KR20130115719A - Grid-tied multistring photovoltaic inverter system - Google Patents

Abstract

PURPOSE: A grid connected multi string sunlight inverter system is provided to lengthen the life of a device by flexibly operating an inverter depending on the quantity of solar radiation. CONSTITUTION: A grid connected multi string sunlight inverter system includes multiple inverters (10, 20), a switching unit, and a control unit (40). The multiple inverters include a PV array, inverter units (12, 22), and AC filters (13, 23). The DC voltage from the PV array is converted into AC power, and the converted power is supplied to a system power source (50). Depending on the voltage of a DC link terminal of the multiple inverters, the switching unit switches the on/off state of the multiple inverters. Depending on the rated output power of the multiple inverters, the control unit controls the on/off operation of the switching unit. [Reference numerals] (11,21) PV array; (12,22) Inverter unit; (13,23) AC filter; (30) Switch unit; (40) Control unit; (50) System power

Description

Grid-connected multistring solar inverter system {GRID-TIED MULTISTRING PHOTOVOLTAIC INVERTER SYSTEM}

The present invention relates to an inverter, and more particularly, to an inverter system applied to a photovoltaic power generation system.

In general, a photovoltaic power generation system is a power conversion device that converts direct current electricity produced by a photovoltaic cell into alternating current electricity, and is composed of a solar cell, a power conversion device, and a grid connection circuit.

In a grid-connected photovoltaic power generation system, inverters are MIC (Module Integrated Converter), string, multi-string, central, multi-centre, depending on the application or combination of solar cell modules. can be classified in a central way.

In the dual-string solar power generation system, two inverters are directly connected in parallel to the DC link stage, and always share the DC link voltage, and are connected at the grid connection point. Sharing the DC link stage can increase the power generation efficiency of the system by operating only one inverter at a low level of solar radiation.

However, when the amount of insolation increases and each inverter is operated, since the DC bus is always connected in common, the maximum power point is tracked in only one array, and as a result, the maximum power point is tracked in each array. There is a problem that the overall efficiency is lowered.

1 is a block diagram of a general grid-connected multi-string solar inverter system, showing a two-level full-bridge (multi-bridge) multi-string solar inverter system.

As shown in the drawings, a conventional grid-connected multistring inverter system includes a photovoltaic (PV) array 110a, 110b, three-phase inverters 120a, 120b, LCL filters 130a, 130b, and a grid. (grid) 200.

The most important part of the multi-string solar inverter system is the power conversion efficiency. In the system as shown in FIG. 1, the DC link stages 140a and 140b of each PV array 110a and 110b are connected in parallel to always provide a DC link. Share the voltage.

That is, in the above system, since the maximum power point is followed by using the entire array as one input, there is a problem that the overall efficiency is lowered.

In addition, in the above system, when one inverter 100a or 100b cannot be used, there is a problem in that power generation cannot be continued.

The problem to be solved by the present invention, by connecting a plurality of independent photovoltaic (PV) array in parallel by using a switch at the DC link voltage stage, to increase power conversion efficiency and stability, extending the life of the inverter It is to provide a multi-string solar inverter system.

In order to solve the above technical problem, the solar inverter system of the present invention, a plurality of inverters for converting the DC voltage from the photovoltaic (PV) array into an alternating current power supply to the grid power supply; A switching unit for switching on / off of the plurality of inverters according to DC link terminal voltages of the plurality of inverters; And a control unit for controlling the on / off of the switching unit in accordance with the rated output of the plurality of inverters.

In one embodiment of the present invention, the plurality of inverters, the PV array for converting light energy into direct current voltage using a photoelectric effect; An inverter unit for converting a DC voltage into an AC power under control of the controller; And an AC filter for improving the waveform of the converted AC power supply.

In one embodiment of the present invention, the control unit, preferably transmits a pulse width modulation (PWM) control signal to the inverter unit.

In one embodiment of the present invention, the switch unit, it is preferable to include a plurality of switching elements for connecting the DC link between the PV array and the inverter unit of the plurality of inverters in parallel, switching a plurality of inverters. .

In one embodiment of the present invention, the control unit, it is preferable to turn on the first switching device connected to the inverter (first inverter) of the high voltage of the DC link among a plurality of inverters, and drive the first inverter.

In one embodiment of the present invention, the control unit, when the first inverter substantially exceeds the first ratio of the rated output, by turning off the first switching element, the inverter of any one of the first inverter and the remaining inverter It is preferable to drive the (second inverter).

In one embodiment of the present invention, the control unit, when the output of the first inverter and the second inverter is substantially less than the second ratio, the second switching device connected to the first switching device or the second inverter By turning on, it is preferable to drive either the first inverter or the second inverter.

In one embodiment of the present invention, the first switching device is a thyristor, the control unit, it is preferable to control to increase the current command of the second inverter until the first switching device is off.

