KR102420070B1 - System and method for controlling interleaved voltatge source inveter comprising coupled inductors, and a recording medium having computer readable program for executing the method - Google Patents

Abstract

Disclosed are an inverter control system, a method, and a recording medium recording a computer readable program for executing the method. The inverter control system is a control system for controlling an interleaved voltage source inverter system including inductors each connected to a parallel-connected voltage-type inductor and magnetically coupled to each other. includes a control unit. The circulating current calculator calculates a circulating current that circulates through the inverters from currents respectively flowing through the parallel-connected inverters, the zero-phase current separator separates the zero-phase current from the circulating current to calculate a cross current, and the inverter current controller divides the cross current and the zero-phase current. Each of the currents is processed in a preset manner.

Description

SYSTEM AND METHOD FOR CONTROLLING INTERLEAVED VOLTATGE SOURCE INVETER COMPRISING COUPLED INDUCTORS, AND A RECORDING MEDIUM HAVING COMPUTER READABLE PROGRAM FOR EXECUTING THE METHOD}

The present invention relates to a power conversion system and method, and more particularly, to a power conversion system and method having a parallel structure in which output signals of a plurality of inverter modules are combined in an interleaving manner using an anti-phase coupling inductor.

Due to the development of power electronics technology and the increase in the capacity of new and renewable energy, the capacity increase is also required for the grid-connected voltage inverter, and in consideration of economic efficiency and efficiency, we respond to this demand by operating multiple voltage inverters in parallel. are doing

According to this parallel operation method, switching patterns between inverter modules can be implemented differently, and accordingly, when interleaving (phase shift of carriers) is applied, filter inductance or switching frequency can be reduced, and as a result, the size of the system can be reduced. reduced and the efficiency can be improved.

However, direct parallel connection of inverters generally generates circulating current between inverter modules. When interleaving is applied, switching harmonic current is converted into separate switching frequency circulating current and circulating current. Since is added, the circulating current becomes larger compared to the case where interleaving is not applied.

The circulating current can be classified into a low-frequency component and a high-frequency component, and must be treated differently. Accordingly, many studies have been conducted to control the low-frequency circulating current by adjusting the zero-sequence voltage of the inverter, which is also known as the zero-sequence current.

The high frequency circulating current cannot be controlled because it consists of the switching frequency and higher frequency components. Accordingly, a coupled inductor (CI) that inserts a high impedance into a path of a circulating current and uses magnetic coupling to implement a high inductance is mainly used.

1 is a diagram illustrating two parallel three-phase VSIs including a filter and CI. In FIG. 1 , two parallel three-phase voltage source inverters (VSIs) are shown that share a dc-link and do not include an ac isolation transformer. In this case, a low-frequency circulating current is generated. When interleaving is applied, a high-frequency circulating current is added. CI is employed for a high-frequency circulating current, and the filter L and CI may be combined.

2 and 3 are diagrams illustrating an inverse coupled inductor in a single phase. 2 and 3 show CIs used for two parallel three-phase voltage source inverters (VSI). CIs are inversely coupled for suppression of circulating current, and are employed for each phase.

However, when CIs are employed in each phase for a parallel three-phase VSI, a low-frequency cross current (cross current) is induced by a parameter mismatch of CI other than the zero-phase current.

In addition, as shown in FIGS. 2 and 3 , fluxes in CI vary according to current characteristics. 2 shows an example in which only leakage fluxes flow by the same current, and FIG. 3 shows an example in which leakage flux and mutual fluxes flow by different currents, respectively.

This phenomenon (cross current and different impedances) causes a difference in impedance for different types of current, which in turn causes stability problems in the conventional control method using only a zero-phase current controller.

The present invention has been devised to solve the above-mentioned conventional problems, and provides a control method and system capable of improving the stability of an inverter system that is deteriorated by mismatch between inductors coupled to each other used for interleaved inverters. aim to do

In order to achieve the above object, an inverter control system according to the present invention is a control system for controlling an interleaved voltage source inverter system comprising inductors each connected to a parallel-connected voltage-type inductor and magnetically coupled to each other, the circulating current It includes a calculator, a zero-phase current separator, and an inverter current controller.

The circulating current calculator calculates a circulating current that circulates through the inverters from currents respectively flowing through the parallel-connected inverters, the zero-phase current separator separates the zero-phase current from the circulating current to calculate a cross current, and the inverter current controller divides the cross current and the zero-phase current. Each of the currents is processed in a preset manner.

