JP2018011405A - Power conversion system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power conversion system which never imposes any constrains on operation of a capacitor for phase modification, and which can avoid harmonic waves' instability phenomenon without a synchronous phase modifier.SOLUTION: A power conversion system comprises: a separately excited converter LCC connected to an AC system A and an AC system B; and a self-excited converter VSC interconnected to the AC system A. The self-excited converter VSC includes: a self-excited power converter 20 connected to the AC system A; a control part 30 which calculates voltage command values V, V, Vto the power converter 20 so that a current output from the power converter 20 to the AC system A follows first current command values i, i; and an active filter 40 which extracts a harmonic voltage component from an AC voltage of the AC system, and outputs a second current command value determined by multiplying the harmonic voltage component by a predetermined gain K. The control part 30 superposes the second current command value on the first current command values i, ito calculate the voltage command values V, V, V.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a power conversion system capable of suppressing harmonics generated by a separately excited converter connected to an AC system.

  Conventionally, there has been known an event (also referred to as a harmonic instability phenomenon) that continues after a harmonic generated by a separately-excited converter linked to an AC system is gradually expanded. This event occurs when a parallel resonance point of a specific harmonic order exists in the AC system.

  In order to avoid such a harmonic instability phenomenon, various solutions have been proposed. For example, it has been proposed to provide a phase adjusting facility with a phase adjusting capacitor in an AC system, and to suppress the generation of harmonics by shifting the parallel resonance point by turning on and opening the phase adjusting facility (for example, Patent Document 1). In addition, it has been proposed to avoid the harmonic instability phenomenon by connecting a synchronous phase adjuster to the AC system.

  However, when the phase adjusting equipment is provided in the AC system, the operation of the phase adjusting equipment may be restricted. For example, the phase adjusting equipment is provided for reactive power compensation, but if this is used to suppress the generation of harmonics, reactive power compensation cannot be performed. When a synchronous phase adjuster is provided in an AC system, a loss occurs when the synchronous phase adjuster is operated, and maintenance thereof is required.

JP 2001-37083 A

  In view of such circumstances, the present invention provides a power conversion system capable of avoiding harmonic instability without causing operational restrictions on a phase adjusting capacitor and without using a synchronous phase adjuster. With the goal.

  A first mode for achieving the above object includes a separately excited converter linked between two AC systems, and a self-excited converter linked to one or both of the two AC systems. The self-excited converter includes a self-excited power converter connected to the AC system, and the power so that a current output from the power converter to the AC system follows a first current command value. A control unit that calculates a voltage command value to the converter, and an active filter that outputs a second current command value that is in phase with the harmonic voltage component of the AC system and whose amplitude is proportional; In the power conversion system, the voltage command value is calculated by superimposing the second current command value on the first current command value.

  In the first aspect, it is possible to suppress harmonics generated by a separately-excited converter linked to an AC system, avoid harmonic instability, and enable stable operation. In addition, since the phase adjusting capacitor connected to the AC system is not applied to suppress harmonics, the operation of the phase adjusting capacitor is not restricted. Furthermore, since a synchronous phase adjuster is not required for the AC system, it is possible to avoid the loss of operating it and eliminate the need for maintenance.

  According to a second aspect of the present invention, in the power conversion system according to the first aspect, the active filter includes a high-pass filter or a band-pass filter that detects a harmonic voltage component from the AC voltage of the AC system, By multiplying a harmonic voltage component by a predetermined gain, a second current command value having the same phase as that of the harmonic voltage component and an amplitude proportional thereto is output.

  In the second aspect, the harmonic voltage is extracted by a high-pass filter or a band-pass filter, and the harmonic voltage component is multiplied by a predetermined gain, whereby the amplitude is proportional to the second phase of the harmonic voltage component. The current command value can be output.

  According to a third aspect of the present invention, in the power conversion system according to the second aspect, the gain is set to a value that avoids a harmonic instability phenomenon. It is in.

  In the third aspect, the second current command value for avoiding the harmonic instability phenomenon can be calculated by setting the gain.

