JP2011039876A - Photovoltaic power generation facility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide photovoltaic power generation facility which prevents efficiency of AC-DC conversion from being reduced due to the reduction of load factor of VSI and maintaining high availability factor. <P>SOLUTION: The photovoltaic power generation facility is constituted by providing a rectifier for linking bus bars between the DC bus bars in two series of solar cells composed of two groups of PV arrays having different rated voltages, two VSIs, and the DC bus bars provided among them and connecting the rectifier in such a way that cathode of the rectifier is on a DC bus bar side where there is the PV array having higher rated voltage and anode of the rectifier is on a DC bus bar side where there is the PV array having lower rated voltage. As a result, this facility can individually perform MPPT control in each series when sufficient and satisfactory sunshine and outputs close to the maximum electric power can be obtained and can make the rectifier for linking bus bars conduct electricity is turned (ON) to operate two groups of PV arrays by one VSI in order to maintain load factor of the VSI, namely, the efficiency of AC-DC conversion when hours of sunshine are reduced. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention comprises a PV array group in which one or more solar cell arrays (PV arrays: Photo Voltaic Modules) are connected in series and parallel. It relates to solar power generation equipment.

  In general, in the field of photovoltaic power generation, a system is constructed with a PV module formed by connecting solar cells in series and parallel as basic elements. That is, an appropriate number of PV modules are connected in series or in series / parallel if necessary so that the required DC voltage and DC current can be output. This DC output is converted into AC power by a voltage source inverter (VSI: Voltage Source Inverter), and this AC power is consumed within the consumer, or surplus power that is not consumed is contracted with the power company and supplied to the power system. The

  Currently, large-scale solar power generation facilities of MW class are expected, and in particular, there are many plans that power companies install in cooperation with local governments. Since such a large-scale photovoltaic power generation facility has a large capacity, a system is constructed with a PV array formed by connecting a large number of PV modules in series and parallel as a basic element. That is, one VSI is allocated to a certain capacity PV array group, and this unit is used as a unit, and a required number of units are installed. And these 1 or several units are integrated and it becomes the structure linked to an electric power grid | system.

  In solar power generation, the direct current output fluctuates due to changes in solar radiation intensity from the sun, changes in solar cell temperature (due to changes in temperature, solar radiation intensity, etc.), and the like. As shown in FIG. 1, the VSI has a characteristic that the AC / DC conversion efficiency decreases as the load factor decreases, so that the AC output further decreases due to the decrease of the DC output.

  In a large-capacity photovoltaic power generation facility, the area where the PV array group is installed becomes large, and the solar array between the PV array or the PV array group due to the influence of the shade due to clouds or the like causes unevenness in the detached house. Compared to small-capacity solar power generation equipment installed on the roof of the city. For this reason, in large-capacity solar power generation facilities, output reduction due to partial load operation of VSI can be a more serious problem.

  Therefore, two series (one series means one PV array group consisting of one or a plurality of PV arrays and one VSI connected by a main line and a return line. In the following, the main line and the return line are called DC buses. A system configuration shown in FIG. 2 is proposed in which the DC bus 5 is connected via a communication line 6 (see, for example, Patent Document 1).

  In this system configuration, the operation when the DC output drop of the PV array group 2a is reduced will be described. When the direct current output of the PV array group 2a is reduced to a specified value or less, either VSI 3a or VSI 3b is gate-blocked (GB: Gate Block). Here, it is assumed that the VSI 3a is GB. Then, the direct current from the PV array group 2a flows into the DC bus 5c on the VSI 3b side via the connection line 6a. The combined DC output from the two PV array groups 2a and 2b is AC / DC converted by one VSI 3b. By doing in this way, the output fall by the partial load driving | running | working of VSI as represented in FIG. 1 can be prevented. Here, the specified value of the output of the PV array group when the VSI is GB is set so that the AC / DC conversion efficiency is maximized from the measurement result of the load factor / AC / DC conversion efficiency characteristic of the actual VSI.

