JP7557768B2 - Power Conversion Equipment - Google Patents

Description

The present invention relates to a power conversion device that converts high-voltage AC power into low-voltage AC power and then generates DC power.

In the past, when generating DC power from high-voltage AC power, a configuration has been widely adopted in which the high voltage is converted to low voltage in a high-voltage power receiving facility, and then DC is generated by an AC/DC converter. For example, in Patent Document 1, DC is generated by using a converter circuit using IGBTs.
There is also a configuration that does not use an AC/DC converter and uses a multi-phase transformer. For example, in Patent Document 2, a six-phase transformer having a star-connected winding and a delta-connected winding on the secondary side is used, and the six-phase voltage is full-wave rectified to generate DC power.

JP 2017-163839 A JP 2008-295155 A

However, when generating direct current using an AC/DC converter, there is a problem that high-frequency switching noise is generated, which adversely affects surrounding electronic devices and communication devices. In addition, the converter circuit requires high-voltage semiconductor elements, which are expensive and difficult to maintain.
Furthermore, although a configuration using a multi-phase transformer does not generate high-frequency noise, the core cannot be made small because each phase has an independent winding, and the transformer is therefore large in size.

In view of these problems, the present invention aims to provide a power conversion device that uses a multi-phase transformer when converting high voltage to low voltage, making it possible to generate DC power without using an AC/DC converter, and further enabling the multi-phase transformer to be made smaller.

In order to solve the above problems, the invention of claim 1 is a power conversion device that converts high-voltage three-phase power into low-voltage AC power using a multi-phase transformer and converts the converted low-voltage AC power into DC power using a full-wave rectifier circuit, characterized in that the multi-phase transformer is a 12-phase transformer having a first winding constituting a primary winding and second to fifth windings constituting a secondary winding for generating four different three-phase phases, the second to fifth windings are all star-connected and the first to fifth windings are wound around a common iron core, the neutral points of the second and third windings are connected to each other, while the fourth and fifth windings branch off from a common predetermined position midway through the third winding and are formed with 0.71 times the number of turns of the third winding , and the second to fifth windings output three-phase voltages of different phases, thereby outputting a 12-phase AC voltage.
According to this configuration, 12-phase polyphase low-voltage AC power is generated and then converted to DC power by a full-wave rectifier circuit, so that DC power with small ripples can be generated, making it possible to generate stable DC power without using an AC/DC converter that switches at high frequency.
In addition, of the four windings that make up the secondary side, the fourth and fifth windings are formed by branching off from the middle of the third winding, so that a part of each winding can be taken over by the third winding, and the number of turns can be reduced compared to when each phase is wound independently, thereby making it possible to miniaturize the multi-phase transformer.

According to a second aspect of the present invention, in the configuration described above , the neutral point of the secondary winding is grounded via a lightning arrester.
According to this configuration, by grounding the neutral point of the secondary winding of the multi-phase transformer via a lightning arrester, lightning damage to the secondary circuit can be minimized with only one lightning arrester.

The invention of claim 3 is characterized in that, in the configuration described in claim 1 or 2 , two DC/DC converters are connected in parallel to the output of the full-wave rectifier circuit, and the outputs of the DC/DC converters are linked to form a bipolar power supply that outputs the same positive and negative voltages to the ground electrode .
According to this configuration, since a bipolar power supply is provided, power can be supplied to any DC load regardless of the load.

According to a fourth aspect of the present invention, in the configuration according to the first or second aspect, the output line of the full-wave rectifier circuit is grounded via a resistance element of 10 kΩ to 50 kΩ.
According to this configuration, when an electrical circuit abnormality such as an electric leakage or electric shock occurs on the DC power output side, the electric leakage current can be suppressed by the resistive element, thereby suppressing damage.

The invention of claim 5 is characterized in that, in the configuration described in claim 4 , it has a current monitoring unit that monitors the current flowing through the resistance element and outputs an abnormality occurrence signal if the current value deviates from a predetermined range, and an alarm unit that receives the abnormality occurrence signal and performs an alarm operation.
According to this configuration, when an electric circuit abnormality such as a ground fault occurs in the DC electric circuit, the current flowing through the resistance element changes, and the alarm unit detects the electric circuit abnormality from the change and performs an alarm operation. Therefore, the occurrence of an electric circuit abnormality can be recognized and it is easy to deal with it.