In one embodiment of the present invention, the first and second switching element is a thyristor, the control unit, it is preferable to control the current command of the first inverter to increase than the current of the second inverter.

In the present invention as described above, a plurality of independent PV arrays are connected in parallel and the inverter operates according to the change in the solar radiation rate to increase the power conversion efficiency, and when the solar radiation is low, the inverter is stopped for the life of the device. It is effective to extend.

In addition, the present invention can perform the maximum power point tracking control related to the generation energy of each solar array, even if the characteristics of the PV array is different, the effect of recovering more energy than the same environmental conditions There is.

In addition, the present invention may require the maintenance and repair of some of the plurality of inverters, or even if a malfunction occurs in some inverters, the inverters that operate normally can continue to generate power at a high energy level, There is an effect that enables a lot of energy recovery.

1 is a block diagram of a typical grid-connected multi-string solar inverter system.
2 is a configuration diagram of an embodiment of a grid-connected multi-string solar inverter system according to the present invention.
Figure 3 is a circuit diagram of an embodiment of a grid-connected multi-string solar inverter system according to the present invention.
4A to 4C are exemplary views illustrating an operation mode of the inverter by the operation of the switch unit of FIG. 2.
Figure 5 is an exemplary view of the waveform of the output current during the multistring operation in the inverter system according to the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the invention is not intended to be limited to the particular embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

Terms including ordinals such as first, second, etc. may be used to describe various elements, but the elements are not limited by such terms. These terms are used only to distinguish one component from another.

When a component is said to be 'connected' or 'connected' to another component, it may be directly connected to or connected to that other component, but other components may be present in between. It should be understood that. On the other hand, when an element is referred to as being "directly connected" or "directly connected" to another element, it should be understood that there are no other elements in between.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprises", "having", and the like are used to specify that a feature, a number, a step, an operation, an element, a component, or a combination thereof, But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment according to the present invention.

2 is a configuration diagram of an embodiment of a grid-connected multi-string solar inverter system according to the present invention.

As shown in the drawing, the inverter system of the present invention includes a plurality of inverters 10 and 20, a switch unit 30, a control unit 40, and a grid power supply 50. In the drawings, the inverters 10 and 20 are not shown separately for convenience, but as shown in FIG. 1, the PV array, the inverter unit, and the AC filter will be described as one inverter. However, it will be apparent to those skilled in the art that other configurations may be included in the inverter.

In addition, although description of this invention demonstrates that a system includes two levels, ie, two inverters, for example, this invention is not limited to the number of levels. That is, it is obvious that a plurality of inverters may be included.

That is, the inverters 10 and 20 include PV arrays 11 and 21, inverter units 12 and 22 and alternating current filters 13 and 23, respectively.

The PV arrays 11 and 21 convert light energy respectively received into electric energy using a photovoltaic effect. Since the general function of the PV array will be as already known, a detailed description thereof will be omitted.

The switch unit 30 switches the PV arrays 11 and 21 and the inverter units 12 and 22 under the control of the control unit 40. Detailed operation of the switching unit 30 will be described in more detail later with reference to the drawings.

The inverter units 12 and 22 convert the input DC voltage into AC power by pulse width modulation (PWM) control of the controller 40.

The AC filters 13 and 23 improve the waveform of the AC power converted by the inverter units 12 and 22 to reduce the total harmonic distortion (THD).

The system power supply 50 supplies the AC power of the inverters 10 and 20 received through the AC filters 13 and 23 to the system.

The control unit 40 controls the switch unit 30 by sensing the voltage of the DC link terminals (not shown in FIG. 2) of the inverters 10 and 20, and controls the PWM control signals of the inverter units 12 and 22. It generates and delivers it to the inverter unit (12, 22). In addition, the phase of the system power supply 50 is controlled.

Figure 3 is a circuit diagram of an embodiment of a grid-connected multi-string solar inverter system according to the present invention. Although the illustration of the controller 40 is omitted in FIG. 3 for convenience, FIGS. 3 and 2 are equivalent, so that FIG. 3 will be described with the controller 40.

The configuration of FIG. 3 will already be described with reference to FIG. 2, and therefore, the switch unit 30 will be mainly described. Hereinafter, for convenience of description, the inverter 10 is referred to as the first inverter 10, the inverter 20 is referred to as a second inverter 20 will be described.

The switch unit 30 of the present invention includes, for example, two thyristors Tt and Tb, and connects the DC links 14 and 24 of the first inverter 10 and the second inverter 20 in parallel. do.

The controller 40 senses the voltages of the DC links 14 and 24 of the first inverter 10 and the second inverter 20 after sunrise, and drives the higher inverter (for convenience, the first inverter 10 To work).

However, the switch device of the switch unit 30 of the present invention is not limited to the thyristor, it is obvious that other power semiconductor devices can be used as the switch device. In addition, the switch unit 30 of the present invention is shown for a two-level multistring structure, it will be apparent that the number of switch elements used will also increase when the number of levels increases.

