Power conversion device and DC power transmission system

Abstract

To provide a power conversion device and a DC power transmission system in which operation continuity of a sound pole can be prevented from being deteriorated while misoperation due to an accident event is avoided.SOLUTION: A power conversion device according to an embodiment has a power converter, a switching control unit, and a protection control unit. The power converter has a plurality of capacitors, and a plurality of switching elements capable of switching charging / discharging of the capacitors. The switching control unit controls the switching elements disposed in the power converter. The protection control unit temporarily blocks the gate of a switching element in the power converter when detecting an accident spreading from an accident having occurred in a different pole.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] FIELD An embodiment of the present invention relates to a power conversion device and a DC power transmission system. [Background technology]

[0002] In recent years, the introduction of high-voltage direct current (HVDC) systems has been expanding for the purpose of improving the stability, reliability, and capacity factor of power systems, as well as promoting the use of renewable energy. Among these HVDC systems, multi-pole HVDC systems with four or more voltage-commutated converters at both ends are required to continue operating the healthy pole even in the event of a DC ground fault or short-circuit fault. However, if a DC ground fault occurs, a potential fluctuation in the ungrounded return conductor due to a fault current causes a potential fluctuation in the healthy pole main conductor on the same side. If this phenomenon causes a significant increase in DC voltage, the current flowing through the healthy pole converter increases rapidly, causing a rise in cell capacitor voltage, which triggers a converter shutdown and potentially disrupts the healthy pole's operation. It is known that if the fluctuation in the ungrounded return conductor potential is too large, the healthy pole converter's own-end protection can quickly and temporarily block the gate (GB) to minimize the impact on the healthy pole's operation.

[0003] The following Patent Document 1 describes a method for fault monitoring on DC transmission lines as a self-end protection method. The method calculates abnormal currents that could cause cell capacitor overvoltages from the instantaneous value and time average of DC current, and detects when the abnormal current exceeds a predetermined threshold. This method, when attempting to detect the spread of a fault to healthy poles in a bipolar configuration based solely on abnormal current, may result in converter gate blocking, even if the converter is capable of continuing operation, potentially reducing its ability to continue operation. The following Patent Document 2 describes a method for suppressing potential fluctuations in the return line to continue operation without gate blocking when a fault occurs on the DC side. However, if the fluctuations in the return line potential due to a DC fault are too large, suppressing potential fluctuations may not be enough to prevent cell overvoltages. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6336236 [Patent Document 2] Patent No. 7379395 Summary of the Invention [Problem to be solved by the invention]

[0005] The problem to be solved by the present invention is to provide a power conversion device and a DC transmission system that can suppress a decrease in the operational continuity of healthy poles while avoiding malfunctions in the event of an accident. [Means for solving the problem]

[0006] A power conversion device according to an embodiment has at least two pairs of DC transmission lines, each pair having a high potential side and a low potential side, and is provided in a multi-pole configuration in which either the relatively high potential side or the relatively low potential side of each pair of DC transmission lines is shared with either the relatively high potential side or the relatively low potential side of another pair of DC transmission lines, and is provided between the high potential side and the relatively low potential side of each pair of DC transmission lines. The power conversion device according to an embodiment has a power converter, a switching control unit, and a protection control unit. The power converter has a plurality of capacitors and a plurality of switching elements capable of switching between charging and discharging the capacitors. The switching control unit controls the switching elements provided in the power converter. The protection control unit temporarily gate-blocks the switching elements of the power converter when it detects a fault spread caused by a fault occurring on another pole. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a diagram showing an example of the configuration of a power conversion system including a power conversion device according to an embodiment; [Figure 2] FIG. 1 is a diagram for explaining an example of an event that occurs when a ground fault occurs in a DC transmission line. [Figure 3] 1 is a diagram illustrating an example of the configuration of a power conversion device according to an embodiment; [Figure 4] FIG. 1 is a diagram illustrating an example of the configuration of a power converter according to an embodiment. [Figure 5] FIG. 2 is a diagram showing an example of a cell configuration according to the embodiment. [Figure 6] FIG. 2 is a diagram illustrating an example of a protection control unit according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, a power conversion device and a DC power transmission system according to an embodiment will be described with reference to the drawings.

