JPH0767351A - Controller for npc inverter - Google Patents

Abstract

PURPOSE:To improve the operating efficiency of a device with an NPC inverter circuit 4, to which DC voltage is input and the neutral-point potential of DC voltage is input which converts DC voltage into AC voltage by pulse width modulation control and outputs AC voltage. CONSTITUTION:A mode discriminating means 11 discriminating four kinds of operating modes from the combination of AC voltage output from an NPC inverter circuit and the code of AC currents or a signal related to these AC voltage and AC currents and a PWM control means 12 outputting a signal switching the NPC inverter in response to the result of the comparison of the decided operating modes, a voltage command and triangular waves for modulating pulse width are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter using PWM control, and more particularly to an NPC (Nutral Point Clampe).
d: Improvement of control device of NPC inverter using control by neutral point potential fixed) method.

[0002]

2. Description of the Related Art The output of a power conversion device using PWM control contains many harmonics, and a control system called NPC inverter is known as a control system for reducing these harmonics. FIG. 6 shows the configuration of a conventional device of this type in the case of driving a motor by converting a DC voltage into an AC voltage.

As shown in this figure, the NPC inverter 4
Is supplied with DC power from between the positive and negative poles P and N of the DC power supply 1, and is connected to the neutral point O divided by the capacitors 2 and 3, and the DC voltage is converted into the AC voltage by the switching signal SS from the control unit 9. To drive the electric motor 5. The position detector (resolver) 6 is connected to the rotary shaft of the electric motor 5,
The rotation speed Nr is fed back by the R / D converter 7 and compared with the speed reference Nr ^* to control the motor speed. The control unit 9 shows an example of vector control. From the torque command Te ^* and the magnetic flux command Φ ^* obtained from the speed deviation, d
The q-axis component current references Id ^* and Iq ^* are calculated and compared with the dq-axis component currents Id and Iq of the current detected by the current detector 8, respectively, and the three-phase inverter voltage commands V _U ^* and V are calculated from the current deviations. PWM control is performed by calculating _V ^* and _VW ^* .

FIG. 7A shows a main circuit configuration of the NPC inverter 4. A transistor, a GTO, or an IGBT is applied as a switching element.
This is shown in the example of TO. In addition to switching elements S11, S12 and flywheel diodes DF11, DF12 that form a normal three-phase bridge, switching elements S13, S14 for connecting to neutral point O, flywheel diodes DF13, DF14, and clamp diodes D11, D12. Is provided for each phase. In the case of the U phase, this circuit is connected in series to the output terminal U at the series connection point of the switching elements S13 and S14, and is antiparallel to the series connection circuit of the switching elements S13 and S14 in which the diodes D11 and D12 are connected in series, that is, energization They are connected in parallel so that the directions are opposite. The series connection point of the diodes D11 and D12 is connected to the power source neutral point O, and the same applies to the V phase and the W phase. The control unit 9 is provided with a PWM control unit 9A that outputs ON and OFF gate signals to the switching elements according to the voltage commands of the respective phases.

FIG. 8 is a time chart showing the gate signals applied to the U-phase switching elements S11, S12, S13 and S14 according to the U-phase voltage command V _U ^* . This chart compares the voltage command V _U ^* and the triangular wave C for PWM control, and illustrates the operation of the PWM control unit 9A that determines the gate signal of the U-phase switching element. However, the same applies to other phases.
Looking at the period from t ₀ to t _{1 in} FIG. 8 (the range in which the triangular wave C is larger than the voltage command V _U ^* ), the switching elements S14, S13
Is on, and S11 and S12 are off. As a result, the output terminal U is connected to the switching elements S14, S13 and the diode D1.
1 and D12 are connected to the power supply neutral point O, and terminals P and N
Are disconnected by the switching elements S11 and S12, so that the output terminal U is fixed at the potential of the power supply neutral point O. This state continues from time t ₀ to t ₁ . At time t ₁ , the triangular wave C becomes smaller than the voltage command V _U ^* , the switching elements S11 and S13 are turned on, and the switching elements S14 and S12 are turned off. As a result, the output terminal U is connected to the terminal P via the switching elements S11 and S13, and is disconnected from the power supply neutral point O and the terminal N by the switching elements S14 and S12, so that the output terminal U becomes the potential of the terminal P. The voltage E _D1 is positive with respect to the neutral point O. This state continues until time t ₂ . Becomes a time t _2, becomes the same state as when the time t ₀ ~t ₁ triangular wave C is the voltage command V _U ^* becomes larger, it lasts until the time t _3.

