KR20140011254A - Inverter system for photovoltaic power generation - Google Patents

Abstract

Provided is an inverter system capable of performing efficient sunlight generation by automatically switching an integrated operation and an independent operation of the inverters according to the voltages and current values of a sunlight panel without a separate communication function. The inverter system for the sunlight generation according to an embodiment relates to an inverter system for switching DC power and AC power outputted from a first sunlight panel and a second sunlight panel. The inverter system comprises a first inverter and a second inverter. The inverter system applies outputs of the first sunlight panel and the second sunlight panel to the first inverter according to the outputs of the first sunlight panel and the second sunlight. The inverter system applies the output of the first sunlight panel to the first inverter and applies the output of the second sunlight panel to the second inverter.

Description

Inverter system for solar power generation {INVERTER SYSTEM FOR PHOTOVOLTAIC POWER GENERATION}

The present invention relates to an inverter system for solar power generation, and more particularly, to an inverter system for operating a small capacity solar power inverter in a multi-central manner.

In the solar power system, the inverter converts direct current (DC) power produced from the solar panel into alternating current (AC) power. The inverter will not _start until the DC input power is above a certain level (W _{in - start} ) necessary for normal operation (starting), and above the maximum input power (W _{in - max} ) it will stop to protect the equipment. Operation stops at an input below the minimum power (W _{in -min} ). Where the values of (W _{in - min} ) and (W _{in - start} ) may be the same or different depending on the inverter.

The efficiency of the inverter is expressed as the ratio of the input power to the output power, which is not always constant over the entire operating range, and varies with the output as shown in FIG. 1. The efficiency of the inverter depends on the structure and control method, but is generally known to be the highest in the 30% to 80% range.

Inverters in photovoltaic systems are module-integrated converters (MICs), strings, multi-strings, centrals, and multi-centrals, depending on the combination of solar panels and arrays. ) It can be divided into inverter.

MIC attaches inverter to each panel, so it is easy to install because there is no need for separate DC line wiring, and maximum energy harvest is possible even when the sunlight conditions between panels are different due to differences in shadow or installation conditions. There is an advantage, but there is a disadvantage that the cost is large and the efficiency is somewhat low when implementing a large capacity. In small systems such as building integrated photovoltaics (BIPV), including homes, it has begun to spread based on the advantages of flexibility and expandability of panel layout.

The string method uses DC / AC inverters per panel series group to control the maximum power point tracking (MPPT) for each string, and can harvest energy relatively effectively against partial shade, but it is a large capacity power plant. When applied to the system, the number of inverters is too large to increase maintenance costs, and the inverter is not centrally controlled, so it is somewhat unsuitable in terms of system protection, such as preventing stand-alone operation, which is suitable for a medium capacity solar power system.

The multistring method uses an inverter or DC / DC converter per panel series group, which combines the advantages of the string method and the central method, but uses a dual power converter.

The central method has a disadvantage in that energy harvesting is rather low due to the direct and parallel combination of all panels. However, it is mainly used as a large-capacity industrial inverter because of the efficiency of the converter and the low cost compared to the output capacity. Such a central method is advantageous in system protection and low maintenance cost because it uses a single inverter, but has the disadvantage that the entire system can not operate in case of inverter failure. Recently, a multi-central method, which is a method of implementing a single high-capacity inverter system by connecting a large-capacity central inverter in parallel, has been developed as a method to compensate for such disadvantages.

The multi-central inverter is a structure in which central inverters are connected in parallel, and when the power generation system is constructed, it is composed of several inverters instead of one inverter. In low solar conditions such as sunrise, sunset and cloudy weather, the power generated by the solar panels is collected to drive only a specific inverter, and when there is a large amount of solar energy, several inverters are operated so that the inverter operates at optimum conditions. It is possible to improve the efficiency of the solar power plant. In addition, it is possible to extend the service life of inverters by operating them in order to maintain the same operation time of inverters, and to reduce energy loss by operating another inverter at high energy level in case of one inverter failure, maintenance and repair. It is beginning to spread to large-scale solar power systems.