When the first inverter 10 is in operation and the amount of solar radiation increases, so that the first inverter 10 reaches a predetermined ratio, for example, 80% or more, of the inverter rated output, the controller 40 controls the current of the second inverter 20. Increase the reference slowly until the T _t switch is off. Current flowing through the T _b in the switch unit 30 is greater than the current flowing through the T _t, T _t The nature of the thyristor switch is turned off. As such, when the T _t switch is turned off, the first inverter 10 and the second inverter 20 operate in parallel.

Then, when the amount of solar radiation decreases so that the output of the first inverter 10 and the second inverter 20 is less than a predetermined ratio, for example, 40% of the rated output, the control unit 40 is the current of the first inverter 10 V increases the command than the current of the second inverter 20, V_{pv _t} <V_{pv _b}Becomes T_b Turn the switch on. Thereafter, when the second inverter 20 current command is increased and the first inverter 10 current command becomes 0, transmission of the PWM control signal to the first inverter 10 is stopped.

4A to 4C are exemplary views illustrating an operation mode of the inverter by the operation of the switch unit of FIG. 2. 4A to 4C, only the inverter units 12 and 22, the AC filters 13 and 23, and the system power source 50 are illustrated for convenience of description.

When the sun rises, the controller 40 measures the voltages of the DC links 14 and 24 of the first and second inverters 10 and 20 until the sun becomes operational, thereby operating the inverter that has been operated first. (FIG. 4A). It will be described that the first inverter 10 operates first.

The first inverter 10 continues to operate at less than 80% of the rated output. When the inverter reaches 80% or more of the rated output, the T _t switch of the switch unit 30 is turned off under the control of the control unit 40 so that the multi-string inverter (Fig. 4B).

Figure 5 is an exemplary view of the waveform of the output current during the multistring operation in the inverter system according to the present invention. In the figure, A is the operating waveform of the first inverter 10, B is the second inverter 20.

The multistring inverter operates only up to a predetermined ratio, for example, less than 40% of the rated output of the first and second inverters 10 and 20, respectively, and when the multistring inverter is less than 40% of the inverter rated output, the control of the controller 40 By this, only one inverter, for example, the second inverter 20, is operated (Fig. 4C). At this time, it is obvious that the first inverter 10 may be controlled to operate alone. The second inverter 20 operates alone until the solar radiation decreases and the inverter stops in an inoperable state.

According to the present invention, it is possible to increase the power conversion efficiency by operating the inverter close to the rating in the multi-string photovoltaic power generation system, and to extend the life of the inverter that does not operate when the solar radiation is low.

Even when one inverter cannot be used, the inverter which normally operates can be used, and thus more energy can be recovered.

While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims. Accordingly, the true scope of the present invention should be determined by the following claims.

10, 20: inverter 11, 21: PV array
12, 22: inverter unit 13, 23: AC filter
30: switch unit 40: control unit
50: grid power

Claims (9)

A plurality of inverters for converting the DC voltage from the PV array into AC power and supplying the power to the grid power;
A switching unit for switching on / off of the plurality of inverters according to DC link terminal voltages of the plurality of inverters; And
And a controller for controlling on / off of the switching unit according to the rated output of the plurality of inverters.
The method of claim 1, wherein the plurality of inverters,
The PV array converting light energy into a DC voltage using a photoelectric effect;
An inverter unit for converting a DC voltage into an AC power under control of the controller; And
A solar inverter system comprising an AC filter for improving the waveform of the converted AC power.
3. The apparatus of claim 2,
A solar inverter system for transmitting a pulse width modulation (PWM) control signal to the inverter unit.
The method of claim 2, wherein the switch unit,
And a plurality of switching elements for connecting the DC link between the PV array and the inverter unit of the plurality of inverters in parallel and switching the plurality of inverters.
5. The apparatus of claim 4,
A solar inverter system driving a first inverter by turning on a first switching device connected to an inverter (first inverter) having a high voltage of a DC link among a plurality of inverters.
The inverter of claim 5, wherein the controller is configured to turn off the first switching device when the first inverter substantially exceeds a first ratio of the rated output so as to at least one inverter of the first inverter and the remaining inverters. Solar inverter system that drives 2 inverters.
The switching device of claim 6, wherein the controller turns on the first switching device or the second switching device connected to the second inverter when the outputs of the first inverter and the second inverter are substantially less than the second ratio. Thus, the solar inverter system for driving any one of the first inverter or the second inverter.
The solar inverter system of claim 6, wherein the first switching device is a thyristor, and the control unit controls the current command of the second inverter to increase until the first switching device is turned off.
The solar inverter system according to claim 7, wherein the first and second switching elements are thyristors, and the control unit controls the current command of the first inverter to increase than the current of the second inverter.

Images