KR101936564B1 - Apparatus for controlling multilevel inverter - Google Patents

Abstract

A multi-level inverter control apparatus is disclosed. In a high-voltage inverter system including a plurality of power cells connected in series and synthesizing a voltage applied to an electric motor, an apparatus of the present invention for controlling the power cell is configured to control a DC voltage stored in a DC link capacitor A plurality of switching elements for converting an AC voltage into an AC voltage and a cell controller for controlling pulse width modulation of the switching element, wherein the cell controller controls a power factor of output power of the plurality of power cells, Respectively.

Description

[0001] APPARATUS FOR CONTROLLING MULTILEVEL INVERTER [0002]

The present invention relates to a multilevel inverter control apparatus.

In the electric power industry, there is a great need to develop an apparatus for operating and configuring an efficient and flexible electric power system such as a flexible AC transmission system (FACTS). In addition, The development of the facility has brought about the high voltage and high capacity inverter which is a power conversion device for variable frequency and DC-AC conversion. In recent years, multi-level inverters have become increasingly popular as topologies for such high-voltage high-capacity inverter systems.

Multi-level inverters are divided into diode clamp inverter, cascade H-bridge inverter, and flying capacitor inverter. Among them, a cascade H-bridge inverter connects low-voltage H-bridge cells in series to adjust the total output voltage by controlling the number of cells according to the inverter's output voltage.

Since the cascaded H-bridge inverters generate the final output voltage by series connection of cells, the stabilization of the output voltage of each cell affects the quality of the output voltage. Since the voltage of each cell is shifted with the phase difference, A phase difference is caused by the difference between the current and the variable voltage. The output power of the cell changes instantaneously due to the phase difference, and if it is corrected in each cell, the output voltage of each cell can be stabilized and output.

The present invention is directed to a multi-level inverter control apparatus for controlling the output power of a unit power cell of a high voltage inverter by controlling power factor, power balance and dead time compensation of each unit cell of a high voltage inverter .

According to an aspect of the present invention, there is provided a high voltage inverter system including a plurality of power cells connected in series to synthesize a voltage applied to a motor, A plurality of switching elements for converting a DC voltage stored in the DC link capacitor into an AC voltage of a predetermined magnitude and frequency; And a cell controller for controlling the pulse width modulation of the switching element, wherein the cell controller controls the power factor of the output power of the plurality of power cells to control the output powers of the plurality of power cells equally have.

에서, 각 전력셀에서 출력되는 전압의 천이값에서, 직렬로 연결되는 전력셀의 수에 따라 α를 가변하여 역률을 제어할 수 있다.In an embodiment of the present invention,

The power factor can be controlled by varying? According to the number of power cells connected in series at the transition value of the voltage output from each power cell.

In one embodiment of the present invention, the cell controller includes: a voltage detector for detecting a voltage of the DC link capacitor; And a compensation unit for determining a compensation voltage to be compensated for an output voltage of the power cell, based on a voltage error of the DC link capacitor, an offset of an input voltage, and an initial voltage detection error.

In an embodiment of the present invention, the compensation unit may be a proportional (P) controller or a proportional integral (PI) controller.

In one embodiment of the present invention, the cell controller includes: a voltage detector for detecting a current flowing in the power cell; A polarity discrimination unit for discriminating a polarity of a current flowing in the power cell; And a compensation voltage determination unit for determining a compensation voltage considering the DC link voltage and the dead time according to the polarity of the current flowing in the power cell to compensate the reference voltage.

In one embodiment of the present invention, the polarity determination unit can detect the polarity of the current by calculating the slope of the current flowing in the power cell.

The present invention can flexibly design a phase displacement transformer used in the input stage of the multi-level inverter, thereby increasing the degree of freedom in system design and achieving stabilization of the output voltage of the entire system. It is effective to raise price competitiveness.

Further, according to the present invention, since the final line-to-line voltage is stabilized, the current ripple is reduced, and when a large capacity is applied, the transient state such as hunting can be easily stabilized and the life of the motor can be increased.

In addition, according to the present invention, stabilizing the electric power that is conducted on the average in the inverter increases the efficiency and life of the system and reduces the capacitance of the capacitor that occupies a large portion of the size of the power cell, Since the size can be reduced, it has the effect of reducing the overall size of the system.

1 is a configuration diagram of a multi-level inverter system to which an embodiment of the present invention is applied.
2 is a detailed configuration diagram of the power cell of FIG.
3 is a block diagram illustrating a voltage controller of the cell controller of FIG.
FIG. 4 is a diagram illustrating a voltage controller in which the cell controller of FIG. 2 determines a compensation voltage.