The invention of claim 6 is characterized in that in the configuration of claim 5 , a circuit breaker is provided on the primary side of the polyphase transformer, and the circuit breaker performs a cutoff operation upon receiving an abnormality occurrence signal.
According to this configuration, when an electric circuit abnormality such as a ground fault occurs in the DC electric circuit, the input electric circuit of the multi-phase transformer is cut off, so that a safe state can be quickly ensured when an abnormality occurs in the DC electric circuit.

According to the present invention, 6-phase or 12-phase polyphase low voltage AC power is generated and converted to DC power by a full-wave rectifier circuit, so that DC power with small ripples can be generated and stable DC power can be generated without using a converter that switches at high frequency. Therefore, noise due to high frequency switching is not generated.
In addition, the number of turns can be reduced in both the 6-phase transformer and the 12-phase transformer compared to conventional transformers, making it possible to reduce the size of the multi-phase transformer.

1 is a schematic configuration diagram showing an example of a power conversion device according to the present invention; FIG. 1 is a diagram showing the configuration of a power conversion device in which a multi-phase transformer is replaced with a 12-phase transformer (high resistance). FIG. 1 is a configuration diagram of a power conversion device in which a DC output is converted into a bipolar output. FIG. 1 is a schematic diagram of a 12-phase transformer. FIG. 5 is a vector diagram of the 12-phase transformer shown in FIG. 4 . FIG. 5 is a diagram illustrating the phases of the secondary side of the 12-phase transformer shown in FIG. 4 . 7 is an explanatory diagram showing a state in which the output between terminals R1 and T1 of the second winding shown in FIG. 6 is applied to a load. 7 is an explanatory diagram showing a state in which an output between an R1 terminal of the second winding and a T3 terminal of the fourth winding shown in FIG. 6 is applied to a load. FIG. 7 is an explanatory diagram showing a state in which an output between an R1 terminal of the second winding and an R2 terminal of the third winding shown in FIG. 6 is applied to a load. FIG. FIG. 5 is a vector diagram showing positions at which the fourth and fifth windings are drawn from the third winding of the 12-phase transformer shown in FIG. 1 is a circuit diagram illustrating a flow of a lightning current when a DC line is struck by lightning in a power conversion device using a 12-phase transformer. 1 is a circuit diagram illustrating a flow of a ground fault current when a ground fault occurs in a DC circuit in a power conversion device using a 12-phase transformer. FIG. 1 is a configuration diagram of a power conversion device equipped with an abnormality detection mechanism. FIG. 13 is a configuration diagram showing another example of a power conversion device in which other DC power generated by private power generation is added to the DC output. 1A and 1B show the external appearance of a power conversion device, with FIG. FIG. 1 is a configuration diagram of a power conversion device in which a multi-phase transformer is a six-phase transformer. FIG. 1 is a schematic diagram of a six-phase transformer. FIG. 1 is a vector diagram of the voltages generated by a six-phase transformer, in which (a) shows the phase voltages of the second winding and the third winding, and (b) shows the relationship between the phase-to-phase voltage of the second winding and the line-to-line voltage of the same phase between the second winding and the third winding. FIG. 2 is an output waveform diagram of a six-phase transformer. 1 is a circuit diagram illustrating a flow of lightning current when a DC circuit is struck by lightning in a power conversion device using a six-phase transformer. 1 is a circuit diagram illustrating a flow of a ground fault current when a ground fault occurs in a DC circuit in a power conversion device using a six-phase transformer.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a schematic diagram showing an example of a power conversion device according to the present invention, and shows a state where it is incorporated into a high-voltage power receiving facility. The power conversion device 1 is housed in a cubicle or the like that houses the high-voltage power receiving facility, and includes a multi-phase transformer 3 to which a high-voltage drop line is connected via a high-voltage AC load break switch (LBS) 2, and a rectifier circuit 4 that rectifies the output of the multi-phase transformer 3.
The power conversion device 1 converts high-voltage three-phase AC power of 6600 V received from a high-voltage three-phase power source 10 into low-voltage AC power of a specific voltage such as 380 V, and then converts the converted power into DC for output.