However, with this system configuration, another problem arises that individual MPPT (Maximum Power Point Tracking) control for each VSI cannot be performed well during two-series operation.
Here, the MPPT control is to perform tracking control to the optimum operating point of the voltage and current at which the direct current output of the solar cell is maximum. For example, the output power obtained at the current operating point and the minute amount from the operating point. Compared with the output power at the new operating point that is varied in the range, select the direction judgment in the direction of increasing the output power, and sequentially move to the optimal operating point that is constantly changing due to changes in solar radiation intensity etc. The method is typical (see, for example, Non-Patent Document 1).

  In this system configuration, two series of DC buses are electrically connected, and the bus voltage is the same. For this reason, the maximum power generation voltage of the PV array group 2a and the maximum power generation voltage of the PV array group 2b are naturally different when the output of one of the two PV array groups is lowered. Control becomes impossible. For example, assuming that the PV array group having the same characteristics shown in FIG. 3A is used, the PV array group 2a is strongly irradiated with sunlight (for example, irradiation intensity of 1.0 kW / m 2), and the PV array group 2b is irradiated with sunlight. When weakly irradiated (for example, an irradiation intensity of 0.7 kW / m 2), the composite characteristics of the two PV array groups are of a bimodal type as shown in FIG. In this case, when the voltage is probed in the boosting direction, the point P2 (maximum point) on the graph of this composite characteristic becomes the operating point, but as apparent from the graph, the optimum operating point is the point P1 (maximum point) on the graph. When operating with the voltage / current at P2, the DC output is reduced by P1-P2 from the maximum value that can be output.

  In this case, it is possible to find the maximum point instead of the maximum point by exploring the voltage in the step-down direction, exploring by combining the step-up and step-down directions, or expanding the voltage range to be searched. It seems like. However, if this is done, it will take time to search for the optimum operating point, and there may be a decrease in output due to fluctuations in the intensity of solar irradiation and cell failures due to aging (particularly cell failures in one PV array group). Therefore, it may be extremely difficult to maximize the DC output due to a control time lag or the like.

  Therefore, in order to solve this problem, as shown in FIG. 4, a system configuration has been devised in which a bus bar connection switch 7 is provided on the communication line 6a. That is, when a certain output is defined as a threshold value and neither of the outputs of the two PV array groups is below this threshold value, the connection line 6a is not conducted (the switch 7 remains OFF). ), One PV array group and one VSI unit are individually operated by MPPT control. When either the output of the PV array group 2a or the output of the PV array group 2b falls below the threshold value, the connection line 6a is turned on (switch 7 is turned ON), and the VSI 3a is set to GB. Then, AC / DC conversion is performed only with the VSI 3b.

JP 199633211

Power Engineering Handbook, Asakura Shoten, October 30, 2005, first edition, first print, 404 pages

  However, in this system configuration, it is necessary to provide a switch driving device 8 for operating the bus bar connection switch 7, and the increase in size of the system and the power loss due to the switch driving device 8 become new problems. As described above, there is a demand for a method of switching the operating VSI from the two-unit operation to the one-unit operation only by controlling the VSI without providing the bus connection switch 7.

Therefore, the inventors have invented a photovoltaic power generation facility having the following configuration in order to solve this problem.
That is, a photovoltaic power generation facility comprising two PV array groups, two VSIs, two DC buses installed between them, and two connection lines between the DC buses,
The two PV array groups are configured with different rated voltages,
One of the connection lines is provided with a bus connection rectifier, and the connection of the bus connection rectifier is such that the cathode of the bus connection rectifier is a DC bus side with the PV array group having the higher rated voltage, and the connection of the bus connection rectifier The solar power generation facility is characterized in that the anode is on the DC bus side with the PV array group having a lower rated voltage.

The effect of the present invention will be described.
When the solar radiation conditions are good and the PV array group is at or near the rated power, the DC bus voltage is approximately the rated voltage, and the PV array group has a different rated voltage. Since it is connected to a low voltage, it is in a non-conductive (OFF) state. For this reason, each series consisting of one PV array group and one VSI is electrically separated, MPPT control can be performed individually for each series, and AC / DC conversion efficiency can be maintained.

