JPH1028378A - Method for controlling power converter and method for controlling power converting system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the operating efficiencies of power converters by reducing the power losses in the converters by controlling the operating states of the converters by variably controlling the switching frequencies of the converters, corresponding to the load quantities of the converters and changing the switching losses of the converters, corresponding to the load quantities. SOLUTION: Detectors 191 -196 respectively detect the load quantities of power converters 171 -176 , and a control section 20 outputs compound signals 211 -216 for the switching frequencies of the converters 171 -176 . The converters 171 -176 variably control their switching frequencies for PMW control, based on the signals 211 -216 . The control section 20 outputs the command signals 211 -216 , so that a flowing out harmonic current can be suppressed to a low value as compared with the total AC power supplied from a AC power source 11, and the switching loss of the whole power converting system can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter and a system control method having a converter for converting AC power into DC power by PWM control, and more particularly to an improvement in its operation efficiency and harmonics flowing to an AC power supply. The present invention relates to a power conversion device and a system control method for suppressing current.

[0002]

2. Description of the Related Art NPC inverters (neutral point clamp type inverters) are used to convert DC power into AC power by PWM control. However, reversible power conversion is possible, and AC power is controlled by PWM control. It can be used as a converter for converting to DC power. NPC
As a literature of the inverter, "The PWM control method and the neutral point potential control for eliminating the waveform distortion due to the pulse width limitation of the NPC inverter" is described in IEICE Transactions D, Vol. 116, No. 4, 1996, pp. 412-419. ing. According to this paper, it is shown that AC power is converted to DC power by PWM control, and GTO is used as a switching element.
It has been shown to operate at a switching frequency of several hundred Hz using a (gate turn-off thyristor).

FIG. 7 shows a power converter using such an NPC inverter as a converter for converting AC power to DC power by PWM control. This power converter converts the AC power of AC power supply 11 into DC power by PWM control by converter 12, smoothes the DC power with capacitors 13 and 14, and smoothes the smoothed DC power again with inverter 15. To AC power and load 1
6.

The converter 12 comprises three sets of power conversion circuits 12
R, 12S, 12T, and each power conversion circuit 12
R, 12S, and 12T have AC terminals R, S, and T, DC terminals P and N, and a neutral terminal C. The AC terminals R, S, and T are connected to the AC power supply 11, and the DC terminals P and N Are connected to the DC buses P and N. Also, P, N is located between the DC buses P, N.
A series circuit of two capacitors 13 and 14 that obtains a neutral point potential C by dividing the DC voltage between N is connected, and the neutral point potential C is connected to the neutral point of each of the power conversion circuits 12R, 12S, and 12T. Terminal C is connected.

The inside of each of the power conversion circuits 12R, 12S, 12T is configured in the same manner,
Switching elements S1 to S4 in which D4 is connected in anti-parallel
Are connected in series and connected to DC terminals P and N, and a connection point between switching elements S2 and S3 is connected to AC terminals R, S and T, respectively. Also, two diodes D5 and D6
Are connected in parallel in the same direction as the diodes D2 and D3, and the connection point of the diodes D5 and D6 is connected to the neutral terminal C
Connected to.

The switching elements S1 and S2 and the diodes D1 and D2 function as positive arms, and the switching elements S3 and S4 and the diodes D3 and D4 function as negative arms. The diodes D5 and D6 function as neutral point clamp diodes. As the switching elements S1 to S4, self-extinguishing switching elements such as GTO and IGBT (gate insulated bipolar transistor) are generally used.

In the converter 12 of the power converter,
The AC power input from the AC power supply 11 can be converted into a desired DC power by the PWM control for selecting and controlling the on / off operation of the switching elements S1 to S4.
In addition, when the AC power is PWM-controlled, when viewed from the AC power supply 11 side, the switching frequency becomes the same as the PWM control at a frequency twice as high as the switching frequency of the switching elements S1 to S4, and flows out to the AC power supply 11. The harmonic current is also reduced.