  According to the present invention, it is possible to avoid the harmonic instability phenomenon of a separately-excited converter without causing operational restrictions of the phase-adjusting capacitor and without using a synchronous phase adjuster, and to stabilize the power conversion system. Can be realized.

1 is a schematic diagram of a power conversion system according to Embodiment 1. FIG. 2 is a block diagram of a self-excited converter according to Embodiment 1. FIG. Voltage _{V sys,} current _i lcc _{+ i acf,} is a graph showing the relationship between the output current _{i vsc.} Voltage _{V sys,} current _i lcc _{+ i acf,} is a graph showing the relationship between the output current _{i vsc.}

<Embodiment 1>
FIG. 1 is a schematic diagram of a power conversion system according to the present embodiment. As shown in the figure, the power conversion system 10 includes a self-excited converter VSC and a separately-excited converter LCC.

  The separately-excited converter LCC is a general separately-excited power converter, and is linked between two AC systems (AC system A and AC system B). The self-excited converter VSC is linked to the AC system A.

  An AC filter ACF1 and a phase adjusting capacitor C1 are connected to the AC system A, and an AC filter ACF2 and a phase adjusting capacitor C2 are connected to the AC system B. These AC filters ACF1 and ACF2 and phase adjusting capacitors C1 and C2 are used to suppress harmonic currents generated by the separately excited converter LCC.

  The AC bus of AC system A is connected to a voltage source and a short circuit reactance (AC Grid), and the AC bus of AC system B is connected to a voltage source and a short circuit reactance (AC Grid). The short-circuit capacity in each AC bus of AC system A and AC system B is simulated by the voltage source and the short-circuit reactance.

  The separately excited converter LCC, the self-excited converter VSC, the phase adjusting capacitor C1, and the phase adjusting capacitor C2 are connected to the AC bus of the AC system A and the AC bus of the AC system B through a transformer (not shown). doing.

Further, the voltage of the AC bus of AC system A is V _sys , the back voltage of AC system A is V _s , the short-circuit reactance in the AC bus of AC system A is L _ac , and the separately excited converter LCC after passing through AC filter ACF 1 _{Is represented} by i _lcc + i _acf , the output current from the self-excited converter VSC is i _vsc , and the power flow from the connection end to the non-connection end of the self- _excitation converter VSC is P _lcc .

  A detailed configuration of the self-excited converter VSC will be described with reference to FIG. FIG. 2 is a block diagram of the self-excited converter according to the present embodiment.

  The self-excited converter VSC of this embodiment includes a self-excited power converter 20 connected to the AC system A, a control unit 30 that controls the power converter 20, and an active filter 40.

The power converter 20 is a voltage type self-excited converter. The power converter 20 is provided with voltage command values V _u ^* , V _v ^* , and V _w ^* for each of the three-phase ACs from the control unit 30, and outputs an AC voltage corresponding to them to the AC system A. Then, the current i _vsc is output to the AC system A by the AC voltage based on these voltage command values V _u ^* , V _v ^* , and V _w ^* .

The controller 30 performs reactive power control (AQR) and DC voltage control (DCAVR) so that the current output from the power converter 20 to the AC system A follows the current command values i _q ^* and i _d ^*. The voltage command value V _d ^* and the voltage command value V _q ^* are output to the power converter 20. Specifically, the control unit 30 obtains the reactive power value _qac from a detector (not shown) that detects the reactive power of the AC system A, and sets the reactive power value _qac and the reactive power command value _qac ^* . A control value such that the deviation is zero is obtained, and a current command value i _q ^* is _obtained by multiplying the control value by a predetermined gain K _aqr .

Further, the control unit 30 obtains the voltage value V _dc from a detector (not shown) that detects the voltage of the AC system A so that the deviation between the voltage value V _dc and the voltage command value V _dc ^* becomes zero. A current control value is obtained, and a current command value i _d ^* is _obtained by multiplying the control value by a predetermined gain K _dcavr . Note that these two current command values i _q ^* and i _d ^* correspond to the first current command value in the claims.

