JP2703258B2 - Power converter control method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a control method of a power conversion device including a converter cascaded in two stages and converting AC power of a constant frequency into AC power of an arbitrary frequency. About.

(Prior Art) A description will be given using a two-stage cascaded circulating current type cycloconverter for driving an induction motor as a conventional power converter.

As shown in FIG. 5, the cycloconverter has two stages of cascade-connected positive-group converters 1 and 2 for passing a positive polarity current of the output AC current and negative-group converters 3 and 4 for passing a negative current.
A U-phase converter is composed of transformers 5 and 6 connected to these positive-group converters 1 and 2 and negative-group converters 3 and 4 and a reactor 7 for suppressing circulating current. The main circuit is constituted by connecting the phase components and the respective outputs to the induction motor 8.

The operation principle of the cycloconverter thus configured will be described below.

The positive group converters 1 and 2 and the negative group converters 3 and 4 simultaneously output the same voltage, and the two average voltages are supplied to the induction motor 8. Also, a sinusoidal current of any frequency can be applied to each phase.
The operation speed of the induction motor 8 is arbitrarily varied by giving a shift of 120 °.

Next, the operation will be described in detail with reference to FIG. The speed reference ω _r ^* is compared with the actual speed ω _r detected by the speed detector 43 of the induction motor 8 and input to the speed control unit 31. The speed control unit 31 includes comparison integration, overshoot correction, and the like, and this output serves as a torque reference T ^* . Flux calculating unit 32 is a control of the automatic field weakening, and outputs the magnetic flux reference φ from the actual speed omega _r. With this output, the torque reference is divided by the divider 33
After dividing by, input to the vector control unit. Torque current component of the primary current vector control unit 34 in the induction motor 8 I _1q ^* and the flux current component I _1d ^* constants of the induction motor 8 (the secondary resistance R ₂ secondary inductance L ₂ mutual inductance M)
Is decomposed using In the magnetic flux saturation function 35a, and outputs a relationship speed φ and the excitation current I ₀ of the induction motor 8 in consideration of the saturation flux, the differentiator 35b is the rate of change of magnetic flux d
Forcing current I for changing magnetic flux from φ / dt
_FOR is output, and the sum of the exciting current I ₀ and the forcing current I _FOR becomes the magnetic flux current component I _1d ^* . The torque current component I _1q ^* is proportional to the output of the divider 33. Further, the slip frequency ω _S of the induction motor 8 is calculated from the torque current reference, the motor constant and the magnetic flux. The torque current component I _1q ^* and the magnetic flux current component I which are the outputs of the vector control unit 34
_1d ^* is compared with the respective actual currents I _1q -F, I _1d -F,
It is input to a current control unit 36 composed of proportional and integral elements. Each of the actual currents (I _1q −F, I _1d −F) detects the input currents I _U , I _V , I _W of the induction motor 8 by the current detectors 37a, 37b, 37c, and performs three-phase two-phase conversion. _Iq which is a two-phase DC quantity via the
-F and _E1d- F. The current control unit 36, and the reference torque current component and the flux current component, the deviation of the actual current are each independently proportionally, are integral control, each of the voltage reference E _q, is output as E _d. In the voltage vector calculation unit 38,
Voltage reference input E _q, the voltage amplitude reference E and a voltage phase theta _V is calculated as follows using E _d.

The three-phase voltage reference calculation unit 39 includes a primary frequency phase φ ₀ obtained by adding the primary frequency θ ₀ obtained by adding the actual speed ω _r and the slip frequency reference ω _S via the integrator 40, and a voltage phase θ _V calculated voltage phase theta _0V from the fixed windings of the voltage reference from, and enter it and voltage amplitude reference E, 3-phase U, V, voltage reference W E _U, E _V, and outputs the E _W. This operation is Are input to the asymmetric control unit 41 as sine waves shifted by 120 °. In the asymmetric control unit 41, the voltage is converted into a voltage signal to be output to each converter of the two-stage cascade-connected main circuit unit. For example, in the case of the U-phase, output voltage references U1SA and U1SB to the first-stage positive group converter 1 and the first-stage negative group converter 3 are given as the first-stage converter output voltage reference U1S. Also, the second stage converter output voltage reference
Output voltage references U2SA and U2SB to the second-stage positive group converter 2 and the second-stage negative group converter 4 are output as U2S.

As described above, a voltage reference is given to the positive group converters 1 and 2 and the negative group converters 3 and 4 so as to output the same voltage, and the average voltage V is output to the induction motor 8 and is expressed by the following equation. .

Although the effective values of the output voltages of the positive group converters 1 and 2 and the negative group converters 3 and 4 are given equally, the positive group converters 1 and 2 circulate through the negative group converters 3 and 4 with the difference voltage generated from the output voltage waveform. An electric current flows, and its magnitude is suppressed by the circulating current suppressing reactor 7. The same applies to the V and W phases.

