JP3585896B2 - Power converter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a self-excited voltage-type power converter suitable for performing voltage adjustment of a power system or for applying to a system stabilizing device or the like.
[0002]
[Prior art]
Conventionally, a static reactive power compensator has been applied to adjust the voltage of the power system, a series impedance compensator has been installed to maintain the power flow appropriately and reduce or stabilize the loss of the power system. A system stabilizing device such as a vessel may be applied.
[0003]
These use a large-capacity power converter using a power semiconductor switch in order to continuously adjust the output reactive power and continuously adjust the impedance and the amount of phase shift. In particular, in recent years, a self-excited voltage-type power converter tends to be applied because of its high control ability.
[0004]
FIG. 7 shows a configuration example of a conventional power converter used for such a purpose. In FIG. 7, reference numeral 1 denotes an inverter, which comprises switching elements 11 to 14, and antiparallel diodes 15 to 18, as shown in the figure. The AC output of the inverter 1 is connected to the power system via the converter transformer 3. On the other hand, a capacitor 4 is connected to the DC side.
[0005]
In the operation of the power conversion device, after the DC capacitor 4 is appropriately charged, the switching element 11 is operated, for example, as shown in FIG. Further, the switching element 12 is operated as shown in (b), and a pulse opposite thereto is applied to the switching element 14.
[0006]
As a result, the output voltage shown in FIG. 8C is obtained on the primary side of the transformer 3 for the converter. Here, the turns ratio of the transformer 3 for the converter is set to 1: 1 for simplicity. However, even if the turns ratio is appropriately changed depending on the voltage class on the power system side and the rated voltage on the inverter side, the operation principle is affected. Clearly not.
[0007]
Here, since the phase and magnitude of the fundamental wave component of the output voltage can be arbitrarily adjusted by adjusting the timing and the conduction period of the pulse given to each switching element, this operation is combined with a higher-level control device. Thus, operations such as the above-described reactive power control, impedance compensation, and phase shift are performed.
[0008]
[Problems to be solved by the invention]
However, it can be easily assumed that a large amount of harmonic components are included in the output voltage of FIG. When this harmonic component flows out to the power system, it may cause a communication failure, or may flow into another device to cause overheating or abnormal vibration. Therefore, it is desirable that the generation amount is as small as possible, and the upper limit is as specified in the guideline for countermeasures for harmonic suppression.
[0009]
Therefore, conventionally, the number of pulses applied to each switching element is increased to suppress the generation of harmonics from flowing out to the power system side, and the winding of a transformer for a converter is performed using a plurality of inverters to reduce the generation of harmonics. A technique of multiplexing by connecting lines is applied.
[0010]
However, if the number of pulses is increased, the loss associated with the switching operation of the switching element increases, and the utilization rate of the switching element decreases due to restrictions on the minimum on-pulse of the element, and it is difficult to construct a system economically. become. On the other hand, in the case of multiplexing, it is desirable to increase the multiplexing number because the generated harmonics are reduced in accordance with the multiplexing number. There was a problem of disadvantage.
[0011]
The present invention has been made to solve the above-described problems, and has as its object to provide a low-cost power conversion device that can reduce generated harmonics while reducing switching loss.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, a power converter according to claim 1 of the present invention is a power converter having at least two inverters and a capacitor on the DC side of each of the inverters. A first inverter operating at a frequency and connected to the grid via a first winding of the transformer for the converter, and a second winding of the transformer for the converter operating at a higher frequency than the grid frequency with medium capacity the and a second inverter connected to the system through, and by the transducer transformer first winding and the second winding of which is series-connected on the primary side, the switching losses It is possible to realize a power conversion device with a small number of generated harmonics.
[0013]
The power converter according to claim 2 of the present invention is the power converter according to claim 1, wherein the DC side of the inverter is connected to each other and shares one capacitor, while the first winding of the transformer for the converter is used. And the second winding has a different turns ratio.
[0014]
According to a third aspect of the present invention, in the power converter according to the first or second aspect, the first inverter and the second inverter are each formed of an NPC inverter having two capacitors. .