Figure 2 shows a configuration in which a 12-phase transformer 3a is used for the multi-phase transformer 3 to output a low-voltage 12-phase voltage, and a set of DC outputs is generated by a rectifier circuit 4 equipped with four three-phase full-wave rectifier circuits. 5 indicates a ground fault protection circuit provided on the DC circuit M1, and the DC circuit M1, which consists of two output lines, is grounded via a resistance element Rr of 10 kΩ to 50 kΩ each.

Fig. 3 shows another configuration of the DC output, which is a bipolar output configuration. As shown in Fig. 3, two DC/DC converters 7 (a first converter 7a and a second converter 7b) are arranged in parallel with the DC output generated by the rectifier circuit 4, and the two outputs can be combined to use as a bipolar power supply.
Here, a configuration is shown in which the first converter 7a and the second converter 7b both have an output of 380V, and by connecting the 0V terminal of the first converter and the 380V terminal of the second converter, a DC power of +380V/-380V can be output.
In this way, a bipolar power supply can be provided, and power can be supplied to any DC load regardless of the load.

The two converters 7 may be used independently without being connected. In that case, if one of them is set to a 100V output, two types of DC power can be output: 380V output and 100V output.

4 to 10 are explanatory diagrams showing a specific configuration of the 12-phase transformer 3a, and the 12-phase transformer 3a will be specifically described below with reference to these drawings.
A configuration diagram is shown in Fig. 4. As shown in Fig. 4, the 12-phase transformer 3a includes a first winding 11 constituting a primary winding L1, and four windings (a second winding 12, a third winding 13, a fourth winding 14, and a fifth winding 15) constituting a secondary winding L2.
Each winding has three winding portions (11a to 11c, 12a to 12c, 13a to 13c, 14a to 14c, and 15a to 15c) corresponding to the three-phase power input.

The first winding 11 is delta-connected, and high-voltage three-phase AC power is connected to three terminals (Rin, Sin, Tin).
The second to fifth windings 12, 13, 14, and 15 are all star-connected (however, as described later, the fourth winding 14 and the fifth winding 15 are not completely star-connected). In the following description, the three phases are referred to as the R phase, the S phase, and the T phase. The first winding 11 may be star-connected.

The secondary side of the 12-phase transformer 3a has output terminals R1, S1, T1 of the second winding 12, output terminals R2, S2, T2 of the third winding 13, output terminals R3, S3, T3 of the fourth winding 14, and output terminals R4, S4, T4 of the fifth winding 15, for a total of 12 terminals.
The star-connected neutral points Q of the second winding 12 and the third winding 13 are connected to each other, and the third winding 13 is wound around the iron core 8 so that its polarity is opposite to that of the second winding 12. The fourth winding 14 and the fifth winding 15 are formed by branching off from the same point in the middle of the third winding 13, and the fourth winding 14 and the fifth winding 15 share the neutral point Q of the third winding 13.

Specifically, the fourth winding 14 and the fifth winding 15 are wound around the iron core 8 as follows. First, each phase of the fourth winding 14 is wound as follows.
The R-phase winding 14a is branched off from the middle of the R-phase winding 13a of the third winding 13 and wound around the leg core 8b common to the primary side S-phase winding 13. The tip of the R-phase winding 14a is the output terminal R3.
The S-phase winding 14b is branched off from the middle of the S-phase winding 13b of the third winding 13 and wound around the leg core 8c common to the T-phase winding 11 (or the T-phase winding 12) of the first winding 11. The tip of the S-phase winding 14b is the output terminal S3.
The T-phase winding 14c is branched off from the middle of the T-phase winding 13c of the third winding 13 and wound around the leg core 8a common to the primary R-phase winding 13. The tip of the T-phase winding 14c is the output terminal T3.

Each phase of the fifth winding 15 is wound as follows: The R-phase winding 15a is branched off from the middle of the R-phase winding 13a of the third winding 13, which is the same portion as the fourth winding 14, and is wound around the leg core 8c common to the T-phase winding 11 (or the T-phase winding 12). The tip of the R-phase winding 15a is the output terminal R4.
The S-phase winding 15b is branched off from the middle of the S-phase winding 13b of the third winding 13, which is the same portion as the fourth winding 14, and is wound around the leg core 8a common to the R-phase of the first winding 11 (or the R-phase of the second winding 12).The tip of the S-phase winding 15b is the output terminal S4.
The T-phase winding 15c is branched off from the middle of the T-phase winding 13c of the third winding 13, which is the same portion as the fourth winding 14, and wound around the leg core 8b common to the S-phase winding 11 (or the S-phase winding 12).The tip of the T-phase winding 15c is the output terminal T4.

