Power Conversion Systems

Abstract

To provide a power conversion system capable of suppressing the DC magnetization of a transformer connected thereto.SOLUTION: The power conversion system is a system to which multiple power converters are connected in parallel to supply the power to a load through a transformer. The power conversion system includes: a current detector for detecting the output current of the power converters; a load current calculator that calculates the current supplied by each of the power converters to the transformer from the output current detected by the current detector; and a biased magnetization suppression controller that suppresses the DC magnetization of the transformer by reducing the high-order components of the output of the load current calculator.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power conversion system including a plurality of power conversion devices connected in parallel. [Background technology]

[0002] A transformer is generally connected to the AC side of a power converter that converts AC to DC or DC to AC in order to obtain a desired AC voltage and current, and to insulate the AC side of the power converter from other devices such as a power source and a load. If the input voltage of such a transformer contains a DC component, the magnetic material constituting the transformer will saturate, causing problems such as a sudden decrease in the excitation inductance of the transformer, an excessive excitation current flow, and a distortion of the output voltage of the transformer, making it difficult to continue operating the power converter. This phenomenon of transformer saturation is called DC bias magnetization. Therefore, a method for suppressing such DC bias magnetization of a transformer is proposed in Patent Document 1. In Patent Document 1, the strength and direction of DC bias magnetization of the transformer is detected from the output current of the power converter, and the AC output of the power converter is corrected based on the detected value to suppress the DC bias magnetization of the transformer.

[0003] In addition, power conversion devices are parallelized in order to increase the power capacity to be output or to provide redundancy for the power conversion devices. At this time, cross current flows between each of the parallel-connected power conversion devices, generating active power and reactive power that are not exchanged with the power source or load connected to the AC side of the power conversion device. When such cross current flows, the power capacity required by each power conversion device increases, and the loss generated by each power conversion device increases. Therefore, Patent Document 2 proposes a method for suppressing cross currents flowing between each power conversion device in a power conversion system composed of multiple power conversion devices connected in parallel. Patent Document 2 shows an uninterruptible power supply system provided with a circuit for detecting cross currents flowing between multiple parallel-connected power conversion devices. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2003-309980 A [Patent Document 2] JP 2005-27436 A Summary of the Invention [Problem to be solved by the invention]

[0005] It is possible to suppress the cross current flowing between multiple parallel-connected power converters by detecting the cross current using a cross current detection circuit shown in Patent Document 2, for example, and controlling the cross current to be small. However, since it is difficult to completely eliminate the cross current, the output current of multiple parallel-connected power converters contains some cross current components. When DC bias magnetization occurs in a transformer connected to the AC side of such parallel-connected power converters, the output current of the power converter is superimposed with a component caused by DC bias magnetization and a component caused by the cross current. In that case, it becomes difficult to detect the strength and direction of DC bias magnetization of the transformer from the output current of the power converter, as in Patent Document 1.

[0006] An object of the present invention is to suppress DC bias magnetism in a transformer connected to a power conversion system constituted by a plurality of power conversion devices connected in parallel. [Means for solving the problem]

[0007] A preferred example of the present invention is a power conversion system in which a plurality of power conversion devices are connected in parallel and supply power to a load via a transformer, the power conversion system having a current detector that detects an output current of the power conversion device, a load current calculator that calculates a current to be supplied to the transformer by each of the power conversion devices from the output current detected by the current detector, and a bias magnetism suppression controller that reduces high-order components of the output of the load current calculator and suppresses DC bias magnetism of the transformer. Effect of the Invention

[0008] According to the present invention, it is possible to suppress DC bias magnetism occurring in a transformer connected to a power conversion system. [Brief description of the drawings]

[0009] [Figure 1] 1 shows the configuration of a first embodiment of a power conversion system in which two power conversion devices are connected in parallel. [Diagram 2] 2 shows the configuration of a load current calculator according to the first embodiment. [Diagram 3] 2 shows the configuration of a magnetic bias suppression controller according to a first embodiment. [Figure 4] 1 shows the configuration of a power conversion device according to a first embodiment in which three or more power conversion devices are connected in parallel. [Diagram 5] 1 shows the configuration of a second embodiment of a power conversion system in which two power conversion devices are connected in parallel. [Figure 6] 13 shows the configuration of a load current calculator according to a second embodiment. [Figure 7] 1 shows the configuration of a power conversion device according to a second embodiment in which three or more power conversion devices are connected in parallel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a power conversion system according to the present invention will be described with reference to the drawings. EXAMPLES

[0011] 1 shows the configuration of a power conversion system according to a first embodiment. The power conversion system 1 of the present embodiment is connected to a three-phase AC system 2 and a transformer 3, and the other terminal of the transformer 3 is connected to a load 4. The power conversion system 1 supplies power to the load 4 via the transformer 3.

