JP2014150598A - Electric power conversion system and determination method for bias magnetism of transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power conversion system, interconnected with a system through a transformer, capable of well suppressing DC bias magnetism of the transformer.SOLUTION: The electric power conversion system includes: AC CTs on the AC system side (hereinafter referred to as "primary side") and power converter side (hereinafter referred to as "secondary side") of a transformer connected with an AC system; means for detecting a difference current of each of the AC CTs; and a function for determining bias magnetism of a transformer core from the difference current.

Description

The present invention relates to a power conversion device and a method for determining the demagnetization of a transformer. In particular, the present invention is connected to an AC power system via a transformer, receives and receives power from the AC power system, and further includes a DC demagnetization of the transformer. The present invention relates to a power conversion apparatus and a method for determining a magnetic bias of a transformer suitable for suppressing noise.

  Usually, a power converter connected to an AC system connects a power converter via a transformer. The power converter is controlled so that no DC voltage or DC current is output so that the transformer core (hereinafter simply referred to as a transformer core) is not saturated by a well-known DC bias. However, due to the offset error of voltage sensors (hereinafter simply referred to as VT (Voltage Transformer)) and current sensors (hereinafter simply referred to as CT (Current Transformer)), the power converter generates unintended DC voltage and DC current. Output. As a result, the saturation of the transformer core due to direct current bias increases the leakage flux that does not pass through the transformer core, overheating the surrounding bolts, etc., possibly weakening the mechanical strength. In addition, when the transformer core is saturated, the exciting current of the transformer increases, and part of the exciting current also flows to the power converter, which may exceed the allowable current value of the power converter. In addition, when the transformer core is saturated, the noise generated by the transformer also increases.

  Since various problems arise when the transformer core is saturated in this way, it is desired to determine the state of magnetic bias of the transformer core. Therefore, for example, a technique is known in which a current transformer is installed on each of the primary side of the transformer and the secondary side of the transformer, and the demagnetization state of the transformer core is determined based on the current from the current transformer. Yes. Such a technique is described, for example, in JP-A-11-162769.

JP-A-11-162769

  The current transformer in the above-described prior art includes not only a signal related to the DC bias magnetism of the transformer but also a signal related to the load current flowing through the transformer, and in particular, this signal includes a DC component. In addition, the component of the exciting current flowing through the transformer due to DC bias is very small compared to the magnitude of the load current flowing through the transformer. When the demagnetization detection / suppression control is performed, there is a problem that the determination of the demagnetization detection / suppression control becomes inaccurate. In addition, it was impossible to distinguish between the components generated by the DC bias of the transformer core and the components generated from the AC system and the power converter, which could lead to erroneous determination.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a power converter that can accurately and accurately detect DC bias of a transformer core and a method for determining the bias of a transformer.

  In order to achieve the above object, in the present invention, a first alternating current is output according to a current flowing from the inverter device to the transformer, and a second alternating current is generated according to the current flowing from the transformer to the power system. Outputs a differential current that is a difference between the first alternating current and the second alternating current, and a direct current component contained in the current flowing from the inverter device to the transformer is suppressed. The alternating current component of the exciting current of the transformer is detected from the differential current, and the magnetic bias of the iron core of the transformer is determined from the alternating current component of the exciting current.

  According to the present invention, it is possible to eliminate the influence of the load current of the transformer and reduce the possibility of erroneous determination of DC bias.

The structure of the power converter device which is the 1st Example form of this invention is shown. The structure of the control apparatus of the power converter device which is the 1st Example form of this invention is shown. An example of the excitation current waveform of a transformer when the transformer is demagnetized to the positive side is shown. An example of the transformer excitation current when the transformer is biased to the positive side shows the waveforms of each component separated into a DC component, fundamental component, and second harmonic component. The structure of the control apparatus of the power converter device which is 2nd embodiment of this invention is shown.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows a power converter according to a first embodiment of the present invention.

  First, the structure of the power converter device which is 1st embodiment of this invention is demonstrated.

