JP5952087B2 - Power converter - Google Patents

Description

  The present invention relates to a three-level converter connected to a power system, and more particularly to a power converter that suppresses an unbalance between a positive-side voltage and a negative-side voltage of the three-level converter.

  In the three-level converter connected to the power system, a state where the positive voltage and the negative voltage are not equal is called neutral point potential imbalance. When neutral point potential imbalance occurs, a DC voltage is generated at the output voltage of the three-level converter, and there is a possibility that the transformer connected to the three-level converter is demagnetized.

  In addition, if the positive electrode voltage and the negative electrode voltage are not equal, the output voltage of the three-level converter includes a low-order harmonic voltage. In general, the harmonic current output from the power converter connected to the power system to the power system may be regulated by a guideline so that it is kept below a predetermined magnitude. May cause conflicts with the guidelines.

  Non-Patent Document 1 below discloses a capacitor (hereinafter referred to as a positive electrode) connected to a positive potential and a neutral potential by outputting a harmonic voltage (hereinafter referred to as a secondary harmonic voltage) that is twice the system voltage frequency. Side capacitor) and a capacitor that is connected to the negative side potential and the capacitor connected to the negative side potential (hereinafter referred to as the negative side capacitor) arbitrarily creates an unbalance of power and suppresses neutral point potential unbalance. It is disclosed.

Matsui, “Stabilization of neutral point potential by harmonic power flow of 3-level PWM inverter system SVG”, National Institute of Electrical Engineers of Japan, 1994, pp. 607-610

  In the technique described in Non-Patent Document 1, the amplitude of the secondary harmonic voltage is determined only according to the magnitude of the neutral point potential imbalance. However, the ratio Kn2 (= ΔE / ΔV2, hereinafter neutral) of the amplitude ΔV2 of the secondary harmonic voltage output from the three-level converter and the change amount ΔE of the neutral point potential compensated by the secondary harmonic voltage. (Referred to as point control gain Kn2) varies depending on the amplitude of the output current, the polarity of the reactive power output by the three-level converter, and the amplitude of the output voltage of the three-level converter.

  Therefore, if the amplitude of the secondary harmonic voltage is determined only according to the magnitude of the neutral point potential imbalance, there is a possibility that the neutral point potential imbalance cannot be satisfactorily controlled depending on the operation state of the three-level converter. .

  The present invention has been made in view of the above problems, and is a unipolar modulation type three-level conversion that outputs a harmonic voltage of an even multiple of the system voltage frequency in order to suppress neutral point potential imbalance. An object of the present invention is to enhance the neutral point potential imbalance suppression effect.

  The power converter according to the present invention makes the amplitude of the even harmonic voltage variable depending on the magnitude of the neutral point potential imbalance, the amplitude of the output current, and the power factor angle.

  According to the power conversion device of the present invention, the neutral point potential imbalance suppression effect is enhanced, thereby reducing the harmonic current output by the power conversion device, and further, the transformer connected to the power conversion device. DC bias can be prevented.

  Problems, configurations, and effects other than those described above will become apparent from the following description of embodiments.

It is a figure which shows the structure of the power converter device 21 which concerns on Embodiment 1. FIG. It is a figure explaining the principle in which the power converter device 21 compensates the neutral point control gain Kn2 in Embodiment 1. FIG. 3 is a functional block diagram showing details of an even harmonic amplitude calculator 14; FIG. It is a control block diagram of the power converter device 21 which concerns on Embodiment 2. FIG. It is a figure which illustrates F (Iq) 61 and G (Iq) 62 which are each neutral point electric potential control gains when the three-level converter 6 outputs two even-order harmonics of different orders. It is a figure explaining the method of preventing a neutral point potential control gain from becoming a negative value. It is a figure which shows a mode that the larger one is used among the absolute values of each neutral point potential control gain. It is a figure which shows the correspondence of neutral point electric potential variation | change_quantity (DELTA) E and the reactive current Iq.