In order to fully understand the structure and effects of the present invention, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described below, but may be embodied in various forms and various changes may be made. However, the description of the present embodiment is intended to provide a complete disclosure of the present invention and to fully disclose the scope of the invention to a person having ordinary skill in the art to which the present invention belongs. In the accompanying drawings, the constituent elements are enlarged in size for the convenience of explanation, and the proportions of the constituent elements can be exaggerated or reduced.

It is to be understood that when an element is referred to as being "on" or "tangent" to another element, it may be understood that another element may be directly in contact with or connected to the image, something to do. On the other hand, when an element is described as being "directly on" or "directly on" another element, it can be understood that there is no other element in between. Other expressions that describe the relationship between components, for example, between 'between' and 'directly between' can be similarly interpreted.

The terms " first, " " second, " and the like may be used to describe various elements, but the elements should not be limited by the above terms. The above terms may only be used for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, a 'first component' may be referred to as a 'second component', and similarly, a 'second component' may also be referred to as a 'first component' .

The singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. The term "comprising" or "having" is used to specify that a feature, a number, a step, an operation, an element, a part or a combination thereof is described in the specification, A step, an operation, an element, a part, or a combination thereof.

The terms used in the embodiments of the present invention may be interpreted as commonly known to those skilled in the art unless otherwise defined.

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

FIG. 1 is a configuration diagram of a multi-level inverter system to which an embodiment of the present invention is applied, and FIG. 2 is a detailed configuration diagram of the power cell of FIG.

As shown in the drawing, a multi-level inverter system 1 to which an embodiment of the present invention is applied includes a phase shift transformer 10, a plurality of unit power cells 20, And a main controller 30 for transmitting a control signal to the electric motor 20. The electric motor 2 can output an AC voltage of a predetermined magnitude and frequency.

Each unit power cell 20 includes a rectifying section 21 for rectifying an AC voltage input from the phase displacement transformer 10, a capacitor 22 for smoothing a DC voltage rectified by the rectifying section, A plurality of switching elements 23 for converting the stored DC voltage into a predetermined AC voltage and a cell controller 24 for performing pulse width modulation (PWM) control of each switching element 23 and a power cell 20 And a current detector 25 for detecting a current flowing in the current detector 25.

The phase displacement transformer 10 can supply the plurality of unit power cells 20 with the phase of the input power source varied. The detailed configuration of the phase-shifting transformer 10 is well known in the technical field of the present invention, and a detailed description thereof will be omitted.

The plurality of unit power cells 20 are connected in series to each phase so that each phase U, V, W of the high voltage inverter 1 can be configured.

The main control unit 30 is for variable speed control of the electric motor 2 and can calculate the voltage and current necessary for the inverter 1 to output and transmit the information of the output frequency and the output voltage to each power cell 20 . It can also perform overall monitoring and diagnostics, monitoring, protection, man-machine interface (MMI), communication and other assistance of the system.

The main control unit 30 and the cell control unit 24 are connected to each other via a high-speed link and exchange data with each other to operate the system. The power cell 20 is supplied with the independent power from the phase displacement transformer 10 regardless of the system voltage, and insulation can be ensured.

The output voltage and the output current of each power cell 20 can be defined as follows.

In this case, φ is the load angle, ω is the operating frequency, t is the time, and V ₀ and I ₀ are the effective values (RMS) of the output voltage and output current. From the above equations (1) and (2), the output power of the power cell 20 can be determined as follows.

As can be seen from Equation (3), the power of the power cell 20 includes a DC component, which is a first-order component that is changed by the load angle, and an AC component that is twice the operation frequency. As a result, in the output waveform of the switching element 23, the output voltage having the pulsation corresponding to twice the output frequency is synthesized. In order to reduce the influence of the pulsation in the normal case, the capacitance of the DC- have.

In such a system, the output voltage of the inverter 1 is equal to the sum of the output voltages of the respective power cells 20, and the output currents are the same. At this time, a difference occurs in the output power generated in the power cell 20 according to the difference of the output voltage.

In addition, the output power is different for each power cell 20 for various reasons. For example, an electrolytic capacitor is mainly used as the direct-current capacitor 22 of the high-voltage inverter 1, and there is usually a capacitance variation of 15 to 20%, which causes the ripple of the capacitor 22 The influence is further exacerbated, so that the output voltage can be changed for each power cell 20. [ In addition, the voltage synthesized in the final power cell 20 can be varied by the difference in the tap voltage of the transformer 10 input to each power cell 20. Although the transformer 10 having a plurality of secondary taps is designed with the same number of turns theoretically, the transformer 10 is actually manufactured with a difference of several percent. When the load is maximized, the voltages input to the capacitors 22 are greater in accordance with the respective taps, and the output power of each power cell 20 is different because the voltage of the same phase is different. If regenerative power is generated, the load energy is concentrated on the side having a small phase voltage. In particular, a voltage is applied to a power having a small output power, which causes a sudden overvoltage.

