WO2021256850A1

WO2021256850A1 - System for converting operation mode of grid-connected inverter

Info

Publication number: WO2021256850A1
Application number: PCT/KR2021/007543
Authority: WO
Inventors: 권민호; 이종필; 오창열; 김기룡; 제갈준혁
Original assignee: 한국전기연구원
Priority date: 2020-06-16
Filing date: 2021-06-16
Publication date: 2021-12-23
Also published as: KR20210155582A

Abstract

A system for converting an operation mode of a grid-connected inverter is a system for converting an operation mode of a grid-connected inverter, the system having an inductor (Li) and a capacitor (Cf) for converting an output voltage of an inverter into a sinusoid, and the system comprising: an output current calculation unit; a voltage control unit; a wavelength control unit; an inductor current calculation unit; and an output voltage calculation unit. The output current calculation unit calculates a real axis and an imaginary axis output current target value (lod*, loq*) of a grid-connected inverter according to power setpoints (P*, Q*), the voltage control unit calculates a real axis current compensation value (ΔId) by using a voltage setpoint of capacitance, the wavelength control unit calculates an imaginary axis current compensation value (ΔIq) by using a wavelength setpoint of a grid, the inductor current calculation unit calculates a real axis and an imaginary axis current setpoint (Iid*, Iiq*) of an inductor by using the output current target values, the voltage of a capacitor, the real axis current compensation value, and the imaginary axis current compensation value, and the output voltage calculation unit calculates output voltage setpoints (Via*, Vib*, Vic*) of the inverter by using a current measurement value of the inductor and the inductor current setpoints.

Description

Grid-connected inverter operation mode conversion system

The present invention relates to a power converter control related technology, and more particularly, to a system for switching an operation mode for seamless voltage control in a grid-connected inverter having a significant load.

As smart grids are emerging to expand the use of renewable energy such as solar and wind power, research on distributed power generation systems based on renewable energy is being actively conducted. At this time, the microgrid, which is a form of a small power grid fused with solar power, wind power, energy storage, etc., is classified into an AC microgrid that is connected based on AC power and a DC microgrid that is linked based on DC power. can

Recently, as technologies of DC-based devices such as solar power, batteries, and LEDs develop, interest in DC microgrids based on DC distribution is increasing. In this regard, it will be described with reference to FIG. 1 .

1 is a conceptual diagram showing a schematic configuration of a DC microgrid. DC microgrid consists of a structure in which several power conversion devices are tied to one bus (DC-bus). Since these devices are separated from each other, a distributed control method that autonomously participates in power control is essential. Here, an energy storage system (ESS) and a grid-connected inverter play a pivotal role in controlling and managing power in the grid.

In particular, in the DC microgrid, the grid-connected inverter is located between the upper grid and the microgrid, and when the microgrid operates in grid-connected mode, it controls the incoming and outgoing power from the upper grid or operates as a basic voltage source of the microgrid.

In addition, if the microgrid is switched to the independent operation mode when the upper system fails, the grid-connected inverter requires a voltage control operation for the AC-side load. In other words, in a grid-connected inverter system to which a critical load that requires stable power supply is connected, when an independent operation situation occurs in which the power supply is interrupted due to a grid error, the load voltage may fluctuate greatly depending on the output power and load conditions. can

In this case, when the grid is restarted, the power conversion system and important loads can be greatly damaged due to the difference in phase and output voltage between the grid and the inverter. It is necessary to disconnect the connection and control the voltage of the critical load. At this time, a mode switching technique that can supply a stable voltage to the power converter and the critical load is essential.

A PPL-based mode conversion method has been proposed as a mode conversion method of a conventional grid-connected inverter. This is a method to select and control the controller to be activated or deactivated according to the operation mode by summing the outputs of the current controller and the voltage controller. However, mode conversion without a transient state is possible, but until the grid-connected inverter recognizes independent operation. There is a problem in that the voltage fluctuation is severe in the time period.

In addition, an indirect current control technique has been proposed as a mode switching technique of a conventional grid-connected inverter. This is a technique that performs voltage control in both the grid-connected mode and the independent operation mode. By performing voltage control even during the independent operation of the grid-connected inverter, the voltage fluctuation is small and mode conversion without a transient state is possible. There is this.