As described above, when the AC power is converted into the DC power by the PWM control, the harmonic current component included in the current of the AC power supply 11 changes, and when the switching frequency is increased, the harmonic current component decreases. Are known.

On the other hand, with the recent spread of power electronics devices, harmonic currents generated from these devices increase the voltage distortion of the power system, and the effects on other devices connected to the power system become apparent. ing. Under such circumstances, the Agency for Natural Resources and Energy is conducting studies to ensure that harmonic countermeasures are promoted smoothly, and enacts “Guidelines for Measures against Harmonics for Consumers Receiving High Voltage or Extra High Voltage” (Heisei 6
It is hoped that each customer side will suppress the harmonic outflow current.

[0010] Each customer must keep the harmonic current flowing into the power system at the receiving point in accordance with the contracted power within the regulation value of the above guidelines. In order to reduce the harmonic outflow current, it is desirable to use a power converter as shown in FIG. 7 to suppress the harmonic current generated by the device itself.

As described above, GTO or IGBT is used for the switching elements S1 to S4 used in the converter 12, and the power loss is reduced by the conduction loss (Pi) generated when current is supplied and the switching element in a non-conductive state. ON loss (Pon) generated when shifting to the conductive state,
It is the sum of three losses (Pi + Pon + Poff) of the off loss (Poff) that occurs when the switching element transitions from the conductive state to the nonconductive state. The conduction loss (P i) is substantially proportional to the flowing current because the voltage drop of the switching element is substantially constant. Switching loss (Pon + P
off) occurs almost every time switching is performed, and is therefore substantially proportional to the switching frequency.

The maximum switching frequency of each of the switching elements S1 to S4 is limited depending on the type thereof, and is several hundred Hz such as 500 Hz for a GTO which is a current-driven switching element, and several KHz such as 5 KHz for a IGBT which is a voltage-driven switching element. It is used at a switching frequency of about 10 to 10 KHz. Further, the switching element S1
The maximum power loss allowed for S4 is limited by cooling conditions and the like.

In a large-capacity power converter, a GTO is often used as the switching elements S1 to S4. For example, when the switching frequency is 500 Hz and a predetermined load is applied, the power loss of the GTO is about 6 KW. The breakdown is as follows.

Conduction loss (Pi) = about 1.8 KW On loss (Pon) = about 1.0 KW Off loss (Poff) = about 3.2 KW

[0015]

The conventional power converter described above has a relatively large switching loss (Pon + Poff) generated in the switching elements S1 to S4 when converting AC power to DC power by PWM control. ,
If an attempt is made to reduce the harmonic current generated by the power converter by increasing the switching frequency, there is a problem that the power loss of the GTO increases and the operating efficiency decreases, and operation satisfying the following conditions is desired. . (1) The switching element is operated at a switching frequency as high as possible so that the harmonic outflow current of each power converter satisfies the regulation value of the above guideline. (2) When the load is low, the conduction loss (Pi) decreases in accordance with the load current, but the switching loss (Pon + Poff) does not change much. Is not reduced. (3) As the load current increases, the harmonic outflow current also increases.Therefore, it is desirable to suppress the harmonic outflow current by increasing the switching frequency as much as possible, but do not exceed the maximum allowable power loss determined by the cooling conditions of the switching element. To

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to suppress a harmonic outflow current of a power conversion device, reduce a total power loss during operation, and improve operation efficiency. It is an object of the present invention to provide a power conversion device and a control method of a system that can perform the control.