[0012] The power conversion system 1 includes a power conversion device 11a, a power conversion device 11b, a voltage detector (voltage detection sensor) 12a, a voltage detector (voltage detection sensor) 12b, a current detector (current detection sensor) 13a, a current detector (current detection sensor) 13b, a current detector (current detection sensor) 14a, a current detector (current detection sensor) 14b, a cross current detection circuit 15, a controller 16a, and a controller 16b. The controllers 16a and 16b include hardware such as a processor and a recording device, and a program recorded in the recording device, and can be configured such that the processor reads out the program and executes the calculation described below. The controllers 16a and 16b are, for example, configured as microcomputers.

[0013] One AC terminal of the power conversion device 11a is connected to the three-phase AC system 2. The other AC terminal is connected to the transformer 3. The power conversion device 11a receives AC power from the three-phase AC system 2, and controls the AC voltage of the AC terminal connected to the transformer 3 to a desired value.

[0014] The voltage detector 12a detects the output AC voltage of the power conversion device 11a. The current detector 13a and the current detector 14a detect the AC current output from the power conversion device 11a.

[0015] One AC terminal of the power conversion device 11b is connected to the three-phase AC system 2, and the other AC terminal is connected to the transformer 3. The power conversion device 11b receives AC power from the three-phase AC system 2, and controls the AC voltage of the AC terminal connected to the transformer 3 to a desired value.

[0016] The voltage detector 12b detects the output AC voltage of the power converter 11b. The current detector 13b and the current detector 14b detect the AC current output from the power converter 11b.

[0017] The cross current detection circuit 15 receives the AC current detection value detected by the current detector 14a and the AC current detection value detected by the current detector 14b as inputs, and detects the cross current flowing through the power converter 11a and the cross current flowing through the power converter 11b. Here, the cross current flowing through the power converter 11a is the current that is the difference between the average value of the AC current detection values ​​detected by the current detectors 14a and 14b and the AC current detection value detected by the current detector 14a. Similarly, the cross current flowing through the power converter 11b is the current that is the difference between the average value of the AC current detection values ​​detected by the current detectors 14a and 14b and the AC current detection value detected by the current detector 14b.

[0018] The controller 16a receives as input the AC voltage detection value Va output by the voltage detector 12a, the AC current detection value Ia output by the current detector 13a, and the cross current detection value Iva output by the cross current detection circuit 15, and outputs a corrected output voltage command Vaq to the power conversion device 11a.

[0019] The controller 16b receives as input the AC voltage detection value Vb output by the voltage detector 12b, the AC current detection value Ib output by the current detector 13b, and the cross current detection value Ivb output by the cross current detection circuit 15, and outputs a corrected output voltage command Vbq to the power conversion device 11b.

[0020] The controller 16a has a voltage controller 161a, a load current calculator 162a, a bias magnetization suppression controller 163a, and an adder 164a. The voltage controller 161a receives an AC voltage detection value Va output from the voltage detector 12a, and outputs an output voltage command Vao so that the detection value of the AC voltage output from the power conversion device 11a becomes a predetermined value.

[0021] The load current calculator 162a receives as input the AC current detection value Ia output by the current detector 13a and the detection value Iva of the cross current flowing through the power conversion device 11a output by the cross current detection circuit 15, and outputs a calculated load current value Iwa supplied to the transformer 3 by the power conversion device 11a.

[0022] The bias magnetization suppression controller 163a receives the load current calculation value Iwa output by the load current calculator 162a, and outputs an output voltage correction command Vap so that the transformer 3 does not suffer from DC bias magnetization.

[0023] The adder 164a receives the output voltage command Vao output by the voltage controller 161a and the output voltage correction command Vap output by the bias magnetization suppression controller 163a, performs an addition operation, and outputs a corrected output voltage command Vaq. The corrected output voltage command Vaq output by the adder 164a becomes a command for the AC voltage output by the power conversion device 11a.

[0024] The controller 16b has a voltage controller 161b, a load current calculator 162b, a bias magnetization suppression controller 163b, and an adder 164b. The voltage controller 161b receives an AC voltage detection value Vb output from the voltage detector 12b, and outputs an output voltage command Vbo so that the detection value of the AC voltage output from the power conversion device 11b becomes a predetermined value.