According to this configuration, by separately processing the cross current caused by the mismatch between the inductors coupled to each other, the stability of the inverter system deteriorated by the mismatch between the inductors coupled to each other used for the interleaved inverters is reduced. can be improved

In this case, the circulating current calculator may calculate the circulating current by the difference between the inverter currents, and the inverter current controller further includes a first proportional resonance processor for proportional resonance processing of the cross current, and a proportional integration processor for proportional integration processing for zero current. can do.

In addition, it may further include a transfer current calculator for calculating a transfer current for transmitting power to the outside, the transfer current calculator may calculate the transfer current by the sum of the inverter currents.

In this case, the inverter current control unit may further include a second proportional resonance processing unit for proportional resonance processing of the transmitted current and a harmonic compensator for compensating for harmonics in the transmitted current.

In addition, the invention implementing the system in the form of a method and a recording medium recording a computer readable program for executing the method are disclosed together.

According to the present invention, the stability of the inverter system deteriorated by the mismatch between the inductors coupled to each other used for the interleaved inverters is improved by separately processing the cross current caused by the mismatch between the inductors coupled to each other. can be improved

1 is a diagram illustrating two parallel three-phase voltage source inductors (VSIs) including a filter and a coupled inductor (CI).
2 and 3 are diagrams illustrating an inverse coupled inductor in a single phase;
4 is a schematic block diagram of an inverter control system according to an embodiment of the present invention;
5 is a block diagram of a conventional current control system for a parallel three-phase voltage source inverter.
6 and 7 are equivalent circuits of a reverse-coupled inductor.
8 and 9 are equivalent circuits of an inverter system that change according to current characteristics.
10 is a view showing an actual implementation example of the inverter control system of FIG.
11 to 16 are graphs showing inverter system output data according to a conventional system.
17 to 20 are graphs showing inverter system output data according to the system of the present invention.

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

4 is a schematic block diagram of an inverter control system according to an embodiment of the present invention. The inverter control system 100 of FIG. 4 is a control system for controlling an interleaved voltage source inverter system including inductors each connected to a parallel-connected voltage-type inductor and magnetically coupled to each other, and a circulating current calculator 110 . , a zero phase current separator 120 , an inverter current controller 130 , and a transfer current calculator 140 .

In FIG. 4 , the inverter current control unit 130 again includes a proportional resonance processing unit 132 , a proportional integral processing unit 134 , and a harmonic compensator 136 , and the proportional resonance processing unit 132 is again a first proportional resonance processing unit 132-1 and a second proportional resonance processing unit 132-2.

In FIG. 4 , each component of the inverter control system 100 may be implemented with only hardware, but will generally be implemented with hardware and software operating on the hardware.

The circulating current calculator 110 calculates a circulating current circulating through the inverters from currents flowing through the parallel-connected inverters, respectively. In this case, the circulating current calculator 110 may calculate the circulating current based on the difference between the inverter currents.

The zero-phase current separator 120 separates the zero-phase current from the circulating current to calculate a cross current, and the inverter current controller 130 processes the cross current and the zero-phase current in a preset manner, respectively.

Accordingly, the first proportional resonance processing unit 132-1 performs proportional resonance processing on the cross current, and the proportional integration processing unit 134 performs proportional integration processing on the zero-phase current.

The transfer current calculator 140 calculates a transfer current for transmitting power to the outside. In this case, the transfer current calculator 140 may calculate the transfer current by the sum of the inverter currents.

The second proportional resonance control unit 132-2 performs proportional resonance processing on the transmitted current, and the harmonic compensator 136 compensates for harmonics in the transmitted current.

Hereinafter, the embodiment will be described in more detail with more specific examples.

5 is a block diagram of a conventional current control system for a parallel three-phase voltage source inverter. In FIG. 5, the conventional current control system consists of controllers for individual inverter currents and a controller for zero current, and a harmonic compensator for compensating fifth and seventh harmonic currents for the two inverter current controllers; HC) is added. The conventional current controller concentrates only on zero current with respect to the circulating current.

The individual inverter currents may be controlled by a PI controller on a rotating dq-axis coordinate system, but both positive and negative dq-axis currents require separate PI controllers, which complicates the controller.