  In the following, first, in the three-level converter controlled by the unipolar modulation method, the neutral point control gain is the amplitude of the output current, the polarity of the reactive power output from the three-level converter, and the amplitude of the output voltage of the three-level converter. The principle which changes with is explained. Then, the structure of the power converter device which concerns on this invention is demonstrated.

<About the principle that the neutral point potential control gain varies>
For example, when the three-level converter outputs a secondary harmonic of amplitude ΔV2, the U-phase voltage v _su of the connection point voltage, the U-phase output voltage v _{cu of} the three-level converter, and the U-phase output current of the three-level converter i _cu can be expressed by the following equation 1 with reference to the phase θ of the U-phase voltage vsu.

For simplicity of explanation, if the zero-phase component, the negative-phase component of the converter output current, and the negative-phase component of the converter output voltage are zero, the neutral point current i _Z flowing into the neutral point potential of the three-level converter. Can be represented by the following formula 2.

  Here, when the electrostatic capacitance of the positive electrode capacitor and the negative electrode capacitor is C, the neutral point potential change amount ΔE satisfies the relationship of the following equation (3).

  When the 3-level converter outputs only reactive power, the phase difference δ = 0 between the interconnection point voltage and the converter output voltage, and when outputting fast-phase reactive power, the power factor angle φ = −π / 2. Yes, the power factor angle φ = π / 2 when outputting the delayed reactive power. In order to simplify the explanation, assuming that the phase difference ψ = 0 between the interconnection point voltage and the secondary harmonic voltage and substituting Equations 1 and 2 into Equation 3, the neutral point control gain Kn2 is expressed by the following Equation 4- 1 and Formula 4-2.

  As is clear from (Equation 4-1) and (Equation 4-2), the neutral point control gain Kn2 varies greatly depending on the amplitude of the output current and the polarity of the reactive power. The control gain Kn2 decreases. This is because even if the 3-level converter outputs a secondary harmonic voltage having the same amplitude, the neutral point potential change amount that can be corrected thereby becomes small, that is, the performance of controlling the neutral point potential is degraded. means.

  Also, when the reactive power is output, the neutral point control gain Kn2 is 0 depending on the amplitude of the output voltage and the output current, that is, the neutral point potential imbalance cannot be suppressed even when the secondary harmonic voltage is applied. there is a possibility. Further, when the polarity of the neutral point control gain Kn2 is reversed, there is a possibility that the neutral point potential imbalance is erroneously controlled in a direction to further expand.

<Embodiment 1: Overall configuration>
FIG. 1 is a diagram illustrating a configuration of a power conversion device 21 according to Embodiment 1 of the present invention. The power conversion device 21 is a power conversion device that receives active power from the power system 1 and outputs reactive power to the power system 1. The power conversion device 21 is connected to the power system 1 via the system impedance 2, and includes an AC voltage detector 3, an associated transformer 4, a voltage type three-level converter 6 (hereinafter referred to as a three-level converter), and a current detection. 5, a voltage detector 9, a control device 10, and a PWM circuit 11.

  The AC voltage detector 3 detects the voltage at the connection point between the power system 1 and the power converter 21. The interconnection transformer 4 connects the two by converting the voltage between the power system 1 and the power converter 21 to each other. The three-level converter 6 includes a positive side DC capacitor 7, a negative side DC capacitor 8, and a switching element. The current detector 5 measures the current output from the three-level converter 6. The voltage detector 9 detects both-ends voltage of the positive side DC capacitor 7 (positive DC capacitor voltage Vp) and both-ends voltage of the negative side DC capacitor 8 (negative DC capacitor voltage Vn).

  The control device 10 converts the voltage Vs at the connection point (hereinafter referred to as the connection point voltage Vs), the current Ic output from the three-level converter 6, the positive DC capacitor voltage Vp, and the negative DC capacitor voltage Vn into three levels. The voltage command value Vc_ref1 instructing the device 6 to output the voltage Vc is output.

  The PWM circuit 11 performs a well-known unipolar modulation based on the magnitude relationship between the output voltage command value Vc_ref1 and a carrier such as a triangular wave, and performs a gate pulse for ON / OFF control of the switching element included in the three-level converter 6. Gate_Pulse is output.