[0025] The load current calculator 162b receives as input the AC current detection value Ib output by the current detector 13b and the detection value Ivb of the cross current flowing through the power conversion device 11b output by the cross current detection circuit 15, and outputs a load current calculation value Iwb supplied to the transformer 3 by the power conversion device 11b.

[0026] The bias magnetization suppression controller 163b receives the load current calculation value Iwb output by the load current calculator 162b and outputs an output voltage correction command Vbp so that the transformer 3 does not suffer from DC bias magnetization.

[0027] The adder 164b receives the output voltage command Vbo output by the voltage controller 161b and the output voltage correction command Vbp output by the bias magnetization suppression controller 163b, performs an addition operation, and outputs a corrected output voltage command Vbq. The corrected output voltage command Vbq output by the adder 164b becomes a command for the AC voltage output by the power conversion device 11b.

[0028] 2 shows the configuration of the load current calculators 162a and 162b. The load current calculators 162a and 162b each have a subtractor 1621. The subtractor 1621 receives the current detection value Ia or Ib and the cross current detection value Iva or Ivb as input, and outputs the difference between them as the load current calculation value Iwa or Iwb.

[0029] The cross current detection circuit 15, which outputs a cross current detection value, outputs to the controller 16a the current difference between the average value of the AC current detection values ​​detected by the current detectors 14a and 14b and the AC current detection value detected by the current detector 14a as a cross current detection value Iva. Similarly, to the controller 16b, it outputs to the controller 16b the current difference between the average value of the AC current detection values ​​detected by the current detectors 14a and 14b and the AC current detection value detected by the current detector 14b as a cross current detection value Ivb.

[0030] 2, the load current calculation value Iwa or Iwb is calculated by finding the average value of the AC current detection values ​​detected by the current detectors 14a and 14b. By performing such a calculation, the load current calculation value Iwa or Iwb does not contain a cross current component, and the influence of the cross current can be eliminated.

[0031] 3 shows the configuration of the magnetic bias suppression controllers 163a and 163b. The magnetic bias suppression controllers 163a and 163b each include a harmonic detector 1631 and a harmonic controller 1632.

[0032] A harmonic detector 1631 detects harmonic components contained in the load current calculation value Iwa or Iwb. A harmonic controller 1632 outputs an output voltage correction command Vap or Vbp so that the harmonic component detection value Ipp output by the harmonic detector 1631 becomes zero.

[0033] When the transformer 3 suffers from DC bias magnetization, the output voltage of the transformer 3 is distorted, distorting the current flowing between the power conversion device 11a and the power conversion device 11b and the transformer 3, and harmonic components are generated in the load current calculation value Iwa or Iwb. The harmonic detector 1631 detects these harmonic components, and the harmonic controller 1632 outputs the output voltage correction command Vap or Vbp so that the harmonic components become zero, thereby suppressing the DC bias magnetization of the transformer 3.

[0034] With the above configuration, in a power conversion system consisting of two power conversion devices connected in parallel, it is possible to eliminate the effects of cross currents by calculating the load current, and it is possible to suppress DC bias magnetism in the transformer connected to the power conversion system.

[0035] In this embodiment, the configuration in which two power conversion devices are connected in parallel has been shown, but a configuration in which three or more power conversion devices are connected in parallel may also be used. Fig. 4 shows a configuration in which three or more power conversion devices are connected in parallel. The load current calculators in the controllers 16a, 16b, and 16c calculate a current value obtained by dividing the current supplied to the transformer 3 by the number of power conversion devices. The configuration shown in Fig. 4 also makes it possible to suppress DC bias magnetism in the transformer connected to the power conversion system. EXAMPLES

[0036] Fig. 5 shows the configuration of a power conversion system according to a second embodiment. The difference from the first embodiment shown in Fig. 1 is that there is no cross current detection circuit. Another difference is that the current detection values ​​output by the current detectors 13a and 13b are input to both the controllers 17a and 17b, and the load current calculators 165a and 165b use the current detection values ​​to output the load current calculation values ​​Iwa and Iwb.

[0037] Fig. 6 shows the configuration of the load current calculators 165a and 165b. The load current calculators 165a and 165b have an adder 1651 and a gain 1652. Two current detection values ​​are input, and the adder 1651 adds the two current detection values ​​Ia and Ib, and the gain 1652 multiplies the result by a gain of 0.5 and outputs the result as a load current calculation value Iwa or Iwb. That is, by performing the calculation shown in Fig. 5, the load current calculation value Iwa or Iwb becomes the average value of the AC current detection values ​​detected by the current detector 13a and the current detector 13b.

[0038] With the above configuration, the calculated load current value becomes the average value of the output currents of the power conversion devices, making it possible to eliminate the influence of cross currents.