The output terminal U is at the potential of the terminal P from time t ₃ to time t ₄ . Thus changes the potential E _U0 at the output terminal U, the switching element S up to the time point t ₆
The switching elements S11 and S14 are turned on and off while 12 is off and S13 is on, and the operation to be either the potential of the power supply neutral point O or the potential of the terminal P is repeated.

During the period from time t _{6 to} t ₇ (the range in which the triangular wave C is larger than the absolute value | V _U ^* | of the voltage command), the output terminal U
Becomes the potential of the power supply neutral point O, and the potential polarity of the output terminal U is inverted during this transition period. That is, time point t
_{At 7} , the triangular wave C becomes smaller than the absolute value | V _U ^* | of the voltage command, and the switching elements S14 and S12 are turned on and S
11, S13 is turned off, the output terminal U is connected to the terminal N,
A negative voltage −E _D2 with respect to the neutral point O. Then, the switching elements S12 and S13 are repeatedly turned on and off while the switching element S11 is kept off and the state of S14 is kept on, so that the potential _EU0 of the output terminal U is either the power supply neutral point O or the terminal N. The same applies to the output terminals V and W, which have that potential. FIG. 9 is a flowchart showing the above-mentioned U-phase switching state.

As described above, in the NPC inverter, the output voltage of the inverter seen from the neutral point O of the DC power source is positive,
It has three voltage states of zero and negative, and in particular, there is a period in which the neutral point of the DC power source is connected to the load via the diode and the GTO, and current flows through the neutral point of the DC power source during that period.

A snubber circuit composed of a capacitor C11, a resistor R11 and a diode D11a is generally connected in parallel to each switching element of the main circuit of the NPC inverter shown in FIG. 7 (a), as shown in FIG. 7 (b). The voltage increase rate dv / dt when the switching element S11 is turned off is suppressed.

This capacitor C11 is a switching element S
While 11 is off, it is charged by the voltage applied to the element, and when the switching element S11 is on, the electric charge of the capacitor C11 is discharged through the resistor R11 and released as heat energy. Therefore, a loss occurs every time the switching element S11 is turned on, and this is referred to as snubber loss.

Further, the loss of the switching element itself includes an on-loss and an off-loss which occur at the timing of turning on and off, and a steady loss which occurs in a steady energized state.
Since the above snubber loss, on-loss, and off-loss increase in proportion to the number of times of switching, the pulse width modulation frequency is limited by the characteristics of the switching element and the method of processing these losses.

[0012]

There is a demand to increase the pulse width modulation frequency in order to reduce the harmonics generated in the switching operation of the NPC inverter and to reduce the torque ripple when driving the AC motor. However, there is a problem in that the cooling device for processing the heat generated by these losses becomes large and expensive, which is not economical. There is also a demand for reducing the loss and improving the efficiency of the inverter at the current pulse width modulation frequency.

The present invention has been made in view of the above problems, and an object thereof is to reduce the number of times of switching of a switching element without reducing the pulse width modulation frequency while maintaining the conventional output characteristics. The purpose of the present invention is to obtain an NPC inverter control device capable of reducing loss and performing operation with improved efficiency.

[0014]

In order to achieve the above object, the present invention converts a direct current voltage and a neutral point potential of the direct current voltage into an alternating voltage by pulse width modulation control. In a device having an NPC inverter circuit for outputting, it is judged to be a mode judging means for judging four kinds of operation modes from the combination of the signs of the AC voltage and the AC current output by the NPC inverter circuit or signals related to them. PWM for outputting a signal for switching the NPC inverter circuit according to an operation mode and a comparison result of a voltage command and a triangular wave for pulse width modulation
A control means is provided.

Further, in the NPC inverter circuit, first, second, third and fourth switching elements each having a diode connected in antiparallel between a positive electrode and a negative electrode of a DC voltage are serially connected in series, and A circuit for obtaining an AC output from the series connection side of the second and third switch elements and connecting a diode between the other side of the second and third switch elements and the neutral point potential for each phase And the PWM control means is
A gate signal for turning on / off the first to fourth switching elements in eight normal modes is output for each phase.

Further, the PWM control means is the NPC.
When the inverter circuit outputs the potential at the neutral point, one of the second switch element and the third switch element is made conductive.