  In addition, when solar radiation decreases, the VSI corresponding to the PV array group with the lower rated voltage is GB, so that the voltage of the DC bus directly connected thereto is boosted, and the anode side of the bus connection rectifier is connected to the cathode side. High voltage. Therefore, the busbar connection rectifier is in a conductive (ON) state. Therefore, the current flowing through the DC bus directly connected to the PV array having the lower rated voltage flows to the VSI corresponding to the PV array group having the higher rated voltage via the connecting line, and the DC of the two PV array groups. By concentrating the output on the VSI on one side, the load factor of the VSI is improved and the AC / DC conversion efficiency of the VSI can be maintained.

  Furthermore, according to the present invention, the switch driving device is not required, and the entire system can be reduced in size and cost.

It is a curve showing the characteristic of the load factor-AC / DC conversion efficiency of a voltage type inverter (VSI). It is a figure explaining the prior art which provided the connecting line between direct-current buses. (A) IV characteristic (parameter: solar radiation intensity) and PV characteristic (parameter: solar radiation intensity) of PV array group, (B) PV which synthesize | combined the output of two PV array groups from which solar radiation intensity differs It is a figure explaining a characteristic. It is a figure explaining the prior art which provided the bus-line connection switch in the connection line between DC bus lines. It is a figure of description of embodiment of this invention. It is a figure explaining the change of the PV characteristic by the change of solar radiation intensity, and the change of an operating point, (C) in the case of (A) P1 (1st state), (B) P2 (2nd state) This is the case of P3 (third state). It is a curve showing the characteristic of the direct current input-AC / DC conversion efficiency of a voltage type inverter (VSI).

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The following embodiments are specific examples of the present invention and do not limit the technical scope of the present invention.

(Description of composition of invention)
FIG. 5 shows a basic configuration of the present invention. Here, the upper PV array group 2a, DC buses 5a, 5b, rectifiers 4a, 4c, and VSI 3a in FIG. 5 are grouped together, and the lower PV array group 2b, DC buses 5c, 5d, rectifier in FIG. 4b, 4d, and VSI 3b may be collectively referred to as a B series.

  In FIG. 5, the rated voltage of the PV array group is set so that the lower B series is higher than the upper A series (for example, PV array group 2a is 380V and PV array group 2b is 400V). Here, the two PV array groups may have the same capacity, or the rated currents may be the same and the rated voltages may be different to have different capacities.

  The PV array group is configured by series-parallel of one or a plurality of PV arrays. To change the rated voltage of the PV array group, the number of PV modules in the PV array may be changed in series. Furthermore, you may select PV module from which the number of series of photovoltaic cells differs. Also, PV arrays or PV modules having two types of specifications may be prepared, and the rated voltages may be different by appropriately combining them.

  Similarly to the PV array group, the VSI capacity may be the same capacity or different capacity. However, in the case of different capacities, the capacity of VSI 3a needs to be equal to or less than the capacity of VSI 3b. This is because, as will be described later, the DC power of the two PV array groups is concentrated on the VSI 3b by GBing the VSI 3a. Therefore, if the capacity of the VSI 3b is small, the rated capacity is exceeded.

  Connection lines 6a and 6b are respectively provided between the DC bus lines 5a and 5c and between the DC bus lines 5b and 5d, and a bus line connection rectifier 9 is installed in series on the connection line 6a. The installation method (polarity) of the bus-connecting rectifier 9 is such that the anode side is the DC bus 5a of the PV array group 2a with a lower rated voltage and the cathode side is the DC bus of the PV array group 2b with a higher rated voltage 5c.

  The DC buses 5a and 5c are provided with rectifiers 4a, 4b, 4c and 4d before and after the connection point with the connecting line 6a. This is to prevent damage to the PV array group and the VSI due to backflow (because the rectifier is overwhelmingly cheaper in price).