[0017]

In order to achieve the above object, a control method of a power converter according to the present invention is provided in which a power converter having a converter for converting AC power to DC power by PWM control is controlled. Means for variably controlling the switching frequency of the converter in accordance with the load of the power converter, changing the switching loss of the converter in accordance with the load, reducing the power loss depending on the operating state, Improve. (Claim 1) Further, a means is provided for reducing the switching frequency of the converter when the load amount increases and exceeds a predetermined load amount, thereby suppressing switching loss at a high load when the load amount exceeds the predetermined load amount. Improve the operating efficiency at high load.
(Claim 2) Further, a means is provided for increasing the switching frequency of the converter when the load amount increases and exceeds a predetermined load amount. The switching frequency at light load is reduced to reduce switching loss and improve operation efficiency. And suppresses the harmonic current flowing to the AC power supply side under a high load. (Claim 3) Further, when the switching frequency is reduced in accordance with the load amount, a specific change ratio (for example, 2/3 or 1.
In step 5), means for changing stepwise is provided to approximate the order of the harmonic current flowing to the AC power supply side, thereby facilitating the configuration of a filter circuit for preventing the harmonic current from flowing out. (Claim 4) A method for controlling a power conversion system according to the present invention is characterized in that:
When a plurality of power converters each having a converter for converting AC power to DC power by control are provided, and each of the power converters controls a power conversion system that operates a load machine, any one of the above-described power converter control methods The switching frequency of each power conversion device is controlled so as to suppress the harmonic outflow current and reduce the switching loss as a whole of the power conversion system based on the load amount of each power conversion device. To improve the overall operation efficiency of the entire system and suppress the sum of harmonic currents flowing out to the AC power supply side. (Claim 5) Furthermore, the power conversion system is configured by adding a second power conversion device having a converter composed of a switching element having a high switching operation speed, and a plurality of power conversion devices are controlled by the above-described power conversion device control method. Controlling the second power conversion device at a switching frequency sufficiently higher than the switching frequency of the power conversion device by controlling at two switching frequencies according to any one of the control methods;
By operating at various types of switching frequencies, harmonic current flowing to the AC system side is finely suppressed and controlled. (Claim 6) Further, a current-driven switching element is used as a switching element of a converter of the power converter, and
A voltage-driven switching element is used as a switching element of the converter of the power converter, and the switching frequency of the second power converter is increased by about 10 times the switching frequency of the power converter, and the harmonic current flowing out to the AC system side Control is finely controlled. (Claim 7)

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for controlling a power converter according to the present invention will be described below. FIG. 1 shows an embodiment according to claims 1 and 2 and claim 4 of the present invention. FIG. 1 shows a relationship between a load Id and a switching frequency f in a power converter, and FIG. The load amount Id is determined by the converter 1
2, and the switching frequency f is a carrier frequency for performing PWM control on the switching elements S1 to S4. As shown in FIG.
In the region where the load amount Id of No. 2 is equal to or smaller than the predetermined load amount In, 1.
PWM control is performed at a switching frequency of 5fc, and when the load amount Id exceeds a predetermined load In, the switching frequency is reduced to fc. FIG. 2 shows the power loss in the switching elements S1 to S4 of the converter 12 when the switching frequency f is changed as described above.
2A shows the case where the switching frequency is 1.5 fc with the load In, and FIG. 2B shows the case where the switching frequency is fc with the load 2In.

5 when the switching frequency is 1.5 fc
Taking as an example a GTO that corresponds to 00 Hz and generates the above-described power loss with the load In, when the switching frequency is 1.5 fc with the load In, the power loss P becomes the conduction loss (P
i), the ON loss (Pon), and the OFF loss (Poff), which are 6.0 KW from the following equation (1). (Fig. 2 (a))

[0020]

P = Pi + Pon + Poff = 1.8 + 1.0 + 3.2 = 6.0 KW (1) If the load is doubled (2In) with the switching frequency kept at 1.5fc, the conduction loss (Pi) becomes about 2 The power loss P becomes 7.8 KW from the following equation (2),
Increase by 1.8 KW.