5 is a vector diagram of each of the secondary windings L2, with each phase shown based on the R1 phase of the second winding 12. As shown in this Fig. 5, the three-phase windings 12a, 12b, and 12c of the second winding 12 output voltages having a phase difference of 120 degrees from output terminals R1, S1, and T1, similar to the input three-phase power.
The phases of the windings 13a, 13b, and 13c of the third winding 13 are opposite to the phase of the second winding 12 as described above, and the phases of the output terminals R2, S2, and T2 have a phase difference of 180 degrees with respect to the phases R1, S1, and T1 of the second winding 12. The number of turns of the third winding 13 is 0.73 times (√3-1 times) the number of turns of the second winding 12.

The phases of the output terminals R3, S3, and T3 of the windings 14a, 14b, and 14c drawn from the third winding 13 of the fourth winding 14 generate voltages in phase with the respective phases of the second winding 12. Furthermore, the phases of the output terminals R4, S4, and T4 of the windings 15a, 15b, and 15c drawn from the third winding 13 of the fifth winding 15 generate voltages in phase with the output terminals R1, S1, and T1 of the corresponding phases of the second winding 12, similar to the fourth winding 14.

FIG. 6 is an explanatory diagram of the secondary side phase, showing the output phases of the third winding 13, the fourth winding 14, and the fifth winding 15 relative to the phase of the R1 terminal of the second winding 12.
6, the absolute value and phase of the voltage between the R1 terminal of the second winding 12 and the T3 terminal of the fourth winding 14 are equal to those between the R1 terminal and the T1 terminal of the second winding 12, and the phase lags 15 degrees. Also, the absolute value of the voltage between the R1 terminal of the second winding 12 and the R2 terminal of the third winding 13 is equal to those between the R1 terminal of the second winding 12 and the S4 terminal of the fifth winding 15, and the phase lags 45 degrees.

7 to 9 are explanatory diagrams showing how the specific interphase voltages shown in FIG. 6 are applied to a load (DC grid) 6, and show how the output voltage (shown as a vector) of the 12-phase transformer 3a is rectified by a rectifier circuit 4 and applied to the load 6.
Figure 7 shows how the output between the R1-T1 terminals of the second winding 12 is applied to the load 6, Figure 8 shows how the output between the R1 terminal of the second winding 12 and the T3 terminal of the fourth winding 14 is applied to the load 6, and Figure 9 shows how the output between the R1 terminal of the second winding 12 and the R2 terminal of the third winding 13 is applied to the load 6.
In this manner, the inter-phase voltages having the same absolute value are rectified and applied to the load 6 .

These phase relationships also apply to the output phases of the third winding 13, fourth winding 14, and fifth winding 15 relative to the phase of the S1 terminal of the second winding 12, and the output phases of the third winding 13, fourth winding 14, and fifth winding 15 relative to the phase of the T1 terminal of the second winding 12, and phase-to-phase voltages of equal absolute value are generated and applied to the load 6.

10 is a vector explanatory diagram showing the positions at which the fourth winding 14 and the fifth winding 15 are drawn from the third winding 13. Hereinafter, the position X at which the fourth winding 14 and the fifth winding 15 are drawn will be specifically explained with reference to this vector diagram. Note that E1 (the voltage between Q and R1) indicates the output voltage of the second winding 12.
If the voltage (A+B) between R1 and T3 shown in FIG. 10 is 200V, then the voltage between R1 and R2 is also 200V, and the voltage E1 becomes 200/√3=115.4V.
And, Voltage A = Voltage B / tan 15°
Also, Voltage A = Voltage B x 3.723
Therefore, when voltage A = 200 - voltage B is taken into account,
Voltage A = 200/(1 + tan 15°) = 157.73 V
It becomes.