[0009] (Embodiment) FIG. 1 is a diagram illustrating an example of the configuration of a power conversion system including a power conversion device according to an embodiment. The power conversion system 1 is an example of a "DC transmission system." The power conversion system 1 illustrated in FIG. 1 is connected between a first AC system E1 and a second AC system E2. The power conversion system 1 includes, for example, power conversion devices 10-1 to 10-4, DC transmission lines 20-1 to 20-3, transformers 30-1 to 30-4, and AC circuit breakers 40-1 to 40-4. The power conversion devices 10-1 to 10-4 are provided at interconnection points between the AC system and the DC system, and convert AC power supplied by the AC system into DC power supplied by the DC system. The AC system may be an AC power source or an AC load, and the DC system may be a DC power source or a DC load.

[0010] The power conversion device 10-1 is connected to the first AC system E1 via a transformer 30-1 and an AC circuit breaker 40-1, and converts AC power from the first AC system E1 into DC power. The power conversion device 10-1 also converts DC power transmitted from the power conversion device 10-3 side via the DC transmission line 20-1 into AC power and outputs the AC power to the first AC system E1. The power conversion device 10-2 is connected to the first AC system E1 via a transformer 30-2 and an AC circuit breaker 40-2, and converts AC power from the first AC system E1 into DC power. The power conversion device 10-2 also converts DC power transmitted from the power conversion device 10-4 side via the DC transmission line 20-2 into AC power and outputs the AC power to the first AC system E1.

[0011] The power conversion device 10-3 is connected to the second AC system E2 via a transformer 30-3 and an AC circuit breaker 40-3, and converts AC power from the second AC system E2 into DC power. The power conversion device 10-3 also converts DC power transmitted from the power conversion device 10-1 side via the DC transmission line 20-1 into AC power and outputs the AC power to the second AC system E2. The power conversion device 10-4 is connected to the second AC system E2 via a transformer 30-4 and an AC circuit breaker 40-4, and converts AC power from the second AC system E2 into DC power. The power conversion device 10-4 also converts DC power transmitted from the power conversion device 10-2 side via the DC transmission line 20-2 into AC power and outputs the AC power to the second AC system E2.

[0012] That is, the power conversion devices 10-1 to 10-4 can perform control as rectifiers that convert AC to DC and inverter control that converts DC to AC. Note that, hereinafter, the control of the power conversion devices 10-1 to 10-4 as rectifiers may be referred to as "Rec control," and the inverter control of the power conversion devices 10-1 to 10-4 may be referred to as "Inv control."

[0013] For example, in the power conversion system 1, of the DC transmission lines 20-1 and 20-2, which are main lines with both ends ungrounded, the DC transmission line 20-1 connecting the power conversion device 10-1 and the power conversion device 10-3 has a positive potential, and the DC transmission line 20-2 connecting the power conversion device 10-2 and the power conversion device 10-4 has a negative potential. The DC transmission lines 20-1 and 20-2 may be expressed as poles. The DC transmission line 20-3 is a return line with one end grounded, and the potential on the side connected between the power conversion device 10-1 and the power conversion device 10-2 is 0 (zero, ground potential). One end of the DC transmission line 20-3 is connected to the power conversion device 10-1 and the power conversion device 10-2, and the other end is connected to the power conversion device 10-3 and the power conversion device 10-4.

[0014] In the pair of DC transmission lines 20-1 and 20-3, the DC transmission line 20-1 is a DC transmission line on the relatively high potential side, and the DC transmission line 20-3 is a DC transmission line on the relatively low potential side. In addition, in the pair of DC transmission lines 20-2 and 20-3, the DC transmission line 20-3 is a DC transmission line on the relatively high potential side, and the DC transmission line 20-2 is a DC transmission line on the relatively low potential side. In addition, the DC transmission lines 20-1 and 20-2 are transmission lines that are not shared with other pairs, and the DC transmission line 20-3 is a transmission line that is shared with other pairs. 1 has two sets of DC transmission lines, each set having a high potential side and a low potential side, and is provided in a multi-pole configuration in which either the relatively high potential side or the relatively low potential side of each set of DC transmission lines is shared with either the relatively high potential side or the relatively low potential side of the other set. Note that it is sufficient to provide at least two sets of DC transmission lines, each having a high potential side and a low potential side.

[0015] The transformer 30-1 converts the voltage of the AC power output from the power conversion device 10-1 to the first AC system E1 and the voltage of the AC power output from the first AC system E1 to the power conversion device 10-1. The transformer 30-2 converts the voltage of the AC power output from the power conversion device 10-2 to the first AC system E1 and the voltage of the AC power output from the first AC system E1 to the power conversion device 10-2. The transformer 30-3 converts the voltage of the AC power output from the power conversion device 10-3 to the second AC system E2 and the voltage of the AC power output from the second AC system E2 to the power conversion device 10-3. The transformer 30-4 converts the voltage of the AC power output from the power conversion device 10-4 to the second AC system E2 and the voltage of the AC power output from the second AC system E2 to the power conversion device 10-4.