The power conversion device 1 of the present embodiment includes a voltage source inverter device (hereinafter simply referred to as an inverter device) 4 having a DC capacitor and a self-extinguishing element such as IGBT or GCT,
A DC voltage sensor 5 for detecting the DC voltage Vdc of the inverter device 4;
A control device 2 that outputs an output voltage command value Vc_ref for controlling the output voltage of the inverter device 4;
A PWM circuit 3 for generating a gate pulse Gate_Pulse for controlling on / off of the self-extinguishing element of the inverter device 4 in accordance with an output voltage command Vc_ref given from the control device 2,
A transformer 6 interconnecting the inverter device 4 and the AC system 9,
An output current sensor 7 which is installed on the AC system 9 side of the transformer 6 (hereinafter simply referred to as the primary side) and detects the current Is1 output from the power converter 2 to the AC system 9;
AC voltage sensor 8 for detecting system voltage Vs of AC system 9,
Primary AC CT11 (or second AC) that is installed on the primary side of transformer 6 and detects current Is1 and transforms the alternating current component of current Is1 to Is2, such as a well-known wire-wound current transformer. (Also called current sensor)
Installed on the inverter device 4 side of the transformer 6 (hereinafter simply referred to as the secondary side), for example, detects a current Ic1 output from the inverter device 4 like a well-known winding current transformer, and detects the current Ic1 Secondary AC CT10 (also referred to as a first AC current sensor) that transforms the AC component into Ic2,
A burden resistor 12 connected in parallel to each output terminal of the primary AC CT11 and the secondary ACCT10;
Current detection means 13 for detecting the difference current Imag between the currents Ic2 and Is2 flowing through the load resistor 12
And is configured.

  Here, the primary side AC CT11 and the secondary side AC CT10 both operate as an AC current sensor and are generated by the time change of the magnetic flux generated by the current flowing to the primary side of each CT from the law of electromagnetic induction. An AC current flows on the secondary side of each CT due to the AC induced voltage. That is, the primary AC CT11 and the secondary AC CT10 cut the DC component that does not change with time and output only the AC component. As described above, the primary AC CT11 and the secondary AC CT10 are used as an AC current sensor that detects only an AC component and outputs a current, such as a wound current transformer, Rogowski coil, Pearson CT, etc. Is used.

  Although the output current sensor 7 is installed on the primary side of the transformer 6 in this embodiment, it may be installed on the secondary side of the transformer 6.

  In addition, the inverse ratio of the current ratio of the primary AC CT11 and the secondary AC CT10 is set so that the differential current Imag flowing through the load resistor 12 becomes the current ratio converted multiple of the excitation current of the transformer 6 excluding the DC component. Is approximately equal to the turns ratio of the primary winding and the secondary winding of the transformer 6.

  Also, when the transformer 6 is not dc-biased, the difference current Imag is 1/10 to 1 / hundredth of the magnitude of the currents Ic2 and Is2. Thus, the AC CTs 10 and 11 are less likely to be saturated than when the differential current Imag is calculated after connecting the burden resistor 12 to the AC CTs 10 and 11, respectively. Therefore, the burden resistance 12 can be increased and detected with high sensitivity.

  In this embodiment, the transformer 6 is a single-phase transformer to simplify the explanation, but the current flowing in the primary winding and the secondary winding wound around each leg of the iron core of the transformer 6 It is characterized by detecting the AC component of the excitation current by taking the difference, and the difference current Imag can be detected in the same manner even if the transformer 6 is a three-phase transformer.

  Also, even in a transformer that has three or more windings wound around the transformer core, the difference between the currents flowing from the primary winding wound around the same leg of the transformer core to each winding can be obtained. The AC component of the excitation current can be detected.

  Next, the operation of the power conversion device 1 according to the first embodiment of the present invention will be described.

  The control device 2 receives the DC capacitor voltage Vdc of the inverter device 4, the system voltage Vs of the AC system 9, the output current Is1 and the difference current Imag of the power converter 1, and the output voltage command value Vc_ref of the inverter device 4 is obtained. Output.

  The PWM circuit 3 outputs a gate pulse for controlling on / off of the self-extinguishing element of the inverter device 4 based on the magnitude relationship between the output voltage command value Vc_ref and a carrier such as a triangular wave.

  The inverter device 4 converts the power with the AC system by outputting the output voltage Vc of the inverter device 4 to the transformer 6 based on the gate pulse output from the PWM circuit 3.

  Next, the configuration and operation of the control device 2 will be described. FIG. 2 is a diagram showing an outline of the control block of the control device 2 shown in FIG.

The control device 2
A voltage command calculator 21 that receives the DC voltage Vdc, the system voltage Vs, and the output current Is1, and calculates a voltage command value Vc_ref0 to be output from the inverter device 4,
The AC voltage Vs and the difference current Imag are input, for example, a known discrete Fourier transform (hereinafter simply referred to as DFT) is performed, and the same frequency as the AC voltage Vs of the difference current Imag based on the phase of the AC voltage Vs. A fundamental phase calculator 22 for computing the phase θ1 of the component (hereinafter referred to as fundamental component);
The AC voltage Vs and the difference current Imag are input, for example, a well-known DFT is performed, and the amplitude component I2 of the frequency component twice the AC voltage Vs of the difference current Imag (hereinafter referred to as the second harmonic component) and the AC voltage Vs A second harmonic amplitude / phase calculator 23 for calculating the phase θ2 of the second harmonic component of the difference current with respect to the phase;
Each of the phases θ1 and θ2 is input, and the polarized-polarity determination calculator 24 that determines the polarized-polarity of the transformer 6 based on the logic of the equation (1),
Deviation between secondary harmonic component command value I2_ref of differential current Imag (usually 0 to suppress DC demagnetization) and secondary harmonic component amplitude I2; For example, a bias control arithmetic unit 25 for calculating a DC compensation voltage Vhos having a polarity opposite to that of the pole polarity in order to suppress the bias of the transformer 6 by performing a well-known proportional integral calculation.
The sum of the output voltage command value Vc_ref0 of the voltage command value calculator 21 and the compensation voltage Vhos that is the output of the bias control calculator 25 is set as a new output voltage command value Vc_ref of the inverter device 4.