Since only two proportional-resonant (PR) controllers are required in an orthogonal stationary reference frame (αβ axis), the current controller can be implemented more simply. Therefore, in the present invention, the system stability is analyzed using the PR controller for the αβ axis current.

A conventional current controller converts a three-phase inverter current into an αβ -axis current and a zero sequence. In this process, only the zero current is extracted, and as a result, the cross current (i _Clα,cir and i _Clβ,cir ) is the inverter current (i _Clα and i _Clβ ). The converted inverter current is as follows.

i _{CI α} = i _{CI α, pow} + i _{CI α, cir}

i _{CI β} = i _{CI β, pow} + i _{CI β, cir}

i _{CI z} = i _{CI z, cir}

However, in an inverter system including a reverse-coupled inductor, current i _{CI αβ, pow} for power flow and the current i _{CI αβ,cir for circulation is} Because they have different inductances, the current controller gain is proportional to each other's inductances. should be chosen differently.

6 and 7 are equivalent circuits of a reverse-coupled inductor, and FIGS. 8 and 9 are equivalent circuits of an inverter system that change according to current characteristics. 8 shows an equivalent circuit for power flow, and FIG. 9 shows an equivalent circuit for current circulation.

However, conventionally, since the αβ-axis current includes both current for power transmission and circulation, it has been impossible to individually select different gains. Although this was not a problem in fundamental current control, the controllers for harmonic compensation such as 5th and 7th made the system unstable.

That is, since the filter inductances are reduced by interleaving, the low-frequency harmonics are seriously raised, and this problem must be solved in order to meet the harmonic limit in the grid.

In other words, the cross current (i _Clα,cir and In the absence of i _Clβ,cir ), the inverter current ( i _{CI αβ} ) can be considered only as a transfer current ( i _{CI αβ, pow} ) for power transfer, but in the _presence of a cross current, this current is _{αβ, pow} ) and flow in a different path. Since this results in a different equivalent inductance, the current controller gain must be chosen for its individual inductance.

However, in the control system of Fig. 5, since the cross current is included in the same control loop as the transfer current i _{CI αβ, pow ,} it is impossible to select a different control gain, and the cross current is the transfer current i _{CI αβ , pow} ) had to take the same control gain.

FIG. 10 is a diagram illustrating an actual implementation example of the inverter control system of FIG. 4 . Fig. 10 shows a proposed current control system for parallel VSI employing a coupled inductor (CI).

As already explained, low inductance can be used by interleaving, but this causes low frequency harmonics like the 5th and 7th harmonics. These harmonics must be reduced to meet the grid's proposed harmonic limits.

However, in the conventional current control method, a resonance controller for harmonic compensation cannot be applied by the coupled inductor CI. The present invention proposes a new current control method to solve this stability problem.

Since the conventional current control method extracts only zero current, a cross current is included in the current control loop for power transmission. Accordingly, the remaining cross current due to insufficient control gain in terms of equivalent inductance causes a stability problem (cf., L cir ≫ L pow).

Accordingly, in the present invention, it is an object of the present invention to remove the cross current from the current for power transmission, and for this, the inverter current is divided into a current for power transmission and a circulating current.

The current for power transfer i _{CI abc, pow} is obtained by summing the individual inverter currents, which are controlled by the PR controllers using the reference currents. The remaining currents are considered as circulating currents ( i _{CI abc , cir} ) that are zero current and cross current, and are obtained by the difference between individual inverter currents. The extracted circulating currents are separated into cross current and zero current, and are individually controlled by PR and PI controllers.

According to the present invention, the use of control gain for individual equivalent circuits is possible, and consequently the cross current by the coupled inductor CI and the stability problem caused by the different inductances are solved by the separation of the currents.

11 to 16 are graphs showing inverter system output data according to a conventional system. Fig. 11 shows the steady-state response before harmonic compensation, Fig. 12 shows the transient response after harmonic compensation for the 5th and 7th harmonics, Fig. 13 shows the magnified waveform at the moment of harmonic compensation, and Fig. 14 shows the magnification after about 18 seconds of harmonic compensation. FIG. 15 shows an enlarged waveform after about 24 seconds of harmonic compensation, and FIG. 16 shows an enlarged waveform after about 25 seconds of harmonic compensation. The sampling rates are different due to different time divisions, which are 12.5 MS/s in FIG. 11 and 200 kS/s in FIGS. 12 to 16 .

