KR102565431B1 - Switching operation method and an inverter device for grid connection considering transformer magnetization - Google Patents

Abstract

A grid-connected inverter device considering magnetization and a method for controlling a switching element are disclosed. A grid-connected inverter device considering magnetization includes an inverter that converts an input DC voltage into an output AC voltage, a transformer that boosts and reduces the output AC voltage of the inverter, a load connection line that connects the output of the transformer to a load, and the output of the transformer to a grid. An electric control switching element controlled to connect, and a controller controlling the inverter and the electric control switching element, wherein the inverter is soft-started by a control operation of turning on the electric control switching element by the controller. It is characterized in that the output of the transformer is switched to the grid and output without need.

Description

Grid-connected inverter device and switch operation method considering transformer magnetization {Switching operation method and an inverter device for grid connection considering transformer magnetization}

The present disclosure relates to a method and apparatus for operating in consideration of magnetization of a transformer without delay when switching between a load and a grid connection in a grid-connected inverter system.

The invention according to the present disclosure was conducted as part of a technology development project supported by the Korea Institute for Advancement of Technology with financial resources from the Ministry of SMEs and Startups (Task identification number: P0009927, task title: 50kW class convergence type (PV with seamless function) PCS + ESS PCS) PCS development).

A grid-connected inverter system is a system that outputs power generated by a small-scale grid rather than a power company, such as renewable energy, to the grid. In many cases, load-tied inverters and grid-tied inverters are often operated separately. In recent years, a system in which load and grid are interlocked with one inverter is often operated as a method of switching between load and grid. In this way, when a magnetic switch is used for switching in an inverter system connected while switching loads and grids, instantaneous switching does not occur and a delay time is required to sufficiently magnetize the transformer, so the inverter must be soft-started.

In an inverter system that is connected while switching loads and grids in the power system, if a magnetic switch is used for switching, instantaneous switching is not performed, and a delay time is required to achieve sufficient magnetization of the transformer, which is a problem that has no choice but to soft-start the inverter. occurs.

A grid-connected inverter field system considering transformer magnetization to solve the above-mentioned problems includes an inverter that converts an input DC voltage into an output AC voltage, a transformer that boosts and reduces the output AC voltage of the inverter, and a load that connects the output of the transformer to a load. A connection line, an electric control switching element controlled to connect the transformer output to a grid, and a controller controlling the inverter and the electric control switching element, wherein the control turns the electric control switching element on by the controller. It is characterized in that the output of the transformer is switched to the grid and output without the need to soft-start the inverter by operation.

In one embodiment, the electrically controlled switching element is characterized in that a thyristor semiconductor element.

In one embodiment, the electrically controlled switching element is characterized in that any one of a GTO element, a transistor, and an IGBT element.

In one embodiment, the controller causes the output of the transformer to be switched to the load and output without the need to soft-start the inverter by a control operation of turning off the electrically controlled switching element.

In one embodiment, the output AC voltage is a three-phase AC voltage, and the inverter includes at least six electrically controlled switch elements to convert the input DC voltage to the three-phase AC voltage.

In one embodiment, it is characterized in that the predetermined time delay for turning on (ON) of the electric control switching element in the off (OFF) state is within 100usec.

In one embodiment, the inverter is operated in current control mode when the transformer output is connected to the grid by the electrically controlled switching element, and when the transformer output is connected only to the load by the electrically controlled switching element. It is characterized in that it is operated in a voltage control mode.

A method for controlling a switching element in a grid-connected inverter system in consideration of transformer magnetization to solve the above problems includes converting an input DC voltage into an output AC voltage by an inverter, step-down of the output AC voltage of the inverter by a transformer and outputting, connecting the transformer output to a load by a load connection line, and turning on an electrical control switch element located between the transformer output and the grid so that the transformer output is removed from the load without the need to soft-start the inverter. It is characterized in that it comprises the step of switching to the grid and outputting it.

According to the present disclosure, if a power semiconductor device is used instead of a magnetic switch (MC) during switching in an inverter system operated while switching between a load and a grid in a power system, instantaneous switching is possible and the magnetization of the transformer is not lost. No starting required.