  Here, the difference current Imag has a waveform from which the DC component has been removed from the exciting current of the transformer 6.

  Note that I2, θ1, and θ2 are constant value signals that do not change with time if the input difference current Imag has the same value in the fundamental wave period.

  The new output voltage command value Vc_ref is, for example, a command value that varies with a period depending on the power system, and includes a slight DC component in a waveform similar to a sine wave.

  In the present embodiment, the configuration of the control device 2 includes a voltage command calculator 21, a fundamental wave phase calculator 22, a secondary harmonic amplitude / phase calculator 23, a polarized magnetic polarity determination calculator 24, and a bias control calculator. Although 25 is described as a functional block, these functions may of course be implemented as a program operation of one or a plurality of electronic computers, for example.

  In this case, in the control device 2, for example, the output voltage command value Vc_ref0 is calculated every predetermined control cycle, and the compensation voltage Vhos is calculated. Calculations according to the expressions (1) to (3) shown below and the determination of the magnetic pole polarity are also performed in this series of control cycles.

  Next, the determination method of the polarized magnetic polarity determination calculator 24 will be described in detail.

  Fig. 3 shows a typical waveform of the excitation current of the transformer with the transformer biased to the positive side. The exciting current is characterized by the fact that a DC component with the same polarity as that of the demagnetized polarity is superimposed, resulting in a waveform having a peak in the depolarized polarity. Fig. 4 shows the waveforms obtained by separating the excitation current into a DC component, a fundamental component with a period T, and a second harmonic component with a period T / 2. As shown in Fig. 4, when the magnet is biased to the positive side, the positive peak of the fundamental wave component of the excitation current and the positive peak of the secondary harmonic component are approximately the same, so the fundamental wave component of the excitation current The second harmonic component has the relationship of equation (1).

Positively demagnetized: θ2−2 × θ1 ≒ 0 (rad) (1)
Since the negative side peak of the fundamental component of the excitation current and the negative side peak of the secondary harmonic component are approximately the same when biased to the negative side, the fundamental and secondary harmonic components of the excitation current are It has the relationship (2).

Negatively magnetized: θ2−2 × θ1 ≒ π (rad) (2)
Therefore, the depolarization determining unit 24 can determine the depolarized polarity, for example, as shown in Equation (3).

Positively biased: Cos (θ2−2 × θ1) ≧ 0
Negatively polarized: Cos (θ2−2 × θ1) <0 (3)
The power conversion device 1 of the present embodiment can remove the influence of the load current of the transformer 6 and can reduce the possibility of erroneous determination of DC bias.

  Further, by applying the power conversion device 1 of the present invention, it is possible to reduce the possibility of erroneous determination of DC bias due to DC component detection error.

  In addition, transformer DC noise can be suppressed with high accuracy, and transformer noise can be reduced.

  FIG. 5 is a diagram schematically showing a control block diagram of the control device 12 of the power conversion device 1 according to the second embodiment of the present invention.

  First, the structure of the control block of the power converter device which is 2nd embodiment of this invention is demonstrated. However, the description of the same or corresponding parts as in the first embodiment will be omitted, and only different parts will be described.

  The control block of the control device 12 of the power conversion device 1 shown in FIG. 5 is not the fundamental phase calculator 22 and the secondary harmonic amplitude / phase calculator 23 but the system voltage 1 of the control blocks shown in FIG. The difference is that the period maximum value calculator 31 and the period minimum value calculator 32 for calculating the maximum value and the minimum value of the difference current Imag are used for each period.

  Next, only the differences from the first embodiment will be described regarding the operation and effects of the control device 12.

  Since the difference current Imag is detected by AC CT, the DC current component is removed from the excitation current of the transformer 6. When the DC bias is not applied, the difference current Imag has a substantially positive / negative symmetrical waveform, and the sum of the maximum value and the minimum value for each cycle of the system voltage is approximately zero.