  The three-level converter 6 outputs the output voltage Vc to the interconnection transformer 2 based on the gate pulse output from the PWM circuit 11. As a result, power is exchanged with the power system 1 where the output current Ic flows to the power system 1.

<Embodiment 1: Configuration of control device 10>
The control device 10 includes a neutral point potential control circuit 16 that controls the neutral point potential of the three-level converter 6. The neutral point potential control circuit 16 includes a phase detector 12, an even harmonic oscillator 13, a power factor angle calculator 15, and an even harmonic amplitude calculator 14.

  The phase detector 12 detects the phase of the interconnection point voltage Vs. The even harmonic oscillator 13 outputs a sine wave signal that oscillates at an even multiple of the frequency of the power system 1. The power factor angle calculator 15 calculates a power factor angle that is a phase difference of the output current Ic with respect to the interconnection point voltage Vs. The even harmonic amplitude calculator 14 outputs the amplitude of the even harmonic to be output to the three-level converter 6 in order to suppress the neutral point potential imbalance ΔVpn which is the difference between the positive capacitor voltage Vp and the negative capacitor voltage Vn. calculate.

  The phase detector 12 receives the connection point voltage Vs as input, and calculates and outputs the connection point voltage phase θ using a known PLL (Phase Locked Loop) circuit, DFT (Discrete Fourier Transform) circuit, or the like. At the same time, the angular frequency ω of the linkage point voltage Vs is calculated and output.

  The even harmonic oscillator 13 receives the interconnection point voltage phase θ output from the phase detector 12, oscillates at an even multiple of the frequency of the power system 1, and has three sine wave signals (hereinafter referred to as 120 ° different phases). 3 phase even harmonic signal).

  The even harmonic amplitude calculator 14 receives the neutral point potential imbalance ΔVpn, the interconnection point voltage Vs, and the output current Ic, and outputs the amplitude V2n of the three-phase even harmonic signal.

  The control device 10 multiplies the three-phase even harmonic signal output from the even harmonic oscillator 13 by the amplitude V2n of the three-phase even harmonic signal output from the even harmonic amplitude calculator 14, and outputs the result to three levels. The value subtracted from the output voltage command value Vc_ref of the converter 6 is newly output to the PWM circuit 11 as the output voltage command value Vc_ref1 of the three-level converter 6.

<Embodiment 1: Principle for Compensating Neutral Point Control Gain Kn2>
FIG. 2 is a diagram for explaining the principle by which the power conversion device 21 compensates for the neutral point control gain Kn2 in the first embodiment. The vertical axis in FIG. 2 indicates a neutral point potential change amount ΔE indicating an amount by which the neutral point potential can be compensated by the even harmonics output from the three-level converter 6. The horizontal axis of FIG. 2 shows the reactive current output from the three-level converter 6. However, in order to simplify the explanation, the amplitude of the output voltage of the three-level converter 6 is assumed to be constant.

  As described in (Equation 4-1) and (Equation 4-2), the neutral point potential that can be compensated when the three-level converter 6 outputs an even harmonic having the amplitude V2n0 is the magnitude of the reactive current Iq. It changes accordingly, and the neutral point potential can be sufficiently compensated by the even harmonics at the time of delay, but the amount that can be adjusted at the time of phase advance decreases. This means that even if the even harmonic amplitude is increased, it is difficult to obtain the effect of compensating for the neutral point potential, that is, the neutral point control gain Kn2 is decreased.

  Therefore, the power conversion device 21 according to the first embodiment calculates the neutral point control gain Kn2 once based on each input value, and compensates for the lowered portion. In the control calculation, the reciprocal of the control gain once calculated may be multiplied with the previous control gain in a superimposed manner. That is, the power converter 21 calculates the neutral point control gain Kn2 that has been used conventionally, and uses the same input value that is used when calculating the neutral point control gain Kn2, and its reciprocal 1 / Kn2. And a control gain obtained by multiplying these values in a superimposed manner may be used as a final neutral point control gain.