1 is a system diagram showing a switching operation between a load and a grid in a power system.
2 is a waveform showing a soft-start operation of an inverter during a switching operation between a load and a grid in a power system.
3 is a system diagram showing a switching operation between a load and a grid according to an embodiment of the present disclosure.
4 is a waveform showing operation of an inverter during a load-to-grid switching operation in a power system.
5 is a flowchart illustrating a switching operation method between a load and a grid in which an inverter does not require a soft start operation according to an embodiment of the present disclosure.

Terms used in this specification will be briefly described, and the present disclosure will be described in detail.

The terms used in the present disclosure have been selected from general terms that are currently widely used as much as possible while considering the functions in the present invention, but they may vary depending on the intention or precedent of a person skilled in the art, the emergence of new technologies, and the like. In addition, in a specific case, there is also a term arbitrarily selected by the applicant, and in this case, the meaning will be described in detail in the description of the disclosure. Therefore, terms used in the present disclosure should be defined based on the meaning of the term and the general content of the present disclosure, not simply the name of the term.

When it is said that a certain part "includes" a certain component throughout the specification, it means that it may further include other components without excluding other components unless otherwise stated. In addition, terms such as "...unit" and "module" described in the specification mean a unit that processes at least one function or operation, which may be implemented as hardware or software or a combination of hardware and software. .

Hereinafter, with reference to the accompanying drawings, embodiments will be described in detail so that those skilled in the art can easily carry out the embodiments. However, this disclosure may be implemented in many different forms and is not limited to the embodiments described herein. And in order to clearly describe the present disclosure in the drawings, parts irrelevant to the description are omitted, and the same reference numerals are attached to the same components throughout the specification.

1 is a system diagram 100 showing a switching operation between a load and a grid in a power system.

In Figure 1, the DC voltage 110 can be seen as alternative power. That is, rather than energy generated by electric power companies, it can be seen as a small-scale power source generated by various power generation means, such as renewable energy, as a kind of smart grid. For example, renewable energy by wind power generation and renewable energy by photovoltaic power generation may be exemplified, but are not limited thereto, and energy generated by a small power company is also a source of small power supply by DC voltage 110. It can be.

The DC link 120 is a capacitor bank that maintains the DC voltage by the DC voltage 110 . The inverter 130 may be referred to as a kind of power conversion system (PCS) inverter. However, according to an embodiment of the present disclosure, the inverter 130 of FIG. 1 is a PCS inverter and an independent inverter connected to the load 180 combined into one inverter. The inverter 130 in FIG. 1 transmits DC voltage to six power switch elements SW1 (1301), SW2 (1302), SW3 (1303), SW4 (1304), SW5 (1305), SW6 (1306) is controlled to convert to 3-phase AC voltage and output it. The controller 140 for controlling the six power switch elements SW1 (1301), SW2 (1302), SW3 (1303), SW4 (1304), SW5 (1305), and SW6 (1306) turns on each power switch element. - Outputs the gate control signal 1401 for turning off. In addition, an MC control signal 1403, which is a control signal of the MC (magnetic contactor) 160 for turning on/off the connection between the output of the transformer 150 and the grid, is output.

The transformer 150 boosts or reduces the inverter output voltage. In practical applications, it is often operated as a step-down transformer (380V:220V), and by connecting △-Y, the output connects R, S, T, and four connection lines (R, S, T, N) can come out. The output of transformer 150 may be connected to grid 170 or to load 180 .

That is, the inverter output may be connected to the grid 170 or the load 180, and the connection between the two is basically switched by turning the MC 160 on or off. When the controller 140 turns off the MC 160 so that the inverter 130 is connected to the grid 170 and switched to the load 180 to function as an independent inverter, the inverter 130 controls the current The operation mode must be changed from mode to voltage control mode. However, at this time, since the MC 160 is a mechanical switch, the switching time takes up to 30 ms to 50 ms, so that the inverter temporarily pauses during the relatively long switching time of the MC. The reason is that the magnetization of the transformer 150 is released during the long switching time of the MC 160, so when the inverter 130 starts the instantaneous voltage control mode operation, the transformer 150 in a non-magnetized state is short-circuited. This is because the same overcurrent as the state flows. Therefore, in order to prevent damage or insulation breakdown due to a short circuit in the transformer 150, the inverter 130 has no choice but to take a method of normal operation after establishing the magnetization of the transformer while sending a low current through a soft start. . As a result, from the point of view of the load 180, there is no choice but to be in an instantaneous power failure state. Conversely, when the inverter 130 turns on the MC 160 while driving the load 180, only the operation mode of the inverter 130 is reversed (voltage control mode -> current control mode), and magnetization in the transformer The same phenomenon of unraveling will occur.