Next, the asymmetric control unit 41 will be described. FIG. 7 shows U
Output voltage reference E of first-stage positive and negative group converters
And _U1S (= U1SA = U1SB), shows a two-stage positive group and negative group converter output voltage reference E _U2S (= U2SA = U2SB) and the total output voltage reference E _U wave transducer, 8 The figure shows a flowchart for realizing the control method. That is, Kojo one side of the transducer to _α maximum voltage E of the converter, as much as possible to control the output voltage of the other transducers on the total output voltage E _U converter.

 The converter output voltage reference for each stage is given by:

When the total output voltage reference of the converter E _U ≒ Esinθ _0V > 0, the first-stage converter output voltage reference E _1U = E _{α The} second-stage converter output voltage reference E _2U = Esinθ _0V -E _α where | Esinθ _0V | by _≦ 2E α E α is given to the transformer output up to indicate a voltage (E _α> 0) _{Further, E U1S = Esinθ 0V + E} α E U2S = -E α thus each transducer when E _U ≦ 0 The voltage applied to the motor is a sine wave as shown in the waveform of FIG.

(Problems to be Solved by the Invention) However, the above-described asymmetric control method can be applied when the frequency based on the output voltage is low, that is, in a low-speed operation region. Since the output voltage waveform of each stage that outputs asymmetric control cannot be correctly output, a sine wave cannot be obtained as an output voltage to the motor, which causes disturbance such as torque ripple. Therefore, this asymmetric control can be applied only in a low frequency region where a voltage waveform as shown in FIG. 5 can be actually realized.

However, in order to realize a cycloconverter that requires a high-speed operation of the motor and has a high input power factor, in a low-speed region where the input power factor is particularly poor (about the base speed), the input is performed using asymmetric control. It is necessary to improve the power factor and use symmetric control in the high-speed region to control the output voltage waveform at the time of high-frequency operation correctly.

Therefore, in the present invention, high-speed operation can be performed without disturbing the output voltage waveform by using asymmetric control and symmetric control according to the operating range of the motor, and particularly by continuously and smoothly shifting the switching of the control method. In addition, it is an object of the present invention to provide a method for controlling a power converter having a high input power factor in a conventional low-speed region.

(Means for Solving the Problems) In order to achieve the above object, in the present invention, a positive converter and a negative converter composed of two sets of cascade-connected converters are connected in anti-parallel via a reactor. In the case of converting a constant frequency AC voltage to an arbitrary frequency AC voltage based on an AC voltage reference, when the arbitrary frequency is equal to or lower than the first frequency, the output of one of the converters is provided. Controlling to the maximum voltage, performing asymmetric control by controlling the other converter so that the combined output voltage of the one converter and the other converter has a value corresponding to the AC voltage reference,
When the arbitrary frequency is equal to or higher than a second frequency higher than the first frequency, an output voltage obtained by combining output voltages of the one converter and the other converter with the same value is a value corresponding to the AC voltage reference. The symmetrical control is performed so that
An AC voltage reference corrected based on the AC voltage reference frequency in the one converter and the other converter so as to continuously shift from the asymmetric control to the symmetric control from a frequency to a second frequency. give.

(Operation) In the above configuration, when the frequency of the AC voltage reference Esinshita _0V is equal to or less than the first frequency omega _alpha, the output of one converter is controlled to the maximum voltage E _alpha or Esinθ _0V + E _α, the output of the other transducer Is controlled to Esin θ _0V -E _α or −E _α so that the combined output voltage becomes a value corresponding to the AC voltage reference to perform asymmetric control, and the frequency of the AC voltage reference Esin θ _0V is the first frequency when higher of the two or more frequency omega _beta than omega _alpha, the output of one of the transducer and the other transducer same value (1 /
2) Esinshita output voltage controlled to synthesized to _0V performs symmetrical controlled to a value corresponding to the AC voltage reference, the frequency of the AC voltage reference Esinshita _0V from the first frequency omega _alpha to the second frequency omega _beta The output of one converter is K · E _α +
((1-K) / 2) Esinθ _0V or (1- (1-K) / 2)
Esin θ _0V + K · E _α and the output of the other converter is (1- (1-K) / 2)) Esin θ _0V -K · E _α or -K
E _α + ((1−K) / 2) Esin θ The value of K is continuously changed from 1 to 0 by controlling to _0V , and the AC voltage reference is continuously changed from asymmetric control to symmetric control. An AC voltage reference corrected based on the frequency is given, the input power factor is improved in the low frequency range, and the operation with good output characteristics is performed without disturbing the output voltage waveform in the high frequency range.

(Example) Hereinafter, as an example of the present invention, an example in which the present invention is applied to a cycloconverter will be described.