Therefore,
Voltage B = 200 - Voltage A = 200 - 157.73 = 42.27 V
Voltage between R1 and X = A/cos15° = 163.3V
Voltage between R2 and X = (voltage between R1 and R2) - (voltage between R1 and X) = 36.7 V
And then,
Voltage between Q and X = (Voltage between R1 and X) - (Voltage between R1 and Q)
=163.3-115.46=47.83V
Voltage between Q and R2 = 200 - (voltage between R1 and Q)
=200-115.46=84.54V
It becomes.
Voltage B = 200 - Voltage A = 42.27 V
The voltage between X and T3 = voltage B x √2 = 59.77V.

From these results,
(Voltage between Q and X)/(Voltage between Q and R2)=47.83/84.54=0.565
That is, position X is a position 56.5% away from the neutral point (point Q).
In addition, looking at the turn ratio of each winding on the secondary side,
Second winding: third winding: fourth winding: fifth winding = 1: 0.73: 0.52: 0.52
It is as follows.

In this way, of the four windings, the second to fifth windings, that make up the secondary side, the fourth winding 14 and the fifth winding 15 branch off from the third winding 13 at a position 56.5% from the neutral point, so they can take over more than half of the area, reducing the number of turns compared to winding each phase independently, and thus making it possible to miniaturize the multi-phase transformer.
The output voltage from the third winding 13 has a phase that is 180 degrees inverted from the output phase of the second winding 12, and the phase of the output voltage from the fourth winding 14 generates a phase difference of 15 degrees with respect to the output of the same phase of the second winding 12. Furthermore, the phase of the output voltage from the fifth winding 15 generates a phase difference of 15 degrees with respect to the output of the same phase of the second winding 12, and generates a phase difference of 30 degrees with respect to the fourth winding 14, sandwiching the output of the same phase of the second winding 12. As a result, a 12-phase voltage is generated.
In addition, since 12-phase polyphase low-voltage AC power is generated and full-wave rectified by the rectifier circuit 4 to generate DC power, it is possible to generate DC power with small ripples, and it is possible to generate stable DC power without using an AC/DC converter that switches at high frequencies. Therefore, noise due to high frequency switching is not generated.

Fig. 11 shows the flow of a lightning current (lightning surge) It when the DC circuit M1 is struck by lightning. The 12-phase transformer 3a is shown by a vector diagram of the secondary winding L2 shown in Fig. 5. Also, P1 is the point of lightning strike, 17 is a switch that opens and closes the output of the 12-phase transformer 3a, 18 is a switch device that disconnects the load 6 from the DC circuit M1, and 20 is a lightning arrester (SPD) provided on the ground wire of the secondary neutral point Q of the 12-phase transformer 3a.
As shown in FIG. 11, when the DC circuit M1 is struck by lightning, the resulting lightning surge It flows out to the ground via the rectifier circuit 4, the 12-phase transformer 3a, and the lightning arrester 20.
If the polarity of the lightning strike is reversed, the lightning surge It flows through the reverse path (through the reverse-polarity diode in the rectifier circuit 4).

In this way, when lightning strikes the DC circuit M1, the lightning surge It can be passed through the rectifier circuit 4 and the transformer 3 to the lightning arrester 20, protecting the secondary side of the transformer 3 from lightning damage. In addition, the 12-phase transformer 3a has a neutral point Q shared by the secondary winding L2, and by grounding one of the neutral points Q via the lightning arrester 20, lightning protection can be implemented with only one lightning arrester 20.

12 is a circuit diagram illustrating a flow of a ground fault current Ig when a ground fault occurs in a DC current path M1. P2 indicates a ground fault point.
12, the ground-fault current Ig flows from the ground through the ground-fault protection circuit 5 to the other DC circuit M1, and then flows to the grounded DC circuit M1 via the rectifier circuit 4 and the transformer 3. At this time, the current is limited by the resistor element Rr of the ground-fault protection circuit 5.
In this way, since the ground fault current Ig is limited by the resistance element Rr of the ground fault protection circuit 5, if an electrical circuit abnormality such as leakage current or electric shock occurs on the DC power output side, the leakage current can be suppressed by the resistance element Rr, thereby minimizing damage.