(Explanation of the operation of the invention (part 1): When the solar radiation intensity is strong)
A state in which the solar radiation is not blocked by the clouds and the solar array is strongly irradiated (hereinafter referred to as the first state or P1. In the following, subscripts and the like are added to P, and operations such as the A series are performed. The operation in the case of points and time series) may be described.
In this case, since the output of the PV array group is in the vicinity of the rating, a voltage difference corresponding to substantially the difference between the rated voltages of the PV array group 2a and the PV array group 2b occurs between the A-series and B-series DC buses. However, in the polarity of this voltage difference (the voltage of the DC bus 5a <the voltage of the DC bus 5c), the back electromotive force is applied to the bus connection rectifier 9 installed between the A series and B series DC buses, and the bus connection The rectifier 9 is in a non-conductive (OFF) state, and the A series and the B series are electrically separated (independent). Therefore, MPPT control can be individually performed for both the A series and the B series, and a relatively high AC / DC conversion efficiency can be maintained.

It is assumed that the B series operates at the operating point Pb1 (output voltage Vb1) in the PV characteristic diagram in FIG. 6A, and the A series operates at the operating point Pa1 (output voltage Va1) in the same figure. Then, the sum Pdc (P1) of the DC outputs from the two PV array groups in this state is
Pdc (P1) = Pdc_b (Vb1) + Pdc_a (Va1)
It can be expressed. Here, the first term on the right side is the DC output at the operating point Pb1 (output voltage Vb1) in FIG. 6A for the B series, and the second term is the operating point Pa1 (the output voltage Va1) in FIG. 6A for the A series. ) DC output.

Since the load factor is defined as load factor (%) = DC input to VSI (kW) / VSI rated input P0 (kW) × 100, the load factor (unit:%) on the horizontal axis in FIG. Convert to DC input (unit: kW), convert the AC / DC conversion efficiency scale on the vertical axis from percentage value (%) to actual value (real value of 0 ≦ η ≦ 1), and measure the characteristic curve in FIG. The efficiency η can be determined by the direct current input (kW) from this characteristic curve. That is, in FIG. 7, the efficiency ηb1 corresponding to the DC output Pdc_b (Vb1) at the B-sequence operating point Pb1 (output voltage Vb1) is ηb1 = η (Pdc_b (Vb1)), and the A-sequence operating point Pa1 ( The efficiency ηa1 corresponding to the DC output Pdc_a (Va1) at the output voltage Va1) is determined by ηa1 = η (Pdc_a (Va1)).
In addition, η (Pdc_b (Vb1)) is expressed as a variable (DC input Pdc on the horizontal axis) and an AC / DC conversion efficiency η (function value) on the vertical axis in accordance with FIG. However, actually, Pdc is a function of the output voltage Vb1, and therefore η (Pdc_b (Vb1)) also has the meaning of a function (function value) with the output voltage Vb1 as a variable.

Since the A series and B series are controlled independently (AC / DC conversion), the sum Pac (P1) of the AC output is
Pac (P1) = Pdc_b (Vb1) × η (Pdc_b (Vb1)) + Pdc_a (Va1) × η (Pdc_a (Va1))
It becomes.

  Actually, the actually measured efficiency curve is digitized and stored in the photovoltaic power generation facility 1, and the value of the efficiency η corresponding to the DC input is taken out whenever necessary.

(Description of the operation of the invention (part 2): When the solar radiation intensity is weak)
The operation when the solar radiation is blocked by clouds or when the solar radiation to the PV array group is weak (hereinafter referred to as the second state or P2) such as a time zone when the morning and evening solar radiation is weak will be described.

  In P2, the B series is at the operating point Pb2 (output voltage Vb2) in the PV characteristic diagram in FIG. 6B, the A series is at the operating point Pa2 (output voltage Va2) in the same diagram, and the operating point at P1. It is assumed that the transition is made from Pb1 and Pa1, and the operation is performed at the operating point.

In FIG. 7, the efficiency ηb2 corresponding to the DC output Pdc_b (Vb2) at the B-sequence operating point Pb2 (voltage Vb2) is ηb2 = η (Pdc_b (Vb2)), and the A-sequence operating point Pa2 (voltage Va2). The efficiency ηa2 corresponding to the DC output Pdc_a (Va2) in) is determined by ηa2 = η (Pdc_a (Va2)).
Then, the sum Pdc (P1) of the DC outputs from the two PV array groups at P2 is
Pdc (P2) = Pdc_b (Vb2) + Pdc_a (Va2)
It can be expressed. Here, the first term on the right side is a DC output at the operating point Pb2 (voltage Vb2) in FIG. 6B for the B series, and the second term is at the operating point Pa2 (voltage Va2) in the A series in FIG. 6B. DC output.