Further, the control unit 30 includes a non-interference current controller 31 (ACR). Non-interference current controller 31, a detector for detecting a current of the AC system A (not shown) to obtain the current value i _d of the d-axis, the deviation between the current value i _d and the current command value i _d ^* A voltage command value V _d ^* that is zero is output. Similarly, the non-interference current controller 31 obtains a q-axis current value i _q from a detector (not shown) that detects the current of the AC system A, and the current value i _q and the current command value i _q ^* The voltage command value V _q ^* is output so that the deviation of the zero becomes zero.

The voltage command values V _d ^* and V _q ^* output by the non-interference current controller 31 are converted to voltage command values V _u ^* , V _v ^* and V _w ^* by the inverse dq converter 32, and the power converter 20 is output.

When the active filter 40 is not functioned, the self-excited converter VSC operates in the same manner as a conventional self-excited converter. That is, the controller 30 outputs a current i _vsc corresponding to the voltage command value V _dc ^* and the reactive power command value q _ac ^* to the AC system A.

Active filter 40 of this embodiment includes a dq converter 41, and a 42d highpass filter, and 42q, and the gain _{K af.}

The dq converter 41 obtains voltage values V _u , V _v , and V _w from a detector (not shown) that detects the voltage of each phase of the AC system A, and performs dq conversion on these based on the fundamental wave phase. , Voltage values V _d and V _q are output. As a result, the fundamental wave frequency components of V _u , V _v , and V _w are converted into direct current amounts at V _d and V _q .

42d pass filter extracts the harmonic voltage component from the voltage value V _d. Similarly the high pass filter 42q extracts harmonic voltage component from the voltage value V _q. Specifically, high-pass filters 42d and 42q with a cutoff frequency of 5 Hz, for example, are used.

Instead of the high-pass filter, a band-pass filter that extracts a specific harmonic voltage may be used. For example, the second harmonic voltage components of V _u , V _v , and V _{w having} a fundamental wave component of 50 Hz are converted to 50 Hz in V _d and V _q , and specifically, the pass band is set to, for example, 45 A band pass filter with a frequency of ˜55 Hz is used.

Harmonic voltage component output 42d highpass filter, from 42q is predetermined gain _{K af} is multiplied. The value obtained by multiplying the harmonic voltage components of the voltage command values V _d and V _q by the gain K _af corresponds to the second current command value of the claim in which the amplitude is proportional to the same phase as the harmonic voltage component.

The control unit 30 superimposes the second current command value obtained by multiplying the harmonic voltage component output from the voltage command value V _d by the gain K _af on the current command value i _d ^* . Similarly, the control unit 30, the second electric current command value obtained by multiplying the gain K _af the harmonic voltage component outputted from the voltage command value V _q, superimposed on the current command value i _q ^*.

Gain _K 1 _{/ K af} is the reciprocal of _af corresponds to the resistance value. By such a control unit 30, the self-excited converter VSC does not respond to the fundamental frequency of the AC system A, but acts as a resistance to the harmonic frequency. The gain _Kaf is set to a value that avoids the harmonic instability phenomenon. Specifically, since the harmonic instability phenomenon occurs when the impedance at the parallel resonance frequency of the AC system A exceeds the equivalent impedance of the separately-excited converter, a value lower than this is set. For example, if the equivalent impedance of the separately excited converter LCC is 500Ω, 1 / K _af is set to 330Ω, that is, K _af is set to 0.003S. Alternatively, a value that avoids the harmonic instability phenomenon by trial and error may be set by experiment or computer simulation.

By connecting such a self-excited converter VSC to the AC system A, the impedance at the parallel resonance point of the AC system A is reduced. As a result, the current i _vsc output from the self-excited converter VSC to the AC system A has a waveform that cancels the harmonic component.