Figure 13 shows a modified example of a power conversion device 1 equipped with an anomaly detection mechanism for detecting a ground fault. Here, the current flowing through the resistive element Rr of the ground fault protection circuit 5 is detected to monitor the state of the electrical circuit. The multi-phase transformer 3, rectifier circuit 4, etc. are the same as those in the above embodiment, but differ from the above embodiment in that a circuit breaker 30 is placed on the primary side of the multi-phase transformer 3 and a current monitoring unit 31 is provided to cause the circuit breaker 30 to perform an interrupting operation. The differences are explained below.

The current monitoring unit 31 measures the current flowing through the resistive element Rr of the ground fault protection circuit 5, and monitors changes in the current. The current monitoring unit 31 also has a notification unit 31a, and is connected to the circuit breaker 30 by a signal line 32 to operate the circuit breaker 30 to break the circuit breaker.
A constant current normally flows through the resistor element Rr of the ground fault protection circuit 5. For example, when a voltage of 380 V is applied to the DC circuit, if the resistor element Rr is 10 kΩ, 38 mA flows at all times, and this current is monitored by the current monitoring unit 31. If this current falls outside a predetermined range (for example, if it falls below 10 mA), it is determined that an abnormality (ground fault) has occurred, and an alarm is issued from the notification unit 31a. At the same time, an abnormality occurrence signal is output to the circuit breaker 30, which then performs a circuit breaker operation.

In this manner, when an electric circuit abnormality such as a ground fault occurs in the DC electric circuit M1, the current flowing through the resistance element Rr changes, and the notification unit 31a detects the electric circuit abnormality from the change and performs a notification operation. Therefore, the occurrence of the electric circuit abnormality can be recognized and it is easy to deal with it.
Furthermore, if an electric circuit abnormality such as a ground fault occurs in the DC electric circuit M1, the input electric circuit of the multi-phase transformer 3 is cut off. Therefore, if an abnormality occurs in the DC electric circuit M1, a safe state can be quickly ensured.

FIG. 14 shows another example of the power conversion device 1, and shows a configuration in which other DC power generated by private power generation is added to the DC output.
In FIG. 14, a DC circuit M1 output from a rectifier circuit 4 is connected to power generated by renewable energy sources such as solar power 23 and wind power 25, as well as EV (Electric Vehicle) output power 24, storage battery power 26, and the like.
When the voltages output by the individual power sources are different, by arranging a multi-input DC/DC converter 27 that converts the voltages of multiple DC power sources into the same voltage and combining them into one output, as shown in FIG. 14, it becomes possible to effectively supply power to a DC load (DC grid) 6 that requires a relatively large amount of power.

If each power source outputs the same voltage as the output voltage of the DC circuit M1, the multi-input DC/DC converter 27 is not necessary, and the power sources can be directly connected to the DC circuit M1, which is the output circuit of the rectifier circuit 4.

Fig. 15 shows an external view of the power conversion device 1, (a) being a plan view and (b) being a front view. In Fig. 15, 34 is an openable and closable panel disposed in front of the multi-phase transformer 3, 18 is a switchgear for disconnecting the DC circuit M1 from the rectifier circuit 4, and 35 is a display for displaying AC/DC voltage, current, etc. The plan view shows the state with the top panel removed.
The rectifier circuit 4 and the switchgear 18 are disposed on the rear side of the panel 34 , and the display 35 is disposed on the front side of the panel 34 .
By arranging the opening and closing device 18 and the rectifier circuit 4 behind the panel 34 which can be opened and closed in this manner, maintenance of the rectifier circuit 4 is facilitated and the opening and closing device 18 is easily operated.

Figure 16 shows another example of the power conversion device 1, and is a configuration diagram of the power conversion device 1 in which the multi-phase transformer 3 is a 6-phase transformer 3b. As with the configuration using the 12-phase transformer 3a shown in Figure 2 above, it is equipped with a ground fault protection circuit 5 and a rectifier circuit 4. However, since the rectifier circuit 4 only needs to rectify the 6-phase AC voltage, the number of diodes is halved.

FIG. 17 is a specific configuration diagram of a six-phase transformer 3b, which has three windings: a first winding 41 constituting a primary winding L1, and a second winding 42 and a third winding 43 constituting a secondary winding L2.
Each has three winding sections (41a to 41c, 42a to 42c, 43a to 43c) for handling three-phase power, and is star-connected.