[0016] The AC circuit breaker 40-1 is provided between the first AC system E1 and the transformer 30-1 and closes or opens the current path therebetween. The AC circuit breaker 40-2 is provided between the first AC system E1 and the transformer 30-2 and closes or opens the current path therebetween. The AC circuit breaker 40-3 is provided between the second AC system E2 and the transformer 30-3 and closes or opens the current path therebetween. The AC circuit breaker 40-4 is provided between the second AC system E2 and the transformer 30-4 and closes or opens the current path therebetween. When a fault that makes it impossible to continue operation of the own pole is detected, the AC circuit breakers 40-1 to 40-4 are opened after the power conversion devices 10-1 to 10-4 of the own pole are stopped.

[0017] Here, for example, if a ground fault or short circuit occurs in the DC transmission lines 20-1, 20-2, which are ungrounded main lines, the operation of the power conversion devices 10-1 to 10-4 or the entire power conversion system 1 may stop, the active power output to the first AC system E1 side or the second AC system E2 side may reverse (power flow reversal), or the stability of the voltage or frequency of the first AC system E1 or the second AC system E2 may be impaired. Below, an example of an event that occurs when a ground fault occurs in the DC transmission line 20-2 will be described.

[0018] FIG. 2 is a diagram illustrating an example of an event that occurs when a ground fault occurs in a DC transmission line. The example in FIG. 2 shows a configuration similar to that of power conversion system 1. Arrows in FIG. 2 schematically indicate the direction of current flow in the power lines. In the example shown in FIG. 2, it is assumed that power conversion devices 10-3 and 10-4 perform Rec control, and power conversion devices 10-1 and 10-2 perform Inv control. In the example shown in FIG. 2, a ground fault occurs in DC transmission line 20-2, so hereinafter, DC transmission line 20-1 will be referred to as a healthy electrode, and DC transmission line 20-2 will be referred to as a fault electrode.

[0019] Although a case where a ground fault occurs in the DC transmission line 20-2 is taken as an example here, a similar event occurs when a fault similar to a ground fault occurs in the power conversion devices 10-2 and 10-4. In other words, a fault at a fault pole includes not only a fault occurring in a DC transmission line but also a fault occurring in a power conversion device at the fault pole.

[0020] When a ground fault occurs in the DC transmission line 20-2 ((1) in the figure), a fault current occurs in the DC transmission line 20-3, which is the return line ((2) in the figure). This causes the return line potential on the ungrounded side (point A in the figure) to rise, and the current in the DC transmission line 20-1, which is the healthy pole, fluctuates ((4) in the figure, the current component that increases after the fault). As the potential on the side (ungrounded side) of the DC transmission line 20-1 (healthy pole main line) connected to the power conversion device 10-3 rises, the current component that flows from the high-potential side of the power conversion device 10-3 toward the power conversion device 10-1 increases.

[0021] The power converters 10-2 and 10-4 of the fault pole detect the fault current and stop operation to protect against overcurrent. Here, for example, if the power converter 10-1 on the ground side of the healthy pole receives excessive active power, the voltage (for example, capacitor voltage, or cell capacitor voltage in the case of MMC, which will be described later) may rise and exceed the protection threshold, potentially causing the power converter 10-1 to stop operation. In this embodiment, the power converter 10-1 performing Inversion control temporarily blocks the gate, thereby avoiding malfunctions during a fault and suppressing a decrease in the operational continuity of the healthy pole. Specific control details will be described later.

[0022] (Configuration of power conversion device) The configuration of the power conversion devices 10-1 to 10-4 will be described below with reference to the drawings. Note that the same configuration can be applied to the power conversion devices 10-1 to 10-4, and therefore, hereinafter, the power conversion devices 10-1 to 10-4 will be collectively referred to simply as "power conversion device 10" except when the power conversion devices 10-1 to 10-4 are to be individually described.

[0023] FIG. 3 is a diagram illustrating an example of the configuration of a power conversion device according to an embodiment. As illustrated in FIG. 3, the power conversion device 10 includes a power converter 110 and a switching control unit 120. The power converter 110 performs Rec control and Inv control based on the control of the switching control unit 120, and converts AC power into DC power and vice versa. The power converter 110 is a circuit configured using self-extinguishing switching elements such as IGBTs (Insulated Gate Bipolar Transistors) or MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). In this embodiment, the power converter 110 is a modular multilevel converter (hereinafter referred to as MMC).