  According to the above method, the neutral point potential change amount ΔE can be kept constant regardless of the polarity and magnitude of the reactive current Iq, as shown in FIG. Below, the specific structural example which implement | achieves the said method is demonstrated.

  FIG. 3 is a functional block diagram showing details of the even harmonic amplitude calculator 14. The even harmonic amplitude calculator 14 includes a positive phase effective value calculator 31, a phase calculator 32, a neutral point controller 33, and a neutral point control gain reciprocal calculator 34.

  The positive phase effective value calculator 31 calculates the effective value of the positive phase component which is a three-phase balanced component. The phase calculator 32 calculates the phase difference between the two three-phase AC signals. The neutral point controller 33 outputs an even harmonic amplitude V2n0 that is increased or decreased according to the neutral point potential imbalance ΔVpn. The neutral point control gain inverse calculator 34 calculates the inverse 1 / K2n of the neutral point control gain.

  The neutral point controller 33 receives the neutral point potential imbalance ΔVpn and outputs an even harmonic amplitude V2n0 for compensating for the neutral point potential imbalance ΔVpn. The positive phase effective value calculator 31 receives the output current Ic and the output voltage command value Vc_ref, and outputs | Ic | and | Vc_ref |, which are positive phase effective values.

  The phase calculator 32 receives the interconnection point voltage Vs and the output voltage command value Vc_ref and outputs a phase difference δ between them. The neutral point control gain reciprocal calculator 34 receives as input the power factor angle φ, the positive current effective value | Ic | of the output current, the effective value | Vc_ref | of the output voltage command value, and the phase difference δ. The reciprocal 1 / Kn2 of Kn2 is output.

  In the even harmonic amplitude calculator 14, the neutral point controller 33 multiplies the amplitude V2n0 by the reciprocal 1 / K2n of the neutral point control gain, and outputs the result as the even harmonic amplitude V2n. Thereby, in the control calculation, as shown in FIG. 2, it is possible to compensate for the decrease in the neutral point control gain and obtain a constant control gain.

<Embodiment 1: Summary>
As described above, the power conversion device 21 according to the first embodiment is based on not only the neutral point potential imbalance ΔVpn but also the interconnection point voltage Vs, the output voltage Vc of the three-level converter 6, and the output current Ic. The amplitude of even harmonics is variable. Thereby, even when the three-level converter 6 outputs the fast phase reactive power, the neutral point potential imbalance can be favorably compensated.

  Moreover, since the neutral point potential imbalance can be satisfactorily suppressed by using the power conversion device 21 according to the first embodiment, the harmonic current output from the power conversion device 21 is reduced, and the regulations set by guidelines and the like are established. Can fit.

  Moreover, since the neutral point potential imbalance can be satisfactorily suppressed by using the power conversion device 21 of the present invention, the interconnection voltage transformation caused by the DC voltage output from the power conversion device 21 due to the neutral point potential imbalance. It is possible to prevent the DC bias of the device 4.

  In the first embodiment, the power conversion device 21 has been described on the assumption that it is a reactive power compensator that receives active power from the power system 1 and outputs reactive power. However, the present invention is not limited to this. The same applies to the following embodiments.

<Embodiment 2>
In the first embodiment, the configuration example has been described in which the neutral point potential change amount ΔE is not decreased by compensating for the decrease in the neutral point control gain Kn2. In the second embodiment of the present invention, a configuration example will be described in which the neutral point control gain is not decreased by compensating each other using even-order harmonics of a plurality of different orders. The description of the same configuration as that of the first embodiment will be omitted, and the following description will focus on the differences.

  FIG. 4 is a control block diagram of the power conversion device 21 according to the second embodiment. Here, only the block diagram of the control device 10 is shown. Since other components are the same as those in the first embodiment, description thereof is omitted. The neutral point potential control circuit 16 in FIG. 4 is the same as that in the first embodiment, except that a plurality of neutral point potential control circuits 16 are provided. Here, two examples are shown. The control device 10 further includes a determination device 42 and a switching device 41.