However, the multi-central approach is inadequate for small solar power systems because it requires the control of several inverters and solar panels, resulting in higher system construction costs and complex control functions, including communication between inverters or between the inverter and the central controller. There is this.

The present invention has been proposed to solve this problem, and in using the MIC in a multi-central manner, it is possible to automate the switching of the integrated operation and the independent operation of the inverters according to the voltage and current value of the solar panel without a separate communication function The purpose is to provide an inverter system that enables more economical and efficient solar power generation.

Inverter system for solar power generation according to an embodiment of the present invention for achieving this object is an inverter system for converting the DC power output from the first solar panel and the second photovoltaic panel into AC power, And a first inverter and a second inverter, and applying the outputs of the first and second solar panels to the first inverter according to the output values of the first and second solar panels, or of the first solar panel. An output may be applied to the first inverter, and an output of the second solar panel may be applied to the second inverter.

In the inverter system according to the embodiment, if the output value of the first or second solar panel is less than a predetermined power value, the output of the first and second solar panels are applied to the first inverter, the first inverter Alternatively, when the output value of the second solar panel is equal to or greater than a preset power value, the output of the first solar panel may be applied to the first inverter, and the output of the second solar panel may be applied to the second inverter, respectively.

The first inverter and the second inverter may be connected only by a wire for transmitting power.

The first inverter includes an on / off first switch connected to the second solar panel to further receive the output of the second solar panel, and the second inverter A second switch connected between the second solar panel and the first inverter to receive the output of the second solar panel or to convert and apply the output of the second solar panel to the first inverter; It may include.

The first inverter may include a first controller for controlling the first switch, and the second inverter may include a second controller for controlling the second switch.

The first and second controllers may monitor the voltage and current input from the first and second solar panels to control the first and second inverters to operate at the maximum power point.

When the output value of the second solar panel is less than a preset power value, the first controller turns on the first switch and the second controller switches the second switch to the first inverter side, When the output value of the second solar panel is equal to or greater than a predetermined power value, the first controller may turn off the first switch, and the second controller may switch the second switch to the second inverter side.

According to the present invention, in a photovoltaic power generation facility in which several small inverters are connected in parallel, the main inverter and the longitudinal inverter are interconnected by only a power line, and when the intensity of sunlight is low, the outputs of the two solar panels are combined in the main inverter. When operating, and when the intensity of sunlight is strong, it is possible to extend the power generation time by operating independently of each other, and at the same time, it is possible to operate each inverter in the most efficient range.

In addition, in the operation of the conventional multi-central inverter, the communication function between the inverters or between the inverters and the central control unit is required for the control of the inverter, but in the present invention, integrated operation and independent operation are possible using the output characteristics of the solar panel. By doing so, since a separate communication device and a communication line are not required, economic efficiency is improved by simplifying the installation.

1 is a graph illustrating a change in efficiency of a general solar inverter.
2 is a graph illustrating changes in output voltage, output current characteristics, and maximum output point of a general solar panel.
3 and 4 are schematic diagrams of an inverter system for photovoltaic power generation according to an embodiment of the present invention operating in different solar conditions.

The above and other objects, features, and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, which are not intended to limit the scope of the present invention. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 illustrates the voltage-current output characteristics of a typical solar panel. The density of solar energy varies with time and weather conditions, which in turn changes the output of the solar panel. The solar power inverter has a maximum power point tracking (MPPT) function to get the maximum power, and it outputs the maximum voltage-current output condition of the solar panel according to the change of solar energy density. Adjust to a value that can produce

At this time, as shown in Figure 1, the inverter is preferably W _{FS - min} to W _{FS - max} The overall efficiency can be improved by operating in a section with high efficiency.

3 and 4 are schematic diagrams of an inverter system for photovoltaic power generation according to an embodiment of the present invention operating in different solar conditions.