Therefore, whenever the MC 160 is turned on or off through the controller 140, the controller 140 has to repeat the operation of restarting the inverter 130 in soft start operation, and from the viewpoint of the load 180, the MC 160 ) during the switching time (30 ms ~ 50 ms), momentary blackout phenomenon inevitably occurs repeatedly.

2 is a waveform showing a soft-start operation of an inverter during a switching operation between a load and a grid in a power system.

Referring to FIG. 2, the Y-axis is the output 201 of the inverter 130 and the X-axis is time (t).

When the inverter 130 is connected to the grid 170 and the connection is switched to the load 180 side, the MC OFF (203) signal turns off the MC 160 simultaneously with the switching signal while operating in the current control mode (205). When outputting , the inverter 130 switches to the voltage control mode 207, and in order to prevent an instantaneous short circuit of the transformer 150, the instantaneous output is set to '0' and a soft start 209 operation is performed so that the transformer 150 prevent short circuits from occurring.

3 is a system diagram showing a switching operation between a load and a grid according to an embodiment of the present disclosure.

Referring to FIG. 3 , in the present system 300 , a switching element 360 such as a thyristor is used instead of the MC 160 used for switching in FIG. 1 . The switching element is a high-power electrically controlled switching element. A thyristor is suitable as a semiconductor element for the high-power electrically controlled switching element, but is not limited thereto. Considering power capacity or other switching environments, GTO, IGBT, power MOSFET, and transistor can be used. However, considering that the system 300 has a large capacity and is connected to the grid 170, a thyristor is the most appropriate switching element.

The controller 140 outputs a switching element control signal 3403 to control the switching element 360 such as a thyristor.

In one embodiment, the inverter 130 connected to the grid 170 and operated in the current control mode turns off the switching element 360 to switch the connection to the load 180. Unlike the MC 160, the switching device has a relatively short switching delay time (hundreds of nsec to several usec). 180, the magnetization of the transformer 150 is not lost. That is, an instantaneous blackout phenomenon as in the case of driving the MC 160 as shown in FIG. 1 does not occur. Therefore, the soft-start operation of the inverter 130 is not required, and the inverter 130 is directly converted from the current control mode to the voltage control mode, as in a stand-up start-up. This is because instantaneous switching is possible even when the switching element 360 is turned on to change from the voltage control mode supplying energy to the load 180 to the current control mode by connecting to the grid 170, so that the transformer 150 loses magnetization. This does not occur and soft-start operation of the inverter 130 is also not required.

4 is a waveform showing operation of an inverter during a load-to-grid switching operation in a power system according to an embodiment of the present disclosure.

Unlike FIG. 2, FIG. 4 is a graph showing a change in inverter output when an electric control switching element is used for grid-load switching, as shown in FIG.

Unlike in FIG. 2, the inverter output 401 turns off the switch 403 of the switching element 360 synchronized with the switching control signal (not shown) during operation in the current control mode 405 when outputting to the grid 170. ) is output from the controller 140, the switching speed of the switch is up to 10 KHz, that is, the switching delay speed is as short as about 100 us, so magnetization loss does not occur in the transformer 150. In addition, since the load 180 side does not regard the same amount of time as an instantaneous power outage, the inverter 130 can momentarily switch the mode to the voltage control mode 407 like a stand-up start-up and output the inverter output as it is. Therefore, since the inverter 130 has no magnetization loss of the transformer 150, there is no need for soft-start operation.

5 is a flowchart illustrating a switching operation method between a load and a grid in which an inverter does not require a soft start operation according to an embodiment of the present disclosure.