[0024] In the example of Fig. 3, a grid-connecting inductor Ltr is provided between the power converter 110 and a grid-connecting point P1. Part or all of the grid-connecting inductor Ltr may be replaced by a leakage reactance of a reactor or a transformer. Also in the example of Fig. 3, a measurement transformer VT is provided at the grid-connecting point P1. The measurement transformer VT measures, for example, the AC voltage of each phase and outputs the measured AC voltage of each phase to the switching control unit 120. The grid-connecting inductor Ltr and the measurement transformer VT may be provided, for example, on the side of the power conversion device 10.

[0025] The switching control unit 120 includes, for example, a DC current control unit 121, an AC current control unit 122, and a gate command generation unit 123. The switching control unit 120 realizes the DC current control unit 121, the AC current control unit 122, and the gate command generation unit 123 as functional units by, for example, a hardware processor such as a CPU executing a program (software) stored in a storage unit (not shown). Furthermore, some or all of these components may be realized by hardware (including circuitry) such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), or a GPU (Graphics Processing Unit), or may be realized by a combination of software and hardware.

[0026] The DC current control unit 121 generates a DC voltage command value based on the DC current Idc of the DC transmission line obtained from the power converter 110 and a DC current command value Idc* (a current command value based on an active current command value). The AC current control unit 122 acquires the AC current (e.g., three-phase AC currents Isr, Iss, Ist) and AC voltage (e.g., three-phase AC voltages Vsr, Vss, Vst) of the AC system (AC power supply) from the power converter 110, and generates an AC voltage command value based on the acquired AC current and AC voltage so as to control the AC current to a predetermined AC current value or AC voltage value.

[0027] The gate command generation unit 123 generates a gate command value to be input to the power converter 110 based on the DC voltage command value output from the DC current control unit 121 and the AC voltage command value output from the AC current control unit 122, and outputs the generated gate command value to the power converter 110. As a result, the power converter 110 switches the switching elements in the power converter 110 according to the input gate command value, thereby adjusting the DC voltage Vdc.

[0028] For example, the gate command generation unit 123 receives as input a voltage specification value (a constant or a value adjusted to control the DC current) output as a DC voltage command value from the DC current control unit 121 and voltage command values ​​Vr*, Vs*, Vt* output as AC voltage command values ​​from the AC current control unit 122. Then, the gate command generation unit 123 calculates and outputs gate commands gtp, gtn, ... to be given to switching elements inside the power converter 110 so that each voltage command value is output in a pseudo manner to the AC terminal and the DC terminal of the power converter 110.

[0029] The gate command generation unit 123 includes, for example, a protection control unit 130. When detecting the spread of a fault caused by a fault occurring in another pole, the protection control unit 130 temporarily gate-blocks the switching element of the power converter 110. The reason for including such a protection control unit 130 is to prevent a decrease in the continuity of operation of a healthy pole while avoiding malfunctions during normal operation or in an accident event (e.g., an AC system fault) in which the switching element of the power converter 110 should not be gate-blocked. In this embodiment, an example in which the protection control unit 130 is provided in the gate command generation unit 123 will be described, but the protection control unit 130 may be provided outside the gate command generation unit 123 or outside the switching control unit 120. Details of the protection control unit 130 will be described later.

[0030] Fig. 4 is a diagram illustrating an example of the configuration of a power converter according to an embodiment. The power converter 110 illustrated in Fig. 4 includes a plurality of legs LG between a positive electrode (terminal P shown in the figure) of a DC system and a negative electrode (terminal N shown in the figure) of the DC system.

[0031] The number of legs LG corresponds to, for example, the number of phases of AC power supplied by the AC system. In this embodiment, the AC system supplies three-phase AC power: a first phase (the R phase shown in the figure), a second phase (the S phase shown in the figure), and a third phase (the T phase shown in the figure). Therefore, the power converter 110 includes a leg LGr corresponding to the R phase, a leg LGs corresponding to the S phase, and a leg LGt corresponding to the T phase. In the following description, when there is no need to distinguish between the legs LGr, LGs, and LGt, they will be collectively referred to as "legs LG."

[0032] A certain phase of the three phases of AC power supplied by the AC system is connected to leg LG. Leg LG may be connected to a certain phase via a transformer as necessary. Specifically, the R phase is connected to leg LGr, the S phase is connected to leg LGs, and the T phase is connected to leg LGt. In the following description, the connection point between leg LGr and the R phase will be referred to as connection point CPr, the connection point between leg LGs and the S phase will be referred to as connection point CPs, and the connection point between leg LGt and the T phase will be referred to as connection point CPt.

[0033] In the following description, a portion having the same potential as terminal P of the DC voltage output by the power converter 110 will also be referred to as terminal P of leg LG, and a portion having the same potential as terminal N of the DC voltage will also be referred to as terminal N of leg LG. The legs LG have similar configurations. In the following description, the configuration related to leg LGr will be designated with the suffix "r", the configuration related to leg LGs will be designated with the suffix "s", and the configuration related to leg LGt will be designated with the suffix "t". When there is no need to distinguish between the configurations related to each leg LG, the "r", "s", or "t" will be omitted. Below, leg LGr will be described as a representative of the legs LG.

[0034] The leg LGr includes two groups of n cells CL (cells CL1-1r to CL1-nr and cells CL2-1r to CL2-nr shown in the figure) and a plurality of reactors RT (reactors RT1r and RT2r shown in the figure). Here, n is a natural number. The cells CL are, for example, half-bridge circuits, and the details of their configuration will be described later. Here, the group of cells CL between the terminal P of the leg LG and the connection point of each phase will also be referred to as a positive-side arm unit. In addition, the group of cells CL between the connection point of each phase and the terminal N of the leg LG will also be referred to as a negative-side arm unit.

[0035] In the positive arm unit of leg LGr, cells CL1-1r to CL1-nr are connected in series in the order shown from terminal P toward node CPr, and these are connected to node CPr via reactor RT1r. In addition, in the negative arm unit of leg LGr, cells CL2-1r to CL2-nr are connected in series in the order shown from node CPr toward terminal N, and these are connected to node CPr via reactor RT2r.

[0036] The interconnection inductor Ltr shown in FIG. 3 is an equivalent interconnection impedance that collectively represents the effective inductance for the AC currents Isr, Iss, and Ist of each phase. If no separate reactor or transformer is provided, this interconnection inductor Ltr is 0.5 times the inductance of the reactor RT for each arm. In other words, if the inductance of the reactor RT is L, then Ltr = L / 2 [H]. Furthermore, if a reactor or transformer with inductance L' is connected to the AC terminal side, then Ltr = L' + L / 2 [H]. If the power converter 110 is not an MMC but has a configuration similar to a general two-level converter without a reactor in the arm, then the inductance of the reactor or transformer leakage reactance connected to the AC side terminal simply matches Ltr.

[0037] FIG. 5 is a diagram showing an example of the configuration of a cell according to an embodiment. As described above, the cell CL is, for example, a half-bridge circuit. The cell CL shown in FIG. 5 includes, for example, a plurality of switching elements Q (switching elements Q1 to Q2 shown in the figure), diodes D (diodes D1 to D2 shown in the figure) in a number corresponding to the number of switching elements Q, and a capacitor C. The switching element Q is, for example, an IGBT. However, the switching element Q is not limited to an IGBT. The switching element Q may be any element as long as it is a self-extinguishing switching element capable of realizing the function of a converter or an inverter. In this embodiment, a case will be described in which the switching element Q is an IGBT.

[0038] The switching element Q1 and the switching element Q2 are connected in series. The switching element Q1 and the switching element Q2 are connected in parallel with the capacitor C. Each switching element Q and a diode D are connected in parallel. Specifically, the switching element Q1 and the diode D1 are connected in parallel with each other, and the switching element Q2 and the diode D2 are connected in parallel with each other.

[0039] The cell CL has a positive terminal connected to the terminal P of the leg LG and a negative terminal connected to the terminal N. The positive terminal of the cell CL is connected to the connection point between the switching element Q1 and the switching element Q2, and the negative terminal of the cell CL is connected to the emitter terminal of the switching element Q2. In the following description, the voltage generated between the positive and negative terminals of the cell CL is referred to as the cell voltage Vo.

[0040] Each switching element Q is provided with a switching terminal (not shown) that switches the switching element Q on and off. The switching terminal is connected to the switching control unit 120, and a control signal is input thereto. Specifically, a gate command gtp is input as a control signal to the switching element Q1, and a gate command gtn is input as a control signal to the switching element Q2. Each switching element Q is switched on or off based on the control signal, thereby charging or discharging a capacitor C included in the cell CL. In addition, the cell CL is provided with a voltage detector (not shown) that detects a capacitor voltage Vc, which is the voltage of the capacitor C.

[0041] The capacitor voltage Vc may be the average of the voltages of all the capacitors C provided in the cells CL of the power converter 110, or may be the average of the voltages of some of the capacitors C provided in the cells CL of the power converter 110. Alternatively, the capacitor voltage Vc may be the voltage of a pre-selected capacitor from among the capacitors C provided in the cells CL of the power converter 110.

[0042] A control signal that turns on the switching element Q is represented as "1," and a control signal that turns it off is represented as "0." When (gtp, gtn) = (1, 0), the cell voltage Vo becomes the capacitor voltage Vc, and when (gtp, gtn) = (0, 1), the cell voltage Vo becomes 0 [V]. In this way, a multi-level waveform can be generated by switching the switching element Q provided in each leg LG.

[0043] Note that setting the switching element Q to (gtp, gtn) = (1, 1) is prohibited because it would short-circuit the capacitor C. Also, to prevent the state of the switching element Q from transiently becoming (gtp, gtn) = (1, 1) during switching, the switching element Q is usually controlled to a transient state (gtp, gtn) = (0, 0) for a very short time (dead time). Also, when the switching control of the switching element Q is stopped, it is fixed to a state of (gt, gtn) = (0, 0). Stopping the switching control of all switching elements Q of the power converter 110 is called gate blocking, and this state is called a gate blocked state.

[0044] FIG. 6 is a diagram illustrating an example of a protection control unit according to an embodiment. As illustrated in FIG. 6, the protection control unit 130 includes a first comparator 131, a second comparator 132, a first computing unit 133, and a second computing unit 134. The first comparator 131 compares the DC current Idc of the DC transmission line obtained from the power converter 110 with a predetermined current reference value Iref. The current reference value Iref is set according to the magnitude of the DC current Idc at which a power conversion device on a healthy pole side (e.g., the power conversion device 10-1) becomes unable to continue operating due to potential fluctuations in the DC transmission line 20-3, which is a return line. Specifically, the current reference value Iref is set to a value that can detect the DC current Idc at which the power conversion device on the healthy pole side becomes unable to continue operating. For example, the first comparator 131 outputs “1” when the DC current Idc is greater than the current reference value Iref, and outputs “0” when the DC current Idc is equal to or less than the current reference value Iref.

[0045] The second comparator 132 compares the capacitor voltage Vc, which is the voltage of the capacitor C shown in FIG. 5, with a predetermined voltage reference value Vref. The voltage reference value Vref is set according to the magnitude of the capacitor voltage Vc at which the power conversion device on the healthy pole side (e.g., the power conversion device 10-1) becomes unable to continue operating due to potential fluctuations in the DC transmission line 20-3, which is the return line. Specifically, the voltage reference value Vref is set to a value that can detect the capacitor voltage Vc at which the power conversion device on the healthy pole side becomes unable to continue operating. For example, the second comparator 132 outputs "1" when the capacitor voltage Vc is greater than the voltage reference value Vref, and outputs "0" when the capacitor voltage Vc is equal to or less than the voltage reference value Vref.

[0046] The first computing unit 133 performs an operation on the comparison result of the first comparator 131 and the comparison result of the second comparator 132. When the comparison result of the first comparator 131 indicates that the DC current Idc is greater than the current reference value Iref and the comparison result of the second comparator 132 indicates that the capacitor voltage Vc is greater than the voltage reference value Vref, the first computing unit 133 outputs a signal indicating that a fault has been detected. For example, the first computing unit 133 outputs "1" when the output of the first comparator 131 is "1" and the output of the second comparator 132 is "1." For example, a "1" output from the first computing unit 133 indicates that a fault has been detected due to a fault occurring on another pole. For example, the first computing unit 133 outputs "0" when at least one of the outputs of the first comparator 131 and the second comparator 132 is "0." Note that the first computing unit 133 may be, for example, a logical product circuit.

[0047] The second computing unit 134 performs an operation on the fault signal transmitted from the power converter of the fault pole (for example, power converter 10-2) indicating the occurrence of a fault, and the signal output from the first computing unit 133. Here, the power converter of the fault pole (for example, power converter 10-2) is configured to be able to transmit the fault signal to the power converter of the healthy pole (for example, power converters 10-1 and 10-3). This is to prevent the pole from being stopped by notifying the power converter of the fault pole from the fault signal, even if the healthy pole power converter cannot detect the fault spread caused by the fault that occurred at the fault pole.

[0048] When the signal output from the first computing unit 133 is a signal indicating that a fault has been detected, or when an accident signal is input, the second computing unit 134 outputs a gate block command to stop switching control of all switching elements Q of the power converter 110. For example, the accident signal is a signal with a value of "1," and the value of the signal output from the first computing unit 133 indicating that a fault has been detected is also "1." When at least one of the accident signal and the signal output from the first computing unit 133 indicating that a fault has been detected is input, the second computing unit 134 outputs a gate block command with a value of "1." Note that, for example, a logical OR circuit can be used as the second computing unit 134.

[0049] Furthermore, after outputting the gate block command, the protection control unit 130 releases the gate block when it confirms that the AC circuit breakers (e.g., AC circuit breakers 40-2 and 40-4) at the fault pole are open. Alternatively, the protection control unit 130 releases the gate block when the time required for opening the AC circuit breakers (e.g., AC circuit breakers 40-2 and 40-4) at the fault pole has elapsed. The time required for opening the AC circuit breakers at the fault pole is, for example, about three AC cycles.

[0050] In this way, when the protection control unit 130 detects an event that may cause a healthy pole to stop protection due to the spread of an accident caused by an accident that occurred at the fault pole, it temporarily gate blocks the power converter 110. Then, when the AC circuit breaker at the fault pole is opened and the accident is resolved, the protection control unit 130 quickly releases the gate block to restore the power converter 110. As a result, in this embodiment, it is possible to prevent malfunctions during steady operation or accident events (e.g., AC system accidents) that should not cause gate blocking of the switching elements of the power converter 110, while suppressing a decrease in the continuity of operation of the healthy pole.

[0051] Although the above-described power conversion system 1 has two sets of DC transmission lines between the high potential side and the low potential side, the embodiment can be applied to a configuration having at least two sets. In other words, the embodiment can be applied to a configuration having three or more sets of DC transmission lines between the high potential side and the low potential side.

[0052] According to the embodiment described above, the power conversion device 10 includes at least two pairs of DC transmission lines 20, each pair having a high potential side and a low potential side, and a multi-pole configuration in which either the relatively high potential side or the relatively low potential side of each pair of DC transmission lines 20 is shared with either the relatively high potential side or the relatively low potential side of another pair of DC transmission lines 20, and the power converter 110 includes a plurality of capacitors C and a plurality of switching elements Q that can switch between charging and discharging the capacitors C, a switching control unit 120 that controls the switching elements Q provided in the power converter 110, and a protection control unit 130 that temporarily gate-blocks the switching elements Q of the power converter 110 when detecting the spread of an accident caused by an accident occurring at another pole. This makes it possible to provide a power conversion device and a DC transmission system that can prevent malfunctions in the event of an accident and suppress a decrease in the continuity of operation of a healthy pole.

[0053] Specifically, in this embodiment, in the power conversion device 10 of the healthy pole, an abnormal current inflow that may cause the voltage of the capacitor C of each cell CL provided in the power converter 110 to become an overvoltage and a resulting rise in the capacitor voltage Vc are detected using the first comparator 131, the second comparator 132, and the first computing unit 133. This makes it possible to appropriately perform temporary gate blocking while avoiding malfunctions during steady-state operation or in accident events (e.g., AC system accidents) that should not cause the converter to be gate blocked.

[0054] Furthermore, in this embodiment, in the event of an accident that threatens the continuation of operation of the healthy pole, a temporary gate block is performed on the power conversion device 10 of the healthy pole based on accident information from the fault pole, thereby avoiding bipolar shutdown. Furthermore, in this embodiment, the gate block is released when it is confirmed that the AC circuit breaker of the fault pole has been opened, or when the time required for the AC circuit breaker of the fault pole to be opened has elapsed. This allows the operation of the power conversion device 10 of the healthy pole to be quickly restored, thereby minimizing the impact on continuity of operation.

[0055] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims.

[0056] With respect to the above embodiment, the following supplementary notes are disclosed as one aspect and optional features of the invention.

[0057] (Appendix 1) The system has at least two sets of DC transmission lines each having a high potential side and a low potential side, and is provided in a multi-pole configuration in which either the relative high potential side or the relative low potential side of each set of DC transmission lines is shared with either the relative high potential side or the relative low potential side of another set of DC transmission lines; provided between the high potential side and the low potential side of each set of DC transmission lines, a power converter including a plurality of capacitors and a plurality of switching elements capable of switching between charging and discharging the capacitors; a switching control unit that controls the switching elements provided in the power converter; a protection control unit that temporarily gate-blocks the switching element of the power converter when detecting a fault spread caused by a fault occurring on another pole; A power conversion device comprising:

[0058] (Appendix 2) In the power conversion device according to Supplementary Note 1, The protection control unit may include a first comparator that compares a DC current flowing between the high potential side and the low potential side of the DC transmission line with a predetermined current reference value.

[0059] (Appendix 3) In the power conversion device according to Supplementary Note 2, The protection control unit may include a second comparator that compares a capacitor voltage, which is a voltage of the capacitor, with a predetermined voltage reference value.

[0060] (Appendix 4) In the power conversion device according to Supplementary Note 3, the protection control unit includes a first computing unit that computes a comparison result of the first comparator and a comparison result of the second comparator, The first computing unit may output a signal indicating that the accident has been detected when the comparison result of the first comparator indicates that the DC current is greater than the current reference value and the comparison result of the second comparator indicates that the capacitor voltage is greater than the voltage reference value.

[0061] (Appendix 5) In the power conversion device according to Supplementary Note 4, The power conversion device includes a transmitter that transmits an accident signal indicating an accident when an accident occurs, the protection control unit includes a second computing unit that computes the fault signal transmitted from another of the power electronics devices and the signal output from the first computing unit, The second computing unit may output a command to temporarily gate block the switching element of the power converter when the signal output from the first computing unit is a signal indicating that the accident spread has been detected, or when the accident signal is input.

[0062] (Appendix 6) In the power conversion device according to any one of Supplementary Note 1 to Supplementary Note 5, The protection control unit may temporarily gate-block the switching element of the power converter, and then release the gate block when it confirms that an AC circuit breaker at the other pole where the accident occurred is open, or when the time required for opening the AC circuit breaker has elapsed.

[0063] (Appendix 7) A multi-pole configuration having at least two sets of DC transmission lines each having a high potential side and a low potential side, in which either the relatively high potential side or the relatively low potential side of each set of DC transmission lines is shared with either the relatively high potential side or the relatively low potential side of another set of DC transmission lines; The power conversion device according to any one of Supplementary Note 1 to Supplementary Note 6 is provided between the DC transmission lines on the high potential side and the low potential side of each of the pairs. DC transmission system. [Explanation of symbols]

[0064] 10...power conversion device, 20...DC transmission line, 110...power converter, 120...switching control section, 130...protection control section, 131...first comparator, 132...second comparator, 133...first computing unit, 134...second computing unit, C...capacitor, Q...switching element

Claims

1. The system has at least two sets of DC transmission lines each having a high potential side and a low potential side, and is provided in a multi-pole configuration in which either the relative high potential side or the relative low potential side of each set of DC transmission lines is shared with either the relative high potential side or the relative low potential side of another set of DC transmission lines; provided between the high potential side and the low potential side of each set of DC transmission lines, a power converter including a plurality of capacitors and a plurality of switching elements capable of switching between charging and discharging the capacitors; a switching control unit that controls the switching elements provided in the power converter; a protection control unit that temporarily gate-blocks the switching element of the power converter when detecting a fault spread caused by a fault occurring on another pole; A power conversion device comprising:

2. 2. The power conversion device according to claim 1, wherein the protection control unit includes a first comparator that compares a DC current flowing between the high-potential side and the low-potential side of the DC transmission line with a predetermined current reference value.

3. The power conversion device according to claim 2 , wherein the protection control unit includes a second comparator that compares a capacitor voltage, which is a voltage of the capacitor, with a predetermined voltage reference value.

4. the protection control unit includes a first computing unit that computes a comparison result of the first comparator and a comparison result of the second comparator, the first computing unit outputs a signal indicating that the accident has been detected when the comparison result of the first comparator indicates that the DC current is greater than the current reference value and the comparison result of the second comparator indicates that the capacitor voltage is greater than the voltage reference value. The power converter according to claim 3.

5. The power conversion device includes a transmitter that transmits an accident signal indicating an accident when an accident occurs, the protection control unit includes a second computing unit that computes the fault signal transmitted from another of the power electronics devices and the signal output from the first computing unit, the second computing unit outputs a command to temporarily gate-block the switching element of the power converter when the signal output from the first computing unit is a signal indicating that the accident spread has been detected or when the accident signal is input. The power converter according to claim 4.

6. 2. The power conversion device according to claim 1, wherein the protection control unit temporarily gate-blocks the switching elements of the power converter, and then releases the gate block when it confirms that an AC circuit breaker for the other pole where the fault occurred is open, or when a time required for opening the AC circuit breaker has elapsed.

7. A multi-pole configuration having at least two sets of DC transmission lines each having a high potential side and a low potential side, in which either the relatively high potential side or the relatively low potential side of each set of DC transmission lines is shared with either the relatively high potential side or the relatively low potential side of another set of DC transmission lines; The power conversion device according to any one of claims 1 to 6 is provided between the high potential side and the low potential side of each of the sets of DC transmission lines. DC transmission system.

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