3 and 4, the inverter system for photovoltaic power generation according to an embodiment of the present invention, the DC power output from the first photovoltaic panel 1 and the second photovoltaic panel 2 alternating current It is an inverter system for converting into electric power, and includes a first inverter 10 and a second inverter 20. In the inverter system according to the present embodiment, the outputs of the first and second solar panels 1 and 2 are all applied to the first inverter 10 according to the output values of the first and second solar panels 1 and 2. Alternatively, the output of the first solar panel 1 may be applied to the first inverter 10, and the output of the second solar panel 2 may be applied to the second inverter 20, respectively.

In the present embodiment, the first inverter 10 and the second inverter 20 are configured as a pair to function as a master inverter and a slave inverter, respectively.

The first and second inverters 10 and 20 respectively include first and second controllers Controller1 and Controller2, and the first and second controllers Controller1 and Controller2 are respectively connected to the first and second solar panels 1 and 2. By monitoring the input voltage and current, each inverter (10, 20) can be controlled to operate at the maximum output point.

In the first and second inverters 10 and 20, when the output value of the first solar panel 1 is less than the preset power value, the outputs of the first and second solar panels 1 and 2 are all the first inverter 10. If the output value of the first photovoltaic panel 1 is greater than or equal to a predetermined power value, the output of the first photovoltaic panel 1 is the first inverter 10, the second photovoltaic The output of the panel 2 can be applied to the second inverter 20 respectively (independent operation).

To this end, the first inverter 10 is an on / off first switch connected to the second solar panel 2 to further receive the output of the second solar panel 2. And (11).

The second inverter 20 receives the output of the second solar panel 2 or converts the output of the second solar panel 2 to the first inverter 10 to apply the second solar panel (2). It may include a second switch 21 connected between the 2) and the first inverter 10.

When there is no separate control from the first controller Controller1 of the first inverter 10, the first switch 11 maintains an open state (Off) as a default value, and the second switch 21 is a second inverter ( When there is no separate control from the second controller (Controller2) of 20) it can maintain the state connected to the second inverter 20 by default.

Hereinafter, in order to simplify the description of the technology, the performance of the first and second solar panels 1 and 2 are the same, and the performance of the first and second inverters 10 and 20 connected thereto are also the same, and the maximum input of the inverter is Assume that it is set to the maximum output of the solar panel. The variation of the output of the solar panel due to temperature change, panel contamination, and secular variation was not considered. Explanation of symbols for explaining the characteristics and operation of the inverter is as follows.

W _{FS - min} : Minimum output power set for efficient operation of inverter

W _{FS - max} : Maximum output power set for efficient operation of inverter

W _{in - max} : Maximum input power that inverter can operate normally

W _{in - min} : Minimum input power that the inverter can continue to operate normally

W _{in - start} : Minimum input power that the inverter can normally start operating in the stopped state.

V _{M - max} : Maximum input voltage of the first inverter 10

I _{M - max} : Maximum input current of the first inverter 10

V _{M - in} : First inverter 10 input voltage

I _{M - in} : First inverter 10 input current

W _{M - in} : First inverter 10 input power, V _{M - in M} I _{M - in}

V _{S - max} : Maximum input voltage of the second inverter 20

I _{S - max} : Maximum input current of the second inverter 20

V _{S - in} : Second inverter 20 input voltage

I _{S - in} : Second inverter 20 input current

W _{S - in} : Second inverter 20 input power, V _{S - in S} I _{S - in}

V ⁱ _OC : Open voltage of solar panel (when sunshine condition is i)

I ⁱ _SC : Short-circuit current of solar panel (when sunshine condition is i)

V ⁱ _pmax : Output voltage of solar panel at maximum output point (when sunshine condition is i)

W ⁱ _pmax : Output of solar panel at maximum output point (when sunshine condition is i)