In step S501, the input DC voltage, such as renewable energy, is converted into an AC voltage output by the inverter 130. This is the traditional function of inverter 130. At this time, the inverter 130 may be connected to the grid 170 and may be connected to the load 180 side by switching.

In step S503, the transformer 150 boosts or reduces the output AC voltage of the inverter and outputs it. In general, transformers often step down to 220V, but are not limited thereto. In addition, the transformer may have a neutral point (N) connection output at the time of output by Δ-Y connection.

In step S505, the output of the transformer 150 is connected to the load 180 by a load connection line.

In step S507, an electrical control switching element located between the output of the transformer 150 and the grid 170 is turned on so that the output of the transformer is transferred from the load 180 side without the need to soft-start the inverter 130. It is switched to the grid 170 side and output. Since the delay time of the electrically controlled switching element is within 100us, the inverter 130 does not require a soft-start operation considering loss of transformer magnetization.

The methods according to the embodiments may be implemented in the form of program instructions that can be executed through various computer means and recorded on a computer readable medium. The computer readable medium may include program instructions, data files, data structures, etc. alone or in combination. Program instructions recorded on the medium may be those specially designed and configured for the present invention or those known and usable to those skilled in computer software. Examples of computer-readable recording media include magnetic media such as hard disks, floppy disks and magnetic tapes, optical media such as CD-ROMs and DVDs, and magnetic media such as floptical disks. - includes hardware devices specially configured to store and execute program instructions, such as magneto-optical media, and ROM, RAM, flash memory, and the like. Examples of program instructions include high-level language codes that can be executed by a computer using an interpreter, as well as machine language codes such as those produced by a compiler.

Although the embodiments have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concept of the present invention defined in the following claims are also within the scope of the present invention. belongs to

100; Grid Tied Inverter System
110; DC voltage
120; DC link
130; inverter
140; controller
1401; gate control signal
1403; MC control signal
150; Transformers
160; MC
170; grid
180; Load
360; switching element
3403; Switching element control signal
201, 401; inverter output
205, 405; voltage control mode
207, 407; Current control mode
203; MC_OFF
209; soft start
403; switch OFF

Claims (8)

an inverter that converts an input direct current voltage into an output alternating current voltage;
a transformer that boosts or reduces the output AC voltage of the inverter;
a load connection line for connecting the output AC voltage boosted/reduced by the transformer to a load as a transformer output;
an electrically controlled switching element controlled to connect the transformer output to a grid; and
Including a controller for controlling the inverter and the electrical control switching element,
A grid-connected inverter considering transformer magnetization, characterized in that the transformer output is switched to the grid and outputted without the need to soft-start the inverter by a control operation of turning on the electrical control switching element by the controller Device. The grid-connected inverter device according to claim 1, wherein the electric control switching element is a thyristor semiconductor element. The grid-connected inverter device according to claim 1, wherein the electric control switching element is any one of a GTO element, a transistor, and an IGBT element. The transformer magnetization according to claim 1 , wherein the controller causes the transformer output to be switched to the load and outputted without the need to soft-start the inverter by a control operation of turning off the electric control switching element. Grid-tied inverter device considering The transformer magnetization according to claim 1, wherein the output AC voltage is a three-phase AC voltage, and the inverter includes at least six electrically controlled switch elements to convert the input DC voltage to the three-phase AC voltage. Grid-tied inverter device considering The grid-connected inverter device according to claim 1, wherein a predetermined time delay when the electric control switching element is turned on from an off state is within 100 usec. The inverter according to claim 1, wherein the inverter is operated in a current control mode when the output of the transformer is connected to the grid by the electrically controlled switching element, and the output of the transformer is connected only to the load by the electrically controlled switching element. A grid-connected inverter device considering transformer magnetization, characterized in that it is operated in voltage control mode. converting an input direct current voltage into an output alternating current voltage by means of an inverter;
step of upscaling the output alternating current voltage of the inverter by means of a transformer and outputting it as a transformer output;
connecting the output of the transformer to a load by a load connection line; and
Turning on an electrical control switch element located between the transformer output and the grid so that the transformer output is switched from the load to the grid and outputted without the need to soft-start the inverter. A method of operating a grid-connected switch element.

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