[0021]

P = 2Pi + Pon + Poff = 2 * 1.8 + 1.0 + 3.2 = 7.8 kW (2) However, if the switching frequency is reduced to fc when the load is 2In, the switching loss (Pon + Pof) is reduced.
f) decreases to about 2/3, and the power loss P becomes 6.4 KW from the following equation (3), and increases only by 0.4 KW. (Figure 2
(b))

[0022]

P = 2Pi + 2/3 (Pon + Poff) = 2 * 1.8 + 2/3 (1.0 + 3.2) = 6.4 KW (3) Thus, the switching frequency f corresponding to the load Id
Is controlled, the load Id increases and the conduction loss (P
i), the ON loss (Pon) and the OFF loss (Pof)
f) can be reduced, and even when the load amount Id increases, the power loss under a high load can be suppressed without increasing the power loss corresponding to the load amount Id. Therefore,
According to the present embodiment, it is possible to suppress the harmonic outflow current at the time of a low load and to improve the operation efficiency of the power converter at the time of a high load.

FIG. 3 shows the first, third and fourth aspects of the present invention.
And shows the relationship between the load Id and the switching frequency f in the power converter, and other conditions are the same as in the above embodiment.

In this embodiment, in a region where the load Id of the converter 12 is equal to or less than the predetermined load In, the PWM control is performed at the switching frequency of fc and the load Id is reduced to the predetermined load Id.
Beyond n, the switching frequency is increased to 1.5fc. FIG. 4 shows switching elements S1 to S1 of converter 12 when switching frequency f is changed in this way.
4 (a) shows the case where the switching frequency is fc with the load In, and FIG. 2 (b) shows the case where the switching frequency is 1.5fc with the load 2In. is there.

The switching frequency is fc with the load In.
In the case of (FIG. 4A), the switching frequency is 1.5 fc.
Switching loss (Pon + P
off) is reduced to about 2/3, and the power loss P is 4.6 KW from the equation (4).

[0026]

## EQU4 ## P = Pi + 2/3 (Pon + Poff) = 1.8 + 2/3 (1.0 + 3.2) = 4.6 kW (4) The switching frequency is 1.5 fc with the load 2In (FIG. 4).
In the case of (b)), the conduction loss (Pi) is approximately doubled, and the power loss P is 7.8 KW from the equation (5).

[0027]

P = 2Pi + Pon + Poff = 2 * 1.8 + 1.0 + 3.2 = 7.8 KW (5) As described above, the switching frequency f corresponding to the load Id.
Is increased, the switching loss (Pon
+ Poff) increases, but the harmonic outflow current can be suppressed corresponding to the load amount Id. Further, at a low load, the switching frequency f is reduced, so that the switching loss (Pon + Poff) is reduced, and the operation efficiency can be improved.

As shown in the above embodiment, when the switching frequency is variably controlled, the switching frequency is reduced to 2/3,
By increasing by 1.5 times, higher-order harmonics become closer, so that it is easy to provide a filter circuit for suppressing the higher-order harmonic outflow current, and to set the switching frequency f to the carrier of the three-phase PWM control converter. The frequency can be optimized, and the beat phenomenon between the power supply frequency of the AC power supply 11 and the switching frequency can be prevented.

Next, an embodiment of the control method of the power conversion system according to the present invention will be described. FIG. 5 shows an embodiment according to claim 5 of the present invention.
7 _{1-17 6} power converter illustrated in FIG. 7, 18 ₁ to 1
8 ₆ load machine such as an electric motor, 19 _{1-19 6} loading I
, A detector 20 for detecting the power conversion devices 17 ₁ to 17 ₁
Controller providing a command signal 21 ₁ to 21 ₆ for determining the switching frequency of 17 _6, 22 is a load line for machining of the material, shows the configuration of a power conversion system for controlling the manufacturing process lines is there.

In the above configuration, each power converter 17 ₁
To 17 ₆ drives the load machine 18 _{1-18 6} power is converted to the desired power of the AC power supply 11, a product by processing while transporting the raw material of the load line 22.