V ₁ : output voltage of the first solar panel 1

I ₁ : output current of the first solar panel 1

W ₁ : output power of the first solar panel 1, V ₁ Ⅹ I ₁

V ₂ : output voltage of the second solar panel 2

I ₂ : output current of the second solar panel 2

W ₂ : output power of the second solar panel 2, V ₂ Ⅹ I ₂

Immediately after sunrise when the sun's altitude is not high until the morning time, as shown in FIG. 3, the second switch 21 is switched to the first inverter 10 side and the first switch 11 is shorted (On). In this way, the first and second solar panels 1 and 2 are connected in parallel, and the output power of the second solar panel 2 is controlled from the DC output terminal 23 on the upper side of the second switch 21. (11) It is applied to the first inverter 10 in combination with the output of the first solar panel 1 via the DC input terminal 13 at the bottom. At this time, the power W _{M - in} supplied to the first inverter 10 becomes V ₁ ⅹ (I ₁ + I ₂ ) = 2 (V ₁ ⅹ I ₁ ). This allows each of the solar panels (12, 3) the output is capable of the minimum start-up of the inverter _- if only at least one-half of (W _{in min)} it is possible to start the first inverter 10 can expedite the start of the development time And each can be operated in a higher operating efficiency range when driving alone.

As the altitude of the sun increases, the output of the first and second solar panels 1 and 2 increases to reach a range in which the first inverter 10 and the second inverter 20 can independently operate efficiently. 4, the first switch 11 is opened and the second switch 21 is switched to the second inverter 20 side. By doing so, the first inverter 10 and the second inverter 21 are separated and each can operate independently.

The time point for switching to independent operation is the minimum power value W _{FS - min} or maximum power value W _in which the power value W _{M -in} input to the first inverter 10 is set for efficient operation of the inverter. _{FS - max} ).

That is, W _{M - in} > (2 ⅹ W _{FS - min} ) while W _{M - in} > W _{FS - max} In the case of switching to the single operation, the operation can be performed in a section in which the efficiency of the first inverter 10 and the second inverter 20 is high.

When the first switch 11 is opened for independent operation of the first inverter 10, the voltage V ₂ of the second solar panel 2 rises instantaneously to the open voltage V ⁱ _OC . The second inverter 20 detects the value of the output voltage V ₂ of the second solar panel 2 and the variation thereof, and when it is determined that the first switch 11 is open, the second inverter 20 removes the second switch 21. 2 Switch to the inverter 20 side and start independent operation.

Apart from this, the second inverter 20 may independently determine the start of the single operation. In a normally operating system, the output voltage of the solar panel is maintained at V ⁱ _pmax by the MPPT function of the inverter. Therefore, the output value (W ₂ = W ⁱ _{pmax )} of the second solar panel 2 can be known from this value. Alternatively, the output current I _{2 of} the second solar panel ₂ may be simultaneously measured to obtain the output value W ₂ = V ₂ V I _{2 of} the second solar panel 2. The output W ₂ of the second solar panel ₂ is W ₂ > W _{FS - min} while W ₂ > W _{FS - max} In this case, the second switch 21 is switched to the second inverter 20 side to start the independent operation.

When the sun's altitude is lowered in the afternoon, as shown in FIG. 3, the outputs of the first and second solar panels 1 and 2 are integrated into the first inverter 10 to increase efficiency and extend generation time. Can be. When the input power W _M-in of the first inverter 10 in the independent operation state falls below a range _in which independent operation can be efficiently performed (W _{M - in} <W _{FS - min} ), the first switch ( 11) is short-circuited to prepare for the integrated operation of the first inverter 10. When the input power W _{S - in} falls below a range _in which the independent power can be efficiently operated (W _{S - in} <W _{FS - min} ), the second switch 20 in the independent operation state also receives the second switch ( 21 is switched to the first inverter 10 side.

At this time, if the first switch 11 is not yet shorted, the voltage V ₂ of the second solar panel 2 rises instantaneously to the open voltage V ⁱ _OC value, and thus, the first switch 11 You can check whether there is a short circuit.

Another method for determining whether the first switch 11 is shorted is to check the voltage at the output terminal 23 output to the first inverter 10. Since the second switch 21 is switched to the second inverter 20 side and the first switch 11 is also open in a single operation state, a voltage is applied to the output terminal 23 of the second switch 21. Not authorized If the first inverter 10 is ready for integrated operation, the first switch 11 is short-circuited so that the output voltage V ₁ of the solar panel ₁ is detected at the output terminal 23 of the second switch 21. . By detecting the presence or absence of this voltage, it is possible to confirm whether the first switch 11 is opened or closed.