SUMMARY OF THE INVENTION The present invention is directed to controlling the power factor and power balance of each power cell and controlling the output power of the unit power cell equally.

From the viewpoint of voltage and current applied to the electric motor 2, the inter-line voltage of the high-voltage inverter 1 must be constant and the phase voltages U, V and W must be constant. In addition, in order for the phase voltage to be constant, the voltage of each power cell 20 must be constant. That is, in order to make the voltage constant, it is necessary to accurately output the voltage to be outputted according to the frequency in each power cell 20. In the case of the cascade-type high-voltage inverter 1 as shown in FIG. 1, since the phase-shifted voltage is applied by the transformer 10, the overall power is instantaneously different. By controlling them similarly, the output voltage of each power cell can be stably synthesized. In the present invention, each cell controller 24 stabilizes the final output voltage.

First, the cell controller 24 controls the output power of each power cell to power factor, and can vary the power by varying the voltage with respect to the same current.

The phase-shift transformer 10 delays and outputs the phase per each output. The power factor of the voltage and current of the power cell 20 of each of the U, V, W phases differs depending on the load of the transformer 10. The output power of each power cell 20 varies do. The final output power in the high voltage inverter 1 is the sum of the output power of each power cell 20 and the power factor of the power cell 20 depends on the output frequency of the inverter and the number of layers in which the unit power cell 20 is disposed .

In such a system, when the power factor of the power cell 20 is not controlled, the output power of each power cell 20 is different, and there is a possibility that voltage regeneration may occur in a specific power cell in a region where the load is small.

Therefore, the cell control unit 24 of the embodiment of the present invention can control the power factor so that the output power output from each power cell is the same. This can be expressed by the following equation.

At this time, n represents the number of the unit power cells 20 in the high voltage inverter 1. [ The 'number of layers' of the unit power cell 20 means the number of unit power cells 20 connected in series. Typically, three or six unit power cells 20 are connected in series in the high-voltage inverter 1. In this case, when three unit power cells 20 are connected in series to form one phase voltage, 3, and six unit power cells 20 are connected in series to form one phase voltage.

Since the current of the system is the same, the cell control unit 24 can vary the output power by controlling the switching element 23 to vary the voltage. The cell control unit 24 can arbitrarily vary? According to the shift value of the voltage output from each power cell 20 according to the total number of power cells. The cell control unit 24 can arbitrarily vary? According to the number of total power cells 20 with respect to the transition value cos? Of the voltage output from each power cell 20 on the basis of Equation (4). The value a according to the number of total power cells 20 can be determined differently for each power cell 20 according to the reference of the uniform current and voltage. In addition, it is possible to synthesize the final output voltage form if the corrected? Is determined based on the power per load. As a result, the output voltage and current a can be varied uniformly for each power cell 20. [

In addition, the cell controller 24 may compensate the reference voltage of the capacitor 22 to supply independent power to the power cell 20. 3 is a configuration diagram for explaining the voltage controller of the cell controller 24 of FIG.

As shown in the figure, in an embodiment of the present invention, the voltage controller of the cell controller 24 may include a compensation unit 3A and a voltage detector 3B.

The reference voltage Vdc (Ref) of the capacitor 22 of the power cell 20 is determined according to the system regardless of the input voltage. The cell control unit 24 receives the actual DC voltage Vdc (fdk) detected at every control cycle from the voltage detection unit 3B and the compensation unit 3A compares the difference between the input voltage and the frequency of the shaking capacitor 22 voltage Depending on the size, the compensation voltage? V to be compensated for the final output voltage can be determined.

When power is first applied to the system, an initial value is calculated from the voltage of the capacitor 22 in each power cell 20, a change of each value is calculated at the output, The output voltage and the current power can be maintained at a constant level. Equation (5) is for explaining the content of generating the reference point for introducing the power concept, and Equation (6) is a reflection on the change for each power cell (20).

The voltage controller of the cell controller 24 of the present invention can determine a compensation voltage that compensates for an unbalanced voltage for each power cell 20 based on the voltage error, the input offset, and the initial voltage detection error of the capacitor 22 .