In such a power conversion system,
Load machine 18 _{1-18 6} sequentially become high load, if all the load machine 18 _{1-18 6} becomes a high load at the same time less, in the normal operating condition becomes part of the load machine only high load, the other Generally, the load machine has a low load. In such an operation state, each of the detectors 19 ₁ to 19 ₁ to
19 Each of the power converter by ₆ 17 _{1-17 6}
The control unit 20 detects the load amount I of the
Switching frequency command signals 21 _{1 to} 21 _{6 according to}
Outputs, each power conversion apparatus 17 _{1-17 6} variably controls the switching frequency f of the PWM control as described above on the basis of the command signal 21 ₁ to 21 _6.

In this case, the control unit 20 controls the switching frequency command signal 2 so that the harmonic outflow current is suppressed to a low value with respect to the total AC power supplied from the AC power supply 11.
1 ₁ to 21 ₆ outputs, also outputs command signals 21 ₁ to 21 ₆ of the switching frequency to reduce the switching loss of the entire power conversion system.

Therefore, according to the present embodiment, the harmonic outflow current to the entire power conversion system is suppressed,
Operation efficiency can be improved. FIG. 6 shows an embodiment according to claim 6 of the present invention, 23 ₁ and 23
₂ is a power conversion device for driving a load machine 24 _{1-24 6} for material transportation, is the same configuration as the power converter illustrated in FIG. 7, the switching element S ₁ to S ₄ of the converter 12
IGBT and MOS operating at high switching frequency
It is configured using a voltage-driven switching element that is turned on / off by a voltage signal such as an FET, and is used in a region where the switching frequency f exceeds several KHz to 10 KHz. Converter 12 of the power conversion apparatus 17 _{1-17 6}
Uses a current-driven switching element that is turned on / off by a current signal such as a GTO or a bipolar transistor. Even if the switching frequency f is higher than several hundreds Hz, 1.
Used in the region of about 5 KHz.

[0034] 25 for detecting the current (load) supplied from at least an AC power source 11 to the power converter 17 _{1-17 6} detector, 20 power conversion apparatus 17 _1-17
Command signal 21 for determining the switching frequency of _6
1 to 21 ₆ and the control unit gives a command signal 26 _{1-26 6} for determining the switching frequency of the power converter 23 ₁ and 23 _2, others are denoted by the same reference numerals in the same as FIG.

In the above configuration, the power conversion devices 17 ₁ to 17 ₁ to
17 ₆ drives the load machine 18 _{1-18 6} power is converted to the desired power of the AC power supply 11, to product processing while transporting embodiment similarly to the raw material of the load line 22 of FIG. The power conversion apparatus 23 ₁ and 23 ₂ to drive the load machine 24 _{1-24 6} for material transportation.

In this case, the controller 20 controls the power converter 17
_{1-17 6} command signal of the switching frequency in the same manner as described above in Example in accordance with the respective detection load I 21 ₁ through 21
₆ is output, the harmonic outflow current is suppressed to a low value with respect to the total AC power supplied from the AC power supply 11, and the switching loss of the entire power conversion system is reduced. By these command signals, the power conversion devices 17 ₁ to 17 ₁
7 ₆ is variably controlled in accordance with the load amount I in the area of at most 1.5KHz position a few hundred Hz.

Further, the control unit 20 when the total current supplied from the AC power source 11 to the power converter 17 _{1-17 6} is detected by the detector 25, monitors the harmonic component of the total current, the AC power source Command signal 26 ₁ for determining the switching frequency so as to suppress the harmonic current flowing out to 11
- 26 ₆ outputs, the power conversion apparatus 23 ₁ and 23 ₂ the number K
The switching frequency f is variably controlled in a range from Hz to over 10 KHz.

Further, the control unit 20 outputs a command signal 26 _{1-26 6} for determining the switching frequency to reduce the predetermined harmonic component flowing into the AC power source 11, suppressing a predetermined harmonic component The switching frequency f can also be variably controlled in such a manner. In this case, by changing the control phase while keeping the switching frequency, it is also possible to perform control so as to absorb a predetermined harmonic current component.