[0039] That is, in this embodiment, in a power conversion system configured with two power conversion devices connected in parallel, it is possible to suppress DC bias magnetism in a transformer connected to the power conversion system without using a cross current detection circuit, as compared to embodiment 1.

[0040] In this embodiment, the configuration in which two power conversion devices are connected in parallel has been shown, but a configuration in which three or more power conversion devices are connected in parallel may also be used. Fig. 7 shows a configuration in which three or more power conversion devices are connected in parallel. The configuration shown in Fig. 7 also makes it possible to suppress DC bias magnetism in a transformer connected to a group of power conversion devices. [Explanation of symbols]

[0041] 1: power conversion system, 2: three-phase AC system, 3: transformer, 4: load, 11a: power conversion device, 11b: power conversion device, 11c: power conversion device, 12a: voltage detector, 12b: voltage detector, 12c: voltage detector, 13a: current detector, 13b: current detector, 13c: current detector, 14a: current detector, 14b: current detector, 14c: current detector, 15: cross current detection circuit, 16a: controller, 16b: controller, 16c: controller, 16 1a: voltage controller, 161b: voltage controller, 162a: load current calculator, 162b: load current calculator, 163a: bias magnetism suppression controller, 163b: bias magnetism suppression controller, 164a: adder, 164b: adder, 1621: subtractor, 1631: harmonic detector, 1632: harmonic controller, 17a: controller, 17b: controller, 17c: controller, 165a: load current calculator, 165b: load current calculator, 1651: adder, 1652: gain

Claims

1. A power conversion system in which a plurality of power conversion devices are connected in parallel and supply power to a load via a transformer, a current detector for detecting an output current of the power conversion device; a load current calculator that calculates a current to be supplied from each of the power conversion devices to the transformer from the output current detected by the current detector; and a bias magnetism suppression controller that reduces high-order components of an output of the load current calculator and suppresses DC bias magnetism of the transformer.

2. 2. The power conversion system according to claim 1, A first power conversion device; A second power conversion device; a first current detector that detects an output current of the first power conversion device; a second current detector that detects an output current of the second power conversion device; a first load current calculator that calculates a current to be supplied to the transformer by a first power conversion device based on currents detected by a first current detector and a second current detector; The second power conversion device is determined based on the currents detected by the first current detector and the second current detector. a second load current calculator that calculates a current to be supplied to the transformer; a first load current calculator that performs calculations using an output from the first load current calculator as an input, reduces high-order components of the output from the load current calculator, and suppresses DC bias magnetism of the transformer; a second load current calculator that performs calculations using an output from a second load current calculator as an input, reduces high-order components of the output of the load current calculator, and suppresses DC bias magnetism of the transformer.

3. 2. The power conversion system according to claim 1, A first power conversion device; A second power conversion device; a first current detector that detects an output current of the first power conversion device; a second current detector that detects an output current of the second power conversion device; a cross current detection unit that detects a cross current flowing between the first power conversion device and the second power conversion device; a first load current calculator that calculates a current supplied from a first power conversion device to the transformer based on outputs of a first current detector and the cross current detector; a second load current calculator that calculates a current supplied from a second power conversion device to the transformer based on outputs of a second current detector and the cross current detector; a first load current calculator that performs calculations using an output from the first load current calculator as an input, reduces high-order components of the output from the load current calculator, and suppresses DC bias magnetism of the transformer; a second load current calculator that performs calculations using an output from a second load current calculator as an input, reduces high-order components of the output of the load current calculator, and suppresses DC bias magnetism of the transformer.

4. 2. The power conversion system according to claim 1, The load current calculator a power conversion system that calculates a current to be supplied to the transformer by dividing the current supplied from each of the power conversion devices to the transformer by the number of the power conversion devices.

5. 2. The power conversion system according to claim 1, The load current calculator The power conversion system calculates an average value of a current output from each of the power conversion devices.

6. 2. The power conversion system according to claim 1, The load current calculator A power conversion system that performs a calculation to remove cross current components flowing between the power conversion devices.

7. 4. The power conversion system according to claim 3, The first load current calculator Calculating a difference between an output of a first current detector and an output of the cross current detector; The second load current calculator A power conversion system that calculates a difference between the output of a second current detector and the output of the cross current detector.

8. 2. The power conversion system according to claim 1, a voltage detector for detecting an output voltage of the power conversion device; a voltage controller that calculates an output voltage command from the output voltage of the voltage detector; a calculator that calculates a corrected output voltage command from the output voltage command and an output voltage correction command from the bias magnetic field suppression controller;

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