The sum Pac (P2) of the AC output at P2 is the same as Pac (P1) at P1,
Pac (P2) = Pdc_b (Vb2) × η (Pdc_b (Vb2)) + Pdc_a (Va2) × η (Pdc_a (Va2))
However, the output of the PV array group is considerably lower than the rating. For this reason, if the A series and the B series are operated independently due to a decrease in the load factor, the overall output is also reduced due to a decrease in the AC / DC conversion efficiency of VSI.

In order to avoid this, the VSI 3a is GB, and the AC / DC conversion in the VSI 3a is stopped (hereinafter referred to as the third state or P3). Due to this GB, the voltage of the A series DC bus 5a rises to a value at which the bus connection rectifier 9 becomes conductive (ON), and the DC current output from the PV array group 2a passes through the bus connection rectifier 9 to the B series. Inflow. The voltage difference between the DC buses of the A series and the B series has a polarity opposite to that when the solar radiation intensity is strong, and slightly becomes the voltage of the DC bus 5a> the voltage of the DC bus 5c.
The DC power output from the PV array groups 2a and 2b is synthesized as shown in FIG. 6C and input to the VSI 3b. As shown in FIG. 7, the operating point changes from P2 to P3, and the load of the VSI 3b The rate is improved, and the AC / DC conversion efficiency of the entire system is improved.

Regarding the transition from P2 to P3, it is necessary to clarify the judgment conditions. This condition is that if the AC output at P3 is Pac (P3),
Pac (P3) ≧ Pac (P2) + Pε3
It becomes. Here, Pε3 on the right side is a constant of zero or more.
Since the change from P2 to P3 is for the purpose of increasing the AC output, it is a natural condition that the AC output of P3 must be larger than the AC output of P2.
In addition, the addition of Pε to the right side is not guaranteed that the operating point of P3 is the optimum operating point as described in paragraph 0009, and this is compensated, and GB and gate deblocking are performed too frequently. This is because a dead zone for not repeating (GDB) is provided to stabilize the operation of the photovoltaic power generation facility 1.

Furthermore, even if this transition determination condition is satisfied, the rated output of VSI3b is set to Pb0.
Pac (P2) ≧ Pb0 (≧ Pac (P3))
If is established, P2 does not change to P3. This is because the output cannot be made equal to or higher than the rated output Pb0 of VSI3b even if it is shifted to P3, and the total output is larger when P2 remains (because there is no restriction due to the capacity of VSI3b). In addition, the capacity of the VSI 3b may be larger than the capacity of the VSI 3a).

Another problem is how to estimate Pac (P3). That is, the operating point of P3 (Vb3, Va3, Vb3 = Va3) is unknown, and an accurate value of Pac (P3) cannot be grasped.
Therefore, first, a method using the following approximate value is conceivable.
Pac (P3) ≈ (Pdc_b (Vb2) + Pdc_a (Va2)) × η (Pdc_b (Vb2) + (Pdc_a (Va2))
This approximate value is based on a combination of the A-sequence DC output Pdc_b (Vb2) and the B-sequence DC output Pdc_a (Va2) at P2 immediately before the transition to P3.

Also, since the PV power generation digital data of the PV array group is stored in advance in the photovoltaic power generation facility 1, if the B series DC output is larger than that of the A series, the operating point at P3 is the output at P2b. As voltage Vb2,
Pac (P3) ≈ (Pdc_b (Vb2) + Pdc_a (Vb2)) × η (Pdc_b (Vb2) + (Pdc_a (Vb2))
It can also be.
Conversely, if the DC output of the A series is larger than that of the B series, the operating point at P3 is set as the output voltage Va2 at P2a.
Pac (P3) ≈ (Pdc_b (Va2) + Pdc_a (Va2)) × η (Pdc_b (Va2) + (Pdc_a (Va2))
It can also be.