However, the indirect current control method is a control algorithm limited to an inverter having an LCL filter, and there is a limitation in that it cannot be applied to a system using an LC filter. In addition, since the LCL filter indirectly controls the grid-side current, not the inverter-side current, the higher the order of the current controller, the higher the controller design difficulty, which makes it difficult to design a wide controller bandwidth. There is a limitation that it is not suitable.

Therefore, there is a need to propose a grid-connected inverter operating method that can be applied to various types of filters and can perform stable voltage control for important loads.

Accordingly, by changing the operation mode by autonomous judgment, the communication dependence is lowered, maintaining the advantage of the distributed control base. Methods are being tried.

2 is a schematic circuit diagram of a general grid-connected three-phase inverter having a significant load, and FIG. 3 is a control block diagram showing a schematic configuration of an operation mode switching system of a conventional grid-connected inverter.

The operation mode conversion system of the grid-connected inverter of FIG. 3 calculates the amount of change in the load voltage according to the grid current before and after the circuit breaker is opened when the breaker of the grid-connected inverter is opened, and based on the amount of change, a plurality of By operating at least one of the external voltage controller loops to control the voltage of the critical load, it is possible to supply a stable voltage to the critical load even in the independent operation mode.

However, according to the system of FIG. 3, when the inverter is switched to independent operation, the capacitor voltage (Vc) value is at the same node as the grid voltage (Vg), so they are measured to be the same as each other, and accordingly, the Vc, q values It has a value as much as ΔV. Therefore, the Vg,q value also has a finite value as much as ΔV, and this value not only greatly changes the frequency (ω) of the output voltage through the controller of the PLL but also saturates the controller.

4 is a diagram illustrating a phase angle calculation process in a PLL technique. In a grid-connected inverter, a technique called PLL (Phase Locked Loop) is widely used as an essential technology to follow the frequency or phase of the grid and create a phase angle (θ) required for synchronous coordinate conversion.

In addition, the system of FIG. 3 can be limitedly applied only in the balanced state of the three-phase load, and there is a problem of causing the unbalance of the three-phase line voltage in the unbalanced state of the load.

The present invention has been devised to solve the above-mentioned conventional problems, and the operation mode conversion system of a grid-connected inverter capable of controlling the output voltage so as not to saturate the PLL controller by diverging the frequency of the output voltage even when switching independent operation is intended to provide

Another object of the present invention is to provide an operation mode switching system of a grid-connected inverter capable of supplying a balanced voltage to a load even when the load is in an unbalanced state.

In order to achieve the above object, the operation mode conversion system of the grid-connected inverter according to the present invention is a grid-connected inverter operation mode conversion system having a capacitor (Cf) and an inductor (Li) for converting the output voltage of the inverter into a sine wave As such, it includes an output current calculation unit, a voltage control unit, a frequency control unit, an inductor current calculation unit, and an output voltage calculation unit.

The output current calculation unit calculates the real axis and imaginary axis output current target values (Iod*, Ioq*) of the grid-connected inverter according to the power command values (P*, Q*), and the voltage control unit uses the voltage command value of the capacitance to calculate the real axis The current compensation value (ΔId) is calculated, the frequency control unit calculates the imaginary contraction current compensation value (ΔIq) using the frequency command value of the system, and the inductor current calculation unit calculates the output current target value, the voltage of the capacitor, the real axis current compensation value, The real axis and imaginary axis current command values (Iid*,Iiq*) of the inductor are calculated using the imaginary axis current compensation value, and the output voltage calculator uses the inductor current setpoint and the measured current of the inductor to calculate the output voltage setpoint (Via) of the inverter. *, Vib*, Vic*) are calculated.

According to such a configuration, the control loop for the grid-connected mode and the control loop for the independent operation mode are integrated into one, and the saturation and activation of the compensator of each loop can be switched autonomously and seamlessly according to the system state.

Therefore, it is possible to independently operate the microgrid without interruption even in case of a sudden power outage, and it is possible to reduce the burden of communication equipment cost due to low communication dependence with the upper controller.

At this time, the output voltage calculation unit converts the real and imaginary axis current command values of the inductor into current command values for three phases, respectively, and the output voltage command value using the converted 3-phase inductor current command value and the inductor current measurement value. It may include a resonance type (PR) control unit to calculate. According to this configuration, it is possible to supply a balanced voltage to the load even when the load is in an unbalanced state by using three PR controllers for controlling the instantaneous value of each current of the three phases.