[0015]
According to a fourth aspect of the present invention, in the power converter according to the first or second aspect, the first inverter and the second inverter are each a single-phase inverter, an NPC inverter, or a three-phase inverter. It is composed of any combination.
[0016]
The power converter according to claim 5 of the present invention is the power converter according to claim 4, which operates at a large capacity and at an operating frequency equal to the system frequency in addition to the first inverter and the second inverter. A third inverter connected to the grid via a third winding of the transformer and a third inverter connected to the grid via a fourth winding of the transducing transformer operating at a higher frequency than the grid frequency with a medium capacity. has a fourth inverter, said third winding and the fourth winding of the transducer transformer are connected in series with the primary side, and is connected to generate a phase difference.
[0017]
A power converter according to claim 6 of the present invention is the power converter according to any one of claims 1 to 4, which operates at a frequency sufficiently smaller than the system frequency with a small capacity, and A third inverter connected to the system via a third winding, wherein the first winding, the second winding, and the third winding of the converter transformer are connected in series on the primary side. Have been.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
(First Embodiment)
FIG. 1 is a configuration diagram of a power conversion device according to a first embodiment of the present invention. Here, 1 is a large-capacity inverter, 2 is a medium-capacity inverter, and includes switching elements 11 to 14, 21 to 24, and antiparallel diodes 15 to 18, 25 to 28, respectively, as shown in the figure.
[0019]
The AC output of the inverter 1 is connected to the power system via a winding 31 of the converter transformer 3, and the AC output of the inverter 2 is connected to the power system via a winding 32 of the transformer 3 for the converter. . The winding 31 and the winding 32 of the converter transformer 3 are connected in series on the primary side. On the other hand, capacitors 4 and 5 are connected to the DC side of inverters 1 and 2, respectively.
[0020]
Next, the operation of the present embodiment will be described with reference to FIG. As shown in FIG. 2A, the large-capacity inverter 1 operates at an operating frequency equal to the system frequency, and its output voltage is 3 Vd. On the other hand, as shown in FIG. 2B, the medium capacity inverter 2 operates at an operating frequency higher than the system frequency, and the magnitude of the output voltage is Vd. Since both outputs are connected in series on the primary side by the windings 31 and 32 of the transformer 3 for the converter, the total output voltage becomes the waveform shown in FIG. .
[0021]
According to the present embodiment, the large-capacity inverter 1 occupying 3/4 of the entire capacity operates at the same operating frequency as the system frequency, that is, at the minimum operating frequency, so that the switching loss can be kept sufficiently small. On the other hand, by operating only the medium-capacity inverter 2 at a high operating frequency, harmonics generated in the entire power converter can be significantly reduced.
[0022]
To obtain the same output voltage only by multiplexing, four windings of four inverters and a transformer for a converter are required. In the present embodiment, two windings of two inverters and a transformer for a converter are required. It is realized by a line, the system configuration is simple, and therefore it can be realized at low cost.
[0023]
(Second embodiment)
FIG. 3 is a configuration diagram of a power conversion device according to a second embodiment of the present invention. 3, description of the same elements as those in FIG. 1 will be omitted, and only different parts will be described. In FIG. 3, the DC sides of the inverters 1 and 2 are connected to each other and share one capacitor 4. On the other hand, the turns ratio of the primary side of the first winding 31 and the second winding 32 of the converter transformer is 3: 1, and the first winding 31 outputs three times the output voltage of the primary side. Is configured to be generated.
[0024]
The switching operation of the inverters 1 and 2 is the same as that of the first embodiment, but differs from that of the first embodiment in that the operating voltage of the inverter 2 increases three times from Vd to 3 Vd while the current increases. Is 1/3. Therefore, considering the turns ratio of the transformer 3 for the converter, the operation when viewed from the primary side, that is, the AC output voltage is the same as that of the first embodiment. According to the present embodiment, the same effect as in the first embodiment can be obtained even with one DC capacitor.