  On the other hand, when DC is magnetized, the difference current Imag has a positive and negative asymmetric waveform with a large peak in the magnetized polarity, and the sum of the maximum and minimum values for each period of the system voltage has a value of the magnetized polarity. .

  The power conversion device 1 of the present embodiment uses the sum of the maximum value Imag_max and the minimum value Imag_min of the difference current Imag for each cycle of the system voltage as the amount of bias of the transformer 6, and the command value Isum_ref of the amount of bias of the transformer 6 (Normally, 0 is used to suppress DC bias), and the deviation is input to the bias control calculator 25.

  By applying the power conversion device 1 of the present embodiment, the influence of the load current of the transformer 6 can be removed, and the possibility of erroneous determination of DC bias can be reduced.

  Further, by applying the power conversion device 1 of the present invention, it is possible to reduce the possibility of erroneous determination of DC bias due to DC component detection error.

  In general, when the transformer is DC-magnetized and saturated, the excitation current increases rapidly, so the sum of the maximum value Imag_max and the minimum value Imag_min of the difference current Imag for each cycle of the system voltage also becomes very large. Therefore, by applying the power conversion device 1 of the present invention, it is possible to detect the DC bias with high sensitivity.

  In addition, transformer DC noise can be suppressed with high accuracy, and transformer noise can be reduced.

  As mentioned above, it is related with the power converter device which has a function which suppresses the direct current | flow magnetism of a transformer among the power converter devices connected to an alternating current system using a transformer.

1 Power converter
2 Control unit
3 PWM circuit
4 Voltage source inverter device
5 DC voltage sensor
6 Transformer
7 Output current sensor
8 AC voltage sensor
9 AC system
10 Secondary AC CT
11 Primary AC CT
12 Burden resistance
13 Current detection means
21 Voltage command calculator
22 Fundamental phase calculator
23 Second harmonic amplitude / phase calculator
24 Polarity judgment calculator
25 Magnetic bias control calculator
31 Period maximum value calculator
32 Period minimum value calculator

Claims (7)

A power converter connected to a power system through a transformer, an inverter device that outputs predetermined AC power to the converter, and a first that outputs an AC current according to a current flowing to the transformer An AC current sensor, and a second AC current sensor that outputs an AC current according to a current flowing from the transformer, wherein the current of the first AC current sensor and the current of the second AC current sensor An excitation current detection unit that detects an alternating current component of the transformer excitation current from a difference current that is a difference, and a demagnetization determination unit that determines the bias of the transformer iron core from the alternating current component of the transformer excitation current. And a power converter configured to suppress a direct current component included in the exciting current of the transformer.
2. The power conversion device according to claim 1, wherein the first alternating current sensor is configured as a first current transformer, the second alternating current sensor is configured as a second current transformer, and The ratio of the current transformer ratio of the first current transformer and the second current transformer is substantially equal to the reciprocal of the turns ratio of the primary side and the secondary side of the transformer, and the first alternating current and the second current transformer The output of the alternating current is connected, a resistor is connected in parallel with the output of the first alternating current and the second alternating current, and the alternating current component of the exciting current of the transformer is detected from the current flowing through the resistor. A power converter characterized by that.
2. The power conversion device according to claim 1, wherein among the alternating current components of the exciting current, a fundamental wave component that is a frequency component equal to a system voltage of the power system and a secondary component that is a frequency component twice the system voltage. The magnetic pole of the transformer core is determined from the phase difference from the harmonic component, and the amount of magnetic bias of the transformer core is determined from the amplitude of the secondary harmonic component of the excitation current. Power conversion device.
2. The power converter according to claim 1, wherein a maximum value or a minimum value of the AC component of the excitation current during one period of the system voltage is calculated, and the polarized magnetic pole of the transformer core is calculated from the maximum value or the minimum value. The power converter characterized by determining the property and the amount of magnetic bias.
The power conversion device according to claim 1, wherein a maximum value or a minimum value of the AC component of the excitation current during one cycle of the system voltage is calculated, and a sum of the maximum value and the minimum value is used to calculate the transformer core. A power conversion device having a function of determining a polarization property and a polarization amount.
2. The power conversion device according to claim 1, wherein at least one of the first alternating current sensor and the second alternating current sensor is a current transformer having an air-core configuration.
A first alternating current is output according to the alternating current component of the current flowing from the inverter device to the transformer, a second alternating current is output according to the alternating current component of the current flowing from the transformer to the power system, and the first Forming a differential current that is a difference between the alternating current of the second current and the second alternating current, and a direct current component included in a current flowing from the inverter device to the transformer is suppressed, and from the differential current, the A transformer demagnetization determination method for detecting an AC component of an excitation current of a transformer and determining a demagnetization of an iron core of the transformer from the AC component of the excitation current.