[0039] According to this embodiment, lower switching frequency reduces the switching losses of the power conversion apparatus 17 _{1-17 6} large-capacity switching loss is large, it is possible to improve the operating efficiency of the entire system, further it can be the switching frequency to absorb a portion of the harmonic current flowing out to the AC power source 11 in the power converter 23 ₁ and 23 ₂ of the higher medium and small capacity switching loss is small.

In the present invention, a normal converter having a 6-arm bridge can be used as a converter for converting AC power to DC power by PWM control, and is not limited to an NPC inverter as shown in FIG.

Further, the embodiment of FIG. 6, to select the control pattern of the PWM control so as to reduce specific harmonic current flowing out in the power converter 17 _{1-17 6,} for example, the low order harmonic current flowing out The increase in high-order harmonic outflow current due to the selection of this control pattern is determined by the power converter 23 _1.
When absorbs at 23 _2, the respective harmonic components of the harmonic current flowing out to the AC power source 11 so as to be within limits for guidelines for selecting controlled by the control unit 20 of the PWM control method for each of the power converter be able to.

[0042]

According to the power converter control method of the present invention, the following effects can be obtained. (1) Since the switching frequency of the PWM control can be variably controlled in accordance with the load amount I, the ON loss and the OFF loss due to the PWM control change in accordance with the load amount, and the power loss of the device is reduced. Operation efficiency can be improved.
(2) By controlling the switching frequency of the PWM control to decrease in response to the increase of the load I, the switching loss is reduced when the load I is large, and the switching element itself is generated. Loss is suppressed. For this reason,
The cooling of the switching element is facilitated, the size of the switching element can be reduced, and the power converter can be reduced in cost and improved in economic efficiency. (Claim 2) (3) By controlling the switching frequency of the PWM control to increase in response to the increase in the load I, the harmonic generation current can be suppressed in accordance with the load I, The switching loss when the load I is small can be reduced, and the operating efficiency at the time of light load can be improved.
(Claim 3) (4) By changing the switching frequency to be variably controlled at a specific ratio, it is possible to make the order of the harmonic outflow current close, and to suppress the harmonic current flowing out to the AC power supply side. The configuration of the filter circuit is simplified, and the PWM
It is possible to suppress the beat phenomenon of the harmonic outflow current accompanying the control. (Claim 4) According to the power conversion system control method of the present invention,
The following effects can be obtained. (1) In an operation state in which all of the plurality of power converters do not simultaneously have a high load, the plurality of power converters are variably controlled in the switching frequency of the PWM control corresponding to each load I, and It is possible to reduce the switching loss of the entire conversion system, improve the operation efficiency, and suppress the harmonic outflow current of the entire power conversion system. (Claim 5) (2) Further, it is possible to control the switching frequency of the power conversion device whose switching operation speed is sufficiently fast so as to suppress the harmonic outflow current of the entire power conversion system,
The harmonic outflow current can be suppressed and controlled finely. (3) In a plurality of power converters, a predetermined order component of a harmonic generation current of a power converter composed of a current driving switching element having a low switching frequency is converted into a voltage driven switching element having a high switching frequency. In the power conversion device configured by the above, it is possible to suppress fine harmonic generation current such as suppression and absorption, and to improve the overall operation efficiency of a plurality of power conversion devices. (Claim 7)

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an embodiment of a control method of a power converter according to claims 1, 2 and 4 of the present invention. FIG. 1 shows a relationship between a load I of the power converter and a switching frequency f. FIG.

FIG. 2 is a diagram for explaining the operation of the embodiment, and FIG.
3B is a power loss diagram when the load I is 2IN, and FIG. 3B is a power loss diagram when the load I is 2IN.

FIG. 3 is a diagram for explaining an embodiment of a control method of a power converter according to claims 1, 3 and 4 of the present invention, and illustrates a relationship between a load I of the power converter and a switching frequency f. FIG.