  Each neutral point potential control circuit 16 outputs three-phase even harmonics of different orders. For example, a combination of a second harmonic, a fourth harmonic, a second harmonic, a sixth harmonic, and the like can be considered.

  The determiner 42 receives the power factor angle φ, the linkage point voltage Vs, the output current Ic, and the output voltage command value Vc_ref, and the absolute value of each neutral point control gain K2n output by each neutral point potential control circuit 16. Which is the largest is determined.

  The switch 41 is connected to the neutral point potential control circuit 16 that outputs the absolute value of the neutral point control gain K2n determined to be the largest by the determiner 42, and outputs the output as the output voltage command value Vc_ref1.

<Embodiment 2: Principle for Compensating Neutral Point Control Gain Kn2>
FIG. 5 is a diagram illustrating F (Iq) 61 and G (Iq) 62 which are neutral point potential control gains when the three-level converter 6 outputs even harmonics of two different orders. The vertical axis of FIG. 5 represents the neutral point potential control gain Kn2, and the horizontal axis represents the reactive current Iq output from the three-level converter 6.

  As shown in FIG. 5, the neutral point potential control gains F (Iq) 61 and G (Iq) 62 are each 0 depending on the value of the reactive current Iq. This is equivalent to an uncontrollable point where neutral point potential imbalance cannot be suppressed no matter how the even harmonic amplitude is set. Further, when the neutral point potential control gain exceeds the uncontrollable point and becomes a negative value, the control is performed in the direction opposite to the originally intended control.

  FIG. 6 is a diagram illustrating a technique for preventing the neutral point potential control gain from becoming a negative value. This corresponds to taking the absolute value of each neutral point potential control gain. The absolute values | F (Iq) | 71 and | G (Iq) | 72 of the neutral point potential control gains F (Iq) 61 and G (Iq) 62 are the same as F (Iq) 61 and G (Iq) 62. Each of them is symmetrically moved in the vertical axis direction with the uncontrollable point as a turning point.

  FIG. 7 is a diagram showing a state in which the larger one of the absolute values of the neutral point potential control gains is adopted. When the larger one of | F (Iq) | 71 and | G (Iq) | 72 is selected, a neutral point potential control gain | H (Iq) | 82 shown by a thick line in FIG.

  FIG. 8 is a diagram illustrating a correspondence relationship between the neutral point potential change amount ΔE and the reactive current Iq. The vertical axis of FIG. 8 indicates the neutral point potential change amount ΔE, and the horizontal axis indicates the reactive current output from the three-level converter 6. The neutral point potential change amount ΔE that can be compensated when the three-level converter 6 outputs an even harmonic having an amplitude V2n0 is the neutral point potential change amount V2n0 × | H (Iq) | 91.

  Since the switch 41 employs the larger one of the neutral point potential control gains output from each neutral point potential control circuit 16, the output of the switch 41 is, as a result, V2n0 × || H (Iq) | ×. 1 / | HI (q) | 92. As is apparent from FIG. 8, this value does not have an uncontrollable point regardless of the value of the reactive current Iq, and does not become a negative value.

  In the above description, a plurality of even-order harmonics of different orders are used to compensate for each other. Similarly, even when a plurality of even-order harmonics of different phases are used to compensate for each other, FIGS. Since the same relationship as described is generated, the same effect as in the second embodiment can be exhibited using this relationship. In this case, each neutral point potential control circuit 16 may output even harmonics having different phases.

<Embodiment 2: Summary>
As described above, since the power conversion device 21 according to the second embodiment compensates for the neutral point potential control gain using a plurality of even-order harmonics of different orders, the power conversion device 21 uses only one even-number harmonic. Rather than compensating the sex point potential imbalance, a stable control operation can be realized.

  The configuration described in the second embodiment can be combined with the first embodiment. For example, when a plurality of neutral point potential control circuits 16 are provided and the neutral point potential imbalance can be sufficiently compensated only by the configuration described in the first embodiment, a single neutral point potential control circuit 16 is used. Then, when a sufficient effect cannot be obtained, it is conceivable to perform the operation of the second embodiment.