11 to 16, the inverter currents in the A phase, the difference between the A phase currents, and the zero phase currents are output. In the figures the inverter current and other currents are stable until the moment of compensation. However, the system is slowly distorted by the 5th and 7th harmonic currents and then trips out by an over current (OC) fault after 25 seconds, and when the system trips all currents are severely distorted. can be checked In addition, the instability phenomenon starts from the circulating current, and it can be confirmed that it affects the current for power transmission.

17 to 20 are graphs showing inverter system output data according to the system of the present invention. Fig. 17 shows the steady-state response before applying harmonic compensation, Fig. 18 shows the transient response after compensation for the 5th and 7th harmonics, Fig. 19 shows the enlarged waveform at about 35 seconds after harmonic compensation, and Fig. 20 shows the harmonics Steady-state responses after compensation are shown respectively.

The sampling rates are different due to different time divisions, which are 12.5 MS/s in FIGS. 17 and 20 and 200 kS/s in FIGS. 18 and 19 . The current controller gain is the same as for the conventional system.

Before harmonic compensation, the current distortion due to the low-frequency harmonic current is similar to the case of the conventional controller, but after that, it is stable unlike the case of the conventional controller. It can be seen that there is no distortion in all currents even after 35 seconds.

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.

100: inverter control system
110: circulating current calculation unit
120: zero current separation unit
130: inverter current control unit
132: proportional resonance processing unit
132-1: first proportional resonance processing unit
132-2: second proportional resonance processing unit
134: proportional integral processing unit
136: harmonic compensation unit

Claims (15)

A control system for controlling an interleaved voltage source inverter system comprising inductors each coupled to a parallel-connected voltage-type inverter and magnetically coupled to each other, the system comprising:
a circulating current calculating unit for calculating a circulating current circulating through the inverters from currents flowing through the parallel-connected inverters;
a zero-phase current separator for calculating a cross current by separating a zero-phase current from the circulating current; and
and an inverter current controller configured to respectively process the cross current and the zero phase current in a preset manner.
The method according to claim 1,
The circulating current calculator calculates the circulating current based on a difference between the inverter currents.
3. The method according to claim 2,
The inverter current control unit comprises a first proportional resonance processing unit for proportional resonance processing the cross current Inverter control system, characterized in that.
4. The method according to claim 3,
The inverter control system according to claim 1, wherein the inverter current control unit further comprises a proportional integral processing unit for proportionally integrating the zero-phase current.
5. The method according to claim 4,
Inverter control system, characterized in that it further comprises a transfer current calculation unit for calculating the transfer current for transmitting power to the outside.
6. The method of claim 5,
The transfer current calculator calculates the transfer current by the sum of the inverter currents.
7. The method of claim 6,
The inverter current control unit further comprises a second proportional resonance processing unit for proportional resonance processing of the transmitted current and a harmonic compensator for compensating for harmonics in the transmitted current.
A control method for controlling an interleaved voltage source inverter system comprising inductors each connected to a parallel-connected voltage-type inverter and magnetically coupled to each other, comprising:
a circulating current calculating step of calculating a circulating current circulating through the inverters from currents flowing through the parallel-connected inverters;
a zero-phase current separation step of calculating a cross-current by separating a zero-phase current from the circulating current; and
and an inverter current control step of respectively processing the cross current and the zero phase current.
9. The method of claim 8,
The circulating current calculating step is an inverter current control method, characterized in that calculating the circulating current by the difference between the inverter currents.
10. The method of claim 9,
The inverter current control step is an inverter current control method, characterized in that it comprises a proportional resonance processing step of proportional resonance processing the cross current.
11. The method of claim 10,
Inverter current control method, characterized in that the inverter current control step further comprises a first proportional integral processing step of proportional integration processing the zero-phase current.
12. The method of claim 11,
Inverter current control method, characterized in that it further comprises a transfer current calculation step of calculating a transfer current for transmitting power to the outside.
13. The method of claim 12,
The method of calculating the transfer current comprises calculating the transfer current by the sum of the inverter currents.
14. The method of claim 13,
The inverter current control step further comprises a second proportional resonance processing step of proportional resonance processing of the transmitted current and a harmonic compensation step of compensating for harmonics in the transmitted current.
A recording medium recording a computer readable program for executing the method of any one of claims 8 to 13.

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