The parameters shown in Table 1 were applied to the power conversion system having the configuration shown in FIGS. 1 and 2, and an instantaneous value simulation was executed. 3 and 4 are graphs showing the relationship _among the voltage V _sys , the current i _lcc + i _acf , and the output current i _vsc from the self-excited converter VSC obtained from the simulation results. FIG. 3 is a case where the active filter of the self-excited converter is not operated, and FIG. 4 is a graph when the active filter of the self-excited converter is operated. The horizontal axis is time, and the vertical axis is voltage or current.

As shown in FIG. 3, when the self-excited converter VSC including the active filter 40 is not operating, the current i _vsc is zero. At this time, as indicated by the voltage V _sys and the current i _lcc + i _acf , it can be confirmed that harmonic currents of 100 Hz and 320 Hz are flowing out from the separately excited converter LCC. In an actual power conversion system, the operation of the separately-excited converter LCC is stopped in order to prevent damage to equipment.

On the other hand, when the self-excited converter VSC including the active filter 40 is operating, the self-excited converter VSC does not respond to 45 to 55 Hz that is near the fundamental frequency of the AC system A, and other frequencies. Acts equivalently as a resistance of 1 / K _af = 330Ω.

Therefore, as shown in FIG. 4, the current i _vsc output from the self-excited converter VSC has a waveform that cancels the harmonic current. Thereby, the influence of the harmonic currents of 100 Hz and 320 Hz on the voltage V _sys and the current i _lcc + i _acf is greatly reduced.

  As described above, according to the power conversion system 10 of the present embodiment, the harmonics generated by the separately excited converter LCC can be suppressed, and the harmonic instability phenomenon can be avoided. Further, in the power conversion system 10, since the phase adjusting capacitor C2 is not applied to suppress harmonics, the operation of the phase adjusting capacitor is not restricted. Furthermore, since the power conversion system 10 does not require a synchronous phase adjuster, it is possible to avoid a loss of operating the same and eliminate the need for maintenance.

In the example shown in FIG. 4, since the reactive power command value q _ac ^{* is set} to zero, the current i _vsc has a waveform based on the command value from the active filter 40. It is not limited. For example, the self-excited converter VSC may operate in the same manner as a conventional fundamental reactive power compensator, and further superimpose a current command value from the active filter 40. By connecting the self-excited converter VSC having such a configuration to an existing power conversion system using the separately-excited converter LCC, the harmonic instability phenomenon of the existing separately-excited converter LCC can be avoided and stabilized. Operation is possible.

  In the power conversion system 10 described above, the active filter 40 including a high-pass filter or a band-pass filter and a gain is used. However, the present invention is not limited to such a configuration. The active filter 40 may have any configuration as long as it can output the second current command value having the same phase as the harmonic voltage component of the AC system A and having an amplitude proportional thereto. When the active filter outputs such a second current command value, a current that can avoid the harmonic instability phenomenon can be output from the self-excited converter VSC to the AC system.

  The self-excited converter VSC is linked to one AC system A, but may be configured to be linked to two AC systems A and AC system B.

DESCRIPTION OF SYMBOLS 10 ... Power conversion system, 20 ... Power converter, 30 ... Control part, 40 ... Active filter, 42d, 42q ... High pass filter

Claims (3)

Separately-excited converter connected between two AC systems,
A self-excited converter linked to one or both of the two AC systems,
The self-excited converter is:
A self-excited power converter connected to the AC system;
A control unit that calculates a voltage command value to the power converter so that a current output from the power converter to the AC system follows a first current command value;
An active filter that outputs a second current command value whose amplitude is proportional to the phase of the harmonic voltage component of the AC system;
The control unit calculates the voltage command value by superimposing the second current command value on the first current command value. The power conversion system according to claim 1,
The active filter includes a high-pass filter or a band-pass filter that detects a harmonic voltage component from the AC voltage of the AC system, and by multiplying the harmonic voltage component by a predetermined gain, the active filter has the same phase as the harmonic voltage component. And outputting a second current command value whose amplitude is proportional to the power conversion system. In the power conversion system according to claim 2,
The gain is set such that a harmonic instability phenomenon is avoided.