The first winding 41 has three terminals (primary side terminals) Rin, Sin, and Tin to which the R, S, and T phases of a high-voltage three-phase power supply are connected. The second winding 42 has three output terminals (secondary side terminals) R1, S1, and T1, and the third winding 43 has three output terminals (secondary side terminals) R2, S2, and T2. The star-connected neutral points of the second winding 42 and the third winding 43 are connected to each other.
In addition, this neutral point Q is grounded via a lightning arrester 20 as described later.

The windings (41a-41c, 42a-42c, 43a-43c) of each winding 41, 42, 43 are wound around three legs of this iron core 48. However, the third winding 43 has fewer turns than the second winding 42, and the second winding 42 and the third winding 43 are wound at a ratio of 1:(√3-1). Specifically, the third winding 43 has approximately 0.73 times the number of turns of the second winding 42.

Fig. 18 shows vector diagrams of voltages generated at six terminals on the secondary side wound in this way, where (a) shows the voltages of each phase and (b) shows the line voltage. As shown in Fig. 18(a), the three-phase voltages V _R1 , V _S1 , and V _T1 output from the second winding 42 and the three-phase voltages V _R2 , V _S2 , and V _T2 output from the third winding 43 have a phase difference of 120 degrees, but the output of the third winding 43 is small in proportion to the turns ratio with respect to the second winding 42. Therefore, the output voltage has a magnitude proportional to the turns ratio.

18B, the line (phase) voltage is such that, for the phase of the voltage V _S1-R1 between the R1 terminal and the S1 terminal, the voltage V _S1-S2 (V _S1 +V _S2 ) between the S1 terminal and the S2 terminal occurs in a 30-based phase and has the same absolute value. Also, for the phase of the voltage V _T1-S1 between the S1 terminal and the T1 terminal, the voltage V _T1-T2 (V _T1 +V _T2 ) between the T1 terminal and the T2 terminal occurs in a 30-based phase and has the same absolute value. Furthermore, for the phase of the voltage V _R1-T1 between the T1 terminal and the R1 terminal, the voltage V _R1-R2 (V _R1 +V _R2 ) between the R1 terminal and the R2 terminal occurs in a 30-based phase and has the same absolute value.

Figure 19 shows the output voltage waveform of transformer 3b with each winding wound in this manner. This output voltage is rectified by rectifier circuit 4, which performs full-wave rectification, generating a direct current consisting of a 12-phase voltage waveform, and a direct current voltage with small ripple can be obtained.

In this way, since the second winding 42 and the third winding 43 which generate the secondary voltage are also star-connected in addition to the first winding 41, it is easy to make the generated voltage constant. And since the third winding 43 is also star-connected, the number of turns can be reduced by √3 compared to the conventional delta connection, and the transformer 3b can be made smaller.
Since the line voltages between the same phases of the second winding 42 and the third winding 43 are generated with a phase difference of 30 degrees with respect to the phase of the line voltage output from the second winding 42, a voltage of six phases can be generated. Then, by performing full-wave rectification, a waveform of a total of 12 phases with a phase shift of 30 degrees can be generated.
Therefore, by generating six-phase polyphase low-voltage AC power and generating DC power by full-wave rectification using the rectifier circuit 4, it is possible to generate DC power with small ripples, and it is possible to generate stable DC power without using a converter that switches at high frequency. Therefore, noise due to high frequency switching is not generated.

20 shows the flow of lightning current (lightning surge) It when the DC circuit M1 of the power conversion device 1 using this six-phase transformer 3b is struck by lightning. However, only the secondary winding of the six-phase transformer 3b is shown. P1 is the point of lightning strike, 17 is a switchgear, 20 is a lightning arrester (SPD), and the neutral point Q of the six-phase transformer 3b is grounded via the lightning arrester 20.
As shown in FIG. 20, when the DC circuit M1 is struck by lightning, the resulting lightning surge It flows out to the ground via the rectifier circuit 4, the six-phase transformer 3b, and the lightning arrester 20.
If the polarity of the lightning strike is reversed, the lightning surge It flows through the reverse path (through the reverse-polarity diode in the rectifier circuit 4).