[0025]
(Third embodiment)
FIG. 4 is a configuration diagram of a power conversion device according to a third embodiment of the present invention. 4, description of the same elements as those in FIG. 1 will be omitted, and only different parts will be described. In FIG. 4, inverters 1 and 2 have a configuration called an NPC (Neutral Point Clamped) inverter, and include switching elements 11 to 14, 41 to 44, 21 to 24, 51 to 54, antiparallel diodes 15 to 18, 45 to 48. , 25-28, 55-58, and neutral point clamp diodes 19, 20, 49, 50, 29, 30, 59, 60. Two capacitors 4a, 4b, 5a, 5b are connected to the DC side, respectively.
[0026]
Although the detailed description of the relationship between the switching element and the output voltage of each inverter is omitted, the inverter 1 outputs +10 Vd, +5 Vd, 0, −5 Vd, and −10 Vd by the combination of ON / OFF of the switching element. The inverter 2 can generate output voltages of + 2Vd, + Vd, 0, -Vd, and -2Vd.
[0027]
By operating the inverters 1 and 2 as shown in FIGS. 5B and 5C, the primary side of the converter transformer 3 has a waveform closer to a sine wave as shown in FIG. 5D. Will be output. Also, at this time, the on / off state of each switching element of the inverter 1 is repeated once in each cycle of the system frequency, that is, the minimum number of on / off states is repeated as shown in FIG.
[0028]
According to the present embodiment, by using the NPC inverters for the inverters 1 and 2, it is possible to obtain a remarkable effect that the harmonics are further reduced by the effects described in the first embodiment.
[0029]
(Fourth embodiment)
Although a configuration diagram and a detailed description of the operation of the fourth embodiment are omitted, the inverter 1 may have the single-phase inverter configuration of FIG. 1 and the inverter 2 may have the NPC inverter configuration of FIG. , By appropriately selecting the ratio of the DC voltage between the inverter 1 and the inverter 2 or the winding ratio between the first winding 31 and the second winding 32 of the converter transformer, and reducing the harmonics on the inverter 2 side. , The same effects as in the first to third embodiments can be obtained.
[0030]
(Fifth embodiment)
FIG. 6 is a configuration diagram of a power conversion device according to a fifth embodiment of the present invention. 6, description of the same elements as those in FIG. 1 will be omitted, and only different parts will be described. In FIG. 6, an inverter 6 operating at an operating frequency equal to the system frequency with a large capacity, and an inverter 7 operating at a frequency higher than the system frequency with a medium capacity are added. Are connected and share capacitors 4 and 5. On the other hand, the AC side is connected to the third winding 33 and the fourth winding 34 of the converter transformer, and the primary sides of the windings 31 to 34 are connected in series.
[0031]
The switching operations of the inverters 6 and 7 of the present embodiment are the same as those of the inverters 1 and 2, respectively. (Ie, the inverter 1 and the inverter 6 are multiplexed, and the inverter 2 and the inverter 7 are multiplexed), and the combined output voltage appearing on the primary side is closer to a sine wave than in the first embodiment. Harmonics are further reduced.
[0032]
According to the present embodiment, by combining with multiplexing, it is possible to obtain a remarkable effect of further reducing harmonics in the effect described in the first embodiment. Further, in the present embodiment, the phase is shifted by changing the winding configuration of the transformer 3 for the converter. However, the winding configurations of all the windings are all the same, and the timing of the pulse applied to each inverter is simply shifted as appropriate. However, the same effect can be obtained.
[0033]
(Sixth embodiment)
Although a configuration diagram and a detailed operation description for the sixth embodiment are omitted, the DC voltage of the first inverter operating at an operating frequency equal to the system frequency with a large capacity is 9 Vd, and the DC voltage is higher than the system frequency with a medium capacity. By setting the DC voltage of the operating second inverter to 3 Vd and the DC voltage of the third inverter operating at a small capacity and sufficiently higher than the system frequency to Vd, the output voltage on the AC side is in the range of +13 Vd to -13 Vd. Can be adjusted in steps of Vd, and the generated harmonics can be significantly reduced.