Furthermore, if the difference between the DC output of the A series and that of the B series is small, the operating point at P3 is set as the average value Vb2 / 2 + Va2 / 2 of the output voltage Vb2 at P2b and the output voltage Va2 at P2a.
Pac (P3) ≒ (Pdc_b (Vb2 / 2 + Va2 / 2) + Pdc_a (Vb2 / 2 + Va2 / 2))
× η (Pdc_b (Vb2 / 2 + Va2 / 2) + Pdc_a (Vb2 / 2 + Va2 / 2))
It can also be.

  Furthermore, the voltage interval [Va2−Vaε, Vb2 + Vbε] (when Va2 <Vb2) or [Vb2−Vbε, Va2 + Vaε] (when Vb2 <Va2) is divided into (n−1) equal parts to obtain n voltages. It is also possible to calculate Pac (P3) at a point. Here, Vaε and Vbε are constants greater than or equal to zero, and the optimum operating point of P3 may not exist in the voltage interval [Va2, Vb2] or [Vb2, Va2]. It is a parameter for taking this value and can be set as appropriate by actual measurement or the like.

  Although it is possible to calculate Pac (P3) finely in this way, there is a possibility that a great improvement in accuracy may not be achieved even if it is calculated too finely considering the time lag of tracking control due to changes in solar radiation intensity, etc. It should be noted that there are.

(Description of the operation of the invention (part 3): When the solar radiation intensity is restored)
An operation in a state where the solar radiation intensity is recovered by moving away from the cloud (hereinafter referred to as a fourth state or P4) will be described.
In this case, the VSI 3a is GDB and the AC / DC conversion operation in the VSI 3a is resumed. With this GDB, the voltage of the A-series DC bus 5a decreases from the optimum operating voltage of the PV array 2b to the optimum operating voltage of the PV array 2a (about rated voltage 380V). For this reason, the voltage difference between the A-series and B-series DC buses generates a voltage difference corresponding to almost the difference between the rated voltages of the PV array group 2a and the PV array group 2b. However, in the polarity of this voltage difference (the voltage of the DC bus 5a <the voltage of the DC bus 5c), the back electromotive force is applied to the bus connecting rectifier 9 installed between the A series and B series DC buses. The bus connection rectifier 9 is in a non-conductive (OFF) state, and the A series and the B series are electrically separated (independent).
Then, the MPPT control is individually performed for both the A series and the B series to maintain a relatively high AC / DC conversion efficiency.

The AC output sum Pac (P4) at P4 is the same as P2,
Pac (P4) = Pdc_b (Vb4) × η (Pdc_b (Vb4)) + Pdc_a (Va4) × η (Pdc_a (Va4))
But
Pac (P4) + Pε4 ≧ Pac (P3)
If the following condition holds, the transition from P3 to P4 occurs. Here, Pε on the left side is a constant of zero or more.

  However, it is actually difficult to derive an approximate value of Pac (P4) from the state of P3. As shown in FIG. 6 (C), in the state of P3, the P-V characteristics obtained by synthesizing the A series and the B series are collectively operated, and from this, the VSI 3a is GDB and the A series and the B series are operated independently. This is because it is not possible to find the optimum operating point.

Therefore, the transition condition from P3 to P4 is considered by paying attention to the elapsed time of output.
That is, in the state of P3, the output exceeding this rated capacity (or allowable short-time rated capacity) cannot be output due to the limit of the rated capacity of the VSI 3b. For this reason, when the output is continued for a certain level or more at this rated capacity, it is determined that the solar radiation is already in a state where the A series and the B series can be independently operated, and the VSI 3a is GDB.

1 Solar power generation equipment 2 PV array group 3 VSI
10 PV array

Claims (1)

A photovoltaic power generation facility comprising two PV array groups, two VSIs, two DC buses installed between them, and two connecting lines between the DC buses,
The two PV array groups are configured with different rated voltages,
One of the connection lines is provided with a bus connection rectifier, and the connection of the bus connection rectifier is such that the cathode of the bus connection rectifier is a DC bus side with the PV array group having a higher rated voltage, and the connection of the bus connection rectifier The anode is on the DC bus side with the PV array group with the lower rated voltage,
A solar power generation facility characterized by

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