When the first switch 11 is in the open state, the second inverter 20 continues the independent operation with the second switch 21 connected to the second inverter 20 side.

After confirming that the first switch 11 is shorted by checking the voltage of the output terminal 23 outputted to the first inverter 10 or after a predetermined predetermined waiting time, the second switch 21 is connected to the first inverter ( 10) You can switch back to the side. The switching operation of the second switch 21 is repeated more than a set number of times, or the input power W _{S - in} If the first switch 11 is not shorted even after being lowered below a predetermined level (W _{FS - min} ), it is determined that the first inverter 10 is broken and the second switch 21 is set to the second inverter 20. It can be fixed to) side to continue independent operation.

If switching is normally performed, the second inverter 20 stops independent operation.

When the driving method is automatically performed by the controllers (Controller1 and Controller2) of the inverters 10 and 20, it is possible to efficiently generate power even when the amount of sunshine of the sun, such as cloudy weather, is lower than usual.

The foregoing description is merely illustrative of the present invention, and various modifications may be made by those skilled in the art without departing from the spirit of the present invention. Therefore, the embodiments disclosed in the specification of the present invention are not intended to limit the present invention. The scope of the present invention should be construed according to the following claims, and all the techniques within the scope of equivalents should be construed as being included in the scope of the present invention.

1: first solar panel 2: second solar panel
10: first inverter
11: on / off switch controlled by the first inverter (first switch)
12, 13: DC power input terminal
14: AC power output terminal of the first inverter
20: second inverter
21: switching switch controlled by the second inverter (second switch)
22: DC power input terminal
23: DC power output terminal
24: AC power output terminal of the second inverter

Claims (8)

In the inverter system for converting the DC power output from the first solar panel and the second solar panel into AC power,
A first inverter and a second inverter,
The outputs of the first and second solar panels are all applied to the first inverter according to the output values of the first and second solar panels, or the output of the first solar panel is the first inverter, The output of the 2 solar panel is applied to the second inverter, respectively
Inverter system for solar power generation.
The method of claim 1,
When the output value of the first solar panel is less than a predetermined power value, the output of both the first and second solar panels are applied to the first inverter,
The output of the first solar panel is applied to the first inverter and the output of the second solar panel to the second inverter when the output value of the first solar panel is equal to or greater than a predetermined power value. doing
Inverter system for solar power generation.
The method of claim 1,
When the output value of the second solar panel is less than a predetermined power value, the output of both the first and second solar panel is applied to the first inverter,
The output of the first solar panel is applied to the first inverter and the output of the second solar panel is applied to the second inverter when the output value of the second solar panel is equal to or greater than a predetermined power value. doing
Inverter system for solar power generation.
The method of claim 1,
The first inverter and the second inverter is characterized in that connected only by a wire for transmitting power
Inverter system for solar power generation.
The method of claim 1,
The first inverter includes an on / off first switch connected to the second solar panel to further receive the output of the second solar panel,
The second inverter is connected between the second solar panel and the first inverter to receive the output of the second solar panel or to convert and apply the output of the second solar panel to the first inverter. It characterized in that it comprises a second switch
Inverter system for solar power generation.
6. The method of claim 5,
The first inverter includes a first controller for controlling the first switch,
The second inverter comprises a second controller for controlling the second switch.
Inverter system for solar power generation.
The method according to claim 6,
When the output value of the second solar panel is less than a preset power value, the first controller turns on the first switch and the second controller switches the second switch to the first inverter side.
When the output value of the second solar panel is greater than or equal to a predetermined power value, the first controller turns off the first switch and the second controller switches the second switch to the second inverter side. doing
Inverter system for solar power generation.
The method according to claim 6,
The first and second controllers may monitor the voltage and current input from the first and second solar panels to control the first and second inverters to operate at the maximum power point.
Inverter system for solar power generation.

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