In this way, when the DC circuit M1 is struck by lightning, the lightning surge It can be passed through the rectifier circuit 4 and the transformer 3 to the lightning arrester 20, protecting the secondary side of the transformer 3 from lightning damage. In addition, the secondary winding L2 of the six-phase transformer 3b has a common neutral point Q, and by grounding this neutral point Q via the lightning arrester 20, lightning damage to the secondary circuit can be minimized with only one lightning arrester 20.

21 is a circuit diagram illustrating the flow of a ground fault current Ig when a ground fault occurs in the DC circuit M1 of the circuit configured as described above using the six-phase transformer 3b. P2 indicates the point of the ground fault.
21, the ground-fault current Ig flows from the ground through the ground-fault protection circuit 5 to the other DC circuit M1, and then flows through the rectifier circuit 4 and the six-phase transformer 3b to the grounded DC circuit M1. At this time, the current is limited by the resistor element Rr of the ground-fault protection circuit 5.
In this way, the ground fault current Ig is limited by the resistance element Rr of the ground fault protection circuit 5, so that damage caused by electric shock accidents and the like can be suppressed.

Incidentally, even in the power conversion device 1 using this six-phase transformer 3b, the DC output can be made a bipolar power supply as shown in FIG. 3 above using the 12-phase transformer 2a, and as in the configuration shown in FIG. 13 above, a circuit breaker can be disposed on the primary side of the six-phase transformer 3b, and the circuit breaker on the primary side can be tripped if the current value of the resistance element Rr indicates an abnormal value.
In the above embodiment, the power conversion device 1 that converts high-voltage AC power into DC power has been described. However, if a consumer has an AC grid in addition to a DC grid, a portion of the low-voltage AC power before rectification can be branched off and supplied to the AC grid.

1: power conversion device, 3: multi-phase transformer, 3a: 12-phase transformer, 3b: 6-phase transformer, 4: rectifier circuit (full-wave rectifier circuit), 5: earth fault protection circuit, 7: DC/DC converter, 10: high-voltage three-phase power supply, 11: first winding, 12: second winding, 13: third winding, 14: fourth winding, 15: fifth winding, 17: switch, 18: switchgear, 20: lightning arrester, 30: circuit breaker, 31: current monitoring unit, 31a: alarm unit, 34: panel, 41: first winding, 42: second winding, 43: third winding, L1: primary winding, L2: secondary winding, M1: DC circuit, Q: neutral point, Rr: resistive element.

Claims (6)

A power conversion device that converts high-voltage three-phase power into low-voltage AC power using a multi-phase transformer, and converts the converted low-voltage AC power into DC power using a full-wave rectifier circuit,
the multi-phase transformer is a 12-phase transformer having a first winding constituting a primary winding and second to fifth windings constituting a secondary winding for generating four different three-phase phases,
the second to fifth windings are all star-connected, and the first to fifth windings are wound around a common iron core;
The neutral points of the second winding and the third winding are connected to each other,
the fourth winding and the fifth winding are branched from a common predetermined position in the middle of the third winding , and are formed with a number of turns that is 0.71 times that of the third winding ,
The power conversion device is characterized in that the second to fifth windings output three-phase voltages having different phases, thereby outputting a 12-phase AC voltage. 2. The power conversion device according to claim 1 , wherein the neutral point of the secondary winding is grounded via a lightning arrester. 3. The power conversion device according to claim 1, wherein two DC/DC converters are connected in parallel to the output of the full -wave rectifier circuit, and the outputs of the DC/DC converters are linked to form a bipolar power supply that outputs the same positive and negative voltages to a ground electrode . 3. The power conversion device according to claim 1 , wherein an output line of the full-wave rectifier circuit is grounded via a resistance element of 10 kΩ to 50 kΩ. a current monitoring unit that monitors a current flowing through the resistive element and outputs an abnormality occurrence signal when the current value falls outside a predetermined range;
5. The power conversion device according to claim 4 , further comprising an annunciation unit that receives the abnormality occurrence signal and performs an annunciation operation. A circuit breaker is provided on the primary side of the multi-phase transformer,
6. The power conversion device according to claim 5 , wherein the circuit breaker performs a circuit breaker operation upon receiving the abnormality occurrence signal.

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