[0017]

In the above structure, the mode discriminating means is the NPC.
The polarity of the output voltage and output current of the inverter circuit
The regeneration state is discriminated by four types of operation modes, and the PWM control means generates a pulse width modulation control signal based on the discriminated operation mode and the voltage command, and switches the NPC inverter circuit. Further, the PWM control means determines any one of eight energization modes from the operation mode determined as the comparison result of the voltage command and the triangular wave for pulse width modulation, and according to the determined energization mode, the NPC inverter. A gate signal is output to each of the first to fourth switching elements of each phase of the circuit. Further, the PWM control means is an NPC.
When each phase of the inverter circuit is in the energization mode in which the potential of the neutral point is output, a gate signal is output to energize either one of the second switch element and the third switch element. Therefore, no loss occurs due to the switching of the other switching element that is not energized.
It is possible to significantly reduce the loss of the entire inverter circuit and improve the operation efficiency.

[0018]

FIG. 1 shows an embodiment of the present invention. In this embodiment, the control unit 10 has a new function in addition to the conventional circuit configuration. That is, the AC output current and voltage command of the NPC inverter circuit 4 are input, and their polarities (signs) are input.
A mode discriminating circuit 11 for discriminating four kinds of operation modes from the combination of the above and a PWM controller 12 for outputting a gate signal of pulse width modulation based on the discriminated operation mode and the voltage command are provided.

As an input to the mode discrimination circuit 11, N
A current reference for each phase may be generated and used instead of the AC output current of the PC inverter circuit 4, and NP may be used instead of the voltage command.
The AC output voltage of the C inverter circuit 4 may be used.

The mode discriminating circuit 11 and PW in the above configuration
A flow chart showing the operation of the M control unit 12 is shown in FIG.
This figure shows the U phase as a representative, and the same applies to other phases.

The mode discriminating circuit 11 operates in the operation mode 1 if the voltage command V _U ^* is positive and the current I _U is positive (double row state).
If the current I _U is negative (regeneration state), it is determined to be the operation mode 2,
Current I _U is negative when voltage command V _U ^* is negative
Then, the operation mode 3 is determined, and if the current I _U is positive (regeneration state), the operation mode 4 is determined.

The PWM control unit 12 determines eight kinds of energization modes according to the magnitude comparison result of the operation mode, the voltage command V _U ^*, and the triangular wave C for pulse width modulation, and determines the U-phase switch elements S11 to S14. Outputs a gate signal for turning on and off.
That is, in the operation mode 1, if the voltage command V _U ^* is larger than the triangular wave C, it is determined to be the conduction mode 1 and the switch element S
11 and S13 are turned on, and switch elements S14 and S12 are turned off. If voltage command V _U ^* is smaller than triangular wave C, energization mode 2
Then, only the switch element S13 is turned on and S12 to S14 are turned off. Therefore, since S14 is not turned on as in the conventional case, the energy of the snubber capacitor of S14 is not discharged and the loss due to the switching of S14 is eliminated.

In the operation mode 2, the voltage command V
_{If U} ^* is larger than the triangular wave C, it is determined to be the energization mode 3, and all the switch elements S11 to S14 are turned off. Conventionally, the switch elements S11, S13 were turned on, but no current flows through S11, S13, and flywheel diodes DF11, DF
Since it flows to 13, the unnecessary gate signal is not given.

In the operation mode 2, the voltage command V
_{If U} ^* is smaller than the triangular wave C, it is determined to be the energization mode 4, and only the switch element S14 is turned on and S11, S13, S12 are turned off. Therefore, since S13 is not turned on as in the conventional case, S
The energy of the snubber capacitor of 13 is not discharged, and the loss associated with the switching of S13 is eliminated.

Further, in the operation mode 3, if the absolute value of the voltage command | V _U ^* | is larger than the triangular wave C, it is judged to be the energization mode 5, the switch elements S11 and S13 are turned off, and S14 and S12.
To turn on. If the absolute value | V _U ^* | of the voltage command is smaller than the triangular wave C, it is determined to be the conduction mode 6 and the switch element S
Only 14 is turned on and S11, S13 and S12 are turned off. Therefore, since S13 is not turned on as in the conventional case, the energy of the snubber capacitor of S13 is not discharged and the loss due to the switching of S13 is eliminated.