  The present invention is not limited to the embodiments described above, and includes various modifications. The above embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to the one having all the configurations described. A part of the configuration of one embodiment can be replaced with the configuration of another embodiment. The configuration of another embodiment can be added to the configuration of a certain embodiment. Further, with respect to a part of the configuration of each embodiment, another configuration can be added, deleted, or replaced.

  Each of the above-described configurations, functions, processing units, processing means, and the like may be realized in hardware by designing a part or all of them, for example, with an integrated circuit. Each of the above-described configurations, functions, and the like may be realized by software by interpreting and executing a program that realizes each function by the processor. Information such as programs, tables, and files for realizing each function can be stored in a recording device such as a memory, a hard disk, an SSD (Solid State Drive), or a recording medium such as an IC card, an SD card, or a DVD.

  1: Power system, 2: System impedance, 3: AC voltage detector, 4: Linkage transformer, 5: Current detector, 6: 3-level converter, 7: Positive side DC capacitor, 8: Negative side DC capacitor, 9: Voltage detector, 10: Control device, 11: PWM circuit, 12: Phase detector, 13: Even harmonic oscillator, 14: Even harmonic amplitude calculator, 15: Power factor angle calculator, 16: Neutral Point potential control circuit, 21: power conversion device, 31: positive phase effective value calculator, 32: phase calculator, 33: neutral point controller, 34: neutral point control gain reciprocal calculator, 41: switch, 42: Determinator.

Claims (6)

A power conversion device that is connected to an electric power system and outputs reactive power,
An inverter device having a switching element and configured as a three-level conversion unit capable of selectively outputting three potentials of a positive side potential, a neutral point potential, and a negative side potential;
A control device for controlling the inverter device by a unipolar modulation method;
A difference between a positive voltage that is a potential difference between the positive potential and the neutral point potential and a negative voltage that is a potential difference between the negative potential and the neutral potential is detected as a neutral point potential imbalance. An unbalance detection unit;
A system voltage detector that detects a system voltage that is a voltage of the power system;
An output current detector that detects an output current that is a current that the power conversion device outputs to the power system; and
With
The controller is
Compensating the neutral point potential imbalance by causing the inverter device to output an even harmonic voltage having an even multiple of the frequency of the system voltage,
The amplitude of the even harmonic voltage is variable according to the magnitude of the neutral point potential imbalance, the amplitude of the output current, and the power factor angle obtained from the system voltage and the output current ,
The control device further includes:
The ratio of the change amount of the neutral point potential imbalance compensated by the even harmonic voltage to the amplitude of the even harmonic voltage is calculated as a neutral point control gain,
The amplitude of the even harmonic voltage obtained once using the magnitude of the neutral point potential imbalance multiplied by the inverse of the neutral point control gain is output as the amplitude of the even harmonic voltage. And controlling the inverter device. The power converter according to claim 1, wherein the control device makes the amplitude of the even harmonic voltage variable according to an output voltage command value of the inverter device. The said control apparatus makes variable the amplitude of the said even harmonic voltage according to the phase difference between the said system | strain voltage and the said output voltage command value. The power converter device of Claim 2 characterized by the above-mentioned. The controller is
Outputting the even harmonic voltages of different orders to the inverter device;
The power converter according to claim 1, wherein any one or more of the even harmonic voltages are selected according to an output voltage command value of the inverter device, the output current, and the power factor angle. The controller is
Outputting the even harmonic voltage of a plurality of different phases to the inverter device;
The power converter according to claim 1, wherein any one or more of the even harmonic voltages are selected according to the system voltage, the amplitude of the output current, and the power factor angle. The controller is
The ratio of the change amount of the neutral point potential imbalance compensated by each even harmonic voltage to the amplitude of each even harmonic voltage is calculated for each even harmonic voltage as a neutral point control gain,
The amplitude of once the even harmonics voltage obtained by using the magnitude of the neutral point potential imbalance, a material obtained by multiplying the largest of the neutral point control gain obtained by each said even harmonics voltage The power converter according to claim 4 or 5 , wherein the inverter device is controlled so as to be output as an amplitude of the even harmonic voltage.

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