In addition, the frequency control unit, the imaginary axis upper control including an imaginary axis upper compensation unit for calculating the imaginary axis upper current compensation value using the frequency error calculated by adding a preset frequency tolerance to the error between the frequency command value and the frequency measurement value of the system It includes a loop and an imaginary axis lower control loop including an imaginary axis lower compensation unit that calculates an imaginary axis lower current compensation value using the frequency error calculated by subtracting a preset frequency tolerance from the error between the frequency command value and the frequency measurement value of the system can do. In this case, the imaginary axis current compensation value ΔIq may be the sum of the output value of the imaginary axis upper control loop and the output value of the imaginary axis lower control loop.

According to this configuration, it is possible to prevent the system from becoming out of control by maintaining the imaginary contraction capacitor voltage (Vc,q) at 0 to diverge the output frequency of the system or saturating the PLL controller.

In addition, the imaginary axis upper control loop further includes an imaginary axis upper limiter for limiting the output upper limit of the imaginary axis upper compensator, and the imaginary axis lower control loop further includes an imaginary axis lower limiter for limiting the output lower limit of the imaginary axis lower compensator can do.

In addition, the voltage controller includes a real-axis upper compensation unit that calculates a real-axis upper current compensation value using the voltage error calculated by adding a preset voltage tolerance to the error between the real-axis command value of the capacitor voltage and the real-axis measurement value of the capacitor voltage. real-axis upper control loop, and the real-axis lower part that calculates the real-axis lower current compensation value using the voltage error calculated by subtracting the preset voltage tolerance from the error between the real-axis command value of the capacitor voltage and the real-axis measurement value of the capacitor voltage It may include a real-axis lower control loop including a compensation unit. In this case, the real-axis current compensation value ΔId may be the sum of the output value of the upper control loop and the output value of the lower control loop.

In addition, the upper real axis control loop further includes a real axis upper limiter for limiting the output upper limit of the real axis upper compensation unit, and the real axis lower control loop further includes a real axis lower limiter for limiting the output lower limit of the real axis lower compensation unit can do.

According to the present invention, the control loop for the grid-connected mode and the control loop for the independent operation mode are integrated into one, and the switching of saturation and activation of the compensator of each loop can be performed autonomously and seamlessly according to the system state.

In addition, by using three PR controllers for controlling the instantaneous value of each current of the three phases, it is possible to supply a balanced voltage to the load even if the load is in an unbalanced state.

In addition, it is possible to prevent the system from becoming out of control by maintaining the imaginary contraction capacitor voltage (Vc,q) at 0 to diverge the output frequency of the system or saturating the PLL controller.

1 is a conceptual diagram showing a schematic configuration of a DC microgrid.

Figure 2 is a schematic circuit diagram of a general grid-connected three-phase inverter having a significant load.

3 is a control block diagram showing a schematic configuration of an operation mode switching system of a conventional grid-connected inverter.

4 is a diagram illustrating a phase angle calculation process in a PLL technique.

5 is a control block diagram showing a schematic configuration of an operation mode switching system of a grid-connected inverter according to an embodiment of the present invention.

6 is a diagram illustrating in more detail the voltage control unit and the frequency control unit of FIG. 5 .

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

5 is a control block diagram illustrating a schematic configuration of an operation mode conversion system of a grid-connected inverter according to an embodiment of the present invention.

In FIG. 5, the operation mode conversion system of the grid-connected inverter is an operation mode conversion system of the grid-connected inverter having a capacitor (Cf) and an inductor (Li) for converting the output voltage of the inverter into a sine wave, and an output current calculation unit 110 , a voltage controller 120 , a frequency controller 130 , an inductor current calculator 140 , and an output voltage calculator 150 .

The voltage control unit 120 again includes a real axis upper control loop 122 and a real axis lower control loop 124 , and the frequency control unit 130 includes an imaginary axis upper control loop 132 and an imaginary axis lower control loop ( 134).

The output current calculation unit 110 calculates the real axis and imaginary axis output current target values (Iod*, Ioq*) of the grid-connected inverter according to the power command values (P*, Q*).

The voltage controller 120 calculates a real-axis current compensation value ΔId by using the voltage command value of the capacitance. 6 is a diagram illustrating in more detail the voltage control unit and the frequency control unit of FIG. 5 .