Further, in the operation mode 4, if the absolute value | V _U ^* | of the voltage command is larger than the triangular wave C, it is judged to be the energization mode 7 and all the switch elements S11 to S14 are turned off.
In the past, the switch elements S14 and S12 were turned on, but the gate signal is not given unnecessarily. In the operation mode 4, if the absolute value │V _U ^* │ of the voltage command is smaller than the triangular wave C, the energization mode 8 is determined and the switch element S13
Only turn on and turn off S11, S14 and S12.

Therefore, since S14 is not turned on as in the conventional case, the energy of the snubber capacitor of S14 is not discharged and the loss due to the switching of S14 is eliminated. FIG. 3 is a time chart showing the series of operations described above in a waveform, and shows an example in which the current I _U is a delayed current.

Further, FIGS. 4 and 5 show the energization modes 1 to 8 respectively.
3 shows a path through which the current I _U of the NPC inverter circuit 4 in FIG. In the example of FIG. 3, times t _{0 to} t ₁
Becomes a conduction mode 4, and t _{1 to} t ₂ become a conduction mode 3 and these two conduction modes are repeated until time t ₃ ′. Time t ₃ '~t ₄ is energized mode becomes 1, t ₄ ~t
₅ becomes energization mode 2 and is repeated until time t ₆ ′. The time t ₆ '~t ₇ next conduction mode 8, t ₇ ~t
₈ becomes energization mode 7 and is repeated until time t ₉ ′. Time t ₉ '~t ₁₀ is energized mode 5 next, t ₁₀ ~t
₁₁ becomes the energization mode 6 and is repeated until time t ₁₂ ′.

[0029]

According to the present invention, the number of times the switching element of the NPC inverter circuit is turned on and off can be reduced without lowering the pulse width modulation frequency, and the loss associated with switching can be reduced. Moreover, it is possible to obtain the control device of the NPC inverter which has an improved operation efficiency without affecting the output voltage waveform.

Further, if the same loss as the conventional one is allowed, the pulse width modulation frequency can be increased, the harmonics of the output voltage waveform can be reduced, and the control device of the NPC inverter with improved output characteristics can be provided. it can.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a control device for an NPC inverter of the present invention.

FIG. 2 is a flowchart for explaining the operation of the main part of the above embodiment.

FIG. 3 is a waveform diagram for explaining the operation of the present invention.

FIG. 4 is a main circuit diagram for explaining an energization mode of the present invention.

FIG. 5 is a main circuit diagram for explaining an energization mode of the present invention.

FIG. 6 is a configuration diagram of a conventional NPC inverter control device.

7A is a main circuit diagram of an NPC inverter, and FIG. 7B is a snubber circuit diagram thereof.

8 is a waveform diagram for explaining the operation of the conventional device of FIG.

9 is a flowchart of switching of the conventional device of FIG.

[Explanation of symbols]

 1 ... DC voltage source 2, 3 ... Capacitor 4 ... NPC inverter circuit 5 ... Electric motor 6 ... Position detector 7 ... R / D converter 8 ... Current detector 11 ... Mode control discrimination circuit 12 ... PWM control unit

Claims (3)

[Claims] 1. An apparatus including an NPC inverter circuit, which receives a DC voltage and a neutral point potential of the DC voltage, converts the AC voltage into an AC voltage by pulse width modulation control, and outputs the AC voltage. Comparison of mode discrimination means for discriminating four kinds of operation modes from the combination of the output AC voltage and AC current or the sign of a signal related thereto, and the judged operation mode and voltage command and a triangular wave for pulse width modulation. PWM for outputting a signal for switching the NPC inverter circuit according to the result
An NPC inverter control device comprising a control means. 2. The apparatus according to claim 1, wherein the N
The PC inverter circuit includes first, second, third, and third diodes, each of which is connected in antiparallel between a positive electrode and a negative electrode of a DC voltage.
A fourth switch element is sequentially connected in series, and the second and third switch elements are connected.
A PWM circuit is provided for each phase to obtain an AC output from the series connection side of the switching elements and to connect a diode between the other side of the second and third switching elements and the neutral point potential for each phase. A control device for an NPC inverter, wherein the control means outputs a signal for turning on and off the first to fourth switching elements in eight energization modes for each phase. 3. The apparatus according to claim 2, wherein the P
The WM control means, when the NPC inverter circuit outputs the potential of the neutral point, the second switch element and the third switch element.
A control device for an NPC inverter, characterized in that any one of the switch elements is turned on.