system 4 (PI+PG
), and when PI=PL, PG=0, and as the output power PI decreases, the power supplied from the power system 4 PG (power received by the general load 17) increases.

However, the output power P from the power conversion circuit 3
I exceeds the load power PL to the general load 17 (PI>P
In case L), excess power (PI-PL) flows into the power system 4, and PG<0, resulting in a reverse power flow. Therefore, in order to prevent reverse power flow, load power P
Countermeasures have been taken, such as installing a dummy load by installing an L power shortage relay, or stopping the power conversion circuit 3.

[0025]

[Problems to be Solved by the Invention] As described above, the conventional power conversion device for solar power generation has a problem in which a dummy load is applied or the power conversion circuit 3 is
Since the output power P of the photovoltaic array 1 cannot be used effectively, there are problems in that reliability is low.

[0026] This invention was made to solve the above-mentioned problems, and it realizes highly reliable grid connection without adding a dummy load or stopping the power conversion circuit. The object of the present invention is to obtain a power conversion device for solar power generation that can reliably prevent the occurrence of tidal current.

[0027]

[Means for Solving the Problems] A power conversion device for solar power generation according to the present invention includes a power detector that measures the power supplied from the power system to the general load, and a power detector that indicates that the supplied power is below a specified value. In some cases, a correction means for reducing the output power of the power conversion circuit is provided.

[0028]

[Operation] In this invention, when the power supplied from the power system falls below a specified value that poses the risk of reverse power flow,
By narrowing down the output power of the power conversion circuit, the power supplied to the power system is increased and the output power of the power conversion circuit is prevented from flowing backward into the power system.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, and numerals 1 to 17 are the same as those described above. 18 is a power detector that measures the power PG supplied from the power system 4 to the general load 17 and outputs a voltage VA corresponding to the supplied power PG;
9 is the minimum specified value PGm of the power supply PG to the general load 17
a bias setting device that outputs a bias voltage VB corresponding to in; 20 an inverting amplifier that inverts and amplifies the deviation voltage (VA-VB) between the power detection value VA and the bias voltage VB; 21;
is a diode that allows only the positive voltage of the output voltage of the inverting amplifier 20 to pass through as the correction voltage Vc. Correction voltage V
c is added to the DC voltage reference value Vdref.

The bias setter 19, the inverting amplifier 20, and the diode 21 are configured to reduce the output power VI of the power conversion circuit 3 when the power system supply power VG is below a specified value (bias voltage VB). This constitutes a correction means.

FIG. 2 shows the power detection value VA output from the voltage detector 18, the bias voltage VB output from the bias setting device 19, the deviation voltage (VA-VB), and the inverting amplifier 2.
The correction voltage Vc output from 0 through the diode 21
FIG. The detected power value VA has a linear correlation with the supplied power PG, and the deviation voltage (VA-VB
) has a waveform obtained by shifting the power detection value VA in the negative direction by the bias voltage VB. Further, the correction voltage Vc is determined when the supplied power PG is a specified value PGmin.
The waveform begins to increase from 0 when it decreases to 0, and has a maximum value GVB (G is the inverted amplification factor) at PG=0.

Next, referring to FIGS. 2 and 4, FIG.
The operation of the embodiment of the present invention shown in FIG. First, the power detector 18 detects the power P supplied from the power system 4.
G (received power to the general load 17), and outputs a power detection value VA in which the power reception direction is positive. Also, bias setting device 1
9 outputs a bias voltage VB corresponding to the minimum specified value PGmin that may cause a reverse power flow to the power system 4, and adds this as -VB to the power detection value VA. The inverting amplifier 20 generates a deviation voltage (VA
-VB) with an amplification factor G, the diode 21
A correction voltage Vc is obtained through the voltage Vc.

If the output power PI from the power conversion circuit 3
increases, the power system supply power PG decreases, and PG≦
When PGmin is reached, the inverting amplifier 20
1, outputs the correction voltage Vc as shown in FIG. This correction voltage Vc is applied in the + direction to the addition point of the DC voltage detection value Vds and the DC voltage reference value Vdref, and equivalently increases the DC voltage reference value Vdref.

As is clear from FIG. 4, increasing the DC voltage reference value Vdref acts in the direction of decreasing the effective power P output from the photovoltaic array 1. Therefore, the output power PI from the power conversion circuit 3 becomes smaller,
Conversely, the power system supply power PG increases, and reverse power flow is automatically prevented.

At this time, as is clear from FIG. 4, even if the DC voltage reference value Vdref is set smaller than Vop, the output power P decreases; however, if the DC output voltage Vd is small, the The inverter cannot operate reliably, and the AC output power PI becomes unstable. Therefore, as described above, it is desirable to increase the DC output voltage Vd and decrease the output power P.

In the above embodiment, the DC output voltage Vd of the photovoltaic array 1 is used as the feedback signal to which the correction voltage Vc is applied, and the output power PI of the power conversion circuit 3 is calculated based on the output characteristics of the photovoltaic array 1 and the DC output voltage Vd. Voltage reference value Vdre
Although the case where the determination is made based on f has been described, the same effect can be obtained even when the output power PI of the power conversion circuit 3 is directly used as a feedback signal. In this case, instead of the DC voltage detector 12 and the DC voltage reference setter 13,
A power detector and a power setter are provided on the output side of the power conversion circuit 3, and the correction voltage Vc is added to the power setting value with a - sign. Furthermore, it goes without saying that the same effect can be achieved even when a battery is installed on the output side of the photovoltaic array 1.

[0037]

[Effects of the Invention] As described above, according to the present invention, there is provided a power detector that measures the power supplied from the power system to the general load;
A correction means is provided to reduce the output power of the power inverter circuit when the supplied power is below the specified value, and when the supplied power from the power system falls below the specified value causing the risk of reverse power flow. In addition, the output power of the power conversion circuit is reduced to increase the power supplied to the power grid, and the output power of the power conversion circuit is prevented from flowing backward into the power grid. This has the effect of providing a power conversion device for photovoltaic power generation that can realize highly reliable grid connection without stopping the system and can reliably prevent the occurrence of reverse power flow.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a waveform diagram for explaining the operation of FIG. 1;

FIG. 3 is a block diagram showing a conventional power conversion device for solar power generation.

FIG. 4 is a characteristic diagram showing the output characteristics of a general photovoltaic array.

[Explanation of symbols]

1 Photovoltaic array 3 Power conversion circuit 4 Power system 17 General load 18 Power detector 19 Bias setting device 20 Inverting amplifier 21 Diode 19-21 Correction means P DC output power PI AC output power PG Power system supply power VB Bias voltage (specified value) Vc Correction voltage

Claims (1)

[Claims] 1. A photovoltaic array including a solar cell, a power conversion circuit that converts DC power output from the photovoltaic array into AC power, and a power conversion circuit that is connected in parallel to the power conversion circuit and that is generally used together with the power conversion circuit. In a power conversion device for solar power generation equipped with a power system that supplies power to a load,
A power detector that measures the power supplied from the power system to the general load, and a correction means that reduces the output power of the power conversion circuit when the supplied power is less than a specified value. A power conversion device for solar power generation characterized by: