Power conversion device

Abstract

The power conversion device (2) includes a power conversion circuit (100A) for performing AC-DC power conversion of the AC circuit (20, 3) and the DC circuit (9) of the power system (1), a braking chopper (10), and a control circuit (30) of the braking chopper (10). The braking chopper (10) includes a series circuit of a switch (11) and an energy absorber (12), and is configured to consume the energy at the connection destination of the braking chopper (10) by energizing the energy absorber (12) during the ON period of the switch (11). The control circuit (30) variably controls the duty ratio of the ON period of the switch (10) according to a power consumption command value based on at least one of the power flowing into the power conversion circuit (100A) and the power flowing out of the power conversion circuit during the operation period of the braking chopper (10) in response to the occurrence of a disturbance in the power system (1).

Description

Technical Field

[0001] The present disclosure relates to a power conversion device.

Background Art

[0002] In recent years, there has been an increasing use case of transmitting power generated by an offshore wind power plant using a high-voltage direct current (HVDC) system including a DC transmission line between an AC / DC converter (converter) on the sea and on land.

[0003] In such an HVDC system, when an AC system accident such as a ground fault or a short circuit occurs near an onshore AC / DC converter, it becomes impossible to send all the generated power of the offshore wind power plant to the onshore AC power system. There is concern that problems such as the AC / DC converter becoming unable to continue operation due to overvoltage protection may occur due to the voltage rise caused by the surplus energy generated at this time.

[0004] For countermeasures against such surplus energy, International Publication No. 2021 / 261041 (Patent Document 1) describes protection control by a braking chopper composed of a series circuit of a switch and a resistor. In Patent Document 1, when the DC voltage (Vdc) at the connection destination of the braking chopper rises during an accident, protection control is described in which the braking chopper is turned on (the switch is turned on) to consume surplus power, thereby suppressing the rise in the DC voltage.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the protection control described in Patent Document 1 operates the braking chopper in response to the detection of an increase in the DC voltage and maintains the operation of the braking chopper until the temperature of the braking chopper reaches the upper limit. Therefore, in the case of a minor system fault, there is a concern that the energy consumption of the braking chopper becomes excessive and the DC voltage drops too much, which may affect the operation of the AC-DC converter, such as the occurrence of hunting. Thus, there is room for improvement in the protection control of the power system using the braking chopper.

[0007] The present disclosure has been made to solve such problems, and an object of the present disclosure is to enhance the protection control by the braking chopper provided in a power conversion device arranged in a power system such as an HVDC system.

Means for Solving the Problems

[0008] In one aspect of the present disclosure, a power conversion device is provided. The power conversion device is connected between an AC circuit and a DC circuit of a power system. The power conversion device includes a power conversion circuit, a braking chopper, and a first control circuit that controls the operation of the braking chopper. The power conversion circuit performs AC-DC power conversion between the AC circuit and the DC circuit. The braking chopper is connected to the AC circuit, the DC circuit, or inside the power conversion circuit. The braking chopper includes a series circuit of a switch and an energy absorber, and is configured to consume the energy at the connection destination of the braking chopper by energizing the energy absorber during the on-period of the switch. The first control circuit variably controls the duty ratio of the on-period with respect to the switching period of the switch according to a power consumption command value based on at least one of the power flowing into the power conversion circuit from one of the AC circuit and the DC circuit and the power flowing out of the power conversion circuit to the other of the AC circuit and the DC circuit during the operation period of the braking chopper in response to a disturbance occurring in the power system.

Effects of the Invention

[0009] According to the present disclosure, since the power consumption by the braking chopper can be variably controlled in correspondence with the difference between the incoming power and the outgoing power in the power conversion circuit, it is possible to avoid the excessive consumption energy of the braking chopper and the excessive decrease in the DC voltage in a minor power system accident or the like, and to improve the protection control by suppressing the voltage rise inside the DC circuit or the power conversion circuit.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

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Figure 14A

Figure 14B

Figure 14C

Figure 15A

Figure 15B

Mode for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the following, the same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated in principle.

[0012] Embodiment 1. FIG. 1 is a schematic diagram for explaining a configuration example of an HVDC system 1 which is an example of a power system in which the power conversion device according to the present embodiment is arranged.

[0013] The HVDC system 1 shown in FIG. 1 is configured to transmit the generated power by the wind power generation device 3, also called a wind farm, arranged offshore, to the general AC system 20 on land. Specifically, the HVDC system 1 includes power conversion devices 2A and 2B, a wind power generation device 3, a bus bar 4, transformers 5 and 15, a transmission line 6, a DC system 9, and an AC system 20. For example, the DC system 9 can be constituted by a bipolar DC cable.

[0014] The generated power of the wind power generation device 3 is output to the bus bar 4 of the offshore substation. The power of the bus bar 4 is transmitted to the transmission line 6 via the transformer 5. The power conversion device 2B has a power conversion circuit 100B connected between the transmission line 6 and the DC system 9. The power conversion circuit 100B performs power conversion (AC-DC power conversion) between the AC power of the transmission line 6 and the DC power of the DC system 9. For example, the power conversion circuit 100B performs power conversion (AC-DC power conversion) with energy balance control for avoiding energy surplus or shortage inside the power conversion circuit 100B. In this case, the power conversion circuit 100B can be controlled so as to control the output current (output power) from the power conversion circuit 100B to the DC system 9 according to the generated power of the wind power generation device 3.

[0015] The DCL (DC reactor) 7 is inserted and connected to the power conversion circuit 100B side (wind power generation device 3 side) of the DC system 9 to suppress the voltage and current vibration components (harmonic components).

[0016] The power conversion device 2A is connected between the DC system 9 and the AC system 20. The power conversion device 2A is connected to the AC system 20 via the transformer 15. The power conversion device 2A has a power conversion circuit 100A for performing power conversion (AC-DC power conversion) between the AC power of the AC system 20 and the DC power of the DC system 9. Each of the power conversion circuits 100A and 100B can typically be configured by an MMC (Modular Multilevel Converter) in which converter cells each incorporating a power storage element such as a capacitor are cascade-connected. The power conversion circuit 100A basically performs AC-DC power conversion with energy balance control for avoiding energy surplus or shortage inside the power conversion circuit 100A by transmitting the power input from the wind power generation device 3 to the DC system 9 directly to the AC system 20 in order to control the DC voltage of the DC system 9 to be constant.

[0017] In the HVDC system 1 of FIG. 1, when a disturbance such as a system accident or load fluctuation occurs in the AC system 20, the power that can be output from the power conversion circuit 100A to the AC system 20 temporarily decreases. On the other hand, assuming a case where the generated power of the wind power generation device 3 is transmitted to the DC system 9 without decreasing, in the power conversion circuit 100A, the outflow power to the AC system 20 decreases with respect to the inflow power from the DC system 9, and surplus energy in which this difference is integrated is generated.

[0018] Due to this surplus energy, there is a concern that the operation of the power conversion circuit 100A may become unable to continue due to the occurrence of an overvoltage (OV: Over Voltage) abnormality as the voltage on the DC side (DC system 9) of the power conversion circuit 100A or the capacitor voltage inside the power conversion circuit 100A increases. As described below, the power conversion device according to the present embodiment is provided with a protection control function by a braking chopper that consumes the above-described surplus energy, and the protection control function can be made highly accurate by optimizing the power consumption by the braking chopper.

[0019] In the following, an example in which the power conversion device 2A is configured by the power conversion device according to the present embodiment will be described in correspondence with the above case. However, each of the power conversion device 2B or the power conversion devices 2A and 2B may be configured by the power conversion device according to the present embodiment. For this reason, in the following, the power conversion device according to the present embodiment will be described as the power conversion device 2 including the power conversion circuit 100 without using the subscripts A and B in FIG. 1.

[0020] That is, in the configuration example of FIG. 1, each of the wind power generation device 3 and the AC system 20 corresponds to an example of an "AC circuit". The DC system 9 corresponds to an example of a "DC circuit".

[0021] The power conversion device 2 according to the present embodiment includes, in addition to the power conversion circuit 100, a braking chopper 10, a chopper control circuit 30, and an MMC control circuit 110. The braking chopper 10 has a series circuit of a switch 11 and a resistance element 12. Hereinafter, in the present embodiment, an example in which the connection destination of the braking chopper 10 is the DC system 9 will be described. When the DC system 9 is a bipolar cable, the switch 11 and the resistance element 12 can be connected in series between the two-pole cables. Alternatively, the switch 11 and the resistance element 12 connected in series can be arranged between the positive electrode side (high voltage side) cable and the ground and between the negative electrode side (low voltage side) cable and the ground, respectively.

[0022] The switch 11 can be configured by a self-extinguishing switching element capable of controlling both the on operation and the off operation, typified by an IGBT (Insulated Gate Bipolar Transistor). The resistance element 12 is an example of an "energy absorber". During the on period of the switch 11, the energy of the connection destination (DC system 9) is consumed by energizing the resistance element 12.

[0023] In the DC system 9, in addition to DCL7 in Fig. 1, it is also possible to additionally connect a DCL on the side of the wind power generation device 3 from the connection point with the braking chopper 10. On the other hand, if a DCL is arranged between the connection point and the power conversion circuit 100A, there is a concern that the effect of reducing the inflow power to the power conversion circuit 100A due to the operation of the braking chopper 10 will decrease. Therefore, it is preferable to avoid arranging a DCL at this part.

[0024] The chopper control circuit 30 outputs a gate signal for controlling the on / off of the switch 11 based on sensor measurement values (voltage, current, temperature, etc.). The switch 11 turns on during the H-level period of the gate signal (described later) from the chopper control circuit 30, while it turns off during the L-level period. The chopper control circuit 30 that controls the operation of the braking chopper 10 corresponds to an embodiment of the "first control response".

[0025] The MMC control circuit 110 generates a control signal for controlling the operation (AC / DC power conversion) of the power conversion circuit 100 based on sensor measurement values (voltage, current, temperature, etc.). Note that an example of the control operation by the MMC control circuit 110 when the power conversion circuit 100 is configured by an MMC will be described in Embodiment 2 below. That is, the control of the braking chopper (also simply referred to as chopper control) described in Embodiment 1 is applicable without limiting the configuration of the power conversion circuit 100. The MMC control circuit 110 corresponds to an embodiment of the "second control circuit".

[0026] Fig. 2 is a block diagram showing an example of the hardware configuration of the chopper control circuit 30 and the MMC control circuit 110. Fig. 2 shows an example of configuring the chopper control circuit 30 and the MMC control circuit 110 by a computer.

[0027] Referring to FIG. 2, the chopper control circuit 30 and the MMC control circuit 110 include one or more input converters 90, one or more sample and hold (S / H) circuits 91, a multiplexer (MUX) 92, an A / D (Analog-to-Digital) converter 93, one or more CPUs 94, a RAM (Random Access Memory) 95, a ROM (Read Only Memory) 96, one or more input / output interfaces (I / F) 97, an auxiliary storage device 98, and a bus 99 that interconnects the above components with each other.

[0028] The input converter 90 has an auxiliary transformer (not shown) for each input channel. Each auxiliary transformer converts a detection signal such as voltage, current, temperature, etc. from each sensor into a signal with a voltage level suitable for subsequent signal processing. The sample and hold circuit 91 is provided for each input converter 90. The sample and hold circuit 91 samples and holds a signal representing the electrical quantity received from the corresponding input converter 90 at a specified sampling frequency.

[0029] The multiplexer 92 sequentially selects the signals held in the plurality of sample and hold circuits 91. The A / D converter 93 converts the signal selected by the multiplexer 92 into a digital value. Note that by providing a plurality of A / D converters 93, A / D conversion may be performed in parallel on the detection signals of a plurality of input channels.

[0030] The CPU 94 controls the entire chopper control circuit 30 or MMC control circuit 110 and executes arithmetic processing according to a program. The RAM 95 as a volatile memory and the ROM 96 as a non-volatile memory are used as the main memory of the CPU 94. The ROM 96 stores programs and setting values for signal processing, etc. The auxiliary storage device 98 is a non-volatile memory with a larger capacity than the ROM 96 and stores programs and data such as sensor detection values.

[0031] The input / output interface 97 is an interface circuit for communication between the CPU 94 and an external device. Note that, unlike the example in FIG. 2, at least a part of the chopper control circuit 30 and the MMC control circuit 110 can also be configured using a circuit such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). For example, the functions of each functional block described in each of the drawings to be described later can be configured based on the computer illustrated in FIG. 2, or at least a part of them can be configured using a circuit such as an FPGA or an ASIC. Also, at least a part of the functions of each functional block can be configured by an analog circuit.

[0032] Next, the details of the control of the braking chopper will be described. FIG. 3 is a functional block diagram for explaining the control configuration of the braking chopper according to Embodiment 1.

[0033] As shown in FIG. 3, the chopper control circuit 30 shown in FIG. 1 includes a duty ratio control unit 40 and a start / stop control unit 50. Also, the AC power system 20 is configured as a three-phase AC power system, and the DC power system 9 is configured by DC cables 9P and 9N on the high-voltage side and the low-voltage side.

[0034] Measurement values by the AC sensor group 21 arranged in the AC power system 20 and measurement values by the DC sensor group 22 arranged in the DC power system 9 are input to the chopper control circuit 30. The measurement values by the AC sensor group 21 include the voltages Vu, Vv, Vw of each phase (UVW) of the three-phase AC and the currents Iu, Iv, Iw of each phase.

[0035] The measured values by the DC sensor group 22 include the DC current Idc flowing through the DC system 9 and the DC voltage Vdc between the DC cables 9P and 9N. As shown in FIG. 3, since the direction of the DC current Idc from the power conversion circuit 100A to the DC system 9 is defined as the positive direction (Idc>0), it is understood that the inflow power from the DC system 9 to the power conversion circuit 100A is represented by (-Idc)×Vdc. As shown in FIG. 3, the DC current Idc is a measured value at a position on the right side in the figure (i.e., the side away from the power conversion circuit 100A) rather than the connection point with the braking chopper 10 (denoted as "BRK" in the figure) in the DC system 9. Therefore, it is assumed that the DC current Idc indicates the current value exchanged between the power conversion circuit 100A and the braking chopper 10 and the DC system 9. Therefore, in FIG. 3, when Idc>0 (the outflow direction from the power conversion circuit 100A to the DC system 9), the DC current Idc does not include the current of the braking chopper 10, and when Idc<0 (the inflow direction from the DC system 9 to the power conversion circuit 100A), it is understood that the DC current Idc includes the current of the braking chopper 10.

[0036] The chopper control circuit 30 includes a duty ratio control unit 40 and a start / stop control unit 50. The start / stop control unit 50 executes a start sequence and a stop sequence, which will be described later, in order to control the start and stop of the braking chopper 10 by control operations described later.

[0037] The duty ratio control unit 40 includes arithmetic units 41A, 41B, a hold unit 42, a safety factor multiplication unit 43, a multiplier 44, a subtractor 45, a duty ratio arithmetic unit 46, a PWM (Pulse Width Modulation) modulation unit 47, and a carrier wave generation unit 48.

[0038] The calculation unit 41A calculates the outflow power Pac, which is the AC power output from the power conversion circuit 100A to the AC system 20, using the voltages Vu, Vv, Vw and currents Iu, Iv, Iw of each phase. Further, the calculation unit 41A calculates the system voltage V1 that reflects the accident state from the voltages Vu, Vv, Vw of each phase. For example, as the system voltage V1, the positive-phase voltage or the minimum-phase voltage, etc. can be used. As is well known, the positive-phase voltage is calculated using the voltages Vu, Vv, Vw of each phase represented in vector form and an operator that rotates the phase of the vector by 120°. Therefore, when the positive-phase voltage is used as the system voltage V1, during normal times (when no disturbance occurs), V1 = 1.0 [p.u.]. On the other hand, when a system accident or the like occurs, the system voltage V1 (positive-phase voltage) decreases (V1 < 1.0) to a value that depends on the mode of the accident (three-phase ground fault, single-phase ground fault, two-phase short circuit, unbalanced accident, etc.) and the position of the accident occurrence point (distance from the power conversion circuit 100A), etc., compared to normal times.

[0039] The calculation unit 41B calculates the inflow power Pdc, which is the DC power input from the DC system 9 to the power conversion circuit 100A, based on the above-mentioned calculation of (-Idc)×Vdc. In this embodiment, the values of voltage, current, and power are shown in dimensionless [p.u.] units.

[0040] The hold unit 42 holds the outflow power Pac calculated by the calculation unit 41A. Thereby, when the braking chopper 10 is activated due to a disturbance such as a system accident, the outflow power Pac0 at the immediately preceding normal time, that is, the outflow power Pac0 immediately before the operation of the braking chopper 10, can be obtained. The output value (outflow power Pac0) of the hold unit 42 is maintained after the activation of the braking chopper 10 until the braking chopper 10 is stopped. It is also possible to use, instead of the outflow power Pac0, the value Pdc0 of the DC power Pdc calculated by the calculation unit 41B at the immediately preceding normal time.

[0041] The safety factor multiplication unit 43 multiplies a predetermined safety factor Ka (for example, Ka = 0.9, Ka < 1) by the system voltage V1 calculated by the calculation unit 41A. The multiplier 44 outputs the multiplication value of the output value (outflow power Pac0) of the hold unit 42 and the output value (V1 × Ka) of the safety factor multiplication unit 43 as the outflow power command value Pac*.

[0042] Therefore, the outflow power command value Pac* can be set to be lower than the outflow power Pac immediately before the occurrence of a disturbance such as a system accident (during normal operation), and the lower the system voltage V1 is, that is, the more the system voltage decreases, the greater the degree of decrease.

[0043] The subtractor 45 calculates the power consumption command value Pbk* in the power conversion circuit 100A by subtracting the outflow power command value Pac* from the inflow power Pdc output from the calculation unit 41B (Pbk* = Pdc - Pac*). As a result, when Pdc > Pac*, Pbk* is set to be > 0. It is also possible to input the above-mentioned Pdc0 (immediately previous normal value) to the subtractor 45 and set Pbk* = Pdc0 - Pac*.

[0044] When the braking chopper 10 operates according to the startup sequence by the startup stop control unit 50, the duty ratio calculation unit 46 sets the duty ratio DTY (0 ≤ DTY ≤ 1.0) according to the power consumption command value Pbk*. Basically, the duty ratio DTY is set to a larger value as the power consumption command value Pbk* is higher. Most simply, the duty ratio DTY can be set within the range of 0 ≤ DTY ≤ 1.0 in proportion to the power consumption command value Pbk*. Alternatively, the duty ratio calculation unit 46 can also calculate the duty ratio DTY from the power consumption command value Pbk* as the output of a control calculation with a predetermined time response characteristic such as a first-order lag calculation or a pseudo-differential calculation including integration with the power consumption command value Pbk*.

[0045] On the other hand, during the stop period of the braking chopper 10, the duty ratio calculation unit 46 basically fixes the duty ratio DTY = 0 in order to fix the switch 11 in the off state. However, as will be described later, in the stop sequence of the braking chopper 10, it is also possible to gradually return the duty ratio DTY to 0 at a certain rate instead of immediately returning it to 0.

[0046] The PWM modulation unit 47 generates the gate signal Gch of the switch 11 of the braking chopper 10 by PWM modulation based on the comparison between the carrier wave CW generated by the carrier wave generation unit 48 and the duty ratio DTY output from the duty ratio calculation unit 46.

[0047] The carrier wave CW is a periodic signal with a constant frequency, such as a triangular wave or a sawtooth wave. The gate signal Gch becomes a pulse signal in which the duration of the H level period is obtained according to the duty ratio DTY within the same period Tc as the carrier wave CW. That is, the switching period Tc of the switch 11 is equal to the period Tc of the carrier wave CW.

[0048] Note that the switching frequency fsw (fsw = 1 / Tc) of the switch 11 needs to be sufficiently high with respect to the time constant by DCL7. On the other hand, if the switching frequency is too high, the power loss at the switch 11 increases. Therefore, an appropriate switching frequency fsw (fsw = 1 / Tc) can be determined considering the trade-off between the two.

[0049] The switch 11 is controlled to turn on during the H level period of the gate signal Gch and turn off during the L level period. That is, the ratio of the duration of the H level period Ton to the period Tc of the gate signal Gch, that is, the on-duty ratio of the switch 11, is controlled according to the duty ratio DTY from the duty ratio calculation unit 46.

[0050] Therefore, during the period when the duty ratio DTY = 0, since the switch 11 is fixed in the off state and the non - energization of the resistor element 12 is maintained, the braking chopper 10 is in a stopped state (power consumption Pbk = 0).

[0051] On the other hand, when the duty ratio DTY = 1.0, since the switch 11 is fixed in the on state and the energization of the resistor element 12 is maintained, the power consumption of the braking chopper 10 reaches the maximum value. In contrast, by setting 0 < DTY < 1.0, the power consumption of the braking chopper 10 can be continuously varied according to the duty ratio DTY.

[0052] In FIG. 3, an example of introducing the outflow power command value Pac* and setting the power consumption command value Pbk* according to (Pdc - Pac*) was described. However, more simply, it is also possible to calculate the power consumption command value Pbk* according to the difference between the measured values of the inflow power Pdc and the outflow power Pac. By setting the power consumption command value Pbk* using such a difference, a part of the inflow power from the DC system 9 during the occurrence of a disturbance in the AC system 20 can be absorbed by the braking chopper 10 (resistor element 12). Or, by setting Ka = 0 and making Pbk* = Pdc (or Pdc0), or by directly using the outflow power Pac0 immediately before the operation of the braking chopper 10 and setting Pbk* = Pac0, it is also possible to set the power consumption command value Pbk* so that all the inflow from the DC system 9 is absorbed by the braking chopper 10 (resistor element 12). Although the power consumption in the resistor element 12 increases and the influence on the frequency fluctuation of the power system also becomes larger, the most stable operation can be achieved for the power conversion circuit 100 (100A). As described above, in this embodiment, the power consumption command value Pbk* of the braking chopper 10 that operates in response to the occurrence of a disturbance in the AC power system 20 can be set based on at least one of the power flowing into the power conversion circuit 100A (100) and the power flowing out of the power conversion circuit. At this time, regarding the power flowing out of the power conversion circuit 100, more precisely, the loss generated in the downstream power conversion circuit 100 and / or the transformer 15 etc. may be considered and corrected by subtracting the loss component.

[0053] Next, with reference to FIG. 4, a configuration example of the start / stop control unit 50 will be described. FIG. 4 is a logic circuit diagram for explaining the details of the start / stop control unit 50.

[0054] Referring to FIG. 4, the start / stop control unit 50 includes comparators 51, 54, 56, 62, a pulse generation circuit 52, a subtractor 53, an integrator 55, logic gates 57 to 58, a start sequence circuit 60, a one-shot pulse generation circuit 61, logic gates 63 to 65, a high-temperature detection unit 68, and a stop sequence circuit 70.

[0055] First, the configuration and operation of the start / stop control unit 50 for starting the braking chopper 10 will be described.

[0056] The subtractor 53 subtracts the outflow power Pac calculated by the arithmetic unit 41A (FIG. 3) from the inflow power Pdc calculated by the arithmetic unit 41B (FIG. 3) and outputs the surplus power ΔP. The integrator 55 outputs the surplus energy ΔE by integrating the surplus power ΔP with multiplication of an integration gain Kp that defines the integration time constant.

[0057] Comparator 54 outputs a detection signal S2 according to the comparison result between the surplus power ΔP from the subtracter 53 and a predetermined determination value Pth. The detection signal S2 is set to the H level when ΔP > Pth, while it is set to the L level when ΔP ≤ Pth. Note that the determination value Pth may be set to have a so-called hysteresis characteristic such that it is set to a lower value when the detection signal S2 is at the H level than when the detection signal S2 is at the L level. The determination value Vth corresponds to an embodiment of the "first threshold value". Also, when having the hysteresis characteristic, the determination value Vth is such that the value when S2 = L level corresponds to the "first threshold value", and the set value when S2 = H level corresponds to the "fourth threshold value".

[0058] Similarly, comparator 54 outputs a detection signal S3 according to the comparison result between the surplus energy ΔE from the integrator 55 and a predetermined determination value Eth1. The detection signal S3 is set to the H level when ΔE > Eth1, while it is set to the L level when ΔE ≤ Eth1. The determination value Eth1 corresponds to an embodiment of the "second threshold value".

[0059] Comparator 51 generates a detection signal S0 according to the comparison result between the system voltage V1 calculated by the arithmetic unit 41A (Fig. 3) and a predetermined determination value Vth. The detection signal S0 is set to the H level when V1 < Vth, while it is set to the L level when V1 ≥ Vth. Regarding the determination value Vth, it may be set to have a hysteresis characteristic such that it is set to a higher value when the detection signal S0 is at the H level than when the detection signal S0 is at the L level. The determination value Vth corresponds to an embodiment of the "third threshold value". Also, when having the hysteresis characteristic, the determination value Vth is such that the value when S0 = L level corresponds to the "third threshold value", and the set value when S0 = H level corresponds to the "sixth threshold value".

[0060] The pulse generation circuit 52 generates a pulse signal S1 whose level transitions in response to the detection signal S0 from the comparator 51. Specifically, the pulse signal S1 transitions from the L level to the H level in response to the output signal of the comparator 51 changing from the L level to the H level, and transitions from the H level to the L level in response to the output signal of the comparator 51 changing from the H level to the L level. Therefore, in this way, the pulse signal S1 is generated to have an H level period corresponding to the period during which a state (V1 < Vth) where the system voltage has decreased due to a disturbance such as a system accident occurs.

[0061] Note that when the pulse generation circuit 52 transitions the pulse signal S1 from the H level to the L level, it is also possible to transition the pulse signal S1 from the H level to the L level by adding a standby time T1 as a delay time with respect to the timing when the output signal of the comparator 51 changes from the H level to the L level. Hereinafter, a control example in which the standby time T1 is added will be described.

[0062] The logic gate 57 outputs a detection signal S4 according to the OR (logical sum) operation result of the detection signal S2 from the comparator 54 and the detection signal S3 from the comparator 56. Therefore, the detection signal S4 changes from the L level to the H level when at least one of a state where the surplus power ΔP corresponding to short-term fluctuations is excessive (ΔP > Pth) and a state where the surplus energy ΔE corresponding to long-term fluctuations is excessive (ΔE > Eth1) occurs in the power conversion circuit 100A.

[0063] Logic gate 58 generates a start command signal Son for braking chopper 10 according to the AND (logical product) operation result of the detection signal S4 from logic gate 57 and the pulse signal S1 from pulse generation circuit 52. When the start command signal Son changes from the L level to the H level, start sequence circuit 60 executes the start sequence of braking chopper 10. The start sequence includes an instruction to start the operation processing of duty ratio DTY for duty ratio calculation unit 46. Duty ratio calculation unit 46 fixes DTY = 0 until it receives the start instruction, while when it starts the operation processing, it executes the calculation of duty ratio DTY (0 ≦ DTY ≦ 1.0) based on the power consumption command value Pbk*. Also, although details will be described in Embodiment 2, when braking chopper 10 itself experiences operation delay or non-operation due to a failure or the like, or when the bypass switch for protection is inadvertently turned on in each converter cell 120 due to an unexpected severe accident, control may be performed to wait for the turning on of the bypass switch according to the detection signal S4 in a state where the surplus power ΔP or surplus energy ΔE is excessive.

[0064] Next, the configuration and operation of start / stop control unit 50 for stopping braking chopper 10 will be described.

[0065] One-shot pulse generation circuit 61 outputs a one-shot pulse signal S5 with a pulse width time length of T0 in response to the start command signal Son changing from the L level to the H level.

[0066] Comparator 62 outputs a detection signal S7 according to the comparison result between the surplus energy ΔE from integrator 55 and a predetermined determination value Eth2. Detection signal S7 is set to the H level when ΔE < Eth2, while it is set to the L level when ΔE ≧ Eth2. Note that the determination value Eth2 is for determining whether the state where the surplus energy ΔE is excessive (ΔE ≧ Eth2) has been resolved, and is set so that Eth2 < Eth1 in order to have a hysteresis characteristic. The determination value Eth2 corresponds to an example of the "second threshold value" or the "fifth threshold value".

[0067] The logic gate 59 outputs a control signal S6 according to the NOR (negative logical sum) operation result of the detection signal S4 from the logic gate 57 and the pulse signal S1. Further, the logic gate 63 outputs a control signal S8 according to the OR (logical sum) operation result of the control signal S6 from the logic gate 59 and the detection signal S7 from the comparator 62.

[0068] Therefore, when starting the braking chopper 10 in which the start command signal Son changes from the L level to the H level, the control signal S8 is set to the L level in response to the pulse signal S1, or the detection signals S2 or S3 being set to the H level.

[0069] After the braking chopper 10 is started, the control signal S8 is set to the H level when the decrease in the system voltage V1 is eliminated and the pulse signal S1 returns to the L level, and the increase in the surplus power ΔP is eliminated and the detection signal S2 returns to the L level, or when the increase in the surplus energy ΔE is eliminated and the detection signal S7 changes to the H level.

[0070] The logic gate 64 outputs a control signal S9 according to the AND (logical product) operation result of the inverted level of the pulse signal S5 from the one-shot pulse generation circuit 61 and the control signal S8 from the logic gate 63. Therefore, the control signal S9 is fixed at the L level during the H level period (time length T0) of the pulse signal S5, while having the same signal level as the control signal S8 in other periods.

[0071] The high-temperature detection unit 68 generates a detection signal Stmp of temperature rise based on the chopper temperature Tbk detected by a temperature sensor (not shown) disposed in the braking chopper 10. For example, when the chopper temperature Tbk rises above a predetermined upper limit temperature, the detection signal Stmp is set to the H level for device protection of the braking chopper 10. The chopper temperature Tbk can be the temperature of the switch 11, the temperature of the resistance element 12, or both of these. Note that the detection signal Stmp for the temperature rise may be generated based on the amount of energy consumed by the braking chopper 10 (resistive element 12) instead of the detection value by the temperature sensor. For example, when the amount of energy consumed (calculated value), which is indicated by the time integral value of the power consumption calculated from the duty ratio DTY, exceeds a predetermined upper limit amount of energy, the detection signal Stmp can be set to the H level.

[0072] The logic gate 65 generates a stop command signal Soff for the braking chopper 10 according to the OR (logical sum) operation result of the control signal S9 from the logic gate 64 and the detection signal Stmp from the high-temperature detection unit 68.

[0073] Therefore, when the detection signal Stmp is set to the H level due to the rise in the chopper temperature Tbk, the stop command signal Soff is set to the H level for protecting the device of the braking chopper 10. On the other hand, when the detection signal Stmp is at the L level, it is set to the H level in response to the release of the abnormal state of the system voltage V1 and the abnormal states related to the surplus power ΔP and the surplus energy ΔE in the power conversion circuit 100A.

[0074] When the stop command signal Soff changes from the L level to the H level, the stop sequence circuit 70 executes the stop sequence of the braking chopper 10. The stop sequence includes an instruction to start the stop process for returning the duty ratio DTY to 0 for the duty ratio calculation unit 46. As described above, the stop process may involve a rate process of gradually returning the duty ratio DTY from the current value to 0 at a certain rate.

[0075] Also, after the execution of the stop sequence of the braking chopper 10, the execution of the next startup sequence process may be prohibited until a predetermined time elapses for cooling the braking chopper 10 or until the chopper temperature Tbk drops below a predetermined reference temperature.

[0076] In a symmetric monopolar HVDC system, when one of the positive and negative DC transmission lines constituting the DC system 9 grounds, the healthy side transmission line on the other side will have an overvoltage more than twice that with respect to the ground. When the midpoint of the braking chopper 10 is grounded, switches 11 are respectively arranged between both sides of the transmission lines and the grounding point. In such a configuration, among the two switches 11, the overvoltage of the healthy side transmission line can also be suppressed by turning on the switch 11 connected to the transmission line on the voltage rising side (overvoltage side). In this case, although not shown in FIG. 4, for each switch 11, control is applied such that when the voltage measurement value of the connected DC transmission line becomes equal to or higher than a predetermined threshold value with respect to the ground, it turns on, and when it becomes lower than the threshold value, it turns off. By doing so, the braking chopper 10 can be operated so that the switch 11 turns on in the overvoltage side transmission line.

[0077] FIG. 5 is a signal waveform diagram for explaining an operation example of the start / stop control unit 50. In FIG. 5, an operation example is shown when the temperature does not rise excessively during the operation of the braking chopper 10 and the detection signal Stmp is maintained at the L level.

[0078] In the example of FIG. 5, in response to a decrease in the system voltage due to a system accident in the AC system 20, at time t1, since the system voltage V1 drops below the determination value Vth, the pulse signal S1 changes from the L level to the H level.

[0079] Then at time t2, since the surplus power ΔP increases in response to a decrease in the power that can be output from the power conversion circuit 100A to the AC system 20 due to the system accident, through the changes in the detection signals S2 and S4, the start command signal Son changes from the L level to the H level. In response to this, a high level period (time length T0) is provided in the pulse signal S5 from the one-shot pulse generation circuit 61. At this time t2, the above-described start processing sequence of the braking chopper 10 is executed.

[0080] From time t2, the operation period of the braking chopper 10 is started, and by providing the ON period of the switch 11, the energy of the connection destination of the braking chopper 10 (for example, the DC cable 9P) is consumed by energizing the resistance element 12. Note that during the time t2 to t3 when the pulse signal S1 is at the H level, the output signal (control signal S9) of the logic gate 64 is forcibly set to the L level. Therefore, it is understood that the operation period of the braking chopper 10 ensures at least the H level period (time length T0) of the pulse signal S5.

[0081] In the example of FIG. 5, at time t4, the system fault is resolved, the system voltage V1 returns (V1 > Vth), and at time t5, it is assumed that the increase in the surplus power ΔP and the surplus energy ΔE has been resolved (ΔP < Pth and ΔE < Eth2). As a result, the detection signal S4 (logic gate 57) returns to the L level at time t5. On the other hand, the pulse signal S1 is maintained at the H level until time t6 when a predetermined standby time T1 has elapsed since time t4, and then returns to the L level at time t6.

[0082] The control signal S8 output from the logic gate 63 is maintained at the L level until time t6 when all of the pulse signal S1 and the detection signals S4 and S7 become the L level. Therefore, the stop command signal Soff is maintained at the L level until time t6 and changes from the L level to the H level at time t6. In response to this, the stop process of the braking chopper 10 is started. At this time, the operation of the braking chopper 10 can be maintained until the standby time T1 has elapsed from the timing (time t4) when the resolution of the system fault is detected. Thereby, during the transient phenomenon period immediately after the system fault recovery, it is possible to achieve a stable recovery from the system fault without rapidly increasing the outflow power Pac from the power conversion circuit 100A (that is, the power supplied to the AC system 20).

[0083] FIG. 6 shows a flowchart for explaining the outline of the protection control by the braking chopper in the power conversion device according to the present embodiment. The control process shown in FIG. 6 is repeatedly executed by the chopper control circuit 30 during the operation of the power conversion device 2.

[0084] As shown in FIG. 6, in the initial state where the braking chopper 10 is stopped, the chopper control circuit 30 determines whether or not the chopper startup condition is satisfied in step (hereinafter simply referred to as "S") 110. The process of S110 corresponds to the determination process for changing the startup command signal Son from the L level to the H level in FIG. 4. While S110 is NO determination, the stopped state of the braking chopper 10 (the OFF-fixed state of the switch 11 due to DTY = 0) is continued, and the determination of whether or not the chopper startup condition is satisfied (S110) is repeatedly executed.

[0085] When the chopper startup condition is satisfied and the startup command signal Son is set to the H level (at the time of YES determination in S110), the chopper control circuit 30 operates the braking chopper 10 in S120 and executes variable control of the chopper power consumption. As described with reference to FIG. 3, the power consumption of the braking chopper 10 can be variably controlled according to the surplus of the input power Pdc with respect to the power (outflow power command value Pac*) that the power conversion circuit 100 can output. For example, as in the example of FIG. 3, variable control of the chopper power consumption can be realized by controlling the duty ratio (DTY) of the switch 11 according to the power consumption command value Pbk* set corresponding to the above surplus.

[0086] During the operation of the braking chopper 10, the chopper control circuit 30 determines whether or not the chopper stop condition is satisfied in S130. The process of S130 corresponds to the determination process for changing the stop command signal Soff from the L level to the H level in FIG. 4. While S130 is NO determination, the operating state of the braking chopper 10 (the state in which the on-period of the switch 11 is provided by 0 < DTY ≤ 1.0) is continued, and the determination of whether or not the chopper stop condition is satisfied (S130) is repeatedly executed.

[0087] When the chopper stop condition is satisfied and the stop command signal Soff is set to the H level (when the determination at S130 is YES), the chopper control circuit 30 executes the stop processing sequence of the braking chopper 10 by S140 to stop the braking chopper 10. As a result, the initial state in which the braking chopper 10 is stopped is formed again. At this time, the startup of the braking chopper 10 may be prohibited according to the temperature measurement value of the braking chopper 10 or the elapsed time since the stop.

[0088] Next, with reference to FIGS. 7 and 8, an operation example of the protection control by the braking chopper in the power conversion device according to the present embodiment will be described.

[0089] FIG. 7 shows a conceptual waveform diagram for explaining a first operation example in a case where a disturbance such that the system voltage such as a three-phase ground fault accident near the power conversion circuit 100 drops to near 0 occurs in the AC system 20.

[0090] Referring to FIG. 7, in the first operation example, it is assumed that the system voltage drops to 0 in response to the occurrence of a system accident at time t1. As a result, in the control configuration of FIG. 4, as the system voltage V1 also drops to 0, the outflow power command value Pac* is set to 0, and the power supply from the power conversion circuit 100A to the AC system 20 is stopped.

[0091] In response to this, after time t1, while the outflow power Pac = 0, in the braking chopper 10, the duty ratio DTY can be set to 1.0 to increase the power consumption Pbk. Thereby, the consumption amount of surplus energy by the braking chopper 10 can be increased, and the voltage rise inside the DC system 9 or the power conversion circuit 100A can be suppressed.

[0092] On the other hand, Fig. 8 shows a conceptual waveform diagram for explaining a second operation example in a case where a disturbance occurs such that the system voltage such as a phase-to-phase short-circuit accident or an unbalance accident remains at a remote point from the power conversion circuit 100 (does not decrease until near 0).

[0093] Referring to Fig. 8, in the second operation example, in response to the occurrence of a system accident at time t1, the amount of decrease in the system voltage is smaller than that in Fig. 7 (the first operation example), and the system voltage does not decrease until 0. As a result, in the control configuration of Fig. 4, since the system voltage V1 also does not decrease until 0, the outflow power command value Pac*>0 is set. Therefore, it is possible to supply a certain amount of power from the power conversion circuit 100A to the AC system 20.

[0094] In such a case, if the braking chopper 10 is operated in such a manner that the switch 11 is simply turned on and fixed as in Patent Document 1, the energy consumption by the braking chopper 10 becomes excessive, and there is a possibility that the voltage inside the DC system 9 or the power conversion circuit 100A decreases excessively. Such a voltage drop is feared to have an adverse effect on the operation of the power conversion circuit 100A after the braking chopper 10 stops.

[0095] On the other hand, in Embodiment 1, in the control configuration of Fig. 4, by setting the outflow power command value Pac*>0, the power consumption command value Pbk* of the braking chopper 10 is set lower than that in Fig. 7 (Operation Example 1). Accordingly, after time t1, while power is supplied to the AC system 20 with the outflow power Pac>0, the braking chopper 10 can reduce the power consumption Pbk by setting the duty ratio DTY<1.0, compared with the operation example of Fig. 7.

[0096] Also, throughout Figs. 7 and 8, even when the system voltage returns due to the elimination of the accident cause such as the disconnection of the accident location at time t4, the operation of the braking chopper 10 is ensured until time t6 when the standby time T1 elapses from time t4, so that the outflow power Pac from the power conversion circuit 100A to the AC system 20 can be maintained.

[0097] Note that in FIGS. 7 and 8, for simplicity of description, the outflow power Pac and the power consumption Pbk of the braking chopper 10 are represented by constant values. However, actually, as the outflow power command value Pac* changes with the change in the system voltage V1 or the inflow power Pdc changes, the duty ratio DTY of the braking chopper 10 changes as the power consumption command value Pbk* changes over time, and the power consumption Pbk changes over time accordingly.

[0098] As described above, according to the power conversion device according to Embodiment 1, the power consumption Pbk during the operation period of the braking chopper 10 can be variably controlled in correspondence with the surplus power in the power conversion circuit 100. Thereby, while avoiding the excessive consumption energy of the braking chopper 10 and the excessive decrease in the DC voltage due to a minor system accident or the like, by suppressing the voltage rise inside the DC system 9 or the power conversion circuit 100A, the protection control can be enhanced.

[0099] In particular, by variably setting the power consumption Pbk in reflection of the degree of decrease in the system voltage V1, by appropriately estimating the power (outflow power Pac*) that can be output from the power conversion circuit 100, the voltage rise inside the DC system 9 or the power conversion circuit 100A can be appropriately suppressed by setting an appropriate power consumption Pbk.

[0100] Also, when the braking chopper 10 stops, the operation of the braking chopper 10 can be maintained until a predetermined standby time T1 elapses from the timing when the system accident is resolved. Thereby, the operation of the power conversion circuit 100A during the transient phenomenon period immediately after the system accident recovery can be stabilized, and a stable recovery from the system accident can be achieved.

[0101] Similarly, since the duty ratio DTY is gradually decreased to 0 by rate processing when the braking chopper 10 stops, the operation immediately after the system accident recovery can be further stabilized.

[0102] Also, in the example of FIG. 3, the safety factor Ka for obtaining the outflow power command value Pac* is set as a fixed value, but the safety factor Ka may be variably set based on the actual output power (outflow power) of the power conversion circuit 100 with respect to the AC power system 20. For example, by monitoring the difference (Pac - Pac*) between Pac* and the real-time outflow power Pac, when the difference is a negative value (Pac* > Pac), in order to increase the power consumption of the braking chopper 10, the safety factor Ka may be decreased from the current value to update and decrease the outflow power command value Pac*.

[0103] Note that the startup condition and stop condition of the braking chopper 10 when a disturbance occurs in the AC power system 20 are not limited to the examples shown in FIG. 4. Specifically, based on at least one of the three determinations used in FIG. 4: (1) excess determination of the surplus power ΔP (ΔP > Pth), (2) excess determination of the surplus energy ΔE (ΔE > Eth1, Eth2), and (3) decrease determination of the system voltage V1 (V1 < Vth), it is possible to arbitrarily determine each of the startup condition and stop condition. For example, in the configuration example of FIG. 4, the logic gate 58 can be an OR gate instead of an AND gate, and the startup command signal Son can be generated according to the OR (logical sum) operation result of the detection signals S4 and S1. In this way, steep power fluctuations that are faster than the detection of fluctuations in the system voltage V1 can also be protected. Alternatively, giving priority to the protection responsiveness, it is also possible to generate the startup command signal Son using only the detection signal S4 based on the surplus power ΔP.

[0104] Modification example of Embodiment 1. In the control of the braking chopper 10 described in Embodiment 1, if the startup of the braking chopper 10 is delayed in the event of a disturbance such as a system accident, the capacitor (not shown) in the power conversion circuit 100 is charged during the delay period, which increases the possibility of overvoltage. Therefore, after an accident is detected, it is preferable that the braking chopper 10 starts up as fast as possible. Regarding accident detection, as described above, high speed can be achieved by differential calculation of the input power and output power of the power conversion circuit 100. On the other hand, when PWM controlling the switch 11 of the braking chopper 10, there may be an operation delay due to the phase of the carrier wave.

[0105] FIG. 14A is a conceptual waveform diagram for explaining an example of the operation delay of the braking chopper 10 depending on the phase of the carrier wave.

[0106] As shown in FIG. 14A, the PWM control of the switch 11 by the PWM modulation unit 47 (FIG. 3) is executed by comparing the duty ratio DTY (0 ≦ DTY ≦ 1.0) set within the range of 0 to 1.0 according to the power consumption command value Pbk* and a carrier wave that periodically changes between 0 and 1.0 corresponding to the setting range of the duty ratio DTY. FIG. 14A illustrates a triangular carrier wave CW0 whose phase is not particularly adjusted.

[0107] When DTY < 1.0, the gate signal Gch is set to the H level and the switch 11 is turned on during the period of DTY ≧ CW, while the gate signal Gch is set to the L level and the switch 11 is turned off during the period of DTY < CW. Thereby, the power consumption of the braking chopper 10 can be made variable by on / off control of the switch 11 according to the power consumption command value Pbk*.

[0108] In the example of FIG. 14A, at the time of starting the braking chopper 10 in response to the occurrence of a disturbance in the AC system 20, at time t0, the duty ratio DTY rises from "0", which is a fixed value in the stopped state, to D1, which is a value set according to the power consumption command value Pbk*.

[0109] However, at time t0, if the value of the unadjusted carrier wave CW0 is higher than D1, the switch 11 is actually turned on, and the power consumption by the braking chopper 10 starts when the value of the periodically changing carrier wave CW0 becomes lower than D1 at time t1. After time t1, the high-level period lengths (times t1 to t2 and t3 to t4) of the gate signal Gch according to the duty ratio DTY = D1 are ensured. Thereby, through the duty ratio DTY, the power consumption of the braking chopper 10 can be controlled according to the power consumption command value Pbk*. However, since the switch 11 is not turned on between times t0 and t1, the above-described operation delay of the braking chopper 10 occurs, and there is concern about voltage increase inside the power conversion circuit 100 and the like.

[0110] Therefore, in the modification of Embodiment 1, in the control configuration of FIG. 3, the duty ratio DTY output from the duty ratio calculation unit 46 is also input to the carrier wave generation unit 48, and the carrier wave generation unit 48 sets the phase of the carrier wave according to the duty ratio DTY.

[0111] FIG. 14B is a conceptual waveform diagram for explaining a first example of PWM control of the braking chopper in the power conversion device according to the modification of Embodiment 1.

[0112] In the first example shown in FIG. 14B, at time t0, when the duty ratio DTY changes from 0 to D1 and the start of the braking chopper 10 is instructed, the carrier wave generation unit 48 generates a carrier wave CW1 as a triangular wave whose initial value at time t0 is equal to T1 and whose value decreases from time t0.

[0113] By using such a carrier wave CW1 of a phase, at time t0, since the gate signal Gch can be changed from the L level to the H level, the on period of the switch 11 can be started. Also, regarding the H-level period length (times t0 to t2a and t3a to t4a) of the gate signal Gch by comparing the carrier wave CW1 with the duty ratio DTY (= D1), it is understood that it is the same as in FIG. 14A (times t1 to t2 and t3 to t4).

[0114] FIG. 14C is a conceptual waveform diagram for explaining a second example of PWM control of the braking chopper in the power conversion device according to the modification of the first embodiment.

[0115] In the second example shown in FIG. 14C, at time t0, when the duty ratio DTY changes from 0 to D1 and the start of the braking chopper 10 is instructed, the carrier wave generation unit 48 generates a carrier wave CW2 as a sawtooth wave whose phase is set so that the initial value at time t0 is 0.

[0116] In the examples of FIGS. 14B and 14C, by setting the phase of the carrier wave CW by the carrier wave generation unit 48, the on period of the switch 11 can be started without delay at the start of the braking chopper 10, so that the occurrence of an operation delay of the braking chopper 10 can be avoided.

[0117] FIG. 15A is a block diagram for explaining a third example of PWM control of the braking chopper in the power conversion device according to the modification of the first embodiment.

[0118] In the third example shown in FIG. 15A, a shaping unit 49 is provided between the duty ratio calculation unit 46 and the carrier wave generation unit 48. The shaping unit 49 takes the duty ratio DTY from the duty ratio calculation unit 46 as an input and outputs a shaped duty ratio DTY♯. The PWM modulation unit 47 generates a gate signal Gch according to the comparison between the carrier wave CW from the carrier wave generation unit 48 and the duty ratio DTY♯ from the shaping unit 49. The phase of the carrier wave CW at this time is arbitrary, and it is also possible to use the carrier wave CW0 with no phase set as described in FIG. 14A.

[0119] FIG. 15B shows a conceptual waveform diagram for explaining PMW control according to the block diagram of FIG. 15A.

[0120] Referring to FIG. 15B, at time t0, in response to the duty ratio DTY from the duty ratio calculation unit 46 changing from 0 to D1, the waveform shaping unit 49 initializes the duty ratio DTY♯ to 1.0 instead of D1. Further, after time t0, the shaping unit 49 decreases the duty ratio DTY♯ from 1.0 to D1 with a predetermined time constant. By such waveform shaping of the duty ratio DTY, also in the third example of PMW control, the on-period of the switch 11 can be started without delay at the start of the braking chopper 10, so that the occurrence of an operation delay of the braking chopper 10 can be avoided.

[0121] Regarding the time constant for the duty ratio DTY♯ to change from 1.0 to D1, if it is set to be shorter than one period of the carrier wave CW, after the second H-level period (time t3 to t4), the on-period of the switch 11 can be provided with a period length according to the originally set duty ratio DTY (= D1).

[0122] In the third example shown in FIGS. 15A and 15B, since the on-period length of switch 11 from time t0 becomes longer than the period length according to the originally set duty ratio DTY (= D1), the braking chopper 10 can be controlled on the safe side with respect to the voltage rise due to surplus energy. Thereby, it can be expected that even if a delay occurs in accident detection, a voltage rise in the power conversion circuit 100 can be avoided.

[0123] Embodiment 2. In Embodiment 2, assuming that the power conversion circuit 100 is configured by an MMC, a method for enhancing the effect of protection control will be described.

[0124] FIG. 9 is a circuit diagram for explaining a configuration example of the power conversion circuit 100 in the power conversion device according to Embodiment 2.

[0125] Referring to FIG. 9, the power conversion circuit 100 is configured by a modular multilevel converter (MMC) including a plurality of converter cells 120 connected in cascade with each other. The power conversion circuit 100 performs power conversion between a DC system 9 which is an example of a DC circuit and an AC circuit (the AC system 20 or the wind power generation device 3 in FIG. 1).

[0126] The power conversion circuit 100 includes a plurality of leg circuits 104u, 104v, 104w connected in parallel with each other between a high-potential side DC terminal Np and a low-potential side DC terminal Nn. Hereinafter, for the leg circuits 104u, 104v, 104w, when collectively referring to them or indicating an arbitrary one, they may also be denoted as "leg circuit 104".

[0127] The high-potential side DC terminal Np is connected to a high-voltage side DC cable 9P, and the low-potential side DC terminal Nn is connected to a low-voltage side DC cable 9N. In the configuration example of FIG. 9, the braking chopper 10 can be connected between the DC cables 9P and 9N. Alternatively, the braking chopper 10 may be connected between the DC cable 9P and the ground and between the DC cable 9N and the ground, respectively. Furthermore, a capacitor 14 for holding the DC voltage Vdc may be connected between the high-potential-side DC terminal Np (DC cable 9P on the high-voltage side) and the low-potential-side DC terminal Nn (DC cable 9N on the low-voltage side) with respect to the MMC.

[0128] The leg circuit 104 is provided for each of a plurality of phases constituting the alternating current. The leg circuit 104 is connected between the above-described AC circuit and DC circuit and performs power conversion between the two circuits. In FIG. 9, the case where the AC circuit is three-phase AC is shown, and three leg circuits 104u, 104v, and 104w are provided corresponding to the U phase, V phase, and W phase, respectively.

[0129] The AC input terminals Nu, Nv, and Nw provided in the leg circuits 104u, 104v, and 104w, respectively, are connected to the AC circuit via the transformer 13. The transformer 13 corresponds to the transformer 15 (on the AC system 20 side) or the transformer 5 (on the wind power generation device 3 side) in FIG. 1. In FIG. 9, for ease of notation, the connection between the AC input terminals Nv, Nw and the transformer 13 is not shown.

[0130] Note that instead of using the transformer 13, the AC circuit and the leg circuits 104u, 104v, and 104w may be electrically connected via a series reactor. Alternatively, instead of the AC input terminals Nu, Nv, and Nw, primary windings may be provided in the leg circuits 104u, 104v, and 104w, respectively, and the leg circuits 104u, 104v, and 104w may be connected to the transformer 13 or the series reactor via secondary windings magnetically coupled to the primary windings. In this case, the primary windings may be the following reactors 108A and 108B. That is, the leg circuit 104 is electrically (i.e., directly or alternatingly) connected to the AC circuit via connection portions provided in each of the leg circuits 104u, 104v, and 104w, such as the AC input terminals Nu, Nv, and Nw or the above-described primary windings.

[0131] The leg circuit 104u includes an upper arm circuit 105 from the high-potential side DC terminal Np to the AC input terminal Nu and a lower arm circuit 106 from the low-potential side DC terminal Nn to the AC input terminal Nu. The AC input terminal Nu, which is the connection point of the upper arm circuit 105 and the lower arm circuit 106, is connected to the transformer 13. Hereinafter, the upper arm circuit 105 and the lower arm circuit 106 are collectively referred to simply as the "arm circuit". Since the leg circuits 104v and 104w have the same configuration as the leg circuit 104u, the configuration of the leg circuit 104u will be typically described below.

[0132] The upper arm circuit 105 includes a plurality of converter cells 120 connected in cascade and a reactor 108A. The plurality of converter cells 120 and the reactor 108A are connected in series. Similarly, the lower arm circuit 106 includes a plurality of converter cells 120 connected in cascade and a reactor 108B. The plurality of converter cells 120 and the reactor 108B are connected in series. In the following description, the number of converter cells 120 included in each of the upper arm circuit 105 and the lower arm circuit 106 is denoted as Ncell (where Ncell ≧ 2).

[0133] The position where the reactor 108A is inserted may be any position of the upper arm circuit 105 of the leg circuit 104u, and the position where the reactor 108B is inserted may be any position of the lower arm circuit 106 of the leg circuit 104u. A plurality of reactors 108A and 108B may be provided respectively. The inductance values of the respective reactors may be different from each other. Furthermore, only the reactor 108A of the upper arm circuit 105 or only the reactor 108B of the lower arm circuit 106 may be provided. Also, by devising the transformer connection, it is possible to cancel the magnetic flux of the DC component current and, by the leakage reactance of the transformer acting on the AC component current, it may be used as an alternative to the reactor. By providing the reactors 108A and 108B, it is possible to suppress a sudden increase in the fault current in the event of an accident in an AC circuit or a DC circuit, etc.

[0134] For the power conversion circuit 100, there are arranged an AC voltage detector 21A and an AC current detector 21B corresponding to the AC sensor group 21 shown in FIG. 3, a DC voltage detector 22A, 22B and a DC current detector 22c corresponding to the DC sensor group 22 shown in FIG. 3, and arm current detectors 109A, 109B provided in each leg circuit 104. The signals detected by the detectors are input to the MMC control circuit 110.

[0135] In FIG. 9, for the sake of easy illustration, the signal lines of the signals input from each detector to the MMC control circuit 110 and the signal lines of the signals input and output between the MMC control circuit 110 and each converter cell 120 are partially described together, but actually, signal lines are provided for each detector and each converter cell 120. The signal lines are constituted by, for example, optical fibers.

[0136] The AC voltage detector 21A detects the voltage Vu of the U phase, the voltage Vv of the V phase, and the voltage Vw of the W phase of the AC circuit. In the following description, Vu, Vv, and Vw are also collectively referred to as Vac.

[0137] The AC current detector 21B detects the current Iu of the U phase, the current Iv of the V phase, and the current Iw of the W phase of the AC circuit. In the following description, Iu, Iv, and Iw are also collectively referred to as Iac.

[0138] The DC voltage detector 22A detects the DC voltage Vdcp of the high-potential side DC terminal Np connected to the DC circuit (DC system 9). The DC voltage detector 22B detects the DC voltage Vdcn of the low-potential side DC terminal Nn connected to the DC circuit (DC system 9). The difference between the DC voltage Vdcp and the DC voltage Vdcn corresponds to the DC voltage Vdc in FIG. 1 and the like. The DC current detector 22C detects the DC current Idc flowing through the DC system 9 (high-potential side DC terminal Np or low-potential side DC terminal Nn).

[0139] The arm current detectors 109A and 109B provided in the leg circuit 104u for the U phase detect the upper arm current Ipu flowing through the upper arm circuit 105 and the lower arm current Inu flowing through the lower arm circuit 106, respectively. The arm current detectors 109A and 109B provided in the leg circuit 104v for the V phase detect the upper arm current Ipv and the lower arm current Inv, respectively. The arm current detectors 109A and 109B provided in the leg circuit 104w for the W phase detect the upper arm current Ipw and the lower arm current Inw, respectively.

[0140] FIG. 10 is a circuit diagram showing an example of a sub-module constituting the converter cell 120 of FIG. 9.

[0141] The converter cell 120 illustrated in FIG. 10 includes a half-bridge type conversion circuit 120HB, a capacitor 124 as a representative example of a "power storage element", a voltage detector 125, and a cell control unit 127.

[0142] The half-bridge type conversion circuit 120HB includes switching elements 122A and 122B connected in series with each other, and diodes 123A and 123B. The diodes 123A and 123B are connected in anti-parallel with the switching elements 122A and 122B, respectively. Hereinafter, when the switching elements 122A and 122B and the diodes 123A and 123B are collectively referred to or any one of them is shown, they are described as the switching element 122 and the diode 123, respectively.

[0143] The capacitor 124 is connected in parallel with the series connection circuit of the switching elements 122A and 122B and holds a DC voltage. The connection node of the switching elements 122A and 122B is connected to the high-potential side input / output terminal 126P. The connection node of the switching element 122B and the capacitor 124 is connected to the low-potential side input / output terminal 126N.

[0144] Typically, the input / output terminal 126P is connected to the input / output terminal 126N of the converter cell 120 adjacent to the positive electrode side. The input / output terminal 126N is connected to the input / output terminal 126P of the converter cell 120 adjacent to the negative electrode side.

[0145] For each of the switching elements 122A and 122B, a self - extinguishing type switching element is used in the same way as the switch 11. For example, the switching elements 122A and 122B can be composed of IGBTs or GCTs (Gate Commutated Turn - off Thyristors), etc. Further, a bypass switch BPS can be provided between the input / output terminals 26P and 26N. The bypass switch BPS is a switch configured to be able to short - circuit both ends of the switching element 122B by closing the contacts, and is capable of conducting fault current. That is, the bypass switch BPS can protect each element (switching elements 122A, 122B, diodes 123A, 123B, and capacitor 124) included in the converter cell 120 from the over - current generated during an accident by short - circuiting the converter cell 120.

[0146] Note that the conversion circuit of the converter cell 120 is not limited to the half - bridge type conversion circuit 120HB as described above. For example, the converter cell 120 may be configured using an upside - down half - bridge type, a full - bridge type conversion circuit, or a three - level bridge type conversion circuit.

[0147] The voltage detector 125 detects the voltage between both ends 124P and 124N of the capacitor 124 (hereinafter, the cell voltage Vcell).

[0148] The voltage of the capacitor 124 as a power storage element (cell voltage Vcell) changes according to the integration of the power balance of the power conversion circuit 100. Specifically, when the inflow power of the power conversion circuit 100 is greater than the outflow power, the cell voltage Vcell held in the capacitor 124 of each converter cell 120 rises. Conversely, when the inflow power of the power conversion circuit 100 is smaller than the outflow power, the cell voltage Vcell of each converter cell 120 decreases.

[0149] The cell control unit 127 generates a gate signal for controlling the on and off of the switching elements 122A and 122B according to the phase-shifted PWM control based on the control command 128 received from the MMC control circuit 110. The cell control unit 127 further transmits a signal 29 including the cell voltage Vcell detected by the voltage detector 125 to the MMC control circuit 110.

[0150] Typically, the cell control unit 127 can control one of the switching elements 122A and 122B to be in the on state and the other to be in the off state. When the switching element 122A is in the on state and the switching element 122B is in the off state, the voltage (Vcell) across both ends of the capacitor 124 is applied between the input / output terminals 126P and 126N. Conversely, when the switching element 122A is in the off state and the switching element 122B is in the on state, the voltage between the input / output terminals 26P and 26N becomes 0 [V] (zero voltage).

[0151] The converter cell 120 can output a zero voltage or a positive voltage (+Vcell) depending on the voltage of the capacitor 124 by alternately turning on the switching elements 122A and 122B. The diodes 123A and 123B are provided for protection when a reverse voltage is applied to the switching elements 122A and 122B. Also, for the overvoltage prevention measures of the capacitor 124, the bypass switch BPS can be controlled by the cell control unit 127 so as to turn on when the cell voltage Vcell rises above a predetermined voltage upper limit value OV1. As described above, the instruction to turn on the bypass switch by the cell control unit 127 may be waited for according to a command from the chopper control circuit 30 or the MMC control circuit 110 in response to the on state (transition from the L level to the H level) of the detection signal S4 indicating that the surplus power ΔP or the surplus energy ΔE is excessive, as described with reference to FIG. 4. Thereby, in the case of an operation delay or non-operation due to a failure of the braking chopper 10 itself as described above, or when the bypass switch BPS is unnecessarily turned on due to an unexpected severe accident, it can be avoided.

[0152] As described above, after waiting for the instruction to turn on the bypass switch BPS in response to the on state of the detection signal S4, if the voltage further rises and reaches the insulation limit of the capacitor or the element, the bypass switch BPS may be turned on to prevent insulation breakdown. In this case, it is understood that the waiting for turning on the bypass switch BPS means changing the threshold voltage for turning on the bypass switch (for example, the above-mentioned voltage upper limit value OV1) in the upward direction.

[0153] The above-mentioned cell control unit 127 may be configured by a dedicated circuit such as an ASIC (Application Specific Integrated Circuit), or may be configured using an FPGA (Field Programmable Gate Array) or the like. Alternatively, it may be configured based on a computer including a CPU (Central Processing Unit) and a memory, or may be configured by a combination of two or more of the above.

[0154] FIG. 11 is a block diagram for explaining an example of the MMC control configuration in the power conversion device according to the second embodiment.

[0155] Referring to FIG. 11, the MMC control circuit 110 includes a subtracter 161, a DC voltage control unit 162, an output current control unit 166, a per-phase voltage command calculation unit 168, and a per-cell DC voltage control unit 169.

[0156] The subtracter 161 outputs a voltage deviation ΔVdc obtained by subtracting the detected value of the DC voltage Vdc (DC voltage detectors 22A, 22B) from the target value (Vdc*) of the DC voltage Vdc. The DC voltage control unit 162 calculates a DC voltage command value Vdcref for making ΔVdc = 0 by performing a control operation (e.g., proportional-integral (PI) operation or proportional (P) operation, etc.) with the voltage deviation ΔVdc from the subtracter 161 as an input.

[0157] As will be described later, the total voltage control unit 200 generates an active current target value Id* so as to perform feedback control of the representative value (e.g., average value) of the cell voltage Vcell of each converter cell 120 to a predetermined cell voltage target value Vcell*. As described above, when the power balance becomes unbalanced and the cell voltage Vcell rises or falls, the total voltage control unit 200 calculates the active current target value Id* in order to adjust the output power of the power conversion circuit 100 to compensate for the increase or decrease amount of the cell voltage Vcell with respect to the cell voltage target value Vcell*. Further, as will be described later, the active current target value Id* may be executed by combining feedback control and feedforward control.

[0158] The output current control unit 166 takes as inputs the active current target value Id* which is the output of the total voltage control unit 200, the reactive current target value Iq*, and the currents Iu, Iv, Iw, and outputs an active voltage command value Vd* and a reactive voltage command value Vq*.

[0159] In the output current control unit 166, the active current Id and the reactive current Iq are calculated from the currents Iu, Iv, Iw of each phase by a three-phase to two-phase conversion, and the active voltage command value Vd* and the reactive voltage command value Vq* are calculated by performing a control operation (PI operation or P operation, etc.) for making the current deviations ΔId (ΔId = Id* - Id) and ΔIq (ΔIq = Iq* - Iq) approach zero.

[0160] Although illustration is omitted, the reactive current target value Iq* can be obtained by a control operation (such as PI operation or P operation) for making the power deviation (Pq* - Pq) between the reactive power command value Qq* given from the upper control device and the actual reactive power Qq calculated from the voltages Vu, Vv, Vw and currents Iu, Iv, Iw of each phase approach zero.

[0161] Each phase voltage command calculation unit 168 calculates the AC voltage command values Vu*, Vv*, Vw* by two-phase - three-phase inverse conversion of the active voltage command value Vd* and the reactive voltage command value Vq* from the output current control unit 166.

[0162] Each cell DC voltage control unit 169 generates a control command 128 for each converter cell 120 by taking as inputs the AC voltage command values Vu*, Vv*, Vw* from each phase voltage command calculation unit 168 and the DC voltage command value Vdcref from the DC voltage control unit 162. The control command 128 is, for example, the voltage command values for the upper arm circuit 105 and the lower arm circuit 106 of each phase of UVW. The voltage command value Vref can be calculated, for example, according to the following formula (1). Vref = (Mau + Mdc)·Vcellarm …(1)

[0163] In formula (1), Mau is a sine wave signal having an amplitude and a phase obtained from the above-described AC voltage command values Vu*, Vv*, Vw*, and is set individually for each phase. Also, Mdc is a DC modulation rate set according to the DC voltage command value Vdcref, and is set with a value around 0.5 [pu] as a reference value. Also, Vcellarm is a coefficient set according to the average value of the cell 1 voltage Vcell in each arm circuit (the upper arm circuit 105 or the lower arm circuit 106 of each phase), and Vcellav = 1.0 when the average value is the cell voltage target value Vcell*.

[0164] The cell control unit 127 (FIG. 10) of each converter cell 120 can generate gate signals (on / off control signals) of the switching elements 122A and 122B (FIG. 10) according to the phase-shifted PWM control based on the voltage command value of the arm to which the converter cell 120 belongs, received as a control command 128.

[0165] Note that the carrier frequency of the phase-shifted PWM is set low with emphasis on reducing the switching losses (of the switching elements 122A and 122B) in each converter cell 120 during steady operation. On the other hand, it may be set high with emphasis on control responsiveness and stability during the occurrence of disturbances or high-output operation. For example, (1) when the excess power ΔP obtained by subtracting the outflow power from the inflow power to the converter exceeds the set range, (2) when the control deviation of the control system such as current, voltage, and power exceeds the set range, (3) when the operating state of the converter such as active power, reactive power, apparent power, current, and voltage output from the converter exceeds the set range, or (4) when the system state such as system voltage, frequency, harmonic components, and unbalance exceeds the set range, etc., it can be determined that it is during the occurrence of disturbances or high-output operation, and the carrier frequency can be set higher than during steady operation.

[0166] By such MMC control, the outflow power (AC active power) from the power conversion circuit 100 to the AC circuit is increased or decreased by the total voltage control for controlling the representative value (for example, average value) of the cell voltage Vcell of each converter cell 120 to the cell voltage target value Vcell*. In the configuration example of FIG. 9, it is understood that the increase and decrease of the outflow power by the total voltage control are reflected, and the AC voltage command values Vu*, Vv*, and Vw* of each phase are generated.

[0167] Also, when the DC voltage Vdc of the DC system 9 rises or falls with respect to the target value Vdc*, command values (Vdcref) for compensating for the rise or fall are reflected, and voltage command values for the upper arm and the lower arm of each phase can be generated from the AC voltage command values Vu*, Vv*, and Vw* of each phase.

[0168] FIG. 12 is a block diagram for explaining a configuration example of the total voltage control unit 200. Referring to FIG. 12, the full voltage control unit 200 includes an average value calculation unit 210, a subtractor 212, a feedback (FB) calculation unit 215, feedforward calculation units 220 and 222, a switching switch 223, an adder 225, a control gain setting unit 230, and a multiplier 235.

[0169] The average value calculation unit 210 receives the cell voltages Vcell of all (Ncell × 3) converter cells 120 included in the power conversion circuit 100, and outputs the full voltage average value Vcellav, which is the average value, as a "representative value". The subtractor 212 subtracts the cell voltage target value Vcell* from the full voltage average value Vcellav (representative value) by the average value calculation unit 210, and outputs the full voltage deviation ΔVcell. The FB calculation unit 215 outputs a feedback term Idfb of the effective current target value Id* by a control calculation (for example, PI control or P control, etc.) using the full voltage deviation ΔVcell as an input. Corresponding to ΔVcell>0, that is, Vcellav>Vcell*, Idfb>0 is calculated so as to increase the effective power flowing out from the power conversion circuit 100.

[0170] The feedforward control may be executed with switching (switching switch 223) of calculations between when the braking chopper 10 is not operating (normal time when no disturbance occurs) and when the braking chopper 10 is operating (when a disturbance occurs in the AC system 20). The feedforward calculation unit 220 calculates a feedforward term Idf of the effective current target value Id* in the normal time (when the braking chopper 10 is not operating) by multiplying the DC current Idc by a feedforward gain Kf1 (Kf1<0). According to the positive and negative directions of the DC current Idc defined in FIG. 3, when power flows into the power conversion circuit 100, for Idc<0, Idf>0 is set. On the other hand, based on the target value Pac* (Fig. 4) of the outflow power during the operation of the braking chopper 10, the feedforward operation unit 222 calculates the feedforward term Idff of the target value Id* of the effective current. Specifically, the feedforward term Idff can be calculated by multiplying the value obtained by dividing the target value Pac* of the outflow power by the AC voltage Vac (measured value) by the feedforward gain Kf2 (Kf2 > 0) (Idff = Kf2 · Pac* / Vac). The switching switch 223 selects one of the outputs of the feedforward operation units 220 and 222 according to the control signal sw corresponding to the presence or absence of the operation of the braking chopper 10, and inputs it to the adder 225. Specifically, when the braking chopper 10 is not operating (normal time) with sw = "0", the switching switch 223 selects the output of the feedforward operation unit 220. On the other hand, when the braking chopper 10 is operating (when a disturbance occurs) with sw = "1", the switching switch 223 selects the output of the feedforward operation unit 222.

[0171] The adder 225 adds the feedback term Idfb from the FB operation unit 215 and the feedforward term Idff from the switching switch 223 to calculate the base value of the target value Id* of the effective current (Id* = Idfb + Idff). The control gain setting unit 230 sets the control gain Kb according to the control signal SKb from the chopper control circuit 30. The multiplier 235 multiplies the base value from the adder 225 by the control gain Kb set by the control gain setting unit 230 to calculate the target value Id* (Fig. 10) of the effective current.

[0172] Fig. 13 is a waveform diagram for explaining an example of setting the control gain Kb. Referring to Fig. 13, the control signal SKb is set to the H level over the time period t2 to t6 (Fig. 5) which is the operation period of the braking chopper 10 and the period from time t5 until the elapse of the continuous time T2 of a predetermined time length (time t6 to t7). During the stop period of the braking chopper 10 excluding times t2 to t7, the control signal SKb is set to the L level.

[0173] During the L-level period of the control signal Skb, the control gain setting unit 230 sets the control gain Kb = 1.0. On the other hand, during the H-level period of the control signal Skb, the control gain Kb = K1 (K1>1.0, for example, K1 is around 1.05) is set. The control gain Kb corresponds to the "adjustment coefficient". In the examples of FIGS. 10 and 11, the effective current target value Id* corresponds to an embodiment of the "command value for output power control of the power conversion circuit".

[0174] As a result, the effective current target value Id* output from the multiplier 235 (FIG. 12) is set as the above base value during normal times including before the operation of the braking chopper 10. On the other hand, corresponding to the operation period of the braking chopper 10, during the period from time t2 to t7, it will be set to K1 times the base value which is the control value during normal times.

[0175] As a result, according to the power conversion device according to Embodiment 2, when the cell voltage Vcell rises due to surplus power during the operation of the braking chopper 10, the effect of increasing the output power from the power conversion circuit 100 by full voltage control can be enhanced. In this way, by increasing the control amount of full voltage control in conjunction with the operation of the braking chopper 10, the voltage rise inside the DC system 9 or the power conversion circuit 100A can be effectively suppressed.

[0176] In particular, even after the time t7 when the braking chopper 10 is stopped, by providing a period with Kb>1.0, when surplus energy is accumulated in the power conversion circuit 100 due to disturbances in the AC system 20, the output of surplus energy by full voltage control can be promoted. As a result, even after the system accident recovery, the effect of suppressing the voltage rise inside the DC system 9 or the power conversion circuit 100A can be enhanced.

[0177] Regarding the cell voltage Vcell, if it goes out of the predetermined voltage range, an OV (Over Voltage) abnormality or a UV (Under Voltage) abnormality is detected, and the power conversion circuit 100 is protected and stopped. However, usually, a larger margin is ensured in the UV direction compared to the OV direction. Therefore, during the operation of the braking chopper 10, even if the control is simply to increase the absolute value of the control amount (effective current target value Id*) by total voltage control regardless of the power direction, the occurrence of the OV abnormality of the cell voltage Vcell can be suppressed.

[0178] Note that in the power conversion device according to the second embodiment, it is also possible to adopt a modification in which the braking chopper 10 is arranged in parallel with the power storage element (capacitor 124) of each converter cell 120 of the power conversion circuit 100 configured by the MMC. Such a connection example corresponds to an embodiment of a mode in which the braking chopper 10 is connected inside the power conversion circuit 100.

[0179] In addition, in this embodiment, an example in which the braking chopper 10 is arranged for the power conversion circuit 100A between the DC system 9 and the AC system 20 has been described. However, it is also possible to arrange the braking chopper 10 for the power conversion circuit 100B between the wind power generation device 3 and the DC system 9.

[0180] In the power conversion circuit 100B, by treating the AC power supplied from the wind power generation device 3 to the power conversion circuit 100B as "inflow power" and the DC power supplied from the power conversion circuit 100B to the DC system 9 as "outflow power", similar to the first embodiment, variable control of the power consumption of the braking chopper 10 can be realized. Also in this case, the power that can be output from the power conversion circuit 100B to the DC system 9 is determined by the power that can be output from the power conversion circuit 100A to the AC system 20. Therefore, the command value of the outflow power from the power conversion circuit 100B (equivalent to Pac* in FIG. 3) for obtaining the command value Pbk* of the power consumption of the braking chopper 10 can be obtained by multiplying the hold value of the DC power immediately before the accident by (V1 × Ka)[pu] similar to FIG. 3. However, in this case, high-speed communication between the power conversion devices 2A and 2B is required. For this reason, as another method, similar to the normal HVDC control that does not correspond to the offshore wind power generation device 3, by restricting the input DC power according to the AC output that the power conversion circuit 100A can output, as a result, it is also possible to cause the outflow power of the power conversion circuit 100B. In this case, based on the difference between the inflow power and the outflow power in the power conversion circuit 100B, using the surplus power ΔP in FIG. 4 and the safety factor Ka in FIG. 3, the power consumption command value Pbk* = ΔP × Ka can be set.

[0181] Also, as described above, for energy balance control, the power conversion circuit 100B operates to control the output current (output power) from the power conversion circuit 100B to the DC system 9 according to the generated power of the wind power generation device 3. Therefore, when performing the above-described full voltage control in the power conversion circuit 100B, a similar effect can be achieved by using the output value of the full voltage control unit 200 as the output current (output power) to the DC system 9.

[0182] Note that as a connection destination of the braking chopper 10, a modification example in which the AC sides of the power conversion circuits 100A and 100B are used is also possible. Including the modification example in which the connection destination of the braking chopper 10 described above is inside the power conversion circuits 100A and 100B (for example, between both ends of a power storage element such as a capacitor of the converter cell 120 constituting the MMC), in these modification examples as well, based on at least one of the inflow power and the outflow power of the power conversion circuit 100 (100A, 100B) corresponding to the connection destination of the braking chopper 10, the duty ratio of the braking chopper 10 can be controlled in the same manner as in the first embodiment.

[0183] In addition, in the second embodiment, although the overall voltage control in the case where the power conversion circuit 100 is configured by an MMC has been described, even if it is other than such a configuration, the same control as that in the second embodiment can be applied. For example, even when the power conversion circuit 100 is configured by a known two-level converter or three-level converter, the output control by the dq-axis current control illustrated in FIG. 13 is applicable. Specifically, for any control of the output power from the power conversion circuit 100 according to the increase or decrease of the accumulated energy in the entire power storage element inside the power conversion circuit 100, by interlocking with the operation of the braking chopper 10 and applying a control gain (adjustment coefficient) set in the same manner as in FIG. 13, the same effect can be enjoyed.

[0184] The embodiments disclosed this time should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is indicated by the claims rather than the above description, and it is intended that all modifications within the meaning and scope equivalent to the claims are included.

Description of Reference Numerals

[0185] 1 HVDC system (power system), 2, 2A, 2B power conversion device, 3 wind power generation device, 4, 6 transmission line, 5, 13, 15 transformer, 9 DC system, 9N, 9P DC cable, 10 braking chopper, 11 switch, 12 resistance element, 14 capacitor, 20 AC system, 21, 22 sensor group, 21A AC voltage detector, 21B AC current detector, 22A, 22B DC voltage detector, 22C, 22c DC current detector, 29 signal, 30 chopper control circuit, 40 duty ratio control unit, 41A, 41B, 215 arithmetic unit, 42 hold unit, 43 safety factor multiplication unit, 44, 235 multiplier, 45, 53, 161, 212 subtracter, 46 duty ratio arithmetic unit, 47 modulation unit, 48 carrier wave generation unit, 49 waveform shaping unit, 50 start / stop control unit, 51, 54, 56, 62 comparator, 52 pulse generation circuit, 55 integrator, 57 - 59, 63 - 65 logic gate, 60 start sequence circuit, 61 one-shot pulse generation circuit, 68 high temperature detection unit, 70 stop sequence circuit, 100, 100A, 100B power conversion circuit, 104, 104u, 104v, 104w leg circuit, 105 upper arm circuit, 106 lower arm circuit, 108A, 108B reactor, 109A, 109B arm current detector, 110 MMC control circuit, 120 converter cell, 120HB conversion circuit, 122, 122A, 122B switching element, 123, 123A, 123B diode, 124 capacitor (converter cell), 124N, 124P both ends (capacitor), 125 voltage detector, 126N, 126P input / output terminal, 127 cell control unit, 128 control command, 162 DC voltage control unit, 166 output current control unit, 168 per-phase voltage command arithmetic unit, 169 cell DC voltage control unit, 200 total voltage control unit, 210 average value arithmetic unit, 220 feed-forward arithmetic unit, 225 adder, 230 control gain setting unit, CW carrier wave, DTY duty ratio, Eth1, Eth2, Pth, Vth determination value, Gch gate signal, Id effective current, Id* effective current target value, Idc DC current, Idfb feedback term, Idff feed-forward term, Iq reactive current, Iq* reactive current target value, Iu, Iv,Iw current (each phase), Ka safety factor, Kb control gain, Kf feedforward gain, Kp integral gain, Nn low-potential side DC terminal, Np high-potential side DC terminal, Nu, Nv, Nw AC input terminals, Pbk* power consumption command value, Pdc incoming power, S1, S5 pulse signals, Soff stop command signal, Son start command signal, T0 time length, T1 standby time, T2 continuous time, Tbk chopper temperature, Tc switching period, V1 system voltage, Vcell cell voltage, Vcellav average value of total voltage, Vcell* cell voltage target value, Vd effective voltage command value, Vdc, Vdcn, Vdcp DC voltage, Vdc* target value, Vdcref DC voltage command value, Vq* reactive voltage command value, Vu, Vv, Vw voltage (each phase), Vu*, Vv*, Vw* AC voltage command value (each phase).

Claims

1. A power conversion device connected between an AC circuit and a DC circuit of a power system, a power conversion circuit for performing AC / DC power conversion between the AC circuit and the DC circuit; a braking chopper connected to an inside of the AC circuit, the DC circuit, or the power conversion circuit; a first control circuit for controlling an operation of the braking chopper; the braking chopper includes a series circuit of a switch and an energy absorber, and is configured to consume energy of a connection destination of the braking chopper by energizing the energy absorber during an on-period of the switch, The first control circuit is during an operation period of the braking chopper in response to occurrence of a disturbance in the power system, a duty ratio of the on-period with respect to a switching period of the switch is variably controlled in accordance with a power consumption command value based on at least one of a power flowing from one of the AC circuit and the DC circuit to the power conversion circuit and a power flowing from the power conversion circuit to the other of the AC circuit and the DC circuit; the first control circuit, during an operation period of the braking chopper, sets an outflow power target value based on a measurement value of the outflow power or the inflow power immediately before operation of the braking chopper and a voltage measurement value of the power grid, and variably controls the duty ratio in accordance with the power consumption command value calculated by subtracting the outflow power target value from the measurement value of the inflow power.

2. 2. The power conversion device according to claim 1, wherein the first control circuit sets the outflow power target value based on a measurement value of the outflow power immediately before the braking chopper is activated and a voltage measurement value of the power grid, and variably controls the duty ratio in accordance with the power consumption command value calculated by subtracting the outflow power target value from a measurement value of the inflow power immediately before the activation, among the measurement values ​​of the inflow power.

3. A power conversion device connected between an AC circuit and a DC circuit of a power system, a power conversion circuit for performing AC / DC power conversion between the AC circuit and the DC circuit; a braking chopper connected to an inside of the AC circuit, the DC circuit, or the power conversion circuit; a first control circuit for controlling an operation of the braking chopper; the braking chopper includes a series circuit of a switch and an energy absorber, and is configured to consume energy of a connection destination of the braking chopper by energizing the energy absorber during an on-period of the switch, The first control circuit is during an operation period of the braking chopper in response to occurrence of a disturbance in the power system, a duty ratio of the on-period with respect to a switching period of the switch is variably controlled in accordance with a power consumption command value based on at least one of a power flowing from one of the AC circuit and the DC circuit to the power conversion circuit and a power flowing from the power conversion circuit to the other of the AC circuit and the DC circuit; The first control circuit variably controls the duty ratio during an operation period of the braking chopper in accordance with the power consumption command value calculated to a value equivalent to the measured value of the inflow power during or immediately before the operation of the braking chopper, or the measured value of the outflow power immediately before the operation of the braking chopper.

4. 3. The power conversion device according to claim 1, wherein the first control circuit reduces the outflow power target value below a current value when the outflow power target value is greater than the measured value of the outflow power during an operation period of the braking chopper.

5. The power conversion device according to any one of claims 1 to 3, wherein the first control circuit sets the duty ratio as an output of a control calculation having a predetermined time characteristic, the control calculation having the power consumption command value as an input.

6. 4. The power conversion device according to claim 1, wherein the first control circuit gradually reduces the duty ratio to zero at a constant rate in a process of stopping the braking chopper.

7. A power conversion device connected between an AC circuit and a DC circuit of a power system, a power conversion circuit for performing AC / DC power conversion between the AC circuit and the DC circuit; a braking chopper connected to an inside of the AC circuit, the DC circuit, or the power conversion circuit; a first control circuit for controlling an operation of the braking chopper; the braking chopper includes a series circuit of a switch and an energy absorber, and is configured to consume energy of a connection destination of the braking chopper by energizing the energy absorber during an on-period of the switch, The first control circuit is during an operation period of the braking chopper in response to occurrence of a disturbance in the power system, a duty ratio of the on-period with respect to a switching period of the switch is variably controlled in accordance with a power consumption command value based on at least one of a power flowing from one of the AC circuit and the DC circuit to the power conversion circuit and a power flowing from the power conversion circuit to the other of the AC circuit and the DC circuit; When the first control circuit detects that the disturbance in the power system has been resolved while the braking chopper is in operation, the first control circuit maintains the operation of the braking chopper at least until a predetermined waiting time has elapsed from the point of detection.

8. The power conversion device according to any one of claims 1 to 3 and 7, wherein, when the braking chopper is stopped, the first control circuit determines whether or not to activate the braking chopper based on at least one of a determination that surplus power obtained by subtracting the outflow power from the inflow power is higher than a first threshold value, a determination that surplus energy obtained by integrating the surplus power is higher than a second threshold value, and a determination that a voltage of the power system has dropped below a third threshold value.

9. The power conversion device according to any one of claims 1 to 3 and 7, wherein, when the braking chopper is operating, the first control circuit executes a determination as to whether or not to stop the braking chopper based on at least one of a determination that surplus power obtained by subtracting the outflow power from the inflow power is lower than a first threshold value, a determination that surplus energy obtained by integrating the surplus power is lower than a second threshold value, and a determination that a voltage of the power system has risen above a third threshold value.

10. the first control circuit, when the braking chopper is operating, determines whether or not it is necessary to stop the braking chopper based on at least one of a determination that the surplus power is lower than a fourth threshold, a determination that the surplus energy is lower than a fifth threshold, and a determination that a voltage of the power grid has increased above a sixth threshold; The fourth threshold is set lower than the first threshold, The fifth threshold is set lower than the second threshold, The power conversion device according to claim 8 , wherein the sixth threshold value is set to be higher than the third threshold value.

11. A power conversion device connected between an AC circuit and a DC circuit of a power system, a power conversion circuit for performing AC / DC power conversion between the AC circuit and the DC circuit; a braking chopper connected to an inside of the AC circuit, the DC circuit, or the power conversion circuit; a first control circuit for controlling an operation of the braking chopper; the braking chopper includes a series circuit of a switch and an energy absorber, and is configured to consume energy of a connection destination of the braking chopper by energizing the energy absorber during an on-period of the switch, The first control circuit is during an operation period of the braking chopper in response to occurrence of a disturbance in the power system, a duty ratio of the on-period with respect to a switching period of the switch is variably controlled in accordance with a power consumption command value based on at least one of a power flowing from one of the AC circuit and the DC circuit to the power conversion circuit and a power flowing from the power conversion circuit to the other of the AC circuit and the DC circuit; The first control circuit is a carrier wave generating unit that generates a carrier wave which is a periodic signal for pulse width modulation; a modulation unit that generates an on / off control signal for the switch according to a comparison between the duty ratio set according to the power consumption command value and the carrier wave, The duty ratio is fixed to 0 when the braking chopper is stopped, and increases from 0 when the braking chopper is started; The carrier wave generating unit sets a phase of the carrier wave so that an on period of the switch starts in response to the duty ratio increasing from 0 when the braking chopper is started.

12. the carrier wave is a triangular wave that rises or falls between 0 and 1.0 in accordance with the setting range of the duty ratio, 12. The power conversion device according to claim 11, wherein the carrier wave generating unit sets a phase of the carrier wave so that, when the braking chopper is started, the carrier wave has an initial value according to the first value and decreases from the initial value in response to the duty ratio increasing from 0 to a first value set in accordance with the power consumption command value.

13. the carrier wave is a sawtooth wave that periodically changes between 0 and 1.0 in accordance with the setting range of the duty ratio, The power conversion device according to claim 11 , wherein the carrier wave generating unit sets a phase of the carrier wave such that an initial value of the carrier wave becomes zero when the braking chopper is started up.

14. A power conversion device connected between an AC circuit and a DC circuit of a power system, a power conversion circuit for performing AC / DC power conversion between the AC circuit and the DC circuit; a braking chopper connected to an inside of the AC circuit, the DC circuit, or the power conversion circuit; a first control circuit for controlling an operation of the braking chopper; the braking chopper includes a series circuit of a switch and an energy absorber, and is configured to consume energy of a connection destination of the braking chopper by energizing the energy absorber during an on-period of the switch, The first control circuit is during an operation period of the braking chopper in response to occurrence of a disturbance in the power system, a duty ratio of the on-period with respect to a switching period of the switch is variably controlled in accordance with a power consumption command value based on at least one of a power flowing from one of the AC circuit and the DC circuit to the power conversion circuit and a power flowing from the power conversion circuit to the other of the AC circuit and the DC circuit; The first control circuit is a carrier wave generating unit that generates a carrier wave which is a periodic signal for pulse width modulation; a shaping unit that shapes a waveform of the duty ratio that is set in accordance with the power consumption command value; a modulation unit that generates an on / off control signal for the switch according to a comparison between the duty ratio shaped by the shaping unit and the carrier wave, the duty ratio is fixed to 0 when the braking chopper is stopped, and increases from 0 to a first value that is set in accordance with the power consumption command value when the braking chopper is started; the carrier wave is generated so as to periodically change between 0 and 1.0 in accordance with a set range of the duty ratio; the shaping unit shapes the duty ratio so that the value of the duty ratio input to the modulation unit has an initial value of 1.0 when the braking chopper is started up, and then decreases from 1.0 to the first value in accordance with a predetermined time constant.

15. A power conversion device connected between an AC circuit and a DC circuit of a power system, a power conversion circuit for performing AC / DC power conversion between the AC circuit and the DC circuit; a braking chopper connected to an inside of the AC circuit, the DC circuit, or the power conversion circuit; a first control circuit for controlling an operation of the braking chopper; the braking chopper includes a series circuit of a switch and an energy absorber, and is configured to consume energy of a connection destination of the braking chopper by energizing the energy absorber during an on-period of the switch, The first control circuit is during an operation period of the braking chopper in response to occurrence of a disturbance in the power system, a duty ratio of the on-period with respect to a switching period of the switch is variably controlled in accordance with a power consumption command value based on at least one of a power flowing from one of the AC circuit and the DC circuit to the power conversion circuit and a power flowing from the power conversion circuit to the other of the AC circuit and the DC circuit; A second control circuit is provided for controlling the AC / DC power conversion by the power conversion circuit, The second control circuit controls the AC-DC power conversion by multiplying a command value for output power control of the power conversion circuit by an adjustment coefficient that is set to 1.0 before the braking chopper is operated and that is set to a value greater than 1 in conjunction with the operation period of the braking chopper.

16. 16. The power conversion device according to claim 15, wherein, when the operation period of the braking chopper is ended by the first control circuit, the second control circuit maintains the adjustment coefficient at a value greater than 1 for a predetermined duration from the end point.

17. the power conversion circuit is configured by a modular multilevel converter having an arm in which a plurality of converter cells, each of which includes a storage element and a switching element, are cascaded; 17. The power conversion device according to claim 15 or 16, wherein the command value for the output power control is calculated so as to feedback control a representative value of the voltage of each of the storage elements of all of the converter cells in the power conversion circuit to a predetermined target value.

18. the power conversion circuit is configured by a modular multilevel converter having an arm in which a plurality of converter cells, each of which includes a storage element and a switching element, are cascaded; the command value for the output power control is calculated by a combination of feedforward control and the feedback control, 18. The power conversion device according to claim 17, wherein the feedforward control is performed based on a DC current flowing between the power conversion circuit and the DC circuit during a non-operation period of the braking chopper, and is performed based on a target value of the flowing power from the power grid during an operation period of the braking chopper.