Since the power does not require a large overall response and is used for instantaneous control, considering the steady state error, the compensation unit 3A may be a proportional (P) controller. In order to synthesize a more stable output voltage and current, the voltage of the power cell 20 must be detected more stably, and a bandstop filter may be further included.

Further, the compensation unit 3A may be a proportional integral (PI) controller. Thereby, the power can be controlled, the balance of the capacitor 22 can be improved, and the effect of the phase transition of the inter-cell voltage and current can be reduced.

In addition, the cell controller 24 of the embodiment of the present invention may determine a compensation voltage considering the dead time according to the polarity of the current, and may generate a new reference voltage by adding it to the reference voltage. The PWM control is performed based on the reference voltage thus generated to control the switching device 23.

FIG. 4 is a diagram illustrating a voltage controller in which the cell controller 24 of FIG. 2 determines a compensation voltage.

As shown in the figure, a voltage controller according to an embodiment of the present invention includes a polarity discriminating section 4A for discriminating a polarity of a current from a current of a power cell 20 detected by a current detecting section 25, And a compensation voltage determination unit 4B that determines the compensation voltage according to the comparison result.

The current detection unit 25 can detect the current flowing in the power cell 20. [ The current detector 25 is, for example, a current transformer (CT). The polarity determination unit 4A can calculate the slope of the current from the current in the power cell 20 detected by the current detection unit 25 and detect the polarity of the current. There are various ways of calculating the slope of the current, and a detailed description thereof will be omitted.

The compensation voltage determination unit 4B can determine the compensation voltage in consideration of the DC link voltage and the dead time. The compensation voltage determination unit 4B can determine the compensation voltage according to Equation (7) when the polarity of the current is positive (+) and the compensation voltage according to Equation (8) when the polarity of the current is negative.

Here, td is the dead time and ts is the switching period. The dead time refers to the time for turning off all the two switching elements connected in series in order to prevent the switching elements of the upper leg and the lower leg from being simultaneously turned on in the process of switching the switching element 23 on or off. Since the two switching elements are simultaneously turned off in the dead time interval, the output voltage command and the output voltage of the actual inverter differ.

The addition section 4C can output the compensated reference voltage Vref 'by adding the compensation voltage to the reference voltage Vref used for the PWM control. The compensated reference voltage Vref 'may be used for PWM control of the switching device 23. [

According to the present invention, the phase displacement transformer used in the input stage of the multilevel inverter can be flexibly designed, thereby increasing the degree of freedom in system design and stabilizing the output voltage of the entire system. This can enhance the price competitiveness of the entire system. In addition, since the final line voltage is stabilized, the current ripple is reduced, and when a large capacity is applied, the transient state such as hunting can be easily stabilized and the life of the motor can be increased.

In addition, according to the present invention, stabilizing the electric power that is conducted on the average in the inverter increases the efficiency and life of the system and reduces the capacitance of the capacitor that occupies a large portion of the size of the power cell, The size can be reduced. Therefore, the size of the entire system can be reduced.

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 protection of the present invention should be determined by the following claims.

1: High-voltage inverter 2: Motor
10: Phase-shifting transformer 20: Power cell
30: main control part 21: rectifying part
22: DC link capacitor 23: Switching element
24:

Claims (6)

An apparatus for controlling a power cell in a high voltage inverter system including a plurality of power cells connected in series and synthesizing a voltage applied to an electric motor,
A plurality of switching elements for converting a DC voltage stored in the DC link capacitor into an AC voltage of a predetermined magnitude and frequency; And
And a cell controller for controlling the pulse width modulation of the switching element,
The cell controller may control the power factor of output power of the plurality of power cells to control the output powers of the plurality of power cells equally,
The cell control unit includes:
A voltage detector for detecting a voltage of the DC link capacitor; And
And a compensation unit for determining a compensation voltage to be compensated for an output voltage of the power cell based on a voltage error of the DC link capacitor, an offset of an input voltage, and an initial voltage detection error.
The apparatus of claim 1,
Wherein the power factor is controlled by varying? According to the number of power cells connected in series at a transition value of a voltage output from each power cell.

delete The control device according to claim 1, wherein the compensating unit is a proportional (P) controller or a proportional plus integral (PI) controller.
The apparatus of claim 1,
A current detector for detecting a current flowing in the power cell;
A polarity discrimination unit for discriminating a polarity of a current flowing in the power cell; And
And a compensation voltage determination unit for determining a compensation voltage considering the DC link voltage and the dead time according to the polarity of the current flowing in the power cell to compensate the reference voltage.
6. The apparatus according to claim 5,
And calculates a slope of a current flowing through the power cell to detect the polarity of the current.

Images