[0034]
As described above, by increasing the number of inverters, the output voltage step can be made finer and harmonics can be suppressed significantly. In general, when a single-phase inverter as shown in FIG. from the smaller the voltage ratio 1: ^3: 3 2: ...: with ^{3 N,} to be the range of the output voltage ⁺ a ^{(1 + 3 + 3 2 +} ... + 3 N) ~- (1 + 3 + 3 2 + ... + 3 N) can, 1 from the smaller the DC voltage ratio in the case of using NPC inverter 4: ^5: 5 2: ...: ⁵ with ^N, the range of the output voltage ^{+ 2 * (1 + 5 +} 5 2 + ... + 5 N) ~ -2 * can ^{(1 + 5 + 5 2 +} ... + 5 N) to be. However, if the number of inverters is too large, it is economically disadvantageous as described in the related art, so that an optimum final configuration is determined in consideration of a target harmonic level and system loss.
[0035]
【The invention's effect】
As described above, according to the present invention, a first inverter that operates at a large capacity and an operating frequency equal to the system frequency, and a second inverter that operates at a medium capacity and a frequency higher than the system frequency are used. Since the output voltage is configured to be connected in series on the primary side of the transformer for a converter, it is possible to provide a low-cost power converter that can reduce the generated harmonics while reducing the switching loss.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a first embodiment of the present invention.
FIG. 2 is a view for explaining the operation of the first embodiment of the present invention.
FIG. 3 is a configuration diagram showing a second embodiment of the present invention.
FIG. 4 is a configuration diagram showing a third embodiment of the present invention.
FIG. 5 is a view for explaining the operation of the third embodiment of the present invention.
FIG. 6 is a configuration diagram showing a fourth embodiment of the present invention.
FIG. 7 is a configuration diagram showing a form of a conventional power converter.
FIG. 8 is a diagram illustrating an operation of a conventional power converter.
[Explanation of symbols]
1, 2, 6, 7 Inverter 3 Transformer for converter 4, 5, 4a, 4b, 5a, 5b Capacitors 11 to 14, 21 to 24, 41 to 44, 51 to 54 Switching elements 15 to 18, 25 to 28, 45-48, 55-58 Anti-parallel diode 19, 20, 29, 30, 49, 50, 59, 60 Neutral point clamp diode 31-34 Transformer winding for converter

Claims (6)

A power converter having at least two inverters and a capacitor on a DC side of each of the inverters, operates at an operating frequency equal to a system frequency with a large capacity, and is connected to a system via a first winding of a transformer for a converter. to a first inverter, operates at a frequency higher than the system frequency at medium volume, and a second inverter connected to the system through a second winding of the transducer transformer and the transducer transformer A power converter, wherein a first winding and a second winding of a transformer are connected in series on a primary side. 2. The power converter according to claim 1, wherein the DC sides of the inverters are connected to each other and share one capacitor, while the first winding and the second winding of the converter transformer have different turns ratios. A power converter characterized by the above-mentioned. 3. The power converter according to claim 1, wherein the first inverter and the second inverter each include an NPC inverter having two capacitors. 4. 3. The power converter according to claim 1, wherein the first inverter and the second inverter are each constituted by an arbitrary combination of a single-phase inverter, an NPC inverter, and a three-phase inverter. apparatus. 5. The power converter according to claim 4, wherein in addition to the first inverter and the second inverter, the power converter operates at a large capacity and at an operating frequency equal to the system frequency, and is connected to the system via the third winding of the converter transformer. A third inverter to be connected and a fourth inverter operating at a frequency higher than the system frequency with a medium capacity and connected to the system via a fourth winding of the transformer for the converter, The third winding and the fourth winding are connected in series on the primary side and are connected so as to generate a phase difference. The power converter according to any one of claims 1 to 4 , wherein the power converter operates at a frequency sufficiently higher than the system frequency with a small capacity, and is connected to the system via the third winding of the transformer for a converter. A power converter comprising an inverter, wherein a first winding, a second winding, and a third winding of the transformer for a converter are connected in series on a primary side.

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