FIG. 4 is a diagram for explaining the operation of the embodiment, and FIG.
3B is a power loss diagram when the load I is 2IN, and FIG. 3B is a power loss diagram when the load I is 2IN.

FIG. 5 is a diagram for explaining an embodiment of a control method for a power conversion system according to claim 5 of the present invention, and is a system configuration diagram of the power conversion system.

FIG. 6 is a diagram for explaining an embodiment of a power conversion system control method according to claims 6 and 7 of the present invention, and is a system configuration diagram of the power conversion system.

FIG. 7 is a main circuit configuration diagram of a conventional PWM control power converter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 ... AC power supply 12 ... Converter 13 ... Capacitor 14 ... Capacitor 15 ... Inverter 16 ... Load P, N ... DC bus C ... Neutral point 12R, 12S, 12T ... Three-level power conversion circuit D1-D4 ... Diode S1-S4 ... Switching elements D5, D6: Diodes R, S, T: AC input terminals 17: PWM control power converter 18: Load machine 19: Load detector 20: Control function 21: Switching frequency command signal 22: Load line 23: PWM Control power converter 24 ... Load machine 25 ... Load detector 26 ... Switching frequency command signal

Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H02M 7/757 8110-5H H02M 7/757 H02P 7/63 302 H02P 7/63 302R // H02J 3/18 H02J 3/18 D

Claims (7)

[Claims] When controlling a power converter having a converter for converting AC power into DC power by PWM control, the switching frequency of the converter is variably controlled in accordance with a load amount of the power converter, and the converter is controlled. A switching loss of the switching element according to the load amount. 2. The control method for a power converter according to claim 1, wherein when the load amount exceeds a predetermined load amount, the switching frequency of the converter is decreased.
A control method for a power conversion device, comprising: suppressing a switching loss of the switching element under a high load to improve an operation efficiency under a high load. 3. The control method for a power converter according to claim 1, wherein the switching frequency of the converter is increased when the load amount exceeds a predetermined load amount.
A control method for a power conversion device, comprising: lowering a switching frequency at a low load to reduce a switching loss of the switching element and improving an operation efficiency at a low load. 4. The method for controlling a power conversion device according to claim 2, wherein when the switching frequency is changed in accordance with the load amount, the switching frequency is changed in a stepwise manner at a specific change rate, and the AC power supply is changed. A method for controlling a power conversion device, wherein the orders of harmonic currents flowing out are brought close to each other. 5. A power conversion system comprising a plurality of power converters each having a converter for converting AC power to DC power by PWM control, each of which controls a power conversion system that receives power from the same AC system and operates a load machine. Is controlled by using any one of the control methods according to any one of claims 1 to 4, and based on the load of each power converter, the harmonic outflow current is suppressed as the entire power conversion system. At the same time, the switching frequency of each power converter is changed so as to reduce the switching loss, thereby improving the overall operation efficiency of the entire system,
A control method for a power conversion system, wherein a total sum of harmonic currents flowing to an AC system side is suppressed. 6. The control method for a power conversion system according to claim 5, wherein the power conversion system is configured by adding a second power conversion device having a converter including a switching element having a high switching speed. The power converter is controlled at two kinds of switching frequencies by using any one of the control methods according to claims 1 to 4, and the switching of the second power converter is sufficiently higher than the switching frequency of the power converter. A control method for a power conversion system, wherein a harmonic current flowing out to an AC system side is finely suppressed by controlling at a frequency and operating at at least three switching frequencies. 7. The control method for a power conversion system according to claim 6, wherein a current-driven switching element is used as a switching element of a converter of the power conversion device.
A voltage-driven switching element is used as a switching element of the converter of the second power converter, and the switching frequency of the second power converter is set sufficiently higher than the switching frequency of the power converter, and harmonics flowing out to the AC system side A control method for a power conversion system, wherein a current is finely suppressed and controlled.