The real axis upper control loop 122 includes a real axis upper compensation unit 122-1 and a real axis upper limiting unit 122-2, and the real axis lower control loop 124 includes a real axis lower compensation unit 124 . -1) and a real-axis lower limiter 124-2, and the real-axis current compensation value ΔId is the sum of the output value of the upper control loop and the output value of the lower control loop.

Real-axis upper compensation unit 122-1 calculates a real-axis upper current compensation value using the voltage error calculated by adding a preset voltage tolerance to the error between the real-axis command value of the capacitor voltage and the real-axis measurement value of the capacitor voltage, The real-axis upper limiter 122-2 limits the upper limit of the output of the real-axis upper compensation part 122-1.

The real axis lower compensation unit 124-1 calculates a real axis lower current compensation value using the voltage error calculated by subtracting a preset voltage tolerance from the error between the real axis command value of the capacitor voltage and the real axis measurement value of the capacitor voltage, and , the real-axis lower limiter 124-2 limits the output lower limit of the real-axis lower control loop 124 of the real-axis lower compensator 124-1.

The frequency control unit 130 calculates the imaginary contraction current compensation value ΔIq using the frequency command value of the system. To this end, the imaginary axis upper control loop 132 includes an imaginary axis upper compensator 132-1 and an imaginary upper limiter 132-2, and the imaginary lower control loop 134 includes an imaginary lower axis compensation unit. It includes a part 134-1 and an imaginary-axis lower limiter 134-2, and the imaginary-axis current compensation value ΔId is the sum of the output value of the upper control loop and the output value of the lower control loop.

The imaginary axis upper compensation unit 132-1 calculates the imaginary axis upper current compensation value using the frequency error calculated by adding a preset frequency tolerance to the error between the frequency command value and the frequency measurement value of the system, and the imaginary axis upper limiter Reference numeral 132-2 limits the upper limit of the output of the imaginary contraction upper compensator 132-1.

The lower imaginary axis compensator 134-1 calculates the imaginary axis upper current compensation value using the frequency error calculated by subtracting the preset frequency tolerance from the error between the frequency command value and the frequency measurement value of the system, and the imaginary axis lower limiter Reference numeral 134-2 may further include an imaginary contraction lower limiting unit that limits the lower limit of the output of the imaginary contraction lower compensating unit 134-1.

Conventionally, when the imaginary contraction capacitor voltage (Vc, q) is not 0, as a configuration for solving the problem that the block PLL for adjusting the phase does not operate, in FIG. 6 ,

is a factor that tracks the phase of the system, and should ideally be 0. In FIG. 6 , Δω among the inputs of the frequency control unit 130 is a value within a preset allowable range, 0 is a system frequency of 60Hz,

may be replaced with the frequency value of each measured system.

When this increases or decreases, the output frequency of the system may diverge. thus,

If ω is maintained near the system frequency of 60Hz by controlling ω to 0, the capacitor voltage becomes zero even when the system goes to independent operation, and the system frequency follows 60Hz well.

The inductor current calculating unit 140 calculates real-axis and imaginary-axis current command values Iid*, Iiq* of the inductor by using the output current target value, the voltage of the capacitor, the real-axis current compensation value, and the imaginary-axis current compensation value.

The output voltage calculator 150 calculates the output voltage command values Via*, Vib*, Vic* of the inverter using the inductor current command value and the inductor current measurement value. At this time, the output voltage calculator 150 includes a coordinate converter 152 that converts the real and imaginary axis current command values of the inductor into current command values for three phases, respectively, and the converted three-phase inductor current command value and the measured current of the inductor. may include a resonance type (PR) control unit 154 that calculates an output voltage command value by using .

According to such a configuration, the output voltage can be stably maintained even when the load is unbalanced by controlling the currents of each phase and simultaneously controlling the positive, negative, and zero sequences. In particular, it is possible to effectively control alternating current by using a resonant controller.

In summary, the present invention integrates the control loop for the grid-connected mode and the control loop for the independent operation mode into one, and by using the limiter and anti-windup technique, the saturation and activation of the compensator of each loop is autonomous according to the system state. and designed to be seamless.

Therefore, it is possible to independently operate the microgrid without interruption even in the event of a sudden power outage, and has the advantage of reducing the burden of communication equipment costs due to low communication dependence with the upper controller.