In this embodiment, the asymmetric control unit 41 shown in FIG.
Determines the phase output based on the sequence shown in FIG.

As shown in Figure 1, it compares the actual velocity omega _r phrase low-frequency operation speed omega _alpha and fast frequency operation speed omega _beta. ω
The _alpha and ω _β, 0 <ω _α <can be set arbitrarily in omega _beta, distortion of the cycloconverter output voltage waveform is the output voltage waveform does not perform no problem frequencies omega _alpha and symmetrical control by performing asymmetric control distorted effect is set to apply non-high-speed operation frequency _ω β as a system such as would at.

Therefore, | in the ≦ omega _alpha region, carried asymmetric control, | | ω _r ω _r | in the area of _{≧ ω β (ω β / 0} ), performs symmetrical control. Further, omega _alpha in the speed range of the region of ≦ ω _r ≦ ω _β, the next so that the voltage reference E in order to migrate the output voltage of each stage continuously smooth symmetrically control from asymmetric control
_U1S and E _U2S are determined.

As can be seen from FIG. 2, in the region of | ω _r | ≦ ω _α ,
In the region of | ω _r | ≧ ω _β with K = 1, control is performed with k = 0, and in the range of ω _α to ω _β , the value of K is 1 to 0.
To change continuously. In this way, as shown in FIG. 2, K is changed from 1 to 0 according to the speed, and 1
Controls stage output voltage E _U1s and two-stage output voltage E _U2S, 2 one transducer total voltage of the output, i.e., so that the voltage supplied to the electric motor to the motor output voltage reference E _U.

Fig. 3 Shows the output voltage waveforms of the first and second stages at the time of
Each output voltage is given by the following equation.

_{However, | E U | ≦ 2E α} | E U1S |, | shows a _{≦ E α E U = E U1S} + E U2S | E U2S.

FIG. 4 shows that when K = 0, the output voltage waveforms of the first and second stages are both the same sine wave voltage. That is, Becomes

_{However, | E U | ≦ 2E α} | E U1S |, | shows a _{≦ E α E U = E U1S} + E U2S | E U2S.

The above equation indicates that the control is symmetric.

As described above, in the asymmetric control unit 41 shown in FIG. 7, the voltage references _EU1S and _EU2S are determined, and _EU1S is defined as the output voltage U1SA and U1SB of the first-stage positive and negative group converters.
_EU2S is the output voltage U2S of the second-stage positive group and negative group converter
A, given as U2SB.

Up to this point, the description has been given for the output voltage of the U phase, but the same can be determined for the V and W phases.

(Effect of the Invention) As described above, according to the present invention, the means for continuously and smoothly shifting the output voltage reference of each converter from the asymmetric control to the symmetric control is provided. The operation can be realized up to the high-speed operation region without improving the rate and without distorting the output voltage.

[Brief description of the drawings]

FIG. 1 is a control flowchart showing one embodiment of the present invention,
FIG. 2 is a graph showing the relationship between the motor speed, the converter output voltage, and the variable K shown in FIG. 1, and FIGS.
FIG. 5 is a time chart showing an output voltage waveform of the power converter when the control method shown in FIG. 1 is applied. FIG. 5 is a circuit diagram of a cycloconverter to which the control method according to one embodiment of the present invention shown in FIG. 1 is applied. FIG. 6 is a schematic diagram showing a control circuit for controlling the main circuit of the cycloconverter shown in FIG. 5, and FIG. 7 is a diagram showing a conventional power converter control method. FIG. 8 is a flow chart showing one control method of the conventional power converter. 1,2,11,12,21,22… Positive group converter 3,4,13,14,23,24 …… Negative group converter 8 …… Induction motor 41 …… Asymmetric control unit 42 …… Main circuit

Claims (1)

(57) [Claims] 1. A power converter comprising: a positive converter and a negative converter comprising two sets of cascade-connected converters connected in anti-parallel via a reactor, and having a constant frequency based on an AC voltage reference. When converting the AC voltage to an AC voltage having an arbitrary frequency, when the arbitrary frequency is equal to or lower than the first frequency, the output of one of the converters is controlled to a maximum voltage, and the output of the one converter and the other converter is controlled. Asymmetric control is performed by controlling the other converter so that the combined output voltage becomes a value corresponding to the AC voltage reference, and when the arbitrary frequency is equal to or higher than a second frequency higher than the first frequency, The output voltage of the one converter and the other converter is symmetrically controlled so that the output voltage synthesized with the same value becomes a value corresponding to the AC voltage reference, from the first frequency to the second frequency Before the asymmetric control The method of the power conversion device characterized by providing a corrected AC voltage reference based on the frequency of the AC voltage reference the one of the transducer and the other transducer to continuously shift to symmetrical control.