The present invention is composed of three PR controllers for controlling the instantaneous value of each current in three phases and a voltage/frequency controller operating only in case of a system failure. A frequency controller is introduced instead of a voltage (Vc,q) controller on the imaginary side. In addition, instead of deleting the synchronous coordinate-based PI controller, a resonance type (PR) controller for controlling the instantaneous current of each phase is introduced.

Conventionally, due to a certain finite value of Vc,q, the output frequency may diverge or saturate the PLL controller to become uncontrollable, whereas by introducing a frequency controller in the present invention, the Vc,q value can be maintained at zero. there will be

In addition, unlike when a synchronous coordinate-based PI controller is employed, which can be used only in the parallel state of a three-phase load, by introducing a resonance type (PR) controller for controlling the instantaneous current of each phase, the balanced voltage is can be supplied to the load.

Although the present invention has been described with reference to some preferred embodiments, the scope of the present invention should not be limited thereto, but should also extend to modifications or improvements of the above embodiments supported by the claims.

Claims

An operation mode conversion system of a grid-connected inverter having a capacitor (Cf) and an inductor (Li) for converting the output voltage of the inverter into a sine wave,

an output current calculation unit for calculating real and imaginary axis output current target values (Iod*, Ioq*) of the grid-connected inverter according to the power command values (P*, Q*);

a voltage controller for calculating a real-axis current compensation value ΔId using the voltage command value of the capacitor;

a frequency control unit for calculating an imaginary contraction current compensation value (ΔIq) by using the frequency command value of the system;

Inductor current calculation for calculating real-axis and imaginary-axis current command values (Iid*,Iiq*) of the inductor using the target output current, the voltage of the capacitor, the real-axis current compensation value, and the imaginary-axis current compensation value wealth; and

and an output voltage calculator for calculating an output voltage command value of the inverter using the current command value of the inductor and the current measurement value of the inductor.
The method according to claim 1,

The output voltage calculator includes: a coordinate converter for converting real and imaginary current command values of the inductor into current command values for three phases, respectively; and

and a resonance type (PR) control unit for calculating the output voltage command value using the current command value of the three-phase inductor and the current measurement value of the three-phase inductor.
The method according to claim 2, wherein the frequency control unit

an imaginary axis upper control loop including an imaginary axis upper compensator for calculating an imaginary upper current compensation value using a frequency error calculated by adding a preset frequency tolerance to an error between a frequency command value and a frequency measurement value of the system; and

An imaginary axis lower control loop including an imaginary axis lower compensator for calculating an imaginary axis lower current compensation value using the frequency error calculated by subtracting a preset frequency tolerance from the error between the frequency command value and the frequency measurement value of the system,

The imaginary-axis current compensation value (ΔIq) is the sum of the output value of the imaginary-axis upper control loop and the imaginary-axis lower control loop.
4. The method according to claim 3,

The imaginary axis upper control loop further includes an imaginary axis upper limiter for limiting the upper limit of the output of the imaginary axis upper compensation unit,

The imaginary axis lower control loop is an operation mode conversion system of the grid-connected inverter, characterized in that it further comprises an imaginary axis lower limiter for limiting the output lower limit of the lower imaginary axis compensation unit.
The method according to claim 4, wherein the voltage control unit

A real axis including a real axis upper compensator for calculating a real axis upper current compensation value using a voltage error calculated by adding a preset voltage tolerance to an error between the real axis command value of the capacitor voltage and the real axis measurement value of the capacitor voltage upper control loop; and

A real axis including a real axis lower compensator for calculating a real axis lower current compensation value using a voltage error calculated by subtracting a preset voltage tolerance value from an error between the real axis command value of the capacitor voltage and the real axis measurement value of the capacitor voltage a lower control loop;

The real-axis current compensation value (ΔId) is the operation mode conversion system of the grid-connected inverter, characterized in that the sum of the output value of the upper control loop and the output value of the lower control loop.
6. The method of claim 5,

The real-axis upper control loop further includes a real-axis upper limiter for limiting the upper limit of the output of the real-axis upper compensation unit,

The real axis lower control loop is an operation mode conversion system of the grid-connected inverter, characterized in that it further comprises a real axis lower limiter for limiting the output lower limit of the real axis lower compensation unit.